91 XR100 bad performance at high speed and pilot/air screw, need help

Sorry that it's a lot to read but I don't want to leave anything out. Thanks for the advice!!

Wathc some riding videos for reference at: http://www.youtube.com/user/jjjrickey


In the spring my brother and I bought a 1991 XR100R from a college guy who had outgrown it, previously it was this guy's uncle's bike so I'm kinda owner 2.5. It ran good when we first had it, but I never really ran it too hard because I was still learning how to ride. Its all stock as far as I know. Everything works okay and it runs, just not great. Problems started after about 3hrs of riding when we washed the bike then put it away, the left hand case cover (alternator cover) had been cracked from a chain break and we had filled it with water while rinsing the sprocket. The coils rusted out, took me a while to find the problem but I cleaned the coils with CLR and it sparked and ran again. I bought a new cover too. Then I decided to adjust the pilot/air screw (the screw on the bottom, front of the carb). I unscrewed it a little, which was stupid of me to touch it. We started blackening plugs and whenever it wouldn't start I would replace the plug and it would start right up. Throttle response was okay (I could wheelie it) but the idle speed was messed up, it would be idling fine then rev way up to 4,000ish, I'd then unscrew the idle speed screw a little to bring it down, then it would start to stall because it was idling so low, I couldn't get it to idle smoothly but it still ran good. I read the manual, realized what I did and turned the pilot screw in a little (completely guessing on the amount). That fixed it for a little while.


I recently bought an iridium plug and I swear it gave it a lot more power, the thing ripped!! I continued adjusting the pilot screw, bought a new air filter, changed the oil every time it burns a tank of gas (every 7ish hours or 1 gallon of gas), I have a tachometer on the bike most of the time, and I use non-ethanol 91 octane gas with 1oz of gumout per 1gal. I followed the manual's instructions for the pilot screw, it seems to run best at 1.0 turns out but still feels like its losing power. I've tried a bunch of settings including 2.5 out, 1.75, 0, 0.5, 1.0. At 0 or 0.5 it idles the highest but backfires (like pop-pop-pop) at WOT,about 9,000 rpm (right before I want to shift). If I undershift it runs good, but not at full power because it is overloaded. Throttle response is best at 0.5 turns out. At greater than 1.5 turns it idles good but chokes out when I give it throttle (even if I ease it up) but it will run okay if its way overloaded (like 3/4 throttle at 3,000 rpm). Yesterday I ran it for a while, mostly at WOT and it preformed good but not great and I think the screw was 1.1 turns out. But I would really have to ease it to idle or it would fall too low and stall. When it was hot I would have to give it a little gas to get it started, when it was warm it would start no problem at idle. The hotter it was the lower it idled. Today when it wouldn't start, I took out the iridium plug and it was black, cleaned it with sandpaper but it still didn't spark. It finally started with an old plug. I rode it for a minute or two then put the iridium plug back it (the old plug had turned black in this time), it started this time and ran good but still not great (it ran overall best at about 1.0 turns out). Oh and whenever its cold there is like zero throttle response until it warms up a bit idling. I gave up and came to TT for answers after I still couldn't get it to peak performance.

Edited by jjjrickey

1.stop changing the oil so much. Maybe after 4 or 5 tanks of gas.

2. take the carb off and give it a good cleaning. Best to find someone that has done it before to give you a hand.

3. do a search for the correct jets. read the number on the jets, stock sizes are a little lean. your bike may have the correct one's but the carb likely has some clogged passages which is why it will not idle consistently.

4. the pilot screw (fuel screw) has a spring a washer and then a little tiny O ring that goes up in the hole. If that O ring is missing or if it is bugered up you will also have idle problems.

Get the carb working correctly and you may never have to change another plug. The spark plug caps go bad after many years. order a new one. they unscrew from the wire and then you trim about a 1/4" of the end of the wire and screw on the new one on.

Thanks! I'm trying to avoid taking apart the carb but I guess I have to. I agree that I change the oil maybe a slightly too often but I ride the bike hard and at low speeds in 80 degree weather so it runs hot (the skid plate is usually too hot to hold my hand on) so I think that's why it uses a lot of oil. The old oil is always dark brown and noticeably thinner. I should add that I live in Buffalo, NY elevation is about 600ft. I also have an inline fuel filter and the petcock screen.


I will definitely get a new spark plug cap/boot because the previous owner had it electrical taped because the wire is worn badly.


I also think the PO had a bigger jet in it because he mentioned taking apart the carb a couple times to get more power. This might be making it run rich and blacken the plugs.

Learn to clean the carb. not real hard at all, in fact it is real easy once you have done it a few times.

The small passageways clog very easily and your bike will run crappy once this happens.

Anyone that has ever worked on any carb will be able to help you.

Also the air filter, be sure to keep it clean and don't over oil it or saturate it in filter spray.

So the bike runs okay now, still haven't cleaned the carb (that along with clutch inspection will be winter projects). If its going to start, it starts first or second kick. If not then it will never start until I change the plug. Frequently when I change the plug the "old" one sparks good but won't start the engine, when I put another plug in it starts right up. It's hard to install plugs, I have to use a wrench the whole way, can't even finger tighten it one turn. Does this mean the threads are stripped or badly cross threaded? Or are the threads dirty, I've never really tried to clean then.


My most recent major problem is that when I was installing a different plug it went in as usual but I didn't use my torque wrench (got lazy because I change the plug so often). I turned it about half a turn past the point of increased resistance, then the plug broke, the hex bold and ceramic part broke off the threads, leaving the threads stuck in the head. I read that an easy-out screw extractor will fix this problem, but what if metal shavings fell into the cylinder?


Question about crankcase breather: My breather is just a hose coming out that faces the ground, when the engine is running air blows out pretty fast. There is no plastic plug thing in the tip of the hose, is this causing low oil pressure or is it normal? I've read in diesel mechanic books that low oil pressure is very bad and will destroy the engine very quickly, does this apply to the xr100?


Thanks for the help!!

So I have a friend who owns a KX100 and he could help me take apart my carb, haha but I keep putting that off. And will Gumout fuel additive clean the carb passages? Or does that stuff just clean jets. I've used Gumout in many outboards with much success over the years, it's like my simple trick to improve overall engine performance but it doesn't seem to be affecting my dirt bike at all, if anything the bike is getting progressively worse and worse.


I'm pretty sure the plug threads on the head are cross threaded because I've tried cleaning then with a shop cloth, and a very fine point tool that I gently scraped dirt out of the threads with. When I install a brand new plug I need a wrench just to get it in, I can only finger tighten it about half a turn before it becomes too hard. Is it a big problem if the threads are cross threaded? Well if it is a problem I'm not going to spend $200+ on a new head.


I still do not understand how dirt in the carb can cause a rich condition. I have the pilot screw turned in to make it more lean, and if there'd dirt then the fuel should be restricted making it more lean right? I also have a decent foam air filter that has about the right amount of oil. Between all these factors I'd think that it's running very lean but the plug and spark arrestor are telling me it's rich. The plug's turn black quickly and frequently are wet when I pull it out. I don't run the bike much but a lot of liquidy carbon builds up in the muffler after just a couple hours of running. I'm pretty lost and don't quite know what to do.


Any advice is greatly appreciated!

I wish to have it running great by New Years because I'm going to my cousin's farm and I want to keep up with the small snowmobiles.

Lots of air flow out of the crankcase vent and the dirty plug all mean that it is time to install new rings and possibly a piston.

You're getting lots of blow by and you're burning oil not fuel.

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