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what are the possible vacuum leak locations?

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andrewmarani View Drop Down
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    Posted: July-29-2019 at 4:59pm
I’ve got low vacuum, runs around 11 PSI at 800 RPM. I checked by installing a T into the line between the PCV valve and the manifold. The Holley Carb was rebuilt this spring. 1986 Silver Nautique, Ford 351. A new gasket was installed between the carb and the manifold and spraying carb cleaner around this gasket doesn’t cause any change in RPM.

Some background. I was having a stumble or stall when hitting the throttle from an idle. Found the timing was at 0, reset to 10 BTDC at 800 RPMs. Had to reset the idle screws on the carb and the curb idle screw. Now idles nicely at 800 when warmed up and accelerates smoothly, but my vacuum is low as noted above. I did have to set the carb idle screws out around 2 ½ turns, which is further than most people seem to suggest here.

My question is are there any other vacuum leak areas to check besides around the carb itself, between the carb and the manifold and the hose running between the PCV valve and the manifold? Checked all those and couldn’t find any leaks.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: July-29-2019 at 7:03pm
From reading your words, it sounds like it runs good right now, so I'm wondering if you know if your gauge is accurate?

Is that 800 rpm in gear or idling in neutral?

You have a gasket between the carburetor spacer and the intake manifold and another between the carburetor and the top of the spacer. Did both get checked/sprayed.

You can also have a leak internally on the gasket between the intake manifold and head .

You can check that the intake manifold bolts are torqued to about 23 to 25 ft lbs (Per my PCM and Ford manuals). You can find the sequence with a Google search.

Have you done a compression test lately?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote andrewmarani Quote  Post ReplyReply Direct Link To This Post Posted: July-29-2019 at 7:28pm
The gauge is about a year old but I don't know how to check it's accuracy.

800 RPM in neutral.

On the spacer, I didn't think I had one (probably showing my ignorance on engines here), is it a cast-iron item roughly 1 1/2" or 2" thick that the line from the PCV valve hooks into? It's directly under the carb. The boat is a half hour away, so I am looking at some pictures of the engine and it looks like there might be a gasket under the fitting point that the PCV line hooks into. If that's the spacer, I don't think it's been removed in the 29 years I've owned the boat. I'm not even sure how to unbolt it. The carb bolts look like they screw into it and i don't remember any other bolts being visible when I pulled the carb off.

If the spacer was leaking, I would have expected the engine to speed up when I sprayed the carb cleaner at the other gasket above it. There's not much space between them.

Thoughts?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: July-29-2019 at 10:09pm
That's the spacer that you're describing.

If it's idling smoothly, responds smoothly and otherwise runs fine, I'd kinda wonder if you have a vacuum leak at all (other than the 11 inch gauge reading)

If you want to pull the spacer off though, take the carburetor off, then the spacer slides off over the carburetor studs

I'd have a couple of the SkiDim gaskets in the link below on hand , one for the bottom of the spacer and one for the top. They're wide and shaped right around the edges so that they seal properly. They're thick too.

link

Here's a picture of the bottom of the spacer itself



I'd try the gauge on another engine to see if it reads low or reads a more normal number as a quick check of the gauge.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote andrewmarani Quote  Post ReplyReply Direct Link To This Post Posted: July-29-2019 at 10:20pm
Got it. I will look for a way or engine to check the vacuum gauge. I probably know someone that has one I can compare it to.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote robertbruder Quote  Post ReplyReply Direct Link To This Post Posted: August-14-2019 at 9:31pm
Andrew,

My boat is doing the same thing. Did you find a resolution?

Mine is idling fine between 700 and 800 RPM depending on idle screw and adjustment screw settings. Just replaced the gasket between the carburetor and spacer as engine would rev when hit with carb cleaner. Vacuum gauge fluctuates between 15 and 17. I am just holding it up to the vacuum port on the spacer so that might account for the fluctuation. It will idle fine, but as soon as I ease it into gear the engine stalls.

I am hoping we are experiencing the same issue and can help each other out and that I am not hi-jacking your thread
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gary S Quote  Post ReplyReply Direct Link To This Post Posted: August-14-2019 at 10:24pm
If your using a Pleasurecraft spacer under your carb or have a Pleasurecraft engine you must use a Pleasurecraft sourced gasket. Aftermarket gaskets ie Holley,Mr Gasket are a tad too small and will leak. I have yet to find an aftermarket that works,there may be one out there but it was easier just to buy the PCM one. Gasket link
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Post Options Post Options   Thanks (0) Thanks(0)   Quote robertbruder Quote  Post ReplyReply Direct Link To This Post Posted: August-14-2019 at 11:42pm
Thanks Gary. That is exactly what I used between the Carb and the spacer. Should have changed the one between the spacer and block at the same time but only thought to buy one originally. I will change it out and let you guys know if that does the trick.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 80SN Barnfind Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 8:13am
A related issue I had a couple years ago HERE

Ken, what do you make of this intake and spacer- automotive?



