2.25p backfire (Zenith 361V, 24V)

[Snagger]

Hi everyone. I have a problem with a 24V 2.25 petrol (FFR Lightweight) with a Zenith 361V carburettor. I'm not that familiar with petrol engines and need your help.

It ran well until some very slightly diesel contaminated petrol (1% diesel at most) was used, after which it became difficult to start and very smokey. Misfiring and backfiring became an increasing issue and it quickly became impossible to drive.

I have drained, cleaned and refilled the fuel system with fresh 95% octane petrol, stripped, cleaned and rebuilt the carb, fitted new manifold and carb gaskets, a new exhaust and new plugs. The distributor has been cleaned, the points cleaned and adjusted and the timing set dead accurately.

The engine now runs like a sewing machine on choke with no smoke or misfire, starts easily and has no smoke, but releasing the choke once warmed up results in the misfire, power loss and stalling again. It runs beautifully with the choke set about 1/2" on the lever, having fit the cable to the carb so the choke valve is fully open against its stop with the lever pressed all the way in. I bought a new (pattern) Zenith carb and it runs exactly the same. I have re-stripped and cleaned the original carb and refit it, still to no effect.

I can think of only two plausible causes:

1) the choke cable should be set so that the choke flap is slightly activated with the lever full home, or;

2) I have deposits in the head that are causing the poor running, which will hopefully burn away if the vehicle is given a long run with some cleaning fuel additive.

Can anyone please advise me of causes and cure?

[secondjeremy]

Sounds like an air leak between the carb and the engine - carb mount? manifold? manifold gasket?

[Snagger]

Sounds like an air leak between the carb and the engine - carb mount? manifold? manifold gasket?

It has had a new exhaust manifold because the studs holding the two manifolds together were severely corroded, so all the mating surf aces are good and new gaskets have been fitted throughout. It's not impossible, though. Maybe the manifold nuts need a further nip if the gasket has compressed while settling?

[Snagger]

Well, the gaskets seem to have compressed - I got roughly another half turn on each nut/bolt, including the carb mountings, so we'll see if it makes a difference. I can't start the engine just now as I have the dash out sort the speedo, dash lights and repant the IR light switch guard.

[Jeff Young]

It does sound like an air leak. Hopefully you've got it sorted....

[Snagger]

Tightening everything up made nod difference. Driving up the road to Rogers was bad - 20mph tops on the stretch of dual carriageway and several cut-outs.

What is potentially encouraging is finding that the output of the electrical choke to the coil is 24.8V - it should be 10V, so the choke is clearly faulty. The spare that the car's vendor included is similarly faulty. The coil has been damaged by the over-voltage to the extent that the bottom end is now slightly convex instead of flat. The coil and choke cost £195+VAT from Blanchards and seem unobtainable elsewhere.

The plan is to fit a voltage reducer and standard coil, which should fit the existing leads and distributor. It'll have to wait a while, though, as I've spent too much this month already.