Topic: #1 and #4 running rich but...

[The French Dude]

Hi everybody.

There's the bike presently:

1977 cb750k

carbs cleaned. 110 main 45 slow.
1.5 turns on mix screw on all carbs.
swaped coils to test it and also test it with multimeter.

And I have #1 and #4 always rich I even turn the mix screw all in and still rich...

I don't know what to do now.

Do yu guys have any idea?

Thanks.

[The French Dude]

Oh and bike is hesitate when acceleration. and I don't have a steady running around 3000.

[calj737]

What are you using for airbox? Or pods? Any changes from stock configuration? Exhaust?

Have you verified the fuel level on all four carbs? Are the coils stock? Are you running points/condensers, or electronic ignition?

These answers will help guide you thru some troubleshooting steps to take?

[The French Dude]

What are you using for airbox? Or pods? Any changes from stock configuration? Exhaust?

Have you verified the fuel level on all four carbs? Are the coils stock? Are you running points/condensers, or electronic ignition?

These answers will help guide you thru some troubleshooting steps to take?

It's an airbox, I have a kreker, I did levels, coils are apparently stock and in good shape and I have an electronic ignition.
And new spark plugs.

Thanks

[calj737]

Ok, if 1/4 are rich, a possible culprit is the coil on that side. The spark plug caps should have a resistance reading of around 5k ohms. You can unscrew them from the plug wires and use a meter and measure them. If they are good, you can trim a very short it of the plug wire back and reinstall the stock caps to help produce a better spark.

I am believing the issue to be spark, not fuel because of the cylinder pairing. Which electronic ignition is it? And can you provide any history or detail on the bike? How long you have owned it, have you done any other service beyond the steps listed in your initial post?

[The French Dude]

Thanks for help @calj737 !

So Ignition is Dyna S I juste checked the settings and it's ok.
I tested caps one of the #1 #4 couple is apparently shot. the other is 5.03k
The other from #2 and #3 are 4.8k

I have the bike since a couple a weeks.
I did oil and filter change, clutch, new sparks, and thats it.

Thanks.

[calj737]

If all 4 caps are there (4.8-5.03) then that is not your problem.

What could be your problem is the dyna S. There have been numerous reports of them failing on one side only and causing poor spark, or lack of spark altogether. These units were about 3-4 years old when the suspect batch was produced.

You can go to the Dyna website and find diagnostic steps to evaluate yours. Again, my suspicion is ignition because of the cylinder pairings. It could well be fuel/carbs, but symptoms suggest spark to me.

[flybox1]

Have you done a bench/vacuum sync of the carbs?

If you have a points plate, install it.

[The French Dude]

Ok, if 1/4 are rich, a possible culprit is the coil on that side. The spark plug caps should have a resistance reading of around 5k ohms. You can unscrew them from the plug wires and use a meter and measure them. If they are good, you can trim a very short it of the plug wire back and reinstall the stock caps to help produce a better spark.

I am believing the issue to be spark, not fuel because of the cylinder pairing. Which electronic ignition is it? And can you provide any history or detail on the bike? How long you have owned it, have you done any other service beyond the steps listed in your initial post?

Actually I told you one of them was shot. Sorry if my english is not so clear
I presently search on google what would be the cause of a failed caps.

Thank you for help

[The French Dude]

Have you done a bench/vacuum sync of the carbs?

If you have a points plate, install it.

Don't have point plate unfortunately.
I thought that vacuum sync has to be done when bike is properly adjusted?

Thank you for help

[calj737]

Old age. I saw what you wrote, but did not see a resistance value for the "bad cap". If you have a bad cap, then I encourage you to replace all 4 with new NGK brand and be riding again!

[PeWe]

Will 'F' 1:4 vs 2:3 align with case mark equally? Full advance ''  '' too?

[flybox1]

I thought that vacuum sync has to be done when bike is properly adjusted?

Thank you for help

Ideally, yes.....but, if you have a vac sync set, hook it up and see if you're even close.  Id be interested in seeing what it reads now.
Anything way out can cause overly rich cylinders.....and is easily corrected.  A (small) step in the right direction.
Hey...if its not too far off...one more thing to check off and point you again back to spark as the culprit. 

[BomberMann650]

Will 'F' 1:4 vs 2:3 align with case mark equally? Full advance ''  '' too?

Yes.  They're marked 180deg apart on the advancer, and have all the timing marks for T F and advanced.

[The French Dude]

Old age. I saw what you wrote, but did not see a resistance value for the "bad cap". If you have a bad cap, then I encourage you to replace all 4 with new NGK brand and be riding again!

Thank you I already ordered 4 caps and see that tomorrow. Do you think that one failing cap can make the whole coil disfunctionning?
And what would be the cause of that broken cap?

Thanks

[calj737]

A single cap can interfere with the entire coil since it's a wasted spark ignition, meaning both leads fire at the same time despite the cylinder not being in compression.

It's really not uncommon at all for 40 year old rubber components to wear, crack or split. The only real design flaw with the ignition system on these CBs is the hard wired plug leads from the coilsemselves. It makes replacing the wires nearly impossible. Some use a splice kit to section in new wire with new plug caps instead of replacing the coils.

Any previous owner could have used pliers to pull off the cap due to it being stuck on the plug and damaged it. It's not really a big deal. I'm just glad you've identified a likely culprit and face a simple, cheap remedy.

[The French Dude]

I'm just glad you've identified a likely culprit and face a simple, cheap remedy.

I hope so.
I will let you know tomorrow if it's good
Thanks again...

[The French Dude]

I'm just glad you've identified a likely culprit and face a simple, cheap remedy.

Well nothing changed.
Bike still hesitate during acceleration.
When cold bike run better. Once hot bike running poorly mainly in low motor revolutions.
I check once again timing, caps, plugs... I don't know what to do...

[flybox1]

keep your idle rpm between 1200-1400
vacuum sync done?
clear tube test your bowls?
idle plug chop?

what are the results of these?

[The French Dude]

keep your idle rpm between 1200-1400
vacuum sync done?
clear tube test your bowls?
idle plug chop?

what are the results of these?

Hi.
I don't have vacuum gauge. I am about to make me one asap
I can easily do a clear tube test. What result do I have to get? I don't find any information about it.
Idle plug chop : I just discovered that test! So I never did it. What the purpose to cut the plug exactly? Sorry for ignoirance

[flybox1]

keep your idle rpm between 1200-1400
vacuum sync done?
clear tube test your bowls?
idle plug chop?

what are the results of these?

Hi.
I don't have vacuum gauge. I am about to make me one asap   Great.  Many here have made their own.
I can easily do a clear tube test. What result do I have to get? I don't find any information about it. fuel level should be 3mm +/- below the top of the carb bowl
Idle plug chop : I just discovered that test! So I never did it. What the purpose to cut the plug exactly? Sorry for ignoirance
No sorrys.  all good questions.   This test allows combustion deposits to form on clean spark plug insulators.  white=lean, black=rich, tan/brown=ideal.  Individually adjust each carbs idle mixture screw to achieve the tan plug color.