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100cc Compression

2K views 29 replies 7 participants last post by  mike in idaho 
#1 ·
Hoping someone can help me with compression here. I just acquired a YL1 that had been sitting for over 20 years. Rebuilt carbs, replaced battery, replaced petcock, sealed tank, replaced plugs and changed all fluids. Started right up after 2-3 kicks and after adjusting idle and air screws it seems to idle ok. I am able to put it gear but it just has no pick-up when trying to take off. If I pull hard on the throttle while in 1st gear it seems to not bog so much. I tested compression of both sides with plugs out, throttle open and engine warm on a cheap compression tester. Got 60 compression of both sides. Next I put some oil into the spark plug openings and tested and now I got 120 on left and 90 on right. This seems off since on the first test they were both 60. Is it possible that this tester is inaccurate or am I doing something incorrectly? This is my first time testing compression on a bike. hoping I don't have to replace rings but am prepared to do so if need be. Thanks in advance. I know this isn't a Honda but I do have 3 other vintage Hondas and am having a hard time finding a forum as good as this one that is as active for Yamahas.
 
#2 ·
The 100 twins don't have a lot of low-end torque(but they really scream on the top end). It starts and idles nice, at least the crank seals must be in decent shape. I would put a few miles on it, then re-check the compression. You don't normally see big numbers on a piston port two stroker, the effective compression ratio, at cranking speeds is only about 8:1.
 
#3 · (Edited)
I ordered new rings, bearings and copper gaskets. The rings that came have 4 rings in a box. Two of one type and two of another. Areyou familiar with this? The box says YL1 but I was under the impression that both the top and bottom rings were the same. I'm assuming the box came with 4 rings so you could replaced the rings on both pistons but I can't find any info on which ring should be top or bottom.


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#9 ·
Thanks TOOLS. Got the pistons and cylinders reinstalled.The bike starts up in one kick and idles fine. I put it on the center stand and put it in first gear with no issues until I press the brake. Once I press the brake with the bike in gear it wants to die. From what I'm reading it may be a clutch issue. Should I go straight for removal of the clutch plates or is there something else I should check first? I have multiple manuals that I can follow for removal but I would rather not remove them if not completely necessary. Thanks in advance.
 
#11 ·
This is only a problem IF you are pulling the clutch lever to disengage the clutch WHILE applying the brake and this happens....
Applying the brake while it's in gear will ALWAYS stall it if the clutch isn't disengaged....
 
#10 ·
Check the adjustment, but you probably have clutch discs that are stuck together. It is common and not hard to fix. Most of the time you can just remove them and pop them apart in the process, and then reinstall them. If you handled the pistons, you can handle the clutch.
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#12 ·
Pulled all the plates apart. None of them really seemed stuck but the metal pieces had a bit of light rust so I sanded with fine grit, lubed and reinstalled. One thing I noticed was these scratches on the outside of clutch basket. Is this abnormal? Also ordered a new gasket for cover since the old one came apart. Will update on the status of the issue once gasket arrives and I can take for a test drive.
 

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#14 ·
I pulled the clutch plates apart wiped them clean and then sanded with fine grit sandpaper. Dipped the in oil and reassembled. I them adjusted the clutch per the manual. I still have the same issue. When the bike is running in neutral i can rev like normal. I pull the clutch and kick into 1st and once I start to release the clutch while giving some throttle it wants to die. Could an air leak cause something like this? This bike also does have an autotune system, which I adjusted last night. Could too much oil coming from autolube be the cause? If I give it way too much throttle it will roll a little and i was able to go down the driveway. One thing i noticed is if I put the bike on the center stand and start it in neutral the rear wheel doesnt spin. If I shift into 1st the rear wheel spins. But if I pull the clutch lever while in 1st the wheel still spins. Not sure if this is normal. The total mile on the odometer at 5500.

To recap...so far I have rebuild carbs, cleaned clutch plates, replaced piston rings, adjusted clutch, replaced petcock, sealed tank, new spark plugs.
 
