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XR 100 idle

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Hey guys, finally completed the rebuild of my 1990 XR 100. I installed a BBR 120 kit as well. Cranked it up for the first time last night and after being apart for i hate to admit almost a year (worked on it off and on) it started up after the 3rd kick !

I still have a curious issue with idle which i had before the rebuild as well.

The bike runs pretty well with the choke on and stays at a pretty consistent high idle. If you move the choke down 1 or two positions the idle drops but sometimes bogs down to the point of almost stalling. If you blip the throttle a bit it will bring the RPMs back up but seems to hang at a high idle. You can adjust the idle screw to a point where it will idle smoothly at each choke position but that doesnt make sense to have to do that. It doesnt necessarily always do this and is a bit inconsistent with how it behaves. Not sure if this is a carb issue or a timing issue or where to begin looking !

Any thoughts ??

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I replaced the air screw O-ring and it fixed my problem.

By replacing the air screw o-ring Lincoln eliminated one of many possible air intake leaks. Judging by your symptoms, I'd suggest this is a possibility.

Get the bike running at a steady idle with the choke off or as close as you can get it. Now spray WD 40 at the carb and intake manifold (not at the intake boot or pod air filter if one has been installed). If the engine rpm changes you've got an intake leak. If so, take the time to replace all the carb o-rings as well as the carb outlet and heat isolator o-rings.

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depending on how old your carb is- the slide could be wore out on the carb, allowing air to bleed past the needle, making your idle all over the board.

I would try with the mixture screw first. My neighbor's XR 200 doesn't have an o-ring on the mix screw, and it was pretty difficult for me to figure out why it was running like how it was. I would get the mix screw set right, ride it, and then it would be off again. I finally noticed that vacuum was pulling the mix screw into the carb body! Vibration would allow the screw to turn itself in. There is no spring, no 0-ring, etc. I took a pair of Dikes, crimped a thread, and got it to stay in place. Runs perfect now.

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I didn't think to mention: My neighbor's XR 200 is a 90 model as well. I know yours is a 100cc, but the carburetor symptoms you describe were identical to his. Just be patient because you're gonna be turning in the mix screw, get it where it sounds good, then mess with the idle screw. The factory manual says to turn in the mix screw until it seats, then turn out two complete revoultions. I'd say start from there. You can listen to the engine change rpms. Once you get the throttle action you like, it comes down fast from high rpms, you've got it right.

Just remember you're gonna be turning the mix screw, then the idle screw.

Keep tinkering with it; you'll get it right. Make sure your air filter is clean.

If you still have the problem, you may need to step up one jet size on the low speed jet. Not sure if the 100 has a pilot AND a main jet, but it probably does. Never hurts to experiment. With the idle being all over the place and being inconstant, more than likely a mix screw problem, or a induction leak. Did you replace the O-ring where the intake boot bolts to the cylinder head? Those are cheap, easy to do, and eliminates the possibility of a vacuum leak.

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Replace your pilot jet first. I can't tell you how many times I have seen this same exact symptom here and on my bikes.

Your pilot jetis cloged and is the cause of your idle being erratic.

Tell us what the fix was when you get it fixed. I'll bet money i'm right.

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Ok, so finally got a chance to look at it this evening. Tested for leaks and made sure everything was nice and tight. I tore down the carb again an inspected all the jets. I pulled out the low speed jet and attempted to blow air through it and no go. Sprayed it with carb cleaner and let it sit for a while still no go. Soaked it in some seafom for a while and then used a tiny strand of wire from speaker wire (yes I know a no no) and finally got it cleaned out. It runs like a top now !! Thanks for the input guys.

Now on to re-jetting. What jets are you guys running on your 120s with original exhaust and carbs ?

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Ok, so finally got a chance to look at it this evening. Tested for leaks and made sure everything was nice and tight. I tore down the carb again an inspected all the jets. I pulled out the low speed jet and attempted to blow air through it and no go. Sprayed it with carb cleaner and let it sit for a while still no go. Soaked it in some seafom for a while and then used a tiny strand of wire from speaker wire (yes I know a no no) and finally got it cleaned out. It runs like a top now !! Thanks for the input guys.

Now on to re-jetting. What jets are you guys running on your 120s with original exhaust and carbs ?

The wire is a great thing to use. Copper wire will not scratch brass. Glad she is running good for you now. Does it have decel pop when you are riding. Get up to high rpms in a gear and let off the throttle and coast IE pop or not. If it does go a little bigger on the main if not she is good.

Don't change the pilot size.

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Idle problem is back :) Tried cleaning out the slow speed jet again to no avail. Tried replacing all card gaskets too and tried the wd40 trick as well and didnt notice any change in idle when spraying all around. I ordered a new needle jet set and slow speed jet of original size as well, should be here next week.

Would an ignition issue cause it to run this way ? I wouldnt think it would but ...

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have you checked where the throttle cable is attached to the slide? there's a v shaped clip in there that can be come dislodged and not allow the needle to seat. that was why my bike idled high.

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have you checked where the throttle cable is attached to the slide? there's a v shaped clip in there that can be come dislodged and not allow the needle to seat. that was why my bike idled high.

Ahhh the little clip that likes to send me on a search and rescue mission every time I try to adjust it ! It appears to be seated evenly though it is so worn in the middle position that it no longer holds the clip ! I got a new one of those on the way too.

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The saga continues. The bike seems to start alright but the idle is somewhat inconsistent, especially if you blip the throttle. If i blip the throttle quickly it bogs and about stalls. I would assume this was a lean condition though i currently have the needle clip set towards the bottom position. If I slowly increase the throttle the idle increases without issue then typically hangs and ever so slowly decreases though sometimes it just stays high. The hanging idle is my main issue i feel at this point as it seems to have no rhyme or reason.

