stalling / hesitation issue

[Mopwr]

Every time I press the accelerator any more than just a touch, my '54 m37 wants to hesitate (sometimes stall when cold). It happens cold or hot. I've rebuilt the carb with a Vintage Power Wagons carb kit. I've rebuilt the fuel pump with a Then & Now Auto pump kit. I've rebuilt the distributor and checked that the mechanical advance weights were clean and lightly lubed to work freely, weight springs seemed fine (don't know if new springs could help?). This problem happened from the day I got the truck to now even after rebuilding / cleaning every associated system (distributor, fuel pump, carburetor, etc.). I intend to pull the carb again and double check everything. I have never checked the outlet pressure on the fuel pump, but have run a few different style inline filters between the pump and carb. Engine starts and idles beautifully and runs at higher RPM nice, the only issue is upon acceleration. Any idea's?

[mix4coal]

When I rebuilt my carb on my CJ2A I used the accelerator pump that came with the rebuild kit. It had bad hesitation.
I reinstalled the old accelerator pump and its been running good since. You never know the quality or the replacement parts these days.
Not much help to you but could be the problem.

[Mopwr]

Thanks, I'll check that out after looking over the carb again. Anyone else have any suggestions?

[RMS]

was the correct carb to manifold gasket used ? i had bad off idle hesitation after installing the wrong gasket. don't forget that the throttle pump is controlled by vacuum not actuation of the throttle. it only takes a bit of foreign matter to jam the ball's. if you had the aluminum plugs out of the carb at rebuild, maybe one is leaking vacuum

[achiem37]

The last time I rebuilt my carb I had the same problem. It turned out to be the "new" leather fuel cup on the accelerator plunger. I reused the old leather one as well and the problem went away. The new leather cup I turned down to the same diameter of the old original up. The new leather cup was too big to fit in the bottom of the accelerator pump well when pushed down by the spring. After the resizing and a test installation in the carb it worked well after that. Terry

[Mopwr]

Thanks for the input guys, I'll hopefully get to check it out this weekend. Anyone else experience this problem or have any suggestions?

[jim lee]

I had the same deal with mine. Figured the accel pump wasn't working correctly. Then I read this from the Power wagon forum..

Written by Dave "Doc" Langford

The square top carburetor has a vacuum operated accelerator pump system. If this does not work properly, the engine will sputter and cough when you try and accelerate it.

The accelerator pump is spring loaded. In other words it is always at the bottom of the fuel bore. When the engine is off the leather piston is at the bottom. When you start the truck, vacuum from the manifold pulls the piston up. This is why you need to choke a cold engine to help it start. The accelerator pump does not work until sufficient vacuum is applied. As you know when you accelerate the vacuum drops way off. If you still have vacuum wipers you know they stop when you hit the gas. So the theory is hit the gas-lose vacuum-spring forces down leather piston-fuel shoots into the engine.

Here's the problem: The rebuild kit that is sold is for a different carb (I don't know which one). They make modifications to the accelerator piston length and add a few new gaskets then sell it to you. Which is OK because you just can't find these kits anymore and all the other parts work fine. The spring in that kit seems too stiff and the kit’s acc pump shaft leather is also too hard and stiff. So two things occur, the vacuum cannot overcome the spring tension and the piston is too tight in the bore. I used an old piston and spring from another carb and the engine came to life. If you have an old carb remove the piston and soak the leather seal in any kind of oil for a day or so. Install it with the used spring and you should be happy. Note: if you need a carb rebuild on your square top, save the old acc pump spring and accelerator pump shaft with its leather seal to use with the new parts.

Also, you need to check the accelerator piston cylinder in the air horn for smooth action of the piston. Sometimes these carbs get oxidation built-up from setting for many years. If the piston does not move up or down without sticking slightly, then you need to take a brake cylinder hone and hone the cylinder until the piston can move freely. If the piston has any oxidation, it to needs to be cleaned with fine sand paper as well. Same for the piston/rod in the carb body, the cylinder has to be smooth and free of any roughness. Two items that are very important; the air/fuel passages must be open (not plugged) and the gaskets must be in good shape

Well, I had an extra carb and when I compared accel. pump pistons, the one on the extra carb was smaller. So I swapped out with my original and presto! The trouble was gone! (To be fair I cleaned up the rest of the carb and made sure the Accel. pump was free to move in its bore at the same time.)

