Suspect M135 vacume advance problem

[Brad]

I really am frustrated by this truck.
It starts right up, and once I baby it thru accelerating it seems to run ok at high rpm.
But it just doesnt want to accelerats from idling. It will cough and back fire thru the carb everytime. Like it doesnt want to take gas.
I checked and I have vacume to the distributer.
Could something be wrong in the distributer that will not advance the timeing when accelerating?
Just to be sure, which line on the distributer is the vacume advance? The top or bottom line. One is vacume advance and the other is for the governor?
If anyone has any ideas I'd sure appreciate the help?
Regards,
Brad Foust

 

[NDT]

Neither line is vacuum advance. Both are for the Centrivac governor. The distributer has centrifical advance. Coughing is a sure sign of ignition issues. I would replace the points and condenser and go from there.

 

[G-Force]

Check the accelerator pump in the carb

 

[Oldfart]

It's almost always one of two things with gas engines, fuel or spark. I am just learning all the special issues about our M220, but I would add the thought that a bad accelerator pump might also cause the same symptom. My first blush response to the described symptoms would be weak spark or fuel starvation.

 

[Bill W]

Could also be bad ( worn out ) distributor bushings which would cause the point gap ( dwell ) to be erratic.

 

[badgmc56]

I would definately check the accelerator pump first. The old pumps are made of leather and when they are sitting around for a while or worn out, they can flip over and not pump any fuel. Also make sure the idle circut is not too lean. Adjust the idle mixture screws at the bottom of the carb. With engine off, turn screws all the way in an back off about two turns. Start engine and one at a time turn screws in till engine starts to sputter. Back out the screws one at a time till the engine smooths out and reaches its highest idle. Then road test!

 

[Brad]

Thanks for all the help fellas.
I should have said earlier that I did rebuild the carb and replaced the pump piston completly with a new one. However I also replaced the idle needles, and they seem to do nothing when adjusted. The old ones I removed did, Perhaps I will put those back?
Regards

 

[WillWagner]

Backfiring through the carb could be a timing issue. Thruogh the carb = advanced and out the tail pipe = retarded, or is it the other way around it's been a while! The advance weights could be stuck or have broken springs. Or it could be lean, use a can of something flammable...brake clean, etc. and spray at the base of the carb and around the intake. If the idle comes up, there's your leak. If you rebuilt the carb, it could be that the body is junk or you didn't get all the crap out of the idle circuit passages. Did it do this before?

 

[Oldfart]

Backfiring through the carb could be a timing issue. Thruogh the carb = advanced and out the tail pipe = retarded, or is it the other way around it's been a while! The advance weights could be stuck or have broken springs. Or it could be lean, use a can of something flammable...brake clean, etc. and spray at the base of the carb and around the intake. If the idle comes up, there's your leak. If you rebuilt the carb, it could be that the body is junk or you didn't get all the crap out of the idle circuit passages. Did it do this before?

Your right! late spark out the exhaust, early spark out the intake. Ditto the rest. Loss of idle screw adjustment would make this more of a carb issue than ignition in my mind. I had this happen to me once on an old 57 Ford Holly 4 bbl carb. I got the secondary and primary jets swaped around during my rebuild. Idle mixture screws would do nothing. I just don't know enough about early deuce yet to have a better idea.

 

[YkDave]

yea, these carbs are a little different that your typical holley 4bbl LOL

But all in all, most of the same principles still exist.


If your idle mixture screws do not appear to be doing anything, i would pull one out and compare it to your old one. maybe the needles are different, maybe the passage in the carb is plugged up.

Though the idle circuits biggest influence is obviously at idle and very low throttle (~1/8 ), the circuit is still active all the time and has a small influence on mixture all the way to full throttle.

Being that you already know you have an issue with the carb, start by fixing that before you go and start chasing down problems that may not exist!

 

[Mastertech]

ditto to everything that they all said. alot of times the symptom you have is caused by the idle to cruise transition circuit and will also show up as a symptom in the idle circuit make sure the float leval is high enough if its not the fuel wont pull over soon enough and you will get the stumble/hesitation.

 

[butch atkins]

have you checked the vaccum at the elbow inthe intake manifold? that will tell you a lot about hte motor problems,did you replace the diaphragm in the governor assy on the carb,if it is leaking you will never get the engine to run correct,check to see if the engine has enough vaccum to operate the governor assy,TM 9-8024 pg 228 tells about this,.i spent alot of time and money only to find i had aleaky governor diaphragm,good luck keep posting

 

[Brad]

M135 is continuing to drive me nuts.
Points are not burned, Coil is not hot.
I checked the gov diaphram, no holes.
Thought my carb float might be sticking open cause fuel pump keeps running.
And can smell gas in crank case oil??
Took that apart and tried adjusting float. Now I got gas going everwhere.
Quit for the night, if not fore ever.
Anyone know of a good mechanic in the western Pa area???
I'm about to give up.
Regards