#15 ·
The 100 twins have pretty much zero bottom end torque, it takes just the right combination of clutch lever and throttle movement to take off smoothly. Having the timing and carb adjustments spot on helps too. The rear wheel will try to turn in gear with the clutch disengaged, that's a normal characteristic of a wet type clutch.
 
#20 ·
After I'd given up motorbikes and bought a car, back in my very early '20's, I had an old and much abused YLI that I used to ride on around my parents garden. Until a neighbour complained about the noise, after which it went to my friends parents farm where it got used as a field bike, before being abandoned in a barn, where its rusty remains probably still lie.

Anyway, the point about bottom end torque - my parents garden wasn't that large, and I used to ride around in 1st gear at walking pace, playing at being a trials bike rider. I always thought that it chugged along quite well at low speed.
 
#16 ·
I am beginning to think it is just running bad and you might not be explaining the problem in a way we were understanding it. Not having power is my clue here. I would start looking at the carbs for vacuum leaks. Did you rebuild them with aftermarket brass parts from a kit? I always replace the crank seal behind the stator on old two-stroke engines. Thes seals will go bad and create a vacuum leak into the engine.
TOOLS
 
#19 ·
TOOLS,

Yes, I rebuilt the carbs with aftermarket brass parts and the replaced the carb gasket. The best way to describe it is low power. It just feels like I am trying to start in 3rd gear.

Chaindrivecharlie,

The compression that I got before the rings was low 60s. After changing the rings I got similar results but I cant rule out that the cheap compression tester I got was inaccurate. I have a new short style tester coming in the next couple of days.
 
#21 ·
As for it's low power, I've got many two strokes from piston port, rotary valve to reed valve, and I can tell you with full confidence that nothing is more gutless off the line than a small cc twin piston port induction bike (such as a YL1) which I also own. They just can't get out of the hole, however do scream if you keep the RPMs way up.
 
#23 ·
TOOLS,

I am going to check the crank cover seal this weekend. Already ordered a new one. Could it be an issue of the rubber gasket that connects the air filter box to the carbs isn't sealed well? that piece is definitely old and brittle. unfortunately i have seen no replacements for that part online.
 
#24 · (Edited)
If the crank seal on the shifter side is out, it will suck air and want to run away at idle. If the seal on the opposite side is bad it will smoke at lot out the right pipe as it burns transmission oil. I assume you cleaned the carbon buildup out of the exhaust ports when you did the rings, did you pull the muffler inserts to remove the carbon buildup there? Did you get the control cable for the autolube adjustment set correctly?
 
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#25 ·
It is definitely smoking a lot. I did clean the carbon buildup out of the exhaust ports and I replaced the little metal ring gaskets where the exhaust connects to the engine. the actual mufflers themselves have a ton of buildup that i have tried removing with WD40 which was only semi successful.

I did attempt to adjust the control cable for the autolube but something odd that I found was that the manual was talking about using a "V" notch to make my adjustments. I did not see a "V" anywhere on the system. The only indicators were two yellow paint marks the I I assumed where supposed to be lined up if it was adjusted correctly. Only issue i noticed with this was that when I lined those paint marks up the adjustment was as far as it could go to one side. I have been wondering if this could be letting too much oil into the gas and causing it run poorly also.

Anyone know any Indiana mechanics that would take a quick look maybe be able to diagnose for some $?
 
#27 ·
The "V" is on the wheel the cable wraps around, it is lined up with the roll pin in the plunger. Use only two-cycle motor oil, stay away from anything marked "outboard motor oil", outboard oil is formulated for water-cooled engines and will leave ash deposits that will stick piston rings and build up carbon.
 
#29 ·
Ok guys, when messing around with it today I noticed a lot of smoke coming from the right exhaust when I pull the throttle and none coming form the left. When I pull the throttle and it seems that the left piston is not pumping but when it idles it seems to pump. I wonder if I have been running on one piston this whole time.
 
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