MacDizzy's Jetting 101 - Every Race Is Won

The Thread Spread

- Jetting 101 -

Every Race Is Won

The question - I have an 88 250R with a national engine kit (bore, port, re-deck, cylinder spacer, 86 piston, re-domed head, K+N,thin head gasket, Ram Valve, 38 mm Air Stryker). I run 110 octane race fuel at 32:1 with Maxima. Right now, I'm running a 182 main, the clip in the middle slot of the needle, and a 52 pilot. The plug comes out a deep, dark chocolate brown like it's supposed to look like, but the bike sputters on the top end.(3/4 throttle to wide open). The filter is perfectly clean and oiled. I'm afraid to drop the main, because the plug color looks perfect. I've heard that running race fuel makes it hard to read the plug. Is this true? Could this be making the plug look lighter than it should? What about trying to run a different length needle (I think length of the needle affects top end, right?) or a different slide? Please help!!-I want my bike to run a little rich so it lasts, but I don't want it to sputter.

The response - Deep dark chocolate is a little rich for my taste. I like my plug to be light tan. I think you should probably try dropping the main and see how it does. Just check your plug after every change. I doubt that you needle has anything to do with it. Your main sounds to rich.

Brandon

Sounds like a nice setup you have yourself there! If you have your plug gap set correctly (somewhere in the .28 -.24 range), then you are probably rich. I say this because a 182 is VERY fat on a 38 mm Keihin. You motor probably does want a pretty fat jet though. I would drop to 180 and see if it helps. DO NOT mess with needle length. Although there is an overlap in each of the areas in jetting, you nailed the problem when saying 3/4 to WOT. It IS the main jet. If it were doing it at around 3/4 but then tapering off to run A-OK at WOT, then you may would want to consider needle - but this ain't the case. And you should be able to read the plug just fine with 110.

Trax310

Do a compression check. With your motor COLD - screw the compression gauge in until it is tight, hold the throttle WIDE open, and kick it as hard as you can until the gauge stops reading any higher. I don't know what the compression is supposed to be with that setup, it depends on the exact exhaust port timing, the deck height, the head gasket thickness, the squish clearance and the size (cc's) of the dome (head). Use a Snap-On compression gauge if you can. The point here is that with 110 octane fuel you should be able to run up to 210 PSI cranking compression (or more like I do) without having to worry about melting down, but you may have ignition problems with cranking compression over (about) 185-190 PSI.

Before leaning out the main jet and making a costly mistake, put in a new spark plug and close the gap on the spark plug to .028" and try it. If you still have problems close the gap to .025" or .026" and try it that way. Be very accurate when gapping - only use a WIRE gauge for checking it. The problem with running high compression is that it can cause the spark to "blow-out" and make you think the motor is just running rich, because it sounds that way. What is happening is that with that much compression the stock ignition system can not deliver enough energy to the spark plug to fire it under such extreme pressure. Therefore you are not getting a complete burn - hence the sputter (blubber).

Your main jet size sounds right. You may find that when your spark comes back you can go to a larger main. When properly jetted you wont have the deep dark chocolate brown - it will be more like coffee - tan. Don't mess with the needle length or slide. See http://www.msdignition.com for a high output ignition unit for the TRX. You might try switching to a BR8ES from a BR9ES (if that's what you're running) for a little additional help without fear. I think the MSD unit is too expensive and requires too much maintenance, not to mention permanent modifications to your flywheel.

It's not a bad idea to get a Digitron Exhaust Gas Temperature Gauge to help with jetting. It costs about $225 with EGT and RPM. Though it's not the final word on jetting, it does eliminate some of the guess work - stay under 1150 -1250(f) and you should be OK. I hope it works out for you. Race fuels have fewer additives and produce cleaner plug readings than pump gas. Many people refer to pump gas, or mixtures of pump gas and race gas as "chemical soup." I tend to agree.

Rick

Back to The Thread Spread index of articles.

Every Race Is Won
The question - I have an 88 250R with a national engine kit (bore, port, re-deck, cylinder spacer, 86 piston, re-domed head, K+N,thin head gasket, Ram Valve, 38 mm Air Stryker). I run 110 octane race fuel at 32:1 with Maxima. Right now, I'm running a 182 main, the clip in the middle slot of the needle, and a 52 pilot. The plug comes out a deep, dark chocolate brown like it's supposed to look like, but the bike sputters on the top end.(3/4 throttle to wide open). The filter is perfectly clean and oiled. I'm afraid to drop the main, because the plug color looks perfect. I've heard that running race fuel makes it hard to read the plug. Is this true? Could this be making the plug look lighter than it should? What about trying to run a different length needle (I think length of the needle affects top end, right?) or a different slide? Please help!!-I want my bike to run a little rich so it lasts, but I don't want it to sputter.
The response - Deep dark chocolate is a little rich for my taste. I like my plug to be light tan. I think you should probably try dropping the main and see how it does. Just check your plug after every change. I doubt that you needle has anything to do with it. Your main sounds to rich. Brandon Sounds like a nice setup you have yourself there! If you have your plug gap set correctly (somewhere in the .28 -.24 range), then you are probably rich. I say this because a 182 is VERY fat on a 38 mm Keihin. You motor probably does want a pretty fat jet though. I would drop to 180 and see if it helps. DO NOT mess with needle length. Although there is an overlap in each of the areas in jetting, you nailed the problem when saying 3/4 to WOT. It IS the main jet. If it were doing it at around 3/4 but then tapering off to run A-OK at WOT, then you may would want to consider needle - but this ain't the case. And you should be able to read the plug just fine with 110. Trax310 Do a compression check. With your motor COLD - screw the compression gauge in until it is tight, hold the throttle WIDE open, and kick it as hard as you can until the gauge stops reading any higher. I don't know what the compression is supposed to be with that setup, it depends on the exact exhaust port timing, the deck height, the head gasket thickness, the squish clearance and the size (cc's) of the dome (head). Use a Snap-On compression gauge if you can. The point here is that with 110 octane fuel you should be able to run up to 210 PSI cranking compression (or more like I do) without having to worry about melting down, but you may have ignition problems with cranking compression over (about) 185-190 PSI. Before leaning out the main jet and making a costly mistake, put in a new spark plug and close the gap on the spark plug to .028" and try it. If you still have problems close the gap to .025" or .026" and try it that way. Be very accurate when gapping - only use a WIRE gauge for checking it. The problem with running high compression is that it can cause the spark to "blow-out" and make you think the motor is just running rich, because it sounds that way. What is happening is that with that much compression the stock ignition system can not deliver enough energy to the spark plug to fire it under such extreme pressure. Therefore you are not getting a complete burn - hence the sputter (blubber). Your main jet size sounds right. You may find that when your spark comes back you can go to a larger main. When properly jetted you wont have the deep dark chocolate brown - it will be more like coffee - tan. Don't mess with the needle length or slide. See http://www.msdignition.com for a high output ignition unit for the TRX. You might try switching to a BR8ES from a BR9ES (if that's what you're running) for a little additional help without fear. I think the MSD unit is too expensive and requires too much maintenance, not to mention permanent modifications to your flywheel. It's not a bad idea to get a Digitron Exhaust Gas Temperature Gauge to help with jetting. It costs about $225 with EGT and RPM. Though it's not the final word on jetting, it does eliminate some of the guess work - stay under 1150 -1250(f) and you should be OK. I hope it works out for you. Race fuels have fewer additives and produce cleaner plug readings than pump gas. Many people refer to pump gas, or mixtures of pump gas and race gas as "chemical soup." I tend to agree. Rick