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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 9:08am
Originally posted by 80SN Barnfind 80SN Barnfind wrote:

A related issue I had a couple years ago HERE

Ken, what do you make of this intake and spacer- automotive?





That's all original PCM /Ford in those pictures.

The gaskets time had come

That's the "good gasket" that gets mentioned frequently that SkiDim sells, it just gets affected in that spot by hot exhaust .

That manifold on a car uses that passage for the heat riser to heat the automotive non electric choke.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 80SN Barnfind Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 12:20pm
Ken, the spacer you show above is solid in the area of the intake heat riser, my spacer doesn't have that-its open all the way around. Did PCM have variations in the spacer at some point? My ultimate fix was machining a slug of aluminum and press fitting into the the spacer corresponding to the heat passage in the intake, essentially creating what you have shown. All good since.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 12:30pm
There were spacer variations but they all like the PCM style gasket because they fit the best
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Post Options Post Options   Thanks (0) Thanks(0)   Quote andrewmarani Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 12:59pm
Robert, the adjustments I mentioned at the top seemed to have solved the hesitation issue. We were out last Friday and it ran well. The first time I hit the throttle it hesitated and stalle, but after that it ran great and didn't have the hesitation issue at all. Might not have been completely warmed up... Idle has creeped up to 900 for some reason but I didn't want to play with the curb idle when I had a boat full of people waiting to be pulled up and down the creek.

I'm going to pull the carb and carb spacer in a couple of weeks when both kids are off to college and replace the gaskets with those noted above. Just ordered two of them. Once I do that, I will check back with the results on the vacuum gauge.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 1:21pm
I don't think replacing the gaskets will change your vacuum because at least for me, when one of those is leaking you have a lousy idle and lousy off idle response and it seems like you're running good with your adjustments.

it wouldn't hurt to replace them though as a preventive measure
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jonny Quest Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 2:04pm
I recently installed an Edelbrock Performer RPM aluminium intake manifold on my 1994 351 High Output. I had a small vacuum leak when I first fired the engine up. I used an unlit propane torch to find the source of the leak. It ended up being a bad rubber hose from the PCV valve to the large vacuum port (full manifold vacuum). I didn't have a carb spacer as it was removed due to the fact the Edelbrock manifold coupled with the QuickFuel carb and spark arrestor would be too tall to fit under the engine cover.

The vacuum leak was due to micro cracks in the hose where the hose was inserted into the vacuum port. So, a new PCV valve and new hose and my vacuum leak was solved.

+1 on the PCM carby gaskets. If you use the cast iron manifold and PCM carby spacer, you need 1 PCM gasket between intake and spacer and another between spacer and carburettor. Also, I beleive in proper torque procecdures. I use an "X" pattern when torquing down the carby hold-down nuts to 60-18 inch-pounds of torque.

Vacuum leak suggestions from Holley:
If a rough idle persists after the engine has been started and the mixture screws adjusted, check for manifold vacuum leaks. These could result from unplugged vacuum fittings or a carburetor flange gasket that was torn during installation. Recheck for proper attachment of all vacuum lines and check the lines for cracks. If the manifold was changed, a manifold vacuum leak could occur at the cylinder head/manifold surface due to damaged gaskets or improper torquing. Frequently, manifold vacuum leaks occur from the valley side of the manifold. These are very difficult to detect, unless a discernible whistle can be heard. In most cases, when rough idle occurs after a carburetor/manifold change, they result from manifold vacuum leaks similar to those described above. Assuring a proper manifold installation, rather than assuming the carburetor is not functioning properly, will ultimately save time.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote woodyelc Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 2:14pm
FYI PCM used the aluminum carb spacer from 1971 thru 1985. Cast iron was used because of the heat riser hole. You will need a cast one if the intake has that hole.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gary S Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 2:53pm
Interesting Woody,I have one of each wondered why they had two types. Well now you know
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 5:25pm
Here are the 2 spacers Woody talks about oriented the same way for the pictures

The wide PCM gasket is more critical on the later cast iron spacer



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Post Options Post Options   Thanks (0) Thanks(0)   Quote gun-driver Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 6:13pm
I would think you could fill the void with JB Weld and be done.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote robertbruder Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 9:17pm
Wow. I just came in from the dock to do some reading up. Thanks everyone for the input. This is my first time actually diagnosing a carburetor rather than just replacing with a new one from National Carburetor.

So I ordered the SkiDim gaskets but I am impatient and also picked up a Mr. Gasket.

Mr. Gasket Part

I took off the spacer and yikes, a gasket change was definitely needed!
Old Gasket Between Spacer and Engine Block


Luckily I have the old style so the Mr. Gasket fits fine.


I opened the adjustment screws up about two full turns.

I was able to get it to start and then it would go up to about 1000 RPM and then work its way down to 500 - 600 RPM and then stumble and die . Next I adjusted the idle screw screw to get the engine to idle without dieing. That put RPM up to about 1000 RPM. After that I slowly tightened the adjustment screws going back and forth until the engine started stumbled and died. Then I opened each up about 1/8th of a turn and restarted.

