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Post by twitch on Jan 23, 2008 18:16:18 GMT -5
Ok, so I got a new exhaust given to me the other day. It is a 1.5" diameter pipe. I got it installed and had some minor backfiring when I let off the throttle. It was suggested to me that I was running too lean and should rejet.
Well I have a spare carb in the closet and figure, why buy a jet when i can drill a jet. I get my smallest bit and drill out the main jet. To look at the two jets side by side, you would barely notice that one is a larger hole. I install the larger jet and take her for a spin.
I can roll around fine at anything 20mph and below, but once I roll the power on she is stumling all over herself. I tried turning the mixture screw all the way out, all the way in, and every point in between with only the slightest difference. I pulled the plug after my test ride and the plug was black as coal. In short, i dont know how many sizes I went up with my jetting, but it could not have been more that 2 sizes. I simply think I really am not running lean enough to justify up jetting.
Any opinions or suggestions are welcome.
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Post by Jacine on Jan 23, 2008 18:19:43 GMT -5
You generally drill your own jet as a last resort. Its a lot cheaper to buy varying sizes for the main in the long run. It sounds like your only using the slow jet from idle to 20 mph and the main jet is not working in properly. You also have to adjust the air fuel screw as well as adjust the PM screw.
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Post by RetroAJ on Jan 23, 2008 18:24:27 GMT -5
It is obviously running very rich now, and the mixture screw will not change that. When you get into hundredths of a milimeter, barely noticeable is a big difference. Two jet sizes is about .05 milimeters. You could have gone up .20 milimeters.
You need to know what the drill bit size figures out to in jet size, and go from there. A wire gauge drill bit set is great for drilling out jets.
*A J*
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Post by twitch on Jan 23, 2008 18:26:04 GMT -5
I only though there was a mixture screw and an idle adjusment on these carbs. Where is this other screw? The one I was adjusting is on the opposite side from the throttle linkage, and a little lower too.
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Post by Jacine on Jan 23, 2008 18:28:44 GMT -5
The screw closest to the air intake manifold is the air mixture screw. The Primary mixture screw or fuel mix varies in location on carbs but is closest to the engine intake manifold. The are being covered over with a soldered brass button on the newer bikes to prevent the user from adjusting them.
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Post by twitch on Jan 23, 2008 18:43:53 GMT -5
I have my (spare) carb in m y hands right now, and only see the screw closest to the engine intake. There is no other place on here for another screw. The only cap I see is coveing the end of the butterfly bolt. I think this style carb only has the adjusment screw next to the intake manifold.
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Post by scottfla on Jan 23, 2008 18:44:53 GMT -5
Ack! You got that exhaust on your scooter? Sheesh!
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Post by twitch on Jan 23, 2008 18:47:28 GMT -5
looks better than that tin can it came with. Now i just need to dial everything in.
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Post by Jacine on Jan 23, 2008 18:53:39 GMT -5
Some carbs it's on the bottom. It may be covered by a solder cap. But generally you should have two screws. The big one for air and a smaller one closest to the engine side for fuel. Who is the manufacturer of the carb?
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Post by twitch on Jan 23, 2008 19:07:26 GMT -5
It has "jinxiong" stamped on it. I think it is a kehien(sp?) clone.
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Post by monty57 on Jan 23, 2008 19:17:46 GMT -5
On the subject of drilling jets......
Don't!
I have to through in my two cents worth here. I have been around for a long time and been involved with engines all of my life. Did some racing in my youth and my dad was a life long mechanic and every knowledgable person that I have came in contact with during those 50 or so years have said never drill a jet.
Jets are machined so that the inside is mirror smooth. When you drill a jet you scratch the surface and can actually cause less fuel to flow from the turbulence created by the scratch and irregular surfaces. Fuel metering needs to be precise in order for the the mixture of air and fuel to be optimal for the combustion. So please do your self and your engine a favor and buy new jets, never drill out the old ones.
