Topic: Static good, strobe show advance

[Heirborn]

I have a '70 cb750. New points plate and good plugs. I set the dwell today and static timing is dead nuts. When i put the strobe on it, it shows to be advanced in front of the F mark. The bike is not running too good obvously. Valves were adjusted today too. Plugs are sooty. Bike pops and stutters under accerlation and idle "sticks". I have not cleaned or rebuilt the carbs and not sure how long it has been since the carbs where gone through.

[TwoTired]

I have a '70 cb750. New points plate and good plugs. I set the dwell today and static timing is dead nuts. When i put the strobe on it, it shows to be advanced in front of the F mark.

The timing should advance as the RPM goes up.  What speed did you measure the timing?  Have you checked the mechanical advance mechanism?  Original springs intact?

The bike is not running too good obvously. Valves were adjusted today too. Plugs are sooty. Bike pops and stutters under accerlation and idle "sticks". I have not cleaned or rebuilt the carbs and not sure how long it has been since the carbs where gone through.

Still have cam chain tension and air filtration to address as part of the tune up.

Where are the air bleed screws set?

When was last carb synch?

Original exhaust and intake components?

Are all the headpipes even in temp?

[Heirborn]

Thought I put that I installed a new filter...must have missed that. New filter installed, cam chain adjuster set, bleed screws down to seat and then backed off one turn (also tried turn and a half), I have never sync the carbs and not sure how long it has been, original airbox, 4 into 2 pipe that is not original, all the head pipes seem to be getting plenty hot. Chokes are opening all the way, slides are free. the carbs were pretty wet when I pulled the airbox off to install the new gasket and filter. 

I was also wondering how hot the engine should get? Is there a "touch" rule of thumb that gives you an idea if it is in range, while out riding?

[Heirborn]

mechanical advancer is intact. not sure how to gauge if the springs are worn out. timing was checked at around 1000 rpm and 2500 to advance.

I am guessing if the springs are worn out then it would allow to advance too much?

[TwoTired]

When i put the strobe on it, it shows to be advanced in front of the F mark. The bike is not running too good obvously.  Plugs are sooty. Bike pops and stutters under accerlation and idle "sticks". I have not cleaned or rebuilt the carbs and not sure how long it has been since the carbs where gone through.

How far off is the timing?  Is it both points with this symptom?  Are they new, or old and crusty?  Crusty points can show a difference between static and dynamic time, by a few degrees.

With the strobe, can you see the advance come in and drop back with RPM changes.  I think it should be all in by 3500.  Does it appear to advance and retard smoothly during the transition?

With the air bleed screws out 1.5 turns, did you notice any performance change?  If you take out the air filter temporarily is there a behavior change?

Is the fuel in the carb bowls clean and clear of debris?

Do you know if the carb float height is correct?  How about floats that have pinholes in them?

Do your carbs have the individual cables (early) or the belcrank on the shaft for synchronization?  I heard the early type did go out of synch more often.  Odd idle behavior is a symptom.

Might have to take the carbs apart, but I think I would try syncing them before resigning to that eventuality. 'Course, I have guages in the garage already.

Don't know about a touch test for temp.  But, if you run it stationary, you should have a fan blowing on the cooling fins at 15 MPH or more.

Cheers,

[6adan]

Just to let you know if you have the original air box DO NOT let raw gas get to it or it will be destroyed. It happened to me. Dannie

[Heirborn]

 
Not sure how many degrees. How can I tell? Points are brand new as are condensors. I replaced the entire plate.
 
advance does come in and drop back. It does seem to retard smoothly.

With the air bleed screws out 1.5 turns, did you notice any performance change?  If you take out the air filter temporarily is there a behavior change?

No change that I could notice at 1.5 turns. It seems to bog out with the filter off.

I have not pulled to bowls off.
 
I have not checked the float height or if they have pin holes. I know that might be a possibility. When I went though the carb on my wifes cl70 there were some holes that i soldered up. I have not checked float height.

 
Mine has the belcrank.

 
I have had a fan running on it while idleing.

[Bodi]

The best way to set ignition timing is to set it at full advance, right on the little line. Let the idle timing be wherever it is with the advance set properly.
99% of the time you're riding it will be at full advance.
If you want to fiddle with a bit more advance to get maximum performance you should definitely do it at full advance, and a dial-back strobe light will be better than guessing how much the mark is ahead of the pointer.

[Heirborn]

The static timing is right on the money. Light comes on right at the mark. When I put the strobe on it the timing is in front of the mark. I am guessing that the spark advancer springs might have lost some of their tension?

[TwoTired]

The static timing is right on the money. Light comes on right at the mark. When I put the strobe on it the timing is in front of the mark. I am guessing that the spark advancer springs might have lost some of their tension?

How far off are we talking in inches if not degrees?  Could the point cam sleeve be worn on it's shaft so that centrifugal force moves it out of axis while running?  Has the advance mechanism mechanism been cleaned and lubed with grease less than 35 years old?

Can you feel any latteral movement of the point cam that would effect points opening?

If timing is smooth and not jumping around, I wouldn't expect that as a cause of your spit/sputter, though.  If you've done all the standard tune tuneup items first, the carb synch would be the next thing to try before going after carb guts.

Process of elimination.  Do you have any compression test numbers to eliminate burned or sticking valve issues?
What is age/mileage of motor?

Cheers,

[Heirborn]

went out this afternoon to monkey around with the bike. Fired it up and took it for a run, seem to run pretty good but not as strong as I thought it should. Noticed the #1 was not as hot as the others. Took it back home an pulled the plug. Intermittent spark. Put a good plug into it and took it for a ride. Runs like a champ.

Heres my thought. The plugs are new but I did run them before I installed the new point, condensors, air filter and set the timing. So I guess it was getting fouled, that coupled with all the idling yesterday trying to get it dialed in.

Took the wife on a long ride and it did great! ONe thing that I did notice is that after riding slow for a while through some neighborhoods it started hesitate a bit again. Once I got back on the road and poured the coal to it, the bike came back to life and ran fine again. Slow speed is ok but the bike does not like too much of it.

Could the slow speeds be causing the plugs to get fouled? Do I need to go to a hotter plug? I live in central texas so heat might be a factor. I also like to push it hard too from time to time. Recommendation? I am running D8 NGK's curretly.

[Heirborn]

Just found the post on adjusting the bleed screws to correct the slow speed fouling. I will try that next time I am out.