Topic: CB500 Overheating-Seizing

[Little_Phil]

CB500 K1 1973
Happens after about 15 miles. Stutters and then locks up. Restarts after 10 mins.
Engine has done about 50 000 miles. About a year ago I put new aftermarket rings in and used a cheap hone to bed them in. The ring gaps were about 0.010 (spec for new is 0.005 to 0.013) so was surprised they were that low. Measured at a few points up and down the bore. The two compression rings looked near identical with both appearing to have a chrome like face. Changed the oil and filter at the same time. Used 10-30 oil as I have for years as its not Texas here.
Now completed 500 miles and it's not getting better.
I have lowered the carb needles in the past a notch because the plugs looked dark. 2,3,4 plugs still look dark and 1 looks black. I don't seem to be using much oil and don't keep a close eye on it, but maybe a little. Never cane the engine and usually keep 4-5000 revs range.
Just to add timing is electronic and spot on.
Perplexed!

[seanbarney41]

That's a lot of miles to try and clean up with just a hone and rings.  I think generally bore is out of round, tapered, and otherwise warped enough to require 1st oversize bore.  It may have spots tighter than where you checked the ring gap.  But have not heard of this resulting in soft seizure.  Maybe try double checking valve timing and clearance.  How does the valve train sound?

[bryanj]

Only time i ever saw a 500 sieze the timing was way off

[CycleRanger]

Are you absolutely sure it's actually seizing up?

[scottly]

The used cylinders and pistons shouldn't be tight enough to seize. If it is actually sticking, check that the cam is getting oil on both sides.

[bryanj]

Yes, far more likely cam siezing due to oil feed blockage

[BomberMann650]

Which aftermarket company led you to believe 5 thou was minimum ring gap?

It doesn't add up, the thermal expansion of old iron and even "nice" rings would need more room than that.

[bryanj]

When talking specs you need to say whether its inch or metric

[Little_Phil]

Are you absolutely sure it's actually seizing up?

Won't kick over or turn with starter when I stop.

The used cylinders and pistons shouldn't be tight enough to seize. If it is actually sticking, check that the cam is getting oil on both sides.

Had the rockers off yesterday. All good swimming in oil.
Same electronic optical ignition 20 years. Check over this time occasionally and again yesterday, always spot on.
Also spent the day changing the carbs for a spare set. Got them on and then remembered that they had no return spring fitted. Off they come. On they go. No 1 overflowing. Whack the bowl with a driver. Problem solved. Then no 2 starts up. (most difficult to get to.) Managed to remove the float bowl and had to replace the needle. All back and running and it's raining hard now!

[bryanj]

If it really has been locking up that hard the pistons/rings are damaged and the cylinders need to come off

[Little_Phil]

Yes, far more likely cam siezing due to oil feed blockage

I'm now leaning more to this. Although the oil is there, maybe not enough at higher revs.

[MauiK3]

Bore scope the cylinders through spark plugs to see if there is any visible damage

[BenelliSEI]

Just read this and I bought a cb750 years ago with the exact same symptoms.
THE VENT ON THE GAS CAP IS RUSTED SHUT! You run out of gas after 15 minutes and grind to a stop? 15 minutes later it starts right up? Here’s  hoping I’m right.

[jonda500]

My Datsun Kingcab was doing that (but I wasn't getting 15 miles before seizing) after I did the head gasket - turned out the shop that resurfaced the head had removed all the bits of rag I blocked up all the holes with to keep it clean - except the oil feed hole - they pushed that piece of rag deep into the hole and didn't tell me. I should have (and always will now!) checked the oil passage was clear before reassembling. The camshaft was seizing from not enough oil, then unseizing as soon as it cooled down a bit. I had to drill a hole in the side of the head to extract the little piece of rag from the oil passage, then tapped the hole and locktited a bolt in the hole after cleaning thoroughly of course. I was very lucky nothing got wrecked! (That engine still runs!)
John

[Little_Phil]

Just read this and I bought a cb750 years ago with the exact same symptoms.
THE VENT ON THE GAS CAP IS RUSTED SHUT! You run out of gas after 15 minutes and grind to a stop? 15 minutes later it starts right up? Here’s  hoping I’m right.

No it's not that I ran it still occurred with the gas cap cracked open with a part filled tank.

[HondaMan]

Try removing the timing plate cover (right side where the electronic ignition lives) and see if it goes further.
Certain kinds of electronic ignitions will 'soft fail' when hot, then work again when cooled down. Removing the cover will cool it while running, as a test.

