I have a 1986 F150 with the 351 cu. in. (5.8L) engine and a Motorcraft 4-barrel carb.

Anyone know what the clearance should be between the choke plate front edge and the carb body with a cold engine and the choke plate in the closed position?

Thanks!

 

really there should be none if its cold. Itll open up when it warms up, or when you give it gas.

 

Won't a completely closed plate keep the engine from getting any air for combustion?

When I used to rebuild my own carbs, it seems that I remember the kit instructions included a specified clearance, but it was only a few thousandths of an inch.

But that was a long time ago, and I could be wrong.

Thanks.

 

The answer actually lies somewhere in between. When off and stone cold, the plate should be completely closed. When you crank the engine, the engine will draw from the carb. That little draw will open the plate just enough to supply the needed air.
Pretty cool, eh?

 

OK, that sounds reasonable.

Yep, it's pretty cool....but I'm glad that the days of carbs are about over!

Had my share of problems with those things....especially the SUs on the Triumph TR4 I used to drive.

 

In my 1983 Ford Shop manual it specifies 0.180 inches. That's probably good for your '86, too. A 3/16" drill bit is very close.

 

The choke should be completely closed when cold. But you are correct, it won't run very long being completely shut, so they have a mechansim in the choke housing called a "choke pull-off", or "qualifier". This is operated by vacuum, and cracks the choke open just a little bit so it will get air and keep running. The measurement you read about was how much the the choke is cracked open. If it's not enough, the engine will load up and run rich. If it's pulled open too far, the engine will quit.

 

I'm gonna differ with you...I have read in automotive books how the suction cranking the engine will pull the plate some. Nothing to do with the choke.

 

The choke pivot is off center, with more plate to one side than the other so the air going by will help, but it's not enough when the choke is fully closed during start-up. Read a little bit more, and do a search on "choke pull-off" in your books and on the web. You will find that all carbs have one and it's mounted outside the carb with a vacuum line going to it, or in the case of some models including holleys, it's a piston built into the choke housing with an internal vacuum source.

 

The reason it's off-center is so the plate WILL open slightly.
Give me a purpose other than that.

 

In my post I agreed with that. I thought in the post before that you disagreed with the existence of the choke-pull off and it's purpose. Is that were we are at now?

 

We're cool.
I've always been impressed in how these things like carbs were engineered.
They served their purpose well for over 80 years.

 

Well, I set the choke plate clearance at about 3/16 in., using a drill bit.

The engine seems to start a little easier.

I'm afraid the engine has a stuck valve lifter. It sounds like it, and I can tell the engine's missing by listening at the tailpipe. The truck set up for about 3 years without being started, which may have caused that. I added Rislone to the crankcase, and then CD-2, when the Rislone didn't help. So far, the CD-2 hasn't helped either. I've tried using a mechanic's stethoscope to pinpoint which cylinder is the culprit, but the sound is about the same at all cylinders.

I just hope it's not a deal like I had on a '79 'Vette several years ago. It had a stuck lifter. I added an oil treatment (can't remember what it was) to the crankcase, but that didn't help. Then, I loosened the rocker arm and rotated it out of the way so I could get to the pushrod. I let WD-40 run down the pushrod and then tried pushing down on the pushrod, hoping the lifter would break loose. It didn't, so I tore into it and removed the lifter. I could tell it had collapsed, so I disassembled it, thinking that some type of foreign material or a rough place on the inside surfaces had caused it to stick. I considered replacing the lifter but knew it wasn't good to have a new lifter against the lobe of a used camshaft.

I found absolutely nothing. All surfaces were smooth, and the spring and check valve were OK.....no varnish, carbon or anything. I reassembled it, primed it with oil and put everything back together. It never stuck again, and the engine was still running fine when I sold it several years later. That one still baffles me to this day.

Thanks for all your help, and I will keep you posted!

 

You might just need to run the engine for awhile. I have used the Rislone with good results for a ticking lifter. It has detergents that loosen up the gum.

It's been my experience though, that the stuck lifter just made a noise, not a dead miss in the engine. If you have a dead cylinder, what you can do is pull each sparkplug wire off one at a time. If the cylinder is good, then the engine will slow down. If the cylinder is dead, then pulling the sparkplug wire off will make no difference.

If you find the dead cylinder, then I would pull the valve cover off and look for a broken valve spring. You could also do a compression test on that cylinder to see if it's low.

 

I've always had good luck with Marvel Mystery Oil in the crankcase for freeing stuck lifters.