Topic: Rough running CB750K1

[AndyH]

Hi Folks,  I recently acquired a nice American import K1.  However, it was running poorly...idling too high, seeming to miss and carry on under throttle etc.  The bike is completely stock, including the 4 into 4 exhaust.

So an initial check revealed a faulty plug cap on #3 cylinder.  After replacing same, the bike still ran poorly.  So I took off the carbs and gave them a full clean (removed and cleaned all brass, ultrasonic clean of carb bodies, slides.)  Reinstalled brass, set float levels (all were out!) and carried out a bench sync.  I also replaced the rubber "insulators" beween carbs and head.  Then I checked and adjusted tappet clearances (they were close to correct).  Primary and secondary coil resistances were checked and were in spec (4.5Ω/14.5kΩ).
Before vacuum sync, I checked that the points timing was correct...it was.  Carbs then reinstalled along with stock airbox and filter.
I proceeded to vacuum sync, and although the sync tool (Syncroking 4 MkII) indicated full carb balance at idle, it still ran a little "lumpy".

Increasing throttle opening still showed very good vacuum balance but bike still didn't sound or feel right.  I double-checked my procedures and compared the Syncroking with a mercury manometer.  Both indicated the same good result.
So I popped the tank back on and went for a quick ride, and it proved that something was not quite right....it felt lumpy unless throttle was cranked open.  Closing the throttle brought the bike back to normal idle, again a little lumpy however.

Sorry for the long ramble but I'm at a loss as to what could still be causing such "rough" running.
There are many on this forum far more experienced than I so I'm hoping someone may have an aswer for me.
Thanks in advance!

[69cb750]

You have done everything you should have, now it is time to guess, I would replace the coils and condensers.

[cooldrum]

You may also consider replacing the point set if they are after market and check all key electrical connections and fuel flow through the petcock.  It's those small things that can effect the smooth running of our bikes.  Keep us posted.

[BenelliSEI]

I chased my tail like this on my K1. Look closely at your points cam shaft. Any chance it is slightly bent? Also, check timing advance return. Does it drop back quickly at idle? Any chance the advancer is sticking?

[Don R]

 Maybe a compression test? Double check the points with a dwell meter? You have been very thorough, that makes it easier to help.

[Kevin D]

What’s the status of your spark plugs?
I have carbon fouled lots of ngk’s.
better with 115/38 instead of 120/40.
better yet with ND X24ESU
Good spark plug caps, good condensers critical.
Clean fuel tank, petcock, screen, bowls.
All this works for me.

[MauiK3]

Cam chain adjusted?

[Stev-o]

No offense to 69, but highly doubt it is the coils.  Aftermarket condensers are crap, check that.

Also, you need to confirm the carb brass is Keihin and not aftermarket.  This happened to me on two different bikes.

Good luck, you'll get there and when you find the issue it will all make sense.

[ekpent]

No offense to 69, but highly doubt it is the coils.  Aftermarket condensers are crap, check that.

Also, you need to confirm the carb brass is Keihin and not aftermarket.  This happened to me on two different bikes.

Good luck, you'll get there and when you find the issue it will all make sense.

I'll give a thumbs up to Steve's comment. I sold a 750 once to a guy I know and he chased his tail for awhile with carbs etc. until he swapped out the condensers and then it was fine. It would idle and rev fine but fall flat on its face under a load.
  Do you have stock brass in your carb and what size is the main jet ? Slow is probably a #40. Are the spark plug black or lean ? Stock exhaust and airbox ?

[HondaMan]

I would suggest this:
Using a strobe timing light, run the engine and look for the timing marks. If they are 'jumpy', shuttling back-and-forth under the light, the spark advancer shaft has been bent, which is caused by turning the engine backward with that nut. Don't do that...you will then have to remove the points plate and straighten the shaft to less than 0.003" runout (use the electric starter to turn the engine while checking this).

Today's condensors are BIG trouble. Don't be in a hurry to change out th eold ones, as they do not 'wear;. When the engine reaches temperature at least 3 times it will tend to make the condensors 'heal' again, IF they are good ones like TEC, Hitachi, FEW, or ND. If they have no ID stamped on them, they are Chinese junk from Daiichi, and then you must find better ones.

[Don R]

 I had a 76F with drilled jets once too, that was rough to find. I got it to run sort of ok, but it was out of gas at 95 miles. Somehow all that gas didn't take out the ring seal, it ran great after that.

[AndyH]

Many thanks for all your suggestions! 
I should have mentioned that all the brass is Keihin.  Mains 115, slow 40.  Also the tank and petcock filter are clean.
I also adjusted the cam chain early on in troubleshooting.
The points plate has "Tec" stamped on it (OEM?) but no markings on the condensers that I can see.
I swapped out sparkplugs for a new set, but no difference.  All plug caps were replaced with NGK caps when I found the faulty #3 cap.
I'll try HondaMan's suggestion with the timing light and report back.

[MauiK3]

Don’t forget condensers

[AndyH]

Many thanks for all your suggestions! 
I should have mentioned that all the brass is Keihin.  Mains 115, slow 40.  Also the tank and petcock filter are clean.
I also adjusted the cam chain early on in troubleshooting.
The points plate has "Tec" stamped on it (OEM?) but no markings on the condensers that I can see.
I swapped out sparkplugs for a new set, but no difference.  All plug caps were replaced with NGK caps when I found the faulty #3 cap.
I'll try HondaMan's suggestion with the timing light and report back.

