Fine tuning main jetting

I'm having trouble with a bog at WOT at 60 mph + on the freeway. When I whack it wide open it does not pull to redline, just feels like it's starving for gas. However I know the main jet must be close since it only seems to happen late at night when the air temps are cool. In the daytime the problem doesn't seem as noticeable.

I currently have VM26 carbs with 120 mains, pods and 4-1 Kerker exhaust. My question is should I just go up one jet size at a time to get it right? I'm getting ready to order some 122.5 mains or should I go with 125's?

Thanks

In the daytime the air density is thinner, therefore thw mixture that gets to your engine is richer. In the evening when the air becomes more dense the bike leans out slightly, just as you suspect.

I would personally go with the 125's, but you may end up needing something larger to obtain max power.

Good luck!

i agree go up 2 sizes at a time, and 1 at a time when u are close.
what did u change to necessitate a jet change?

Thanks for the input guys. Not sure what changed really. I did clean the UNI foam pods recently and did not oil them as much as previously so they may be flowing better. Other than that I'm not sure. Like I mentioned the problem only seems to really show when at night on the freeway when air temps are cool.

I have the same setup and same jetting and have no bog at anytime.. Pulls red great

savedrider wrote:

Thanks for the input guys. Not sure what changed really. Other than that I'm not sure. Like I mentioned the problem only seems to really show when at night on the freeway when air temps are cool.

Do u see what i see?

I did clean the UNI foam pods recently and did not oil them as much as previously so they may be flowing better.
If they are indeed FLOWING BETTER, then u would be leaner leaner. try adding choke, to see if it helps.
:silly:

chance wrote:

I have the same setup and same jetting and have no bog at anytime.. Pulls red great

Well I sure hope the jetting fixes it, if not then I'm not out much $$$$. Maybe I should try adding choke like mark1122 says before spending any cash.

savedrider wrote:

I'm having trouble with a bog at WOT at 60 mph + on the freeway. When I whack it wide open it does not pull to redline, just feels like it's starving for gas....

If not already done, would test run with fuel tank cap ajar, to determine whether there might be a tank venting problem.

Good Luck!

Man these are all great ideas!

I actually had worked on a fuel leak I had on the gas tank fill neck recently. It always leaked fuel around the cap if I filled the tank past a certain level. Trouble was it had a ding in the fill neck and wasn't sealing properly.

Fairly recently I replaced the cap seal and did some bodywork to the fill neck. It doesn't leak gas anymore and I can fill it to the top now. Now I am wondering if the tank vent is working. That might make a lot of sense since I've had some starting trouble lately too and it seems like fuel wasn't getting to the bowls. Got a brand new Pingel fuel tap so the fuel tap definitely isn't the problem. Question, how do I go about getting the fuel cap to vent properly if I can determine it's not doing it's job?

The plot thickens even more...I was coming home from a short errand the other day and it starting cutting out. This wasn't at WOT, instead just normal street riding. I knew the tank was getting low on fuel so I just switched to reserve. However, it never recovered and ran like crap for the next couple blocks till I made it home. You could be on to something with the tank venting issue. Looks like I've got some testing to do.

More thoughts regarding possible fuel starvation - - -

Any foreign matter suspended in the fuel, such as paint over-spray, may hover about the petcock pick-up tube screens, and impede fuel flow into the petcock, especially under acceleration.

Petcock screens may quickly clog.
Could remove petcock and assure screen is clean.

And also flush tank by removing tank and swishing fresh gasoline inside and fast-dumping gas out the filler hole.

A clogged inline fuel filter may also cause fuel starvation.

Good Luck!

can u open the tank lid while riding? if it has a lock, u would need a spare key. if u are riding and experience this issue, u could open the tank lid to provide air as a test.
u should be able to unscrew the bottom from the tank cap and disassemble it, to check the vent.