Topic: 1976 CB 750K Idle rattle and Idle mixture/rpm question

[RobL]

Hi, I'm working on a friends 76' CB750 ( pretty clean 3000 mile bike,) and have a qouple questions if anyone can help.  He rebuilt the carbs with the "K" brand aftermarket kit, and quickly became unable to tune/synch the carbs to satisfaction.  Long story short, I just went through them, reinstalled the original parts and the now synch is almost close, ( I have some more tuning to do.)  The rod clips are set in the middle setting and the idle air screws are about 3/4 out. 

This is the first bike I've tuned so my learning curve has been fast and steep.  My first problem was the Clymer manual stating the 8" vacuum target when synching, thanks to the sohc4.net search functio, I am no longer fixated on that as a target vacuum.  My second problem is the spark plug coloring at extended idle.  Since my bud didn't mess with the idle air screws, I was able to do some tuning and make some progress beyond the plugs running completely fouled.   Here is the rub, since I don't ride yet, all I can do is tune for acceptable idle and stationary throttle response.  My plan was to get it running well/synched and let him road test and go from there.   I was going though the posts regarding idle/rpm/carb tuning and came across a quote by TwoTired:

"Good write up.  But, if I could, I'd like to offer this little correction.  >Idle Mixture Screws< are adjusted for maximum idle.  >Idle Air Screws<, are not.  The reason is that, lacking an accelerator pump, these early carbs rely on a rich idle mixture for good low speed throttle response under load.  Opening the idle air screws too far will make the engine idle much higher (and leaner), but the engine will cough and wheeze when cracking the throttle open.  The Idle Air screws are only turned out far enouugh to make low speed throttle response under load, tolerable."

Since these carbs don't have accel. pumps, the idle has to be a little on the rich side to transition to load throttle demands.  So, an extened idle will lead to darker plug readings and the need to blip the throttle to clean it out, etc?  Does that sound correct, since I'm new to motorcycle tuning, I don't know if this is normal practice on this vintage bike. 

What is an appropriate idle rpm?  I'm not done tuning yet but, the lowest it seems happy at is about 1300 RPM, does that seem right? 


My other concern is, at idle, the transmission seems to make a hell of a racket, clutch in is a little better, but not much.  Once again, perhaps I'm just not familiar with this vintage of a bike, but it seems a little excessive.  FWIW, the cam tensioner & valves have been adjusted. Are there other "chain" adjustments that I'm missing?

Thanks for any help, and kudos to the moderators here, the site is well organized and I was able to pick up quite a lot of info with the FAQ section, < now someone will quote a fact post that answers my above questions......>

[TwoTired]

So, an extened idle will lead to darker plug readings and the need to blip the throttle to clean it out, etc?  Does that sound correct, since I'm new to motorcycle tuning, I don't know if this is normal practice on this vintage bike. 

No, it shouldn't be that rich.  Well, unless your extended idle is measured in days.    It should idle for hours without fouling plugs.  If you do that, put a big window fan blowing on it to keep it cool.

What is an appropriate idle rpm?  I'm not done tuning yet but, the lowest it seems happy at is about 1300 RPM, does that seem right? 

No. The shop manual says 850-950 RPM.  I might accept 100 RPM more on an old bike.

My other concern is, at idle, the transmission seems to make a hell of a racket, clutch in is a little better, but not much.  Once again, perhaps I'm just not familiar with this vintage of a bike, but it seems a little excessive.  FWIW, the cam tensioner & valves have been adjusted. Are there other "chain" adjustments that I'm missing?

With a good carb synch and all the cylinders firing evenly (full tuneup) the noise should reduce considerably.  If the carbs are fighting for crankshaft RPM dominance, trans gears (always meshed) constantly change their tooth face contact sides. 

Cheers,

[Markcb750]

Deja vue...all over again.


My '76 caused me the same concern.

Clean the carbs.

Much better after sync.  I YouTubed CB750's and listened to an assortment of videos. to convince myself the noise is normal.

If you are accustomed to water cooled engines the noise of the valve train operating on these engines can be disconcerting. 

BTW..Clean the carbs. then clean them again. Lots of good information on the forum about the passages in the low speed circuit. One of us even uses his used guitar strings as a tool to clean passages.

