Single throttle body vacuum leak / poor vacuum

[kjetil4455]

Dear friends,

I posted a thread here yesterday https://yamahasupertenere.com/index.php?threads/airscrew-adjustment-to-fix-off-idle-stumble-and-throttle-body-sync-w-pigtails.1973/page-27
as I failed to sync my TB well, and could only achieve this by opening the leading screw (white) by 2.5 full turns. This reduced the TB vacuum in both cylinders to about 18 cm Hg. The right cylinder (dominant) was initially about 24-25 cm Hg, which is nearly spec (26.5-30). However, turning the left (non-dominant) cylinder's screw did not change the vacuum pressure what so ever, and I could only calibrate by lowering the right's.

As I have continued to read, I have realized that this is probably indicative either of a left cylinder poor compression problem, or perhaps most likely, a vacuum leak on the left side --> or a problem with the left screw. We already did TB cleaning (forgot about the left screw though), and this didn't fix it. In the end, I was somewhat able to calibrate, but it is calibrated on a very low vacuum, way below spec. The problem, reiterated, is the left cylinder that does not respond to turning of thebleeding screw; its pressure does not increase.

The attached pic shows how it looked BEFORE I opened the dominant (white) screw by 2.5 turns.

Has anyone encountered this problem? I am working with a Yamaha mechanic, but because I live in south america, they have virtually no experience with super bikes. Almost everyone drives a 125cc here. Would be very grateful for tips and tricks. Thanks so much.

 

[Jlq1969]

Just to rule out: -) how many km/miles does the S10 have?
-) the valves are within the specifications
-) at what height above sea level did you synchronize the TB?...
-) since the synchronization is "only for idling".."closed throttle"...The TB are clean Does the butterfly seal?…if you turn the screw and the vacuum does not change, or is the air intake for idling blocked…or is the throttle open at idle

 

[kjetil4455]

Just to rule out: -) how many km/miles does the S10 have?
-) the valves are within the specifications
-) at what height above sea level did you synchronize the TB?...
-) since the synchronization is "only for idling".."closed throttle"...The TB are clean Does the butterfly seal?…if you turn the screw and the vacuum does not change, or is the air intake for idling blocked…or is the throttle open at idle

Thanks for your reply!

1. 40 000 km
2. They sound ok according to the mechanic but we don't know for sure
3. 950 meters above sea level (that's approx 3100 feet)
4. The butterflies seal well and the cylinders are cleaned (we didn't clean the screws as we didn't know about this problem). The air intake seems ok because the parallel cylinder has good vacuum. The throttle does not seem open at idle, the RPM is low around 1000-1100 (which is spec)

Appreciate your time

 

[Jlq1969]

With 40,000 km (you would be in the km of the first check, which always gives a tight exhaust valve)..., having differences in the valves, which could affect the vacuum between both cylinders. And the height above the sea, 1000 meters “is not little”….if you compare it with the vacuum generated at sea level…at 1000 meters, the cylinders would generate approximately 12/15% less vacuum than at sea level sea, because there is less atmospheric pressure. (do not compare vacuum values with other users, without taking into account the height above sea level)..

 

[kjetil4455]

With 40,000 km (you would be in the km of the first check, which always gives a tight exhaust valve)..., having differences in the valves, which could affect the vacuum between both cylinders. And the height above the sea, 1000 meters “is not little”….if you compare it with the vacuum generated at sea level…at 1000 meters, the cylinders would generate approximately 12/15% less vacuum than at sea level sea, because there is less atmospheric pressure. (do not compare vacuum values with other users, without taking into account the height above sea level)..

Interesting my friend. Thank you.
Still, the 1st cyl has normal vacuum pressure. And turning the left screw did NOTHING (low vac cyl), whereas the right screw works fine --> thoughts? I will have the valves checked for sure!

