How much does the fuel screw impact jetting on a Viper?

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dawolf

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In the hope of understanding cause and effect of jetting a bit better I'm posing this question.

I have a Viper ER with the Hauck CPR pipes. Last year it was rich with no power. As this is a newer sled to me I ventured into the carbs this year to see what everything was set at (wouldn't have tried it without this forum) I found 150 mains, 50 pilots, the needle at 3rd pos. with both clips on top and the fuel screw at least 3 turns out.

I've seen the post by Mr Viper (thanks for all of the knowledge you share) regarding a good setup for these pipes being 160, 160, 162.5, 47.5 to 50 on the pilots and needle in the 4th pos. with fuel screws at 1.5.

My question is, would having the fuel screws out 3+ turns really have produced the rich condition I experienced even with the jets undersized?

I'm trying to learn how much of an impact the fuel screw has on tuning. After I read all the posts regarding this engine pipe setup I'm thankful I didn't burn up the motor with the jet sizes I have. Did that super rich fuel screw really save my bacon or is something else possibly at play here?

Thanks!!!
 

It is vented to the box and the box has not been modified.

Got another question while I've got everyone. When I'm cleaning the carbs and I shoot carb cleaner into the pilot, how much of a stream should blow out of the front of the carb? I'm getting dribble right now, which I believe means the passage isn't clean.

Many thanks .
 
you should get a nice fine spray out of the tiny hole in the bottom of the carb after the slide. the front of the carb is the airjet, you will get carb cleaner out of there too when you spray carb cleaner into the pilot circuit. You should be getting more than dribbles. Are you doing this with the pilots still installed?
 
I've pulled all of the jets and am getting ready to install the suggested 160s and 162.5 mag

When I pulled the carbs the jets themselves were open and clear, but I'm guessing I've got a bunch of gunk inside the carb itself. I'll keep hitting it with the cleaner.

When I pulled the carbs I was expecting to find like 180s in there based on how rich everything was last year. But to find 150s I was surprised.

The fuel screw was the real screwy thing it was at least 3 turns out from seated. Maybe this sled had been set up for grass drags or something before I purchased it?

I just want it to get ripping and be somewhat reliable. Seems I spent most of the short season we had on the side of the trail or eating snow dust from my brothers ski don't 800. Don't want a repeat.
 
What Staggs is telling you is that the air screw only affects idle and just off idle running so it's not really a problem except at low speeds. The way it works is low speed is controlled by the fuel screw and pilot circuit, medium speeds are controlled by a combination of the needle and main jets, wot is the main jet only as the needles are out of the way. If your fuel screw was at 3 turns it might be rich at idle, but if your mains were the right size it should clear up as you get underway. Hope that helps some.
 
When u say it was rich, what did the plugs look like. ?? Were u fouling plugs?? With 150's on a piped viper, u should be lean. Fuel screws the performance after u get rolling. Maybe u have some weak spark plug boots causing issues. I would think with 150's vented to air box, u should have burned it up. As vents going to air box will lean it even more. Any pics of plugs or jet wash. Hows the compression. ?? Depending on your brother sled, u may have a tough time with him even in top running condition.
 
I didn't foul any plugs but they were black. I haven't tested compression but will once it's back together. It would backfire at idle not loud more of a quieef. I checked the vforce reeds and there is no damage. When it was wot it seemed it was just making a lot of noise and flat.

I've got all the bigger jets ordered and moved the needles to the 4 position. I'm going to throw everything together and run it on the grass one of these cold nights and see what I get.

I agree I don't know how the thing didn't burn down. I was thinking the rich fuel screw settings saved it, but it doesn't sound the case. The power valves were so dirty they were stuck and I barely got them out. Maybe that had a part?

All this is learning to me and I really appreciate you guys taking the time.
 
northern srx said:
The pilot circuit provides 3 - 5 % of the total fuel flow regardless of throttle position.
Click to expand...

agreed, but fuel screws 3 turns out isnt going to make a big enough difference to make it rich on top with those mains.

Sounds like you a few issues adding up here. if your powervalves were stuck that would definately make it flat on top. Get those carbs squeaky clean and the right jetting in it. Clean adjust your powervalves, and check the pug caps and compression like ottawaair suggested. If those check out see how she goes and report back. One other thing, what are you running for clutching and what rpm were you at before. Those pipes make power in the 9100 range. If you werent pulling those rpms it would have certainly been flat.
 
Early in the season I was able to hit 9100+ (It seems it has some Hauck clutching parts in it but I'm not sure exactly what) and it was fairly quick. I was happy with the thing the first time I rode it, compared to the SXr 600 I had previously.

It sat for about a month before I got to ride it again (Jan 2012). When I took it out the second time it was flat etc. would only pull about 8700-8800. Do carbs gum up that quickly?

I'm taking everyones advice and setting things with the recommended jets etc, clean the heck out of everything etc.

I'm still just scratching my head on how I didn't melt this thing earlier in the year with the 150s and the needles in the 3rd pos. I even had it at Rich's yamaha in Lockport IL at the beginning of the season to go through it and they didn't say a word about the jetting. They probably didn't look.

I guess I should just be thankful and drive on.

New jets etc. should be here today or tomorrow. I'll get everything put back together, let it rip, and report back.

Thanks guys
 
Sounds like most of the issue was in the powervalves. Wont pull up top without those open. If the plugs were black, then I'm not sure u should jump up 10 sizes. I will have to look at my notes on my buds piped viper. not sure what mains i put in there. I will look and give you a baseline from his machine. This one definately runs good.
 
I have 162.5's in my piped/ported Viper so you probably dont need that big with stock cyls but it wont hurt to start high and work your way down (other than the cost of jets)
 
Just throwing this out there as I havent and dont own a viper. Seems I read on here that some guys were backing the screws out 2-2 1/2- 3 turns to resolve a idle hang issue rather than moving needle/clip positions.
 
New jets showed up today . I noticed the new jets have an 8mm head and the 150s that came with the sled have 6mm heads. N100 vs. n102 jet.
 
No they are the rounds. Mikuni makes two styles of the round n100 and n102. The bigger headed ones I think are for use on the 33 carbs the viper has. I found the small headed ones in the sled though
 
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Hmmmm. Now I'm super confused. I ordered jets from two different suppliers, told them what the sled was and what carbs it had and they both sent the big heads. The Mikuni parts diagram shows the big head main jets as well.

I put the bigger headed jets in the carb and they fit fine. According to the website the only difference between the two is the bigger head. The jet opening would be the same comparing 150 to 150 given they are both Mikuni brand jets.

Thanks for the info.
 
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