Fulvia - misfire when hot / traffic jam

[Paul_lancia]

I have encountered a misfire when stuck in traffic jams and the engine compartment becomes very hot. (Occurred quite a few times last summer and on the way back from the AGM on the M25 - nice). I had, at the time, already replaced all the ignition parts* - and am wondering if this might be a hot fuel issue, perhaps a blocked fuel return pipe to the tank. Any ideas ?

* Coil, leads, points, condenser,  distributor cap, plugs.

[lancialulu]

It is very rare for a correctly set up Fulvia to get any form of fuel vaporisation. Equally a blocked return line unless it has been accidentally crushed. Series 1 did not have a return line but I believe it was more a cost effective way of managing the fuel pressure (c3psi). A simple pressure test on the fuel input line when the engine is running would show.

How does the misfire manifest itself?

[Paul_lancia]

When in a traffic jam and the engine becomes hot the misfire occurs on acceleration. In normal use without stuck idling there is no issue.

[lancialulu]

When in a traffic jam and the engine becomes hot the misfire occurs on acceleration. In normal use without stuck idling there is no issue.

What fuel pump are you running? Have you checked fuel levels in carbs? Simple way with small cable tie. Remove small top cover on each carb and remove one emusion tube from each carb. Fuel (after running the car) should be 19mm ifrc from top of emulsion tube mounting. A low fuel level may give misfire under hot conditions - maybe…..

[Paul_lancia]

I will check these levels - running the standard mechanical fuel pump, but an electric one in parallel on crank, and for priming as the fuel seems to run back to the tank overnight.

[GlynW]

When in a traffic jam and the engine becomes hot the misfire occurs on acceleration. In normal use without stuck idling there is no issue.

You & Tim seem to have already identified all of the likely culprits - the only other causes of rough running that I have experienced were due to
(1) intermittent shorting between the spark plug caps and the cylinder head cover - obvious when I lifted the bonnet at night!  I know you have fitted new leads but just check that the connections to the (long) caps are OK.
(2) a fractured spade connection to the coil (hidden inside its sleeve, so only obvious when it broke completely).

Good luck!

[waterman42]

Perhaps when hot under bonnet the plug leads soften, become more floppy, and if you haven't got plenty separators to keep them properly in place (not just relying solely on the factory fitted "comb"), one lead is brushing a metal part?

When idling the fuel level in the float chambers should be as high as they are going to be, any evaporation due to elevated temperature would be small compared with inflow fuel delivery capacity (if it is half decent). My money is on ignition!

[Paul_lancia]

Thanks - will look at the HT lead supports as well. Any recommended Ht(plug) lead brand / supplier?

[nistri]

I would also recheck the coil electrical connections. What make is it? Have you measured the resistance between the two terminals?

[Wangler]

When it’s been misfiring, have you stopped and checked the plugs to see if they are fouling? (Not always easy of course, especially on the M25!)

[Paul_lancia]

I did check the plugs after the issue and they’re fine - no sooting.

[GG]

Check the coil - can have losses when hot if very old. Had a similar problem once.

[Paul_lancia]

It’s a new coil a few months ago and seemed to help but not fully address - I had a worse misfire, now only this hot engine case. (Coil is a Viper VCS MB 2007 - resistances 2.7 ohms and 20K ohms). I did notice the some HT leads were touching the head so moved them away. Issue is hard to reproduce in this weather so I will do what I can for test later. Plan to measure the carburettor fuel level as well after a run.

[Keithver]

I have had a misfire when hot.
Someone recommended that i up my plug gaps to 0.7mm. I eventually reset the Champion plugs to the recommended .55 and the misfire went away