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Yamaha dt3 breaking up with throttle. Please help!

17 Sep 2017 07:05
360jeepboy
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Yamaha dt3 breaking up with throttle. Please help! #1
So I got this thing back together and thought I had everything worked out but that's not the case. The bike starts up easy, will idle, but when I take off on it and give it throttle it breaks up like it's missing.

I have checked my points gap and it is right at .014" and I'm pretty sure I got the opening right. I can't figure out a method with my volt meter to check when they open, but I used deets home made tool method to verify .126" before tdc. I eye balled the opening and I feel it is close, close enough to run for sure.

I did rebuild the carb but made sure everything was clean and used new gaskets and a new main jet o ring. I've had something similar happen on my at2 and the o ring was the bad guy.

I'm trying to get this thing going so I can break in the new rings and bore and ride and enjoy it!

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17 Sep 2017 13:16 17 Sep 2017 13:48 by FrankC.
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Yamaha dt3 breaking up with throttle. Please help! #2
Have you considers the pipe. Might be clogged. A clogged pipe effects the top end powerband first because engine needs more air. Also the filter effects this area first as well. I have heard on the foam filters even if clean, if they are old the cells can break down. Try once without the filter if this type is on the bike. Good luck! Not much of an ignition man and have not done points accept on my old 68 Gto 30 years ago.

Edit- A lot of problems start with the carb a good cleaning as you have done. Sounds rich but after a cleaning on the old car carbs, then working on the points as you did, and perhaps a look at the condenser or bad coil. A good tune up always entails replacement of the spark plug. Might be a bad insulator there or fouled.

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17 Sep 2017 16:37
MarkT
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Yamaha dt3 breaking up with throttle. Please help! #3
You may know this already... point gap is not as important as timing. On most of these bikes, you set the timing and let the gap fall where it falls...

Member Hammer has a method of putting an AM radio tuned to static near the magneto and listening for a "pop" when the points open.

Then after timing is set, you measure the max gap and if it doesn't fall within the specified range then you probably need to replace the points...

Or in a pinch, you can gap the points and the timing is usually close... I would gap at 0.012" to start.

In my experience, "looking" to see when the points open usually ends up with advanced timing... I'd try closing the points up a little

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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17 Sep 2017 16:47
RT325
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Yamaha dt3 breaking up with throttle. Please help! #4
Good idea to put a video up if you can organise it. One of reasonable length. Wait till night time & look in through the flywheel while running it at the missfire point & see if sparks are jumping all over the place. You'll see minimal spark at the points but if an arc like a welder i'd go look at condenser or soldering on condenser. Anyway--probably nothing to do with any of that. All a bit of a guess though & need audio sorry. Think exhaust was mentioned but would have to be 'bad' to stuff it from not far up from idle if that's the case.
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17 Sep 2017 20:03
360jeepboy
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Yamaha dt3 breaking up with throttle. Please help! #5
Thank you for the info guys!

I already pulled the air filter and it is doing the same thing. I don't think my exhaust is plugged. I tried setting the timing to a little retardation and it still happens.

I don't think anything is going on with the carb but I'm fixing to tear into it and see if I can find anything. I cleaned it pretty good. (Ultrasonic)

I'm just at a loss here.... trying not to get frustrated. I just have so much time and money tied up in this bike and I just want to ride it.

I will see if I can make a video of what is happening .
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18 Sep 2017 11:23 18 Sep 2017 11:26 by 360jeepboy.
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Yamaha dt3 breaking up with throttle. Please help! #6
Ok I made a quick video of it breaking up with just revving it. It becomes worse while in motion and does it at about 2500rpm and up.

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18 Sep 2017 11:29
360jeepboy
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Yamaha dt3 breaking up with throttle. Please help! #7

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18 Sep 2017 12:37
360jeepboy
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Yamaha dt3 breaking up with throttle. Please help! #8

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18 Sep 2017 15:27 18 Sep 2017 15:28 by RT325.
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Yamaha dt3 breaking up with throttle. Please help! #9
Not sure what to think sorry. Sounds quite healthy at idle but not enough to hear higher up to say if too rich on the needle [which it sort of sounds like] or if it's heading into an electrical misfire. Idealy just hold it on full song & see if it clears or gets worse--worse as in drowning from the little Oring not sealing. If 'that' & you feel the Oring is good then 'is' the thick washer missing from where the Oring fitting screws into the needle jet. If so--then the Oring won't go far enough into the lead into the hole to fully seal. That's a wild guess because i can't judge what's happening with a short one or two revs on the first video. Thanks heaps for the vid as know it's not easy to do [not for me anyway lol] Longer vid & on the road under full power or stationary & see if it clears. Think about that Washer. No6.
www.partzilla.com/parts/search/Yamaha/Mo...ARBURETOR/parts.html

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18 Sep 2017 15:43
360jeepboy
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Yamaha dt3 breaking up with throttle. Please help! #10

RT325 wrote: Not sure what to think sorry. Sounds quite healthy at idle but not enough to hear higher up to say if too rich on the needle [which it sort of sounds like] or if it's heading into an electrical misfire. Idealy just hold it on full song & see if it clears or gets worse--worse as in drowning from the little Oring not sealing. If 'that' & you feel the Oring is good then 'is' the thick washer missing from where the Oring fitting screws into the needle jet. If so--then the Oring won't go far enough into the lead into the hole to fully seal. That's a wild guess because i can't judge what's happening with a short one or two revs on the first video. Thanks heaps for the vid as know it's not easy to do [not for me anyway lol] Longer vid & on the road under full power or stationary & see if it clears. Think about that Washer. No6.
www.partzilla.com/parts/search/Yamaha/Mo...ARBURETOR/parts.html


Yea the driving video was something I wanted to get but isn't happening with out me rigging up a mount.

I put a brand new oring in place and yes the thick washer is there, I just pulled it apart today.

One thing I will say is there is a slight scale looking substance that is in the whole where the o ring goes into. I tried like hell to clean it up during the carb rebuild but didn't make any headway. I still don't think it is that though because I took my filter out and it didn't improve at all.

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