The Stovebolt Site Forums The Shop Area Engine Shop Where do I start to diagnose a Carburetor Issue issue

So let me start here. I am trying to get my truck ready for the road as in a Daily driver. Had her running last fall or so I thought. It was hard to start (clue #1). Well I go to star it this year and it seems even harder to start. Well once I get it started (when I can) it wont run long with out me keeping the accelerator going and giving it gas when it wants to die. So I figured I need to check the timing. Well I can only time it manually since it won't run long enough to put a light on it. Once I get it (the ball) in the right position I check the Dist. Sure enough I am off a tooth on my timing the rotor is too far advanced. Fiddle with the dist and Oil pump line up and it location the #1 cyc. and finally it is in the right spot. Not 100% sure at this time because my timing light isn't working. Once I do get it started ( it is easier now) I cant seem to get the motor to stay running. I believe it is the carb. but I am not sure what is the best way to diagnose this. If I had a spare carb i would just swap it out and find out.
I didn't last year replace the fuel pump so that may be a contributing factor.

So I don't want to just jump around and try to find it. I need help to step by step go through and figure out where the problem or problems are.. TIA

Do you have a fuel filter? If so, when is the last time you change it? Have you check to see that your fuel pump is pumping enough juice to the carburetor? Just some things I would usually check.

Will it run better with the choke partially closed? If so, you might have debris or corrosion blocking the main jet in the carburetor. The gasoline available these days can get stale and difficult to light off in a matter of weeks under some circumstances. Be sure you have fresh gas (and enough if it).
Jerry

Not only today's gas can go bad quickly, it will also destroy rubber fuel lines and any rubber in a carb.

So here is what i know / have. New tank, new filter and I use only Ethanol free gas and there is 5 gallons of fresh gas in it now.

Jerry it will run somewhat better with some choke. If the main jet is blocked do I need to rebuild the whole carb or can I clear the main jet alone? Clean it with carb cleaner or compressed air ?

Canine Man. The little rubber that is in the fuel line are new and minimum. Two small pieces before and after the fuel filter and then a small piece right before the carb.

Brian is this a Rochester carburetor? There are some things unique to that carburetor that will cause it to run too rich or too lean. The too lean part often centers around vacuum leaks...of any kind (and there can be a few). For the Rochester or any other carburetor make, it also can come from float problems...often one that's incorrectly adjusted so as to shut off fuel flow too soon. And it can be a failed or failing fuel pump. You mentioned that and it could be your problem.

Incorrectly adjusted idle mixture valve can cause it to run too lean or too rich at idle but this will not be a problem while driving. If your problem is only worse while idling or when you let off the gas at a stop sign, this could be a clue.

And also ignition timing could be a problem, but yours sounds to me more like the carburetor either isn't getting enough fuel, isn't regulating it correctly or is getting too much air due to a vacuum leak. If the problem is a vacuum leak, it probably isn't going to be a tiny leak, but I wouldn't rule it out. My vote is you aren't getting enough fuel. If you're not married to the idea of having a mechanical fuel pump, I've had great luck with a cheap electric pump since 1997 and I'd be happy to tell you more.

Brian, your first post shows you are in the right state of mind. You have a hard start condition. You do not want to start guessing or changing things like timing until you do some tests or you will create two or three problems that you didn't have when you started.
1. You first have to determine what system has a fault. Fuel, ignition, spark, timing, compression,vacuum.....
2. If you "had it running" last year it's probably not timing, you may have to put it back where is was, but not yet. May not be a spark problem either. We'll see.
3. Your symptoms seem like fuel pump, so first thing is after trying to start and it won't start or it dies, is to try keeping it running by squirting some gas down carb before it dies and see if you can gently keep it running. If not look down carb and push on throttle and see if you see a squirt of gas as you operate throttle. If none of that gives a clue or results, do a pump test.
4. Pump test: take off center tower plug wire on distributor. Take off carb supply hose/tubing at carb inlet. Put hose/tube into a clear bottle. Use helper or remote start button or hand operate foot start to crank motor and watch to see a good steady pulse of gas start to fill the bottle. Crank for 15 seconds. Use a clock or timer.
5. Report all test findings.

I wouldn't move forward til we see these results.

Please let us know if you have done anything else besides timing/rotating distributor recently or last year.

The logic on these fuel tests is if you don't have a steady pulse/good flow, then we know it is either fuel pump or a blockage or tank venting. If good flow and no or dribble/squirrt test, it's carb/acceleraror pump, needle stick, etc.
Being able to keep it running by squirting gas with a squirrt bottle confirms a fuel delivery problem. You are bypassing the entire fuel delivery system.

