Vintage and Classic Hondas

missing on 2 cylinders

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trikeboiler

missing on 2 cylinders

Hi all, my cb750kz, 1979 model has now decided not to fire on cylinders 3 and 4, I have a good spark, the carbs have been off twice to be cleaned and checked over and all appears ok, it was firing on all four about 3 weeks ago but was revving up as if she was sucking air, this is the job I was about to do today when this problem started. the timing is ok, the coils are ok, there is fuel in the float chambers. has anybody got any ideas whats causing this, unless there is a blockage in the carbs that feed pots 3 and 4. any info will be much appreciated. thanks.

TOOLS1

Re: missing on 2 cylinders

Administrator

Since 3 and 4 are on different coils I would suspect a fuel problem, or vacuum leak. Could be the pilot jets have gotten plugged, or vacuum leaks, or both.
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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trikeboiler

Re: missing on 2 cylinders

HI, thanks for the reply, when you say vacuum leak, where would I be checking for a leak, cheers.

TOOLS1

Re: missing on 2 cylinders

Administrator

The rubber boots between the carbs and head, and some of the seals/O-rings in the carbs can cause this.
TOOLS

trikeboiler

Re: missing on 2 cylinders

Thanks Tools, I shall be having a look this weekend and I shall let you know how in get on.