At this time i have replaced the following to eliminate air leaks -

entire carb gasket kit

new intake boot (black boot that connects the carb to the head) and gasket

I have replaced the following additional carb parts -

low speed idle jet

main jet (went up to a 105 main)

needle jet

needle

New plug, gas and clean air filter.

I have also sprayed wd40 around the carb to see if the idle would be affected indicating an air leak but nothing changed.

I went to restart it and take a video to share, started it and the idle raced, i touched the idle adjust screw and barely turned it and the bike died.

Would this indicate a worn slide? Possibly an ignition issue ? (it has a points style ignition)

I am at a loss !!! It ran awesome for a few weeks after the rebuild now I am scratching my head ! :)

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:thinking:Some things I'd check that can cause problems.

One would be to re-check the valve clearance. What cam are you running and what clearance is called for? A tight intake can cause problems.......but check both.

Another would be to pull the flywheel and check out the spark advancer. Make sure the springs are good, it has a little lube, it's not sticking and returns smooth. Have a good "clean" set of points.....? What method are you using to check ign timing.....?

Also on the carb, did you happen to check the float level.....? Do you have throttle free play, how much......?

Old School Al

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Hi Al, thanks for the tips.

I have a BBR cam that came with the BBR 120 kit that I installed. My understanding is that it is the same grind as the OEM cam. The instructions that came with the cam called for .02 exhaust and .03 intake I beleive (might be the other way around, dont have the sheet with me currently). The clymer manual calls for .02 on both sides. I had just performed a valve adjustment prior to some of the weird idle issues happening but have double checked after words and they seem to be in spec according to BBR. I might try reverting back to the .02 that clymer recommends.

I hate to admit i am not too well versed in timing adjustments on this type of motor. The points as far as I know are likely the original ones :) I will need to check on the advancer, what type of lubricant do you recommend ? The one part of the bike which really hasnt been gone through is the ignition as I must admit i am not very familiar with it.

The throttle cable itself appears to have decent amount of freeplay and returns to closed position with hanging or getting caught in place. Float level is also set to .50 inches from the lip of the carb which is what was recommended in the clymer manual.

Thanks again !

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Hi Al, thanks for the tips.

I have a BBR cam that came with the BBR 120 kit that I installed. My understanding is that it is the same grind as the OEM cam. The instructions that came with the cam called for .02 exhaust and .03 intake I beleive (might be the other way around, dont have the sheet with me currently). The clymer manual calls for .02 on both sides. I had just performed a valve adjustment prior to some of the weird idle issues happening but have double checked after words and they seem to be in spec according to BBR. I might try reverting back to the .02 that clymer recommends.

I hate to admit i am not too well versed in timing adjustments on this type of motor. The points as far as I know are likely the original ones :) I will need to check on the advancer, what type of lubricant do you recommend ? The one part of the bike which really hasnt been gone through is the ignition as I must admit i am not very familiar with it.

The throttle cable itself appears to have decent amount of freeplay and returns to closed position with hanging or getting caught in place. Float level is also set to .50 inches from the lip of the carb which is what was recommended in the clymer manual.

Thanks again !

Strange the BBR cam would be stock grind. I would think clearance would be .002 intake and .003 exhaust. Honda spec manual shows .002 for both with stock Honda cam. Changing a thou shouldn't really make a difference in the way it runs.

Have to make "sure" the ignition is in "top condition" with points. We may not be working on the Space Shuttle, but as with the Shuttle ........ it's the details that count!:) "Everything" inside left case needs to be "clean, clean, clean" and cover needs a good gasket seal. Points need a good contact surface..... clean, unburnt (minor pits after cleaning ok). "No" (NONE) traces of dirt, oil, or grease left on them when you're done adjusting. Point lube on the rubbing block, "tiny" amount of oil on the wick. Point cam need to be smooth (no rust) "tiny" amount of point lube on surface. A "tiny" bit of point lube can be used on the all the advancer wear points. As it may no longer be possible to get point lube, a high temp grease would also work. Main thing to "avoid" is lube slinging onto the point contact surface. Time staticly, points should break contact on the flywheel F mark. Check advance with a strobe light if you like.

Honda Spec manual shows float level at 12.5mm or 0.49". Would be meassured from tip of float, to float bowl gasket surface, float needle valve closed. Without compressing spring in float needle valve......?

Old School Al

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Did you check and see if your slide is pitted where the idle screw hits it? Becareful with taking off your flywheel, if it revs nice and doesn't back fire, you could be opening a big can of worms. Points initions can be a nightmare once you fuss with them. CHeck your valves. Call BBR on their specs. They change witth different cams. With our custom webcams, we go .005 for both. Remember your carb is over 20 years old. It's probably that.. Hey Al, I don't remember the space shuttle having points igntion. I surprised you didn't tell what air pressure to have in the air hose when removing it or putting it back on or any torque specs. Hehe

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Did you check and see if your slide is pitted where the idle screw hits it? Becareful with taking off your flywheel, if it revs nice and doesn't back fire, you could be opening a big can of worms. Points initions can be a nightmare once you fuss with them. CHeck your valves. Call BBR on their specs. They change witth different cams. With our custom webcams, we go .005 for both. Remember your carb is over 20 years old. It's probably that.. Hey Al, I don't remember the space shuttle having points igntion. I surprised you didn't tell what air pressure to have in the air hose when removing it or putting it back on or any torque specs. Hehe

:thinking:sorcal, You do come off with some good ones.........! In your case though I would agree, "you" shouldn't take the flywheel off (or put it back on either).:)

Why you getting so sore about me using the "Space Shuttle".............. aren't you the one that brought it up in the first place? I liked it and went with it.......!:)

Old School Al

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