XR100 engine trouble

[jdscom]

Hello,

First time on this site hoping someone could help me with my Honda XR100R bike.

The bike started running rough intermittently, fouled a plug. I replaced with new and seemed fine for a few hours. Then started rough again. Now it seems to idle fine but has absolutely no power and feels like it’s firing every other combustion stroke when under load. When idling sounds ok as soon as throttle it bogs and sputters, however if I throttle up very gently I can get the revs up pretty high. Once in gear there is no hope of getting going.

Tried new plug again, gas should be fine as its same as other bike which runs great (Honda XR80 same year).
I'm not looking for high performance, just regular running and up hill power. Right now I can't even go 1st gear on flat.

Any thoughts on where to start would be appreciated.

 

[23jayhawk]

Sounds like a weak spark, but maybe check some of the easier things first - fuel tank vent plugged, obstruction in airbox or carb inlet tract, main jet plugged.

If nothing there, then I'd guess something in the coil/igniter circuit is breaking down. Not enough power to create a healthy spark when the cylinder loading starts increasing with heavy throttle.

You might be able to swap some parts back & forth with the XR80 to isolate what the problem is, once you rule out fuel delivery problems. Not sure, but I bet there is a lot of crossover between them.

 

 

[atc3434`]

Choke isn't stuck on, is it?

 

[jdscom]

THanks for your post. The thought of coil entered my mind, but I tested the spark grounded to the bike and it looked Bule and strong. But who knows I'm new at this and don't have much bike mech. experience.

Any idea how much a coil runs for?

Thanks for your help.

 

[atc3434`]

http://www.ericgorr.com/twostktech/carbtuning.html

Thats the link to Eric Gorr's Carb Tuning guide. I'm guess with a good hot spark you electical system isn't the trouble. I would think you have a carberator problem somewhere. Most likely your running rich, if you were able to foul a plug on a 4-stroke. Perhaps a sticky float. Anyways, check it out, and if your up to it, go ahead and dig in to the carb and have a look. Or you can find somebody who knows their stuff to help you out. I do that myself. I tear a carb or a motor apart, but when it comes to doing silly things like changing a tire, I need help. Go figure!

 

[Glitch]

You should really try out the easier things first. Like is the choke on, is the bike running lean or rich, maybe try messing around with jetting. But seeing as how is an '85, electrical might just be the problem.

 

[atc3434`]

Good call Glitch. Definatly check the easy stuff first. I'm still thinking maybe the choke stuck. But get the simple stuff covered first. I once rebuilt the carb on my buddies XR200 before I check the plug and found out it wasn't firing nicely. New plug and it lite right up! Grr...

 

[jdscom]

OK.. thanks for the suggestions. I'm sure the choke is off cause I tried it on and engine cut out imed. But maybe its stuck on internally (not sure if that possible). Come to think of it I haven't needed the choke at all even when cold, running rich is sounding more like it. I was just reading through the manual about the pilot screw. It might be in too far according to spec. I always thought to start 1.5 turns from seat, but book says 2 5/8, i will try this next.

Would the pilot screw effect the throttle when opened, or is this for idle only?

anyways learning lots. thanks for all the info, this is a great site.

 

[atc3434`]

Yeah, it would definatly affect your power, especially as your open it up. Bring that out some, and it may help. It is seeming more likely that something like that is the problem, as you can cold start with no choke. Maybe a stuck chokce too, a possibility.

 

[cr25096er]

is it a breaker point model? if so i would check them. my 86xr80 would run like you described. start w/o choke but idle high(does yours?) then have no power and die eventually.also if its an older bike and the points are out of adjustment, dont bother readjusting them alot. just buy new ones. my xr's would go out of adjustment every 10 mins till i replaced them.

 

[jdscom]

Checked the points and they seemed out of wack, not sure though, I'm trying to get my hands on a timing light to check the timing correctly. I wanted to make sure the advance time was right to. Looks like i'm heading in the right direction.

