Hi folks. So I have been struggling with getting this 87 CBR 600 running perfect for a long time. Started with a missing carb parts, dirty carb and so on. Then the battery would suddenly lose power resulting in missing, sputtering and backfiring; which of course I diagnosed as carb issues. Then discovered the RR wasn't really charging. Battery is now happy, I have a new MOSFET RR (thanks for recommendations on this forum) charging to 14v even at idle, I have two carb racks both cleaned and with new needles, some new jets etc. One has 105 jets and the other has the 112 jets the bike came with. They run different; but they both have the same MISS during acceleration and a bit while cruising. It seems to get worse after the bike is warmed up. So I am down to a handful of parts to consider as I seriously doubt two carb racks would show the same behavior with completely different main jets, variations in float levels due to age and so on. So it could maybe be fuel pump or tank vacuum; but I don't really see how this wouldn't manifest itself differently vs. just a miss. I would think I would lose power, stall out and so on. So, that leaves me with Sparker, the timing sensor, the coils and the spark plug wires (and of course wiring on the bike itself). So my question here is simply if anyone has ever seen a sparker or timing sensor produce an intermittent behavior like this? As with everything else on this bike, all I can do is buy used parts so fool proof parts swapping is a difficult. The bike has moments of feeling pretty good; it pulls really well from low RPM (even though it will sometimes miss), doesn't backfire or ping and accelerates hard (especially with the 112 main jets carb installed, which I am guessing was tuned for the modified exhaust). The 105 main jet carb also idles well and both carbs start easily.

 

Have you tried adding washers or raising the notches on your needles?

 

Don't know if this helps or not, but it's what I use when I am thinking through carb issues. It helps me keep the different circuits straight.

 

The 112 main jets have longer needles that can be adjusted. I have a single washer. The 105 main jets have the needles from the standard rebuild kit with a single washer. Both carbs show this pulsing miss more notable as the bike runs a few miles. Problem is better or worse from day to day; aka. inconsistent. I guess we don't think I have a sparker or purse generator problem?? Raising needles will give me more gas; in our other threads, we decided that a problem that gets worse after the bike runs a few miles must be a RICH condition. I would think if this kind of rich condition was really the problem; the 112 jets would make the problem much worse; although those do have longer and thinner needles and I believe came with some kind of performance carb adjustment kit that was probably installed on the bike when the muffler was changed. If anything, the 112 carb rack may run a little better; but since the problem isn't 100% consistent; that is hard to be certain of.

I will be happy to try raising the needles. I can make that change in under 10 minutes these days

 

What does 'throttle valve cutaway' refer to?

 

the curved part at the bottom of the slider.

I would add a washer to each needle just to see what happens. It may take more than 1 but the good news it is the easiest thing on a carb to change out.

 

Right. Can play with needles while the carbs are on the bike

 

One more question. Do you think the pilot jet/mixture screw could result in a 'miss'? I have the screws at 2 turns out vs. the 3 they were at when I got the bike. Since the thought was a rich condition, I wanted to go on the lean side. The miss is like a pulse that happens as I accelerate or cruise. Not a flutter, bogging, backfiring or pinging condition.

 

Misfire under a load is typical of a spark plug wire being bad.

 

That is my theory. the wires had to be repaired as best I could. One of them was pretty messed up.