[RobL]

TwoTired, Markcb750, thanks for the great info/tips.  Clearly I have more work to do, but now I have a target rpm and plug condition to shoot for.  A fan is used for idle tuning as well as monitoring cyl head temp. 

BTW, I have cleaned the carbs thoroughly, pressurized shop air through all passages, and run wire through all of the passages permittable. 

I have youtubed as well and most were pretty quiet when compared to mine.  I'll get back on it in a day or too and report back. 

Thanks again.

[RobL]

UPDATE:

After scraping my inconsistent set of mish-mash vacuum gauges, I purchased a Motion Pro Synch-Pro and got back to work.  I spent the greater part of yesterday afternoon twisting and turning carb slides and got it pretty close.  Today, I fired it back up wanted to double check all my previous night's work, however, much to my dismay, the gauge was all over the place.  The first thing that came to mind was the 15 degree temperature difference, so, no big deal, it should be calibrated for a warmer day such as today anyway, so, I started by recalibrating my Sync-Pro,  only to discover what I thought was a problem.

I have a Sun machine with a controlled vacuum output that I've used for calibration, so, I set it at say 5" and equalize the Sync-Pro, then, as I turn up the vacuum to say 6", the manometer goes out of calibration.  I tried this at various settings and always the same result, calibrate it for one vacuum setting, raise the setting and the calibration is kaput.  Knowing that I was the problem and not the tool, I gave Motion Pro a call and talked to a gentleman, ( did not get his name but he was an excellent example of customer service,) who explained the basics of manometer calibration and the need to calibrate everytime it is used, etc.    Although I have never used mercury manometers, he explained the difference and how the overall confusion generally becomes enraged swearing. 

For anyone else who might like to know, ( and if this is not right please correct me,) Just like the instructions say, you need to calibrate this to one carb/cylinder on the bike and also mark where this setting is.  This is where the gauge is calibrated and it will be incorrect if you read the setting anywhere else on the gauge.  This is where I was making my error.

I was a little disappointed that I just spent x amount of dollars to purchase this when I might be more content with a set of calibrated vacuum gauges.   I did choose this gauge due to the same day purchase availability and compact viewing angle of where I'm tuning to, vs. a wide view of multiple vacuum gauges, however, I think that with a little perseverance and witt, I shall tune to success.  I'll get back at it tomorrow and report back.   I'd appreciated and thoughts or comments, thanks.

Rob

[Markcb750]

Bummer.

I would have expected the four tubes to be extremely close to "equal" for any given temperature & humidity and the variance between them when attached to the individual carburetors due only to the vacuum in that carburetor.

[RobL]

Update:

I have become quite comfortable with the idle synch procedure and have had improved driveability from it.  I am still fighting fouled plugs.  According to my bud who originally started working on and continues to road test for me, "it has never run better, it accelerates smoothly, power is good, but after about 15-20 minutes of riding, the idle raises from 1100 rpm to about 2000 rpm."  When I pull the plugs, they are black except for the surface of the center electrode. 

It has a new airfilter, stock air-box, stock exhaust, stock everything. I have polished the float pivot pins/rods so they should not be hanging, they are all set .5mm below spec, ( it has been a couple weeks, I think it was 26mm.)  I also raised the rod clip by one spot, so it is only 1 above stock/middle setting.  The only thing I can think is that these floats are "heavy" and are more soaking than floating.  I do have intermittant fuel spill from the vent tubes, although I have not isolated which cylinder or cylinders it is coming from. 

I believe I am covering the basics as far as understanding carburetor theory and operation, but am inexperienced in condemning "heavy" floats and their ill effects.  Any disagreements or ideas with what I am guessing/experiencing? 

As usual, all help is appreciated. 

Can you even buy new floats for these carbs???

[RobL]

"Bump" for more attention..............

[ev0lution7]

when you do the carbs set everything to spec... check float levels... and if you think they are getting wrong after you install them see how the float in some gas. see if they resist sinking...

check the manual for float levels it is very important!

Ohms check your primary side and secondary side of your coils
also check dwell (very important) you might not be firing off the cycl those 76's run VERY lean check you timming after dwell settings...

it is very odd that a 76 would rich foul a spark plug...