Topic: 3/4-full throttle: sputtering and VERY low power

[BlackMax]

Okay, I just got my bike back today, from my mechanic. He was to put my new head on the bike and tune it "to perfection".  

Yea, well, it starts up nicely and sounds good.....revs well, etc.   However, when I jumped on it for a short ride, I cracked open the throttle only to have it basically die, then slightly catch for a second then stumble and sputter.......no power!  It acts a little like a bike would with the choke on.  However, my choke is not on, in fact, if I pull it out, the engine dies.  So, it's not the choke.

I am leaning towards the idea that somehow, the thing is not getting enough air.....but maybe I'm looking in the wrong place.  I guess it could be anything including a clogged petcock screen or something like that.   Maybe I should even fill the tank with fresh gas, but that's a long shot.  What would cause a complete lack of power when under load and with the throttle 3/4 or so, or more open???

Yes, the mechanic should be responsible for making it right.....but that's a whole other story, that I may go into at some point.  I basically got screwed and am trying to cut my losses.  Tip-  NEVER pay someone before the job is 100% done to your satisfaction, no matter how much you think you trust them.    NEVER do it.  

[mlinder]

Firing on all 4?

Whats in your airbox?

[BlackMax]

4x K&Ns

[Really?]

Maybe you are getting too much air and not enough fuel.  Maybe put the stock airbox back on see what happens.

[mlinder]

OK, check to make sure all 4 are firing, and then lets have a look at your plugs.
Did this guy do a carb synch?

[DavePhipps]

3/4 to full throttle would be the main jet. That is if everything else checks out alright.

[bikerbart]

that happened to me recently and it turned out that if I switched to reserve I would suck dirty crap from my flaking tank into the petcock and clog the screen.I fixed it and no problems since.

[BlackMax]

very interesting.  I happen to be running on the reserve setting, since I left the guy very little fuel in the tank.   I am betting it's getting leaned out, most of the time, but a little gets through, thus the "catching" i'm experiencing.   

Let's hope that's it.   Heading down to fill 'er up, and switch back to "on" from "reserve"

[KRONUS0100]

long shot.......compression check.  i just went thru this(seeposts)...and come to find out I had a blown head gasket cause the head bolts were loose and 4 missing.....led to broken rings.turns out my carbs were fine.

[BlackMax]

filled up and switched to "on" from reserve.  It cleared up a LOT; but, it still isn't quite right.  It seems pretty lean above 5000 or so, with a WOT.  It's just not pulling as hard as I'd expect. 

I suspect this is one of the problems with guys (or "mechanics") who tune your bike without ever puting it in gear.  No load is a completely different game than with load.    Great, and I happen to not be the best in the world when it comes to tuning carbs (just something I never jumped into).

On another note, the rings evidently aren't seating yet.  I knew that was going to happen.  Let's hope they find their spot in a few miles. 

[BlackMax]

OK, check to make sure all 4 are firing, and then lets have a look at your plugs.
Did this guy do a carb synch?

Yea, all 4 are firing, at least when there's no load. I'll have to pull the plugs and take a look.  I'd expect them to be white.  But then, the rings also haven't seated yet; so, they could also be a bit fouled....but not enough to cut cylinders out.  There just isn't that much smoke.

[the technological J]

i had a similar problem one of my floats was a hair low and when it ran out of fuel useally trying to push it in fifth it would lose all kinds of horse power.. noob suggestion by the way what is "WOT"..... heres one from left field.. could the timing be set to the wrong mark and as it advances it acually misses around 5000?

[mlinder]

OK, check to make sure all 4 are firing, and then lets have a look at your plugs.
Did this guy do a carb synch?

Yea, all 4 are firing, at least when there's no load. I'll have to pull the plugs and take a look.  I'd expect them to be white.  But then, the rings also haven't seated yet; so, they could also be a bit fouled....but not enough to cut cylinders out.  There just isn't that much smoke.

You can tell the difference between rich condition and burning oil pretty handily. Please go try to flog it, shut it off, and lets have a look at the plugs, wh?

[d_soles]

WOT = Wide Open Throttle

A.K.A. - The slides in the carbs are fully elevated.

Doug

[BlackMax]

OK, check to make sure all 4 are firing, and then lets have a look at your plugs.
Did this guy do a carb synch?

Yea, all 4 are firing, at least when there's no load. I'll have to pull the plugs and take a look.  I'd expect them to be white.  But then, the rings also haven't seated yet; so, they could also be a bit fouled....but not enough to cut cylinders out.  There just isn't that much smoke.

You can tell the difference between rich condition and burning oil pretty handily. Please go try to flog it, shut it off, and lets have a look at the plugs, wh?

It's still doing it.  Starts up instantly; idles nicely; revs like a champ; sounds good....ON THE KICKSTAND.   Then, I take it out for a ride, and give it gas.  It's still dogging and missing all over the place when you hit the throttle. 