Thanks

 

[jdscom]

Hello

If your not all sick of me I could really use some more help. I figured running rich was a high possibility. Checking the jet needle assembly I found it on the top groove which is the leanest setting. Also with the pilot screw seated the bike runs the best, but still crappy.
If I choke it dies instantly. If I gun the throttle it dies as well. Is there a way of leaning the fuel more, without changing the jet?

If the bike was running fine I can’t see why a jet change would be necessary. My plug after 10 minutes of running was black dry fluffy carbon. Also, my manual refers to using a “hotter” Plug. Is there some reference somewhere on plugs and so forth for the Honda 4 strokes?

All the help is greatly appreciated. Thanks

 

[atc3434`]

I'm thinking maybe try checking out your main jet passeges. And float height. IF your getting an idle ok, but it dies on throttle, I'd have to think lean. Definatly if its running best with the air screw all the way in. That means she doesn't have much fuel to burn with the air, so you way lean if you've got it screwed in all the way and it runs the best. Perhaps you have an air leak somewhere. If it was running fine, I'd guess you've got something gummed up somewhere. I'd disassembly the entire carb, and clean all out with carb cleaner. Remove the jets, clean them all. Make sure you get the little passages in the front bottom throat of the carb, people sometimes forget these. Then check you float height. Get he back together and see what you got. And check and make sure you have fuel flowing free from the tank.

 

[jdscom]

If I take the carb apart do I need a new gasket set? Or does this depend on the condition of the gasket? Is it advisable to take the bottom float bowl off the carb while still on the bike to try and clear any junk. or is it best just to tear it down.

ALso, would a week spark or to cold a plug give the same symptoms as running rich or lean?

 

[atc3434`]

I can't speak to the question about the spark plug, but as the gasket set, it depends. I've never had a problem, but I suppose if the carb has been together for a long time, you could tear a gasket a little, and that would not be good. You don't want to tear the carb down on the bike. Its not too hard to get off.

First you want to loosen the collar on the airbox with a screwdriver. I like to get it pretty loose so I don't have to figth it putting it back on. Make sure when you do put it back together you get it all over the carb mouth good, and none of its folded under. You want a good seal. Next you can remove the two bolt on the front of the carb that hold it to the intake piping. Make sure your gas is off, and then unhook your gas line. Now you can slid the carb back off the studs on the intake.

Now you need to unscrew the top of the carb. This contains a spring and the slid, as well as the needle. Be careful, it will sometimes pop open quickly, you don't want anything to go flying. Now I usually just let the slid and needle dangle, but you can tie it up out of the way if you want.

Now you can go ahead and take off the float bowl. Once you get inside, you should take out the pilot and main jets. I then clean everything quiet liberally with carb cleaner. Jets too, they often have the clogging material stuck to them. Make sure you spray all the passages in the carb body. The main, the pilot, as well as the two air guide passages on the intake throat of the carb. I like to use a little compressed air to blow the passages out after I shot them some with cleaner. It can help, if you have the access to a compressor. Clean the fuel inlet as well. Make sure you don't lose any of the little pieces with the float when you tear it down.

Then reassemble. When you put the slid back in, it has to align correctly. You'll notice it has two slits in it. If you look down the carb body where the slid goes, you'll see there is a guide on one side, and the idle speed screw on the other, both at different heights. So make sure you line the slid up with them, it can be tricky. Then finish your assembly.

If you did everything right, you'll know soon enough if your carb was your problem. She'll either be running good, or just the same as before. Make sure you set the air screw back to the stock recomendation after the cleaning. Actually, you should back it out quite a bit before the cleaning, so it gets a good shot of cleaner too. Well dude, I hope this helps. If you have any trouble, we'll do our best to help.

 

[cr25096er]

did you adjust the points? if not they can mimic bad jetting. all you need is a feeler guage.

 

[magneto]

Is the air filter plugged up? What kind of oil did you use on the foam?

 

[jdscom]

Haven't yet oiled the air foam, it actually doesn't look to bad.
What type of oil would you suggest? Also, to clean what type of solvent
works best? any thoughts?

thanks

 

[atc3434`]

I would go NoToil! Its my personal preferance. But there is a big debate on this topic. I'll give ya the link.

