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The saga of my DT175MX 1981 rough running over 4500rpm


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Calling all DT experts, I have joined up YOC after I stumbled upon it while googling for ideas what may be wrong with my '81 DT175MX which has done 50,000 miles. I have had it for the last 24 years. I have a picture loaded on my profile.

It looks like the knowledge in the workshop forum is a pretty vast resource, whereas my knowledge is pretty much exhausted with this particular fault.

It is a pretty epic story, and goes back over a year, I struggle to recall the exact sequence, but as far as I remember it goes something like this......

The bike had been parked up over winter and refused to go in the spring - a surprise as it is usually a great starter.

Initial thought was stale petrol, so drained tank and tried fresh. Not starting.

Had a look in the airbox and found the foam filter cone was disintegrated.

Cleaned out the airbox and the carb, replaced the foam air filter, new NGK spark plug.

Good fuel supply to carb - full bore of pipe.

Engine now starting well, idling well, revving cleanly to around 4500rpm, after that it fires intermittently & backfires occasionally.

Drop it below 4500rpm and it is fine again.

The ignition switch was worn out - the barrel lifted in and out with the key sometimes.

I had read that a faulty ignition switch can fry the CDI unit so I resorted to a made in Taiwan CDI off ebay.

Advertised as fitting an ,81 DT175 MX it is a 6 wire type from Yuniworld in Derby, 18G-85540-00

I replaced the ignition switch with a pattern part at the same time.

Still, above 4500rpm it fires intermittently, backfires occasionally, drop it below 4500rpm and it is fine again.

Still thinking it was CDI related, I armed myself with the Haynes manual and did some testing with a multimeter.

The bike is the type with F3T251 4 slot flywheel, single source coil and 6 wire CDI.

The resistance for the source coil looked below spec so I sourced a made in Taiwan one off ebay - Yuniworld again. No better.

Tried the rest of the resistance test at the CDI wires.

All ok, except I was getting minus18 ohms at the black/white kill switch wire.

I decided to run a new wire up the frame and plugged it direct to the kill switch at the connector in the headlight, thus bypassing the ignition switch.

As I expected, the engine now starts and runs with ignition switch turned off, and the kill switch can be used to stop the engine.

Thinking I may well have fried the Taiwan CDI, I ordered another one off ebay Yuniworld - no better.

I decided I couldn't be that unlucky with CDI's so I looked elsewhere.

The reeds didn't quite seem to sit fully closed on the reed block, so I tried some new reeds - no better.

I then thought it could be air leaks at the crank seals or the crankcase gasket.

Engine out, strip down. Bore ok, rings a bit gummed up.

New main bearings, oil seals all round.

Freed off rings, cleaned piston grooves.

Reset autolube pump setting.

Renew autolube pipes & clips with clear pipe and spring clips.

Woodruff key OK.

Checked exhaust clear.

Clean up engine mount earthing positions.

Start and run - just the same.

At least splitting the engine allowed me to cure a longstanding oil leak by the sump plug with some liquid metal in a hairline crack.

For good measure new pattern part ignition coil, HT lead, NGK plug cap.

Also replaced pulser coil with pattern part, then removed it again as resistance was higher than tolerance.

Refitted original pulser coil.

Thought maybe the Taiwan CDi's were no good, so I bought a second hand Yamaha one off ebay marked 3J0-20 1.9.24.

This significantly changed the characteristic of the engine.

It starts and idles, but will not rev above 3000pm at all.

Reverted back to one of my the Taiwan CDI's, back to 4500rpm then intermittent firing, occasional backfire.

Electrical tests carried out results (spec):-

Ignition coil & HT

Resistor plug cap 5230 ohms (5000)

Ignition coil primary 0.9 ohms (1 +/-10%)

Ignition coil secondary 5920 ohms (5900 +/-20%)

At the loom connectors to the CDI

Orange HT primary 1 ohm (1 +/-10%)

Brown magneto charge coil 279 ohms (300)

White/red magneto pulser coil 10 ohms (10)

Black female 0.1 ohms (0)

Black male 0 ohms (0)

Black/white ignition on/kill switch run position - open circuit (open circuit)

Black/white ignition on/kill switch kill position - minus 19 ohms (0)

If I touch the multimeter leads together I get either 0 or 0.1 ohms.

