Carburetter, Ignition or what?

[mole42]

Since I put the engine back in my car I've had a weird problem with it. Starts perfectly well, runs well on the choke, warms up, all good. In traffic, if I push the accelerator a bit too fast, the engine backfires and can stall. Prior to the layup the engine was perfectly well behaved.

So far I've replaced the electronic ignition, coil, distributor cap, rotor arm and leads, timing is spot on 12 degrees. I've checked the carbs for leaks, dashpot oil is correct, made sure they are balanced and the mixture is good. I've checked the cam timing and valve clearances.

What have I missed?

[trunt]

All I can offer is vac advance and centrifugal advance?

[jbuckl]

Check the vacuum capsule / baseplate movement…. As trunts says.

[Rigid]

I think I had similar symptoms when I reinstalled the carbs and had the choke cable outer clamped in the wrong place - it was too ON - so running too rich once warm.
Worth double checking all the gaps on the cable are correct?

[DZP]

The only thing I can think of is the firing order!!
Did you double-check that you have the plug-leads located on the disti cap in the correct order, as shown on the inlet manifold.
Den

[trunt]

The only thing I can think of is the firing order!!
Did you double-check that you have the plug-leads located on the disti cap in the correct order, as shown on the inlet manifold.
Den

Yes.. good one.

I once had 2 and 4 switched, kinda ran but noticed it most when accelerating

[MikeParker]

If the carb breather valve gets sticky then the mixture goes very lean when the carbs heat up. Just make sure the brass plunger is moving freely when you press the accelerator.

[jbuckl]

If the carb breather valve gets sticky then the mixture goes very lean when the carbs heat up. Just make sure the brass plunger is moving freely when you press the accelerator.

Which brass plunger is that?

[Goldstar]

Firing order or carb diaphragms would be my first port of call, then vacuum advance.

[mole42]

So I've a list of checks for tomorrow - I've been at work today :-(

Check the firing order again

Check the carburetter diaphragms again (I wonder if I should replace them anyway just to be sure?)

Suck the vacuum pipe and make sure the baseplate moves.

[022Dave]

Richard,

Some years ago, I had an ongoing ignition problem, which turned out to be the conical screw that connects the vacuum arm to the base plate. It had sawn through almost completely, as if a hacksaw had been used. In essence, the vacuum was working but wasn’t advancing the ignition because of the cut: the baseplate was just being left behind when the capsule pulled back & forth. I put it down to the baseplate not being lubricated, and putting up too much of a fight when mechanical cooperation was needed. A new screw fixed my problem.

Dave

[mole42]

Plug leads connected to the correct plug ✓
Vacuum pipe moves the baseplate ✓
Baseplate screw in good condition ✓
Carburetter diaphragms have no splits and correctly installed ✓

I think I'm going to view the ignition system as operating correctly and take the carburetters off again for a review - it is 9 years and about 35,000 miles since I completely rebuilt them so maybe there's room for improvement.

[marshman]

Have you checked the o ring on the carb pedestal is OK and that the pedestal is properly seated & securing nut is tight, if not could cause an air leak and hence weak mixture. Also have you checked the carb pistons lift and fall freely, worth removing the elbows and watching them move as you rev the engine.

[mole42]

Yes, Roger, I've been there - the pedestal 'O' ring is new, the nut is properly torqued and the pistons rise and fall as expected.

The only slight doubt about an air leak is that there's a very tiny amount of water appearing in the V - so there must be a leak in the inlet manifold gasket somewhere, which could also mean an air leak.

I have a replacement Torquatrol to fit next weekend, so as I have to take the radiator out I'll take the inlet manifold off the see what's leaking there at the same time.

[MikeParker]

Which brass plunger is that?

Its the plunger that operates the float chamber vent valve. If that get's stuck, it can wreck the mixture when the engine gets hot. It was a problem with my car (about 30 years ago) , but spotted immediately by Tony Hart. Grabbed the plunger with long nose pliers and wiggled back and forth a few times with some WD40 applied and all came good.

See page 28 of: https://socforum.com/forum/filedata/fetch?id=776923

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