Topic: 750K6: no power in Gears 4 and 5

[run-tmc]

Hi everyone,

I am experiencing a strange problem on my K6 that I described in a thread below called "CB750 Tank Mileage" which I cannot currently access from my phone (stuck in the airport on my way to Phoenix).

I pulled the carburetors off and adjusted the needle clips to the middle position from the second from the bottom (assuming bottom is the needle point) position. The bike was a bit tough to start then stronger through half throttle, as I had hoped. The trouble is that while I have no problem ripping up to 10k rpm in gears 1-3, the bike struggles to accelerate at all from 5k and up in 4th gear. It isn't even worth attempting to accelerate in 5th unless it's down a hill. Twisting the throttle around 5k rpm in either of the top gears does bring a noticeable, high pitched sucking sound from the carbs. Is this simply a need for bigger main jets? If that is the case why would this issue not present itself in gears 1-3? I'm wondering if the transmission is part of the issue, as I missed a few gears the other day when not making substantial shifts. I can still get up to 90 mph in third if need be for passing on my upcoming road trip from Calgary to Vancouver then down the coast to Portland, but I sure would have more confidence in the bike if it actually ripped in the top gears. Would a chain being too tight and restricting rear wheel spin cause this? I read a comment in a recent post indicating that if the rear wheel can't make an entire rotation after a manual spin then it may be causing issues. Perhaps dragging rear brake shoes that can be overcome in torquier low gears?

Jets? Transmission? Rear wheel being restricted? Help!

For reference:
New 530 Oring Chain, slack about 1" if I remember correctly
New sprockets F/R, OEM tooth counts
New rubber F/R, BS Spitfires
New wheel bearings
Rims trued
Carbs cleaned and rebuilt
40 slow, 105 main, needle clip in 3rd position
New airbox and intake boots
OEM airbox with new OEM filter
Pamco ignition with HO coils
New NGK DR8ESL plugs
New plug wires
New battery
Running 87 octane gas
Motul 10W40 semi-synthetic oil
Elevation in Calgary is 3200 ft
I weigh 165 lbs
Bike has not been loaded yet beyond my own weight

I apologize for the redundancy as this is all listed in the "...mileage" thread.

Any help is much appreciated!
Thanks
Tom




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[pamcopete]

Here are some suggested things to check:

Bike goes slow.

1. Low battery / charging system. A low battery will produce a low ignition voltage at the plugs which is most evident at higher RPM's.
2. Timing. Did you use a timing light to set the timing?
3. Stuck or no advance. When checking the timing with the timing light, did you check for maximum advance at 3,000 RPM.
4. N/A
5. Fouled plugs. Did you install new plugs? Check them after you got it running?
6. Plug wire not making contact in the coil. Measure the resistance from plug cap to plug cap to ensure that both plug wires are making contact in the coil.
7. Brakes dragging.
8. Speedo out of calibration. You are actually going faster! Use your GPS to check the speedo.
9. Tire pressure. Pump up to 32 PSI for high speed.
10. Drive chain too tight. That will rob you of power to the wheels.
11. N/A
12.Choke left on on partially on.
13. Weak or loose advance springs. Weak or loose advance springs cause the advance to use up some of the available movement because you have to essentially retard the advance to get it on the idle advance mark, so there is less movement available at higher RPM's.
14. Timing chain needs adjustment.
15. Cheap or bad gas.
16. Low octane gas.
17. Water in the gas.
18. Partially blocked pet cock or fuel filter.
19. Paper fuel filter that has low flow when the tank is not full.
20. Try a run with a full tank of high octane, fresh, expensive gas.
21. Blocked fuel cap vent.
22. Incorrect sprockets. What RPM are you getting at 60 MPH.
23. Rear tire rubbing on brake rod or swing arm.
24. High wind resistance. Are you a large person?
25. Throttle cable not fully opening the throttles.
26. Blockage in the muffler(s)
27. Dirty or blocked air filters
28. High altitude
29. Very hot air, like 95+F
30. Very cold air, like 32F
31. Low oil
32. Really dirty oil
33. Gas in the oil

[iron_worker]

Why are you revving your engine to 10k RPM on what sounds like stock internals?

IW

[ekpent]

Engines have a higher "load" in the taller gears and problems with performance can manifest themselves even more in that range.

[martin99]

Why are you revving your engine to 10k RPM on what sounds like stock internals?

IW

That was my first thought, to be honest. If that's how it's ridden in normal service I wouldn't pin hopes on it lasting too long, 40yr old motors need to be treated with a bit of respect. TBH, I don't remember revving them like that when they were new!

