Joined: Jan 16, 2006 Posts: 38 Location: Lake Charles, Louisiana
Posted: Tue Jul 17, 2007 7:57 am Post subject: Engine hesitation on acceleration on '53a1 F-head
This is something that I have been fighting for some time now and have almost replaced everything but the engine itself.
It starts fine, idles, and if you press the gas slowly it will accelerate slowly. If you try quickly it will stall and almost die and has no power. I have changed all electrical (including plugs, wires, distributor, cap&rotor, coil) , compression is 112 on all 4, new rebuilt fuel pump, new tank and pickup, new inline fuel filters, new carb, PCV valve and blew out fuel lines with air compressor. I have also tried spraying carb cleaner around for vacuum leaks and found nothing. If I use the manual primer lever on the side of the fuel pump it squirts plenty of gas out. My next step was to get a fuel pressure gauge to put between the pump and the carb because I'm curious if the foot shaped engine lever that activates the pump could be worn to a degree that the engine is not pressing enough on the pump when running. I would assume that this is possible but has it ever happened in the past to anyone?
My next step might be to check for sticking valves but if I go this far, I'm going to pull the engine and have it rebuilt.
Thanks in advance for all your knowledge,
Lee _________________ 1953 M38A1 F-Head
Joined: Jul 07, 2007 Posts: 19 Location: "SUNNY" WESTERN AUSTRALIA
Posted: Wed Jul 18, 2007 5:46 am Post subject:
Hi, sounds like you've done just about everything! You say it starts & idles ok & will rev if you do it slowly, now how far will it rev? If you accelerate slowly and get the revs up will it hold the revs or does it die? If it holds the revs I doubt it is supply problems, it would probably be the accelerator pump not working properly to give it the extra boost of fuel needed for rapid acceleration, I know you said you changed carby, was it reconed? if so had it been sitting for a while maybe the pump leather has hardened up a bit or the pump jet might be blocked. As far as the valves go unless it is misfireing or poping back through the carby I doubt you should go down that path, especially since you have pretty good compression across the board. Hope this is of some help, if you can give me a bit more info I will try to help nut this problem out if I can. Good luck Bear
Joined: Apr 03, 2005 Posts: 271 Location: Chester County, PA
Posted: Wed Jul 18, 2007 6:25 am Post subject:
Hi Lee,
I know you said you replaced the sparkplugs, but have you checked the gaps of the electrodes on them? If the gap is too close or too far, it is likely that a sparkplug isn't even firing, thus running on 3 cylinders, thus causing hesitation upon acceleration.
This has happened to me before when I first got my 134 running. I forgot to gap one of the sparkplugs. Started and idled fine, but if you gave it gas quickly, it hesitated. _________________ Bob Collins
1954 M38A1 MD79056
Joined: Jan 16, 2006 Posts: 38 Location: Lake Charles, Louisiana
Posted: Wed Jul 18, 2007 7:55 am Post subject:
Bear,
It will rev all the way without a problem as long as it is not in gear. In gear it has very little power. My new carb is new and my old carb did the same thing. I haven't noticed any firing through the carb so maybe I should drive it around the yard some more. Thanks for this tip.
loopbackzero,
I have changed the plugs twice in the past year as I have been tinkering with this F-head and it still runs the same...no power. I will check the plug gaps this weekend if I get a chance.
Please also note that I have removed all of the 24v parts except the starter and replaced them with new 12v parts in thinking it would help which has not . And I would like to sell to someone who still wants 24v parts but I'll probably post these items later.
Thanks again,
Lee _________________ 1953 M38A1 F-Head
The two primary causes of hesitation off idle are:
1-The accelerator pump inop or leaking diaphram.
Carbs rebuilt with NOS kits having the old style rubber material will suffer early failure of the diaphrams from the todays fuels. This is simple to check and does not require carb removal.
This is easy to overlook. The springs and counterweights are hidden under the plate the points are on. These can malfunction even on NOS distributors that have sat a long time. Pull the distributor outer cap and grasp the rotor and it should rotate about 5 to 10 degrees in one direction and when released snap right back. or you can check with a timing light. If your initial timing of 5 degrees BTDC is set correctly and the RPM is at idle (650) then you should see the timing marks lined up. When you advance the throttle steadily you should see the timing advance another 11 degrees as you approach 2000 RPM.
Yes there are other possible causes such as sticking valves, fuel flow restrictions (this includes plugging of the fuel tank vents)(this can be detected by trying to run it with the fuel cap off), low float level, lean idle mixture setting and low fuel pressure.
