Topic: Bike is faster at less than full throttle....

[Caferacernoc]

So I am out doing a WOT plug chop and I noticed something strange. When I back the throttle off from wide open I can feel it pick up power around 7/8 to 3/4 throttle. It's a '72 CB750 by the way. Anyway, I eventually found a good stretch of road to do a proper plug chop at WOT where I did not back off before hitting the kill switch and pulling in the clutch and the plugs didn't look particularly rich or lean. Maybe they are lean but certainly not rich. The electrodes looked light tan, the porcelin white, and the threads a little black. Is that a little lean? The bike runs pretty good with some midrange stumble if I give it a fistful plus the newly discovered weirdness of being faster at 7/8th's throttle over wide open.
When doing the plug chop I had fully warmed up the bike and was doing over 100mph after a good 5th gear pull.
Thanks for any info.

[kghost]

Yeah your lean.....

Classic case ...where it runs better at 7/8th then WOT....

Plug porcelin should be tan....

Threads always seem black

[Caferacernoc]

So lean then?
Man, 130 main jets already!

[Johnny5]

What's your intake and exhaust setup?

[Caferacernoc]

The bike has a 4 into 2 system for exhaust. The mufflers look like tapered HD Sportster mufflers. Minimal baffle, really loud. Intake is velocity stacks with the little screens. I've also tried pods. I don't have the factory airbox to try. This is how the bike came. With the 130 main jets, too. First bike I've owned that didn't need lots of TLC right away. And like I said, it does run good. Starts easily, warms up good. Runs strong. It does hesitate in the midrange if you wack the throttle too fast. And now I notice the lack of power at WOT compared to 3/4 or 7/8ths. I've checked for air leaks. Played with air screws, timing, valves have been adjusted.......
If I just chug around town the plugs look spot on and if I richen the idle circuit they will even foul. Plus I was assuming the 130 mains are big enough. But now that I've done a WOT plug chop it seems to me I need bigger main jets or a factory air box.

[Johnny5]

I run 130 mains in my bike and it's perfect at WOT. I run pods, I imagine the velocity stacks are a little less restrictive so maybe 135 mains will works nicely for you. Let me know, I have 4 new main 135 jets I havent used.

[Caferacernoc]

I run 130 mains in my bike and it's perfect at WOT. I run pods, I imagine the velocity stacks are a little less restrictive so maybe 135 mains will works nicely for you. Let me know, I have 4 new main 135 jets I havent used.

Sweet. I'll pm you.

[michrobi]

I swapped my 115 jets out for 120s and produced a similar result except that above 7/8ths it bogged out really bad, like something was really really wrong. You're running 130s now, what was in there originally?. I had to put my 115s back in.

[eurban]

Yeah your lean.....

Classic case ...where it runs better at 7/8th then WOT....

Actually according to Kevin Cameron's sportbike tuning book you are wrong sir . . . "Main jet tuning methodology depends on having some place to run at full throttle, in top gear, long enough to perform a simple engine diagnostic (for slide carburetors), or long enough to color a spark plug (any type of fuel system).  The diagnostic is to slightly close the throttle, and note whether there is a slight gain in maximum revs.  If there is, the engine is rich and should be jetted down."

Mr Caferacernoc, your bike's behavior would seem to indicate that at WOT you are rich, not lean.

[mattcb350f]

That's a lean plug for sure. The white/light tan has crept back all the way on the electrode to the base at the threaded part. The insulator colour suggests lean also.

I'd be real surprised if it turns out to be rich.

 Matt.

[Old Scrambler]

Both cases (lean or rich) may be indicated by your set up. Less than a minute of time on the plugs at WOT may not be showing the true condition. You have plenty of air and all is good at normal riding. I would go down to 125s first.  I had the same problem on an 836 kit with pods and 120s.  I went down to 117s and it ran better. Then installed standard airbox and K&N filter. Now runs sweet in all conditions.

[eurban]

Both cases (lean or rich) may be indicated by your set up. Less than a minute of time on the plugs at WOT may not be showing the true condition. You have plenty of air and all is good at normal riding. I would go down to 125s first.  I had the same problem on an 836 kit with pods and 120s.  I went down to 117s and it ran better. Then installed standard airbox and K&N filter. Now runs sweet in all conditions.

Right, to perform a proper plug chop for MAIN JET selection involves starting with new plugs in a warmed up bike and quickly reaching WOT in top gear.  You will need to maintain this for 20-30 seconds and then release throttle, pull in the clutch and hit the kill switch.  You would then need to look at the carbon ring on the insulator deep down inside the plug with a flashlight and magnifying glass.  How many people actually do this?  Racer's do.  Basically used plugs will not magically clean themselves of over rich deposits or change from looking lean (if for instance the carbs are lean in the throttle positions where the idle circuit or the needle position is dominant.) For proper carb tuning you want to choose your main jet based plug appearance from actual WOT conditions not based on overall riding conditions where you spend almost no time at WOT.  Your new plug after a properly performed plug chop will never resemble a plug in one of those plug color charts. . . . . .If all this sounds like a PIA then you can perform the "diagnostic" previously mentioned where you partially close the throttle from WOT and see if revs are gained.  If they are, like the poster says they are, then guess what.  Main jet is too large!  This says nothing about: whether or not his needle is in the correct position, whether or not his accelerator pump is functioning, the adjustment and cleanliness of his idle circuits, whether air leaks are present, or float levels are set correctly. . . . .

[Caferacernoc]

Thanks for all the info.
I actually have 135's on the way to try. If that makes it worse then I'll know to go the other way.

[Pinhead]

WOT for 20-30 seconds in 5th gear... Does anyone do this? If I did that on my bike I'd be running 100mph plus!

[Caferacernoc]

Yeah. I did. WOT in 5th gear starting at 45mph and held open to the plug chop. Well over 100mph.