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Wed Aug 04, 2010 11:24 am

Puzzled by this one. Twin carbs. I've put fibre gaskets between my inlet manifold and head - put them on dry, torqued to 14 lbs and I've got a vacuum leak on both sides.

Replaced them with a fresh pair, coated with a permatex type sealant (called wellseal) - and they leak as well - although not so bad. I'm using the incense stick to check and you can clearly see the smoke flowing between the seal.

The manifold surfaces are clean and flat and heads are new, no nicks or burrs.

Does anyone recommend using an RTV type sealant on these? I don't know whether it's suitable to use it where gas is involved?

Wed Aug 04, 2010 11:26 am

Sure you don't have a piece of tin pinched under the manifold?

Wed Aug 04, 2010 11:30 am

gosh, that's a hard one to check, I'll have a look - thanks!

Wed Aug 04, 2010 12:52 pm

I had a corner of the tin under the intake.

RB

Wed Aug 04, 2010 1:10 pm

OK jut checked and there's no tin snagging up on the intake.

What's the general view - sealant or dry?

Wed Aug 04, 2010 1:11 pm

huh - fibre gaskets there? I used the aluminum "crush" seals... no?

Wed Aug 04, 2010 1:13 pm

So do the aluminium crush seals work better than fibre?

Wed Aug 04, 2010 1:14 pm

that I dont know - perhaps the fibre squishes so much under the 14ftlbs that its breaks up/cracks a little?!

hahah, now i'm getting carried away!

Wed Aug 04, 2010 1:18 pm

Driving me mad....those aluminium gaskets are just so expensive here.

Wed Aug 04, 2010 1:39 pm

kubicixfactor wrote: Driving me mad....those aluminium gaskets are just so expensive here.

i feel your pain. Same here.

Have you tried the tears of defeat?

i have awful luck with the crush gaskets, and permatex didn't work so hot for me either.

Is there any contamination around the studs?
Is the metal on the head clean enough to do surgery on? especially that small bit under the port. the tiniest particles seem to get dragged into the mating surface from there when the manifold is being put on.

I just warped my freshly resurfaced intake runners by torquing to 14ft/lb if it makes you feel better.

the incense trick is a good one. I'll have to try that. Less dangerous than shooting aerostart at a running engine via one of those little tube things that go in the nozzle of the can.

To be honest I raised the white flag and bought some thick gasket paper and a craft knife. Made a stencil and made my own gaskets. Worked out a lot better for me than those %$*$ crush gaskets.

Wed Aug 04, 2010 2:33 pm

Good idea - interested to know if that worked. I'm going to get the manifolds resurfaced and try again - a bit less pressure this time than 14 lbs at first and gradually tighten it down. Think this is the cause of my hot heads so at least I know what I need to focus on.

Wed Aug 04, 2010 2:34 pm

What do you me an by fiber gaskets? You mean "paper" gaskets?

If so, the best way is to use RTV...but install them "dry" or cured.

Here is the problem.....if you have nicely machined mating surfaces on both sides (manifold and head) there is very little texture for teh paper gaskets to crush down into and seal. Add to that the fuel soaks through them and allows vacuum to suck through.
This is also partly the fault of most paper gaskets being too smooth and hard in texture.

What you want is a crushable, flexible layer on both sides of the gasket to seal the microscopic crevices.

Go to an art supply shop and buy a rubber or plastic roller about 3-4" wide used for inking rubber stamps or block printing plates. Should cost you about $6.
Squeeze out some good quality, high temp, fuel and oil reistant RTV...like Permatex ultra.....onto a sheet of glass, plastic or wax paper on a flat surface. Work the roller back and forth in it until you have a uniform sheet sticking to the roller.

Roll one side oft hegasket back and forth. It wil create a micro-thin uniform layer of gasket material....usually less than .001" thick. Let it dry until it is able to be picked up without putting finger marks in it.
Flip it over.....do teh other side and wait until it drys again.

Install this...and it will not leak. Ray

Wed Aug 04, 2010 8:08 pm

fyi - fuel eats RTV. using it anywhere fuel lives is a no-no.

Wed Aug 04, 2010 8:56 pm

You've got to use the cb performance gaskets. -They're thicker to compensate for their manifolds not being true! The aluminum crush ones or the paper ones will cause vacuum leaks with the crappy imprted manifolds that are about the standard.

Thu Aug 05, 2010 1:53 am

My paper gaskets are at least 0.8mm thick. Not thin incompressible things. they worked a treat for taking up the slack where things didn't join so perfectly.

i got some permatex form-a-gasket today. Might go for round 3... 4 maybe of trying a goo only solution.

You have no idea how tempted I have been to say F... it and cover the lot in J.B. weld.

OP. I just resurfaced mine with sandpaper, glass and a flat surface for sitting the glass on. Works fine for pretty much anything. A mirror may be slightly preferable as they have been made to be distortion free but I didn't have any so the front of our old TV cabinet did the job.

This is the way I have always resurfaced / trued things. My manifold is bent because I think 14ft/lbs is too much torque for the Scat weber runners.

Thu Aug 05, 2010 2:11 am

When I tuned up the intake gasket surfaces on mine I used fine wet or dry abrasive taped onto quarter inch plexiglass on a good flat table with lots of water, got the head ends of the manifolds <dual port factory looking aluminum> clean and flat along with the carb surface on the center portion of the manifold, and used a set of ~2mm thick paper based seals between the head and intake, they felt like they would conform to whatever untruth there might be between otherwise smooth and clean surfaces as long as they were kinda parallel. This seems to have worked for me, the biggest improvement I have seen was swapping out the as got Brosol H30-31 and factory vacuum only distributor for a 34 PICT3 and SVDA set from aircooled... now the motor is starting to ask for better brakes!

Thu Aug 05, 2010 7:23 am

I also have made my own gaskets with thicker gasket material. I'm running one of those weber progs., and I really had to find all the leaks to get it to tune up properly. I don't use RTV as gas eats it up, as was stated earlier.

Thu Aug 05, 2010 7:46 am

I had the same problem. I eliminated the gaskets, and used Reinzosil. It's pricey, but seems to be holding up well.

Thu Aug 05, 2010 2:35 pm

So I carefully flattened my manifolds tonight and I'll fit them tomorrow with thick paper gaskets.

Can't seem to get Reinzosil over here - looks like it's resistant to petrol so it should be ideal. I can't find any petrol resistant sealant anywhere :(

Thu Aug 05, 2010 2:41 pm

raygreenwood wrote: What do you me an by fiber gaskets? You mean "paper" gaskets?

I'd be willing to bet he's talking about the phenolic spacers used on fuel injected type 4 motors. He should be using the proper aluminum crush gaskets and he wouldn't be dealing with these problems.