CB750 K3 - lack of power

[garyjpaterson]

Yeah throttle was wide open. I checked them in reverse order, so 4 first and 1 last, so the battery wasn't able to spin it quite as fast by the end, so that probably has a small effect

[Martin6]

Was the airbox still on?

[garyjpaterson]

I took the lower half and filter out, left the top half on.
But as I said, i'm not overly concerned with the overall numbers, it seems quite normal to be down on brand new compression after 50 years.
Its just one cylinder being a bit down on the other that i something i'll need to keep an eye on.

[Trigger]

I took the lower half and filter out, left the top half on.
But as I said, i'm not overly concerned with the overall numbers, it seems quite normal to be down on brand new compression after 50 years.
Its just one cylinder being a bit down on the other that i something i'll need to keep an eye on.

That is the correct way to do it, no air filter fitted. 

[garyjpaterson]

the new plugs and caps seemed to make a huge difference, power seems to be much closer to what I would expect. Really chuffed with that!

Now i need to focus on getting the idle mix sorted. Its a bit slow to return to idle, so I checked the plugs and 1 to 3 look potentially a little lean, 4 is very fouled though. All were just 1 turn out

[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  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]

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.

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?

[garyjpaterson]

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.

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?

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 

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]

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.

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]

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.

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?

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 

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.

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?