SOHC.co.uk Forums > CB750

miss at 3.8K

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billy192:
well i have done 600+ miles on mostly A roads. today I went on the Mway only to find at 3.8K i get a miss it keeps missing all the way up to 5K, not taken it passed 5K as it will not rev passed this under load.
when you slow done to 45 ish its fine.

I thought it was fuel starvation but fuel is flowing OK. checked the points, plug gaps all spot on.

a great pity as everything is great but for this miss.

Billy

MarkCR750:
Hi Billy, I had very similar symptoms on a ZXR400, it turned out to be the float heights, i.e when the engine demanded more fuel at higher revs the level in the bowls was too low because the floats were cutting off the fuel supply before they should, trial and error re-setting the float heights fixed it, I couldn't use the factory settings because the engine/intake and exhaust were non standard.

Bitsa (Ralph Wright - RIP):
Billy
Pull yer plugs and see what the colour is like white weak black rich then you will know if its carb problem
Cheers
Bitsa

billy192:
I pulled the plugs (burning hands) and they were white ish, i did try opening the choke but it did not make a difference. I am going to check the float level AGAIN. should i not lift the needle in each carb to rich top end?
Billy

K2-K6:
Are you certain that the ignition advance is working correctly, if no advance then it would struggle to get much above 5000 at all.

Lifting the needles will richen mid range (throttle not fully open), top end is main jet size but I don't think you'd be finding that out if you are not getting the slides fully open anyway....I'm making the assumption that you have not got the throttles fully open as you've not exceeded 5thou, though if you are saying that it won't go higher AND the throttles are fully open then that would suggest something else assuming the carbs are near enough right.

White plugs do suggest lean running......I'd make sure of the ignition advance and timing first as it's easy to do, before taking the carbs down again.

Also are you sure of your method for setting float hieghts?

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