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Old 08-26-2013, 03:28 AM   #46
Twin-shocker
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The resistance specs of the ignition dont mean a great deal as in many cases these will check out fine, yet a bike still wont run properly.
I would suggest finding someone with a scope and getting the system checked out.

If the ignition proves to be ok when properly tested, I would suggest trying another carb from a bike which is known to run ok. In regard to very old bikes which wont run properly, its also worth taking a close look at the exhaust.

Finally a crankcase pressure test is a very good idea if none of the above suggestions helps, and personally is something I would do before anything else, as its not something that involves new parts, and can be carried out in 30 minutes.
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Old 08-26-2013, 08:24 AM   #47
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Quote:
Originally Posted by Inane Cathode View Post
Well, got the new cdi for ac two stroke motors hooked up and got spark. Taped the whole thing together so it wouldnt fall off, hopped on for a spin down the block. Runs just as shitty as ever, now with heavy pinging at low RPM.

I don't know what to do at this point, I'm all out of ideas. I'll throw the new choke plunger i got in there, maybe it's floating open at high rpm or something but for god sake i've tried everything.

Next step is filing the entire ignition in the circular file bin and going to points. I'm sure it'll still run shitty then too, but at least it'll be one more fix to try.

You are not out of ideas to explore at all! Those crankseals are 40 yrs old. Have you done a pressure test on the lower end at all? Crankseals will drive you crazy and give you all sorts of lean/rich running issues. You will likely find at least part of the problem is the mag side seal leaking compression.
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Old 08-26-2013, 10:41 AM   #48
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Substitute known working carb, check exhaust is clean and free from blockages, check ignition system on a scope with bike running.

Other than very obvious mechanical issues, the problems with this bike will be directly related to one of the 3 things mentioned above.

As someone suggested earlier its a far better idea to actually work out whats wrong, and get it fixed, rather than simply changing parts with no idea of whether they will cure the problem or not.
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Old 08-26-2013, 11:15 AM   #49
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Originally Posted by buls4evr View Post
You are not out of ideas to explore at all! Those crankseals are 40 yrs old. Have you done a pressure test on the lower end at all? Crankseals will drive you crazy and give you all sorts of lean/rich running issues. You will likely find at least part of the problem is the mag side seal leaking compression.
The seals in the bike are about a month old, no worries there.

Ran the bike with the exhaust loose by about an inch. Louder, but still crappy. Pulled the exhaust, banged on it, nothing but dust, torched the silencer, everything is unblocked.

The source coil and pulser coil are putting out volts under load as they should be, as well as passing resistance tests. Haven't scoped it yet (don't have one) but i dont have anything to base it off of, unfortunately. Yamaha doesnt get into deep details in that respect.

Pulled the head to check things out in there, looks ok to me. Little striated but nothing that would leak me to think it's too messed up to run good.

Plan of action at this point is to put it back together and just run it. Running funny or not. I wish i had a neighbor with one to ride to see what theyre supposed to be like, alas.

I'm not trying to be snotty, please understand. I've got probably a half dozen written pages of what i've done so far to diagnose and fix possible problems that covers a lot of territory that includes the majority of the fixes people have thrown out so far. When someone just skips to the very end of the thread and throws out ideas with no context, it dilutes the actual helpful content other people are working on.
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Old 08-26-2013, 12:17 PM   #50
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Other then what you have tested the only thing I can think of is maybe it has a super funky and or wrong needle and needle jet combo ??

When running 4 strokes on VM's I have had the needle and needle jet give me fits on the top end..even with a proper main .

but your bike sounds fairly original so it probably has the right stuff in it.
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Old 08-26-2013, 01:52 PM   #51
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I would suggest fitting trying a carb that is known to be good. If that doesnt cure it, then take the bike to someone with a scope, who will find it very easy to find out if there is a problem with the ignition. If neither of those things clarifies the problem, then you will need to do a crankcase pressure test.

Its worth remembering though that these old bikes are never going to run like a more modern machine, and if it will get to around 60mph on the road, and isnt completely gutless when it comes to climbing hills, then there may well be nothing wrong with it at all.

I used to have a later DT400 that had very low mileage, and to be honest when I first rode it did think there was something wrong with it as it seemed so very slow, and didnt seem to rev properly.
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Old 08-26-2013, 03:22 PM   #52
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Originally Posted by JeffS77 View Post
Other then what you have tested the only thing I can think of is maybe it has a super funky and or wrong needle and needle jet combo ??

When running 4 strokes on VM's I have had the needle and needle jet give me fits on the top end..even with a proper main .

but your bike sounds fairly original so it probably has the right stuff in it.
Word, i'll see if i can get a hold of a good vm32 to swap in.

Quote:
Originally Posted by Twin-shocker View Post
I would suggest fitting trying a carb that is known to be good. If that doesnt cure it, then take the bike to someone with a scope, who will find it very easy to find out if there is a problem with the ignition. If neither of those things clarifies the problem, then you will need to do a crankcase pressure test.

Its worth remembering though that these old bikes are never going to run like a more modern machine, and if it will get to around 60mph on the road, and isnt completely gutless when it comes to climbing hills, then there may well be nothing wrong with it at all.

