Vacuum Leak Under Carb
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=1621
Printed Date: November-22-2024 at 3:31pm
Topic: Vacuum Leak Under Carb
Posted By: Jim_In_Houston
Subject: Vacuum Leak Under Carb
Date Posted: May-07-2005 at 5:07pm
While flushing my engine and spraying my carb bolts with WD-40 I discovered I have a vacuum leak between the carb and the manifold. I have a 1" aluminum spacer (with a PCV hose fitting) between the carb and the manifold so two gaskets are required. Both gaskets appear to be leaking. The gaskets are new, the fittings are clean, and the carb is well tightened down. The new gaskets I installed on my recent carb upgrade seem very thin. Evidently this has been leaking ever since I installed the carb. (No wonder I couldn't get my idle down.)
Does anyone here use a sealant on the carb fitting? I have never, in my life, had to but I either need thicker gaskets or some kind of sealant. Anyone?
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Happy owner of a '66 and a '68 Mustang
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Replies:
Posted By: jimbo
Date Posted: May-07-2005 at 5:39pm
Hey Jim. I replaced those gaskets a couple of weeks ago. It made a world of difference. I ordered the gaskets from skidim. They were a little thicker than the standard automotive ones I've seen and they weren't paper.
My friend's father was an auatomive mechanic in the 70s. He told me to make sure the spacer plate is completely flat. He said it's aluminium and can warp. He told me to lay a piece of sandpaper on the garage floor and slide the plate across it then look at the plate. It should have tiny scratches evenly across the surface. If it's warped, the lower points won't contact the sandpaper and won't be scratched. If that's the case, he said to keep rubbing it on the sandpaper until it becomes completely flat.
Now, having said all that, I didn't do it. I put the gaskets on, tightened everything down and it didn't leak so I didn't worry about it.
What do y'all think of the above method?
A new spacer plate is only around $20.
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Posted By: jameski
Date Posted: May-08-2005 at 1:46am
Excellent advice Jimbo. Jim, I'm sure that the vacuum leak is the source of your high idle - and it should run much better when you get it sealed tight. I think most people don't use any sealer on those flange gaskets, but I do (I don't EVER want to have another vacuum leak!). I use the copper gasket spray aerosol and I only spray on a light mist. I don't use it on some carbs. It depends on whether there are any small passages on the base plate that could be clogged - if so, I will spray the gasket with armor-all or wipe it with motor oil (anything to make it swell a little bit before you smash it). Also, if you spray them with copper gasket spray you can probably forget about ever removing and re-installing them during future maintenance. On some carbs I'll oil one side and spray the other - that way I can remove it later and possibly re-install it (I always do cork valve cover gaskets this way).
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current boat
http://www.correctcraftfan.com/diaries/details.asp?ID=1977 - 94 Sport Nautique
previous boat
http://www.correctcraftfan.com/diaries/details.asp?ID=601 - 78 Martinique
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Posted By: David F
Date Posted: May-08-2005 at 11:44am
Jim:
Jimbo is right on about how to ensure the wedge is flat. However, I would suggest you put the sand paper on plate glass as your garage floor is probably not very flat. Use wet/dry paper with plenty of water.
If you want to use a sealant, then I suggest Hylomar. It is a blue sealant that never gets hard and you can easily remove the carb and reseal many times. It was designed for such a purpose.
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Posted By: Guests
Date Posted: May-08-2005 at 12:51pm
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I would really avoid the sealent. Most of the time it will squeeze into the vacuum chamber/channel of the plate and restrict air flow and over time a piece can break free and glog the vacuum port completely. Never use sealent on a carb, Always use Holley gaskets for holley carbs and make sure the mating surfaces are clean and flat and you will not have a problem.
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Posted By: David F
Date Posted: May-11-2005 at 3:01pm
79...:
I generally agree with you, but if Jim wants to try a sealant, then Hylomar will avoid the pitfalls you mention. Only a very thin film would be required and Hylomar never changes properties so nothing would be able to "break loose" and clog anything. What it will do is fill the voids caused by uneven surfaces. You can even remove and refit the parts without reapply the sealant and it will seal again.
FWIW: Hylomar was developed for the avaition industry. Specifically service covers on jet engines that must be sealed. The sealant allows the covers to be removed and reinstalled with reapplying the sealant. It is truely amazing stuff.
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Posted By: GottaSki
Date Posted: May-11-2005 at 4:00pm
Fel-pro makes some very nice gaskets for these flanges, gotta be 3/32" thick or more. Better than the paper gaskets in the rebuild kits.
Check the throttle plate for flatness too..too much torque will warp it.
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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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Posted By: 79nautique
Date Posted: May-11-2005 at 4:05pm
I've never usaed the stuff but it sounds like it is more of a rubberized coating than sealent. It's just most of the time everyone gets a little happy with the RTV and applies more than is needed, just a word of caution more than anything else. Usually if you use quality gaskets and prep the surfaces correctly then there isn't a problem with leaks.
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http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique
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Posted By: Jim_In_Houston
Date Posted: May-11-2005 at 10:17pm
Problem solved. I picked up two new Fel-Pro gaskets and installed. Leak is gone. I had to re-adjust the idle screw and idle mixture screws. I did sand a little on the aluminum spacer. It was not perfectly flat. The engine nows idles smoothly at 600 RPM. I was attributing the high rough idle speed to my new cam. Sometimes you're just wrong. (Hard to beleive huh?) It makes me wonder why those paper thin gaskets get shipped with the carb. Maybe now I can get my wake down and put an end to all the people shaking fists and hollering at me at the marina. Thanks fellows.
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Happy owner of a '66 and a '68 Mustang
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