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1978 Honda CB400T
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Discussion Starter · #1 ·
Hello, I am having a little trouble setting up the Mikuni VM30s on my '78 CB400T. Any guidance would be very much appreciated!

Here's the situation:

The carbs came with the following parts: 6F5 needle, 159 P-5 needle jet (“emulsion tube”), AB160 main jet, and a 25 pilot jet. I have the carburetors installed and synced. The compression of the motor is perfect on both cylinders (150psi), spark/voltage is strong, valves are adjusted correctly, and timing is is spot on too. There are no intake or exhaust leaks either. The carbs have been vacuum synced and we have adjusted the speed and mixture screws accordingly. The bike will only run when the carbs are choked or a hand is held in front of the intake. The bike will idle and ride pretty well when choked, but the moment the choke is taken off, the bike quickly dies and is seemingly starved of gas, even when held at full throttle. I have raised the needle up all the way by lowering the clip and have also bored the AB160 main jet ID to 1.295MM by drilling. However, the same thing happens and the bike is unable to start, idle, or run without the choke.

Any suggestions?

Thanks,
Max
 

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Hello, I am having a little trouble setting up the Mikuni VM30s on my '78 CB400T. Any guidance would be very much appreciated!

Here's the situation:

The carbs came with the following parts: 6F5 needle, 159 P-5 needle jet (“emulsion tube”), AB160 main jet, and a 25 pilot jet. I have the carburetors installed and synced. The compression of the motor is perfect on both cylinders (150psi), spark/voltage is strong, valves are adjusted correctly, and timing is is spot on too. There are no intake or exhaust leaks either. The carbs have been vacuum synced and we have adjusted the speed and mixture screws accordingly. The bike will only run when the carbs are choked or a hand is held in front of the intake. The bike will idle and ride pretty well when choked, but the moment the choke is taken off, the bike quickly dies and is seemingly starved of gas, even when held at full throttle. I have raised the needle up all the way by lowering the clip and have also bored the AB160 main jet ID to 1.295MM by drilling. However, the same thing happens and the bike is unable to start, idle, or run without the choke.

Any suggestions?

Thanks,
Max
Pretty sure the pilot jet is too lean. How many turns out is your air screw?
 

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1978 Honda CB400T
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12 Posts
Discussion Starter · #4 ·
Leon, thanks for the reply. I am running an exhaust without an air box. I spoke with Sudco, the guys I bought the carbs from and we got them dialed in. You were right, the 25 pilot jets were too small. We ended up going with a 27.5 pilot and also increased the needle a 6F4 and the needle jet to a 159 P-6.

For anyone else dealing with Mikuni VM30 issues on a CB400T without an air box, I would highly suggest the jet sizes discussed here. I was having a lot of issues, but dropping in these new jet sizes seems to have really helped a lot. The bike idles great and pulls well though all the gears.
 

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Sounds like a pretty good sized air leak somewhere, did you use the original manifolds or new Mikuni pieces?
 
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