SOHC.co.uk Forums > CB750
CB750 K3 - lack of power
Nurse Julie:
Turn 1 and 3 air screws in by about ½, giving more fuel, and 4 out by about 1 turn, giving more air. Clean plugs, go for a hard ride and re check plug colours after.
garyjpaterson:
Will do. Need to get it insured and put my reg plate on first :-X Been testing on "private ground" up until now...
Trigger:
Nice to hear that it is going a lot better after a little fettling ;)
garyjpaterson:
First proper voyage complete, really good fun. Sounds and feels great, lovely smooth engine.
I think (and yes this will have been contributing to my earlier problem) that the speed is reading significantly low. The bike feels every bit as punchy as I'd expect it to, yet the speedo seems to tell a different story. Went through a radar speed sign - it was telling me I was doing 49, and my speedo was saying 40! And I'm quite sure the discrepancy gets worse the faster I go.
Usually cars/bikes will over-read, not under!
Seems odd, as the odometer seemed to track roughly the right amount of miles, so I'm not sure what's happening.
Otherwise, I still need to dial the carbs in more, still not always happy to return to idle. I'm wondering if this is a sync issue (not yet done). Bit concerned about the vast difference in plug fouling of cyl 4 compared to the rest.
Overall very chuffed though. Still need to get the cosmetics sorted (I daren't show what it looks like atm), but despite it looking so rough I had a guy gleefully asking questions at the fuel station!
Quick clip, uphill, starts in 2nd gear. Poor quality image but hopefully you can see (and hear) that it pulls well, but does the speedo seems quite lethargic? 7k rpm in 3rd gear must be about 80mph, speedo reads 70
https://streamable.com/qd19qj
Martin6:
--- Quote from: Nurse Julie on August 18, 2023, 04:40:13 PM ---Turn 1 and 3 air screws in by about ½, giving more fuel, and 4 out by about 1 turn, giving more air. Clean plugs, go for a hard ride and re check plug colours after.
--- End quote ---
Hi,
"and 4 out by about 1 turn":
That's interesting. I have fouling on no.1 after about 100 miles on my K6, it comes on at low throttle openings, behind traffic and clears for a while once i give it some beans. It's dry soot and brushes off easy, not oily. No. 4 is good, so not timing / ignition. The workshop manual suggests a range for the air screw of 7/8, to 1 1/8 out. Going by the manual, I've been at 1 1/8 out with no difference. I've tried slightly more, but never a full turn further out. I'll try that. 👍
However, my suspicion is it is related to my leaking head, which I plan to do this autumn. But it's not a wet sootiness?
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