misfire when hot

Hi,
I have a 81 302 Ford small bolck in my P5b hotrod. It has developed a misfire when hot and under load. I have changed plugs, leads, pickups, ig box. It pull about 19 psi vac at idle. I have tried blowing propane over the inlets to look for vac leaks and found non. Has anyone got any suggestions on what is causing the misfire.
Thanks,
Fletch

 

Plug wire shorting out or breaking down when it gets hot? Or the coil starting to get weak and break down under a load when it's hot?

 

Yes I thought that but it has got new coil and leads and plugs.

 

Was it sudden onset or gradual ?
Was it fine before ?
Did you do anything (no matter how minor ) before it started acting up ?

There is no misfire under load while its cold only hot - is this correct ?

New parts never means good parts.

 

Cap? or is this electronic ignition?

 

Hi,
It came on all of a sudden. The car runs fine when cold and warming up. I have also rebuilt the Holley fuel pump the carb (looks to be a late 2150). Plugs are a nice colour. Idles fine even when hot. I agree new parts do not men good parts, but the car is behaving the same with the new and old parts.
Fletch

 

Hi,
She has the org Duraspark ignition system, but now with new pickups and I have tried two different ign boxes and fitted a new vac advance unit. I have not changed the rotor arm and cap could be that.
Fletch

 

Just to be sure, are we talking a misfire as in one cylinder is not firing, or a misfire as in a backfire through the carb?

 

Hi,
When hot and under load (pressing acc peddle) missing on a number of cylinders engine stutters pops and bangs with loss of power. Lift of and engine runs fine genle throttle and she runs OK boot it and she dies. This evening it even popped back through the carb. I have not checked the timing as someone has removed the marker (I will have to take out the rad to set it statically). I am even thinking it might be fuel starvation but the plugs are a nice light brown on the tips and insulator with carbon on the base of the threads.

 

I bet there's cauliflour growing in your cap.

 

Hi,
If you mean the dizzy cap, that was cleaned up when the new pick was fitted as was the rotor arm.

 

Did you check the accelerator pump for good volume?

 

Yep, two nice jets of fuel when the throttle is pressed. I have not change the power valve though.

 

Yup.......I have seen a power valve act that way.

 

Can a power valve be serviced or is it put in a new one? The reason this has not been done yet is I tried to find one and a service kit for the carb in the UK it has been like looking for rocking horse poo.

 

Clues I see
Misfire under load when hot only
Fine on easy throttle
Popping thru carb.
Random miss

Sounds like its going lean on the throttle.
Choke is richening things enough.
Vacuum goes low with increased throttle and fuel flow should increase.
Your accelerator pump covers the transition momentarily only.

Take plug reading like this,
clean a few plugs , run it wide open and shut it off. Pull a plug on the side of the road and see what it looks like.

 

Needs to be replaced...Do some research because there are a number of different ones available depending on your setup and engine vacuum.

 

Yes I found that out and I can get one for almost all other engine sizes and years but not for my 81 302.

 

The power valve is not servicable, just replace it. That's what it sounds like. At idle you won't see a difference, moderate to heavy acceleration you will. Try to get the exact part number of it through the numbers off your carb. Look for it on a US site like Rock Auto, maybe they ship to the UK.

Bob

 

I will try US automotive, real steel and rodley motors again in the UK.
If they have no joy I will have to look to the States.

 

Coil or fuel boil in carb are you running a spacer under the carb,are you running hot?

 

There is a carb spacer looks to be about 1/2 inch thick. It has a new coil and this behaves like the old one. The temp gauge never gets fully into the green so I do not think she is too hot. Is there any way to check for boiling fuel?

 

Has the electronic pickup been replaced in the duraspark distributor?

 

Hi,
Yes new pick up.

 

We use spacers under carbs for reasons other than fuel boiling in the bowls. If you believe you are experiencing heat soak up and into the float bowls, you need something like this: a Undercarb heat Shield. Jeggs Part # 510-108-70. I believe the Holley part # to be 108-70. They also have a Insulating Carb Gasket Heat Shield that works well too. # 108-51 or from Jeggs: 510-108-51. The first number I gave you can be made at home as good as the Holley part number. Hope that helps, TR

 

It could be your dist bearing on shaft be worn or loose.
I would remove it and check and oil it.

 

I will look into the dizzy bearing, but I can not see why this would cause the misfire.
I do know that it gets hot under the bonnet as I can see the fule boiling in the filter after getting the engine nice and hot.

 

I'm pretty sure Holly runs that same power valve in their carbs. Try a #6.5 With 19 in. of vacuum that should work good. Any backfire threw the carb will distroy a power valve.

 

Thanks for the info I will make a note of the number.

 

Heres some good info on Ford duraspark and points ignitions:
http://www.fordmuscleforums.com/ignition-articles/485095-troubleshooting-your-ignition-system.html