1981 Yamaha G1-A2 With Engine Noise

Cisgww

Member
Hello and thanks for the add. This is a great forum!

Need help determining cause of engine noise on a 1981 Yamaha G1-A2. I'm not afraid to wrench on anything, but never opened up a 2 stroke/cycle engine. Been in family for about 35 years. Likely due for more than a tune-up.

Cart #J10-0385744, Engine #S215-035744
Engine "clangs and bangs". Sound seems internal to engine, not external
Starts fairly well with some choke, but crank seals don't seem overly worn. Starting fluid sprayed at crank seals changes nothing.
Compression test with a squirt of oil into cylinder was about 115, assuming gauge is accurate.
Much slop in throttle linkage starting at connection to governor.
Motor mounts good.
Carb needs rebuild as gas coming out of overflow port/vent.
FYI: Starter/generator belt damage during power washing.

How do I post a video for you to see and hear this engine?

Here is a link to the video via Dropbox:
 

Zman

Cartaholic
That is a bad noise for sure, Primary clutch bolt is tight i assume ? I think you have a rod problem and if you keep running it a crankshaft problem. All of this is lubricated by oil/gas mixture and i assume you have either a premix fuel or are using the oil injection. Time to tear it down because 115 lbs is a little low and i am assuming you have the throttle plate wide open during test.
 

Cisgww

Member
Clutch bolt was tight. Oil injection is working.
I didn't have throttle plate wide open, but I think engine was warm.

More videos to follow.
 

Cisgww

Member
Bolt and mounting tube under starter generator had some slop in it, but only noticeable when tensioner and belt removed. I doubt that is source of noise. This video leads me to think a connnecting rod to crank bearing or the piston wrist pin bearing.


The Facebook link above shows my process of elimination and videos should the Dropbox links not work.
 

Zman

Cartaholic
Pull the motor and tear it down, you need a crankshaft and rod. Not sure where you can find one unless there is some at vintage carts.
 

Cisgww

Member
I think I concur. Waiting for clutch puller to continue disassembly. I haven’t measure any thing yet, but Do I need a new piston too? Standard Size with honing or oversized With boring of cylinder? I’ll post video soon

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HotRodCarts

Cartaholic
The piston has some scoring on the skirt. You won't really know if you can use a standard piston or oversized until you check the piston to wall clearance.
 

Cisgww

Member
As best as my tools and talent allow, I get the following sizes for piston and cylinder:
Cylinder = 2.677” at both top and bottom using both General digital caliper and Starrett 124-A bore micrometer.
Piston with General digital caliper = 2.671”. With Central Tool I-beam micrometer (which I think measures high) = 2.675. The piston was measured .39” above skirt end.
Thus I get clearance of .002 to .006 due to difference in piston measurements.
If I am reading Service manual correctly, it states maximum skirt clearance for standard size is from .0012-.0014.

If I understand this correctly, my piston has too much wear and cylinder needs boring for an oversized piston?
If cylinder needs to be bored, any estimate as to cost to have bored oversized?
Seems if I have to have it bored, I’d go .50mm oversized?

Your Feedback greatly appreciated
 

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HotRodCarts

Cartaholic
I think your measurements are off on the piston to wall clearance. Your best bet would be find a local machine shop and and let them check it for you. You'll need a machine shop either way to either hone or bore and hone the cylinder. You can also buy a rebuild kit from Golf Cart Tire Supply.

Have you found what was knocking? If not I'd dig a littler deeper before spending money on machine work or parts. :twocents:
 

Cisgww

Member
I agree that someone with better tools and talent should do a measurement as well. That said, if cylinder has little to no wear, me thinks it just needs honing and I should give it new standard size piston and rings. Is my logic correct?

Thanks for the link. I have been on their website and I already had that added to my shopping cart...just in case.
 

HotRodCarts

Cartaholic
Yes your logic is correct if the piston is standard now and the cylinder can be honed without winding up with to much piston to wall clearance.
 

Cisgww

Member
Just rechecked: cylinder is round. Almost every measurement at top or bottom and any direction of radius is 2.677.

My clutch puller came today and it came of easily. Ready to measure rod play if I can figure out my dial/runout gauge.
 

Cisgww

Member
Here are 3 readings of axial play of the small end of the connecting rod. The dial says 1-1000 IN. Do these 3 average .058” or is it .58”? I’ve got the tools and ability to use them, just need help with the interpreting the results.
The service manual says max play is .79”
 

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Cisgww

Member
The clearance at the thrust washer on big end of connecting rod is .019.
Service manual says .010-.030 is acceptable
 

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1YAMAGUY

Cartaholic
I haven't heard the video but if it were me I'd be closely looking at all moving parts both in and out for a sign of what you think your hearing, that is if your rotating parts are within tolerance and not show any obvious wear signs you should be able to see something, like maybe an idler pully, belt slapping, loose motor mount or bushing.
Just my thoughts, good luck n keep us posted.
 
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