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Jets for 4 into 1 exhaust.

Stoney

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Hi everyone.
I recently purchased a k7 SOHC running Keihin round-top carbs and a 4 into 1 exhaust with stock airbox.
It starts well and gets good all-round performance (smooth with good MPG) but I've been having a few issues:

Boggy low revs
SLIGHT spitting/popping if held at 3k +
SLIGHT smoke from exhaust if held at reasonably high revs
Dry black carbon on all plugs (1 and 4 slightly worse)
Idle will creep up slowly from 1k to 1.5/2k when fully warmed up, blipping the throttle brings it back down most times.

So far I've changed out the air filter, plugs, cleaned the carbs a half dozen times, set the air mix (2 turns out seems to provide highest idle, much leaner/richer and it's rough) and also replaced the spark plug caps and ht leads (the bike is fitted with dyna coils). Checked for vacuum leaks and replaced all hose clips around carb boots as some were worn out and loose. Checked air box for leaks.

I'm yet to check points/valves and they are next on the list, but I'm fairly sure I need to re-jet for the exhaust and need some advice as to the correct idle and main sizes. Also, if anyone could advise on float height that'd be a huge help!

Many thanks in advance.
:thumbsup:
 
Last edited:
Set float height to stock height. As far as jetting, every motor will be different. There are too many factors that affect jetting, engine condition, intake setup, exhaust setup, altitude, temperature, fuel quality. There is no set jet size for a given modification, the only way is to try different jets and see how it runs. When you change jets go in two size steps. You need to see what is in there for jet size so you have a starting point. Then you will have to see where the needle is set as far as position. With just exhaust you shouldn't have to change idle jets. Check and sync carbs.

First you need to make sure your ignition is in good condition and in tune. Check to make sure the ignition advance is free and springs are attached and not stretched...which can cause erratic idle and running problems.
 
Thanks dirtdigger. I'll get the float height and ignition looked at/adjusted first then.

If they check out ok, can I assume that the bike may be running a little rich given the black plugs and 4 into 1?
Also as far as establishing the current main jet size, is there a measuring tool you'd recommend?

Thanks now.
 
There is a number stamped on the main jets indicating the size. It will be stamped on the end or on the side. If it is black then it would indicate rich...but you dont know if it is rich at idle, mid range or wide open throttle. I suggest googling a "plug chop" as most people call it to see how the main jet is looking for size. Thing is if they are already black then you will need to put a fresh set in so you can see how it is burning at wide open throttle.
 
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