Weber running lean

7

72CS

Active Member
Site Donor
Messages
51
Reaction score
22
Hi all,

I recently installed a rebuilt M30B35 engine with a pair of Weber 38/38 carbs. I, and my BMW mechanic of many years, are having trouble getting them tuned. I have looked at various threads on this site and believe we have done all the tuning basics. The issue is that the back carb is running lean at idle. The rear bank of cylinders are running hot and the engine is occasionally dieseling and backfiring through the rear carb when the engine is stopped. The idle jets are 45 on one side of the carb and 55 on the other (the previous owner had them set up this way for a M30B34 engine). At idle the barrel closest to the block is hissing and the syncrometer shows it drawing easily more than double the air as compared to the other throat on the same carb. Both barrels on the front carb draw similar volumes of air. This bit seems to me to be significant but I'm not sure why...

The car seems to run ok at part and full throttle
The Weber setup instructions have been followed carefully (many times)
The carb has been taken apart and all passages checked for obvious blockages.
The throttle butterflies are working normally
Vacuum leaks have been checked for
Valves have been checked and adjusted, timing etc... have been checked
We have swapped the idle jets front to rear with no change, and we have even put both 55s on the rear carb, and the 45s on the front, again no real change.

We are leaning towards something funny going on with the idle circuit internal to the carb but beyond that we are at a loss.

Has anyone seen anything like this before or have any ideas on what the cause might be?


Thanks in advance
 
You probably tried this , but how about swapping the carbs- that would tell you if it is the carb or something below. It has been suggested to me that I should install a balance tube between mine which I plan on doing tomorrow. I hope to get a chance to do this tomorrow and I can post my results.
 
the shaft

is tweaked causing the rear carb barrel to be more open. Tweak it back. ( Get two wrenches , hold one side of the shaft , etc.) Common ailment back in the day and how you balance a Porsche that originally came with carbs. (356, 912, early 911).

You'll never get the jets right until you get the air draw right.

Usually on an M30 the rear carb runs slightly rich. The heat raises the fuel level ( density) and it runs rich. M30's like to run a bit towards rich is my experience.
 
Wow, thank you for the incredibly quick response.

With all the troubleshooting that has been done we have definitely narrowed it down to the one carb.

We will give it a "tweak" and see what happens.

Cheers
 
You might also take a look at your intake manifold. On the setup that I had previously, the intake manifold had a smaller bore on one side vs the other (inner vs outer) which I believe was designed to accommodate a 32/36 setup (or something with dis-similar bores). I think you can probably balance this out with different jetting inside vs outside, but ideally you probably want those intake bores to be similar to the 38/38 diameter.
 
balance tube

seems like the original dual and triple car systems didn't need balance tubes to run well.
If you need one to get it to run well it seems like your just covering up the true root cause.

John
 
You must log in or register to reply here.
Back
Top