Idle and Air Screw Adjustment

Lonewulf71 · Jun 14, 2011

The 86 I bought is a 265 and has a bored out Keihin carb, however I don't know for sure what size the carb was to begin with. I was just wondering if there's any difference or anything different to take into consideration when adjusting the air screw and the idle speed. It's runs fine, but we couldn't really get it to idle, and it was stalling when I was going slow. We played around with it a bit, and it's not stalling but it still won't idle for very long just sitting in neutral. We didn't mess around too much because it's got a rather nasty looking foam filter on it. I'm going to put a K & N on it before we work on adjusting it again.

This is probably a dumb question, but what's the purpose of boring the carb? Is it just a cheaper way to get a bigger carb? Also is there a way to identify what size the carb was to begin with? Also we did notice that the spring on the air screw was heavier and maybe a little longer than the one on my 88.

acecarlos · Jun 14, 2011

There are a series of numbers on the side of the carb, next to the float…tell me the numbers and I will tell you what it started out as. If it bored out to a 40.5MM, it likely started life as a 38MM or 39MM…

The purpose of boring a carb to a larger intake diameter is to increase air flow, on larger displacement engines, modified 250CC or BB cylinders (310 on up), fuel and air starvation reduces top end performance.

As an example; boring a Keihin to 40.5MM gives you an increased top end at a slight loss of low to mid range response & performance.

Your air screw should be set at 1 ½ to 2 turns out, and your idle screw 3 turns out to start with…if your engine does not idle, your pilot jet could be off as well. Of course; all of this assumes everything else in the engine is in good working order…

Carlos.

Lonewulf71 · Jun 14, 2011

QUOTE(acecarlos @ Jun 14 2011, 02:32 AM) index.php?act=findpost&pid=73298There are a series of numbers on the side of the carb, next to the float…tell me the numbers and I will tell you what it started out as. If it bored out to a 40.5MM, it likely started life as a 38MM or 39MM…

Carlos.

I think this is what you want, if not I'll look again tomorrow when I have more light. S66A2 UHP

I really hope it's not the pilot jet. I have a jet kit from Duncan Racing that I faithfully carry with me, have never used, and avoid like the plague. So I should have the jet if I need to change it. I haven't really gotten to ride it that much, but from what little knowledge I have the motor seems to run really well. It has a little lower sound than my 88 which is a Duncan national kit. This 86 has an FMF Fatty pipe on it with the FMF Power Core 2 silencer. It seems to kick about the same as my 88, starts without kicking more than a couple of times. There's no weird vibration or anything. This has the head stay on it, my 88 has a cool head without the stay. As far as the vibration or lack there of, they seem about the same. I'm dying to get out and ride it on a track/course!

Air screw and Idle, I can handle....jetting make me very nervous.

diesel · Jun 14, 2011

Air screw and Idle, I can handle....jetting make me very nervous.

The best procedure I've found for adjusting the air screw is to get the engine up to operating temperature, and then rev the quad out smoothly & quickly (in neutral) - when you let off the throttle what do the RPM's do? If the RPM's hang up high, you're lean - turn your air screw in to richen the pilot circuit. If the RPM's drop below idle and then come back up to idle, or if the engine stalls, you're rich - turn your air screw out to lean the pilot circuit. Repeat the test until the engine returns smoothly to idle. Follow Carlos' advice on pilot jet sizing...if you're too far in/out on the air screw, adjust pilot size accordingly.

acecarlos · Jun 14, 2011

I think this is what you want, if not I'll look again tomorrow when I have more light. S66A2 UHP

I really hope it's not the pilot jet. I have a jet kit from Duncan Racing that I faithfully carry with me, have never used, and avoid like the plague. So I should have the jet if I need to change it. I haven't really gotten to ride it that much, but from what little knowledge I have the motor seems to run really well. It has a little lower sound than my 88 which is a Duncan national kit. This 86 has an FMF Fatty pipe on it with the FMF Power Core 2 silencer. It seems to kick about the same as my 88, starts without kicking more than a couple of times. There's no weird vibration or anything. This has the head stay on it, my 88 has a cool head without the stay. As far as the vibration or lack there of, they seem about the same. I'm dying to get out and ride it on a track/course!

