Topic: 1978 Honda CB750K Rich Condition

[njlantis]

Well I just finished doing a compression test, this was done with the engine slightly warm but cold enough that I can touch it without any chance of being burned. The numbers are as follows:

1: 120
2: 110
3: 110
4: 118

For a cold compression test I've seen similar numbers for this engine, however I plan to do a hot test tomorrow to confirm

The spark plugs were pulled as well and the pictures can be seen below with cylinder 4 on the left and 1 on the right. Cylinders 1 and 4 were much darker than 2 and 3. It also seemed like there was a little bit of oil on #4

[smee]

Well I just finished doing a compression test, this was done with the engine slightly warm but cold enough that I can touch it without any chance of being burned. The numbers are as follows:

1: 120
2: 110
3: 110
4: 118

For a cold compression test I've seen similar numbers for this engine, however I plan to do a hot test tomorrow to confirm

The spark plugs were pulled as well and the pictures can be seen below with cylinder 4 on the left and 1 on the right. Cylinders 1 and 4 were much darker than 2 and 3. It also seemed like there was a little bit of oil on #4

What's the difference between the plugs in the picture? The ones in the back are really rich, while the ones up front are different looking.

[HondaMan]

Are your carbs marked "PD42A" or "PD42B"?

[njlantis]

Well I just finished doing a compression test, this was done with the engine slightly warm but cold enough that I can touch it without any chance of being burned. The numbers are as follows:

1: 120
2: 110
3: 110
4: 118

For a cold compression test I've seen similar numbers for this engine, however I plan to do a hot test tomorrow to confirm

The spark plugs were pulled as well and the pictures can be seen below with cylinder 4 on the left and 1 on the right. Cylinders 1 and 4 were much darker than 2 and 3. It also seemed like there was a little bit of oil on #4

What's the difference between the plugs in the picture? The ones in the back are really rich, while the ones up front are different looking.

Sorry, the ones in the back are the old plugs, taken off before I correctly set the float heights. The ones in front are the new ones and were installed the same time I put the carbs back on. They have only been on the bike while it idles for a short amount of time and then took 2 or 3 rides up and down my road.

[njlantis]

Are your carbs marked "PD42A" or "PD42B"?

They are 42B's

[scottly]

The spark plugs were pulled as well and the pictures can be seen below with cylinder 4 on the left and 1 on the right. Cylinders 1 and 4 were much darker than 2 and 3. It also seemed like there was a little bit of oil on #4

You have fixed the rich condition caused by the high float levels, which is why the #2&3 plugs look better, with no carbon. The wet #1&4 spark plugs indicate an ignition issue; no spark, no fire, no carbon. Do you have the stock points ignition?

[denward17]

Also tell us how many miles on the engine and if it has had any engine work performed since in your possession.

[scottly]

Also tell us how many miles on the engine and if it has had any engine work performed since in your possession.

Why?? He's already fixed the carburetor issue and has narrowed the next problem down to the 1-4 side of the ignition system.

[denward17]

Also tell us how many miles on the engine and if it has had any engine work performed since in your possession.

Why?? He's already fixed the carburetor issue and has narrowed the next problem down to the 1-4 side of the ignition system.

Well, I was thinking if the engine had a lot miles and no engine work has been done, it could be valve guides/seal issues in conjunction with a possible ignition issue.

[scottly]

Please don't introduce purely speculative wild-assed guesses into an ongoing trouble-shooting process, as it only tends to confuse the issue, rather than help solve the problem.

[denward17]

Please don't introduce purely speculative wild-assed guesses into an ongoing trouble-shooting process, as it only tends to confuse the issue, rather than help solve the problem.

Understood......

[maxheadflow]

Is the ignition advance mechanism working correctly?

Have you checked it with a strobe light?

[scottly]

Is the ignition advance mechanism working correctly?

Have you checked it with a strobe light?

Sigh... If it was a problem with the advance mechanism, it would affect all four cylinders, not just 1&4. The symptoms indicate a weak and/or intermittent spark: if the bike has the stock points ignition, a bad condenser may be the culprit.

[smee]

Yes, I would agree with some of the comments here. Check the points/condensers. Do they close/open properly? Are the contact points still flat or are they angled at all? Are your condensers good? Is the wiring from the condensers into the plug boots good? Good contact with the tip of the spark plugs?

Other observations from your photos:
- You've lost most of the color on the threads of your new plugs
- The base ring of your newest plugs looks like they don't have must carbon at all on them. Are they really clean?
- It's a bit hard to read across all the plugs, but to me I'd check your static timing again to make sure it's consistent for both sets of cylinders. The ground straps on the 4 newest plugs don't look equal to me (just my opinion).

[njlantis]

The spark plugs were pulled as well and the pictures can be seen below with cylinder 4 on the left and 1 on the right. Cylinders 1 and 4 were much darker than 2 and 3. It also seemed like there was a little bit of oil on #4

You have fixed the rich condition caused by the high float levels, which is why the #2&3 plugs look better, with no carbon. The wet #1&4 spark plugs indicate an ignition issue; no spark, no fire, no carbon. Do you have the stock points ignition?

