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Dt175 misfire and runaway revs

11 Oct 2020 07:57
Digger65
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Dt175 misfire and runaway revs #1
Hi forum
Chasing my tail on an issue am having with my 76 DT175
It developed a misfire when engine under load
Also erratic and runaway revs
Have changed the stator crank seal but yet to get to the clutch side but it doesn’t seem to be burning trans oil, levels not going down
Have just been checking points timing and noted the points don’t appear to be opening and am now thinking it could be worn cam follower on points
Also if anyone could tell me what the device in pic is?
I think it’s advance regard mech?
Many thanks
J

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11 Oct 2020 08:42
Pedalcrazy
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Dt175 misfire and runaway revs #2
You are correct on the device in the pic. Provides retarded timing at startup then advances when running. The points don't open very far....012 to .014 of an inch. You really have to watch very closely. On your misfire and erratic revs...it sounds more like an air leak. I'd be looking for a leaking gasket around the cylinder and especially the intake. There are experts on this forum that are more familiar with the smaller bikes and you'll hear more I'm sure. Good luck!

1978 DT400E
1976 DT400C
1973 RT3
1971 RT1B

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11 Oct 2020 08:46
MarkT
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Dt175 misfire and runaway revs #3
Read the manual... you need to lock the advance open when setting timing.

I agree that runaway is likely an air leak...

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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11 Oct 2020 10:18
Digger65
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Dt175 misfire and runaway revs #4
Manual is on the list
In meantime though may i quickly ask how you lock advance retard open to set the point gap?
Thanks

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11 Oct 2020 10:25
Digger65
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Dt175 misfire and runaway revs #5
Thanks

Piston, rings and gasket on order
And thinking on it I am now pretty certain I saw exhaust gassed escaping from the head so that could well be the cause of the air leak
The misfire started intermittent but is now all the time I think I am chasing two separate issues

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11 Oct 2020 10:28
Gr8uncleal
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Dt175 misfire and runaway revs #6

Digger65 wrote: Manual is on the list
In meantime though may i quickly ask how you lock advance retard open to set the point gap?
Thanks


The manual is probably in the Tech Library on this forum (see top toolbar) and is free to download - you don't have to buy one.
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11 Oct 2020 11:41
MarkT
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Dt175 misfire and runaway revs #7
Yes, manual is in the Tech Library here. :likey

Site does accept donations... it's expensive for owners to provide all this great stuff for free.

yamaha-enduros.com/index.php/community/d...ite-maintenance-fund

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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11 Oct 2020 17:40
Eazy-X
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Dt175 misfire and runaway revs #8
If you've not torqued the head properly then I'd expect a loss of compression, maybe it's bad bottom gasket or the surfaces aren't flat. Might be worth spraying some "Easy Start" or "Start Ya Bastard" around the inlet rubbers see if there's a reaction.

Try and fix one thing at a time. "Runaway revs" sounds like an air leak and probably the easiest to fix first off.

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11 Oct 2020 20:17
RT325
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Dt175 misfire and runaway revs #9
With it on retard you'd have to have the flywheel slot a long way around to see them opening. Hold it on advance then set the gap within spec, not over spec, & should be close for timing. But--yes, airleak, had the base gasket on my old husky chainsaw get sucked in or blown out & similar problem with getting higher in the idle revs, no idle just getting faster.

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12 Oct 2020 14:51
Digger65
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Dt175 misfire and runaway revs #10

RT325 wrote: With it on retard you'd have to have the flywheel slot a long way around to see them opening. Hold it on advance then set the gap within spec, not over spec, & should be close for timing. But--yes, airleak, had the base gasket on my old husky chainsaw get sucked in or blown out & similar problem with getting higher in the idle revs, no idle just getting faster.


Thanks, to clarify I have to hold the adv/retard on flywheel in open position to set point gap?
Or ensure it’s open before putting flywheel back on?
Sorry dark arts of points completely new to me only ever having cdi bikes prior to this one
D

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