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Troubleshooting an acceleration bog/hesitation

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    Posted: August-10-2022 at 3:40pm
Prelude: I have found quite a few posts on diagnosing and fixing various forms of acceleration bogs/hesitation, but many of them fail to follow-up with solutions, so I thought I'd solicit the collective expertise of the forum with the promise of following up with whatever ends up solving my issues.

Background: 1989 Ski Nautique 2001, original PCM 351 with just over 500 hours, Holley 4160, prestolite distributor with breaker points, initial timing is 10* BTDC at ~700 RPM. When I bought the boat last fall, it didn't want to idle, especially when warm, so previous owner had cranked idle speed up (I think it was around 1000 RPM or so) I think in an attempt to compensate. Also had an off-idle/part throttle hesitation or bog where the engine wouldn't respond right away when either taking off under hard acceleration, or when getting back on the throttle after slowing down. I suspected the idle issues were due to a vacuum leak around the carb base/spacer, and indeed when I got the original cast iron spacer off and started to machine it flat it was pretty obvious it was a ways out of flat. So I ordered a new phenolic spacer from Summit, and the thick carb gaskets from Skidim, which I think is what fixed the idle issue. But I also rebuilt the carb while I had it off the intake with a Holley marine rebuild kit from Summit, did not find any clogged passages, and it was pretty obvious someone had been in there before me, lots of tool marks from scraping and such on gasket surfaces and passages on metering block and carb body. Got it all back together and it ran much better, no more idle issues when warm, and no longer noticed the hesitation. One problem fixed, but now noticed that my idle speed would hang up after a hard pull until I'd shut the key off and turn it back on. Mostly with the help of searching this forum, I traced this to the secondary butterflies hanging up, due to a loose return linkage, probably because previous mechanics had to adjust it when they cranked down the primary curb idle screw to compensate for the idle issues. After reading some more about carb tuning and such (always a dangerous thing), I realized that I probably didn't set up the primary butterfly position for proper transfer slot exposure, so wanted to pull the carb back off anyways to check and adjust. Readjusted primaries to just expose the small square of transfer slot, and just barely had to crack the secondary off the stop to get the idle speed right. And after bending back the secondary stop linkage, my idle now returns back down to 700 ish every time. Yay. Fast-forward to...

The problem: I am now noticing quite a bog/hesitation on a hard acceleration from idle, and to be honest I'm not sure if it has been there the whole time or if I'm just now noticing it, as I hadn't had the good fortune to pull an adult slalom skier out of a deep water stop until a few weeks ago. What happens is, if I start at idle and just "drop the hammer", the engine just starts to accelerate for a split second, and then stops suddenly for a second or two, then catches up and pulls hard. It feels as if someone actually pulls back on the throttle for a second, then slams it back forward. No audible missing or backfiring, no black smoke, just seems like it loses power for a second or two. The boat otherwise runs fine; will pull up to 47 MPH on a GPS speedometer, idles fine in and out of gear. I can repeat this without a skier in the water too if I just throw the throttle down hard to try to accelerate as fast as possible (in gear only, throttle seems to respond normally in neutral and will rev nice and crisp).

What I've tried: I checked timing with a timing light, 10* initial, and I do see it advance smoothly as I rev the engine. Pulled breaker plate off distributor and inspected advance weight operation, they seem to move nice and freely without binding. Points gap set to .018, new points set and condenser last season (a story for another day, I had originally put an EI module in there, reverting back to points fixed another weird issue I was having where the engine would feel like it would "skip" occasionally when cruising), timing checked after adjusting points of course. Also played with moving timing up or down but the engine seemed to want to idle best around 10 so left it there. I did check for vacuum leaks with some carb cleaner around carb base after the last time I had the carb off to adjust the primaries, but admittedly I need to do a more thorough job of this (with a spray bottle of water this time probably as I've learned since then). I did adjust the idle mixture, but only in the driveway, have NOT had a chance to do that in gear like I know I should yet, will do that this weekend when we take the boat up to the lake for the week. In neutral, they do seem to like to be the default 1.5 turns out though, and if I turn them in much more than that the engine tries to die on me, so I know the engine is idling off the idle circuit and so I think the mixture screws are responsive enough (this also got a lot better after I had re-adjusted the primary butterflies to expose less of the transfer slots, and idle also feels a lot strong in gear now with this adjustment).

