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My CB350K4 and its problems
- Thread starter bladen
- Start date
stl360+450
Veteran Member
I'm glad the charging system is in good shape.
Do you experience the same flat spot in all gears? I feel like chops are usually done in a higher gear, but I've never done them aside from a little two-stroke moped I had at one point.
Were those fresh plugs before you did the plug chop? If the main jet is too rich the motor will kind of bog down at WOT, falling flat on its face. I guess the idea is to keep increasing the main until this happens and then drop back down a couple of sizes. That's how I approached it with that moped.
Your plugs seem to show more heat, as in a lean condition, which I guess you already suspect based on earlier posts. For 8000 RPM and possibly having not been fresh, though, they don't look bad to me.
Sorry if this has already been discussed, but have you verified the spark advance angle using a dynamic test light? The high rpm timing depends on the proper function of that mechanism.
Do you experience the same flat spot in all gears? I feel like chops are usually done in a higher gear, but I've never done them aside from a little two-stroke moped I had at one point.
Were those fresh plugs before you did the plug chop? If the main jet is too rich the motor will kind of bog down at WOT, falling flat on its face. I guess the idea is to keep increasing the main until this happens and then drop back down a couple of sizes. That's how I approached it with that moped.
Your plugs seem to show more heat, as in a lean condition, which I guess you already suspect based on earlier posts. For 8000 RPM and possibly having not been fresh, though, they don't look bad to me.
Sorry if this has already been discussed, but have you verified the spark advance angle using a dynamic test light? The high rpm timing depends on the proper function of that mechanism.
bladen
Well-known Member
Same flat spot in all gears, yes. It’s very noticeable and feels kind of like one cylinder falls off. I verified advance yesterday while doing timing, but I was not as thorough as I could’ve been due to the noise, I’ll do a better test soon. It appeared to work fine.
The plugs are old, about 2500 miles on them, they’ve been through a lot.
I feel the plugs being lighter after running at high rpm shows that it is not bogging due to richness but to a lean condition, or lack of spark.
I suspect either I need a larger jet in the main, which I will try next, or that my condenser and coils need to be updated. For now, this is out of my price range and it will need to wait a couple weeks.
stl360+450
Veteran Member
Keep in mind that at high RPM there is way more heat and that's what ultimately cleans the plug. So it's perfectly normal to see a difference like that between idle and 8000 RPM. I do agree that it's not rich at high RPM, so a larger main might help.
A dynamic test light should show that the spark is occurring at an angle between the two full advance lines. Advance timing is intended to optimize power, so if it were off substantially it could explain a flat spot.
Coils won't hurt since you have originals, but sounds like you're fine up to 8000 RPM until you can get new ones. That's plenty of bandwidth for street riding.
A dynamic test light should show that the spark is occurring at an angle between the two full advance lines. Advance timing is intended to optimize power, so if it were off substantially it could explain a flat spot.
Coils won't hurt since you have originals, but sounds like you're fine up to 8000 RPM until you can get new ones. That's plenty of bandwidth for street riding.
bladen
Well-known Member
Yep! I’ll try the larger mains, and update. If the coils are the cause, I’d have them fixed not because I want to reach 8k often, but because I feel this bike needs to run perfect, for my own satisfaction haha. Thank you for your help.
bladen
Well-known Member
Update on the bike. Found some more clues. One, I can smoothly accelerate through to 9k rpm if I am barely holding the throttle, only at WOT will I encounter the stumble, which to me points away from ignition and into carburetion. Two, it’s only in the low gears WOT when I encounter the stumble, so it has to do with the rate of acceleration. So I’m thinking trying bigger main jets (120) may fix the issue, or at least I hope - I don’t know why it wants so much fuel in the high rpm, 115 is already much larger than the FSM calls for.
I put the new air filter housings on, little if any change. Did a highway plug chop and here are the plugs:
Left:
Right:
View attachment IMG_6850.jpeg
WOT around 7.5k rpm
I put the new air filter housings on, little if any change. Did a highway plug chop and here are the plugs:
Left:
Right:
View attachment IMG_6850.jpeg
WOT around 7.5k rpm
stl360+450
Veteran Member
bladen
Well-known Member
You know, it did mean that. Turns out, even though I did plug chops at 7-8k rpm that resulted in whitish plugs, it was running rich. I suspect I either was doing them incorrectly or that modern gas can give misleading readings.
I bought 120 jets, but today when I got to work I figured I’d try the 105 jets just to set a baseline since it’s been a while, and now I have a perfectly running CB350.
Thank you for the suggestion. She’s running wonderfully now!
stl360+450
Veteran Member
No problem. I'm glad the bike is running well!