Maybe this is the deal with yours? Maybe not. But its another avenue to check.

Hope this helps!

-jim lee

[Mopwr]

Looks like I'll begin with the accelerator pump, I'm sure I still have the original one somewhere and a spare carb on my parts truck. Now I'm just waiting for my new coil to ship to the house. In a hurray, I left the ignition switch on the other night and I appear to have toasted the coil confirmed by testing. Thanks for all the input!

[Mopwr]

Well, I had the carb apart about 4 times today. I started by comparing 3 accelerator pump pistons (2 old ones I had, plus the new one that had been installed in the carb giving me trouble). All were more less the same except the new ones leather cup had a slightly more snug fit as the 2 old ones were slightly dried out. All 3 springs seemed to be about the same tension.

On the carb, the bore on the piston end of the accelertor pump was perfectly clean and smooth and the piston was perfectly smooth as well. I then replaced the aluminum plugs on the carb with new to make sure it wasn't a vacuum loss issue. I recleaned all carb ports and passages which were all wide open. I inspected the gaskets which were still in great condition.

Before reassembly of the carb, I lightly sanded the leather cup on the new pump piston to lower its drag in the bore. After installing the carb, I had the exact same issue as before. I then pulled the carb again and installed one of the old accelerator pumps that had no drag sliding into its bore. After reinstalling the carb a second time i still had the same issue.

My hesitation issue is basically non existent with the choke pulled out about 1", leading me to believe this has to be an accelerator pump issue as everyone suggested (instead of a distributor mechanical advance issue, etc.), but what else can I / should I do? Am I missing something?

[RMS]

when you rebuilt the carb did you service the governor? i have not serviced a governor but i have been told they can be tricky and cause problems like you are describing. since the throttle plate is controlled somehow by a spring and vacuum and is not directly connected to the gas pedal leads me to believe things can go wrong between the linkage and the throttle plate.

when i remove the throttle return spring on my truck the carb goes to wide open throttle. there is a slack area of movement (around 4 minutes on a clock) were there is free movement with no spring action from the govener at wide open. so with the return spring removed and actuating the linkage by hand starting at idle with the screw hitting home @ 30min on the clock i can feel constant even tension coming from the governor spring as i allow the throttle to open till it reaches 20min at this point i can no longer feel tension from the spring and there is fee unbound moment from 20 to 16 minutes on the clock.

just checking but did you use the correct gasket between the carb and manifold? the reason i keep coming back to this is a fel-pro engine gasket kit comes with 4 or 5 different carb gaskets. the correct gasket should have 4 long notches in it at 11,1,5 and 7 o'clock. below is a pic with four gaskets. starting on the left. no 1 is wrong. 2 is correct, except for the staples . 3 caused my hesitation problems and 4 has no provision for the vacuum signal

the difference between 2 and 3 is small but will cause problems as you can see when compared with the vacuum port location on the carb base.



if your confident that the governor and gasket are good its on to scary stuff...... have you hooked up a vacuum gauge ? what kind of vacuum are you pulling? is the signal strong enough to pull the plunger down at idle? if you do have low vacuum but your compression tester says 100+ i would check for cracks in the intake manifold with the aid of a propane torch. i have seen two intakes that were cracked on the bottom of the middle runner where it attaches to the center distribution box.
my 251 @ 8.3to1 pulls a supper steady 21" at idle, will drop to 3". pulls up some hills @ 16"and hits past 25" on decell. i try to keep between 16" and 21" on hwy to keep my fuel consumption down. if i had a specialty plate it would read SLOW2GO...... well except for when im lined up at the lights with a dump truck or supper B

[achiem37]