What I know I have left to check: I've been reading a ton about what causes hesitation in these engine/carb combos, and of course all this sounds like a problem with the accelerator pump circuit. So I've verified that my accelerator pump linkage is adjusted so that the pump arm is touching the linkage at idle, and that there is no slop between the throttle linkage and accelerator pump arm (i.e., the pump arm moves as soon as the throttle is moved). I get a good squirt of gas from both nozzles as soon as I move the throttle from idle, so that adjustment seems to be okay. However, I did notice last night that the pump cam (it's the white colored cam, FYI) was set on the #2 position as opposed to the #1 position. Based on my reading, the #1 position should be used for idle speeds <900 RPM to bring the full lift of the pump cam in sooner, while the #2 position should be used for idle speeds >900 RPM to delay the lift profile of the pump cam to compensate for the higher idle position. I assume that the pump cam was moved to the #2 position by a previous mechanic to compensate for the high idle speed, but who knows. I moved it back to the #1 position last night but haven't had a chance to water test yet. I think my expectations for this having much impact are low though, since most folks seem to say that pump cam adjustments can help you go from good to better, but not from bad to good. I also need to retune the idle mixture in gear, but the boat does shift pretty nicely in and out of gear now without much change in idle speed so I suspect this is fairly close as-is, and since this should only affect the idle characteristics and not so much the acceleration, if I understand correctly, my expectations are also low for that adjustment having too much impact. Please correct me if I'm wrong! I also haven't tested with a hard pull since adjusting the secondary stop linkage; I don't expect this will have a big impact, but I had thought that when that linkage was loose, maybe that would have allowed a little more air into carb leaning it out a little when the vacuum dropped on a hard pull before the secondary circuit actually kicks in. 

So next up is a water test to check performance after the adjustment to the secondary stop and the accelerator pump cam position, then adjust idle mix in gear and re-test to rule that out. What am I missing? Any other ideas? Since the boat seems to run strong up to 47 MPH at WOT I don't really think the fuel system (before the carb) is to blame, but I did recently replace the (very scary and degraded) fuel line, the fuel/water separator (replaced the old black and orange FRAM can with a spin-on canister type), and the anti-siphon valve was checked last season (but haven't had that apart yet this season since I haven't had reason to suspect it).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: August-10-2022 at 8:27pm
It sounds to me like you've done plenty of research, checked lots of things, straightened out lots of things and you should continue on with the plan you have Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MrMcD Quote  Post ReplyReply Direct Link To This Post Posted: August-11-2022 at 12:28am
If it still bogs on hard acceleration as you mentioned open the engine cover on the water and watch your secondaries as you accelerate.  Makes me think they are opening way too early causing the bog.  Not sure what could allow this?   Not too many things can allow a great idle and a great cruise or wide open throttle but bog so severely.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brhillman Quote  Post ReplyReply Direct Link To This Post Posted: August-11-2022 at 12:43am
Thanks for the vote of confidence Ken. Mark, that's a good idea, I'll definitely do that this weekend if it's still bogging after my adjustments.

After speaking with Johnny Quest today, I couldn't leave well enough alone and pulled the carb back off to check and blow out the accelerator pump circuit, and confirm the nozzle size and power valve that were installed in it. Also sought out the Holley listing chart (http://documents.holley.com/techlibrary_carb_numerical_listing.pdf), which shows that, based on my list number (50463), this carb should have originally come with a 0.025 nozzle, and a 125-25 (2.5 inHg) power valve. The nozzle on there now is indeed stamped 25, and I had installed the 5.0 power valve that came in my 703-29 rebuild kit, which I think should be a better fit for this application than the lower vacuum 2.5 power valve that Holley lists. Based on what I've read and heard, it sounds like a 6.5 or so would be even better, but maybe 5.0 is close enough for the moment.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote steve600 Quote  Post ReplyReply Direct Link To This Post Posted: August-11-2022 at 7:33am
Keep focused on the accelerator pump, sounds like you’ve done a thorough rebuild and lots of research. My experience in off idle hesitation on three different 4160’s has been solved with small adjustments to accelerator pump and cam while out on the water. I have really no technical facts of what helped each one, just learned on each that very small adjustments to the arm made a big difference, the cam in my experience didn’t really change alot. All power valves were 6.5. I think what helped alot was tuning the carbs under load. Not sure if this will help much but just my experience with hesitation on these carbs.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brhillman Quote  Post ReplyReply Direct Link To This Post Posted: August-11-2022 at 10:29am
Thanks Steve, that’s helpful to know.

Started on the hose this morning to check for fuel leaks after getting the carburetor back on last night. Wanted to check for vacuum leaks while I had it running, so sprayed water all around the carburetor base and could not notice any change in idle RPMs. Didn’t trust it so I repeated with carburetor cleaner and again could not detect any noticeable change in RPM, so I don’t think I’m looking at a vacuum leak there.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brhillman Quote  Post ReplyReply Direct Link To This Post Posted: August-11-2022 at 8:00pm
Some updates and thoughts.

