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Yamaha GT 80 HELP

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I just bought a GT 80. It ran when I went to get it. When I got it home I was having trouble starting it. I took the carb off and cleaned it and reinstalled. Am I missing something?

Could someone help me with the carb adjustments. There is an air/fuel screw and what I think is an Idle screw. What should I have these set at? Also, there is not air cleaner on it because it was missing that piece. Is there a possibility that is why it is not starting.

Also the vent lines. I have one on the bottom/floats and one on the side. Neither are connected to anything. Is this right? I would appreciate any help. Thanks.

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Depending on the year it may still have points. if they are dirty/pitted then clean them with a business card rubbed between the closed points. If they are badly pitted time to replace. I had an SWM 320 that would run fine and then wouldn't start it was usually the points. I used to carry abrasive paper to clean them with. (not recommended as you should file them but It will work for awhile)

Or make sure the gas is turned on, and the kill switch is turned off. You would be amazed how many times that fixes things LOL (It happens to all of us. )

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Its a 1980 so no points as far as I know. Here is the update. I cleaned the carb last night and then went this morning to do a little more work on it. It started via full choke and a twisting of the throttle.

So I finally got it started and warmed it up for about 5-10 by lightly blipping the throttle. After that, it was actually idling and sounded really good. I actually rode it around the yard a bit. All told, I had it running for about 20-25 minutes. It didnt die and idled really nicely.

I then decided to turn it off. Several minutes later it fired right up. NO choke, NO throttle. It worked like a dream.

Went to the hardware store to get some stuff and let it cool down. Now im having trouble getting it going again. It appears to just be really hard to start. Once its going and warm it works awesome.

What could the hard start be attributed to. Is the fact that I have to use throttle to get it started right? If anyone could help me with some carb settings it would be great. This is obviously a fuel problem in my mind. Oh yeah and I noticed some fuel leaking from the carb somewhere. The float bowl gasket I think. Could this cause an air leak and therefore a hard start? Any help would be greatly appreciated.

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check your compression. does it push start easily? If is push starts easily but wont fire with the kicker your compression is prob low.

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I haven't checked the compression with a guage. But I have put my finger over the plug hole and kicked it over. It popped my finger right off. I think that is enough to rule out a compression problem. Am I wrong?

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Yes, the pressure of a bad cylinder is still enough to push your finger. Think about it, 80 PSI (lousy for most bikes) is like holding an 40 lb weight with your thumb.

Also, check your plug gap. I had a brand-new looking plug a PO had stretched the gap out really far. The bike wouldn't kick-start but would always push-start. Took me a while to figure that one out.

Try this to sort out your carb:

Cleaning

Adjustment/jetting

Those articles are specific to Mikuni Round Slide, but the overall process is the same for almost any slide carb, in fact, you could probably use the same process on a butterfly multi-barrell automotive carb. Come to think of it, I have. Although it wasn't on my computer screen at the time. I was trying to figure out where the tiny ball bearing I dropped when I took it apart went.

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Im going to go get a compression tester. Does anyone have a ballpark idea of what the compressions should be? I checked specs on the bike and found nothing. I've never had a bike with this small of an engine. Thanks.

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ok. got the compression tester. On the low side I got 90psi. On the side I got around 105-110 psi. This should be enough compression to at least fire right? Im sure its not ideal compression but I think it should be enough for it to fire up. Any thoughts?

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Is this one of the odd-ball dual-spark plug single cylinder machines? If so, you should get the same numbers on both plug holes.

Typically 105 is low, 90 would be really low. You need to find the specs though, this machine may be normal at 105.

How are you doing the test? The standard method is to screw in the tester, open the throttle the whole way and kick it until the needle stops moving.

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No. Its really simple. Single cylinder, single carb. Which makes it more frustrating. I have looked for specs and all I can find is stuff similar to my bike (the only thing I can see is compression ratio 6.8:1). Im going to retest the compression. Also one more thing, there is gear oil leaking I believe from behind the small sprocket. Would this cause some sort of an air leak making it hard to start?

