Topic: Losing 2 cylinders when warm - CB750K2

[gotorx7]

Hey everyone..

New (to me) bike that I've serviced - timing, tappets, carb sync, oil, filters and some cosmetic stuff.

I took her out for a test drive late last week, did about 15Kms, and it was running great, I was very happy.

As I got close to home and started the climb up the 1000 ft hill to get there, I lost power.  Struggled home slowly, and thought (belatedly) that it might be a vacuum lock (cap not venting).

So cleaned the cap in the ultrasonic bath, blew it all through with compressed air, and it seems to be venting just fine. 

Test Drive #2 lasted only a half kilometer or so, and I lost power again.  Lifted the fuel cap, no difference, so not fuel starvation from the tank at least.

Nursed her back home again, pulled the plugs, and 1 & 4 were fueled up ( 2 & 3 looked great).

I'm getting spark at both sets of points, and at all plugs when I turn it over with plugs out - but obviously the bike is cool now.

I have a theory, but before I blow $$$ un-necessarily has anyone come across this or have any thoughts on other tests I can run please?

I'm nervous about taking it out on the road again as this hill I'm talking about will cause me a MAJOR PITA if the bike stops completely, as I currently have no way of hauling it around.

Hope you enjoyed the novel 

Cheers, Dave

[newday777]

Yes that wouldn't be fun Dave.
You say you are getting spark at both points. Are they visably sparking while it is running in a dark garage?
Did you put in aftermarket points and condensers?
Have you tested the coils resistance when hot?
Have you tested the spark plug caps for resistance off the wires?
Did you clip off 5-6mm of plug wire so there is fresh wire to screw the plug caps back into?

[newday777]

If you don't know how to do the tests Brickhouse Builds goes through them in this video, on a K2.

[gotorx7]

Yes that wouldn't be fun Dave.
You say you are getting spark at both points. Are they visably sparking while it is running in a dark garage?
Did you put in aftermarket points and condensers?
Have you tested the coils resistance when hot?
Have you tested the spark plug caps for resistance off the wires?
Did you clip off 5-6mm of plug wire so there is fresh wire to screw the plug caps back into?

Hello again Stu 

I am seeing spark when opening the points manually. I'll try it with the thing running (will probably be a few days until I can get to it)

And No to all the other questions, but will follow those up next.

Thank you very much for the advice, you're worth your weight in gold 

And thanks for the video link, I haven't seen that one.  I will absorb it now!

Cheers, Dave

[newday777]

Part 2 doing the work

[newday777]

Take your time Dave. I hope it is a simple fix for you. They are great bikes to play with.

[scottly]

pulled the plugs, and 1 & 4 were fueled up ( 2 & 3 looked great).

When you say "fueled up", what do you mean? Are those two plugs wet with fuel, but relatively clean, or black and wet, or sooty black and dry?

[HondaMan]

Hey everyone..

New (to me) bike that I've serviced - timing, tappets, carb sync, oil, filters and some cosmetic stuff.

I took her out for a test drive late last week, did about 15Kms, and it was running great, I was very happy.

As I got close to home and started the climb up the 1000 ft hill to get there, I lost power.  Struggled home slowly, and thought (belatedly) that it might be a vacuum lock (cap not venting).

So cleaned the cap in the ultrasonic bath, blew it all through with compressed air, and it seems to be venting just fine. 

Test Drive #2 lasted only a half kilometer or so, and I lost power again.  Lifted the fuel cap, no difference, so not fuel starvation from the tank at least.

Nursed her back home again, pulled the plugs, and 1 & 4 were fueled up ( 2 & 3 looked great).

I'm getting spark at both sets of points, and at all plugs when I turn it over with plugs out - but obviously the bike is cool now.

I have a theory, but before I blow $$$ un-necessarily has anyone come across this or have any thoughts on other tests I can run please?

I'm nervous about taking it out on the road again as this hill I'm talking about will cause me a MAJOR PITA if the bike stops completely, as I currently have no way of hauling it around.

Hope you enjoyed the novel 

Cheers, Dave

Does this bike, by chance, have brand new condensors in it?
If so, get some old, used ones. They never wear out, despite being often changed. In the past 10 years or so, 100% of the condensors except the ones from TEC Japan (from Honda) are badly made, and fail in just a few minutes of run-time. This will act much like what you describe.

[gotorx7]

The condensers do not appear to be new.  I'll check on the brand next time I'm out at the bike.

[gotorx7]

pulled the plugs, and 1 & 4 were fueled up ( 2 & 3 looked great).

When you say "fueled up", what do you mean? Are those two plugs wet with fuel, but relatively clean, or black and wet, or sooty black and dry?