Air screw and Idle, I can handle....jetting make me very nervous.

Diesel's approach is a good one; follow his advice to see if you get a response.

I always state 1 ½ to 2 turns out on the air screw. Engines will run fine with the air screw further out or in, however; I use the 1 ½ to 2 turns as a base line to determine my pilot size.

If the engine runs good with the air screw outside of 1 ½ to 2 turns, then I know the pilot is either too small or too big and adjust accordingly until my air screw falls within the 1 ½ to 2 turn range.

Don't be afraid of jetting…you need to learn how to jet, it is not difficult, yet it is a very important part of keeping your 250R healthy.

I will PM you later today on the #S66A2 UHP (need to look at the Keihin's we have in stock in the shop).

Carlos.

diesel · Jun 14, 2011

To build upon what Carlos said, I'd add that going beyond 2 turns out risks the air screw not making good contact with the spring behind it...and the spring tension is what keeps the air screw from vibrating out.

Lonewulf71 · Jun 15, 2011

I will PM you later today on the #S66A2 UHP (need to look at the Keihin's we have in stock in the shop).

Carlos.

Here's a couple of pictures, it's a PWK. I've found a couple of references to S66A2 but not with the UHP. I've looked at Sudco's website and see the parts list and exploded views for the PWKs but nothing that tells me for sure what size it is. I forget for sure what model is on my 88, but it only has I think 3 vent pipes. This thing has vent pipes all over the place that I need to tame with some zip ties.

acecarlos · Jun 15, 2011

S66A2 UHP is a 38MM PWK Airstriker; at least that is what it started life as.

Now, how do you know it has been bored out, and to what size? If you remove the filter side boot and look at the intake bell, does it still have two vertical fins at the bell curve?

If it has been bored out, it likely was to 40.50MM from 38MM...IMO

Carlos.

Lonewulf71 · Jun 15, 2011

S66A2 UHP is a 38MM PWK Airstriker; at least that is what it started life as.

Now, how do you know it has been bored out, and to what size? If you remove the filter side boot and look at the intake bell, does it still have two vertical fins at the bell curve?

If it has been bored out, it likely was to 40.50MM from 38MM...IMO

Carlos.

Awesome, thank you so much! I'm just going on what the guy I bought it off of told me, we haven't actually had it off the quad yet. My husband works shift work, so we probably won't mess with the carb again until Friday evening. I do most of the work on the quads, but there are some things I don't mess with if I don't have to. I'm going to change the filter on it and make sure all the vent tubes are properly in place and not plugged up or anything. My husband seems to think that the spring on the air screw is longer than what's on my 88, but that may just be the difference from one carb to the other. Hopefully a clean filter, and the air screw adjustment will fix the issue. Also I didn't ride it very long before we messed with it. I basically have a large gravel driveway, so I'll have to make a few laps to get it up to operating temperature, lol.

I'm just trying to get it in order enough to try out on a Grand Prix track on Sunday. We'll see if I get it ready to my satisfaction or not. I'm dying to really ride the thing, and I'm pretty sure my husband's getting tired of hearing about it, lol.The guy I bought it from was going to race it this weekend if I didn't buy it, although I'm sure he would have come back without most of the plastic. I guess I'm paranoid about the whole idle thing because of motor cross racing. In Grand Prix racing it's a dead start, so not killing it on the line isn't really an issue. It seems to be about like my 88, depending on the temperature it starts on the first or 2nd kick.

acecarlos · Jun 15, 2011

Good to see you are into your ‘R’…they are the greatest quads ever!

Keep working on it, don’t shy away from getting deeper into the engine, card, electrical, ect…it is the best way to learn.

Don’t worry about all the questions, just keep asking what you need to know…good luck Sunday on your track run.

Carlos.

Idle and Air Screw Adjustment

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