Misfire after 20 minutes

[GORDON STREETER]

Dave, have you actually checked the dizzy mechanical advance to make sure its getting all its movement ?

[DaveGT6]

Originally Posted by GORDON STREETER

Dave, have you actually checked the dizzy mechanical advance to make sure its getting all its movement ?

Ah, hadn't thought of that. I'll add it to the (now considerable) list of things to check through! Thanks Gordon.

[Al Weyman]

Could'nt sitting in the garage without regularly turning the engine could cause problems with valve springs.

[GORDON STREETER]

Al. The fact that its only after 20 mins would rule out valve springs IMHO.
In situations like Dave is in its annoying to keep fitting different things and can't find a cure. At the end of the day in the past I have had an engine in house dyno tested so we could isolate the fault and make sure its not car related. I know of one engine where the mechanical advance had seized up at about three quarters advance and would start and run OK. But at the top end was running retarded, getting hot and the plugs were breaking down.
Obviously this would show up with a timing light but not everybody checks some of the basic things and will go off and buy expensive bits like electronic ignition boxes etc for no reason when desperate. We have all been there !

[Al Weyman]

Al. The fact that its only after 20 mins would rule out valve springs IMHO.

I do agree actually and would be low on my list of things to check, just wanted toi make the point about them sitting around though.

I had a similiar problem on my yellow car and still dont know for sure if I cured it as it went pop after a couple of slowish laps after I set the plug gaps smaller but seem to recollect it sounded OK. I am now building a totally different spec unit for it which will eliminate mechanical things so if I still have the misfire it will be something electrical as I know its not fuel as I have the actual carb fitted to the black car and run that clean up to 7200 rpm if required. I would be interested to know what plug gap David is using but as you say its the coming in after 20 minutes thats the rub here which what leads me to suspect something breaking down after getting hot like a coil???

[DaveGT6]

Originally Posted by Al Weyman

I would be interested to know what plug gap David is using but as you say its the coming in after 20 minutes thats the rub here which what leads me to suspect something breaking down after getting hot like a coil???

Al,

They're set at 0.65mm.

I have to say my money's on coil or leads. Changed coil yesterday along with dizzy cap and rotor arm. Checked mechanical advance and all looks OK. All wiring good. Fuel tank breather good. Just waiting for the plug leads to turn up. I won't know if I've cured it until next weekend at Oulton.

[tristancliffe]

Are things like mechanical advance still good when everything is nice and hot - thermal expansion can and will cause things that are free and good (and pass all checks) when cold to stop working.

Get your engine nice and hot (to the same condition as it would be after 20 minutes) before checking everything. And wear heat-proof gloves!