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That leak from the sprocket just means the counter shaft oil seal needs replacing. Nothing to do with starting problem. Think, if the crankcase has a breather hose or vent, which it does, you cannot have an air leak from the crankcase, only from the crank seals. Right?? I'll bet you a dozen donuts the top end is smoked causing the compression and poor kickstarting. My 79 RM100 is the same way. Easy fix, its air air cooled mini, couple nuts and the entire cylinder is off.

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I did some more tinkering with it today. Did another compression test and managed to get 115 psi. I feel like while the compression should be better, it should be enough to at least start. I ran it down a hill and tried to bump start it. It sounded like it wanted to start and then choked out. I have taken the carb on and off about 6-7 times. I am almost 100% sure there is no blockage of any kind in the carb. In my mind one of three things is going on.

1. The wrong parts jets/needle etc are in the carb

2. Wrong carb settings- screw, floats

3. The top end needs replaced.

Here is a link to the same carb I have.

80-_W0QQcmdZViewItemQQhashZitem2564506d36QQitemZ160596782390QQptZMotorcyclesQ5fPartsQ5fAccessories'>http://cgi.ebay.com/ebaymotors/CARB-CARBURETOR-CARBIE-CARBY-1974-74-YAMAHA-GT80-GT

80-_W0QQcmdZViewItemQQhashZitem2564506d36QQitemZ160596782390QQptZMotorcyclesQ5fPartsQ5fAccessories

Could someone tell me exactly what the screws do and maybe suggest some settings? I have tried almost everything.....

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I decided to be proactive and look into the top end. Here is what I found.

photo4.jpg

Doesn't look very good. Looks pretty scoured. So it looks like im going to replace the top end. Do I need to get the cylinder honed? Would it prevent it from scouring the piston like the old one? Once again thanks for any help. BTW on the top of the piston it says .50mm which I am assuming means .50mm over standard. Is this why is was scoured up?

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What does the cylinder look like?

The overbore isn't excessive, I don't think that caused your problem, however, if it was not done correctly, it could.

It's probably a good idea to step up to 1.0 simply because you don't know the history of the bike. If you don't want to do that, at least have a machine shop measure the "ovalness" of the cylinder so you know what you're dealing with.

It should start on 115 psi compression. Did you try starting fluid? What did it do? How do your reeds look (or is this a piston port motor?)

Edited by Smacaroni

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The cylinder looks great. The reeds look good. No chips or cracks. I didnt try starting fluid because there has to be something deeper going on. I had it running for over 25 minutes at one point. I haven't gotten it started since.... I am at a loss

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I picked up a service manual for my 1979 GT80 and this is what is says on doing the adjustments.

Carburetor

1. Pilot air screw: Turn air adjusting screw until it lightly seats, then back it out 1-1/2 turns. This can be done with the engine off.

2. Start the engine and let it warm up.

3. Throttle stop screw: Turn throttle stop screw in or out to achieve smooth engine operation at (idling speed: 1250-1400 rpm

Note: The pilot air and throttle stop screws are separate adjustments but they must be adjusted at the same time to achieve optimum operating condition at engine idle speeds.

Not much information besides that on setting the two screws.

Did you use a carb rebuild kit? Check your floats also:

Checking the float height:

Hold the carburetor in an upside down position. Hold the floats with tang just touching the float needle, measure the distance from the top of the float to the float bowl surface (gasket removed). If the distance is not correct , adjust the float height to the specification. Float height: 22.0 mm +- 2.5mm (.87 +- 0.1 in) To correct the float height, remove the float assembly and slightly bend the tang as required. Both Floats must be the same height.

Hope that helps.

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Hello all. I just bought a 1973 G. Its either an 80 or 100. I checked the engine compression and it was only 30. I put some oil in and got the compression to 60. 

Any recommendations on where I should go next to get the compression higher? 

 

Alex 

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Pull the barrel, it needs rings at a minimum and a bore, piston and rings worse case.

You could just try and clean out the ring lands but... I doubt that will fix it. It is an iron bore and may have rusted.

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