Jetting... It's not as simple as a plug chop.

Jonny Appleseed /

A55 70cc airsal

14mm phva 76 main jet

Biturbo

Uni pod

22t rear sprocket

Top speed = 40-42

Read in several places plug chops are unreliable on auto clutch bikes. So why do we keep using them?

How can I jet without using plug chops? I think I may be rich because of awful gas mileage, gas all over the tip of the exhaust silencer, and piss-poor top speed.

How should I test jetting without running plug chops that are clearly unreliable? How would I know of I was lean and in seizure zone ?

I wish there was a more straightforward and reliable way to test jetting on a moped.

Re: Jetting... It's not as simple as a plug chop.

One of the other ways shown in the wiki is "two under 4-stroke", basically go big until it 4-strokes and then back off 2 sizes. Personally, I think it has a lot to do with feeling. Call me crazy but I've gotten "in tune" enough with my bike that I can feel when it is right or not. That and I can tell by feeling the heat coming off the cylinder/head with my hand (not actually touching it) whether it is abnormal or not. Oh, and I can hear changes in the exhaust a bit if it is running a bit rich or lean.

But seriously it has a learning curve to it, like everything else. You won't be a master tuner in a day... I'm still no master but I'm getting there.

Re: Jetting... It's not as simple as a plug chop.

Probably Fred /

Yep it's a feel thing

Re: Jetting... It's not as simple as a plug chop.

Jonny Appleseed Wrote:

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> I wish there was a more straightforward and

> reliable way to test jetting on a moped.

there is.

get a head temp. gauge.

Re: Jetting... It's not as simple as a plug chop.

Jonny Appleseed /

Unfortuneatly head temp gauges aren't 100% accurate either.

Re: Jetting... It's not as simple as a plug chop.

Ken Roff Wrote:

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> Yep it's a feel thing

yep

Re: Jetting... It's not as simple as a plug chop.

classic carl jepsen /

jetting is an instinctive intuition you gotta feel it thing. Like how I can tell from here that your bike is too rich. Its a skill you develop, and by the looks of your setup you are just starting out.

Re: Jetting... It's not as simple as a plug chop.

i like the 4 stroke method. or, if your cheap, or can't wait for shipping, put whatever old jet in there (within reason) and run it w/ no airfilter. do the "indian hands" where you reach down and cover some of the mouth of your carb. if it runs better with part of the carb covered your too lean. or you can just try it with, and without airfliters, then put some tape over your airfilter, see what performs best.

i have never actually "chopped" a plug. but i do check them to see if the color looks good. it's usually just whatever performs best, untill you start getting really big stuff and really fast. then they kinda want to blow themselves up.

and ALWAYS blast a can or two of carb cleaner/starting fluid/whatever on all mating surfaces of new builds a few times. hose it down, fix it up, run it up to temp, cool it off, check torques, repeat. i blew up a gila because i was having so much fun i kinda neglected that part.

temp gauges are good too, and make sure you time it good. the phva carb leads me to think your on a tomos, so get a cheap timing light and do it right. timing lights are awesome. they make your flywheel appear to stop turning while your bike is running. totally bugs me out every time.

Re: Jetting... It's not as simple as a plug chop.

I do the 2 below 4 stroke, it is good insurance against seizing when randomly trying jets. Trouble often with random jets is that a bike will usually go stupid sick fast just before it locks up solid, so you could be down jetting figuring you are going for the gold when you are heading for disaster.

Plug chops will tell you if you are close range or not, even with automatics, check the color and burn pattern on the plug to clue you for jetting close and plug heat range.

Re: Jetting... It's not as simple as a plug chop.

jeff cartwright /

im old school and ive always used plug chops/ partial chops and just learn to read the plug period...it will tell u what going on inside your motor...if you do this as others stated, you will slowly learn the the fine tuning sound of the exhaust, feel of the throttle and the way the bike feels/runs...u cant go wrong... u have to learn someway, somehow to be competent in what your doing... try all the ways mentioned and do what feel right for you.. sometimes learning several ways also makes you a better mechanic as iopposed to one way only...do what comes the easiest/natural for you, not necessary whats joe blow says is best...