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Post by Jacine on Jan 23, 2008 21:19:39 GMT -5
If its a keihin, typically they have the primary mix screw on the bottom of the carb, usually closest to the intake manifold, and just under the or around that idle jet thing that sticks up into the throat of the carb.
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Post by mgreenfield on Jan 23, 2008 21:43:16 GMT -5
My scoot has a 24mm Keihin knock-off that I (unfortunately) know very well inside and out. Just one screw on the outside for idle mixture, but x2 changeable jets inside, currently a Keihin #38 (slow) and a #120 (fast), which seem to be just right.
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Post by Jacine on Jan 23, 2008 21:57:03 GMT -5
This is a weber carb Note the two screws.  This is a Keihin Carb, note the PM Screw on the bottom left  This one has the PM screw on the bottom and the Air adjust screw on the left of the pic.  here is a view from the bottom. Note the PM screw on the center bottom left. This may be soldered over by the manufacturer to prevent alteration with a plug. The plug has been removed. The idle adjust screw is on the bottom right.  |
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Post by earlwb on Jan 23, 2008 22:43:41 GMT -5
Well like the others stated, you don't just drill out the jet. Its better to order them. it sounds like you drilled it out too large. But there is a way to do it OK, but it is something of an art form. To properly do it, you have to have the proper sized drill bit, it is actually slightly smaller than the hole you are drilling as the drill bit makes a slightly larger hole. Then you have to use a high speed drill press so you can get the rpms up high so the tiny little drill bit won't break on you. At high rpms the tiny drill bit doesn't get a chance to bite in and snap off on you. A mini or micro drill press works for this. Then you carefully and slowly drill out the jet. If you drill it out too large, you can fill it in with solder real good and drill it out again. Oh yeah, a good set of tiny drill bits aren't cheap either. This is a small sample of what they look like.  If the carb is the CV type clone of a keihin carb then:   |
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Post by twitch on Jan 24, 2008 9:33:59 GMT -5
Earl, that is my carb you have in those pics. I suspected it only had the one screw. Well, I have to admit I made a rookie mistake then by drilling my jet. The plan was to see if it helped any, and then order a new jet. Well, at least I used the jet from my spare carb, and saved to original so I can put it back in and have a running, albeit somewhat lean, bike. I guess I wil be ordering 2 or 3 new jets from oregon vintage next week. Do I need to up my low speed jet as well even if it seems to be ok?
Oh yea, Jacine, have a Karma on me. You really hung in there on this thread and I appreciate it. And thanks to everyone else who helped.
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Post by earlwb on Jan 24, 2008 10:16:43 GMT -5
No, your idle jet is OK.
Some folks after installing the biggest big bore kit they can get , a hot cam, and a stroker crank with high performance exhaust and a bigger carb might have to go up a size in the idle jet, but otherwise it is very rare to need a larger idle jet.
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Post by Jacine on Jan 24, 2008 10:58:42 GMT -5
I think there is a plug in that carb covering the pm screw to the upper left next to the left screw hole.
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Post by earlwb on Jan 24, 2008 11:07:05 GMT -5
That is a cap over the throttle butterfly shaft end. it's to help keep the air from leaking in.
They press it on tight, it is thick metal of some sort.
I think the "Pilot mixture" and "idle air mixture" means the same thing.
Both refer to the low speed jet or idle jet.
My Harley has the carb like you depicted above. It had a thin aluminum cap pressed in over the air mixture screw to prevent tampering. It was easy to pry off using a awl. But on my Harley it is a bear to make the adjustment as the crossover exhaust pipe is right where you want to put your hand to make the adjustment.
It does get really hot too.
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Post by twitch on Jan 24, 2008 11:10:54 GMT -5
Jacine, I see what you are talking about, but it is part of the casing. It is not a weld or a solder, but part of the form. And the spot in the center is not a cover either. I actually drilled into that one and found out it is just a hole filled with solder. I made sure to stop just short of breaking through into the butterfly tube.
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Post by Jacine on Jan 24, 2008 11:25:00 GMT -5
Must be something completely new then. I guess the did away withe the fuel adjustment screw.
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