OK, so with the timing light, the marks are rock steady from idle up to near WOT.
I also found a spare (old) Tec points plate so I swapped the condensors from it over to the bike....no difference in operation. 

I'm starting to suspect the carbs again!

[BenelliSEI]

When you installed the new NGK caps, did you cut off a bit of the plug cable, so the caps are screwing into fresh copper wire? The ends of the wire can deteriorate and cause a poor spark,

[Don R]

 Usually when I've had a spark issue, I could see it in an erratic timing light.

[HondaMan]

Usually when I've had a spark issue, I could see it in an erratic timing light.

This is kind of my thoughts, too...
I'm leaning toward a possible vacuum leak issue, despite having new intake hoses?
A question: when you tighten the hose clamps on those new hoses, do the hose clamps completely close to a fully-tightened screw on each one? Reason I ask: since about 2006 or so, all the hoses being made are about 2mm too small on the OD dimension at the head. Thus, using the standard hose clamps at the head doesn't usually seal the hoses. I struggled a while with this on my own bike when I replaced those hoses, circa 2012. In the end I installed Oetiker's aircraft clamps #050-9 (available at Aircraft Spruce online), although I must add this warning: if you install these, they are aircraft clamps with tremendous torque, and it is possible to cut all the way thru the hose with the clamp (don't ask, I won't tell you how I know this...). So, watch instead for the rubber to show signs of being squeezed firmly. I installed mine sans carbs and tightened them just a little past where the hoses could be rotated on the head by hand, and left them there for 1 year. Then I went another 1/2 turn on each screw.

[AndyH]

Usually when I've had a spark issue, I could see it in an erratic timing light.

This is kind of my thoughts, too...
I'm leaning toward a possible vacuum leak issue, despite having new intake hoses?
A question: when you tighten the hose clamps on those new hoses, do the hose clamps completely close to a fully-tightened screw on each one? Reason I ask: since about 2006 or so, all the hoses being made are about 2mm too small on the OD dimension at the head. Thus, using the standard hose clamps at the head doesn't usually seal the hoses. I struggled a while with this on my own bike when I replaced those hoses, circa 2012. In the end I installed Oetiker's aircraft clamps #050-9 (available at Aircraft Spruce online), although I must add this warning: if you install these, they are aircraft clamps with tremendous torque, and it is possible to cut all the way thru the hose with the clamp (don't ask, I won't tell you how I know this...). So, watch instead for the rubber to show signs of being squeezed firmly. I installed mine sans carbs and tightened them just a little past where the hoses could be rotated on the head by hand, and left them there for 1 year. Then I went another 1/2 turn on each screw.

Hi HondaMan, the clamps don't close right up, but there's not a lot of gap between the nut and screw side of the clamp.
We sprayed carb cleaner (pretty flammable) around both ends of all the rubber hoses but it didn't make any difference to RPM.

Usually when I've had a spark issue, I could see it in an erratic timing light.

The timing light fired on every ignition pulse on all 4 leads Don.

One interesting thing I've noticed is that after revving the motor a few times, the choke lever rises up a little.
Despite that, if I push it fully down again, there's still no real change to the "rough" running.
Also, if I pull the choke lever up fully after the bike's warmed up it won't kill the motor,  just a slight reduction in idle speed.
NOTE:  Correct choke operation was confirmed when the carbs were on the bench.

[PeWe]

Remove airbox to see that choke is OK on all 4.
It might be problem with them, not open as they should. It can happen if problem with linkage or arm to choke hooked off.

[AndyH]

Remove airbox to see that choke is OK on all 4.
It might be problem with them, not open as they should. It can happen if problem with linkage or arm to choke hooked off.

Thanks PeWe, I thought of that.
I'm fortunate enough to get another set of fully cleaned and bench-synced carbs to fit up on Monday or Tuesday as a comparison.
As before, I'll report findings!

[Keith]

I had a similar problem with my K2. The needles were adjusted so they were fully raised. Reset, and switched to ND plugs per Hondaman! Fixed!!

[AndyH]

I had a similar problem with my K2. The needles were adjusted so they were fully raised. Reset, and switched to ND plugs per Hondaman! Fixed!!

Hi Keith, thanks for info.  When you say "..needles were fully raised. Reset..."  what were they reset to?
I finally received the 2nd set of carbs this afternoon (Perth time UTC +8) so will not do anything today.

Cheers
Andy

[PeWe]

Needle setting depends on your exhaust system and filter.

My K2 that has good flowing 4-4, Yamiya no numbers. Copy of HM300.
(As K1 had stock)

- Needles 1 step richer than K2-K6 with stock HM341 4-4.

4th notch from top instead of 3rd (middle).
This with stock airfilter.

Main jet 115
(105 with HM341)
Air screw 1 turn out.

[Keith]

Yes, I put mine in the middle position.

[AndyH]

Needle setting depends on your exhaust system and filter.

My K2 that has good flowing 4-4, Yamiya no numbers. Copy of HM300.
(As K1 had stock)

- Needles 1 step richer than K2-K6 with stock HM341 4-4.

4th notch from top instead of 3rd (middle).
This with stock airfilter.

Main jet 115
(105 with HM341)
Air screw 1 turn out.

Thanks info PeWe!

Yes, I put mine in the middle position.

Thanks Keith.