Before I threw anymore money at shotgun parts replacements I would get serious about a regimented troubleshooting attempt. Start with items 1 & 2 above. Then do a good set of vacuum gage tests. Then a fuel pump output test with a calibrated pressure gage and a graduated collection bottle to measure pressure and flow rate.
Joined: Jul 07, 2007 Posts: 19 Location: "SUNNY" WESTERN AUSTRALIA
Posted: Wed Jul 18, 2007 8:21 am Post subject:
G'day Lee, if it will rev quickly and hold high revs out of gear, then it doesn't sound like a fuel problem, is the timing set ok it could be retarded, is points gap set right? Vacuum advance working ok? It is hard to say without actually being there and listening to it, but if you want keep filling me in and I will keep trying to help. Cheers mate Bear
Joined: Jan 16, 2006 Posts: 38 Location: Lake Charles, Louisiana
Posted: Wed Jul 18, 2007 9:09 am Post subject:
Bear,
It will only rev at a slow pace. I don't know much about the vacuum advance process and suppose I should look into it.
Wes,
Thanks for all the info. I will print this out and see if I get anywhere. After reading the page on vacuum testing, do you think it could be something as crazy as excessive exhaust backpressure? Heck I could just saw off the muffler since it does look like a hunk of rust anyway. _________________ 1953 M38A1 F-Head
The engine must breath to function. A loss of air in or exhaust out capability will have the same effect. I have seen many a rusted and plugged up muffler cause many a dollar in shotgun troubleshooting parts replacements by owners. I have seen owner installed exhaust plumbing causing the same problems and I would open them up and find the shipping caps and plugs still stuck in them.
The bottom line in troubleshooting is to use a checklist and FOLLOW it. They are designed to hit the most likely and least expensive areas first then work slowly into the less common and more complex solutions. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Jan 16, 2006 Posts: 38 Location: Lake Charles, Louisiana
Posted: Wed Jul 18, 2007 10:11 am Post subject:
Thanks Wes. I am not at all worried or angry at the items I have purchased to replace because they are all pretty ruff looking and did all appear to have problems of their own. The fuel pump had a busted diaphram, when I tried to unscrew the drain on the tank I removed about a 3" diamater of the tank, the fuel pickup was a rust pole with a pine cone on the bottom , the wiring was completely shot (2 lights worked but blinked sometimes and were hooked to some trash knobs, the voltmeter was the only gauge working, etc.), to this day I have not been able to figure out what the old carb went to (my only guess is an old small ford car or possibly a tractor). I am however surprised to come to a belief that the voltage regulator, generator regulator, 24v dist, and possibly the generator may have actually still been working . _________________ 1953 M38A1 F-Head
The most common carb swap on the M38A1 is the civvy YF series Carters found on the CJ3B and CJ5 F134's.
The next is the Solex conversion.
And the third is the old Ford Falcon 170 CI 6 cylinder carb from the 60's. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Jan 16, 2006 Posts: 38 Location: Lake Charles, Louisiana
Posted: Fri Jul 27, 2007 9:58 am Post subject:
Well fellas, my problem was not in the exhaust can as I cut it off this morning and it still runs the same. I also checked for intake leaks for a fourth time this morning by spraying carb cleaner all over the carb and intake and the engine never changed idle speed.
I did also notice that sometimes when I press the gass quickly that it would backfire out the carb, killing the motor. Guess I may have a sticking valve also.
I guess I'll be looking for a pressure gauge to put on the fuel line between the pump and carb. _________________ 1953 M38A1 F-Head
Lee if you have confirmed the operation of the ignition governor (centrifugal advance inside the distributor) then the only test left is the Vacuum gage analysis. Have you completed a vacuum gage test process yet?
Joined: Jan 16, 2006 Posts: 38 Location: Lake Charles, Louisiana
Posted: Sat Jul 28, 2007 7:33 am Post subject:
I had replaced the military dist with a rebuilt IAD-4008 civy dist from John at Midwest Mil which runs the exact same as the military one did so I just kinda knocked that one off the list.
I don't have a vacuum gage yet but would like to get one and run the tests.
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. And I would like to sell to someone who still wants 24v parts but I'll probably post these items later.
, the wiring was completely shot (2 lights worked but blinked sometimes and were hooked to some trash knobs, the voltmeter was the only gauge working, etc.), to this day I have not been able to figure out what the old carb went to (my only guess is an old small ford car or possibly a tractor). I am however surprised to come to a belief that the voltage regulator, generator regulator, 24v dist, and possibly the generator may have actually still been working 
.
