Topic: HUGE!! Flat spot when accel hard from idle

[Quail "Owner of the comfortable k8"]

Note the following was done for the fist time by ME correctly/without error.

I checked the cam tensioner it was still tight from an adjustment 100 miles ago.
I set the valves. the intakes were worn/misadjusted to about 5 and the exhaust were at 4. now 2 and 3.
The timing was advanced 10 degrees???  I set it on the f.
The points were at 12.  I set at 14.  (could not use my dwell meter it was at work)
Plugs gaped and replaced

Now it feels like it almost has a "comes and goes" miss at 2000, jerky accel (lean type jerky)  at 3000 it runs smooth.  It I hit the throttle hard at 1000 it falls on its face until I release the throttle and accel slower.  I know it would run better if I bumped the timing back to where it was but my question is:  could advancing the timing cover up a carb problem?  #1 pipe is a little blue at the header. It has not been smooth below 2700 rpms in the 300 mile I put on it this year. 3000 and up it runs like a raped sheep, or so I have been told. 

If there are any PETA people out there I would not rape a sheep. 

[74cb750]

Raping sheep is approved in the Kingdom of Vermont as long as you have paid for your permit.

[Bulldawg750]

I am a rookie/virgin SOHC 4 owner.  I have a 1972 CB 750.  My father in law is selling it to me, but is trying to get it running perfect beforehand.  When I read your comment, I was intrigued because we have had the same lag in the RPMs when going from idle to higher acceleration.  It just dies off and has a harsh lag.  We have been to three different mechanics and have rebuilt the carbs, bought new (rebuilt carbs), had them tuned and re-tuned, had the rings looked at, and now have been told that the engine won't accelerate like we want it to, if the battery is weak.  Give me some more insight, because it is 75 degrees here in Idaho and I am itching to ride.

[Harry]

At low revs your idle circuit is contributing significantly to overall fuel:air supply. Heres something I picked up from the net:

"Coughing when you go to full throttle fast, after a prolonged idle, means your idle mixture is too rich. Fuel builds up, and the engine is trying to clear
it out. Dieing clean, with no hesitation is a sign of being too lean. If you are way too rich, it can act like it's too lean, so the problem gets harder to diagnose. Look for puffs of smoke, raw fuel out of the muffler as other idnicators of being too rich at idle. Here's the pro way to set your engine:

At idle, use your fingers, a forceps, a needle nose or whatever is safest to pinch the fuel line at the carb. If it dies really fast, you're too lean. If it increases RPM, you're too rich. The goal - do the pinch, and get little or no change in RPM, yet the engine stays running for a few seconds. If you are
rich, and fix it, you may find that you have to reduce the idle speed. Redo the pinch test, again. The goal - lowest possible idle speed, with the perfect mixture.

Do the fast open from idle test as a final check. Response should be immediate."

[TwoTired]

At idle, use your fingers, a forceps, a needle nose or whatever is safest to pinch the fuel line at the carb. If it dies really fast, you're too lean. If it increases RPM, you're too rich. The goal - do the pinch, and get little or no change in RPM, yet the engine stays running for a few seconds. If you are
rich, and fix it, you may find that you have to reduce the idle speed. Redo the pinch test, again. The goal - lowest possible idle speed, with the perfect mixture.

Do the fast open from idle test as a final check. Response should be immediate."

I do the pinch test on my RC engines all the time for adjustment and it works very well.  But, those engines don't have float bowls, with a reservoir of fuel right at the carb. (good thing, they spend half their time upside down)

Seems like, on an SOHC4, you'd have to pinch the feed tube long enough for the bowl to empty to a level where the slow jet finally starved, 1/2 to 2/3 down into the carb bowl reservoir.  This could take quite a while at idle speeds.

Further, fuel requirements differ when the engine is under load and making power.  With the model airplanes there is the propellor supplying a static load, even though the plane isn't moving.  With a motorcycle, acceleration is the load.  Let's see, "while pinching the tube accelerate from idle at the point where idle jet is starving for fuel..."   I question the practicality of this test procedure...

