leak Down test Yamaha G2 1986

don gould · 05-10-2016, 12:21 PM

Need some Ideas here before i tear this engine down

Performed a leak down test.

With 100 PSI into the engine I could not get the piston to hold at TDC it rotated around to the bottom of the hole. When we do a regular V8 we bring the piston to TDC and it hold there while we pressurize the cylinder. The engine had a 70% leak at the bottom and we could hear the air escaping from the intake valve and out through the carb which should be normal as the intake valve was starting to open.

We rotated the engine by hand and it operated as normal on the intake side but when we got to the exhaust stroke, with the exhaust valve wide open it only had a very small hiss through the exhaust, we cracked the muffler loose and we could here a slight hissing but not like it should have been with 100 PSI blowing in the spark plug hole.

We could not get the engine to rotate as the pressure was to high to turn it by hand when it started up on the compression stroke. It almost seems like the exhaust valve port it plugged.

This engine loses all power in about 20 seconds of running, it pops through the carb when trying to start and backfires intermittently through the carb when running and dies.

It builds huge crankcase pressure when it does run and after a few seconds it starts to blow oil out the Valve cover breather hose.

We tried a Compression test but it would only bounce the needle to about 40 PSI and back to zero we could not get it to hold pressure. There is too much compression to turn the motor over by hand.

Our diagnosis at this point is the exhaust port must be plugged up somehow (Carbon?) We check the valve lash to .004, Oil level is right at the top of the stick marks. Plenty of fuel, good spark. Last year it ran fine but pushed oil out the breather hose, then it just lost power and these other issues started.

We are planning on pulling the head and checking that exhaust port and doing a valve job..... are we missing anything????

We work on race engines everyday but this little 1 cylinder has us all confused. Why won't the exhaust valve allow the air to blow freely out of the exhaust port???

don gould · 05-10-2016, 02:44 PM

So heads off and everything looks fine, a little carbon build up but nothing extreme. So why won't it flow through the open exhaust valve?

Mike Mac · 05-10-2016, 03:49 PM

Then chase it down streaml, take the muffler off and blow air through it.

And, are you sure the exhaust valve was opening, before you pulled the head?

don gould · 05-10-2016, 06:52 PM

Air blows freely through the muffler, valve was definitely opening wide, Checked the valve and it shows a good seat all around. Piston looks fine no visible cracks and cylinder walls look fairly good no deep scratches. i put my fingers on the lifters and they are opening and closing all in time with the stroke.

don gould · 05-10-2016, 06:54 PM

I can tune a 318 Dodge to make 600 HP and run 10 second 1/4 mile passes but I can't figure out this *** 1 cyl Golf Cart engine.... I'll find the issue and fix it, will just take time and patience or a 1/2 stick of stump powder.

Do you guys lock the crank somehow at TDC to do a proper leakdown test?

smallblock450sl · 05-10-2016, 06:55 PM

Did the head gasket look like it was leaking? Did the head bolt threads look "brown" and "corroded"?

don gould · 05-11-2016, 10:46 AM

Front center bolt is brown and coated in carbon trace. All others look fine
The head gasket shows a very small carbon trace in one small area

05-10-2016, 12:21 PM	#1
don gould Not Yet Wild Join Date: Jun 2015 Posts: 5	leak Down test Yamaha G2 1986 Need some Ideas here before i tear this engine down Performed a leak down test. With 100 PSI into the engine I could not get the piston to hold at TDC it rotated around to the bottom of the hole. When we do a regular V8 we bring the piston to TDC and it hold there while we pressurize the cylinder. The engine had a 70% leak at the bottom and we could hear the air escaping from the intake valve and out through the carb which should be normal as the intake valve was starting to open. We rotated the engine by hand and it operated as normal on the intake side but when we got to the exhaust stroke, with the exhaust valve wide open it only had a very small hiss through the exhaust, we cracked the muffler loose and we could here a slight hissing but not like it should have been with 100 PSI blowing in the spark plug hole. We could not get the engine to rotate as the pressure was to high to turn it by hand when it started up on the compression stroke. It almost seems like the exhaust valve port it plugged. This engine loses all power in about 20 seconds of running, it pops through the carb when trying to start and backfires intermittently through the carb when running and dies. It builds huge crankcase pressure when it does run and after a few seconds it starts to blow oil out the Valve cover breather hose. We tried a Compression test but it would only bounce the needle to about 40 PSI and back to zero we could not get it to hold pressure. There is too much compression to turn the motor over by hand. Our diagnosis at this point is the exhaust port must be plugged up somehow (Carbon?) We check the valve lash to .004, Oil level is right at the top of the stick marks. Plenty of fuel, good spark. Last year it ran fine but pushed oil out the breather hose, then it just lost power and these other issues started. We are planning on pulling the head and checking that exhaust port and doing a valve job..... are we missing anything???? We work on race engines everyday but this little 1 cylinder has us all confused. Why won't the exhaust valve allow the air to blow freely out of the exhaust port???
don gould Not Yet Wild Join Date: Jun 2015 Posts: 5

05-10-2016, 02:44 PM	#2
don gould Not Yet Wild Join Date: Jun 2015 Posts: 5	Re: leak Down test Yamaha G2 1986 So heads off and everything looks fine, a little carbon build up but nothing extreme. So why won't it flow through the open exhaust valve?
don gould Not Yet Wild Join Date: Jun 2015 Posts: 5

05-10-2016, 03:49 PM	#3
Mike Mac G8 Specialist Join Date: Jun 2009 Location: Phoenix Posts: 7,799	Re: leak Down test Yamaha G2 1986 Then chase it down streaml, take the muffler off and blow air through it. And, are you sure the exhaust valve was opening, before you pulled the head?

05-10-2016, 06:52 PM	#4
don gould Not Yet Wild Join Date: Jun 2015 Posts: 5	Re: leak Down test Yamaha G2 1986 Air blows freely through the muffler, valve was definitely opening wide, Checked the valve and it shows a good seat all around. Piston looks fine no visible cracks and cylinder walls look fairly good no deep scratches. i put my fingers on the lifters and they are opening and closing all in time with the stroke.
don gould Not Yet Wild Join Date: Jun 2015 Posts: 5

05-10-2016, 06:54 PM	#5
don gould Not Yet Wild Join Date: Jun 2015 Posts: 5	Re: leak Down test Yamaha G2 1986 I can tune a 318 Dodge to make 600 HP and run 10 second 1/4 mile passes but I can't figure out this *** 1 cyl Golf Cart engine.... I'll find the issue and fix it, will just take time and patience or a 1/2 stick of stump powder. Do you guys lock the crank somehow at TDC to do a proper leakdown test?
don gould Not Yet Wild Join Date: Jun 2015 Posts: 5

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05-10-2016, 06:55 PM	#6
smallblock450sl Sometime's............ Join Date: Oct 2007 Location: Tacoma Washington Posts: 11,875	Re: leak Down test Yamaha G2 1986 Did the head gasket look like it was leaking? Did the head bolt threads look "brown" and "corroded"?

05-11-2016, 10:46 AM	#7
don gould Not Yet Wild Join Date: Jun 2015 Posts: 5	Re: leak Down test Yamaha G2 1986 Front center bolt is brown and coated in carbon trace. All others look fine The head gasket shows a very small carbon trace in one small area
don gould Not Yet Wild Join Date: Jun 2015 Posts: 5

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