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Air Bypass Screws

JasonC

JasonC

Cruisin' Guzzisti
GT Contributor
Joined
Jun 5, 2021
Messages
371
Location
Katy, Texas
Gentlemen, I've read several pages of threads about the air bypass screws and I'm now totally confused. I've read that both should be set 1/2 turn out from seated; I've also read they should be 1 turn out; and then I've read one screw should be closed all the way and the other open; and lastly I've read that you simply twiddle these screws to obtain the desired idle speed.

So, I have two questions: what is the purpose of these screws; and how should they be set/adjusted?

Thanks!

Jason
 
These screws are used to set the balance at idle with the engine warm, this is what my Breva 1100 manual says
Completely close off both by pass screws
Run the engine at idle and check the vacuum between the two cylinders is balanced
If this is not the case open ONLY the by pass screw with the highest underpressure to obtain equal values between left and right.
I use a manometer that I made and it is very accurate.
 
Kevin.NZ said:
These screws are used to set the balance at idle with the engine warm, this is what my Breva 1100 manual says
Completely close off both by pass screws
Run the engine at idle and check the vacuum between the two cylinders is balanced
If this is not the case open ONLY the by pass screw with the highest underpressure to obtain equal values between left and right.
I use a manometer that I made and it is very accurate.
Click to expand...

Thank you for that response Kevin. I need to add it to my list of air bypass screw purposes. Notably, one post I read said that when balancing the throttle bodies you should close both air bypass screws and then perform balance adjustments using the threaded rod that connects the two throttle bodies.

Jason
 
What bike do you have? You mentioned procedures for 3 or 4 different models. If you have a single TB V7 there is no such thing to do.
 
As V700 Steve says, this is only for engines with 2 throttle bodies, also do not adjust the threaded rod between the throttle bodies as you will have a hell of a job to get them synchronised
 
V700Steve said:
What bike do you have? You mentioned procedures for 3 or 4 different models. If you have a single TB V7 there is no such thing to do.
Click to expand...
I have a V7 with two throttle bodies. But I would think that the purpose of the air bypass screws should be the same, unless there is no air bypass screw on a single throttle body.

Both of my bypass screws are backed out one (1) full turn from seated. But as I bought my bike used, I don't know if this is correct or not.

Based on Steve's post, one (1) turn out from seated may be OK so long as the two throttle bodies are balanced.

Jason
 
Last edited:
Kevin.NZ said:
One should be shut and adjust one for balance otherwise you end up chasing your tail. Do you have a tool to check the balance, you cant do it "by ear"
Click to expand...

Thanks Steve, I'm beginning to get it now.

Yes, I have a gauge setup for balancing the carbs on a KZ-1000, which will work on the V7 Cafe Classic. So essentially the purpose of the bypass screws are for balancing two throttle bodies and are obviously not required for a single TB.

Jason
 
JasonC said:
Ok, I understand now; they are simply used for balancing the TBs.

Twin TBs are new to me, so pardon my ignorance.

Jason
Click to expand...

You were too fast on the draw there! I was still editing. Go back and read the rest please.

If you understand what “balancing the throttle bodies” really means then you understand perfectly why only 1 air bleed is ever open and the other closed, and that anything that says 1 turn or 3/4 turn etc is dead wrong.
 
scottmastrocinque said:
You were too fast on the draw there! I was still editing. Go back and read the rest please.

If you understand what “balancing the throttle bodies” really means then you understand perfectly why only 1 air bleed is ever open and the other closed, and that anything that says 1 turn or 3/4 turn etc is dead wrong.
Click to expand...

Scott, thanks again; I think I fully understand now.

When balancing, or matching downstream vacuum, you start with both screws seated and adjust as necessary the air bypass screw on the TB that, I assume, has the higher vacuum. I'm thinking that opening the air bypass will lower the vacuum on that particular TB. I'm going to check my TB balance today.

Jason
 
EXACTLY.

This is why one is ALWAYS CLOSED. You start the process with both closed and then open ONLY the side with the greater manifold vacuum at IDLE and only to the point where both levels on your synchronization tool read the same value for both the higher and lower side, and NOT 1/2, 3/4, 275 turns etc. :rofl:

Well done
 
Jason. What year and model motorcycle fo you have?

I suspect that you have a V7 Sport from 1971-1974 and just may be posting in the wrong forum.
 
On the V7 Classic 2 TB it's easy to follow the manual, it is easily understandable & both air screws are open about 1 turn-3/4 to get your 1100 rpm back after having them closed to balance at higher speed. Only read manual for your model w/15RC ECU which is like a Cali EV. All models w/5AM are closed all the time but one side may be open to sync at idle. Different procedures for different ECU's.
 
V700Steve said:
On the V7 Classic 2 TB it's easy to follow the manual, it is easily understandable & both air screws are open about 1 turn-3/4 to get your 1100 rpm back after having them closed to balance at higher speed. Only read manual for your model w/15RC ECU which is like a Cali EV. All models w/5AM are closed all the time but one side may be open to sync at idle. Different procedures for different ECU's.
Click to expand...

I have a 2010 V7 Cafe Classic; does it have a 15RC ECU?

Jason
 
Those screws change your mixture, when closed it will turn your spark plugs black. The 5AM ecu doesn't do that and is made for closed screws.
 
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