Need Some Help

[BillinSC]

I just picked up a 1973 CB750 as a project bike. The guy I bought it from was running a Mac 4 into 1 exhaust without the baffle and still running the stock airbox. He said he had it re-jetted and tuned but Im not sure about that (in fact Im kinda certain). The bike idles well and sounds awesome but it has stutter and breaks up as you slowly roll the throttle. The bike had been sitting a while and I got it cheap so I took it home any way thinking it might just be a simple Fuel, spark or Air issue that I could fix. Once I got it home and started looking at it more I found that there was a small fuel leak carb #2 (looked to be from the float bowl gasket and cylinder #3 wasn't firing (#3 exhaust wasn't getting hot). Removed the plug and found there was no spark. Replaced all four plugs and thought I had the issue solved but, while all cylinders are now firing, the stutter/break-up is the same. So next I drained all the gas and started in on the carbs. The gas wasn't terrible (he said it was fresh non-ethanol 91) but I did see some signs of water in the gas when draining the carb bowls. Anyway, got into the carbs and found the following...

1) Three of the four float bowl gaskets were home made crap and the one that was leaking had a tear
2) Overall pretty clean but could use some cleaning
3) Idle and Main jets were clear
4) Floats are in good shape but havent measured the height yet
5) Idles jets look pretty new and are stock #40
6) Main jets are not new and are #110 (which I've read were stock on certain engines depending on serial number which mine is one of)
7) the spark plugs I removed were pretty black and sooty
8) The exhaust pipes for cylinder #1, #3, #4 and blued pretty good indicating a lean condition. #2 is barely discolored

So, I'm a little confused. I was expecting to see at least #115 or #120 main jets if the previous owner actually had it re-jetted and tuned for the Mac exhaust without baffle and the spark plugs are telling me that they're running rich not lean? I also have yet to see/find an obvious smoking gun for the stutter/break up unless it could be bad gas?

Obviously I need to continue cleaning and rebuilding the carbs 'cuz they need it but If anyone has any ideas/suggestions for me regarding other possible causes for the stutter/break-up I'd appreciate some feed back.
Also, what would be the recommended main jet size for this set up (4 into1 exhaust without baffle and stock airbox). I'm thinking that the current #110 are too lean as indicated by the pronounced exhaust pipe blueing but the black/sooty plugs are telling a different story.

Any feedback would be greatly appreciated, Thanks!
Bill

 

[UK K6]

Similar story as mine, except I have stock pipes. I went very carefully through the carbs with cleaner and compressed air in every tube and each circuit in the carb body. I did replace the brass, but went back to the original, as the replacements (Keyster brand) made mine run rich. Their gaskets were fine though. Float heights were all over the place too! Once up and running, I bought a carbtune, to balance all 4. They were out, but I can't say I noticed any difference balanced. But probably better for the longterm health of each cylinder.

The stutter could be needle position, due to the exhaust, but it could also be retarded ignition timing, points gap or a stuck mechanical advance (which provides some advance even at idle, vs. static timing, unless it's stuck or sticky).

Worth a quick compression check, to see if no.2 is similar to the other three. Also no.2 spark plug is the one I find stiffest to screw in on mine and when I bought my bike, I suspect the plug was not fully seated in the head. Replace the plugs, check the leads and caps, even snip off 1/4" from the end of the lead and reattach the cap, to get a nice connection.

Initially on yours, once you get no.2 firing, I'd read the plugs rather than the exhaust header colour, which may have been from how it ran a long time ago. I find mine works best with each carb's idle screw adjusted to the stock position (I think 1 turn out from lightly seated IIRC). It's easy to get them sooty or lean at tickover with very little adjustment from there. As this affects the fuel mixture on throttle transition off idle, idle screw adjustment alone might solve your stutter.

Your rich running might be due to poor condition of the carbs / float heights etc. It may also be a previous owner assuming the needles needed to be raised, when a stock set up would still have worked. I'd get it all cleaned up, tuned and start from the factory settings. One other point, to move the needle needs some care with the little screws for the needle holders in the top of the carb. Get proper fitting (JIS) screwdrivers and don't grip the carb too tightly, or you could put the slider out of shape. One of mine had been damaged.

 

[BillinSC]

Thanks for the feedback and suggestions. I will be cleaning the carbs, replacing some seals and checking/adjusting float height. I also plan to change the main jet to #120 since that seems to be a common suggestion when running higher flow exhaust (#40 idle jets should be fine). After I get that straightened out I'll look at the other issue you mentioned like timing and points.

I also read something that seemed to be a good explanation for having excessive exhaust pipe blueing (lean) and black sooty plugs (rich). Is explained that both lead and rich conditions can exist simultaneously if not tune properly. Could be lean at higher RPMs due to the #110 main jet and too rich at idle due to incorrect mixture screw adjustment. Something else to look at when I get things running again.

 

[barney905]

Billie, something else you might want to consider are your valve guide seals. I have a 750k5 with almost 100k on it. Original owner. I don't ride it much anymore, maybe 100 miles a month. It sat for many years before my son and I got it running again. It kept fouling #1and 2 spark plugs. I finally took the head off and looked at the valve seals. They were hard and brittle and literally fell apart when removing them. After cleaning everything up and a lap job on the valves, it stopped fouling the plugs. Runs pretty good now. I have to remember to not ride it like my Harley. I need to keep the rpms up and not lug it like the other bike. Totally different bikes. My Honda is a member of my family. I toured just about every state west of Nebraska on it. Lots of good memories.

 

[barney905]

Billie, this is my old friend.

 

[Irish Dan]

Billie, this is my old friend.

Beautiful