Webers not behaving themselves.

[fingersnfumbs]

I have an N/A Cosworth and have recently fitted a new cylinder head.

On starting it seems No 3 is not firing properly as it makes very little difference when the plug lead is removed. The mixture screw No 3 makes little or no difference when adjusted.
I have Misab washers fitted which seem OK but was wondering if I possibly have an inlet leak eirther on one of the misabs or maybe manifold.

Now the strange bit. If I put my hand over No 3 velocity stack it really doesnt seem to have much suction, also when I remove my hand engine revs rise and No 3 seems to fire for a while before returning back to running on thee cylinders.

Any ideas anybody?

[Graham]

I have an N/A Cosworth and have recently fitted a new cylinder head.

On starting it seems No 3 is not firing properly as it makes very little difference when the plug lead is removed. The mixture screw No 3 makes little or no difference when adjusted.
I have Misab washers fitted which seem OK but was wondering if I possibly have an inlet leak eirther on one of the misabs or maybe manifold.

Now the strange bit. If I put my hand over No 3 velocity stack it really doesnt seem to have much suction, also when I remove my hand engine revs rise and No 3 seems to fire for a while before returning back to running on thee cylinders.

Any ideas anybody?

sounds like you have an air leak on no3

[rallyrob]

sounds like you have an air leak on no3

And it sounds to me that, in asking the question you've also more or less given yourself the answer too.

[fingersnfumbs]

Yup,
Just thought the symptom of speeding up when I cover the inlet would ring bells with someone.
I will check gaskets and seals etc.

[haz87]

Might also be worth compression testing just to make sure the fault is upstream and not with the head/issue with the head gasket

[modifyitmike]

Sounds like air leak , you say misab washers and o rings are OK, you cant really tell by looking they must be removed and inspected for cracking on the o rings, you could try spraying carb cleaner around them when she,s running, to find any leaks, revs will rise if any leaks detected by the spray. but be careful of fires. HOPE this helps,, and as haz87 replies , check compression for other issues.

[fingersnfumbs]

Sounds like air leak , you say misab washers and o rings are OK, you cant really tell by looking they must be removed and inspected for cracking on the o rings, you could try spraying carb cleaner around them when she,s running, to find any leaks, revs will rise if any leaks detected by the spray. but be careful of fires. HOPE this helps,, and as haz87 replies , check compression for other issues.

Misabs are new but airboix is fairly heavy hanging out there so is distorting the O Rings.
Has anybody used solid gaskets instead?
They are readily available so I would have thought its feasable?

[katana]

Provide support to the airbox would seem a sensible solution first off? I must say though, Misabs 'seem' to cause no end of problems! Not personally knowing why they are used, I suspect some form of 'flexible joint / isolation? In which case why? I've used DCOE's with a rigid mount and pretty much every other makers carb installations are rigid mount so why the aggro?

[onecamohv]

Provide support to the airbox would seem a sensible solution first off? I must say though, Misabs 'seem' to cause no end of problems! Not personally knowing why they are used, I suspect some form of 'flexible joint / isolation? In which case why? I've used DCOE's with a rigid mount and pretty much every other makers carb installations are rigid mount so why the aggro?

high frequency vibration can foam the fuel in the bowls....... and although i use misabs, ive also ran DCOE's on my own car years ago on solid machined nylon spacer plates with no ill effects because they came that way from the manufacturer. ran it for years like that in all conditions no issues so...

maybe its certain types of engines that suffer from vibration foaming, and we've all just adapted it as the norm ..

[modifyitmike]

As Katana says.
Provide support to the airbox would seem a sensible solution first off, i would support this idea. as they can at times foam up the fuel in the bowls as described before, and that is no good for power.
due to flex at the misab washers , and sometimes suck in air due to flexing, hope this helps.

[fingersnfumbs]

So just to tie this thread up.
As suggested I have fabricated a support for the carbs and airbox.
I have fitted solid spacers and so far it runs perfect.

I can only assume the running on three cylindrs was down to an air leak.

Thanks all for usefull advice.