Topic: Plugs fouling.. need help?

[CBRat]

Hi everyone, new to these forums but I am having a problem with my '73 CB750.  When I put in a fresh set of plugs I get about a few days of great performance out of the bike, but eventually the plugs get fouled up.. the #4 in particular(carbon fouled), and that cylinder will cut out.. Sometimes it won't start at all until I clean up the plugs, but even then the #4 cylinder wont fire properly.  A new set of plugs solves the problem.. for a short while. Anybody have an idea what I could be facing here?  Thanks.

[Bob Wessner]

What is the history on this bike, was it fine last year and this just started? What is the tune-up history? Have you made any alterations re: airbox, exhaust, etc.? Really need more detail, but is sounds like carb issues, specifically being rich that is. When this occurs what sort of riding are you doing, putt-zing around town, highway cruising? Did you ever do a plug chop at various throttle openings?

[750deepsouth]

All other things being equal - turn out the air screws on each carb, start with an extra 1/2 turn, even an extra full turn.
I went Through the same issues with my K2 and resolved it almost totally with leaning the idle mix by a full turn.
The problem I think is related to modern fuel, octane's etc, the recommended idle screw setting leaves the motor running wayyy too rich on the idle jet.
Even so I sometimes get #4 to foul up a little, but overall it starts on all 4 pots 99% of the time. I've found that when starting from cold, full choke but only a little throttle. Starts almost straight away.
It runs much more smoothly at lower rev ranges ie 2500-4500rpm. The bike used to run a little unevenly at around town speeds - now it is very smooth.

[CBRat]

Thanks Bob, Andy.  Actually, the problem happens mostly when city driving and has been going pretty much since I bought the bike a year ago.  After a more thourough search of the older posts on this board I have found some similar threads with some good ideas to try so I'll avoid a redundant thread here.  I will try turning out the air screws, and the plug chop sounds like a good idea too, thanks again.

[750deepsouth]

Carb threads are never redundant   

My probs were town only as well. Once you are up to open road speeds the main jets are in play. Town speeds, up to 4ooo revs or so, mostly idle jets. That's where your problem will be most likely to lie.

Only took me two years to figure it out  and town riding is now a pleasure 

[Bob Wessner]

Since the problem occurs at the lower speeds, I agree it sounds like the idle mixture (i.e., air screw settings). I spent a fair amount of time sorting mine out at idle and found that very minor adjustments can make a big difference. For example, 1/8 turn at a time. A pain with some plug cleaning in between checks, but worth it in the end. Make sure you have the bike thoroughly warmed and also make sure you use a fan to blow air across the cylinders/heads while doing this. Once the idle end is sorted out, go on to the plug chop at higher throttle settings. You can mark the throttle grip with some tape at 1/4, 1/2 and 3/4 settings. All very tedious, but worth it. Also, have you ever sync'd the carbs?

[eldar]

One thing you did not mention was the plug number? You may be using the wrong plug, either it is too cold or it may be a resistor plug.

You should use either ngk d7ea or d8ea or a comparable plug.

[CBRat]

I am using d8ea's in there now.. I tried some hotter plugs (d7's)  last year before I parked it for the winter, and they seemed to have a bit more life but still there was carbon fouling on #4 eventually.  I went with the d8's again this time just because of availability at the local hardware store.  The bike does run great on the highway, and when I really open it up for a while it will sometimes blast all the carbon off and start running normal again  . The more I think about it idling seems to correlate with the fouling.  I'll give the air screw a try today and maybe check to make sure the choke isn't sticking or something.  Thanks for all the (quick!) replies. 

[CBRat]

Just to update this thread, turning out the idle airscrew 1/4 turn seemed to do the trick and I've been running great all spring.  I still haven't got around to doing a plug chop yet (to busy riding  ) Thanks again for the good advice!