PLUGS: are dry and the color of charcoal....so, incomplete or improper burning of fuel (rich).   I immediately go to the Main Jet or main air bleed, but......what about starting simpler.   The wires are good I think.  The plugs are new.  What about the coil??  Wouldn't a bad coil produce a good idle and all would seem fine on the kickstand (rev like a champ, etc.); but, under load, as when actually driving, all would fall apart?  Maybe I swap out the coil first?  (also- the points are new and were just adjusted). 

Just a thought....

[HondaMan]

I vote for the air bleed to the emulsifier tubes being partially blocked, if the carbs sat more than a year. I see this one a lot: dark, sooty plugs, good low RPM operation, gargles on too much fuel above 1/2 throttle.


Obviously, your mechanic is like the modern mechanics I'm seeing in this area: I've received 2 SOHC bikes now that were worked on by them, only to come out running so badly that they needed a tuneup. And almost everything was adjusted wrong. It makes me fear for those who have to take their bikes to them!

[Johnie]

Just to back up a bit to mlinders post, if it runs nice at idle is there anyway you can check the carb sync on it?  If those slides are way off you could be experiencing this condition.  You could have 1 slide way out of wack here.

[BlackMax]

I vote for the air bleed to the emulsifier tubes being partially blocked, if the carbs sat more than a year. I see this one a lot: dark, sooty plugs, good low RPM operation, gargles on too much fuel above 1/2 throttle.


Obviously, your mechanic is like the modern mechanics I'm seeing in this area: I've received 2 SOHC bikes now that were worked on by them, only to come out running so badly that they needed a tuneup. And almost everything was adjusted wrong. It makes me fear for those who have to take their bikes to them!

Yea, I was afraid of that.  It sat for a loooooong time before I bought it.  I'm so p*ssed I actually paid a lot of money to have this thing "fine tuned" TWICE!  The first time, when the bike came back, I found the compression was 0 on #4, but the mechanic didn't mention that.  He said he had checked the compression (with his finger) and declared "they are all good."  Turned out the head was basically junk.    Now I have the bike back again with new head and cam and another "fine tune".   Oh brother......    

Now, I'm a rookie with the carbs and cringe at the idea of working on 4 of them.  Any suggestions?  Anyone wish to coach me through checking and correcting a clogged main air bleed to emulsifier tube??    

[JS550]

 Maybe your main jets are clogged? If you had crud in the tank I would bet its in the mains now. Does it run better with the choke on? If so clean the mains.

[Spanner 1]

Your petcock is blocked with rust and can't supply enough fuel at WOT to keep the bowls filled...need to clean the tank ( for rust ) and take the petcock apart and clean...IMO.

[BlackMax]

Your petcock is blocked with rust and can't supply enough fuel at WOT to keep the bowls filled...need to clean the tank ( for rust ) and take the petcock apart and clean...IMO.

Another good thought...start simple and work to complex

I did check the tank. It looks pretty good, except for a few rust colored spots, here and there.  Mainly though, it looks shiny.  Also, the petcock screen or whatever you call it (the tall cone-shaped thing IN the tank), is white and looks pretty good.  Still, I'll yank the tank and see if there is any blockage from the top of the carb up to the fuel source.

 If I crack the throttle to WOT, while in neutral, it will stall the engine instantly.  I'm still leaning towards it being clogged air bleeds.....but looking for simple first.  What about timing? ....doesn't seem likely though.   

[Johnie]

Yah, it is a pain in the butt, you have to just keep checking stuff and eliminating things until you find it.  That is the fun with these bikes.  Must have been a bear to do this without the computer community in the early years.

[Spanner 1]

Hold on, we don't know what bike !!!
What bike is it ?...if PD carbs and you crack the throttle and it dies......then it's likely the accelerator pump not working, usually the carb throat nozzles blocked.....

[bender01]

"Must have been a bear to do this without the computer community in the early years."
  I threw away a beautiful cb360 in 1997 Because it wouldnt start!
 I did sell the title for a buck. Which was the same amount of money I had in it.

[BlackMax]

Hold on, we don't know what bike !!!
What bike is it ?...if PD carbs and you crack the throttle and it dies......then it's likely the accelerator pump not working, usually the carb throat nozzles blocked.....

It's a '78 supersport with 150# compression in all cylinders, new head and cam, new points and "tune" (but obviously not really).  The carb. is from who knows what.  I think it's off an auto.

Yes, when I quickly go from idle to WOT, it will instantly die as if I turned the key off.  But also, it has zero power above maybe 3500 rpm....it just won't do much under load.  

I came across another guy, on another site who seemed to have the IDENTICAL same problem.  He said he fixed it by:  

1) accel. pump springs replaced with stiffer ones
2) he said all the needle clips were on the highest setting (leanest).  He said he lowered them.    Now, he claims all is well.   I'm trying to get more info.  

I'll probably replace the coil first, just for kicks.  If it's still a problem, I guess I move to yanking the carb.   Ain't it great!? I just paid a guy $300 to tune the bike "to perfection".  Now, i have to tune the bike myself.  AWESOME!