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MG MGB Technical - Hesitation / loss of power, Weber carb.

1977 mgb roadster with a manual choke Weber DGA carburetor.

Today after going around a sharp curve the car suddenly developed a hesitation / loss of power / "drowning out" issue. Step on the accelerator and the car hesitates to the point of stalling. I feathered the accelerator and limped home.

Another symptom is the idle increased by 2500 rpms. I can't see any binding of the accelerator or choke cables.

All ideas are most welcome.



Brian Denis

One other symptom.

I removed the Weber to do a rebuild and there is excess gas pooling in the manifold. Not surprising I suppose considering the other symptoms.

I will continue with the Weber rebuild kit and see what that does.

Other ideas are welcome.

Thanks.
Brian Denis

I've identified a likely suspect.

The butterfly disc on the larger of the two chambers remains partially open when pressure is off the throttle. So the linkage is binding.

My premise is gas getting through that chamber results in poor carburation and higher idle speed.

I'll continue with the carb rebuild and post the results.
Brian Denis

Brian
You have a fairly common problem there
It usually happens to a car that doesn't get over 1/2 throttle very often
The problem is the secondry shaft gets stuck or stiff from little use and then when it is used it sticks and if it stays slightly open it will run terribly rich and rough Answer is to free it up and lube it and all should return to normal Cheers Willy
WilliamRevit

Brian. Perhaps. But, the Weber DGV series has some problems which could also cause the effect you are seeing. First, there is a power valve which is located in the top "lid" section. It is held off by vacuum. When the diaphragm fails, the power valve dumps lots of extra fuel into the system and can cause some problems due to an excessively rich mixture. The second thing to examine is the brass tubes which run across the top of the venturis. These have been known to shift resulting in the end coming out of the casting and allowing excessive amounts of fuel to get into the system. Had this happen with my current daily driver.

The older rebuild kits did not contain a new power valve assembly. The new (Chinese) rebuild kits do. Take a very close look at what is happening and what the parts look like, both as installed and when you remove them, to see what may be happening. The screws holding the throttle plates (butterfly) to the throttle shaft can loosen, allowing the plate to move and hold the throttle open.

I have used the Weber on two of my cars and on my daughter's car with good results (i.e. it allows the cars to pass emissions testing). The Holley 5200 carb is a licensed version of the Weber DGV and much of the information on the Holley applies to the Weber even if they are of slightly different format. (The Holley has five screws holding the top cover on. It was used on a number of American cars over the years.)

Les
Les Bengtson

I finally feel I've licked the problem. For archive purposes I'll document what I found.

I'm confident the main issue was the secondary butterfly valve not closing 100% when I was off the accelerator pedal. I cleaned up the linkage and lubed it as Willy suggested.

But I also found the carb didn't have the recommended jets for my area and driving habits.... I'm at sealevel and just do cruising. The recommendation I got for jets was: Main: 140 primary, 135 secondary. Idle: 60,50. Air correction: 165, 160. I purchased and installed new jets.

I aslo rebuilt the carb with the standard rebuild kit including the power valve Les mentions in his posting.

Then I went through the recommended tuning process per the Redline Weber website technical information.

Time will tell, but based on a modersate shakedown cruise all appears to be well.

Thanks to Les & Willy for their contributions.

My original posting has a typo in the carb model #, it's a Weber 32/36 DGV.




Brian Denis

This thread was discussed between 19/07/2010 and 27/07/2010

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