Intake manifold mismatches

[Peter Cripps]

Yesterday I started to notice a "cheep-cheep-cheep" noise, very cute, but not supposed to be there! It turned out to be a small leak at the join between the inlet manifold and the rubber spacer.

So today I took off the carbs and spacer, cleaned everything up and reinstalled with new gaskets and Hylomar. Cheeping noise has gone.

But I noticed that there was a rather large mismatch between the rubber spacer and the inlet manifold. In fact someone had already been in there to smooth it out. (Not a very good job, though.) There was also a noticeable discontinuity at the junction between the manifold and the head:





I did smooth out the first mismatch a bit more, with paper towels stuffed down the intakes to stop shavings getting where they shouldn't. But since I didn't have a new manifold-to-head gasket available, I didn't remove the manifold and deal with the second mismatch.

Two questions:

1. Are these mismatches 'normal' or is there something funny about my engine?

2. Apart from making me feel better, is it worth taking everything apart again to deal with the second set of mismatches?

Thanks,

Peter

[P. de R. Lelercq]

Yes, sadly these mismatches are normal. Great improvement in flow can be achieved by rectifying these things. As an example I recall being told that matcthing the inlet manifold resulted in a 17% improvement in measured flow on an Evo engine - far more than was achieved by gas flowing.

As I wrote in my blog "Fulvias in France" ports sgould be matched and then the manifold doweled to preserve align,ent.

I used to cure the rubber problem with a flap wheel in a die grinder.

Paul

[Huib]

Air leaks often happen around the carb rubber.

New original ones are very expensive if you can find them. Beware of imitation ones. On the ones I have seen the rubber collapsed from the inlet vacuum.

It is best to take good care of the one you have.
The rubber tubes have a flange which sits in a chamber in the metal base plates. The rubber has shrunk and come loose. Air leakage is usually around this flanges. Either use silicone instant gasket (probably what you call Hylomar) or cut rubber rings of thin petrol resistant rubber as a filler between gaskets and the metal base plates of the carb rubber. An idea I have been playing around with is to cut the carb to rubber gaskets and rubber to manifold gaskets from petrol resistant rubber.

Vital is the support under the air filter box. Often the rubber at the bottom of the support has gone and the metal bush inside has worn. If the metal bush is damaged you are not going to win. Neither are you going to win if the hole in the support has grown.

From the top of my head, the metal bush is 8,1 x 11 x 7 mm (ID x OD x H).
You have to find the right rubber grommet.

On both sides of the grommet there should be the special washers with the edge bent inwards. Rubber is a funny material. It cannot be compressed. If you squeeze it in one place it expands somewhere else. The bent edges on the washers prevent the rubber from expanding radially outwards if your tighten the bolt so that there is pressure on the support and the bush is kept in place. The dimensions of the grommet have to match the hole in the support, the OD of the bush and the shape of the special washers.

The mismatch on the manifold is normal. If you are happy with the car, don't loose any sleep over it.
Porting and flowing are only important if you drive full throttle at max revs. If the throttle is anything less than fully open the throttle is the most limiting link in the chain. If the revs are less than max the displacement per unit of time is the limiting factor. In fact the extra turbulence from the mismatch may be helpful in creating a more homogeneous mixture. If so, it would follow that porting and / or flowing require re-jetting.

[Peter Cripps]

Paul and Huib, thanks.

The rubber spacer piece was replaced by the previous owner, or at least his mechanic. I do have the old one, which has failed in just the way you describe, Huib. The new part still looks to be in good condition.

I did find the source of the leak -- the very thin paper gasket between the rubber and the inlet manifold was torn just where the 'cheep, cheep' was loudest. The new gasket that I used is of much thicker material.

Hylomar isn't silicone, at least not as I understand it. Its big advantage is that it never sets completely, so you can easily separate flanges and gaskets, if necessary.

Not sure how to insert a URL, but here goes:

Hylomar details


The support under the air filter seems to be OK, but next time I'm under the bonnet I'll inspect the parts you describe, Huib.

As for the value of port matching, I must admit that I've never noticed any significant difference when I've done this on other cars. But then I've never done before-and-after dyno tests, either. On the other hand, since only a few minutes work with a Dremel tool was needed to smooth out the rubber-to-manifold transition, it seemed worth doing. I don't think I'll bother with the manifold-to-head step until I have that apart for some other reason.

Peter

[Huib]

Thanks for the Hylomar link. It is in fact the stuff I have been using for some years. I was under the impression it is silicone stuff which indeed it isn't.

[P. de R. Lelercq]

Yes I know Hylomar well - it is the correct stuff to use on the lower joint of the gearbox casing.

I think that your decision is wise Peter, to clean up the rubber.

As I always say: "Every little bit helps"

Paul