• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

1980 CB750f - Cyl 2&3 cutting out after 10min.

Curt

CB750 New member
Messages
4
Reaction score
0
Points
1
Location
Dorchester, ON
Hey guys, I'm hoping someone can help me out with a problem I'm having on my 1980 CB750F.
The bike fires up without any issue and drives fine for about 10 minutes, then I'll lose cylinder 2&3! I need to let the bike cool down, then it starts up fine again...but after another 10 minutes or so the same thing happens.

I started working my way backwards from the engine, and I've replaced the plugs, wires, and coils. I have also swapped the CDI leads to see if one of them might be bad, but so far I'm out of luck.

I'm almost positive it's an electrical-related issue as it seems to happen once the bike gets warm. I figured if it was Carb-related, I'd have problems starting or idling the bike at start up.

I'd love to hear your thoughts, if you've had something similar happen.
 
Lemme guess....................you've NEVER set the valves have you?

The bikes do NOT have CDI.

I've only had the bike for 3 months, and it had been running fine......there's never been any reason for me to check or set the valves. Do you think that the clearance being off would cause Cyl. # 2 and 3 to work fine when it's cold but stop opening/closing once the bike warms up?

It continues to start without any issue and it will run fine until it gets warm (usually within 8-10 km), so that's why I thought something in the electrical system was expanding with the heat.
 
Did you ever stop to think of engine parts expanding with heat? That effect is 10X what ignition parts are.

An almost 40 year old engine needed the valves set likely 30 years ago and what people do to these all day long. And cannot tell how like crap they are running. I see it all day long. I'd say swap coils but if you've changed them that is likely out as a fix.

A compression test will show you like it does everybody else. Or it'll show me I'm wrong. So far I'm batting pretty much 1000 on this website.

Possible pulser as they lie in hot oil, but not a common problem.
 
Did you ever stop to think of engine parts expanding with heat? That effect is 10X what ignition parts are.

An almost 40 year old engine needed the valves set likely 30 years ago and what people do to these all day long. And cannot tell how like crap they are running. I see it all day long. I'd say swap coils but if you've changed them that is likely out as a fix.

A compression test will show you like it does everybody else. Or it'll show me I'm wrong. So far I'm batting pretty much 1000 on this website.

Possible pulser as they lie in hot oil, but not a common problem.


I checked compression as soon as I picked up the bike. It looked pretty good (Cyl 1= 141psi, Cyl 2 = 148psi, Cyl 3 = 151psi, Cyl 4 =142psi).

I picked up a set of feeler gauges yesterday, so I'll check the valves this weekend to see if you're still batting 1000!
 
I checked compression as soon as I picked up the bike. It looked pretty good (Cyl 1= 141psi, Cyl 2 = 148psi, Cyl 3 = 151psi, Cyl 4 =142psi).

I picked up a set of feeler gauges yesterday, so I'll check the valves this weekend to see if you're still batting 1000!

Looks like your batting avg. might not take a hit here...Cyl. #2 and #3 had clearance on two of the intake valves of 0.002" so I'm going to do some math and figure out the new shims. (the rest of the clearances were 0.004"-0.005")

I'll wait until I get the new shims in before I officially thank you for pointing me in the right direction!
 
Great. Ignore the service manual thing of .003", they run far longer at .005" as the go-to number. Any 4's are OK but you really don't want 3 or lower. 2 is essentially zero, you can easily burn valves with that.

Why?

The cams are looser in the head (up to .008") than the valves are, the cams then get pushed around by the springs to have lower valve numbers that are not real at all. You basically lose .002" of any valve clearance number in the wash of that. Honda never accounted for that in their setting of the spec but it has proven out over thousands of engines.
 
Look very close at the entire ignition, waste spark system using pairs, you lose one cylinder electrically it often leads to the other one of the pair failing ignition as well. One plug of each pair fires backwards, it provides the secondary ignition ground. Meaning BOTH sides of each pair have to work right. You can fail a resistor in like plug caps to make a problem like yours. BTDT. Or bad wire connection plug wire to coil. The ignition pairs are 1-4 and 2-3. Why your issue can be ignition too. The normal check there is to swap ignition coil wiring to swap coils and see if the problem switches to the other pair.
 
Back
Top