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1978 Yamaha DT175E: HELP NEEDED

01 Jun 2020 12:00 01 Jun 2020 12:12 by BikeBound.
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1978 Yamaha DT175E: HELP NEEDED #1
Hi Everyone!

I've owned a 1978 Yamaha DT175E that's been my daily transportation for the past eight months or so.

A few weeks ago, it developed a high idle and would "hang" on revs during deceleration, then begin to lose power and/or stall while riding. It turned out to be a base gasket leak. I replaced the base gasket, head gasket, and both crank seals with the help of a local shop whose owner is kind enough to let me use his space and give guidance when necessary. The piston, rings, and cylinder looked good. Compression is up 150.

Now, the bike no longer has the high idle, but it's still losing power and stalling. It will start first kick every time, idle fine, and run like an absolute top while riding, then suddenly start to lose power and want to stall -- the only thing to prevent it dying is revving it to 6000+ RPM and keeping it there. After stalling, it will generally re-start after 5-6 kicks -- or first kick, if it's allowed to cool a little.

I've swapped out the CDI, coil, and various plugs to no avail. We made sure there were no fuel starvation issues at the petcock or at the carb. Gas tank interior is in good shape, with an inline fuel filter. Oil pump and carb are both adjusted to spec. As I said, the piston, rings, and cylinder walls look good, as do the stator and clutch. I've also replaced the airbox joint boot, air filter, and cleaned the carb thoroughly. The previous owner is a retired professional motorcycle mechanic and two-stroke aficionado who originally built the bike for his son, who never rode it.

A number of experienced vintage moto guys hang around the shop, and they're beginning to scratch their heads, as is the previous owner, who's been keeping in touch.

Has anyone had a similar experience or any additional ideas to try? The bike has been down for most of the Covid lockdown and it's driving me bonkers not to be able to ride! :)

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01 Jun 2020 15:38 01 Jun 2020 15:38 by RT325.
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1978 Yamaha DT175E: HELP NEEDED #2
Just when i thought i knew everything--i know nothing lol. Could it have popped the left seal out behind the mag, if it was ok initially after seal & gasket swap then went like it again. Electrical will be more instant misfire cut in cut out type thing, where fuel starvation is more gasping for fuel but still trying--unless of course the bowl goes totally dry but that should give a hint of starving before stopping. CDI stator source coil can fail & come to life when left for a bit in some cases, but doesn't really sound like that. Nothing untoward in the carb!!. I'm no help sorry, but know how annoying it can be.
I Know--it's broken a reed petal off--yep that's it--go fix it. I'll eat my hat--Go a stack of hat here to eat if i'm wrong.
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01 Jun 2020 17:29
MarkT
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1978 Yamaha DT175E: HELP NEEDED #3
Have you made sure the vent hose on the fuel cap is clear and not pinched? Can you blow air through the vent freely?

Hmmm.. so when you hold it at 6,000 RPM plus to keep it running does it resolve and then start running okay? Or is it destined to stall and you're just prolonging it by keep revs up?

When you say you swapped out the CDI, what exact part or parts are you talking about?

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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01 Jun 2020 18:56 01 Jun 2020 19:03 by BikeBound.
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1978 Yamaha DT175E: HELP NEEDED #4
Thank you both for your input!

@MarkT: Indeed I checked the vent hose. Even tried unscrewing the cap while the problem was happening and it didn't help.

When you're holding it at 6,000 RPM plus, it's much more like it's destined to stall and you're just delaying it by keeping the revs up. Sometimes, despite throttle, it will slowly drop from 6000 down to like 3000, then hit power again -- doing that a few times before it finally dies. Crank it up less than a minute later and it's good for a little while again.

As far as the CDI, I swapped out the "CDI Unit Assembly" (2A7-85540-20-00 ) and also the Ignition Coil Assembly from a friend's same bike to no avail -- behaved exactly the same.

@RT325: I should check the reeds -- that's one thing I haven't checked! They're Boyesen and pretty new. I don't think the left crank seal could've slipped out since it was still problematic right after. We also did a double-lip seal on that side just to be sure.

It sounds more like fuel starvation when you can describe it to someone, but when you feel it, it's more like a "weight" is pulling down the motor/power -- you're just fighting it till it drops all the way.

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01 Jun 2020 20:53
MarkT
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1978 Yamaha DT175E: HELP NEEDED #5
My best guess from what you're describing is the "low speed" ignition source coil on the stator is failing...

On the first couple of monoshock DT175 model years, Yamaha had this "dual" source coil set up with a low and high speed coil.

While I've seen some modern aftermarket ignitions claim this type of set up is the "bee's knees", in my experience it can be problematic... and I wonder why if it was so great did Yamaha go to a single source coil in 1980?

Rambling aside... maybe your buddy will let you try swapping stators? On those, they had slotted mounting plates so if the factory marks are not visible, make new ones so you can put it back in the same place... or you can use a dial indicator to time it. (Much quicker and easier to use the marks.)

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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02 Jun 2020 03:56
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1978 Yamaha DT175E: HELP NEEDED #6
Oh! This is something we hadn't thought of! My buddy's bike is actually an '80 or '81 -- but I found a stator base plate assembly from a '78 on ebay, complete with the low speed ignition coils and wiring. Fingers crossed!

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02 Jun 2020 07:00
MarkT
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1978 Yamaha DT175E: HELP NEEDED #7
I'm confused because the 80-81 CDI box is different than the 1978. So maybe someone has swapped your ignition already? Later box does not have the wire for the extra source coil so not sure how you could have used it to test your bike.

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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02 Jun 2020 08:22 02 Jun 2020 08:38 by BikeBound.
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1978 Yamaha DT175E: HELP NEEDED #8
The CDI box was indeed different. We noticed the different size and that it didn't have the extra wire and were about to scrap the idea of swapping it out. However, one of the more experienced guys at the shop said we should just try it anyway and see what happened.

Since the bike behaved exactly the same with the 80-81 CDI box, could that be indicative that the issue is indeed with the low speed coil?

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02 Jun 2020 10:00
MarkT
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1978 Yamaha DT175E: HELP NEEDED #9
Possible... but just a guess. I suppose it could be the low speed circuitry in your CDI box too.

IF you can talk your buddy into letting you try his complete ignition, that would be the best. You need flywheel, stator, and CDI box.

If it runs good, then at least you know for sure it's ignition and not some oddball fuel issue.

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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02 Jun 2020 10:52 02 Jun 2020 10:54 by BikeBound.
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1978 Yamaha DT175E: HELP NEEDED #10
My buddy has been damn kind to let me swap out parts, but I'm reluctant to ask him if I can swap the whole ignition.

That said, I ordered a complete '78 stator base plate on ebay. It still won't rule out an issue with the low-speed circuitry in the CDI box, but at least we'll know if it's the low-speed coil.

I can't thank you enough for the help. I was plumb out of leads and I'm excited to have some things to track down. Learning a lot about these bikes in the process, too.
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