I discovered today that I *can* reproduce the hesitation (and even get a backfire) when running with no load on the hose if I try to rev the engine too quickly from idle. I never noticed this before, probably because I'd never tried to go from idle to WOT immediately when running without a load on the engine. I don't think this is expected behavior, but correct me if I'm wrong; I don't think I've ever gotten a backfire out of a car by revving it too fast. Armed with the knowledge that I seem to be able to reproduce the issue in the driveway and don't have to put the boat in the lake to test every time I make a change, and knowing that I still apparently have an issue and my previous efforts have failed, curiosity got the better of my and I tore back into the carburetor. I was also a little worried about the two little o-rings that go on the ends of the accelerator circuit transfer tube (probably not calling that by the correct name, it's the piece the goes between the metering block and the body of the carb and feeds the accelerator nozzle, Holley part # 26-23 I think it is) that looked a little torn. So, I pulled the carb back off, replaced the o-rings (had to buy a Holley rebuild kit to get them, my local Napa didn't have the o-rings themselves and we're taking the boat to the lake this weekend and don't have time to order, so those were $50 o-rings), put a tiny amount of lube on them so they wouldn't tear putting the metering block back on (the mistake I made last time I think), and replaced the power valve with a 6.5 while I was at it for good measure. Checked out the pump diaphragm and everything looked fine there, no tears or anything. It's also a new green one out of the rebuild kit I put in last fall. Put it back together and...still hesitates and wants to backfire on me if I open the throttle too quickly. I pulled the flame arrestor off and worked the throttle a bit (being very cautious to keep everything away from the top of the carb and to not work the throttle to point of backfire) and do see a nice spray of fuel out of the nozzles when working the throttle, right up till the primary venturis start spraying as well. Observing a carb in action is kind of a beautiful thing...

Anyways, still seem to have a hesitation. Thinking about trying a different pump cam, but I don't really think that's going to solve the issue, and I'm starting to wonder if it's not carburetor related after all. So 1) I think I need to take timing measurements at various RPMs and make sure my timing is advancing in a reasonable way to make sure advance weights aren't sticking or something (even though I already checked this), and probably first verify my TDC mark is correct (thanks JQ for that suggestion) but 2) random long-shot, would an exhaust restriction present this way? I started to notice a strange occasional and irregular knocking sound, and today while running the boat on the hose I think I narrowed the source down to the muffler (my 89 has the invertaflow muffler). Kind of sounds like something is floating and bumping around in there, so I'm wondering if maybe some baffle came loose (I have no idea what these things look like inside) and is partially blocking the flow through the muffler.

Thoughts?

Sidebar: Good news is I'm getting better at swapping the carburetor on and off and pulling the bowls off, but I still hate that inverted flare fitting on the fuel inlet.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote swilliams Quote  Post ReplyReply Direct Link To This Post Posted: August-11-2022 at 11:27pm
Have you checked your throttle shafts for excessive play? I ran into the same situation and found that they were pulling in too much air, creating a stumble.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brhillman Quote  Post ReplyReply Direct Link To This Post Posted: August-12-2022 at 1:41am
That’s a good thought, I haven’t really checked that, but they are loose enough that if I fiddle with the accel pump too much with the engine off without opening the primaries and let fuel pool on the primary butterflies it will leak out a little through the throttle shaft. How did you tighten that up on yours when you had that problem?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote swilliams Quote  Post ReplyReply Direct Link To This Post Posted: August-12-2022 at 6:57am
Mine was a Quadrajet so it entailed having the throttle plate re-bushed. I am not that familiar with Holleys but the same principle applies. You could check while engine is running the same way you checked for gasket leaks. Worth checking out as it could be that simple!?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brhillman Quote  Post ReplyReply Direct Link To This Post Posted: August-12-2022 at 9:12am
Yeah for sure, thanks for the suggestion!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brhillman Quote  Post ReplyReply Direct Link To This Post Posted: August-16-2022 at 12:57pm
A small update: the big hesitation and backfire through the carb when revving fast in neutral seems to have gone away since replacing the ignition coil, which is good news. I am just left with a slight hesitation on acceleration under load. Tuning the idle mixture under load didn’t seem to have too much impact, but did notice when I tuned the mix screws that they didn’t have too much of an impact passed about a full turn or so out. I have an assortment of accelerator pump cams arriving today, will see if swapping the white to blue makes any difference.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote steve600 Quote  Post ReplyReply Direct Link To This Post Posted: August-16-2022 at 12:59pm
Great news! Definitely keep playing with the accelerator pump, you’ll get it
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brhillman Quote  Post ReplyReply Direct Link To This Post Posted: August-29-2022 at 5:30pm
A small update, I'm still chasing this issue. I had ordered the assortment of accelerator pump cams, thinking maybe I needed a bigger pump shot as the blue cam might give, thinking I had the white cam on there which is one of the smaller cams. Well, after pulling the cam out, while it was white in appearance it was in fact a 427 cam, same as the blue. So, that's probably not the issue, but might try swapping to a smaller pump cam anyway just to see if there's any change.

