SOHC.co.uk Forums > CB750
CB750 K3 - lack of power
garyjpaterson:
--- Quote from: Martin6 on August 20, 2023, 11:53:42 AM ---
--- 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?
--- End quote ---
Mine seems very similar to yours, dry soot, cleans up easy. Haven't done a high speed chop, so not sure if it clears up, but it feels very healthy beyond the idle circuit.
Some big (half and full) turns haven't done a whole lot for me, I think i'll revert and just try and tune it by ear. I dont like one being so different from the rest, and overall it feels quite lean still so I'd rather bring them all to the richer side for now.
Was able to verify with GPS that the speedo is off quite a bit. Actually, that's a bit unfair, it can be quite accurate, but its slow to react, especially beyond 50-60mph. Its often 10-15mph behind on acceleration, but will catch up eventually.
My guess is the needle pivot is sticky, maybe it'll free up with more use at those higher speeds 8)
Rode it back to back with the CB350 twin, was really fun doing a direct comparison. I'm in love with the smoothness of the four cylinder, and it sounds just sublime to my ears.
The drum front brake on the 350 is far far better at coping with its mass though. And of course overall the little one is a lot more agile. Struggles on bumps though, like a bucking bronco, the 750 is a lot more stable. Fun pair, looking forward to many miles on both.
Nurse Julie:
If the speedo reading doesn't settle down, consider letting forum member Peter Horton (Kent 400) give it a service over winter.
garyjpaterson:
Oh good shout Julie. I had was going to have a little look myself but couldn't even work out how to take the casing apart without breaking anything.
Nurse Julie:
--- Quote from: garyjpaterson on August 20, 2023, 07:22:45 PM ---Oh good shout Julie. I had was going to have a little look myself but couldn't even work out how to take the casing apart without breaking anything.
--- End quote ---
Yes, I tried once and it ended in disaster, luckily I was using a totally wrecked clock that was beyond repair. Since then, Peter has been my go to man for clocks 😊😊😊
Martin6:
--- Quote from: garyjpaterson on August 20, 2023, 06:43:07 PM ---
--- Quote from: Martin6 on August 20, 2023, 11:53:42 AM ---
--- 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?
--- End quote ---
Mine seems very similar to yours, dry soot, cleans up easy. Haven't done a high speed chop, so not sure if it clears up, but it feels very healthy beyond the idle circuit.
Some big (half and full) turns haven't done a whole lot for me, I think i'll revert and just try and tune it by ear. I dont like one being so different from the rest, and overall it feels quite lean still so I'd rather bring them all to the richer side for now.
Was able to verify with GPS that the speedo is off quite a bit. Actually, that's a bit unfair, it can be quite accurate, but its slow to react, especially beyond 50-60mph. Its often 10-15mph behind on acceleration, but will catch up eventually.
My guess is the needle pivot is sticky, maybe it'll free up with more use at those higher speeds 8)
Rode it back to back with the CB350 twin, was really fun doing a direct comparison. I'm in love with the smoothness of the four cylinder, and it sounds just sublime to my ears.
The drum front brake on the 350 is far far better at coping with its mass though. And of course overall the little one is a lot more agile. Struggles on bumps though, like a bucking bronco, the 750 is a lot more stable. Fun pair, looking forward to many miles on both.
--- End quote ---
On the speedo, it possibly might not be the speedo itself. I had a similar thing on my Commando, also with some oscillation of the needle. I pulled the inside of the speedo cable and it was mucky. Cleaned up the cable, flushed out the sleeve and put a light coat of grease on the cable and in the drive housing (not oil). It's been fine since (couple of years and 5,000 miles later). Cheap to give it a try.
Puzzling about the sooty plugs. Have you checked compression on that cylinder?
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version