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'79GT80 I had to buy a Mikuni VM18 carb to replace the stock 16mm carb

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I finally got the basket case put together. And it runs good once you get it going and warmed up, but it is hard to start. Stock carb is a 16mm Teikei and no longer available. I bought a brand new Mikuni VM18 (they used to sell a VM16 but no longer available). It comes supposedly jetted for an average 2 stroke engine that it would fit on. I had to make a PEEK plastic adapter to clamp it on the stock intake. I am having to push start it to get it going. It has great spark, and I've messed with choke/no choke, no throttle, little bit of throttle, WFO throttle til I'm blue in the face. I've turned the air screw out 1 full turn.

At this point, I'm thinking of going down a size on the pilot jet.

Has anyone been down this path before, or remember reading on here something similar?

BTW, my 10 year old son is so happy. His neighbor friends all have new 4 wheelers, and he'd just stand around with his hands in his pockets wishing he had something. (I was him back in 1971)...
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03 Nov 2020 11:20 #1

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Firstly has it got reasonable compression to feel by hand on the kick start. Being an 80cc it should feel pretty good. Secondly will it idle or just fade away--like low comp. Also when running does it respond to the idle mixture screw which if right home "screwed home lightly" it should richen up noticeably. When running does applying the choke make it want to stop, or make it run faster. If the choke jet drilling in the bowl is blocked the opening choke just ads more air & no fuel, making a bad situation even worse. Mind you, a new carb shouldn't be blocked but was a thought. Points set to 14 thou. Wouldn't go chasing timing yet as will be close enough & only altered by points gap. Old points can close right up over time & fiber heal wear which retards the timing & knocking the pony power right back. Doesn't sound like that though. Also i'm hung up on blocked exhausts lately but only from a top end pony power point of view & once again doesn't sound relevant in your case.
Great little bikes by the way.
Last edit: 03 Nov 2020 14:53 by RT325.
03 Nov 2020 14:52 #2

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Thank you for your reply.

Compression is suspect. It has a new piston and rings and bore, but it was done years ago, and I worry that the cylinder may have rusted??? I have read about trying to check compression online here, and they say it is hard to get a good reading on small 2 stroke engines with a compression tester with a quick connect. (which is what I have). With the motor cold, not squirting oil down the cylinder, full throttle opening, I could only get it up to 30 psi. Then I tested my small 2 stroke shindaiwa weed eater, and it only tested less than 3 psi, and it runs fine. The GT80 runs great, when it is running. If you wick it in first gear in it's power band, it will pull up the front wheel.

But yesterday, I still had to push start it, and clear it out/warm it up. While it was running good, I was trying to teach my son to ride (only his 2nd time on it), and it sputtered and died. I guess we ended up fouling the plug to the point it wouldn't run anymore. It was a Nippondenso with only about 45 minutes of run time.

I have a UNI foam filter on it, and I oiled it and wrung it out.

I can't really tell if the air mixture screw is working, as it doesn't idle well. I tried holding it at a set rpm, guessing about 3000, and turning it in and out. No major difference. Weird.

I might try to lower the fuel level in the bowl. I see evidence of a bit of fuel coming out the overflow hose after riding it.

The choke seems to have no evidence of changing the way it runs, when it's running and I blip the throttle. I admit, I bought the VM18 brand new, and just bolted it on after making the adapter sleeve. I did not take it apart. I need to take it apart, the choke is suspect.
08 Nov 2020 06:16 #3

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Here are the stock VM18 size jets as supplied.
08 Nov 2020 07:33 #4

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Simply for comparison sake, I will do the exact same test on my GT80 using a similar quick connect style compression tester. I believe my GT is completely original with about 2,300 miles on it. I will reply this evening with my results.
Schu

CT175B, CT175C, JT1' JT2' DT360A, GT80B, DT100B, DT125B,
DT175B, DT175C, DT250B, DT400B, Z50, SCR950

Someday, you'll own some Yamahas
08 Nov 2020 07:36 #5

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From what you're describing, sounds way rich on the pilot. You could try temporarily removing the air screw completely to see if it got any better at starting.

Also on the filter, again you might try removing it as a temporary test. Especially if you used the sticky "air filter oil". The foam doesn't need much oil and I've seen it many times that rich issues are caused by an over-oiled filter... I like the Twin Air oil or the old school thing we used to do was wash the filter in old 20:1 premix and let it dry.. and not adding more oil. (not recommending the premix method, but it did seem to work okay.)

I'd start with a 20 pilot. That's what Yamaha used on the 1960's 72cc bikes that came with VM15 or 16 carbs.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
08 Nov 2020 10:15 #6

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Ok, mine peaked at 115 PSI cold, WOT, no oil squirt.
Schu

CT175B, CT175C, JT1' JT2' DT360A, GT80B, DT100B, DT125B,
DT175B, DT175C, DT250B, DT400B, Z50, SCR950

Someday, you'll own some Yamahas
08 Nov 2020 13:35 #7

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Wow! Thank you for this. I wonder why it runs so good once it's started?
16 Nov 2020 18:56 #8

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I believe there's a reason Yamaha didn't publish compression specs... it's extremely unreliable on these small engines.

My DT50 won't hit 50 psi on a new top end with my gauges. With Schu's gauge it might hit 130? Runs strong, nothing wrong with it.

The combustion chamber is less than 10cc's and you're adding several more cc's in hose volume not to mention the schrader valve restriction and location can affect the readings significantly. None of this matters much on a bigger displacement engine that the compression gauges were designed for.

In my experience, the only thing a compression gauge is good for is comparing readings to see if compression is dropping over time or comparing similar bikes... like a GT80 to a GT80... as long as you always are using the same exact gauge.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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16 Nov 2020 19:10 #9

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Good info Mark!
My reason for owning a compression tester in the first place is to monitor any downward progression over extended time and usage. Or variance in multi cylinder engines.
I do have another compression tester that is a simple device that you have to press into the spark plug hole by hand with enough pressure to ensure a good seal on the rubber tip.
Now I'm curious to see the variance between that gage and the one with the quick connect fittings on rubber hose.
Stay Tuned!
Schu

CT175B, CT175C, JT1' JT2' DT360A, GT80B, DT100B, DT125B,
DT175B, DT175C, DT250B, DT400B, Z50, SCR950

Someday, you'll own some Yamahas
Last edit: 17 Nov 2020 09:20 by Schu.
17 Nov 2020 09:19 #10

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