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Low Speed Missing

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nadores

Low Speed Missing

Just got my 1977 CB750K running after having the carbs cleaned and synched . Bike runs great at mid range and full throttle but I have a low speed miss. Only modification is that previous owner put 4 into 2 side exhaust pipes on it. I understand that can cause a lean condition requiring bigger main jets. Could that be the cause of the low speed miss? I checked the plugs after I ran it today and they are a little on the light side. Any suggestions as to the cause and what to check?

shinyribs

Re: Low Speed Missing

Administrator

Pipes alone rarely cause you to need to rejet. Might have to play with the idle mixture screws a tad, but that's about it.

Whatever you are running for an air filter may require rejetting, especially with free flowing exhaust being used at the same time.

My $0.02 would be to add a little fuel with the idle mixture screws. Run them all the way in til they are LIGHTLY SEATED(*) and count how far they are turned out now. Then start turning them in 1/4 turn at a time and see if it helps any.

Turning in= richer
Turning out= leaner

*OVER TIGHTENING CAN RUIN THEM.

TOOLS1

Re: Low Speed Missing

Administrator

77 should have keyhole carbs, and they have a "fuel" screw, not an "air" screw. So they are the reverse of what Shiny said.
Out=Richer
In=Leaner.
TOOLS

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1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
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shinyribs

Re: Low Speed Missing

Administrator

BAM! Tools to the rescue! Thanks for catching my mistake.

TOOLS1

Re: Low Speed Missing

Administrator

That's what I'm here for.

TOOLS

Re-run

Re: Low Speed Missing

Administrator

In reply to this post by nadores

How light is light? Is the insulator white? If so, too lean then. But if it is a very light tan to medium tan, it is just fine. These carbs are meant to run leaner than the older carbs, which is the reason they have the accel pump. If possible, get a pic of the plugs and post it.

I would also check plug gap. .024 - .028 is where you should be set, especially if you still have points. Slightly higher gap can result in occasional misfires.

The ride IS the adventure. The destination is just to get gas!

nadores

Re: Low Speed Missing

In reply to this post by TOOLS1

Thanks everyone for your input. I'll give it a shot.

gkmaia

Re: Low Speed Missing

In reply to this post by nadores

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Re-run

Re: Low Speed Missing

Administrator

Actually, the idle mix affects to about 3/4 throttle. i would check plugs before doing anything else. You can have idle issues and be running rich. Always check plugs before assuming.

The ride IS the adventure. The destination is just to get gas!