Some other thoughts though and an update on my thinking: I had the carb apart on the workbench again a couple weeks ago, and noticed one of the high speed air bleeds on the second side appeared a little varnished over. I cleaned these out pretty easily with the bristle from a brush and blew out with carb cleaner. I finally got a chance to test this again yesterday and...no change, BUT this did get me thinking about the secondary circuits and what else could be causing this bit of bog/hesitation on hard acceleration. To be clear, this hesitation I'm experiencing is not just a failure to immediately accelerate from idle, but rather it seems like the boat actually starts to accelerate for a split second, then falls flat, then picks back up. Does this sound like it could be an issue with the secondary circuit? I think Mark suggested earlier visually inspecting the carb operation under load to see when the secondaries open, with the thought that perhaps they could be opening way too early, which I think would create an extremely lean condition initially since the vacuum wouldn't be there yet to start pulling fuel through the secondary jet holes in the secondary metering plate. The engine would then pick back up as vacuum in the carb caught up to start pulling fuel down through the secondary venturi's. I haven't done this check yet, but it's next on my list for the next time I can get the boat out. I've also heard of folks testing this by temporarily wiring the secondary linkage closed. If the hesitation goes away, then that seems to point to an issue with secondaries opening. On thoughts as to what would cause this though, perhaps a too light or worn out spring in the secondary vacuum diaphragm assembly? I think a ruptured or poorly sealing diaphragm would just mean that the secondaries never open, which I don't think would cause this issue but rather a limiting of top end, is that right? One last thought, when I rebuilt the carburetor, the secondary sealing plate (the thin stamped plate that goes between the metering plate and main body to seal off the power valve cavity) was in pretty bent up shape, probably from a previous mechanic prying on the plate trying to pull it off an old gasket surface. I tried my best to bend it back into as flat a shape as possible, but now I'm wondering if there's maybe some kind of internal leakage here between the metering plate, sealing plate, and main body of the carburetor.

Any other thoughts much appreciated, will update again once we get the boat back out on the water without our toddler running around so I can do the secondary test with the engine cover up.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KevinGodwin Quote  Post ReplyReply Direct Link To This Post Posted: January-25-2023 at 6:09pm
Wow - glad I stumbled on this thread!  I have a '90 SN with the same problem. I have had the boat for 17 years and this problem only started this past summer and I have spent too much money at the marina trying to fix.  Monkeying around with the accelerator pump did improve things but the damn thing isn't consistent.  Some times its smooth then all of a sudden its rough. The secondary idea sounds interesting.  Please let me know what the fix is when you find it!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brhillman Quote  Post ReplyReply Direct Link To This Post Posted: January-25-2023 at 6:42pm
For sure. Didn’t solve the problem last summer, planning to pull the carb back apart before recommissioning this spring. Will definitely update this thread when/if it’s resolved.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mille1sj Quote  Post ReplyReply Direct Link To This Post Posted: March-26-2023 at 12:27pm
I had the same issue on our 77 Ski Tique after a rebuild. While at idle or low cruising speed, it would bog or hesitate going to WOT. I change the accelerator pump nozzle and the issue was fixed. Didn’t end up needing to change the cam on the carburetor. I ordered a few different size nozzles to try out.

