1980 cb750k loss of power

[surfish95747]

Alright guys, I need help getting pointed in the right direction.

My 1980 cb750k seems to choke when I push it pass 50 mph and get to 5000 rpm. If I push the bike hard and for too long, white smoke will be coming out of the exhaust pipes. While I'm riding, if I let off the throttle, the left side back fires out the left exhaust and let's out white puffs of smoke with each back fire. Then when I pull it over and idle, white smoke continues to come out, not terrible, but it is visible. The engine just seems a bit gutless. I have rebuilt and synced the carbs both bench and vacuum sync. All four cylinders compression test at 110 psi +\- 2 psi. It seems like the plugs get a bit blacker than I would like, but they are dry (no oil) when I remove them. The boots in between the air box and carburetors have cracks in the rubber (I have new ones, just have not put them on yet).

I've been thinking about doing a valve job (adjusting valve clearance and changing the valve seals), changing the cracked rubber and getting new plugs, then resync the carbs. Beyond that I'm not sure what should be done. Any ideas?? Thanks!

[shinyribs]

I hate to say it, but 110 psi is pretty low compression. That, and white smoke ( probably blow-by/ oil passing the rings) points to rings. Black plugs = rings, too. Ever if they don't actually feel oily.

Even if your boots being cracked are causing immediate issues, it still sounds like your due for a minimum of new rings and hone the cylinders. Possibly pistons and a rebore if it's too far outta spec.

[surfish95747]

Thats what I was afraid of. I've been looking for piston ring kits and this one is on ebay. It's only $58, should I stay away from something that cheap? Everything else I've found for the DOHC is around $150.

www.ebay.com/itm/Honda-CB750-Piston-Ring-Set-x4-Standard-1979-1980-1981-1982-1983-DOHC-750-/381320875987?hash=item58c87ecfd3&vxp=mtr

[TOOLS1]

I would use that set of rings.
TOOLS

[surfish95747]

Thanks Tools.

[shinyribs]

I'd get the jugs off and have a look before ordering rings. If you need to rebore and use larger pistons then you'll need the matching larger diameter rings as well.

[surfish95747]

Good thinking! What would I do without you guys. Thanks!

[surfish95747]

If I don't change the rings immediately, is there any kind of permanent damage that I could do by waiting till the winter to change the rings etc? I just don't particularly want to do it now (waiting for next years tax return is ideal ;)

[shinyribs]

The absolute worst thing that could happen is a ring could break if it's severely worn. That could result in 1 of 2 typical things:

1: Severely score the cylinder/piston. No biggie if you plan to rebore rather than just re-ring.

2: Seize then engine, which can lock up your rear tire at speed. Just pull the clutch in if this happens.

But since I hate alarmists on forums, those are very slim chances. Chances are you're just gonna burn a little oil. No harm done there.

 

[nilsthenomad]

The trick the guys here taught me was to pour a cap of oil down the cylinder and then run the compression test. If your compression improves for a minute then it's the rings. Certainly the smoke would point to burning oil leaking from the pistons.

If it's not the rings, I just had similar problems and worked through them in this thread:

http://honda-cb750-s.456789.n3.nabble.com/Stutter-and-loss-of-power-at-high-speed-and-high-rpm-td4057076.html