Topic: Jetting

[Johnny340]

I would say you need bigger idle jet, but is anything bigger available?
Any chance you got confused on the clip position and put it all the way at the top(leaner) instead of all the way at the bottom(richer)?
Something is not right here
Have you had a look at the plugs?

I agree. I believe there are bigger Pilot jets available. I definitely have the clip on the bottom (which helped). Funny that the pipe supplier says I shouldn't even need to change the pilot jets from stock..

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[seanbarney41]

last bike I jetted for aftermarket intake was a Kawasaki with Mikuni's.  Everyone said I would not have to change pilots.  Tried everything with similar poor idle, hanging idle, etc.  Finally upped the pilot and bam, everything else fell right into place.  So if you can get a 47.5, do it.  Keep going up on the pilot jet until it will idle with the pilot screws turned out to the 3/4 to 1-1/4 range.  Then you want to re-check mid throttle positions as you might want to go back a notch on the needle.

[Johnny340]

last bike I jetted for aftermarket intake was a Kawasaki with Mikuni's.  Everyone said I would not have to change pilots.  Tried everything with similar poor idle, hanging idle, etc.  Finally upped the pilot and bam, everything else fell right into place.  So if you can get a 47.5, do it.  Keep going up on the pilot jet until it will idle with the pilot screws turned out to the 3/4 to 1-1/4 range.  Then you want to re-check mid throttle positions as you might want to go back a notch on the needle.

That sounds like excellent advice which I will try next. Thank you!

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[seanbarney41]

good luck...never heard of anyone needing to go that big on the pilot

By the way, where are you buying pilot jets?

[Johnny340]

good luck...never heard of anyone needing to go that big on the pilot

By the way, where are you buying pilot jets?

I think it was from 4into1 before. Suggestions?

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[seanbarney41]

good luck...never heard of anyone needing to go that big on the pilot

By the way, where are you buying pilot jets?

I think it was from 4into1 before. Suggestions?

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My local Parts Unlimited dealer can order a wide selection of Keihin and Mikuni brand jets...other than that, everybody likes JetsRUs, but I have not used them.

[Paintedseat]

What happens if you go 2 turns out? Are you buying real keihin  jets or the cheap knockoffs

[Johnny340]

What happens if you go 2 turns out? Are you buying real keihin  jets or the cheap knockoffs

2T out makes it hang up really high like it's super lean. It runs best when the air screw is between 1/4 to 1/2T out. I checked and despite that Keihin logo on my jets, I have some cheap knockoff ones . Now I will order some genuine ones. I wonder if I should get genuine 45s or go right to the 48..

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[Paintedseat]

I would go with 48 and lean it out if needed with the air screws

[Johnny340]

These carbs were giving me inconsistent behaviour with each jetting change so I decided to try a set of 657A carbs from another project. It's raining out so I haven't road tested it but in the garage it seems great with:
125M
40P
Clip on 4th
1T out.
The revs don't hang up before returning to idle like with the other carbs.
I noticed the mixture screws on the original 064A carbs don't have a great bottom out feel like they do on 657A carbs.
Are the mixture screws supposed to be the same shape (with the hole through them) with just a spring (no o-ring or washer) for both sets of carbs?


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[seanbarney41]

no washer or o-ring, just spring, on all cb750 roundtops...the shape of the mixture screw varied over the years, probably another very important factor as to whether any given bike will idle properly with pods

[Johnny340]

no washer or o-ring, just spring, on all cb750 roundtops...the shape of the mixture screw varied over the years, probably another very important factor as to whether any given bike will idle properly with pods

Ok thanks for confirming that. I need to check that the mixture screws fit properly and bottom out nicely on the 064A carbs.
The other thing I noticed was the amount the slides were open at 'idle'. The 657A which idled perfectly had them hardy open at all and the 064A were about twice as open. Hmm..

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