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350 GM Tune issue popping back thru Carb |
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MrMcD 
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Joined: January-28-2014 Location: Folsom, CA Status: Offline Points: 3590 |
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 Topic: 350 GM Tune issue popping back thru CarbPosted: July-14-2021 at 4:08am |
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Trying to help a friend with his Pro Am Skier, a boat not well known, not many sold. A 20" tournament ski boat, Center mount engine GM 350 SKI rated 260 HP I believe a 1990 model year, runs a 13 x 13 prop standard rotation. My buddy bought this boat brand new at the Sacramento Boat show and it has been a pretty good boat for him since. I did not check but guess his hours are near 1,000.
He started having issues recently, boat won't run quite right. Running rough at idle, won't pull a load, stumbles and spits back through the carb. My buddy put a new Distributor in, a GM HEI knock off. He also installed a new rebuilt carb, new spark plugs. Still had issues. He put it in a Marine Shop local to his home and they charged $250 and told him the connection to his HEI was the only problem. It was not fixed and runs the same. I listened to it as we followed his boat down the lake. Asked him to bring it by my house and we looked at it. Right away we found 2 wires crossed, #3 and #6 in his Chevy. ( boat shop did not find that issue) The boat fired up and ran really well with the wires fixed. Timed it and it ran even better on the Driveway. Decided to check total timing, revved the engine from idle up to 4,000 RPM watching the timing . Timing was jumping, not consistent, I saw 10* variance. Pulled the chinese knock off distributor and re installed his old GM HEI factory unit. Spark was no longer jumping and engine sounded great. I believe the 1990 GM 350 used an iron camshaft sprocket not the Nylon trouble sprocket so I did not expect timing wear as the issue. The GM HEI Distributor only offers 20* advance so to give him 34* total timing I had to set initial to 14*. At 14* the engine would pop on all quick throttle applications. Popping back through the carb. I backed the timing off to 8* at idle which limited his Total to 28*. Not good but without modifying the distributor that is where we set it. It ran fantastic at idle and revved good without popping back through the carb. Carb on this is a Rochester not a Holley and these tests were only on my driveway. He took the boat for a water test, I was not able to join him. He says boat now starts perfectly and runs fantastic up to 2,600 RPM, where it starts popping, once past this point it runs good again, the 4 barrel opens and it revs to 4,000 max. I think the 4,000 RPM limit is caused by the 28* total timing limit. On the water he advanced the timing again to check top speed, told me the engine popped much worse so he put it back to 8* and left it. The 3 and 6 plugs were both black when we took them out, after discovering the crossed wires. We cleaned them up and re installed these plugs. Pulled all 8 plugs after this water test and all 8 look perfect, very light in color almost white. LEAN BURN??? My buddy is installing 8 new plugs and new plug wires as he is fed up with the issue. I am not sure that is the cause and wanted to ask for advice and experience from this group. Maybe a plug is mis firing but it should have been at least a little black when pulled? I am thinking the crossed wires probably happened when he installed the new Chinese HEI and the problem he was fighting probably existed and still exists before and after the new HEI and Carb. We have a camp trip coming up, 5 days on the water and I want to help him get it running great again. He says it has never been overheated, gets fresh oil every season, new plugs every season and even new it never revved over 4,200 RPM with the 13x13 prop. I think the boat runs heavy with a lot of hull drag and with only 28* advance the engine is held back. I have driven it only once or twice in 30 years and skied behind it a couple times. Ski wake was good in this boat. I wish we had done a compression check while I had it. He said all the popping seemed to come out one exhaust, not both. Wide open to any ideas, would like to fix it before our next trip. Thanks Mark
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TRBenj
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Joined: June-29-2005 Location: NWCT Status: Offline Points: 21107 |
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Posted: July-14-2021 at 8:53am |
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14 initial shouldn’t cause any popping. 28 total shouldn’t cause a significant lack of top end. 14/34 should make it happy, and if not, I would not suspect a timing issue. All new plugs is a good start, I’d put new wires on also. Does the recently reinstalled “old hei” have a fresh cap and rotor? Do that if not.
Carb has been consistent through all this? I’d dump it if cap/rotor/wires don’t cure it. With today’s gas, it’s hard to read plugs beyond extremes, but if it’s clean to light brown then it’s fine… specks of aluminum would be bad. |
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GottaSki
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Joined: April-21-2005 Location: NE CT Status: Offline Points: 3327 |
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Posted: July-14-2021 at 9:27am |
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Yes what Tim sez.
Now, are these Vortec heads? i understand they don't like big final advance numbers. Could the pops be a lean pop from buggared up primaries? |
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"There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing."
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TRBenj
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Joined: June-29-2005 Location: NWCT Status: Offline Points: 21107 |
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Posted: July-14-2021 at 9:53am |
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Valid point with the Vortec heads not needing/wanting too much timing… though the ones I’ve run seem to take low 30’s just fine. That shouldn’t be the case on a 260hp from 1990 unless swapped in though, ‘96 was the year the vortecs showed up in marine engines and would’ve been rated 290+.