So you think that I'm not getting any spark on 1 and 4? Would the engine even run if that were the case? And yes I do have the stock points ignition and I recently set ignition timing and points gaps

[scottly]

I think you are getting some spark, but not enough or not all the time. First thing I would do is remove the points cover and check for excessive arcing at the points, which indicates a bad condenser.

[njlantis]

Yes, I would agree with some of the comments here. Check the points/condensers. Do they close/open properly? Are the contact points still flat or are they angled at all? Are your condensers good? Is the wiring from the condensers into the plug boots good? Good contact with the tip of the spark plugs?

Other observations from your photos:
- You've lost most of the color on the threads of your new plugs
- The base ring of your newest plugs looks like they don't have must carbon at all on them. Are they really clean?
- It's a bit hard to read across all the plugs, but to me I'd check your static timing again to make sure it's consistent for both sets of cylinders. The ground straps on the 4 newest plugs don't look equal to me (just my opinion).

I'll tear back into points tonight and confirm timing is all set properly and I'll post pictures of the points tonight. The new plugs are really clean as they were not run on the bike for very long, only a little bit of idling and two drives up and down my street. Can you explain what parts you are talking about when you say base ring and ground strap?

[scottly]

First just look for acing at the points with the engine running.

[njlantis]

First just look for acing at the points with the engine running.

Should I be able to see any arcing or should I not see any arcing at all?

[scottly]

Normally there is some arcing, but it will be much worse if the condenser is bad. Compare the 2-3 points to the 1-4 points. If there is a 3-blade propeller symbol on the end of the condenser, that brand is notorious for failing.
 

[scottly]

First just look for acing at the points with the engine running.

Should I be able to see any arcing or should I not see any arcing at all?

Watch this video:

[njlantis]

I was able to dig into the points and ignition last night and took the attached video of the bike running with the cover off. Please let me know any comments on how the arcs look. The condition of the points looks good in my opinion, I don't see any pitting and the points make flat contact when closed. I did however measure the gaps and the 1-4 point is way out of spec, the gap being more than double what it should be. Not sure how this happened as I previously went through and set points gaps and static timing. Maybe I set the gap when it wasn't fully open. Regardless, going to set the gap again and then set the static timing again and see what happens.

https://clipchamp.com/watch/CYzQrYO0Mrn

[scottly]

I did however measure the gaps and the 1-4 point is way out of spec, the gap being more than double what it should be. Not sure how this happened as I previously went through and set points gaps and static timing.

A large point gap would reduce the amount of time the points are closed, called dwell. During this time, the coil "charges up", and then discharges when the points open. A very short dwell time would produce a weaker spark, which would get worse as RPM increased.
The 2-3 points seemed to arc more than the 1-4 points?
It looks like the 6mm stud that holds the advancer onto the crankshaft is wobbling? The stud can get bent by turning the engine over by the big "nut" on the end of the advancer, especially with the spark plugs installed, and might cause such a large change in the points gap as well as ignition timing.

[newday777]

I did however measure the gaps and the 1-4 point is way out of spec, the gap being more than double what it should be. Not sure how this happened as I previously went through and set points gaps and static timing.

A large point gap would reduce the amount of time the points are closed, called dwell. During this time, the coil "charges up", and then discharges when the points open. A very short dwell time would produce a weaker spark, which would get worse as RPM increased.
The 2-3 points seemed to arc more than the 1-4 points?
It looks like the 6mm stud that holds the advancer onto the crankshaft is wobbling? The stud can get bent by turning the engine over by the big "nut" on the end of the advancer, especially with the spark plugs installed, and might cause such a large change in the points gap as well as ignition timing.

I agree the 2-3 points are arching more, bad condenser. Are those aftermarket points and condensers?(if so you need to order the points plate assembly from Honda.)

https://www.southsoundhonda.com/--xpartsstream#/Honda_Powersports/CB750KA_(76)_750_FOUR%2c_JPN%2c_VIN%23_CB750-2540001_TO_CB750-2575894/POINTS/b3d05c73-22e7-4620-b018-132301d96b6e/80fb05af-446d-41cb-a88f-21b6993df839/y

And yes the points advancer shaft is bent. (Never turn the motor with the 17mm nut with the spark plugs in place!)
It needs to be straightened. I just straightened one on a K4 I was tuning on a friend's bike, he is the original owner of the bike. It was @.015 out.
I got it to within .001, Hondaman says anything less than .006 in within spec. I used a clamp on dial indicator from HF and a few taps with a ballpeen hammer on the high points finally brought it within spec.

https://www.harborfreight.com/clamping-dial-indicator-63656.html

[maxheadflow]

Is the ignition advance mechanism working correctly?

Have you checked it with a strobe light?

Sigh... If it was a problem with the advance mechanism, it would affect all four cylinders, not just 1&4. The symptoms indicate a weak and/or intermittent spark: if the bike has the stock points ignition, a bad condenser may be the culprit.

Maybe it does.. At the same time the timing is different between the 2 pairs?

So the bad condenser on 2-3 is killing 1-4?