Weber Balance Question

digginfool · #1 (**permalink**) 03-07-2011, 07:07 PM

Over the weekend, I had an issue where I had lost a cylinder. Through some fiddling around (and mostly luck) the issue resolved itself and was most likely dirt in the idle circuit of one barrel. During my process of elimination, I purchased an infrared thermometer to identify which cylinder was misfiring by taking the temps of the header at each cylinder. After the engine cleared itself out, I took the temps of all eight header pipes and was surprised by the disparity. On the driver side, temps of the front and rear cylinder were about 30 degrees cooler than the center two. On the passenger side, all four header pipes were within 10 degrees of each other and 50 degrees warmer than the hottest pipes on the driver side. This was after about 20 minutes of driving that included highway speed followed by in-town driving, without shutting down engine. This makes me believe the carbs probably have very different A/FRs. I just acquired the car two weeks ago and have only put 200 miles on it. The previous owner stated he spent a small fortune having the carbs set up. The engine runs smooth, pulls very hard but with such low seat time, I couldn't possibly know if I'm missing out on some significant performance gains. Any suggestions?

*13* · #2 (**permalink**) 03-07-2011, 08:52 PM

Your question is confusing. Suggestions about which?

digginfool · #3 (**permalink**) 03-07-2011, 10:05 PM

The only question was comments on the information. Am I correct in theory that with such disparity in cylinder temps that the carbs are not in balance? Since Weber experts are difficult to come by, any suggestions on where to start (e.g. books, specific threads, known experts I can talk to, etc.) in determining whether or not my carbs are in balance? Could the different temps be a product of variations in the thickness of the ceramic coating on the headers? The engineer in me says no but I'm civil, not mechanical. So, any suggestions?

wolf k · #4 (**permalink**) 03-08-2011, 12:10 AM

As a mechanical engineer who has worked with large marine diesels engines all my adult life, a spread of 80 degrees F. at the exhaust is not out of the ordinary.
Couple of points:1) To get true accurate exhaust temp readings, you need thermocouples installed directly in the header pipes. An infrared gun is helpful in troubleshooting, but not for absolute numbers. Your readings can be affected by the thickness of the coating, positioning of your gun, and airflow from cooling fans across the primary pipes.
2) In large diesels, exhaust temps are monitored only when the engine is under a moderate load to full load. At idle, and low loads the exhaust temps are usually way out of balance and the alarms are automatically inhibited. When you measured your temps, your engine was not under load at the time, exhaust temps fall rapidly when unloading an engine.
3) It is combustion pressure that we try to balance in diesel engines, sometimes exhaust temp and combustion pressure do not coincide due to several factors. Exhaust temps can be a tell-tale to combustion pressure but it is not an absolute to tuning an engine. Manufacturers give acceptable exhaust spreads.

So, if this information crosses over to gasoline engines, which I believe it would, I would not be alarmed at your readings, A/F measuring would probably be more helpful, or just reading your plugs at this time. Hope this helps, Wolfgang

*13* · #5 (**permalink**) 03-08-2011, 06:25 AM

Who did the Weber Tuning? If it's a reputable mechanic with experience in this such thing you may want to give him a call & ask him a few questions. If the car is running smooth & there are no issues, it's likely they are pretty well tuned.

Rick Parker · #6 (**permalink**) 03-08-2011, 11:53 AM

If the air volume through the carbs has been balanced then it would be a A/F mixture variance. ALSO it is possible that the volume draw through the throats of and individual carb are not equal for varying reasons. Sometimes a little gentle tweaking of the throttle shafts can correct this, just be carefull. FWIW the amount of time the engine spends idling is of little significance. As mentioned above, it is when then engine is experiencing load that is of importance.

bobcowan · #7 (**permalink**) 03-08-2011, 06:23 PM

I spent a lot of time getting my stacks exactly balanced. they're all so close to each other, I'd need a digital gauge to get them closer. But, I'v noticed the same thing about the header temps. 30-40* variance is about what I have. I wouldn't worry about it.

xb-60 · #8 (**permalink**) 03-08-2011, 06:59 PM

You mention that you had lost a cylinder? When this has happened to me it's always been a fouled plug (or two). If you pull them all, are they all a similar colour? That will give you at least some idea as to whether you have mixture variations.
There's an old saying...if you think it's a carb problem, check the electrics, and if you think it's electrical, check the carbs.
My experience with multiple (sidedraft) carbs is limited to Alfas, and when you get the setup of the Webers right, you know it was worth it. They hold their tune well too.
Glen

Caprimaniac · #9 (**permalink**) 03-27-2011, 01:37 AM

Agree on Wolf and BobC. So no big issue if car is running great.

You could buy a cheap syncrometer to check if the carb flow is synced.
If they are and you still have large differences in temperature on individual cylinders, it might be due to small differences in fuel flow in carbs or minute differences in cylinders. You could do some adjustments by using the idle mixture screws. More fuel = colder (until plug fouling ..)

But, there's only one way to know for sure: Install temp AND A/F sensors in all 8 exhaust runners and go to a dyno or do live mapping....

This is what the pro's do.