Bonneville America Forums Forums Technical Forums Performance Mods CV Carb Tuning

Update:
Removed the shims and installed the #152 mains Thursday evening. The test ride was my commute in to work Friday morning...
In town low speed low RPMs running up through the gears everything was fine! The larger jets seemed to have the bike running better - like it had a more available power and no issues with acceleration or run quality.

The problems began to exhibit when I got onto the highway and got up to about 65 miles an hour where, once again, the engine run quality was very bad - cutting out and sputtering like it wasn't firing right and the rpm's falling off like it was out of fuel. At one point about halfway to work it was running so badly I backed off the throttle to slow down below 50-55mph (where it had seemed to run better). But when I twisted the throttle to accelerate back up to 55mph - the engine completely bogged out like it was out of fuel!
I slowed to the point that I began to pull out of the right lane onto the shoulder when it started running normally again (although still not well), so I continued on to work at a reduced pace.

At the end of the day I took the uni filters off (to verify the slides were moving up and down normally), and also disconnected the throttle position sensor at the connector (thinking that might have had something to do with how the bike was not running well at speed). Well, that didn't have any effect (!) because on the way home it started running so poorly that I had to limp it along at 50 mph down the freeway - with diesel truck and car traffic passing me - like I was on a moped! 😖😤

Got it home and put the fans on it to cool it down. Once it was cool I put it up on the jack and pulled the tank to remove the tops of the carbs and check that I hadn't inadvertently covered up the vacuum slide bleed holes with the little footed piece that goes over the top of the needles... causing the "even worse" running issue... that wasn't the case as both holes were unblocked and open.

I put everything back together, put gas in my temporary fuel bottle and hung it from the rafters; connected the carbs and fired the bike up and turned up the idle knob so the motor was turning ~4500rpm. Using carb cleaner with an extension tube - I sprayed around the manifolds and spacers; around the rubber boots that connect the carbs to the manifolds; at the vacuum caps; and, all around the carb bodies high and low on all sides.

At one point - spraying the inner side of the L/H carb - the engine bogged down!

What I believe I've discovered is: there a significant vacuum leak which only occurs at higher RPMs and steady throttle! It's possibly at the carb fuel bowl vent T-pipe where it passes into the body of the L/H carb. So either the tube is cracked or there's a leak at the U-clip where the tube connects with the body the carb!
Without removing the carbs again and completely tearing them down I'm not sure if that's exactly the case (I've ordered a new T-pipe and new U-clips). So, when everything gets here sometime next week it'll be another carb teardown!

After thinking about this some more...
It's possible the leak may not be at the vent t-pipe at all. It could be the throttle shaft!

Is there some type of o-ring seal on the throttle linkage shaft where it passes through the body of the carb to the butterfly plate?
If so, is this a serviceable item (I don't see a throttle shaft O-ring/seal in the carb parts diagrams)?

I'm now thinking a leak at the throttle shaft is the more likely culprit for what I've been experiencing.

What do you think?

Regards,
CoolFogMan (John)