engine and drivetrain 38 DGAS Idle Adjustment

[landon]

I am having some trouble getting the idle adjusted on my weber 38 DGAS. This is really a self induced problem as it was running decent until I tried to fine tune it.

 

Background;

Idle was last adjusted at a shop a couple of years ago and was running good. Stock cam 9.5 pistons, 180 k motor. The carb never idled silky smooth but was acceptable.

The carb has always had the typical flat spot in the transition. 

High speed operation is fine.

Cold starts are fine.

Hot starts are fine.  In fact these are much better now since I installed a Jeep filter bypass and return line to the tank. No more flooded hot starts and fuel smell.

I have averaged 22-26 mpg in mixed driving. Plugs always looked good.

 

I noticed after the bypass install that the flat spot seemed a little more pronounced so thinking that the mixture may have leaned out a bit I decided to see if idle adjustments would make a difference. I did not note the original settings.

I have read numerous posts regarding the adjustment and it all seems pretty simple but I am just not getting this dialed in.  

 

I started with the redline weber baseline settings

idle speed  = 1/2 turn in

idle mixtures = 1 turn out

 

The car will not not idle on it's own at this setting so I go to 1 1/2 turns out on the mixture screws to keep it running.

I leave the idle speed alone and adjust mixtures only turning CCW (richer) until it runs better. I go in 1/4 turn increments equally on both sides. I get the highest idle speed at about 2 turns out. The engine shakes pretty good at this point. I get a little smoother idle by richening another 1/2 turn or so and then there is not much difference in running after that. Both screws are adjusted equally. Now we are running about 1200 rpm. I turn down the speed screw to about 3/8ths of turn in to get 1000 rpm. It runs much rougher. I go back to the mixture screws and adjust slightly either direction but I can't seem to get a smoother idle. Low speed running at this point is rough and it feels like the engine is struggling to get to the transition. It runs fine past the transition.

 

Some questions;

 

1. According to the weber directions it seems that the mixture screws are turned out too far and the idle jets may be too small? I have not pulled them to check the size. The shops adjustment seemed OK though and they did not indicate the need for re-jetting. At this point i am just trying to get it back where it was.      

2. Am I too lean or too rich? Does the fastest idle speed correlate to the optimum mixture? Should I be lean of this or rich of this? Generally, the exhaust smells rich. How can I distinguish between too rich and too lean when I feel the roughness at low throttle openings?

3. Somewhere in the process I pulled a plug and it indicated a pretty lean mixture. No other bad signs of lean condition such as pinging or high temperature. Can damage be done with too lean an idle setting?

 

Thanks for any tips or comments.

[flagoworld]

Do you have a vacuum gauge? Adjust the mixture to the highest vacuum. You may also double check timing as incorrect timing might effect idle speed IIRC. If timing is correct and the highest vacuum doesn't run well, you can look at jets. If they are correct according to how other people run their jets, check out your dwell. I was getting some pretty rough idling due to my points not being adjusted correctly. It took a bit of frustration with nothing else seeming to work, but once dwell was adjusted correctly, everything fell into place and she ran pretty good. Don't bother adjusting dwell with a feeler gauge. Use a meter. When I used a feeler gauge it was WAY off. Due to wear, i guess.

 

** Editing: Use a feeler gauge to incrementally shift your adjustment, but do not rely on what the spec says for clearance between the points. Use the meter for that.

Edited November 2, 2016 by flagoworld

[xr4tic]

I'm going through the same thing, rebuilt my carb and am having issues adjusting it per the Weber instructions.

 

You'll need to confirm your idle jet sizes.  This is fairly easy to do, two plugs on either side of the carb and then read the tiny number on them.

 

Mine were .045 and I had to have the idle speed screw at 2.5 turns to get it to idle.  I ordered some .050 and .055s, plugged the .050s in, and was able to get it to idle at 1.5 turns.  I'm going to try the .055s next.

