NGC, 1972 Cb 750 chopper cylinder 3 not getting hot at idle???

[bobrebos1]

But it is getting hot under throttle.  Carb low speed idle jet, spark plug?  What would you check first?  1972 cb750 inline 4, 4 carbs.  Thanks for any help. 

[faffi]

The idle mixture screws on these should regulate air. So if all adjustment screws are set equally, it seems logical to expect a blockage in the idle circuit in the carb of the affected cylinder. Could be the jet, and/or the circuits in the carb body itself. I usually spray carb cleaner into on end of the circuit and check that the cleaner exit the other end to verify if it is blocked or clear.

[bobrebos1]

The idle mixture screws on these should regulate air. So if all adjustment screws are set equally, it seems logical to expect a blockage in the idle circuit in the carb of the affected cylinder. Could be the jet, and/or the circuits in the carb body itself. I usually spray carb cleaner into on end of the circuit and check that the cleaner exit the other end to verify if it is blocked or clear.

That’s what I thought.  Hoping it’s something as easy as a clogged jet.  Il tear into carb next week. 

[Scout63]

Good luck. I would imagine that getting to the carb will be harder than checking the jets.

[guzzisteve]

Could be a bad plug, put a timing light on it to see if sparking. Then move to carb.

[Moparnut72]

I would add a bunch of Seafoam to the tank before I started tearing things apart. I live in California where non-ethanol fuel is non-extent where I live. I have trouble with engines that are only run seasonally even though I take steps to prevent the problem. Most of my yard equipment are the main trouble makers. Seafoam will usually get them running right again. It is usually the idle circuit that acts up.
kk

[bmc5733946]

I was taught by a fella I respect to rev the engine high (near redline) and close the choke while holding the throttle open. He told me that occasionally it would suck something out of the jets and allow better running. I won't say it works, I have done it one my 1978 750-4 and it seemed to help. Costs nothing and is less work!

Brian

[chuck peterson]

Spark.  Timing light blinking? At idle…high rpp

Compression. Pull plug, thumb or gauge

Gas. Can you open a drain plug? Does moving the petcock on/off/res effect the flow of fuel thru the drain..

After that I would need more tools, like beer.
 

[bobrebos1]

Il check spark at idle first, then float bowl fuel level using the clear tubing method, then float level, then low speed idle circuit.  Out of state for another week then will get to it!  Woo hoo!

[Grabcon]

A few of things on the SOHC CB750. As mentioned the spark plugs. Check the spark plug caps, they are screwed on to the plug wire. Unscrew and cut no more than 1/4" for a fresh end put back on plug cap. The wires are typical short  so trimming to much will give you grief. The plug wires cannot be extended they are molded into the coils.

Now to the carbs. If you have a cold cylinder pull the carbs off. pull them apart on the bench. Everything. Clean the passages and make sure that the emulsifiers are pulled and cleaned. If parts are needed, gaskets and needle valves and seats only buy keihin carb parts do not buy aftermarket kits. More money yes but less grief.

Make sure the float levels are properly set. Fully assemble on bench put the carbs level in a flat pan wide enough to hold are carbs. Hook up a gas supply and check to make sure the floats and needle valves are working and there are no leaks out of the overflow tubes. 

If all that tests good put back on the bike. If you have a center stand put the bike on the center stand and turn on the gas. Give it five mounts and check for overflow leaks. When they are apart polish the pin that hold the float in place. Once there is not issue, fire up the bike and again check for your cold cylinder.

When you park the bike overnight or longer period, turn off the petcock and run the carbs dry. this help keeps the ethanol from reacting with the pot metal of the carbs, and keeps the corrosion free.

If you have spark the issue on cold cylinders on these SOHC motors is always carbs.

Here is an excellent SOHC forum. http://forums.sohc4.net. Hondaman is the guy that is the technical guru.

Hope this helps.

[John Croucher]

Float level

[bobrebos1]

A few of things on the SOHC CB750. As mentioned the spark plugs. Check the spark plug caps, they are screwed on to the plug wire. Unscrew and cut no more than 1/4" for a fresh end put back on plug cap. The wires are typical short  so trimming to much will give you grief. The plug wires cannot be extended they are molded into the coils.

Now to the carbs. If you have a cold cylinder pull the carbs off. pull them apart on the bench. Everything. Clean the passages and make sure that the emulsifiers are pulled and cleaned. If parts are needed, gaskets and needle valves and seats only buy keihin carb parts do not buy aftermarket kits. More money yes but less grief.

Make sure the float levels are properly set. Fully assemble on bench put the carbs level in a flat pan wide enough to hold are carbs. Hook up a gas supply and check to make sure the floats and needle valves are working and there are no leaks out of the overflow tubes. 

If all that tests good put back on the bike. If you have a center stand put the bike on the center stand and turn on the gas. Give it five mounts and check for overflow leaks. When they are apart polish the pin that hold the float in place. Once there is not issue, fire up the bike and again check for your cold cylinder.

When you park the bike overnight or longer period, turn off the petcock and run the carbs dry. this help keeps the ethanol from reacting with the pot metal of the carbs, and keeps the corrosion free.

If you have spark the issue on cold cylinders on these SOHC motors is always carbs.

Here is an excellent SOHC forum. http://forums.sohc4.net. Hondaman is the guy that is the technical guru.

Hope this helps.

Outstanding!  Thanks!!!!

[Huzo]

Have you said whether you have checked the carb balance ?
Is the slide for #3 lagging behind such that it does not provide fuel/air at idle, but chimes in to some degree at more open throttle ?
Can you detect the cylinder “coming alive” suddenly as revs rise, or does it just blend in ?
I guess I’m asking, at idle it noticeably running on three cylinders ?

[Grabcon]

Have you said whether you have checked the carb balance ?
Is the slide for #3 lagging behind such that it does not provide fuel/air at idle, but chimes in to some degree at more open throttle ?
Can you detect the cylinder “coming alive” suddenly as revs rise, or does it just blend in ?
I guess I’m asking, at idle it noticeably running on three cylinders ?

Carb sync will not affect the cylinder running cold. Syncing is for balancing the carb flow at idle.

The only year of the CB750 where the carb slide would lag or get stuck is the first year of production. One throttle cable split into four. All the years after that had a linkage that fixed all slides to move together without one or more getting stuck or moving at differently.