 

[Oldfart]

Take a deep breath and exhale slowly. Sometimes when I am so deeply into a problem, I over look something. I find a good nights rest and a fresh start from the beginning will help. ~~~ In an effort to be positive, your symptoms do not sound like a major problem, just a gremlin or two that are impacting the engine tune. I would start by going back to the last know good running condition and then examining what changes have been made. From what you have posted, it would seem the sputtering on acceleration started after a carb rebuild. ~~ Is there any chance you can borrow a carb off a similar truck around your area. If you can find a know running carb to test with, you should be able to determine if your rebuilt carb is the problem (make sure carb base plate gaskets are not leaking air into the intake.) Only make one change at a time so that you are not evaluating the results of several changes and not knowing which one made a difference. ~~~ Too bad you are so far away from us. There just has to be someone in your area that has experience to lend you.

 

[jollyroger]

These carbs are a little different than a 4bbl Holley but........the basic principles are the same. Diagnostics are taken on a system by system basis. Float system. Idle system. Mid range system. Wide open throttle system. Governor system.

First we need to have adequate fuel delivery from the tank. Sounds like you have that.

Second we need to control the fuel into the float bowl. This is not happening. 3 basic things with this system...... Inlet opening. Needle and seat. Float...... Since you have overflowing of the float bowl we can assume the inlet opening is clear. So you either have a float that is sinking in the bowl, is set too high or the needle and seat has debris in it or is sticking.

Take the carb back apart and see if the needle and seat is stuck or sticks open. If yes you need to clear/clean debris or replace needle and seat. If no, operate the needle and seat with the float and see if it sticks or operates freely. If it sticks replace. If it operates freely move on to the float.

Take the float out and rattle it next to your ear. If you hear any rattling or sloshing the float leaks and is bad. Replace accordingly. If you don't hear anything drop the float into a cup of gas and see if it floats , barely floats or sinks. Leave it in the gas for about 15 minutes. DO NOT use water for the float test. The surface tension is different and it may not leak bad enough to sink in water.......Ask me how I know...... It should remain high in the cup of fuel. After 15 minutes redo the rattle test and check for results.

Once you know you have a good needle and seat and float you need to set the float level. There are guages that show the neccesary fuel level and I am sure the TM's describe what it should be somewhere but.......

Install the float etc. into the carb top. flip the top over like it was on the carb. Look at your float drop. Most carbs have a limit to how far the float will drop. You don't really want the float to sit on the bottom of the float bowl. It is better if it drops to just above the bottom of the bowl. The reason is two fold. That way the float does not get stuck to the bottom of the bowl and corrode the float when not in use and you don't want the needle to drop too far as it will cock in the seat and stick.

Once drop is set keep top right side up like it was on the carb. Push the float up by hand till the needle seats in the seat. DO NOT push up too hard. You just want the needle to touch not run aground. You don't want the float to come in contact with anything keeping the needle from seating. Keep in mind that the carb top is usually the "top" of the float bowl so you don't want the float level to be higher than the line between the carb top and bottom. It should usually be about an 1/8 to 1/4 inch below that line.

To adjust the float drop there should be a tab at the back of the float saddle that runs up against the seat. Bend it against or away from the seat accordingly. To adjust the float level gently bend the float down or up with the seat gently seated in the needle.

This was basic but should get you to the point of having an operable float system so you can get back to tuning the other systems.

 

[topo]

could this motor have been put together with out the metal sleeves that go between the intake manifold and the head . a new intake gasket will work for a little while but then it will start sucking air . I had this happen on gmc 302 and chevy 235 motors

 

[WillWagner]

Carburetion diagnostics is a lost art!

 

[Oldfart]

Carburetor diagnostics is a lost art!

You mean wipe the exhaust pipe with your finger and examine the soot, feel for oil and then smell to check how complete the combustion? Then adjust accordingly !!

I rebuilt an old Allis Chalmers WC tractor carb a couple of years ago. Simple updraft carb that just would not keep running. I took it apart several times and cleaned it, checked the float settings, put in the one gasket between the float bowl and carb top and never could solve the problem. Until the rebuild which included new jets and idle needle as well as float needle and seat. I could find nothing wrong with the old parts, but the new stuff fixed the problem. What ever it was it had to be microscopic or magical.

 

[Mastertech]

Dont get frustrated. you just need to think thru the problem like everyone is saying.
get the fuel level correct like Jollyroger said and move forward from there. alot of good info here
Just remember you are doing this for fun. Ive done alot of carb work in my life and the thing is
someone built it so all that needs to be done is make it work like its supposed to. if you need
someone to talk you thru it let me know and I will PM you my #. Im on the west coast
so 3 hours ahead but Im usually up late

 

[Mastertech]

Well one thing is for sure, It aint Magic Just seams that way untill you figure it out.
Then it will make perfect sense.