Hey there all...

I have a 2005 Vinson auto that I installed a Dynojet jet kit (stage 1) per their instructions (recommended main, clip on 3rd position, fuel screw 3 turns out) and it has absolutely no power; would be lucky if I could hit 30 mph. It starts and idles good without hesitation off idle with that set up, but has no top end/power. I have tried changing out the mains (both increasing and decreasing the size), adjusting the needle clip setting, and nothing has seemed to resolve the issue. I tried returning it all back to pretty much stock (original needle and main and adjusted the fuel screw from 1.5-2 turns out, with #38 pilot), and that resulted in an idle issue and off idle stumble. I have tried many different configurations with this jet kit, and nothing seems to work, so needless to say I am getting a little frustrated.

The only modifications/changes to the ATV are that I have removed the spark arrestor, installed #38 pilot and has a different air filter (not a K&N or high flow filter). The ATV has low miles, clutches are clean, carb is clean, and ran great prior to this 'tweaking'. I live in SE MN, so no elevation concerns here or where I ride. Looking for any recommendations you guys may have on how to remedy this issue.

Thanks...

 

How did it run before the Dynajet kit? Place it all back to stock ( not "mostly" ) but leave the #38 pilot. Adjust the mixture out 1 to 1.5 turns from seat...try it and...are you sure you don't have the starter jet ( choke jet ) and the main jet installed backwards....in the wrong places? They are the same threads ( and about the same size OD ) if I remember correctly.

 

It ran well before, just looking at installing the jet kit to get a bit more out of it. I am fairly certain I don't these switch, but will double check. I will return everything back to stock settings, keep the #38 pilot in, adjust the fuel screw and see what happens.

I do have a message into Dynojet regarding my issues to see if they have any recommendations.

 

Yep, go back stock and get back with us. And hey, check the diaphragm for any holes/tears/is seated correctly and ensure the jet needle, and holder go back in correctly and the slide is moving freely. I mentioned keeping the #38 because the stock #35 is a bit lean. Be ready to adjust the mixture screw ( leaner ) after the bike is fully warmed up. May only need about 1 to 1.5 turns out from seated .....might be less, just have to see. Good luck and keep us posted.

 

Think about installing a wideband O2. I finally bit the bullet and installed an AEM set up. What it taught me: I don't know squat about carb tuning. My Arctic Cat is running very strong with a Keihin 38 pilot jet and Keihin 155 main jet and Dynojet needle and spring set to Dynojet's advice. Surprise, I was worried it was running lean, nope A/F at light throttle is around 11.5/1 and even wot at 50 mph is 11.9-12/1. Now I'm anxious to really fine tune this beauty for max efficiency/power. Then I will put it on a Mustang Dyno.

 

IMO, it takes more than jetting to make things go faster (like the response above).
If you want "a bit more", I'd start with a aftermarket pipe. Then you may be able to utilize the jet kit.

 

Put everything back to stock (main and needle), kept the #38 pilot in, and have the fuel screw about 1/2 turn out from seated, and it is better, but still does not have the power it should. I don't want to go much more in on the fuel screw, do I? I wonder if the slide diaphram is not getting fully seated, as it is a bit of a pain. Do you think that is the issue? Any tricks on working it around in the seat when installing the cap?

 

Adjust the mixture screw for max, warmed up idle. Where is the power loss...bottom, mid, top? If you don't think the diaphragm is seated correctly, you can:
- place it in the freezer for a couple of hours...the temps will cause it to
shrink a bit or
- apply a little grease all the around the groove in which the diaphragm
sits in...the grease will help hold it in place...

 

The loss is on the top end. With the fuel screw, I went from 1 turn out to 2 and no big change to get the top end back. I did use a little grease to seat the diaphragm, which I believe did the trick there, but still no top end. I am wondering if the diaphragm has a very small tear/hole in it...? I will need to look closer at that.

 

More than likely the diaphragm does not have a hole it....you would feel it any time you nailed the throttle...it would pop and probably backfire. The main jet controls the top end speed. With the stock main ( and jet needle ) now installed...you should have the same performance you had before installing the Dynojet kit. Have you done anything else...like intalled larger tires or anything? Added any weight of any kind? Good charge on the battery?