Now it will start and idle fairly well but there is an occasional drop in vacuum pressure followed by a jump in RPM. Whenever I ease it into gear it immediately stumbles and dies Does this indicate there is still some sort of air leak? The vacuum measures 5 in Hg / 19 cm Hg.

I am thinking that the suggestion about the hose coming out of the spacer or pcv valve at the exhaust manifold/header may be a factor? So I put a hose clamp on that pcv valve and as part of my vacuum gauge set up I put a brand new the pcv hose coming off the port on the spacer to a T-Adapter. The old hose is still inline going from the T-Adapter to the pcv valve at the exhaust.

I am wondering if I even have the correct PCV valve? I didn't see one on SkiDim. Can someone tell me the correct PCV valve make/model?

Here is a link to some pictures of before and after, my vacuum gauge set up, and some video of it starting, idling, vacuum gauge, and then dieing when put into gear.
Full Album Carb Gasket Change

I have been searching for videos and reading material if anyone has any suggestions on what I should read to learn more.

I kind of feel like I am chasing my tail at the moment. Can someone point me in the right direction as to whether I should continue to check for leaks, focus on idle adjustment, or rebuild the carburetor?

Thanks!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote robertbruder Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 9:43pm
Found this video link regarding tuning the carb mixture screws in another thread and it has proven very helpful.

video
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Post Options Post Options   Thanks (0) Thanks(0)   Quote robertbruder Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 10:15pm
So I just plugged the side of my T adapter that went to the old hose and the PCV Valve on the exhaust and saw readings in the 15 in Hg range. So it looks like I will be picking up some PCV tomorrow.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote robertbruder Quote  Post ReplyReply Direct Link To This Post Posted: August-15-2019 at 10:36pm
Looking through some Forums, Keno answered my question about the PCV valve. "Tell the parts guy it's for an 84 F-250 with a 351W".

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Post Options Post Options   Thanks (0) Thanks(0)   Quote robertbruder Quote  Post ReplyReply Direct Link To This Post Posted: August-16-2019 at 6:56pm
Ok so I have replaced the gaskets per above and just replaced the PCV valve and PCV Hoses. Vacuum Gauge reads steady at 15 in Hg. Unfortunately Tach holds steady at 1000 RPM. The curb idle screw is all the way backed up. Backing off the ignition adjustment screws (leaning the mixture) any further results in a drop in vacuum.

Is this still a carb problem at this point? Float adjustment? Choke adjustment?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8122pbrainard Quote  Post ReplyReply Direct Link To This Post Posted: August-16-2019 at 7:51pm
Originally posted by robertbruder robertbruder wrote:

Ok so I have replaced the gaskets per above and just replaced the PCV valve and PCV Hoses. Vacuum Gauge reads steady at 15 in Hg. Unfortunately Tach holds steady at 1000 RPM. The curb idle screw is all the way backed up.

Have you tried disconnecting the throttle cable? Wondering if the throttle cable is holding it back any?


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gary S Quote  Post ReplyReply Direct Link To This Post Posted: August-16-2019 at 8:27pm
+1
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Post Options Post Options   Thanks (0) Thanks(0)   Quote robertbruder Quote  Post ReplyReply Direct Link To This Post Posted: August-16-2019 at 8:28pm
Thanks Pete. The throttle cable looks lined up as far as I can tell and I did have it idling at around 600 but it stalls in about the time it takes me to turn the ignition determine it is idling and get to the vacuum gauge. I maybe get 30 seconds on the vacuum gauge before it stumbles and dies unless I turn out the adjustment screws to about 2 full turns.

Given the shape of the gaskets I doubt the carburetor has ever been renewed. I just happen to have a renew kit so headed out to get some carb cleaner. First carb rebuild!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rbuk Quote  Post ReplyReply Direct Link To This Post Posted: August-16-2019 at 9:14pm
Hey all. I just picked up a 1984 Ski Nautique and Im chasing some vacuum issues as well. Not to derail but can someone please tell me where I am supposed to have the line running off the top drivers side of my carb running to? I currently have it capped off as it just sucks air inn. I have no idle issues but know it's supposed to go somewhere thanks !
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Post Options Post Options   Thanks (0) Thanks(0)   Quote robertbruder Quote  Post ReplyReply Direct Link To This Post Posted: August-16-2019 at 10:43pm
Hey rbuk.

I am copying Keno from another thread here, but is this possibly the port you are referring to?

"The small port to the left of the choke has a very small flow thru it to provide cooler air to the choke element housing to prevent the element from overheating itself. It's a built in small vacuum leak by design "

"Not a good spot to hook a vacuum gauge. "
Thread from quote
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rbuk Quote  Post ReplyReply Direct Link To This Post Posted: August-16-2019 at 11:59pm
Thanks Robert!

That a makes sense due to the idle not being affected by this "built in vacuum leak" Always something to learn...
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