Damp and dark, but not fouled..

1-4, left to right

[gotorx7]

Spark plug caps have about 5kOhm resistance, +/- 10%

Resistance between leads 1-4 and 2-3 is approximated 15kOhms.

There is spark visible at both sets of points with the motor turning over with the starter.

I'm about to head off for a few days. When I get back I'll put everything back together and run until warm and do some more tests.

Really appreciate the input and advice given, thank you.

[newday777]

Dave
Another thought, how are the fuel lines run, are there in line aftermarket fuel filters added and the lines run low below the carb bowls? Going up a steep uphill might get starvation with improperly run fuel lines. I've seen some people run their fuel lines way down under the carbs and have running problems. Had to ask.

[gotorx7]

Dave
Another thought, how are the fuel lines run, are there in line aftermarket fuel filters added and the lines run low below the carb bowls? Going up a steep uphill might get starvation with improperly run fuel lines. I've seen some people run their fuel lines way down under the carbs and have running problems. Had to ask.

Good thought!  That would explain it...

No filters, and the fuel lines don't drop down, they JUST reach the petcock..

[newday777]

Dave
Another thought, how are the fuel lines run, are there in line aftermarket fuel filters added and the lines run low below the carb bowls? Going up a steep uphill might get starvation with improperly run fuel lines. I've seen some people run their fuel lines way down under the carbs and have running problems. Had to ask.

Good thought!  That would explain it...

No filters, and the fuel lines don't drop down, they JUST reach the petcock..

👍

[scottly]

pulled the plugs, and 1 & 4 were fueled up ( 2 & 3 looked great).

When you say "fueled up", what do you mean? Are those two plugs wet with fuel, but relatively clean, or black and wet, or sooty black and dry?

Damp and dark, but not fouled..

1-4, left to right

I would replace those two plugs.

[gotorx7]

pulled the plugs, and 1 & 4 were fueled up ( 2 & 3 looked great).

When you say "fueled up", what do you mean? Are those two plugs wet with fuel, but relatively clean, or black and wet, or sooty black and dry?

Damp and dark, but not fouled..

1-4, left to right

I would replace those two plugs.

I have ordered some of the Nippon Densos,

Thank you!

[scottly]

Are the plugs all the same heat range? BTW, if a plug is damp, that cylinder wasn't firing. Given a lack of soot, suspect ignition.

[gotorx7]

Are the plugs all the same heat range? BTW, if a plug is damp, that cylinder wasn't firing. Given a lack of soot, suspect ignition.

I agree.. I suspect the coils, as it is heat related, and I never heard of points or condensors being sensitive to heat (correct me if I'm wrong!)..

Anyway, I've ordered a few bits - new plugs, coil packs, leads and plug boots.      Also a chinese dynatek CDI copy to try if the coil packs/leads etc don't fix it..

Cheers, Dave

[bryanj]

Condensers can d do fail when hot then recover

[gotorx7]

Thanks Bryan,

I've never seen that personally but a good mechanic friend agrees with you, hence I bought the CDI.

My plan is to replace the coils, leads etc and if I still have a problem then do the CDI, rather than change too much at once.

Cheers, Dave

Condensers can d do fail when hot then recover

[gotorx7]

Update :

I've replaced the coils, leads and spark plug connectors with new units from Rick's..

For informational purposes, they are NOT a direct replacement (despite claims).. The coils are about an inch longer (but still fit thanks to the rear mount being movable).  And the Wiring supplied has two male connectors, not a male & female as per stock - I just used the old connectors.

Just waiting on new Spark Plugs and I'll fire her up.

[gotorx7]

And to close another one out 

I just couldn't get the points/timing combination right despite frigging around for many hours, so bit the bullet and bought a Dynatek clone..

With that fitted and timed (had a coupel issues getting enough adjustment range, but a file on the plate fixed that), the bike fired up right away.

I just got back from a ride to a beach further up the coast from me, and happy to report that it purred like a kitten - I am VERY happy now 

Attached are a couple of pics from my destination, Waitpinga beach!

[newday777]

Good to hear you are back up and running great Dave

[gotorx7]

Thanks Stu, and for your help!

[PeWe]

The issue to not get timing right could have been the advancer unit's springs. Too soft.
Both my CB750 have the springs cut. Cut one of the loops that attach spring to advancer. Bend out a new.
Used with points and Dyna-S.

Check if advancer has a play when the M6 nut is loose. Move max counterclockwards and tighten it.
Markings will be better, T at real TDC.

Redo the timing at ca 1200 rpm with a timing light.
You will notice a better more stable idle.