Topic: 1973 cb750 tuning and dies if choke is on

[Lucassayers7]

Hello all, I have a 1973 cb750 motor and trans in my hard tail that I've been messing with wondering if I need to make any changes i believe it is running a bit lean on the low end due to dying on quick snaps but an odd thing is if I use the choke no matter if the bike was or is running or cold it dies instantly I do have the choke set perfectly so there is no gaps when on. How can I fix this and it starts awesome, when cold I got to hold the throttle a tiny bit to keep it alive, thanks for your time.

Main jets: 117.5
Slow jets: 40(should i try 42 or 48?)
Exhuast: 2 to 1 mac side exit on each side baffled
Intake: pods
Needle: middle or 3rd position
A/F screw:1/4 turn out

[ekpent]

 Basically these early 750's will not run if the choke is left full on. It is used for an initial start when the engine is cold and 'tickled' on and off as needed. I also slightly hold the throttle open while warming and once warmed up after a little riding it will settle into a good idle. Its all pretty primitive stuff with no fast idle circuit etc..

[flybox1]

+1
choke goes completely off once the bike warms up.
for best throttle response, ensure that the carbs have been properly vacuum sync'd, and the IMS is set for optimal mixture(check plugs)
beyond that, these are carb'd engines, and rolling on the throttle yields the best result.

[evanphi]

You're probably running lean at idle. The quick snaps open will kill it, especially if not warmed up enough. I have 42 pilots in mine. Runs perfect.


ekpent is correct about the choke, also. I flipped my choke on one time when riding on a highway. I was scratching my calf and my glove flipped the lever up. Killed the bike pretty quick!

[flybox1]

Only go up in pilot jet size if you first cannot tune out the lean-ness with the IMS.
That alone might save you $40   

[Lucassayers7]

Thanks for the replies I can't actually start the bike with the choke on makes it harder and how many turns is to many and turning it out will make it richer correct? Also will the IMS effect light throttle at all or is that just the slow jet? I did a bench synch will get the tools to do a full synch soon

[Lucassayers7]

Wondering if I manage to get a stock air box will it run right with the stock setting or would I need a stock exhaust to do that. Thinking I might have it straight pipe for a bit because they hit if I corner too low wondering how that will effect the carbs as well.

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[Don R]

 Do the choke butterflys have the spring loaded flaps in them? If so, make sure they open with vacuum.