Topic: CB750 lean on 2&3 Cylinders

[Johnny5]

Hey everyone,

I've a done a few things to my bike so I will list them since I cant just say I have a 77 CB750.

Engine is from a 77 with a 74 cam installed. I like the lower end grunt. I also put on the older style 657B carbs with 110 mains, needle in the stock position, air screw out 1 turn. I have installed a new K&N air filter in the stock airbox. And 4-1 exhaust.

I adjusted the cam chain tensioner, adjusted the valves, Point gap is good and timing is spot on - checked it the a timing light gun, carbs are clean and checked that the floats are all set at 26mm.

The bike runs well and will idle fine, but feels a bit flat. Plugs 2&3 are on the lean side and plugs 1&4 are sooty. From what I read this is probably the timing being off, which it's not or the coils might be failing. Can I check the coils by switching the blue and yellow leads from the points to the coils? Anything else I should check?

Thanks,

[TwoTired]

Can I check the coils by switching the blue and yellow leads from the points to the coils? Anything else I should check?

Yes, if you switch the blue and yellow leads, also swap the plug leads for 2&3 with those on 1&4.

Did you check the carb parts for proper numbers jet size and slide needle?
Did you make sure that the metering devices in the carbs were unmodified (drilled)?

Are you certain there are no air leaks in the carb duct path?

Cheers,

[Johnny5]

Correct, verified there were no air leaks - sprayed the boots on both sides of the carbs with WD-40 while running. No change in idle.

The carb parts looked unmolested. No extra holes drilled. From what I could tell all the needles and jets were stock and there were no signs of man-handling.

[Johnny5]

Just checked the coils. Both are reading 5 ohms.

The middle plugs look good. They are a light tan. I guess they arent as lean as I originally thought, but compared to 1&4 they are. It's just that 1&4 are really sooty. I just checked the pilot jets for carbs 1 and 4 and they were clear.

Crap, I guess the only thing I havent done today is a carb sync. I just did one about a month ago, so I dont think it'd be that far off. Could that cause this issue?

...off to check the sync!

[mrbreeze]

I would go with an ignition problem being that it is 1 & 4. I was having problems with mine dropping cylinders and at first I thought it was some crap in the tank I bought from you. One night when it acted up.....I rode home on 2 & 3. 1 & 4 were cold. Next day a found a dodgy connection on one of the wires from my pickups to the ignition box (dyna 2000). I put a better connector in and it has been fine ever since. I need to remember to bring my soldering gun home from work and fix those wires better w/ some shrink wrap.

[Johnny5]

Yeah, I seem to be narrowing it down to something electrical. I just finished vacuum syncing and it wasnt off by much at all.

Back to shed to investigate more!

[Johnny5]

Man, I got nothing. I took the plugs out and grounded them to the engine. All the plugs produced a nice spark.

[tlbranth]

My experience with the old bike is that bad running is almost never due to a single problem and so there's no magic bullet to fix it. It's a lot of tweaking and adjusting - and it's constant. I think you're running rich on 1 & 4 and I'd check fuel levels and for blockages. You don't have to be off by much to cause lousy running. This is true of fuel and timing.

[Johnny5]

Found the issue. Looks like it was a weak connection with one of the wire connections from the points to the coils. I took them apart and cleaned them up and voila, all the plugs are now the same color: tan. Beautiful!

[mrbreeze]

Found the issue. Looks like it was a weak connection with one of the wire connections from the points to the coils. I took them apart and cleaned them up and voila, all the plugs are now the same color: tan. Beautiful!

Thats what I mean!!! Mine wasn't obvious either. I cut into the harness on the Dyna 2000 to lenghten it. I did that back around 2003 when I still lived in Wyoming. I should have soldered and shrink wrapped but I was in a hurry to ride so I butt connected and electrical tape. Over the years I forgot about it and it decided to bite me in the a$$. Glad you got yours straightened out.