Help with a carburetor.

fordwheelinman

fordwheelinman

Well-Known Member
Joined
Feb 11, 2009
Location
Randolph County
I finally got a chance this past Sunday to try and get my old 78 Ford qork truck project running. After replacing the fuel pump, I still have the problem I had 7 years ago, only now I have more information.

The truck will crank with the choke shut, runs great with the choke on, flip the choke flap open, and it dies. Choke it, and it cranks up. Will drive around the yard with the choke on. Same as years ago. However, I did learn sunday that you can give ot partial throttle and it will stay running, but refuses to idle.

I had tried the flammable spray trick to find a vacuum leak at the carb and on the intake runners, which resulted in nothing found. I put trans fluid on the top of the intake at the heads, cranked it, no results.

My theory is this, my aluminum intake is slightly warped, and the thin metal valley pan is not thick enough to seal it, and it is sucking air from the inside of the motor. Does this sound feasible or do I have a bad "rebuilt" carb. Carb was supposedly rebuilt when I bought it, put it on after having issues with an old carb, and it wouldn't run of choke. Granted after 7 years, it probably needs going through again, but like I said, same symptoms as before.

Any help is appreciated, my carburetor knowledge is limited.
 
I'd take it apart and look for trash in the jets. After 7 years of sitting with fuel in the bowl I'm sure that's nasty also. Start with the carb fresh and go from there. Make sure the fuel is 100% fresh also.
 
1) RPM at idle with choke closed?
2) Hot idle rpm?
3) Valley pan and 78 ford makes me think 351m/400? Did you make sure you didn't spit the rubber end pieces on either end of the intake?
4) If an automatic with power brakes, vacuum leaks can occur to the booster and trans modulator and are pretty common
5) Did you happen to put a vacuum gauge on it, you'll wanna be somewhere around 15-16" with a stock engine...and if you are, it's not a vacuum problem.
6) Did you happen to check float levels and mixture screws? Fuel should just barely dribble out of the sight window and mixture screws should be at 2ish turns out
 
Similar to the problem I chased for several weeks, after getting my rebuilt motor running. Used same carb, & even had it gone through. I finally ran a smoke test on it, with a borrowed smoke can. I Found the vacuum leak in 15 seconds. But I wouldn't expect a 7 year old carb, to run right either. Good Luck!
 
@willness33 I have no doubt that the carb could use some help, but I had the problem when it was fresh. Not saying it isn't a problem, just not likely to be the root cause

@UTfball68 Unfortunately the truck does not have a tach, and I didn't put a timing light on it to see, but ear tachometer says it's north of 1000 when warm qith the choke closed. It doesn't idle at all, warm or cold, with the choke open so a regular warm idle is impossible to determine. It is a 400, I know the front is still in, I don't know about the back, haven't verified. It does have power brakes, and is an automatic, HOWEVER the truck ran perfectly with the factory 2bbl on it. This problem only presented itself after switching the intake manifold. Bolted on a random holley that was too much for it to make it run, and it wouldn't. Bought this carb, that was rebuilt, from a member here, it kicked around about a year, and I bolted it on. That got it to run, but it wouldn't idle. No I have not had a vacuum guage on it as I don't own one, and without it wanting to idle non-choke, I would assume the reading would be skewed. Or am I just over thinking that part?
 
OK, lets start with WHAT CARB (don't know, post a pic)... if any of the Motorcraft carbs (2100/4100) the idle tree can get stopped up (not as famous as Carters for that, but possible)...
 
My .02 FWIW is this. What did you change first that has caused issues originally and after 2 different 4bbl carbs? The intake. I’d start by pulling the intake and checking the mounting surfaces for “true” if they are, I’d reset it with new, good gaskets. Then move onto a fresh carb (whether rebuild the one you have or something else).
 
^What he said. I've had some issues on previous SBC builds with janky-ass fleaBay used intake manifolds leaking vacuum in the lifter valley. The only way I got them to idle properly was getting stupid with silicone all around ALL the ports....top, bottom, the thin wall between, etc. Carburetor never gonna fix an intake leak, no matter what you do to it.
 
I appreciate all the help. Messed with it today. Turned the idle set screw to get it to idle without the choke, sprayed syrarting fluid and found a vacuum leak. 20 yr old brake booster hose...dumbass. Anyway, also decided that I needed to check the timing since I never did after having the distributor out. Couldn't find the timing marks, so I turned it a little till it got better. Ugh, moron.

The plugs are probably junked, the carb (Motorcraft) definitely needs replaced, the brakes are crap, the tires are dry rotted to hell, but it moves and half ass idles. Still probably going to pull and check the intake just to know it's usable, I'll clean up the balancer so I can ACTUALLY set the timing, do a tune up, and see where it goes from there.

Picture just because
 

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fordwheelinman said:
the carb (Motorcraft) definitely needs replaced
Click to expand...
Congrats on getting it fired!

IMHO, unless the throttle shaft bores are severely worn (does it wiggle? vacuum leaks there?) or physically broken/holes stripped, Motorcraft carbs are amongst the simplest and typically respond nicely to an uber cleaning + $30 rebuild kit...
Plus, 95% of any "new" carb will be either remanned (and of questionable rebuild quality) *or* crap Chiwanesse knock-offs...
 
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