I used to have a later DT400 that had very low mileage, and to be honest when I first rode it did think there was something wrong with it as it seemed so very slow, and didnt seem to rev properly.
Haha, that may be true. I was thinking that while riding, it might actually be topping this thing out and not even realize it, the taco-meter doesnt really read higher than 3k rpm so theres no way to tell how fast the motor is spinning. It very might well be topped out and i dont realize it. I should have kept my tt500, at least it ran good ><
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Old 08-27-2013, 10:27 AM   #53
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Got a different main jet to try and noticed something weird. The manual states this should have a "5F21" needle in the fourth clip position.

What i just now pulled out is a "6F9" needle in the fifth clip position.

According to a chart, there is no 6f9, but extrapolating what it would look like measurement wise, here's a comparison:



Here's the chart he gives telling you what those numbers mean:



I don't know if this could be a thing, i'm not terribly hopeful given the string of "theres yer problem" moments i've had with this thing. My interpretation of using the wrong needle (especially this one) is it's not only longer, but also thicker. I have to be honest and say i know almost nothing about needles other than i guess they can be interchanged with different sizes. I've never heard of anyone doing that on purpose outside of getting, say, a honda shadow to run right with a pipe, but even then it's essentially "heres this needle, you put this in and it'll run better".
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Old 08-27-2013, 11:39 AM   #54
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I run a 6F8 in my YZ465 w/a 38MM VM Mikuni. 'Tis a bit leaner than OEM but the YZ was jetted way too fat and would blubber until the fat part of the powerband and literally explode into life.

That's how I got it so cheap and in great condition. 1st owner broke his femur and pushed the bike over into the corner for a decade. My ex bought it for 500 US and gave it to me for XMas.

Oh and replacing that needle with the stock may just give you a good running bike after all.
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Old 08-27-2013, 11:41 AM   #55
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Why should the needle affect running at wider throttle openings? It might be a good idea to let someone who is familiar with these bikes to have a ride? Its never going to run like a more modern machine, and if you can get 65mph out of it on the flat, I would suggest its probably ok.
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Old 08-27-2013, 04:04 PM   #56
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Originally Posted by Twin-shocker View Post
Why should the needle affect running at wider throttle openings? It might be a good idea to let someone who is familiar with these bikes to have a ride? Its never going to run like a more modern machine, and if you can get 65mph out of it on the flat, I would suggest its probably ok.
I'm going to put in a stock needle mostly just to get it to stock condition, but also because as far as measurements go, this needle is 6mm longer and quite a bit wider. If it's too long i can see the end of the needle not clearing the jet enough and choking off the fuel supply. Feels pretty red-flaggy to me but like i said i felt that enough already, haha.

I don't know anyone local with one of these, might have to put out some feelers in regionals.
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Old 08-27-2013, 04:22 PM   #57
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Why should the needle affect running at wider throttle openings? It might be a good idea to let someone who is familiar with these bikes to have a ride? Its never going to run like a more modern machine, and if you can get 65mph out of it on the flat, I would suggest its probably ok.

Because the needle is 5MM longer than the one intended for that needle jet leaning fuel delivery at WFO. Among other things. Didn't anyone ever teach you fuel circuits are additive?

That doesn't mean he's found the problem but he's found a problem.
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Old 08-27-2013, 04:43 PM   #58
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5 series needles in the smaller carbs. 6 series needles in the bigger ones. you can run 6's in a 32, but it's gonna get lean..
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Old 08-27-2013, 05:42 PM   #59
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Motorcycle Specifications says 87 MPH top speed for the Monoshock DT400's. I don't think there were any changes to the later motors.
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Old 08-29-2013, 10:34 AM   #60
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So, cool. I have the right needle in there, one size leaner from factory (165 versus 170, im at 5k feet or so, seems about right). Runs better but still 'burbly' and unable to rev out. Riding it with the sense of a four stroke, it's burbly and terrible but still puts out power where it should, runs out of steam much above that (with a huge increase in burbly-ness).

Ran out of gas 4 blocks downhill, had a good opportunity to think about it some more:

1) For the two seconds before it finally quit when it ran out of gas, it ran PERFECTLY. So it's pulling fuel out of the bottom of the bowl, it almost runs out of gas, starts sucking air up and fuel at the same time, emulsifying it.

2) It runs perfectly just on the pilot, i take that as meaning the pilot circuit and related passages/jets are bueno.

3) It seems to be using a bunch of gas

Those three things sure make me think there's something wrong with the main circuit on this thing some how. I think back to the needle jet and those emulsifying holes. It's sealed so you can't actually directly see the holes in the brass dealy but i did blow it out quite a bit, ultrasonic clean it and such. I can't tell where air is supposed to come into from to the needle jet to mix with the fuel. Is there supposed to be a main air bleed on this? I can't see where the inlet is on the tower where the needle jet screws into, and blowing air into the air bleeds on the front doesnt seem to reach the needle jet. Looking at the VM manual there is an air jet, and it does connect to the main, but way up high. I wonder if it's blocked still, it's pretty far in there and i don't remember ever mechanically clearing it with a piece of wire...

TL;DR Where's the air bleed for the main jet on these carbs? Theres something wrong with the main circuit and it's not the fuel part, i suspect air isnt getting to it because when you run it out of fuel it runs really good for just a couple seconds while it's pulling air and fuel off the bottom of the bowl.
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