plugs coked up

[neat street]

Hi All
can anyone advise what I would need to do with the spark plugs getting coked up? I have fitted a new exhaust which I know will be the cause, and not sure whether I should re-jet the carbs (if so anyone any idea of what size I should get(CB750F1)), or could I just re-set the needles as it is obviously running rich. any help as always, gratefully received.
Thanks
John

[K2-K6]

Assume that you've got the engine hot and completely off any choke with a rfeasonable length run to be able to assess the mixture? if not you'll need to wait until you can get like that to check properly.
Change of exhaust can affect jetting but doesn't usually alter that much, was the jetting standard before you started?

[neat street]

Hi, yes had a good run about 10 miles, nothing fierce though around 30 to 40mph most of the time, but up to 60mph. jetting standard before the new exhaust, 4 into 1 as before, but without baffles, just a cone in the rear, so still quite loud. thanks for your reply
John

[MarkCR750]

I thought removing baffles/back pressure tends to make an engine run lean?, wouldn't swear to it but that was my understanding, have you changed the headers and silencer or just the silencer?, if just the silencer could you slip the old one back on to prove it's the exhaust causing the issue?.

[K2-K6]

I'd go along with Mark on that, usually with less pipe restriction you'd end up chasing round to get more fuel into it to make it work well.

As Mark suggests, can you use the old pipe to give yourself a comparison?

Also at slower speeds and no real wide open running you are looking more at the low end jetting and idle circuits to give you plug colour....check you set set at something correct for the idle screws if you haven't done that.

Basically if you want to see the mixture under full load (mainjet size) you'd have to warm it and really cane it and then turn it off while its pretty close to flat out to read anything from the plugs for that jetting area.

I suppose basics must be covered first, timing, ignition advance, needle position, float height and idle screws set within range to give you a decent base from which to work and then see what you've got.

[neat street]

Thanks for your help, the new exhaust is a one piece 4 into 1 SS, and the old one has since gone as it was crap. I will start from the basics and work my way through.
thanks
John