Create an account Home  ·  ·  Forums  ·  ·  Articles  ·  ·  Downloads  ·  ·  Photo Gallery  
Login
Nickname

Password

Don't have an account yet? You can create one here.

Navigation
· Home
· Article Archive
· Article Submit
· Downloads
· FAQ
· Forums
· Members List
· Photo Gallery
· Private Messages
· Web Links
· Your Account

Search Articles



Forums

1953 M38A1 PCV Question
M38A1 PCV vacuum line routing
Three M38A1's for sale - Westfield Massachusetts -$6500
Profile problems
M38 Fender mount Blackout Headlight with bracket
Wes K. and Ryan M. Appreciation Day
Production month/year of my NEKAF M38A1
Request for private photo gallery
M38 Fuel Strainer Extension Tube wanted.
11" Brake parts identification help

Willys M Jeeps Forums


willysmjeeps.com :: View topic - Severe carb flat spot off of idle
 Forum FAQForum FAQ   SearchSearch   UsergroupsUsergroups   ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

Severe carb flat spot off of idle

 
Post new topic   Reply to topic    willysmjeeps.com Forum Index -> Technical Knowledge Base
View previous topic :: View next topic  
Author Message
1948willyman
Member


Joined: Jun 18, 2009
Posts: 45
Location: North Carolina

PostPosted: Mon Mar 01, 2021 7:49 am    Post subject: Severe carb flat spot off of idle Reply with quote

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?
Back to top
View user's profile Send private message
wesk
Site Administrator
Site Administrator


Joined: Apr 04, 2005
Posts: 15332
Location: Wisconsin

PostPosted: Tue Mar 02, 2021 12:31 am    Post subject: Reply with quote

You posted this in the wrong forum:
Quote:
This Web Site
A place to log any issues, ask questions, and post general comments or suggestion regarding this web site.
Moderators Moderator, TomM


I have moved it here to the Technical Forum.

Flat spots, especially on acceleration are usually caused by a malfunctioning accelerator pump.



Check the condition of the accelerator pump diaphragm and insure it is assembled in the correct order. Unlike most other vehicles for this period which use a mechanically operated accelerator pump this carb uses a vacuum operated unit. The rubber diaphragm rots with age and if running modern gas station gas the original material diaphragm do not last very long.




The discharge check ball and the intake check valve ball are easily dropped, lost or misplaced during carb rebuilding. If these are missing or installed incorrectly they will cause flat spots as well.


Make sure you are using this carb base to manifold gasket which improves fuel atomization. The metal "V" faces down when installed. Many folks unknowingly use the plain thin gaskets in place of these.







Finally make sure you manifold heat valve is working properly
_________________
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
Back to top
View user's profile Send private message Send e-mail Visit poster's website
1948willyman
Member


Joined: Jun 18, 2009
Posts: 45
Location: North Carolina

PostPosted: Tue Mar 02, 2021 6:26 am    Post subject: Reply with quote

Will check all this out
Thanks so much
Back to top
View user's profile Send private message
Eades
Member


Joined: Jul 10, 2018
Posts: 87
Location: Western Washington

PostPosted: Fri Mar 05, 2021 9:52 am    Post subject: Reply with quote

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
Back to top
View user's profile Send private message
wesk
Site Administrator
Site Administrator


Joined: Apr 04, 2005
Posts: 15332
Location: Wisconsin

PostPosted: Fri Mar 05, 2021 10:29 am    Post subject: Reply with quote

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
Back to top
View user's profile Send private message Send e-mail Visit poster's website
1948willyman
Member


Joined: Jun 18, 2009
Posts: 45
Location: North Carolina

PostPosted: Sat Mar 06, 2021 7:12 pm    Post subject: Reply with quote

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?
Back to top
View user's profile Send private message
wesk
Site Administrator
Site Administrator


Joined: Apr 04, 2005
Posts: 15332
Location: Wisconsin

PostPosted: Sun Mar 07, 2021 12:04 am    Post subject: Reply with quote

Quote:
Did not know if small check ball was suppose to be in there or not?


If the valve is the original part number (ORD) 5336400 (WO) A6895 (AC) 1543018 there is no ball. Just the spring and metering plate.

The M38 does not offer a good illustration of this valve in it's manuals but the M38A1 does in it's TM9-8015-1 page 156.


Make sure it is pointing the correct direction!

Partial choke and pedal pumping to increase RPM smoothly on acceleration is indicative of:

1 - Accelerator pump not provided needed boost in fuel.

2 - Metering rod incorrect dimensions.

3 - Vacuum leak.

It is tough to verify item 1 without a small flex head borescope to let you see the carb's venturi while you accelerate from idle to see if you are getting the additional squirt.

Metering rod dimensions are illustrated below:



Vacuum leak detection methods vary but one that normally works well when properly and carefully performed is the propane enrichment test.



Any plugging of ports in this drawing will give you an inoperative accelerator pump.

Finally are you absolutely certain you metering and accelerator pump diaphragm springs are arranged per this CORRECTED drawing?


_________________
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
Back to top
View user's profile Send private message Send e-mail Visit poster's website
1948willyman
Member


Joined: Jun 18, 2009
Posts: 45
Location: North Carolina

PostPosted: Mon Mar 08, 2021 8:40 pm    Post subject: Reply with quote

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.
Back to top
View user's profile Send private message
1948willyman
Member


Joined: Jun 18, 2009
Posts: 45
Location: North Carolina

PostPosted: Tue Mar 09, 2021 9:16 am    Post subject: Reply with quote

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.
Back to top
View user's profile Send private message
wesk
Site Administrator
Site Administrator


Joined: Apr 04, 2005
Posts: 15332
Location: Wisconsin

PostPosted: Tue Mar 09, 2021 9:44 am    Post subject: Reply with quote

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
Back to top
View user's profile Send private message Send e-mail Visit poster's website
1948willyman
Member


Joined: Jun 18, 2009
Posts: 45
Location: North Carolina

PostPosted: Tue Mar 09, 2021 8:16 pm    Post subject: Reply with quote

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.
Back to top
View user's profile Send private message
1948willyman
Member


Joined: Jun 18, 2009
Posts: 45
Location: North Carolina

PostPosted: Sat Mar 13, 2021 10:28 am    Post subject: Reply with quote

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?
Back to top
View user's profile Send private message
wesk
Site Administrator
Site Administrator


Joined: Apr 04, 2005
Posts: 15332
Location: Wisconsin

PostPosted: Sat Mar 13, 2021 1:16 pm    Post subject: Reply with quote

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
Back to top
View user's profile Send private message Send e-mail Visit poster's website
Display posts from previous:   
Post new topic   Reply to topic    willysmjeeps.com Forum Index -> Technical Knowledge Base All times are GMT - 6 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum

Powered by phpBB © 2001 phpBB Group
Forums ©

 



PHP-Nuke Copyright © 2005 by Francisco Burzi. This is free software, and you may redistribute it under the GPL. PHP-Nuke comes with absolutely no warranty, for details, see the license.