Here for more Carb tuning advice

[tdubsy]

Hey all,

Looking for a little more advice on getting this bike running nicely. I Currently have the bike idling and accelerating fairly well. But what seems off is if I give it throttle just a little too quickly from lower Rpms the bike bogs down. I'm starting to think it's time to adjust the needle height with shims. Is it possible to tell off the video if this indicating a lean or rich mixture? Pulling the choke seems while doing this seems to do little effect.

I had always thought this was just a normal thing with an older bike but after seeing another cb750 video the other day they did not seem to have this issue

Things to note,
- Carpy's 4 into 1 exhaust
- K&N pod filters
- Carbs Vacuum synced
- 118 Main Jet
- Bouncing back between 135 and 138 slow jets. Have tried 140 and its wayy too much
- Dyna S electronic ignition
- Accel Pumps squirting with both check valves working properly
- Float height set with height tube to fuel 3mm below gasket rim

 

[hinksy]

My F2 does the exact same thing . Accel pump is working and it is spraying good , but I have a bog if I snap the throttle . I also have a DRZ400E with an FCR carb and it will do exactly the same if you stand still , but not under load . Very confusing . I thought it might be the transition of the low to main air ( thought that is what the pump was for ) and either not enough or too much fuel . When I went to look I found out some putz has put non adjustable F3 needles in my F2 . So still waiting to get some very expensive adjustable needles ( 266 of your dollaroos ) and I will have a play . But before that I am going to play with the accel pump timing . The settings you are meant to have leaves a bit of play in it , so maybe this is causing a delay in the squirt . Am going to go through it and tighten her up to see , but man they couldnt make those adjuster arms any thicker could they ?

 

[tdubsy]

My F2 does the exact same thing . Accel pump is working and it is spraying good , but I have a bog if I snap the throttle . I also have a DRZ400E with an FCR carb and it will do exactly the same if you stand still , but not under load . Very confusing . I thought it might be the transition of the low to main air ( thought that is what the pump was for ) and either not enough or too much fuel . When I went to look I found out some putz has put non adjustable F3 needles in my F2 . So still waiting to get some very expensive adjustable needles ( 266 of your dollaroos ) and I will have a play . But before that I am going to play with the accel pump timing . The settings you are meant to have leaves a bit of play in it , so maybe this is causing a delay in the squirt . Am going to go through it and tighten her up to see , but man they couldnt make those adjuster arms any thicker could they ?

Could this maybe be considered normal for these bikes? I wasn't alive back in the late 70's to try one of these bikes out for any kind of reference.

I've also thought about the accell pump timing thing as per the manual my gap is set correctly.

Should you even have adjustable needles? I know some years didn't. But you can also use shims to adjust the needles in these carbs

 

[hinksy]

I had one in 78 when I was 19 and at no time did it ever bog or play up , except for the valve guides failing within 3 months then the cam ran dry after the bike shop fixed the valve guides . Other than that , it didn`t miss a beat and was always reliable , just turn the key and go . Earlier F2`s were adjustable but the later F3`s came with a fixed needle and as you say you could just shim it , but I want the original needles in case the profile/taper is different

 

[dirtdigger]

Free revving means nothing. How it runs under a load is what is important. If you close the choke partially it will probably stop doing it. I have wrote on here many times to go take it for a ride....if you have a drivability issue at a certain point ride to that point hold the throttle and start closing the choke if it gets better it is lean, if it gets worse then you are very close or already too rich on the jetting. Then you can look at what jetting areas you need to look at. But double check that your ignition timing is correct and that the advancer is working correct to and advancing at the correct rate.

 

[tdubsy]

Free revving means nothing. How it runs under a load is what is important. If you close the choke partially it will probably stop doing it. I have wrote on here many times to go take it for a ride....if you have a drivability issue at a certain point ride to that point hold the throttle and start closing the choke if it gets better it is lean, if it gets worse then you are very close or already too rich on the jetting. Then you can look at what jetting areas you need to look at. But double check that your ignition timing is correct and that the advancer is working correct to and advancing at the correct rate.

Yeah I get that. But the bike should not be doing this anyways so i am trying to figure out the root cause. Ignition timing is set correctly and running a Dyna S ignition system

 

[81cb750f]

I don't have much experience with the SOHC 750 carbs so just a shot in the dark here.
Are those the stock jets? Seem rather large. My 81 has jets of 68 and 100.