M38A1 carburetor question

BigJim · Post by **BigJim** » Thu Jan 13, 2011 8:08 pm

Hey guys.I finally got motivated and got my 55 M38A1 running again.It has been sittting up awhile and the gas had gone bad.I cleaned the tank and replaced the fuel filter and had to put a new fuel pump(electric in line) on it as well.Now it is running good but it will not idle.It has never idled right.When it comes back to idle it has always run rough and will eventually cut off.Now it won't idle at all.I have rebuilt the carburetor in the past and even then it did not idle correctly.I imagine the carburetor could stand to be rebuilt again since the bad gas sat up in it to but even so how do you make it idle correctly.I have adjusted the air/fuel mixture both ways and it never seemed to make a difference.Any suggestions as to what my problem may be.Thanks Jim

BigJim · Post by **BigJim** » Tue Jan 18, 2011 4:21 pm

Anybody got any thoughts on why my jeep won't idle.See post above for info.Thanks Jim

Post by **wesk** » Tue Jan 18, 2011 9:42 pm

Electric fuel pumps often exceed the ability of the float inlet valve to fully close. Anything over 5 PSI is too much. You may need to install a pressure regulator. I personally do not see a need for add on electric fuel pumps. A properly rebuilt, installed and maintained mechanical fuel pump is more than adequate for the F134.

Rough idles are usually caused by overly rich or overly lean mixtures. Your first step in troubleshooting is to determine if the idle mixture is too rich or too lean.

Too rich is usually caused by over filling the bowl (IE. electric fuel pump pressure too high) or cracked/warped bowl leaking additional fuel into the idle circuit or the venturi.

Too lean is usually caused by air leaks into the carb's metered air system. Worn throttle shafts at the lower housing shaft holes can cause this. Bad PCV valves can also cause this. This is easily troubleshot using a propane enrichment test. Plugged internal passageways can cause this also.

BigJim · Post by **BigJim** » Tue Jan 18, 2011 9:52 pm

Thanks Wes.I am pretty sure it is running to rich by the smell of the exhaust.Sounds like a lot of the problem could be in the electric fuel pump.I may just go back to the original mechanical style pump.Jim

oilleaker1 · Post by **oilleaker1** » Thu Jan 20, 2011 8:12 pm

If you have the original military carb, there is a long brass tube attached to the underside of the top of the carb. This tube dips down into the bowl and picks up the gas for the idle mixture. It has two very small holes in it's side. If they are not clean and unobstructed you will have a irratic idle or no idle no matter what. I found this a problem on a rebuild until I discovered that my idle smoothed out when I sprayed carb clean into the hose between the air filter and the carb. Ah Ha! no fuel at the idle speed. I found a very small brass cutting stuck in one of the two little side holes. Do not ream out the holes! Wes is right about too much fuel pressure with most modern electric fuel pumps. You need to buy one regulated for the correct pressure. They sure run good when you get it right though. Enjoy, John

BigJim · Post by **BigJim** » Thu Jan 20, 2011 9:31 pm

Thanks John.I know exactly what you are describing.How would you go about cleaning those ports.Jim

Post by **wesk** » Thu Jan 20, 2011 9:42 pm

Left the air horn off and blow the tube out in reverse flow.

You don't want to dump the trash back in the tube's shaft or the bowl.

oilleaker1 · Post by **oilleaker1** » Fri Jan 21, 2011 7:43 am

Hello Jim! I have a set of pilot orifice reamers for working on very small natural gas or propane pilot orifices. If you as old as me you have to wear glasses to see it. I gently worked at it with the reamer, but I think the air blast did the most to dislodge the cutting. I had the top of the carb in my hands. Do not use a welding tip reamer or you will ruin the original size. Some guys will soak the parts gummed up in laquer thinner and then blow it out. No rubber or paint with this technique. Napa ( no auto parts available

) used to sell a adjustable fuel pressure regulator to solve the pressure issue also. The double acting fuel pump is quite a job to rebuild and the mass of lines is a challenge. Education is expensive as a machine shop owner once told me during another education I had. You'll get it purring! John

BigJim · Post by **BigJim** » Fri Jan 21, 2011 11:00 am

Thanks John,I have a friend in the propane business that may have just the tool.I am looking in to the pressure regulator so may be the combination of the 2 will get her going.I began the sanding process last night fo her new paint job.Hopefully she will in good form shortly.Jim

Post by **wesk** » Fri Jan 21, 2011 11:55 am

The mechanical fuel pump is not very difficult to overhaul when compared with single action style mechanical pumps. If you can overhaul the carb you can handle the pump. The internal diaphragm link can be patience testing but the entire procedure is very clearly explained in the pump overhaul TM loaded on our downloads page and Bob Westerman has an excellant step by step guide on his CJ3A web site. http://www.cj3a.info/v35/site.html

http://www.cj3a.info/tech/fuelpump.html
http://www.willysmjeeps.com/v2/modules. ... it&lid=109

Stay away from NOS rebuild kits for any fuel system part. Unless you can be sure it has late style rubber parts dated newer than the mid eighties.

Bob also addresses the plumbing mystery there.

oilleaker1 · Post by **oilleaker1** » Fri Jan 21, 2011 1:58 pm

I found the double acting pump not too bad, but the mounting of the pump to the block with all the limited access and tight quarters to be the hard part. If you took the left front fender off, it would be much easier, and you could adjust your valves while off also. You can sit in a chair then!!!!!!!!!!

John

BigJim · Post by **BigJim** » Fri Jan 21, 2011 3:07 pm

Thanks guys.Gonna try and clean those ports first.I have ordered a pressure regulator so maybe both of those will help.If it still acts up I will try my hand at rebuilding the fuel pump.I went to change the oil tube to the back of the oil pressure gauge and realized my wiring is in pretty bad shape.I will be tackling that soon also.I am sure I will be asking for info on that too.This sight is the coolest thing I have seen lately.I am completely addicted.I have always been a little afraid to tear in to some of these things for fear I would break something then not know how to fix it or not be able to find the part.You guys have eliminated that.Thanks again.Jim

oilleaker1 · Post by **oilleaker1** » Fri Jan 21, 2011 7:13 pm

Thanks for the kind remarks. Wes has helped me many times and He really shines with his pictures of the item under question. I've done the wiring too so yes, we can help. Sticker shock for a harness will be the first experience. Keep us posted. John

BigJim · Post by **BigJim** » Fri Jan 21, 2011 10:01 pm

Hopefully won't have to do the whole wiring harness.I am not trying to restore back to original military specs(I know that may be heresy) and there really isn't much electrical I am needing to power.A few gauges,starter solenoid,fuel pump and head and tail lights is gonna cover me for now.I hope I can kinda do it one piece at a time.I am gonna try and take some pictures along the way.Thanks again. Jim

BigJim · Post by **BigJim** » Sat Jan 22, 2011 6:33 pm

Well we are getting there.I installed the pressure regulater and blew out the ports on the little brass tube in the carburetor.I put her back together and the good news is that it appears to be getting better.It will idle now but very rough.It runs smooth as silk when you accelerate it but it just runs rough as a cobb when it comes back to idle.It is running EXTREMELY rich.It will make your eyes water when you stand around it because it is so rich.I can turn the air/fuel mixture screw either way and it makes no noticeable difference.I don't think it is catching air anywhere else at least not that I can tell.What I am I missing here.Something I was reading on one of the posts said something about a pcv valve needing to be replaced.Does my jeep have one and if so where is it.Thanks guys.