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Balancing Throttle Bodies

6K views 10 replies 4 participants last post by  lee 
#1 · (Edited)
Man I've gotta' get a life. This forum is becoming addictive.

Anyway- If the purpose of balancing the bodies is to achieve the same airflow through the plates then why couldn't one set the airscrews at the same opening (say 1.5 turns out on each), hook up the manometer and adjust the balance screw/stop screw to equalize the plate position as close as possible and then go back and adjust the airscrews to get it all even. This asumes that the airscrew circuts are identical or very close. 1 & 2 and 3 & 4 are tied together so you wouldn't be able to get them all individually but it seems to me they could be gotten fairly close. The airscrews are for setting idle/off idle mixture and for compensating for unequal plate position on the shafts. Correct me if i'm wrong.

The main problem I see here is being able to adjust the CO (mixture)
due to the CO bung on the MC pipes being so far back that it would be really hard, if not impossible, to read CO's at idle. And I'm not so anal that I'm going to add bungs to each header. So one would have to play "suck it and see" and tweak each screw an equal amount to get it set right. Bogging/stumbling on initial throttle opening = rich, and popping out the pipe on decel=lean. There's a fair allowable range for the screw openings so one should be able to get it acceptable. Also the further open the throttle the less effect the airscrew circut has on the overall A/F ratio. What do you think? Am I getting too fussy here?

On another note is there a fuel filter, other than the MV, that will do the job? Economy sucks, and I'm trying to save money. Ain't we all. Except for CAG. He's loaded from his parts business.

What's the diff between the BMC "race" and standard air filter. They're available on the web for 1/2 the cost from the dealers so that's a no brainer.
 
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#2 ·
i set the balance screw between the pairs with the air bleeds closed, then wind the air bleeds out to get the approximate idle speed i want and then balance the vacuum using the air bleed screw for each cylinder as required. once the vacuums are balanced i then use the idle trimmer in the ecu to set the idle mixture.

idealy you would use the air bleeds to set the mixture in each cylinder via a port on each header, but with only 1 port in the collector you simply do what you can. on the brutales with the two ports - one each on the 1-2 and 3-4 collectors you can do them that way - often the vacuum on 1-2 and 3-4 will be quite different to get the mixture the same in both paired collectors (themselves not paired by injector driver/map anyway), so it's kind of a crap shoot as you say.

the air bleeds will lose their influence over mixture once the manifold vacuum drops quite a bit i would think, but i would expect them to have an influnce at all normal cruising speeds.
 
#3 ·
The air bleeds compensate for differences in cylinder efficiency at idle. Valve lift and duration (varying clearances normal in manufacturing tolerances and shim adjust method), valve sealing, piston ring condition, combustion chamber deposits and so forth all affect each cylinder's manifold air pressure slightly at idle. The butterfly valves in the throttle bodies all need to be opening equally and evenly for off idle (normal riding) synchronization, but to get smooth idle the pressures in the inlet manifolds can be equalized (synchronized) by the air bleeds. This also provides a convenient method of correcting/setting idle speed without changing butterfly position and therefore Throttle Position Sensor setting which is critical for ECU calculations. The TPS is one of 3 major determinants of injector pulse duration and timing. The others being the Manifold Air Pressure Sensor and Crankshaft Position Sensor.

The ECU idle CO trim should be a combination of all cylinder's efficiency and is a base line setting the ECU then uses for all injector duration calculations when the throttle is twisted. You can just probe the tailpipe for that reading. Looking at the workshop manual, you will note that the 2 gas probes installed on the factory bungs are going to a Y connection and feeding all 4 cylinder's gasses to the analyzer. The only key on a tailpipe probe is that you insert it far enough in that the reverse pulse waves drawing clean air in to the tailpipe cannot cause erroneously lean readings. Using the bungs provided on the exhaust manifolds eliminate this possibility.

Old carbureted bikes with individual fuel mixture screws needed individual cylinder CO readings to adjust each carburetor properly, but this is not the case with EFI systems. If you adjust the air bleeds for idle CO on each cylinder you are not using them for their intended purpose.

Now if you are doing running condition diagnostics then being able to read each cylinder's CO percentage as well as other exhaust gasses can be very helpful to determine which cylinder is causing the problem.
 
#4 ·
Here's the dope of the day award. Being so used to working with carbs I just assumed the air bleeds varied the mixture by regulating the air mixing with fuel from a pilot jet. Not so with this set up. All they do is vary air. Now how dumb is that? Makes it a newer and much simpler ball game. The older I get the less I know. But I'm trying.

Thanks for your excellent explanation.

Lee
 
#7 · (Edited)
Old carbureted bikes with individual fuel mixture screws needed individual cylinder CO readings to adjust each carburetor properly, but this is not the case with EFI systems. If you adjust the air bleeds for idle CO on each cylinder you are not using them for their intended purpose.

There is NO benefit to trying to get exactly the same CO readings on each cylinder at idle using the air bleeds. What is important is getting the overall CO% correct as this is the ECU's baseline. The ECU cares not what any individual cylinder is doing. By tweaking each cylinder you affect the overall which is the key once you twist the throttle. Smooth idle is a function of equalized manifold air pressures...that is the purpose of the air bleeds given that the throttle butterflies are all set to exactly the same angle.
 
#8 ·
on the 4 cylinder bikes it may not have as much effect due to the smaller volume per cylinder to total capacity ratio, but it can have a large effect on the twins.

it can have a noticeable effect on the low speed running of the brutale when you do each pair individually - which is still a compriomise, but more accurate. sometimes the brutale ends up with quite a vacuum difference between 1-2 and 3-4, but the mixture will be the same in both collectors.

welding bungs onto ti headers would be a bit of a nightmare, yes.
 
#9 ·
I think the vac diff between 1-2 and 3-4 can be adjusted with the balance screw to bring them much closer. Then it's a simple matter of final balancing the vacuum with the airscrews, setting the idle CO with the pot on the ECU and setting the idle rpm with the throttle stop screw as long as the TPS reads 150mv with the plates fully closed. From what I gather the fully closed voltage is the baseline setting from which the TPS and ECU vary the fuel delivery according to throttle opening. That's the procedure I'm going to follow. If anyone disagrees please say so and give your recommendation.
 
#10 ·
I wouldn't recommend using the throttle plate balance screws to adjust idle vacuum. The adjustment between the throttle plates should be factory set. The throttle bodies are assembled as a unit and synchronized that way. Adjusting idle vacuum should be done with the bypass screws. On something like my 748, where the two throttle bodies are separate assemblies, I synchronize the throttle bodies using the linkage at about 3500 rpm, then adjust idle balance using the bypass screws.
 
#11 · (Edited)
Point well taken. It assumes that the bodies were properly set up at the factory. I still feel that if 1-2 and 3-4 are far enough apart bringing them back into line with the balance screw would be acceptable before proceeding with the finer points. How in the world are you able to hold the rpm steady at 3500 long enough to make the required adjustments? This engine is so "twitchy" and sensitive The only way I can get a steady rpm is with the Throttlemeister locked and at that it still drifts. I think we are promoting the same thing, just at different rpm's.
 
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