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Dinyoclese

1975 Honda XL100 timing question

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Hi all,

I'm a total noob to fixing vintage bikes and could use a hand if anyone has any knowledge they could share about timing for my 1975 XL100. I recently purchased it from a guy and the bike ran. After riding it around for about an hour the bike would not idle. I checked out the plug, replaced it and it would run again for about an hour. I thought maybe I got a bad plug and did this a third time before I finally accepted that there was a problem. I have gone though quite a bit of this bike at this point. I have replaced the  rectifier, the coil, and the condenser (The points look good). I have found a few electrical problems mostly in the way of corroded connections and have replaced them to ensure that I am getting proper spark and that the charging system is functional. I replaced the battery. I replaced the carburetor, because I couldn't get the stock carburetor to idle properly not matter how I adjusted it. Finally after all of this I could get the bike to idle properly, but I couldn't get the bike to allow for the throttle to be opened beyond 1/4. I assumed this was likely a lean mixture. I enriched the mixture with the jet needle, no help. I tried spaying starter fluid into the air intake to prove or disprove that I had a lean condition. Starter fluid did not resolve the problem.

I read somewhere that sometimes the centrifugal advancer will get stuck and cause this sort of problem, so I took off the points plate and took a look at it. The advancer seems to be freely moving, though I admit I wouldn't know if it is free enough to work. I then put the points plate back on. I have looked at the timing and feel reasonably confident that the timing was right. I later read that sometime old engines need to have their timing advanced beyond factory specs. I advanced mine by a degree-ish. Now I can get to 100% throttle for anything below about 5000RPM without issue. Anything above 5000 RPM  and it sputters, doesn't matter the throttle opening. Anyone have any idea what I am up against? Should I advance the timing further? Is my centrifugal advancer broken? Can I test it? I'm at a loss at this point and could really use some help.

I really appreciate any advice! 

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Hey there. Though I can't seem to help you with that particular problem, I wanted to touch base about the centrifugal advance. I have a 1976 XL100. I Had an idle problem where it would stick into high speed idle after throttling up. It would go back down to the correct idle speed if I bogged the engine in gear while idling. But stick again if I hit the throttle. Did all of those common things to make sure it wasn't the cable etc... Even bought a new carb thinking the slide was hanging up in the carb's bore. Nope, that didn't fix it. 

I found out it was the centrifugal advance springs that had slop in them right at the beginning where they where to open. (turning the rotor slightly and noticing no spring tension at the very beginning) While the engine was running, they opened up just enough to speed the idle up about 300-500 rpm. I took each spring off and shortened their ears (hooks) so that the spring immediately was in action when slightly turning the rotor.  Idle problem solved. So you should feel spring tension as you turn the rotor by hand and make sure the tension starts immediately as you turn. 

I did set the timing to the shop book and all seems fine. No sputter. So not sure why you need to deviate from the book.  Did you follow the book's how to? I can be a little confusing.

Bob

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Sorry for the slow response and thank you for your response @Galaxyflyer1. I went back and checked the timing per the instructions again and I brought it back to standard timing. It's still doing the same thing. I found a thai made wiring harness and replaced all the wiring. I also put in a used stator to see if that changed things because I was getting some weird readings from the original. It runs better to be sure but I'm still stuck with the lack of high RPM bit. I still haven't replaced the points. Probably will do that next as it seems to be the only thing left and it's not too expensive.  I took another look at the timing advance per you suggestion and I think it's OK. Seems like not too much play and the appropriate amount of movement to advance the timing when necessary. I'll report back after replacing the points.  

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THE ONLY WAY YOU CAN CORRECTLY SET THE TIMING IS WITH A STROBE TIMING LIGHT AT 4000RPM OR SO 

STATOR COVER REMOVED TO ACCESS THE FULL ADVANCE MARKS WHICH IS ALL THAT MATTERS 

ALSO GET NEW POINTS 

DO NOT GET CHINESE SHIT 

GET NOS HONDA OR MADE IN JAPAN 

TAKE THE RESISTOR OUT OF THE PLUG CAP 

OR GET A NEW NGK CAP

 

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Thanks @Ken Nickerson. I got a timing light and new points. I removed the resistor. Timing was set pretty much right on with the light falling between the two advance marks. I messed with the timing advance with the engine at high RPMs it ultimately didn't make much difference. Things got a little worse and a little better depending on how close to proper timing I was but no amount of advancing or retarding resolved the high RPM issue.

So, I went back to the carburetor. I adjusted the jet needle to make the bike run more lean. After that I got a similar problem at lower RPMs. I set the jet needle back to it's original position and tried bringing the RPMs up to failure and popped the choke on to enrich the mixture. No effect. I'm now leaning toward getting a bigger main jet and seeing what effect that has. Any thoughts?

Thanks for all the input so far.

 

 

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Just a thought, and wouldn't hurt to check anyway: Have you checked/adjusted valve lifter clearances? 

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Yes, I checked out the valve clearance and one was a bit off. I adjusted it to the proper specs a few weeks back.

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first pul the float bowl turn on gas tap and makes sure a steady trickle comes out

use a catch can amnd drain a pint or more this verifiers flow and gas cap venting

 

does it have a supertrap ?the discs need cleaning quite often

'

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Gas flows well. I've tested it a couple times. Also have 2 carburetors for it. I rebuilt the original and bought a new because I couldn't the original to idle properly no matter what I did. I put the old carb back on with the original main jet. Runs about the same but smells more rich. It does have a super trap and I have not cleaned the discs. I will do today and report back.

 

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