The importance of jet size.


Active Member
Local time
11:07 AM
Nov 6, 2007
Southeast Pa
Hello all.
I have been playing with a small issue with the 22mm carb on my Whizzer. I have (had) a little stumble just off the idle when going for moderate or strong acceleration. Not much of a stumble but very consistent. After it would get past the little stumble it would just flat out run so I figured it was just a matter of getting the correct jet size and setting the needle.
The original hi-speed jet was way to big. A #95 I think.
Now you may be thinking why is he playing with the hi-speed jet if it has a low speed stumble? Good question. Well to make a long story short the pilot jet, the needle, and hi-speed jet all effect each other to a certain degree. I have played with the air mixture setting, float level, and anything else I could think of but I could not make the stumble go away. Quenton has been a great help with his info and a selection of hi-speed jets to nail down the problem. I found as I went a little smaller with each jet the problem was getting a little better. The reason I started with the hi-speed jet is because it was running very rich almost to the point of it acting like the choke was on even when it was not. I didn't even need the choke to start it when it was cold.
I ended up going from the #95 jet down to a #88 jet. It does not sound like much but of a decrease in size but what a difference it has made with the Whizzer. The idle is perfect and the stumble is 99.9% gone too. Every once in a great while it will hic-up but not enough for me to worry about it. The performance now with the correct jet is just all I could hope for with such a small engine. I got a chance again last evening to get out and ride. Wow. I can make the belts squeal even though they are new and tight. The power comes on and it just keeps wanting to go faster and faster. I am very happy with the way it runs now. A big thanks goes to Quenton and all his modified parts. I don't have all the possible mods but I have enough to make me smile :D As soon as I loose some more weight I'll be able get the most out of this bike.
The moral of this story is make sure you have the right jets in your carb. It will make a night and day difference in the way it runs and if you are running too lean, you will damage your engine. Being too rich can be bad too because of all the raw fuel. It can wash the oil off the cylinder wall causing excessive wear to the rings and cylinder.
Well that's it. Good luck everyone and ride safe.

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Hi Jim - when you say "stumble just off the idle when going for moderate or strong acceleration" - can you describe that stumble feeling? Is it sort of like it bogs down and loses power for a few seconds...not all of the power...but very sluggish until it picks up and then it's ok?
In my case it was sort of like a cough and spit. No power to speak of at all until it stopped coughing. If I would hold the throttle there, it would keep doing it until I moved on up with the throttle.

Hi Jim,

Thanks for sharing the important information about jetting. Often the main jet is too large, and is difficult to troubleshoot. And also thanks for pointing out that too rich can be as damaging as too lean in some cases.

Have fun,
I am going to check my main jet size tomorrow on the 22 mm carb on my 07 NE5. I purchased that 26mm carb - but since it isn't the one that Whizzer was using I am not going put it on my Whizzer - but I did take it apart to get more familiar with how the carb is put together and works - I learn very well by reverse engineering.

The main jet in this 26mm carb is 88 - I thought it was interesting that the larger carb has a smaller jet than the 22mm carb....
That 88 may work for you if the jet in the 22mm carb is too large. You may also want to look at the needle on the slide. Usually it works out to be set in the second notch from the top. That is what works for me now. It's nice to have something to practice on when your unsure of yourself.
Good luck
I am glad the long weekend is here - but we are getting thunderstorms for the next week - and smack dab in the middle of tornado season here in Kansas - so I will get some garage time in to tear into the carb.

One thing I did was adjust the air mix so that it isn't running as rich - then my idle was crapping out on me - so I adjusted that as well - now she runs much better.

Funny thing, gas hit $3.90 here today - and one of my co-workers who drives a Yukon Denali was complaining about how much she now spends a week on gas - I pointed out the window to my Whizzer and said, "Yeah, these gas prices are awful. I am so glad I am getting over 100 MPG" Her jaw dropped....

I <3 my Whizzer!
I just wanted to share some of my findings/problems.

I am at about 2300 ft and my whizzer seemed to need the 92 main to idle.
I could not make a smaller jet work at all no mater what. My performance was poor and it would not go over 3000 rpm it seemed to bog from 2000 up. The idle mixture screw was 1/4 or less turned out and any more than that and it would die at less than 1500 rpm. My engine ran worse with every time I made a change to a smaller jet size. It idled better with a 95 but blew heavy black smoke above 1500 RPM. I put in an 88 closed the mixture screw set it at 1200RPM and rode it to work. I could have sworn that I could here popping in the carb under load. On my lunch break at work I re-jetted back to 92 to go home with no improvement in how it ran. I had close to 300 miles and yes I had adjusted the valves at 50 & 100 miles as recommended.

I finally figured out that I had the #30 or #32 idle jet and not the #35 I should have had by trying a known good running carb on mine then putting that idle jet in my carb proving that it was a wrong unmarked jet (possibly a 30 or 32). I ordered the correct #35 idle jet and put it in.

Now I was doing fine or so I thought. I started to do the jetting procedures and by the time I got down to the 84 jet and had put the c-clip at the top I was getting a slight popping noise under load again but louder. Going back up in jet size did not help to return it back to how it was running before the last jet change. I got it fully warmed up again however and it seemed to quit popping through the carb and the idle was perfect @ 1-1/2 turns out with a smooth 1100 RPM.

Off to work I went but by the time I got to work it was popping through the carb and would not go over 2000 rpm. I was sure it must be fuel starvation. I checked everything but there was nothing anywhere in the fuel system wrong. I was told to check the cam gear to see if it had slipped but it was fine. I was lost. I rode it to work again thinking that I would diagnose it on the fly. It is 14 miles one way but after about 12 miles it would just bog, slow down and stall. Sitting for 10 minutes was the only way it would start and run again but not very well. I made it to work and was more puzzled than ever now. It felt like a fuel problem but there was nothing wrong with the fuel system at all.

Time to make the trip back home but now it would not start. It had little or no change in pedal resistance now with the compression release on or off. Was it a slipping one way clutch bearing? NO. Slipping belt? NO. I pulled the spark plug and found little or no air coming out as I pedaled. Now out of desperation I pulled the breather cover expecting to find the valves not lifting because the cam was not turning. To my surprise the exhaust valve gap looked to be almost .50 and the intake valve was close to .20 or more.

I adjusted the valves but found that the washer for the compression release would not allow the breather cover to go on without lifting the exhaust valve up to clear it so it would fit on. Baffled now I Called my wife to come get me in the truck. The next day I found the lifters had been hammered into a v shape at the base and were much shorter than they were when new. After a very long delivery delay I got my mushroom lifters and sent them to Quenton for reworking. I cleaned all the silver colored metal sludge out of the bottom of the engine case and put the mushroom lifters in. I am now a happy camper as far as how it runs now that is. I'm still working the other bugs out and finalizing the jetting. I was at a #84 jet with the needle clip at the top but still had a dark brown plug. Now I'm down to a #82 and the needle in the middle but have yet to test it at that setting. I have almost 600 miles on it now and can honestly say the motor runs very well.

My point in posting this is to demonstrate that a cut and dry lean engine symptom of popping back through the carburetor under load may in fact be the exhaust valve not opening due to lifter wear because the lifter failed to get hardening treatment when manufactured. So folks be sure to get your mushroom lifters and put them in ASAP. Put in those lifters first and be sure everything is properly adjusted before you begin jetting your carburetor or making any final adjustments. Do not trust that your parts just because they are new are correct size, application, fit etc.

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