Good luck!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brhillman Quote  Post ReplyReply Direct Link To This Post Posted: March-26-2023 at 8:43pm
Was the pump nozzle noticeably restricted? Mine looked fine last time I had it apart but I’ll definitely check it again when I get it back on the bench.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mille1sj Quote  Post ReplyReply Direct Link To This Post Posted: March-27-2023 at 8:07am
Mine was not noticeably restricted, but I went with a larger size which seemed to give it the extra fuel it needed at the moment you put the throttle down.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jonny Quest Quote  Post ReplyReply Direct Link To This Post Posted: March-28-2023 at 2:56am
My "list" of possible suspects:

1.  Accelerator pump (diaphragm in particular)
2.  Accelerator pump linkage that is not properly adjusted
3.  Nozzle / squirter size

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Post Options Post Options   Thanks (0) Thanks(0)   Quote KevinGodwin Quote  Post ReplyReply Direct Link To This Post Posted: June-18-2023 at 9:32pm
Hi BR - any luck getting to the bottom of it? I thought my stuttering was gone but it came back today. It’s been running great through May and early June, but today was warm and humid. It feels like it’s missing, vibrations very noticeable. Wonder if it’s electrical. LMK if you have made any headway.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brhillman Quote  Post ReplyReply Direct Link To This Post Posted: June-19-2023 at 12:41am
Hey Kevin, no I haven’t we just had our second baby a couple weeks ago so haven’t had time to get the boat out more than once. I went back through the carb again before getting it back on the water this spring, and everything looked fine and seemed to be operating properly. Got it all back together, adjusted the timing, and took it out a month or so ago. Ran great other than that persistent hesitation under hard throttle. I too was starting to suspect something electrical or timing. It seemed to improve a bit when I changed the ignition coil last summer.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KevinGodwin Quote  Post ReplyReply Direct Link To This Post Posted: June-19-2023 at 7:26pm
Congrats on baby #2!

Coil was my next stop and another few cans of sea foam! Keep up the good fight.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brhillman Quote  Post ReplyReply Direct Link To This Post Posted: June-20-2023 at 12:48am
Thanks!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KevinGodwin Quote  Post ReplyReply Direct Link To This Post Posted: July-20-2023 at 2:32pm
Hi BR, hope the new baby is doing well.  

After cleaning the carb, which didn't help but was a good learning exercise, I changed the coil (used Flamethrower brand) and spark plugs and that seems to have done the trick!  I am still having a little stuttering at the very bottom end but is getting better as I dial the carb in which has been harder than I thought it would.  Fingers crossed it holds.  I hope you get yours fixed.

K




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Post Options Post Options   Thanks (0) Thanks(0)   Quote brhillman Quote  Post ReplyReply Direct Link To This Post Posted: August-01-2023 at 5:35pm
A small update. First, I took some video that illustrates the issue. Showing RPM on a hard acceleration from idle. The quality of the video is terrible because of conversion from iPhone video to gif for hosting on imgbb to embed here, but you get the idea: RPM initially jumps as the engine gets that shot of fuel from the accelerator pump, then drops, then climbs back up and will continue to climb all the way up to top end without issue. My impression is that this might indicate that the duration of the pump shot isn’t long enough. I tried moving the pump cam from the #1 to the #2 position (and of course then readjusting the pump linkage to zero clearance). This didn’t have much impact, maybe a little bit better but that might just be wishful thinking. Any thoughts?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote brhillman Quote  Post ReplyReply Direct Link To This Post Posted: August-01-2023 at 5:39pm
Also, the idea that maybe pump shot duration isn’t long enough has me wondering if maybe I should try a SMALLER pump nozzle. The engine absolutely had a vacuum leak when I bought it, which I fixed (carb spacer leaking when engine would warm up), so I wonder if someone tried to drill out the pump nozzle at some point to compensate for the extra air getting into the mixture. When I had last pulled the nozzle I didn’t have pin gauges to verify size, so this is an unknown variable I guess. It definitely wasn’t restricted.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bwinn Quote  Post ReplyReply Direct Link To This Post Posted: August-01-2023 at 6:17pm
I’m no help but I gotta say, I don’t think I’ve ever seen a video embedded in a post like that. Kudos!
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Joined: April-21-2005
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GottaSki Quote  Post ReplyReply Direct Link To This Post Posted: August-02-2023 at 8:41am
Yes
Seeing your PV is now 6.5  as recommended,  go smaller ap nozzle, textbook for a longer duration shot
"There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing."

River Rat to Mole
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 91NaughtyQ Quote  Post ReplyReply Direct Link To This Post Posted: August-03-2023 at 11:50am
Originally posted by Jonny Quest Jonny Quest wrote:

My "list" of possible suspects:

1.  Accelerator pump (diaphragm in particular)
2.  Accelerator pump linkage that is not properly adjusted
3.  Nozzle / squirter size

JQ

1 and 2 specifically 1 and making sure linkage adjusted properly has always fixed this issue when it pops up on my 91.  It seems I get about 2 years out of the diaphragm before it looses some "spring".
1991 Ski Nautique
(Previous)1984 Ski Nautique 2001
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