 

[Bill_C]

I would consider testing for vacuum leaks. You can use a lot of different methods. I prefer using propane because it won't damage anything and doesn't create a mess. If you have a small propane touch, just open the valve but don't light it (obviously). Then sweep the nozzle around the base of the TB, the rubber hoses, etc. If the RPMs spike, you've found your leak. No spike = no leak.

Once you have that option eliminated, get those valves checked and adjusted.

Sent from my SM-S906U using Tapatalk

 

[kjetil4455]

I would consider testing for vacuum leaks. You can use a lot of different methods. I prefer using propane because it won't damage anything and doesn't create a mess. If you have a small propane touch, just open the valve but don't light it (obviously). Then sweep the nozzle around the base of the TB, the rubber hoses, etc. If the RPMs spike, you've found your leak. No spike = no leak.

Once you have that option eliminated, get those valves checked and adjusted.

Sent from my SM-S906U using Tapatalk

Thanks a lot, Bill. Must be isolated to the left cylinder then! I will see if I can find a propane torch

 

[Jlq1969]

I would consider testing for vacuum leaks. You can use a lot of different methods. I prefer using propane because it won't damage anything and doesn't create a mess. If you have a small propane touch, just open the valve but don't light it (obviously). Then sweep the nozzle around the base of the TB, the rubber hoses, etc. If the RPMs spike, you've found your leak. No spike = no leak.

Once you have that option eliminated, get those valves checked and adjusted.

Sent from my SM-S906U using Tapatalk

 

[Bill_C]

View attachment 95210

LOL! It's far safer than that. No different than using carb cleaner only less mess.

FYI - Just don't crank it all the way open. You're just cracking it open a little, trying to enrich the air/fuel mixture by introducing a little fuel via the vacuum leak.

Oh, and while it should go without saying, don't go chasing vacuum leaks with anything flammable until you've checked your ignition/coils/plug wires for cracks and grounds. If your ignition is old and cracked and grounding against the block then you've got an ignition source to repair before you can safely use propane, carb cleaner, starting fluid, etc... again, you probably already know that but thought I'd add that little safety warning before you end up looking like the guy in the picture.

Sent from my SM-S906U using Tapatalk

 

[kjetil4455]

LOL! It's far safer than that. No different than using carb cleaner only less mess.

FYI - Just don't crank it all the way open. You're just cracking it open a little, trying to enrich the air/fuel mixture by introducing a little fuel via the vacuum leak.

Oh, and while it should go without saying, don't go chasing vacuum leaks with anything flammable until you've checked your ignition/coils/plug wires for cracks and grounds. If your ignition is old and cracked and grounding against the block then you've got an ignition source to repair before you can safely use propane, carb cleaner, starting fluid, etc... again, you probably already know that but thought I'd add that little safety warning before you end up looking like the guy in the picture.

Sent from my SM-S906U using Tapatalk

Yep, I saw it done on youtube, I know where you're coming from. Will try this, definitely prefer the less messy - and less rubber damaging - approach.

 

[kjetil4455]

UPDATE: We have solved the problem.

Thanks to a kind sir on the facebook groups, I was able to get some pointers. Due to the fact that turning the left screw did literally nothing, he said it must be an issue with the screw or its seal. And indeed, so it was. The screws were nice and clean and were not the problem. However, the tiny o-ring (I didn't even see it until he asked for it; We had to shine light into the hole to see it), which didn't even look damaged, had been malformed by the pressure and lack of variation.

We took out both o-rings, bathed them in warm water, and applied a very small amount of high temp tolerance grease. And with this, the baseline pressures increased to about 20 and 24cm Hg, and the left screw worked again. We set the right TB cyl to 3/4 from closed, and then calibrated the left cyl. We were able to perfectly balance the TB, ending with both cylinders at about 24 cm Hg.

The engine stuttering, and not to forget my main problem - low idling power and motor stalling when releasing the clutch - seems to have been eliminated.

Hope this will help someone in the future.