 

Here's a good thread with lots of information in it on trying to tune the Weber 38 - 

 

 

 

[landon]

3 minutes ago, flagoworld said:

Do you have a vacuum gauge? Adjust the mixture to the highest vacuum. You may also double check timing as incorrect timing might effect idle speed IIRC. If timing is correct and the highest vacuum doesn't run well, you can look at jets. If they are correct according to how other people run their jets, check out your dwell. I was getting some pretty rough idling due to my points not being adjusted correctly. It took a bit of frustration with nothing else seeming to work, but once dwell was adjusted correctly, everything fell into place and she ran pretty good. Don't bother adjusting dwell with a feeler gauge. Use a meter. When I used a feeler gauge it was WAY off. Due to wear, i guess.

Yes,the last attempt I tried a vacuum guage. It generally confirmed what I was hearing. At highest vacuum setting I still have the roughness coming off idle.

 

Timing check was fine. I did not touch it and did not have any reason to. No points - pertronix. Only thing to change here between good running and poor running was my touching the carb adjustment.

[flagoworld]

That pretty much leaves either turning the wrong screws or incorrect jets, I would think. And valves are correctly adjusted? I had a worn rocker arm that could not correctly set an intake valve and caused that one cylinder to not idle well.

[esty]

adjust the mixture to highest idle then adjust your idle

[landon]

55 minutes ago, flagoworld said:

That pretty much leaves either turning the wrong screws or incorrect jets, I would think. And valves are correctly adjusted? I had a worn rocker arm that could not correctly set an intake valve and caused that one cylinder to not idle well.

Yes, turning the screws incorrectly. Valves were adjusted at the beginning of the season probably about 8k miles ago. They are pretty consistent from year to year with only minor adjustments needed. The rough idle started after I adjusted the carb. 

[landon]

15 minutes ago, esty said:

adjust the mixture to highest idle then adjust your idle

Yes, I read all the archives and you make it sound so easy. I obviously don't  have the touch. Highest idle with the mixture screws in my case does not seem to result in the smoothest idle. I understand it is iterative - adjust mixture to high idle, re-adjust idle speed, go back to re-check mixtures, etc.  

[landon]

6 hours ago, xr4tic said:

I'm going through the same thing, rebuilt my carb and am having issues adjusting it per the Weber instructions.

 

You'll need to confirm your idle jet sizes.  This is fairly easy to do, two plugs on either side of the carb and then read the tiny number on them.

 

Mine were .045 and I had to have the idle speed screw at 2.5 turns to get it to idle.  I ordered some .050 and .055s, plugged the .050s in, and was able to get it to idle at 1.5 turns.  I'm going to try the .055s next.

 

Here's a good thread with lots of information in it on trying to tune the Weber 38 - 

 

 

 

Thanks John. I had read thru that post and there are quire a few things at play. I will be checking the size of the idle jets. Per the original post I just want to get back to the starting point without introducing other variables and wanted to get a check on the procedure I am using. 

[BarneyT]

I'm having the same trouble getting my 38/38 to idle well...attached is a picture of the idle screw/needle can someone confirm that this is the correct idle needle as it's different from all the others I have seen...

[flagoworld]

Yeah either that's the wrong needle or you don't actually have a 38/38...

 

That needle seems to belong in 40s or 45s

http://www.ebay.com.au/itm/WEBER-MIXTURE-SCREW-KIT-WEBER-40-DCOE-45-DCOE-40-IDF-CARBURETTOR-CARBY-64750-003-/191346123266

and more similar looking:

https://www.pegasusautoracing.com/productdetails.asp?RecId=5962&utm_source=google&utm_medium=cpc&utm_campaign=64750.003&gclid=CPKYxKe6j9ACFQSUfgodILAJnQ

 

The correct needle is this guy

http://www.opelgtsource.com/store/9000/9080.html

 

And the one for 32/36 is this

http://www.opelgtsource.com/store/9000/9079.html

Edited November 4, 2016 by flagoworld

[BarneyT]

flagoworld....thanks for the heads up, the one I have matches the one listed from pegasusautoracing it has a seat at the end of the tapper...looks like it's meant for a 40...I'm ordering new needles and jets today...

[landon]

Well, solved my adjustment problem. Vacuum leak. (2) intake manifold gaskets were leaking. I had checked for vacuum leaks all around the base of the carb and vacuum lines but did not think of the intake. I took it back to the shop that had adjusted it a couple of years ago and they found the leak. All good tips above about checking the not so obvious and despite my many good repairs on this car over the years, sometimes it takes the help of a pro.