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Old 03-02-2021, 09:08 PM   #1
Daniel.smith6
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Default G1 lack of crankcase pressure

I have a 1982 Yamaha G1 gas cart. It has not ran for over ten years. I am a student in a Marine and Powersports program and we are taught to do three or four tests on every thing before deciding to fix something. here is what we did and results:

Spark plug test it has great spark
Compression 125psi
Leakdown test less than 10%
Crankcase pressure zero nothing zilch
Crankcase vacuum again zero, nothing zilch
This thing should not run at all.

Here is the problem it runs great no issues. I am going to clean the carb, drain the gas add fresh, but it this one has everybody perplexed. We did the usual stuff plugged everything, closed off everything but still no pressure nothing. What are we missing (12 students, 2 instructors) Thanks in advance

Last edited by Daniel.smith6; 03-02-2021 at 09:14 PM.. Reason: wrong forum
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Old 03-03-2021, 02:26 PM   #2
slonomo
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Default Re: G1 lack of crankcase pressure

You guys know it's a 2 stroke right?

The G1 likes to eat crank seals. When someone did the last rebuild they might have used aftermarket crank seals, they never last long. Look for oil around the crank, that's a sure sign.

I had a G1 that the clutch side crank seal would pop off and rattle around there behind the clutch lol. It still ran.

Now you say that it runs great, but I'm betting it will run a lot better when you replace the crank seals (assuming that's where the pressure leak is).

If there is no crank pressure/vacuum the oil injection system will not work correctly. If you haven't already, disconnect the oil injection system and premix with a good quality air cooled synthetic 2 cycle oil to 125:1.
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Old 03-03-2021, 09:00 PM   #3
Daniel.smith6
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Default Re: G1 lack of crankcase pressure

When we did the pressure test we sprayed soap and water around all the seals, hoses, and any fuel, vacuum lines, around the oil pump, fuel pump even around the head gaskets to check for escaping air. We even checked to make sure the Booster Box was intact and not leaking. We also listened for any sounds of leaking air from around the motor.

But I have to redo the clutch so I will look around the seals for any leaking oil on both sides of the crankcase.

It's a two stroke, one stroke for the piston going down and one stroke for it coming back up. Right???? Just kidding that's why we did the crankcase pressure test.
Thanks
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Old 03-03-2021, 09:14 PM   #4
meimk
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Default Re: G1 lack of crankcase pressure

I've never really checked crankcase vacuum / pressure on a 2 stroke. The vacuum would pull open the reed valves to draw air & fuel through the carb. The pressure pushes that fuel air mix up into the cylinder. I'm not sure how much would register.
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Old 03-05-2021, 08:44 AM   #5
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Default Re: G1 lack of crankcase pressure

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Originally Posted by meimk View Post
I've never really checked crankcase vacuum / pressure on a 2 stroke. The vacuum would pull open the reed valves to draw air & fuel through the carb. The pressure pushes that fuel air mix up into the cylinder. I'm not sure how much would register.
I was thinking along the same lines. I'm no professional mechanic, but I know 2 strokes operate a little differently than a typical 4 stroke, and crankcase pressure is likely a much different thing. I also know there are guys like Don Plowman that have been working on 2 strokes for 30-40 years or more that understand the air flow cycle on this engine far better than anyone here, and probably better than any small engine instructor. You could always call him. Look in the site sponsors page for Plowman's Carts.

Not saying your instructor doesn't know what he's doing, but can I ask....

"Why is your instructor looking to measure crankcase pressure on a 2 stroke golf cart that runs good?"

If the engine is running good, there absolutely has to be vacuum/pressure. The engine relies on this vacuum/pressure to function properly.

I'm thinking you are missing something when you are plugging all the holes up. When you did the leakdown test, you had good results? What % was the loss of pressure? What pressure were you using to test?

I'm thinking the error is in the method of testing the crankcase pressure/vacuum.
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Old 03-05-2021, 09:42 AM   #6
rstacy
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Default Re: G1 lack of crankcase pressure

I checked the G1 and G2 service manuals. They do not mention doing a crankcase pressure test anywhere.

It has not been my experience that any of the Yamaha 2 stroke engines like to eat crank shaft seals but after many years the seals can fail. I have a couple of 82s here running great on the original seals.
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Old 03-05-2021, 03:25 PM   #7
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Default Re: G1 lack of crankcase pressure

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Originally Posted by rstacy View Post
It has not been my experience that any of the Yamaha 2 stroke engines like to eat crank shaft seals but after many years the seals can fail. I have a couple of 82s here running great on the original seals.
I agree with you 100% on the Yamaha seals, they have been known to last 20 yrs plus. My beef is with the aftermarket seals. They get eaten.
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Old 03-08-2021, 01:40 PM   #8
smallblock450sl
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Default Re: G1 lack of crankcase pressure

Yes crank seals/ base or head gaskets could be a cause. Never did a test like that on a 2 stroke. If you think about it, you have piston and cylinder ports and reed valves. If your blowing air into the engine to check, where is the air going to go, threw the the least resistance. I say if it runs good....don't mess with it
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Old 03-10-2021, 09:03 AM   #9
AGELE55
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Default Re: G1 lack of crankcase pressure

I’m wondering if the crankcase ever really “pressurizes”. Would it be more like a constant flow thing?...like air moving through a fan? Lots of volume, but little pressure. Just thinking out loud.
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Old 03-10-2021, 10:20 AM   #10
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Default Re: G1 lack of crankcase pressure

Quote:
Originally Posted by AGELE55 View Post
I’m wondering if the crankcase ever really “pressurizes”. Would it be more like a constant flow thing?...like air moving through a fan? Lots of volume, but little pressure. Just thinking out loud.
I don’t know for fact, I’ve never tried on a 2 stroke. But I tend to agree with you. Any amount of “pressure” in a crankcase of a 2 stroke would be minimal. It’s not going to measure like a 4 stroke.

In a 4 stroke the compression/combustion chamber is completely separate and sealed from the crankcase via piston rings. The crank case has pressure/vacuum pulse because the piston moves up and down which chances the displacement of air inside the crankcase. The only relief mechanism is the PCV valve.

In a 2 stroke the compression/combustion chamber is not sealed from the crankcase. It inhales air through the reeds and carb on the side of the case, circulate through the crankcase and then directed into the compression/combustion chamber through a port in the cylinder. So because it’s drawing the air in, there would be a vacuum pulling the air in through the carb/reeds. But at the point that the piston would travel downwards (this is where the 4 stroke creates pressure) a port in the cylinder opens up allowing that vacuum (and the air/fuel mix) get drawn into the cylinder instead of turning into “pressure”.

So while a 2 stroke will create vacuum, any positive pressure would be eliminated because of how a 2 stroke operates.
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