82 guru

Re: Jetting... It's not as simple as a plug chop.

Jonny Appleseed /

instead of the "Indian hands" method, could you alternatively just pull the choke?

I took the UNI pod off and have made short runs with no air filter, and the bike runs remarkably better. It's faster, torquey-er and smoother. When I pulled the choke, it slows down leading me to believe that it's not lean and fuel isn't the issue. I'm going to downjet to a 74 and run the UNI and see how it goes.

I'm pretty sure it's rich though. The plug is dark brown and a little wet around the edges, and the tip of my silencer is covered in oil.

With the 78 jet and the UNI it seems to run slightly rough at WOT and cuts back And 4 strokes when I choke it. pretty obvious symptoms of rich mix.

Other then temps, what are obvious symtoms of a lean mixture?

Re: Jetting... It's not as simple as a plug chop.

temp. gauges aren't 100% accurate but they give you a darn good approximation to warn you if things are getting out of control. nothing regarding two-strokes is 100% accurate.

obviously the only way to know whether or not you're bikes running right is to check the plug for that milky brown color we've all come to love... a temp. gauge will let you down-jet with confidence though, because you can easily see if your engine is running significantly hotter - at all throttle ranges.

like the others have said, it will come with time. use every technique you can to make the best educated guesses, and go from there.

Re: Jetting... It's not as simple as a plug chop.

can't always use choke at wot. like bings and stuff.

Re: Jetting... It's not as simple as a plug chop.

You are running rich. Hope you don't have the jet kit you told others to use. Try the stock idle jet and infuser and between 68 and 72 main jet. Good luck and hope you don't seize. 14 mm carb is too low volume induction for a kit. 16 mm and larger work better. I have seen more 14 mm carbs with kits seize than anything else.

Re: Jetting... It's not as simple as a plug chop.

classic carl jepsen /

why would a smaller carb seize a kit? You have probably seen that because experienced people tend to use a larger carb, and they know how to tune. Air/fuel ratio and oil content doesn't change with bigger displacement, with the lower volume of air, it just wouldn't rev as high.

Re: Jetting... It's not as simple as a plug chop.

"why would a smaller carb seize a kit?"

Smaller carbs introduce specific cases where its far easier to seize your engine. "airspeed over main jet port in the carb" and "amount of fuel flowing out of said port" is not a linear relationship. A larger carb will reduce the airspeed, making it far easier to tune over the entire range of air flow rate.

"Air/fuel ratio and oil content doesn't change with bigger displacement, with the lower volume of air, it just wouldn't rev as high."

what you said it true, but it doesnt represent the problem. Carbs are all about flow rate of volumes... (edited)

Re: Jetting... It's not as simple as a plug chop.

Plug color can tell you if you have too much compression or ignition advance too. A plug thats white or grey can indicate this. Too much heat burns the color right off. If you time it right from the outset and check compression with a compression gauge you won't run into this but its good to know when someone asks you to look at their plug color and who hasn't done these steps

Re: Jetting... It's not as simple as a plug chop.

Zeke Rigg /

also are you running oil injection cuz that screws up plug chops like nuts. you will always get a black wet plug with those. plug chops are like cheap rulers they both have a big error margin but if you want to know the general idea they get the job done. a cheep ruler won't mess up by more than an inch and a plug chop will tell you if you are off and in what direction

Re: Jetting... It's not as simple as a plug chop.

Jonny Appleseed /

no oil injection for me :D

Re: Jetting... It's not as simple as a plug chop.

i wish i could have an oil injection switch. like still premix, but pump some extra oil in whilst in power band.

Re: Jetting... It's not as simple as a plug chop.

Jonny Appleseed /

you could probably do that if added a petcock or valve of some sort on the oil line going to the carb. You'd just have to reach down to switch it on or off.

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