I've actually done this with a 74 CB550:
With the bike on the test stand, and an exhaust gas analyser probe stuffed up it's...er, tail pipe,  I adjusted the idle air bleed screws for minimum hydrocarbons.   (Yes, I have that test instrument.)  This is a good, efficient mixture setting...for idle.  Throttle pickup was dismal, even on the centerstand.  There wasn't enough fuel to overcome the extra load of simply increasing the engine speed.  So, I adjusted the air bleeds in to the point where I got good engine pickup.  But, it was still a much lower hydrocarbon reading than when the screws were at factory settings.  Time for a test ride.  I don't think I made it half a block before turning back.  There was nearly zero acceleration power.  So, I screwed in the air bleeds a little more, ride test, repeat, ride test, etc.  Each time acceleration improved, until I had a very good acceleration from idle in any gear.  I checked the air screw settings.  And, lo and behold, they were at Honda shop manual settings.  A revelation, the Honda engineers had it right!  Who knew?  Well, this WAS a stock bike, after all.

Now, the idle acceleration test is how I find the idle air screw settings on bikes with modified exhaust and air filters.  Settings on these bikes DO vary from book values.

Cheers,

[migopod]

i've been fighting the same issue for a few weeks here.  It idles great and runs well at higher RPMs, but right between
the idle and main it just goes flat.  It's not exactly falling flat on its face since i backed the needles out a notch, but it's
really lagging right at the low-mid throttle position.  I'm in the process of dipping the carbs, replacing the intake manifold
o-rings and double-checking the float height, so i'll probably not know until tonight (crossed fingers) if it's improved at all,
but if you get yours cured i'd really love to hear what fixed it!

-migo

[eldar]

Well the pinch test my work on the sohc but with R/C engines that are nitro based, when you pinch the line, as fuel is cut off, the RPMs climb rapidly before the engine dies. This is a common way of shutting down these engines. So I dont know about that method.

As harry said though, it is going to be with the idle circuit more than likely. Since you said that #1 header turns blue, that is a sign of high heat caused usually by running too lean. The leaner you run, the more heat that is generated. I cant give you a proper running heat range but I would say to possibly richen your idle mix. Do what tired says, turn your screws a bit and ride, and just experiment. I would say start with quarter turn on 2 - 4 and half on 1 since it is getting blued more than the others it sounds like.

[057 adnoh]

 check to make sure your advance is working . hook up a timing light and check to see if you are getting full advance and also that it is returning to F when the revs drop. you can take out your advance unit and check its operation. sometimes the unit will stick.

[migopod]

I just got done with dipping and reassembly, but it got late and dark and i didn't get to completely reinstall
tonight.  One thing that I found that was rather interesting has to do with the little brass plugs that are pressed
into the inside parts.  It looks like the carbs are all probably cast from one mold with different machining and
fittings depending on which carb it's supposed to be.  The fuel passage that has the threaded hole for the pilot jet
has a plug at either end to seal the ends of the drilled shaft (i presume) and force the brunt of the vacuum
to the pilot jet when idling.  Somewhere along the line carb #2 lost the brass plug that is on the downhill end
of the shaft. 
It seems to me that this would have caused at least some really interesting conditions in that particular carb,
and i sincerely hope that:
 a) plugging it with some bronze rod and a gentle hammering ought to do something.
and
b) that it really was supposed to be plugged and the something that it will do is a good something.

Any good theories?

Also, any advice on how to get my hands to quit smelling like carb dip (except to say that gloves would have
been wise)?

-migo

[eldar]

Well you could rub fish on your hands. I think that overpowers anything! Except maybe skunk.

[Quail "Owner of the comfortable k8"]

Also, any advice on how to get my hands to quit smelling like carb dip (except to say that gloves would have
been wise)?

-migo

Boy this could turn into a funny thread now.  Let me get this straight the new question is how to get that smell off your hands.  The best part is fish entered the subject and with a straight face!

[migopod]

i was going to try the fish suggestion, but the little devils are too quick.  Fortunately i think that i've exfoliated enough that the smell has gone
away on its own.  Now they just smell like rubbing compound.