I need some assistance from the smart people.


I have a mostly stock 1986 351w sitting in my 55 f100. The engine is mostly stock except for Summit Racing intake, holly street avenger and an MSD distributor (mechanical advance) / and MSD box. I did remove the smog pump.


The problem I'm having is ignition timing and idle which I've always had with this setup.


I have the engine timed at 10 degrees and my idle mixture screws adjusted to optimal vacuum. with this setting the engine idles at about 1000 RPMs with no idle adjustment available, can idle up but not down, idle screw is backed all the way off and I have a hesitation when accelerating until I get to about 1800 RPMs then she takes off like a rocket.


If I advance the timing to say 20 degrees of course no chance of idling down but she runs good just very rich (you can smell it, will burn your eye sockets out), no hesitation but idles extremely high (of course)


I've not done anything to the distributor meaning no recurving, using springs and stop bushings that it came from factory.


My biggest issue is the idle I can't seem to adjust the idle down unless I retard the timing below 8 degrees but at that setting she won't run, backfires etc. The other thing is if I do time it down to 8 or less degrees she idles very smooth but of course won't pull itself out of the driveway.


Any assistance would be appreciated.


thank you.

 

Sounds like a vacuum leak. Did you plug the air holes in the head after removing smog pump?

 

Well I was thinking vacuum leak and also thinking that the timing and the idle are probably two different problems.


I did plug the two holes in the back of the heads, had to buy the special plugs from Ford.


I only have a couple vacuum lines, Brake Booster, Trans module and PCV. I will check those this weekend and see if for some reason any of them are leaking.


thanks for the reply.

 

Definitely check the brake booster near the master cylinder. Also try a return spring for the throttle.

 

I read something this morning on Holley's forum about the fast idle setting being too high on that carb from the factory, so may play around with it also after I check for vacuum leaks.

will keep the forum updated.

thanks.

 

The idle is affected by the timing. The more timing you give it, the faster the idle will be. I'd look for a vacuum leak too. It should idle at 500 rpms with 10-12* timing (BTC) Set the timing first, then the idle. Make sure the secondary throttle shaft is closed too.

 

When I set to 10 / 11 it will idle at 1000 in park, so I'm thinking I either have a vac leak or my carb is jacked up. I'm going to get the timing dialed in this weekend then focus on the carb / vac leak.

Will update on status sometime this weekend, thanks for the responses and help.

 

I have figured out the problem but not sure how to fix. the high idle is caused from the choke. When I start the truck, Choke kicks in and she idles high which is expected, when I hit the gas pedal, the choke comes off 1 notch meaning the butterflies are still partially closed. The only way to get them fully open is to use my finger and push down on the choke lever which the butterflies open fully and the truck idles at 600 RPMs with plenty of idle adjustment in both directions but when I hit the gas, the choke kicks back on and the butterflies are partially closed again and back to 1000 RPMs.

I'm probably not explaining this right, but there are 3 notches, 1) high RPM, choke fully engaged, 2) hit the gas, choke comes off and the truck idles down to about 1000 RPMs and 3) only if I use my finger and turn the choke off manually but idles at 600 RPMs.


After further research, it's the choke cam that I'm flipping down to come off high idle. Hitting the gas pedal once brings the came to the second position which is about 1000 RPMs, it wont come off any further unless you manually push it off, but as soon as I hit the gas again, it goes back to the second position.

 

I see you're in Texas. And This is a truck that's only going to be driven occasionally in good weather. Remove the whole choke assembly and be done with it. That's the first thing I did with all of my carbs. Pump the go pedal twice, baby the throttle a few minutes after it starts and you're done.

 

Not a bad idea, I have another carb just like this one but has manual choke, I may switch out and hook my old school choke cable up again. I've been searching around and this seems to be a known problem.


thanks.

 

Yea, the manual choke would be an improvement.

 

With the carb off, flip it over and see if the Secondary Throttle plate adjustment screw is set to high. The secondary plate adjustment can only be seen from the underside of the throttle plate.If the Secondary throttle plates are set wrong you will experience high idle .

 

Smitty has very good advice.

Back when I had a Holley 4V on my car I made up a little screwdriver so I could adjust the secondary adjustment screw without taking off the carburettor. It was a short length of 1/8" steel rod ground to a screwdriver tip and brazed into a nut. There's not much room under there.

I recall you turn in the screw until it just touches the throttle plate leg and then turn it an extra quarter or three quarters (it was a long time ago-memory is vague) turn.