No offence intended.

[evanphi]

To get this thread going again... I've just noticed the same. I can't seem to get higher than 6K-ish in 4th or 5th.

This is at full throttle on a highway... I'm going to take a wild guess (but not really) and say I need to up my jetting, since I am running stock jets on a 4-1 with baffled reverse cone muffler.

[run-tmc]

Iron Worker and Old Skool, thanks for the feedback, I do appreciate it. I will admit that I was blindly thinking of the fabled K0 development where Bob Hansen talked of being unable to make the engine fail after riding pinned for miles in the salt flats - more than 40 years ago. This is pretty stupid in the best of circumstances, let alone with my bike that sat for six years prior to me buying it. With 4th and 5th not pulling, I was using 3rd for all of my acceleration onto freeways. I will tone this down on my test ride tonight, where I hope to find that I've solved my troubles.
 
The front brake was dragging considerably on the rotor, so I backed out the adjuster screw a bit.
 
I had overtorqued the rear axle nut as well. I believe the acceptable range is 58-79 ft. lb. and I was at the upper end. As soon as I pulled that back a bit the wheel began to spin significantly better.

Both wheels now spin (by hand) about 1.5-2 full rotations compared to .5 on either end before I made these changes. I also cleaned up and greased the axles. Fingers are crossed for tonight...

As far as larger jets go, does it make sense that gears 1-3 have no problem accelerating past 5k rpm but 4th and 5th are stagnant? That doesn't make sense to me, but I'm a rookie. I would think the jets should affect that range in all gears.

Thanks again for the feedback guys.

[ekpent]

You mentioned the front brake was dragging. Have you removed the caliper yet and gave everything a thorough cleaning. The problem usually is that the piston and or rusty pads do not retract correctly anymore thus causing the drag, especially if the bike had been sitting for 6 years.

[run-tmc]

You bet ekpent, pulled the front Caliper apart, used a dentist's pick to clear out the recessed area where the seal rests, then cleaned up the seal and reused it. It wasn't cracked or anything. Then I cleaned out the small holes in the front master cylinder with the fine E string from my guitar and bled the brake for a long time. I believe the adjuster screw for the fixed pad side was simply pushing too tightly against the front wheel. On another note, does anyone else identify with the 530 chain either rubbing against the lowest portion of the left spring or missing it by a half mm tops? Seems very close on mine. I will upload a pic.


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[run-tmc]

Here is the gap or lack thereof between my 530 chain and the left spring. Pretty tight...


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[scottly]

In general, a lack of power in higher gears may be due to lack of fuel flow at the higher loads. Try cracking the filler cap open; if it gets better, you may have a plugged vent in the cap.

[PeWe]

Check that the adv unit is correctly set and corresponds with case index mark.
T on 1-4 side should align when pistons 1-4 is at top.
My adv unit have a play of ~3 degrees. When turned max counterclockwise it is correct. When turned max clockwise it will cause advanced timing which is wrong. I have it turned max clockwise and have therefore added an alternative case index mark 3 mm after the std one. Engine now Ok, before it fould plugs and I jetted carbs too lean due to that.

[seanbarney41]

 
The front brake was dragging considerably on the rotor, so I backed out the adjuster screw a bit.
 
I had overtorqued the rear axle nut as well. I believe the acceptable range is 58-79 ft. lb. and I was at the upper end. As soon as I pulled that back a bit the wheel began to spin significantly better.

Both wheels now spin (by hand) about 1.5-2 full rotations compared to .5 on either end before I made these changes. I also cleaned up and greased the axles. Fingers are crossed for tonight...

that axle torque issue is troubling.  Torque specs are good to use, but you should be able to torque your axle as tight as humanly possible with a big wrench and it shouldn't affect how the wheel spins.  You need to tear that down and find the source of the drag...bad bearings, incorrectly spaced bearings, incorrect wheel spacers etc. ...something is not right...especially considering your chain/shock clearance issue.

[ekpent]

Yep that chain is waaay to close. Something is assembled wrong like a missing spacer. You should have enough clearance to be able to get a stock chain guard in there and the chain centered in it so nothing is rubbing.

[oldhatt45]

I was reading a thread the other day that was talking about the K6 if I recall correctly.
It was something about the cam and piston parts.  If I remember right, they were saying something like:
If you have a K cam and F pistons you have a dog.
If you have a F cam and K pistons you have a crotch rocket.

I may not have remembered it right, but what's left of my memory said that your case sounds a little like what the other thread was talking about.

I just tried looking for that thread so you can read it but can't find it but I know it was in June of this year.  Sorry.