CB750 K3 - lack of power

[garyjpaterson]

So over the past few months, when time permits, I've been going through my very scabby 750. But once getting it out on the open road, I quickly noticed it lacked a lot of power. Low and mid range seems decent enough, but there doesn't seem to be a lot happening beyond half throttle.

I have 657A carbs, but it came with 130 main jets. I replaced them with stock size 110s (i'm running a stock airbox now) as well as moved the needles to position 4, and that improved things significantly - its not completely stumbling at half throttle any more - it has a little more punch at the top but it still feels very weak. Can't really get over 60mph

Low end punch seems mostly fine - right now I know its not running perfect, i haven't balanced the carbs beyond the best i can do by eye, nor set the idle mixture beyond the stock 1 turn. But as far as i know, that shouldn't affect much once you are deep into the throttle?

I haven't done a compression check - perhaps a big mistake. I don't have a gauge yet, but maybe i should've thought about getting one before spending money on a bike! Though it doesn't seem to gallop, so I'm guessing if its down on compression, they are all roughly equal.

Since last putting the carbs on, the idle has been hunting quite badly, I think I have an intake leak, though the heavy throttle behaviour has been the same before and after this, so I'm not expecting that to be the solution - though of course I need to fix that asap.

I've been through the carbs fairly thoroughly I think, I'm quite sure everything is clear, emulsion tube holes etc.
Tank has been cleaned, and new filter sock over the petcock (I have the threaded on style petcock, took me a while to find the part)
Spark advance is free, so that should be fine. Static timing should be fine, though perhaps I should try and verify with a timing light

Could it simply be that, for whatever reason, stock main jets aren't close enough? Or can anyone think of anything else that might create this sort of behaviour?

[K2-K6]

Lift the choke lever while running when you get to the bit you feel it's tapering off giving you more power, it'll tell you something about mixture.

If it flies, then it needs more fuel than it's getting, slows more, then looking rich anyway.

Quick and dirty assessment to see if it'll tell you something either way.

[taysidedragon]

Check valve clearances as well to make sure valves are closing properly,  or opening as much as they should.
Is the spark strong? Try new plugs and/or caps.

[Martin6]

Lift the choke lever while running when you get to the bit you feel it's tapering off giving you more power, it'll tell you something about mixture.

If it flies, then it needs more fuel than it's getting, slows more, then looking rich anyway.

Quick and dirty assessment to see if it'll tell you something either way.

That's clever.

If you find it needs more fuel, have you double checked, then checked again, the float height?  Also, if you're still on points, check the auto advance is moving freely? Have you cleaned the fuel pipes?
Edit: sorry, just saw you've already checked the timing and advance.

[garyjpaterson]

Checked resistance on the caps - i'm getting about 12-13k ohm on the two outer caps, and just under 9k ohm on the middle ones. They should all be roughly the same, no? Not different by that much.
And I'm using DR8ES plugs atm - should i be looking into non R plugs?

[Nurse Julie]

Checked resistance on the caps - i'm getting about 12-13k ohm on the two outer caps, and just under 9k ohm on the middle ones. They should all be roughly the same, no? Not different by that much.
And I'm using DR8ES plugs atm - should i be looking into non R plugs?

Yes, change plugs to D8EA or Denso equivalent X24ESU

[Trigger]

Checked resistance on the caps - i'm getting about 12-13k ohm on the two outer caps, and just under 9k ohm on the middle ones. They should all be roughly the same, no? Not different by that much.
And I'm using DR8ES plugs atm - should i be looking into non R plugs?

Too many ohms. I think Ash advised me on 5 ohm 

[Bryanj]

Yes the caps should be the same and you should not have resistor plugs with resistor caps

[Oddjob]

NGK caps are normally 5K ohms resistance, those are either buggered or poor copies.

[garyjpaterson]

Plugs and caps are here, waiting for new clamps for the intake boots still as the old ones were very stretched, so hopefully that'll stop any leaks.
In the meantime, I did what I probably should've done ages ago - a compression test!

Would've liked a little more uniformity, but I think its just about acceptable. Engine wasn't properly hot, just slightly warm. I checked valve clearances before, they were just fine.

cyl 1: 130psi
cyl 2: 145psi
cyl 3: 145psi
cyl 4: 150psi

I'll certainly keep an eye on cylinder 1, and check in future once some miles are on it. Hoping that the rings are a little stuck or something

[Trigger]

should be 175psi on a new or rebuilt engine. Looks like you have quite a few PSI missing across the cylinders, which could be anything from worn bores, worn rings, bent valve or not seating properly 

[Pauarc]

You could try doing a leak down test it could show if it valves or pistons

[garyjpaterson]

should be 175psi on a new or rebuilt engine. Looks like you have quite a few PSI missing across the cylinders, which could be anything from worn bores, worn rings, bent valve or not seating properly 

Yep, though given its not rebuilt, I'm happy enough with the overall compression. I'm only really concerned with the ~15% dip in cyl 1.

[Trigger]

should be 175psi on a new or rebuilt engine. Looks like you have quite a few PSI missing across the cylinders, which could be anything from worn bores, worn rings, bent valve or not seating properly 

Yep, though given its not rebuilt, I'm happy enough with the overall compression. I'm only really concerned with the ~15% dip in cyl 1.

I would back the tappets off a little, put some oil down the bores, get it hot and take another reading 

[Oddjob]

Did you remember to open the throttle fully when doing the compression test?