Severe carb flat spot off of idle

1948willyman · Posted: Mon Mar 01, 2021 7:49 am Post subject: Severe carb flat spot off of idle

1952 m-38 with ys carb . Severe flat spot off of idle with two replacement kits put in it. Am I missing maybe a small passage in it somewhere that is stopped up? It seems to help if you pull choke half way out to cut off some of the air intake. Almost like it is running too lean? Thought about putting a lawn mower gas tank above the carb somewhere to rule out fuel pump problem although it has been rebuilt also and seems to be throwing fuel out of the line good. Anyone have any thoughts on this what might be going on? Did not know if valve stems pulling too much air around them might contribute to this problem or not?

wesk · Posted: Tue Mar 02, 2021 12:31 am Post subject:

You posted this in the wrong forum:

1948willyman · Posted: Tue Mar 02, 2021 6:26 am Post subject:

Will check all this out
Thanks so much

Eades · Posted: Fri Mar 05, 2021 9:52 am Post subject:

I rebuilt the stock carb on my M38 with an Ebay kit. Note that I contacted the vendor because one diaphragm was assembled backwards (overseas junk). They sent a new kit. It was a bit doggy off idle and stumbled. I had replaced everything I could from the provided parts. I thought it could have something to do with the accelerator circuit. I tore apart and inspected the parts old vs new. I swapped out the new diaphragm spring for the old which appeared slightly different spring diameter and length. The verdict now is it accelerates a better and no stumble.
_________________
Rick Eades
1951 M38 & 1967 M715

wesk · Posted: Fri Mar 05, 2021 10:29 am Post subject:

The obvious difference in springs "X" accelerator pump and "BB" metering rod is often overlooked during inexperienced reassemblies. Also I have yet to see a new carb kit with the exact same springs that match original "X" and "BB" springs perfectly. But at least it is a simple task to experiment with different length and tension springs till it behaves well.
_________________
Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php

1948willyman · Posted: Sat Mar 06, 2021 7:12 pm Post subject:

I checked out everything in the carburetor and all seemed to be ok.The check ball and weight were in place and the diaphragms seemed to be fine with the springs in the correct locations.Float adjustment also was within specs. The kit came from Pete Debella some time back and seemed to be excellent quality.Checked out the heat riser and it was working good. I had repaired it some time back also and got it in good working order. The carburetor base gasket was not correct but I had one of these in my junk parts pile and replaced it with a thin gasket on both sides also. Still has that flat spot though just as it did before putting a new kit in the carburetor.In trying to check for air leaks, located a minor air leak in the pvc plumbing due to a cross threaded connection although do not believe enough to cause an issue? In checking the pvc valve out,it has about a 1/16 open hole through it. Did not know if small check ball was suppose to be in there or not? Could the engine timing be the cause for this? Thought about advancing it a little just to see.It runs good down the road once up to speed but if you hold just a light foot on the gas pedal from start,it will just die unless you are pumping the pedal to get it rolling good. Engine does have a little spit or miss to it at idle though.Running engine with choke half way on does seem to help the problem. Thought about playing with that spring size on the pump also, just to see if it would change anything. Any one else had this problem?

wesk · Posted: Sun Mar 07, 2021 12:04 am Post subject:

1948willyman · Posted: Mon Mar 08, 2021 8:40 pm Post subject:

Just an update on the carb problem with a flat spot. Used a Carter Model W-O
carb to see if engine ran better. I rebuilt it almost ten years ago and placed on the shelf in my building. To my amazement the engine ran very well with no flat spot at all. I guess I am back at it trying to see what is wrong with the YS carb.Least I know now that is where the problem lays and not somewhere else in the engine. I did check the springs and both are where they should be. Will check the dimensions on the metering rod next to make sure it is correct but I believe it is. Best I can tell all vacumn and gas ports are clear but will re-check.Just dont know what is going on with this carb? Trying not to let it beat me.

1948willyman · Posted: Tue Mar 09, 2021 9:16 am Post subject:

Best I can tell the metering rod is very close to the drawing. Also I forgot to mention that the gas level in the carb was one inch below the top. Should they not be a little more in the reservoir? Only other thing I can come up with is maybe a hairline crack in the carb pump plate that mounts on with the four screws on the vacuum side of it causing not enough pull against the spring to push fuel in when vacuum is low? That is the way I understand the pump system to work? Must be very little travel in the diaphragm though to push fuel in. I blowed cleaner through the jet nozzle and it seems to be open. just do not know what else could be going on with it. In looking at the drawing all parts seem to be in place. I am going to mic the pump ball and weight also to see if it checks out. I forgot to do that.

wesk · Posted: Tue Mar 09, 2021 9:44 am Post subject:

One of the biggest problems with field carb overhauls is getting all the pressure & suction passageways unplugged. The better carb kits come with replacement plugs so you can remove all the passageway plugs and insure all passageways are unobstructed. Sprinkling carb cleaner over the bodies of the three carb sections does not insure passageways are clear.

_________________
Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php

1948willyman · Posted: Tue Mar 09, 2021 8:16 pm Post subject:

I do have a kit like that and worked on a couple of these today. Will let you know how it goes. One passage did seem to be partially blocked.Going to try and work on some more Wednesday. Carb is completely torn down once again.
Thanks for your input.

1948willyman · Posted: Sat Mar 13, 2021 10:28 am Post subject:

Took the old jeep up the road this morning and it done very well with the YS carb back on it. Did find one passage behind a plug that was almost blocked.Got it cleaned out good and jeep is doing much better with no flat spot,I put the longer spring on the pump side that your picture shows in your kit. Took the shorter stiffer spring back out that was originally in there. Thanks Wesk for all your help in trying to figure this problem out. Want to try and get it out some this spring and summer for a drive. Now to start back on my trailer, I have torn down to pull with it, although it is a Vietnam war era M416 I think?

wesk · Posted: Sat Mar 13, 2021 1:16 pm Post subject:

Glad to hear you are winning. Good luck.
_________________
Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php


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