7 replies to this topic

[insertnicknamehere]

Hi I have a 1275cc A+ engine which is surging on idle, it is virtually impossible to set the mixture. It has an HIF44 carb, ADL needle, a mild cam (so I have been told) and an LCB manifold.

I can start the engine with the mixture screw set two full clockwise turns (to richen) from the jet sitting level with the bridge as stated in my manual, however when I come to weaken the mixture it races very high and then sometimes cuts out or just runs very rough until the revs start building up again of their own accord. There is no consistency in whereabouts the adjustment range this problem occurs. The adjustment screw is a lot more sensitive than normal.
If I richen the mixture too much it just dies.
It seems to point to an air leak on the intake after the carburetor so I changed the exhaust/intake gasket and fitted gaskets to the carburetor abutment plate as these were missing.
The exhaust gasket did show signs of leaking but the problem persists.
The carb is still impossible to set without the engine racing away, I have tried the ignition timing all through its adjustment and I have replaced the plugs and coil.
But still the same.
I have changed the carburettor for a rebuilt one but still the same.
I have set the tappets. The compression is 130psi across all cylinders.
Things to note, it does not have the plastic spacer from the inlet manifold to the carburetor.
The fuel pump seemed a little weak so I have bypassed the whole fuel delivery system with an electric fuel pump and a Jerry can. But still the same.
Is there any other way this engine could be drawing in air or another cause of the surging that I have missed??

Regards and thanks.

[bmcecosse]

Compressions of 130 psi are desperately low.......... are you sure?? How are you taking the readings?

[insertnicknamehere]

P.S the distributor is the late electronic metro type, I am going to remove the inlet manifold again tomorrow and see if it has been drawing in from the bottom, I have had a few where the alloy inlet manifold touches the top of the exhaust manifold and makes it impossible to seal but I anticipated this and flied the area down which touches.

[petey81]

defo sounds like an air leak, i used two gaskets between exhaust manifold and carb bracket. the bracket didint fit right so i straightend it out,

[insertnicknamehere]

Thanks
with a compression tester, re tested the compression now I get these readings,

4 3 2 1
140 140 145 145

Its not too low, do you think that will have much to do with the surging?

Edited by insertnicknamehere, 29 June 2012 - 09:47 PM.

[insertnicknamehere]

Thanks Pete I will have a look at those gaskets again and use some RTV on them too.

[bmcecosse]

These are slightly better - but still not 'good'. The surging will be an air leak on the inlet side.

[insertnicknamehere]

Just an update for those interested, I have sealed all possilbe leaks on the intake side of things, gaskets on abutment plates and a bit of RTV etcetera, I have re built the HIF44 with new seals gaskets and jet, once I had the new jet held up against the old one it was pretty clear that it was worn, the new float valve in the re build kit also contains a rubber tapered seal instead of the brass one which seems the seal much better. I have also replaced the needle with a shiny new BDL incase the needle itself was worn too, even if all the things I have tried so far have not helped solve the inital problem they have undoubtly improved the running of the engine. I am also going to try placing a restriction before the air reaches the jet bridge to increace the pressure as I mentioned before that when holding the piston up the engine idles fine so perhaps the seal around the piston is worn and just needs a helping hand.

I will let you know how it goes tomorrow night.

Regards.

Edited by insertnicknamehere, 01 July 2012 - 11:54 PM.