I've posted the proper fuel pump test procedures, and they get ignored every time I do. It's a 3-step test that checks pressure, vacuum, and volume. If anyone has actually followed that procedure in the 20-something years I've been posting it on this forum, I'm not aware of it!
Jerry

Jerry I learned a lot of this with your help long ago. I tend to offer my version for guys like me who don't have any equipment or gauges. Just some tools and a bottle. At least we can get a good idea, if folks are willing to follow ideas. Your procedure is very good, just want to let you know it did help me to diagnose many times, but I do it without any real equipment.

Brian, here is Jerry's procedure. Mine is the shade tree version.

Very helpful. Thanks for sharing again.

Why in the wild, wild world of sports is this procedure not in the Tech Tips?

Great idea Carl! I just turned it into a “Tech Tip”!
You can read “Testing a Stovebolt Fuel Pump” here.

Thanks to everyone! Jerry this is what I am looking for . I will be working on this tomorrow! Thanks again for everyone's input

Re. ignition timing: Centrifugal advance starts at 400-450 RPM. I can't get my engine to idle that slow or at least I'm unwilling to muck about with the idle stop screws on 2 Zenith carbs with loose throttle shafts to get it to turn that slow. So I only use the static timing method because advance is well along at the 800 RPM idle speed and I have no idea what the timing should be at that RPM. This is a perfectly adequate method assuming a distributor in good condition.
Just to review:
1) Set the points at factory spec if you're using the original ignition
2) Set the "octane selector" to zero and tighten it down (its marks line up with a mark on the block).
3) rotate the engine to the 0 TDC mark (may be the "UC" line or a triangle depending on flywheel year - not the pressed in shiny ball) look it up. Some marks are for checking cam timing.
4) Loosen the screw clamp on the distributor housing and rotate the housing until the points just break open (You can watch for spark with the ignition on. Don't do this with a HEI ignition. The shock can be lethal.
You can use a cigarette paper in the points to determine when they open.
5) Tighten the screw clamp.
6) Loosen the octane selector hold-down bolt and move the assembly to the 5 degree of advance mark.
7) To check your work, rotate the engine till the silver ball lines up with the pointer and using the cigarette paper or spark method check that the points just open. I believe the Ball is at 5 degrees on all of them but I invite someone else to verify this.
8) Loosen the hold-down bolt and using the vernier scale on the selector move the assembly to the advance you want.
9) Tighten the hold-down bolt.

I don't know it the above will work when using an HEI with a Hall-effect or reluctor-wheel pickup. My MSD uses the original points as a trigger. One may have to have the engine running to trigger those systems.

Well here is an update. So while trying to test the fuel pump I burned up my starter. Yes I should have been more careful but frustration is a fickle ...... I have a spare that I put on and then life got in the way. So in the mean time I ordered a rebuild kit from Then and Now. Well it showed up today and the weather cooperated in canceling my work for the day. So rebuild the fuel pump did I. Put it on tonight and what do you know truck fired right up. So I wasn't able to Diagnose the old pump because it would not allow the truck to run enough to test. Of course now I have a dragging starter bendix. I am going to look at the new vs old starter and see if swapping the arm to the bendix will solve the problem.

Did that spare starter happen to come from a later model engine- - - -55 to 62 models have 12 volt starters and require a different flywheel. The quick way to check is the diameter of the armature shaft- - - -a 12V starter is about 1/2" in diameter, and a 6V will be between 9/16" and 5/8". Both drive gears have the same number of teeth (9, I think), but the tooth angle is different.
Jerry

The very end of a 6v starter shaft (inside the bushing) is 1/2". Forward of that bushing, the shaft is 0.565+/-" (where the gear slides.)

More info on 12V vs 6V starters and flywheels HERE.

So I swapped the Bendix and nose of the old to the newly rebuilt starter. I have put new plugs in as the old ones were old. I now have an accelerator linkage issue (I think) as the pedal doesn't seem to move smoothly. It wants to catch toward the end of the stroke. So if someone with a Rochester carb could post their linkage picture I would appreciate it.
Thanks again to everyone that has offered up advice. I am hoping I am making some progress.

Thanks
Brian

So here is the latest update. Today has been a good day. This morning went to Two car shows and that helped to raise my motivation to get my truck up and running. Came home and figured I would look at the linkage and see if I could see the problem. I could. Used a cotter pin that was too long and the legs were interfering with the spring at the end of the stroke. clipped them and that problem was solved. Next I was talking to a fellow truck owner and he suggested putting an electric pump on (temporarily) to help diagnose the problem. Well we have our answer. I messed up the rebuild of my fuel pump. As once the electric pump was on the truck started right up and ran fine. So I will send off the other spare Pump that I have to have it rebuilt "professionally" and Install it once I get it back. Thanks to all that offered their knowledge and assistance. I truly appreciate you all!

Where are you sending it? I can highly recommend Then and Now Automotive. 781-335-8860

Otto thanks that is where I am going to send it.

They rebuilt this one 11,000 miles and 4 years ago. They also did one for another car of mine. Top notch.