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Unread 06-23-2010, 09:55 PM   #1
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Default Freshly Rebuilt G1 w/90 lbs Compression

Well, I have put my engine back together. New cylinder, piston, rings, crankshaft, bearings and gaskets. So basically a brand new engine. Problem is I am only reading 90 lbs compression at WOT, warm engine. Head torqued to 18 ft-lbs.

I may have dinked my seal on the clutch side a bit. It started in wrong and I kinda got a little over anxious. I did however put a little grey sealant on the dink. I do not think it was bad enough to cause that much problem Could be wrong though. Are these seals that touchy?

I sealed my cases with a little grey Permatex so I dont think that would be the problem.

I guess I will by two more just to be safe.

Other than that what are my options? Spray carb cleaner near seals?
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Unread 06-23-2010, 10:39 PM   #2
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Default Re: Freshly Rebuilt G1 w/90 lbs Compression

Compression is derived from the piston compressing the air/fuel mixture in the top of the cylinder, not the lower crankcase area so any issues below the piston won't effect it.
I would be more concerned with compression loss or blow by from improper ring end gap, cylinder wall to piston clearance, poorly seated rings from improper honing.
Can you elaborate on the build? Was it bored?, cross honed?, specs verified?
Any chance you have the piston in backwards?
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Unread 06-24-2010, 07:21 AM   #3
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Default Re: Freshly Rebuilt G1 w/90 lbs Compression

Quote:
Originally Posted by crash12888 View Post
Compression is derived from the piston compressing the air/fuel mixture in the top of the cylinder, not the lower crankcase area so any issues below the piston won't effect it.
I would be more concerned with compression loss or blow by from improper ring end gap, cylinder wall to piston clearance, poorly seated rings from improper honing.
Can you elaborate on the build? Was it bored?, cross honed?, specs verified?
Any chance you have the piston in backwards?
I used a new cylinder and top end kit from site sponsor. No boring or honing. The piston was installed with "EX" toward exhaust. I did not check the ring end gap because I forgot in my excitement to get the cart up and running. If everything was new in a kit, I would like to think I would not need to worry about it.

So I guess my rings are not seated?
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Unread 06-24-2010, 06:12 PM   #4
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Default Re: Freshly Rebuilt G1 w/90 lbs Compression

How does the engine run? If it runs OK, I wouldn't worry about a possible low compression reading at this time. The rings have not seated in yet, and compression guages can vary a lot in accuracy. Unless you have a noticeable problem, don't worry about it. Go drive that buggy!
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Unread 06-25-2010, 08:00 AM   #5
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Default Re: Freshly Rebuilt G1 w/90 lbs Compression

Well it does have more power than before but not getting the higher speeds from it. The engine torques up really well on the low side. I put a torque kit in it too though. I will give it a few more days and see. I ran it hard yesterday. Probably drove 3 miles WOT and let it sit to cool for 2 hours ablnd then drove back home the same.

My plug wire is "wallered" out. :-) I need to replace the end on it. Is this just simply pulling the end off and replacing it?
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Unread 06-25-2010, 08:02 AM   #6
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Default Re: Freshly Rebuilt G1 w/90 lbs Compression

Well it does have more power than before but not getting the higher speeds from it. The engine torques up really well on the low side. I put a torque kit in it too though. I will give it a few more days and see. I ran it hard yesterday. Probably drove 3 miles WOT and let it sit to cool for 2 hours ablnd then drove back home the same.

My plug wire is "wallered" out. :-) I need to replace the end on it. Is this just simply pulling the end off and replacing it?
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Unread 06-25-2010, 12:11 PM   #7
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Default Re: Freshly Rebuilt G1 w/90 lbs Compression

Ok Just an update. I measured my piston to cylinder clearance and thought it was good when I put it together. Come to find out, my tolerance is 0.004" max. I measured first what I thought to be 0.001" but turns out it was really 0.012". I missed a decimal place. This is a brand new cylinder and piston kit from Everything Carts. I have called them and await the technical support to call back. I hope they will warranty it. Its bad when I have spent so much money replacing everything on this cart except the engine cases. I have replaced everything new.

Seems like E-Carts would inspect the dimensions to be sure they meet the Yamaha specs before sending this to their customers. I guess the cheapest place to by parts is not always a good idea.

Oh and I fixed my coil wire. Just crimped the end a little more so it grabs the threads on the top of the plug.

I will keep everyone updated on how it goes.
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Unread 06-26-2010, 04:59 AM   #8
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Default Re: Freshly Rebuilt G1 w/90 lbs Compression

Where are you measuring the piston to cylinder clearance? It should be measured at the piston skirt. If you are measuring it at the piston head, you will get readings like you describe. The top of the piston expands a lot under combustion heat and pressure, so it will be sloppy when cold.
These aftermarket parts are made in China or Taiwan, so quality control is not the best. If you really do have a quality issue,that's the manufacturer's fault. The seller can't be expected to open and reinspect every box they are shipped for resale.
If you put a torque spring in the secondary clutch, it will knock the stuffing out of your top speed. Gobs of torque, but no top end. I have one in my G1 yard cart (1 1/2" lift, tractor tires on stock wheels), where climbing hills with a load is important and top speed doesn't matter. In my son's G1 (4" lift, 20" tires) I took the torque spring out and shimmed the original spring with a piece of PVC waterpipe connector. By varying the shim thickness you can balance out the power/speed you want. Try sticking the original spring back into the secondary and take it for a spin.
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Unread 06-26-2010, 09:30 AM   #9
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Default Re: Freshly Rebuilt G1 w/90 lbs Compression

Well it looks like the piston and cylinder are both undersized. Standard piston size is 68mm. The piston measured on 67.7mm. The cylinder measured 67.86-67.9mm.

Everything Carts was very helpful. They are going to send me a new piston and I will hone the difference to get it just right. I highly recommend the technical support from Everything Carts. I do not like the quality of the parts you get these days. Everything is subpar quality. My old piston has years of abuse and it looks good. I ran the replacement for about 2-3 hours total and it looks horrible. Like piston slap times 10.

I am getting the exact same speed from my cart with either spring. Stock tires and wheels I get 21mph consistently. Unless I go down a hill, I hit a staggering 23mph! :-)

I really like the low end torque I was seeing. Even with the 90 lbs of compression I was getting a little tire spin on gravel. Can't wait to get the new piston in.
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Unread 06-26-2010, 12:40 PM   #10
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Default Re: Freshly Rebuilt G1 w/90 lbs Compression

Good on EC for taking care of your problem. I know what you mean about substandard quality these days. There is a lot of pressure to sell stuff cheap, and it sometimes shows in the quality of what you get. 21 MPH in a basically stock buggy is not bad. See here:
82' G1 17.5 MPH top speed
I assume you have bypassed the governor. With stock wheels, tires and engine that's about all you get. Bigger tires and wheels should probably be your next step for more speed. You can get more out of the stock engine with modification, search this forum to see what other owners have done.
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