Welcome to our Site for MG, Triumph and Austin-Healey Car Information.
MG MGB Technical - Weber 32/36 DGV idle issue
|1977 MGB with a Weber 32/36 DGV carburetor with a manual choke and a 25D distributor.|
I'm having two issues: poor initial idle until warmed up (2-3 minutes) and a moments hesitation when accelerating from a stop or up-shift. I installed a rebuild kit on this Weber DGV carb two years ago and got a professional recommendation on which jets to install.
With the idle issue, a cold start begins with the choke out, no gas pedal, turn the key and the car starts 1st or 2nd time. The challenge is to keep it running while it warms. Left to it's own the idle is less than 500RPM and more often than not would stall w/o feathering the gas. Once warm the idle is smooth at 900RPM with no choke.
As for the acceleration hesitation, unless I'm very gentle with the accelerator the car will hesitate for a fraction of a second then respond... makes for a dicey trip in stop and go traffic.
I've been through the set-up procedures for the idle speed and mixture screws and I think I've done it correctly, especially since the car idles well when warm.
Jets installed: Main: 140 PRI / 135 SEC, Idle: 60 / 50, Air Corr. 165 / 160
All suggestions and recommendations are most welcome.
|Adjust choke linkage to open throttle, initial movement should open butterflies before closing choke flaps. (primary only) This will allow you to control tickover speed during warm up without excessively rich mixture. Have you checked the throttle pump is functioning correctly, a blocked jet or split diagphram could be the culprit. I would suggest best way to set slow run mixture is with warm engine (900 revs is fine ) then turn screw in until engine slows, then turn out until engine smooths , continue until engine slows or staggers, and then in again just enough to run smoothly. This should find the richer setting where the engine is smooth, do the same with the secondary mixture screw. All this will help with better pickup up to approx 3000 revs. You will need to make the adjustments quickly and with confidence, take too long and it will be inacurate.|
|Brian, I had a similar problem (32 36 dgev - electric choke). But I only have one mixture screw. I discovered that the accelerator pump diaphragm was split. Changed it and it was much better...|
|Jim and Moss, thank you for your input. Tomorrow I'll remove the carb, replace the accelerator pump and clean out all the passages. I was thinking of doing the work with the carb mounted but I've thought better of it. I'll post again and let you know how I made out.|
I forgot to mention one other symptom. Occasionally when I get the hestitation I get a small backfire from the exhaust. This makes me think the carb is running rich, but the other issues seem to me to imply lack of fuel. Confusing.
|Brian. Did you replace the power valve inside the fuel bowl? This is held in the off position by high manifold vacuum (idle and cruise). When the vacuum drops, the valve opens (acceleration or bad diaphragm) and allows significantly more fuel into the primary venturi.|
I am not sure why you have the larger jet on the primary (i.e. smaller) venturi and the smaller jet on the secondary (i.e. larger) venturi. I will try to check my notes to see what I am using.
One quick check is to start the engine and allow it to idle with the air filter removed. Look into the primary venturi and see what is happening. If you see fuel running into the venturi, the power valve is not functioning properly, resulting in an excessively rich mixture. Older rebuild kits did not contain this component and I have seen them go bad in use. The latest kit I purchased (EMPI) do contain a replacement power valve.
|I can second Les' comment re the power valve (and it was Les that helped with my carb issue a few months ago - thanks Les...)I would recommend replacing regardless...whilst you have the carb off and dismantled.|
|Les and Moss, thanks again for the input. Yes, I did replace the power valve when I went over the carb yesterday. Disappointingly I did not find anything obviously wrong.... no ripped diaphragms, no gunk in the passages. I have remounted the carb but not fired it up yet due to rain, such is life w/o a garage!|
Les, I'd be most interested in knowing what jets you're using, although I believe the recommendation could be different between you in AZ and me in MA.
I came across some interesting info however. One article I read talks about if you have to open the throttle significantly in order to get a good idle you uncover the "transition ports" (I'm clueless on that one)and that implies your idle jet is too lean. One symptom is hesitation. I've always thought my idle speed screw was in much more than recommendeded. I've ordered a 65 idle jet to try as an experiment. Obviously this does not explain why the performance had deteriorated in the last 10 days.
|Probably not relevant but on my 32 36 dgev I discovered that the second throttle valve was open a fraction when at idle - the flap that opens for the last 1/3 of the movement when one accelerates (excuse my poor description, cannot rememebr the correct terms) There is a very small screw on the carb base that can be adjusted to close or open this flap. Not yet worked out why this should be adjustable but it is. I'll try and take a pic later. The point is that it made a difference to idle.|
|My car is now running like a champ thanks to the suggestions I received here and from Bob of Brit-tek. I believe both my issues had to do with getting the proper setting for the idle mixture and speed screws. |
The best advice I received was, from the intial set-up settings, to make 1/16 turn adjustments to the mixture screw and wait 10 seconds until the car adjusted to the new settings. I was amazed at the difference a small adjustment like a 1/16 turn in the mixture screw made in performance. I received advice elsewhere that I should make quick adjustments and that didn't work for me. One shakedown cruise with two pit stops to refine the mixture screw setting was all I needed.
Moss, while a stuck 2nd throttle wasn't my issue this time, two years ago I had that happen. 2,500 rev's at idle was the "symptom". The cure was to lube the linkage.
Thanks again to Jim, Les and Moss for their time and input.
This thread was discussed between 23/07/2012 and 30/07/2012
MG MGB Technical index
This thread is from the archives. Join the live MG MGB Technical BBS now