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Discussion Starter · #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.
 

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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.
 

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Discussion Starter · #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.


s-l1600 (1).jpg

 

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"Chrome or shiny edge are top rings, grey or darker edge are bottom rings IIRC...... Be sure to install with makers marks up.....
 

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Discussion Starter · #6 ·
Thanks for the info. Sounds like I installed them correctly.
 

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Discussion Starter · #7 ·
What method is best for reinstalling the piston and cylinders. Do I install the piston first and then try to slide the cylinder on top while compressing the rings?
 

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Discussion Starter · #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.
 

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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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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.
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....
 

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Discussion Starter · #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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Those scratches will not hurt anything as long as whatever created them is not in the engine. Probably from someone working on it in the past.
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Discussion Starter · #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.
 

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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.
 

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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.
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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.
Those scratches are from after they cast the hub. Then they grind the sprues off on a big grinding wheel.

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Discussion Starter · #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.
 

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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.
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.
 
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