[bryanj]

You said it would not turn with kickstart when it "siezed"  and that suggests something mechanically serious, you really need to take it apart again and check

[Don R]

 I have an 836 that's done the same thing both before and after I honed and ringed it. I can ride all day but if I go over the speed limit on the 4-lane coming home it will die at the stop sign at the bottom of the ramp. It's coming apart after I get a K0 all bolted together to sell.

[HondaMan]

I have an 836 that's done the same thing both before and after I honed and ringed it. I can ride all day but if I go over the speed limit on the 4-lane coming home it will die at the stop sign at the bottom of the ramp. It's coming apart after I get a K0 all bolted together to sell.

My [first] 750K1 would do that if I ran over 100 MPH for about 5-6 miles and then stopped (street or track). It turned out to be a slight kink in the longer fuel line to the "far side" carbs, they weren't refilling fast enough at high speed. It was real annoying on the faster (oval) roadrace tracks, too, as it would drop top speed after about 3 minutes of 100+ MPH running. A new (proper length) fuel hose fixed that one, though.

[Killer Canary]

Just for thoroughness' sake; could one of the brakes be dragging? Just a thought.

[Don R]

I have an 836 that's done the same thing both before and after I honed and ringed it. I can ride all day but if I go over the speed limit on the 4-lane coming home it will die at the stop sign at the bottom of the ramp. It's coming apart after I get a K0 all bolted together to sell.

My [first] 750K1 would do that if I ran over 100 MPH for about 5-6 miles and then stopped (street or track). It turned out to be a slight kink in the longer fuel line to the "far side" carbs, they weren't refilling fast enough at high speed. It was real annoying on the faster (oval) roadrace tracks, too, as it would drop top speed after about 3 minutes of 100+ MPH running. A new (proper length) fuel hose fixed that one, though.

 This one is a cammed and ported 836 with 29 mikuni smoothbores on a single feed petcock. Maybe I should clock the flow.  I may be able to shorten the fuel line between the petcock and tee and keep it more downhill.

[Little_Phil]

You said it would not turn with kickstart when it "siezed"  and that suggests something mechanically serious, you really need to take it apart again and check

It's coming apart tomorrow. I have another bike to use.

[Tracksnblades1]

You said it would not turn with kickstart when it "siezed"  and that suggests something mechanically serious, you really need to take it apart again and check

It's coming apart tomorrow. I have another bike to use.

Better safe than sorry..😇

If it was your ring’s end gap butting, your piston(s) ring land(s) will more than likely be bad..
When a ring butts from heat/expansion it keeps getting hotter causing more expansion, clear to seizure..
The ring(s) that is/are butting get so hot that they micro weld the ring to the piston’s ring land on the way up.
Only to get torn off of the bottom side of the ring land on the way down…over and over until seizure….

If they experienced micro welding in the ring land(s) it’ll be an oil burner if you reuse them.…Similar to a bead blasted piston..  The rings will never experience a load during normal operation that can flatten down the torn up ring land enough to seal..Excepting maybe the top compression ring.. The oil rings will never seal after bead blasting….even standard tension ones…

Your culprit(s) piston’s may now have excessive ring to land side clearance too..Things really get hot right before it seizes..  If it’s a piston seizure it’ll be easy to spot.. 😩

[HondaMan]

I have an 836 that's done the same thing both before and after I honed and ringed it. I can ride all day but if I go over the speed limit on the 4-lane coming home it will die at the stop sign at the bottom of the ramp. It's coming apart after I get a K0 all bolted together to sell.

My [first] 750K1 would do that if I ran over 100 MPH for about 5-6 miles and then stopped (street or track). It turned out to be a slight kink in the longer fuel line to the "far side" carbs, they weren't refilling fast enough at high speed. It was real annoying on the faster (oval) roadrace tracks, too, as it would drop top speed after about 3 minutes of 100+ MPH running. A new (proper length) fuel hose fixed that one, though.

 This one is a cammed and ported 836 with 29 mikuni smoothbores on a single feed petcock. Maybe I should clock the flow.  I may be able to shorten the fuel line between the petcock and tee and keep it more downhill.

Maybe also check the bowl vents: I think the CR29 set I (only once, maybe twice) worked on had little vent holes in the sides of the carbs instead of having some way to attach a vent hose (which is a much better vent, IMHO). This made it hard to supply still-quiet air to the float bowls, not a good thing at all when running at speed. Turbulent air is low-pressure air, and having low-pressure air in the float bowls impedes fuel feed up from below.

[Don R]

 Thanks, all good ideas that may apply to both my bike and Phils. (sometimes, a hijack can be helpful)