Is the brass plug attached to the top of the accelerator plunger smooth in its bore in the top cover?. Is the brass plug really loose or just a slight drag when moved in the bore?. The brass plug should be a slightly snug fit and has the coating worn off of the top covers bore in this area. Maybe the brass plug is sticky?. Is the accelerator plunger spring in good shape and do you have to push the top cover down with a little effort to install the top screws?. Perhaps the wrong pump spring got installed somehow and it is too strong and not allowing the vacuum to pull up the brass plug?. Is your idle mixture control screw set about 1.5 turns out or so from bottomed position and if you adjust it in or out does the idle speed change?. If your idle speed is too high the throttle plate will be open too far to allow the mixture screw to effect the idle speed. My truck if it idles too fast will not be affected by the idle mixture screw and will stumble on acceleration. The vacuum should be around 20 inches plus on a good engine. Is the fuel bowl level set at the right level and not too low?. Just some ideas...Terry

[Mopwr]

RMS, I'll have to check the gasket I have. It looks like 2 or 3 you have pictured, but I guess I've never checked whether the vacuum port on the carb is exposed. Also, I haven't done anything with the governor, but I'll have to check that out as well. As far as vacuum testing goes, I get an erratic reading that wildly jumps from 15 to 21-22 in/hg. However, I hadn't put much stock in that reading as my vacuum gauge is a cheap POS, getting a new vac gauge is on my to do list. The engine was compression tested, all cylinders were over 100psi and with only a couple of % variation. My intake manifold is brand new, I checked it for cracks before installation with none found.

Achiem37, the brass plunger and spring seem fine, I inspected them closely with no wear present in the bore and they're not sloppy or loose or too tight. Also, the fuel float was just recently adjusted, so I'm confident that's not the issue. I'm gonna check my mixture screw, but I did have it adjusted to 1.5 turns out.

I've got some more detective work to do now, thanks a lot guys!!! I really appreciate it!!! I have a couple m-37's, but this is my first that I'm going through 100% for a restoration. I'm decent at figuring most stuff out, but I'm grateful to have your help on the stuff that stumps me!!!

[Mopwr]

A couple new things, I am yet to check out the governor, but pretty much explored all other avenues. 1st and most important, I re checked the intake with a respectable vacuum gauge and found the reading to be erratic as I stated before. It was wilding jumping from 6-7 to 18-19. I checked the carb, air horn, and intake with carb cleaner for leaks with none found. I also rechecked the compression of the engine as the last test was done 2 yrs ago and the recent results sucked. 1-95, 2-82, 3-95, 4-87, 5-95, 6-97. I rechecked all cylinders with ATF and all improved drastically (around 110-115 isn) except cylinder 2 which showed no improvement so I adjusted the tappets and haven't retested yet. The carb to intake gasket was verified correct. I am planning to rebuild the engine as I will be completely restoring this truck and don't intend to ever part with it, but a rebuild will be a bit down the road. Other than the governor which I am yet to check (and admittedly no little about), any other ideas what could be causing the hesitation? She also seems to be missing a little and I know the ignition components, timing, carburetor, etc. are in tip top shape. Could OK to poor cylinder condition be the culprit? not creating the vacuum I need to lift the accelerator plunger and possibly causing the miss? Any ides on how to narrow things down a bit further?

[oilleaker1]

I have had this exact same issue and sometimes still do. What I did was remove the carb and left the fuel in it. Took off the top so you can look down in the float bowl. If you have anything in it, like fine rust dust or dirt, it screws up the fuel delivery through the two ball checks that the accelerator pump ejects the fuel through. I also soaked the leather cup in vineager to swell it some, and then neatsfoot oil. Once it made a seal with the bore it pumps in, I manually by hand pumped the accelerator pump.( you hold the piston assembly and stroke down into the bore with the leather cup) At first it didn't work. Being mad , I then stroked it hard and walla, fuel suddenly started squirting into the carb throat. Worked great until more dirt. I wish the pump was a manual type and not a vacuum type. Then you could just pump it into submission. A pre filter ahead of the carb inlet is needed. John

[oilleaker1]

A miss can be caused by a fouled plug, you said the ignition timing etc. was in good shape, but if you choke it too much, you can fowl a plug. Also mine started missing after re-build and it was a broken (brand new) valve spring. The lowest compression would be the first place to look. That also makes sense with a wild vacuum gauge reading and apparant leaks. Burned exhaust valve also possible. Enjoy, John