The Filter Oil Poll/Debate

 

[jdscom]

Ok I still need help please.

I took the carb off and totally apart, cleaned ever piece, jets are stock according to manual. All back together and back on, when I fired it up it started and ran great. Purring nicely and reving up fast when twisting the throttle. I was in heaven for a short few minutes.

After about a few minutes when the engine warmed up it started back to the same misfiring, bogging, and cutting out when the throttle pulled hard. No power under load.

The air screw goes into seat and no change, plug looks dry but black a sutty. A few questions if anyone could help.

1. What is an easy way to check for an air leak?

2. I noticed when I took the carb apart the pilot screw did not have the little washer that the diagram described. Could this be my problem?

3. The diagram also shows a small plastic looking thing on the main jet, called a jet holder. Looks like it clips on. My carb didn’t have one of these either. ??

4. I think I adjusted the float level correctly, again according to manual, I’m wondering if it is too high. Could this cause that much trouble.

Sorry for the long post but I’m stumped and need help. The last thing I want to do is take this into a shop.

Thanks in advance.

 

[23jayhawk]

From your initial post, I inferred that the bike had previously run well for you, and then began giving problems. If that is not the case, please advise.

If that is the case, I guess I would still be thinking you've got some ignition problems, perhaps a coil that is breaking down between windings. Does that year XR have CDI? Could also be an ignitor breaking down when warm.

Plugs foul for only 2 reasons - too much fuel or too little spark. It would seem you've eliminated the fuel related possibilities.

Have you tried swapping coils, plug wires, or mag with your XR80 to run some cross-diagnostics? I think those engines are almost identical in most ways, and you may be able to avoid going to the shop.

 

[jdscom]

Your comments are logical, and i thank you. I thnink I should check the coil next. My XR80 is at my cabin aout 300KM away, so I'll have to find a meter or something to check.

I still can't get over how well it ran when I first put the carb back on. Like you say the problem is when the bike warms up. thanks again.

any other thoughts are always appreciated.

 

[qwchrbeliakn]

I still can't get over how well it ran when I first put the carb back on. Like you say the problem is when the bike warms up. thanks again.

It ran good for a few minutes after the carb maintenance...a few possible causes come to mind.

1.) If the floats are adjusted such that too much fuel is in the float bowl, the engine would run good in the initial minutes while the float bowl was filling up...but not quite full. When the float bowl was almost full the engine will start to run richer and cause the misfire situation you're experiencing. When the carb is apart you also need to test the float valve (do this by blowing into the fuel inlet tube and toggling the valve open and closed). You can test if your problem is float bowl related (i.e., incorrect float level or bad float valve) by draining all the fuel in the float bowl with the fuel tank valve (petcock) off. Then start the bike and see if it runs good for a few minutes...as before.

2.) Another possible cause that came to mind is with the fuel cap. If the vent hose is not allowing air to enter the tank a vacuum will develop in the tank. This vacuum will hinder fuel flow into the carb. This cause is consistent with what you found by turning your airscrew in all the way to get a better idle.

3.) If the bike runs good for a few minutes, but only when the bike is cold, I would suspect the coil. I have seen many coil failures with this symptom. You could take the coil to a shop for testing if you do not have an ohmeter. A good looking spark when viewed with the spark plug hanging outsite the cylinder is not evidence enough that the plug is firing good when inside the cylinder because more voltage is required to spark the plug when the plug is exposed to increased pressures.

Tom

 

[jdscom]

I was thinking float problem, mainly cause I''m not sure how to set the thing. I'm following the manual instructions but the diagram shows the float as a circular ball. In my carb its like a small boat. Can't figure out where to measure the 12.5mm to!

I read through the float level guide on the just KDX site, but the way it shows to set the level seems way out of wack compared to when I took the carb apart. Can't see how it could have gone that far out of adjustment, so i'm questioning whether the example is correct for my carb.

I checked the Keihin web site but there is no info on this old PD36B carb.
If anyone has info on this I could use help.

Thanks