Throttle chops, do they have to be done at speed?

[weekendrider]

Do you have to be rolling for a successful plug reading?
Or would running the motor(under a fan) at a certain rpms 1200, 3500 and 5500 for a minute give a reliable plug color reading?
Guess I could just do it and see, lazy and 2 cheap to buy 3 extra sets to compare road versus shop at a given rpm.
And yes at 5500 it would have to be anchored or it would dance out the shop door.
At 3500 simply holding the front bike keeps it from walking backwards.
Not sure if it would shake its butt sideways at 5500.
Surely I'm not the first to think of this. Probably the first to not think it though tho.....

 

[TeeCat]

I would think that having the bike under load at x rpm would be the best indicator of what's truly going on in there. I don't know that mere rpm alone would render a real-world reading. Just my take, though.

TC

 

[grizld1]

In some vacuum carburetors, rpm's need to be up for slides to rise. Apart from those, rpm has nothing to do with tuning a carburetor, with the exception of selecting the main air correction jet in applications that require it; and that has to be done at varying rpm's under full throttle. What signifies in carburetor tuning is throttle position. If you're trying to tune a carburetor by looking at the color of the insulator, you're fooling yourself.

Throttle chops only signify in reading the smoke ring on a fresh plug after a full-throttle run. The smoke ring is a thin ring of soot that forms at the very base of the insulator, and a thickness of .020" is too fat. You can't get meaningful information on the mains from a plug that's seen mixed use. Read Gordon Jennings' article on plug reading at www.strappe.com .

Can you jet a carb decently without getting into 3-digit speeds or dyno runs? Sure you can! Get rolling with revs up around 4K in 2nd or 3rd, and roll the throttle wide open--a fast roll, not a snap. If the engine cuts out sharply or falls on its face, step up the mains until it responds cleanly and then, if you wish, go one size for safety. If you get a soft sputter or weak response, try stepping down the mains until lean symptoms occur, then work back up.

Roll-off tests can refine your results, and these are best done at speed on a grade. With rpms up in the slide-lifting range (well north of 4K) and the throttle wide open, roll off to 3/4 throttle. If the engine accelerates (surges), that's a lean signal. If response is as it should be, roll back up to full throttle, then roll off to 7/8. If the engine sputters a bit, that's an indication that mains are too fat.

In aftermarket carbs, if you get both rich and lean signals thrown at you in roll-offs, it's often an indication that needle taper isn't what it should be; you'll often get those mixed signals running 6DP1's in VM carbies, for example.