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GottaSki
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Posted: July-14-2021 at 10:30am |
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Thanks for providing those details Tim
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"There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing."
River Rat to Mole |
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MrMcD
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Joined: January-28-2014 Location: Folsom, CA Status: Offline Points: 3590 |
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Posted: July-14-2021 at 11:54am |
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The heads are the older 76cc truck style pre Vortec heads so old style.
My friend already bought new plugs and wires yet to be installed. To run 4,000 with the 4bbl opening I think it must be close on the tune. Maybe the previous fouled plugs are hit and miss. My buddies wife is patient but now worn out with boat trouble so pressure is cranking up. We used the new cap on the old HEI unit, his old cap looked old with some frosting. The caps on these include the coil and we stayed with the new one off the Chinese HEI unit. I struggle with it starts and runs perfect to 2600 then starts popping and hesitating before running hard again. He says it could pull wakeboards but no skiers running like this. I do appreciate the advice. Plugs that came out after recent lake trip with engine popping. All 8 look the same but I am having photo issues loading. My Malibu ran this identical engine, it ran great and I found it liked 34* timing. At 36* I had some pinging. I thought about the float level but I don’t know the Rochester at all to even know if it could starve on primary’s and still be good on secondary’s. He says his idle sounds great in and out of gear and does not smell rich at all.  |
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MrMcD
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Posted: July-14-2021 at 5:17pm |
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Updated previous note.
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KENO
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Posted: July-15-2021 at 8:17am |
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I think I'd get the boat back and do that compression check you wish you had done along with the previously mentioned new plugs and wires.
Now for a silly question or 2..............he replaced the distributor with a Chinese HEI, was it a marine distributor? Is the GM HEI distributor a marine unit or something automotive that got swapped in at some point over the years. From what I remember, the Pro Am boats used Mercruiser engines and Mercruiser didn't use that distributor in any form as a marine unit.. So it makes me wonder, is it a marine replacement Quadrajet, jetted right for the application or an automotive carburetor that might be jetted for automotive fuel economy on those tiny primaries?
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MrMcD
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Posted: July-15-2021 at 12:55pm |
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Ken I know both Distributors were Automotive. Can’t say on the carbs but I will check with my bud Brian he is out of town for a few days. He installed the first HEI in 1990 right after boat purchase.
Fingers crossed on plugs and wires fixing it but I am not optimistic. You remember the Pro Am brand? Brian’s boat is the only one I have seen. No idea how many were made. I think the plant was in Oregon.
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KENO
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Posted: July-15-2021 at 3:53pm |
We've seen the same number of them then.  I don't think there were many made. I think all the Pro Am models had the 260 HP Mercruiser packageThe company was Bluewater boats in Oregon. The gel coat work looked great, like most of those boats from that country you call home where they seemed to be masters of glass work long before anybody else came close There's a little info on the internet, some of it hints that they were one of the numerous brands built by Fineline Industries for other companies. I think a lot of the companies getting into or trying to get into the ski boat market at that time saw the success of a little homegrown operation called Malibu and wanted a piece of the action
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MrMcD
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Joined: January-28-2014 Location: Folsom, CA Status: Offline Points: 3590 |
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Posted: August-18-2021 at 1:11pm |
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Final Wrap, we just got back from a week long Ski and Camp trip. The Pro Am ran like brand new once again. Brian found his spark plug wire boots were long enough that the actual clip inside was not clipping on to all the spark plugs. He cut them all about 1/4" shorter and re installed plus he chose to clean re gap and use the same plugs since they only had 10 hours on them. The boat runs great, he only ran it up to 4,000 RPM since they have had issues for 3 years and did not wish to spoil a good thing. Translation is they don't trust it yet and keep waiting for it to break down. This is after we found he had plug wires 3 and 6 swapped on the distributor cap, and he had been fighting the lousy running condition for almost 3 years, new distributor, Rebuilt Carb, etc. The only thing I did not like was his OEM GM HEI spark advance comes on to fast. If the distributor was set so total advance was 34 the engine started acting up on acceleration so he pulled it back to 28* total and idle set at 8* and it ran great. I think the distributor needs new springs but he is so happy to have a useable boat that he chose to leave it be. His boat, his decision. With a carburetor that dang boat will start every time by just hitting the key, no pump of the throttle to prime it. Dead cold it does this and the exhaust does not run rich. Acts like it has a good fuel injection system on it.
Brian installed the HEI distributor right after buying this boat back in 1990, I asked what his wide open RPM was and he said it never tached over 4,200. I bet this HEI limited his spark advance to 28* this whole time. I was curious to see what his RPM would be if we advanced so it had at least 34-35*. My Malibu with the same engine and prop would rev to at least 4,600 RPM. I mentioned to Brian that he might have slipped the plug wires into the boot a little deeper rather than cut them shorter. He said it works and he is done. The good news is he got a great week on the water with no issues. And my Nautique Ran like a star the whole week. Adjusted the prop shaft packing for the first time since install 7 years back and the bilge stayed dry. With the Acme 422 it tops out at 4,800 RPM with only one person in the boat but is smooth and pulls well.
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MrMcD wrote: