General carb tuning info

[salty_monk]

Chef,

Could it be the pilots?
I would try to richen up the pilot circuit using the screws first, I read somewhere on here that having them slightly rich helped the transition from no throttle to some throttle (makes sense cause more petrol in there already when you open it up then).

Certainly having those screws too lean on my bike made the idle hang as you describe. Something to try before you mess with the needles again...

Dan

[chef1366]

Chef,

Could it be the pilots?
I would try to richen up the pilot circuit using the screws first, I read somewhere on here that having them slightly rich helped the transition from no throttle to some throttle (makes sense cause more petrol in there already when you open it up then).

Certainly having those screws too lean on my bike made the idle hang as you describe. Something to try before you mess with the needles again...

Dan

Could be but since he wasn't popping on decel I didn't consider it. I jumped from stock 45 pilot to 47.5 pilots but I also jumped up to 170 on the air jet from 160.

[the_journeyman]

OK, I'll go another 1/2 on the screws and see where that happens. I couldn't find any intake leaks and and just checked the valves (all in good shape). If the pilots don't do it, I'll richen the needles by one clip.

I set everything per the Dynojet instructions, so if they tend to be lean, I didn't use the pilot air jet they say is optional. According to the instructions, I don't need it, AND it leans out the bottom.

I haven't done a Synch, which I know might smooth things out a bit more. I don't have the equipment nor the money to purchase so I'll have to do without for now.

Thanks for the input!

JM

[chef1366]

I did not use the Dynojet air jet either.
Sync those carbs. Could be the issue.

[salty_monk]

There's a link on here for making your own. I downloaded the PDF. Basically a board, some tubes & some connectors. Should be cheap enough.....

Let me know if you want me to email the PDF to you.

Dan

[the_journeyman]

I read that link, but couldn't get a handle on making it. Maybe I should look again.

JM

[salty_monk]

The way I read it it's very simple. Just a board, 4 tubes, 4 smaller tubes & some connectors + some old oil.

Depends how valuable your time is I guess. Is there nobody on here near you that you could tap up to take it over too for a quick test, I don't *think* it takes too long.

Dan

[KEITH KRAUSE]

The carbs must be synched well to accurately re-jet.
Otherwise, you may find yourself re-jetting for what may be a vacuum related issue.
The slow to return idle (as long as the throttle plates are operating smoothly/correct throttle cable action) is generally a lean condition but a poor synch will aggravate things.
The slight stumble could be caused by a poor synch. Basic tuning, which includes synching, really must be done so you know any problems are jetting related.
Hard to say without being able to ride the bike, but a "stumble" could also be poor throttle response. Very often with the CV carbs, you need to drill out the vacuum orifice to the diaphragm/piston chamber to improve throttle response. Sometimes lighter springs will fix it too.
Stock air jet may be a problem too.
I'd test at 1/3 throttle and read the plugs before changing jet needle position. But like I said, without a balanced synch, you may get uneven plug reads that just add to frustration.

[the_journeyman]

Ok, I'll work out something about synching, maybe a small shop locally can give me a hand or something like that. I did drill the slides per Dynojet's instructions, so the vacuum orifice has been enlarged from stock ~

JM

[tone]

Would that be the DynoJet 138 or Mikuni 138? :shock:

I am under the impression that they are different sizes and not interchangeable. Yes, they are interchangeable in the sense that you can install the DJ jets in a Mikuni carb, but the DJ 138 jets are NOT the next size larger than a Mikuni 135. Not sure how they relate, but they are not the same.


.

Correct :-D one is measured by the actual size of the hole & the other by the flow rate ! ie a 138 DJ jet is NOT the same size as a 138 mikuni jet the seat where the needle sits is also a different shape & the needles themselves have a different taper

mixing the 2 types will have you chasing your tail for ages (from experiance)