Carburettor Mikuni VM24SS 3U500

Main jet 160

Main nozzle 267 10-8

Pilot jet 20

Float height 21mm measured with carb upside down - gasket mating face on carb body to top of float.

Throttle valve 1.5

Jet needle 4-JC

Needle clip always used to run fine with it in top groove, have also tried it in middle and bottom groove.

All standard I think, and unchanged since it used to run fine.

Decided to strip the carb and give it a few goes in my little ultrasonic cleaner from Aldi with some washing up liquid.

No surprise, but no better.

Other info, it does seep a drip of blackish oil out at the exhaust gasket.

Spark plug is generally slightly wet oily black rather than biscuit coloured.

So here I am, with a box full of spare parts that probably were not needed, and a bike much the same as it was over a year ago, apart from curing the sump plug oil leak.

On the plus side I am now much more familiar with almost every engine, fuel and electrical component on the bike!!

Does anyone now if CDI's can be tested, or is it a case of plugging it into a "good bike" to try it?

Why am I getting minus ohms reading at the black/white kill wire and is this killing my CDI's? (I have tried not connecting the kill wire at the CDI and just contacting the black/white from the CDI to earth to stop the engine)

Does anyone have any other ideas or suggestions I should be looking at?

Sorry it is such a long post, I said it was an epic.

Please help, wife is now saying it might be kindest thing to let the DT slip quietly away - unbelievable!!!!!!!!!!!! :shakeno:

eden1 (David)

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  • Moderator

Wow David you are having fun eh?

Looks to me like you just about tried everything...one thing springs to mind though..what silencer are you using? is it aftermarket? can it be stripped and cleaned / re-packed etc?

I'm wondering if there could be some carbon build up / blockage in there

Neddle jet 4JC? a typo?, 4J6

I know from experience that silencer packing too tight or old and crusty will badly affect performance

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  • Moderator

Hmm, have you rewired the connections to the cdi, I mean the actual wires. When I rewired my DT I found several instances where wires had been used that were too short and had connections within the loom to use 2 shorter bits of wire.

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Hi, thanks for the pointers

The silencer seems to blow through without restriction. I think it would mean drilling out rivets to dismantle the silencer and draw out the contents.

Quite correct, my needle is a 4J6 not the 4JC I said it was.

Several years ago I had to extend some wires in the harness to the magneto in the area below the petrol tap, as they had rubbed through on the frame, but they seem ok.

Pilot jet is 20, which I think is right for my carb, plus it has run fine with it for over 20 years.

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  • Moderator

Couple of points after rereading.

You mentioned the pulser coil resistance being different, you didn't mention if it ran differently.

The other  elephant in the room for me is the 1500rpm change from the cdi change. Could it be that the ig timing has slipped, or changed through age. I would bet when you stripped the motor for the rebuild you carefully marked the ig as is proper. Did you actually check the ig timing?

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Hi Cynic, I think I will try and check the timing. The Haynes manual isn't to clear so if you have any pearls of wisdom on the subject, they would be appreciated.  I did check the crankcase and stator marks were aligned before and after the engine strip down. I will try and compare the Taiwan CDI 's with the secondhand genuine Yamaha CDI.

Have you had any experiences with the Taiwan CDI's in the past?


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So you did set it back exactly as it was, rather than actually re time the bike. Why not just advance it a little maybe 2mm. Be sure of your start point and be logical.

Whatever the effect move it as much again and see if it increases any effect then make a judgement on what to move it to next.

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I have fashioned part of a micrometer into a depth gauge and checked the stamped timing mark on the casing and the rotor and is 1.8mm btdc and it is bang on. I made a couple of other temporary marks at the upper and lower timing tolerances of 1.65 & 1.95mm btdc for reference.

Using a xenon strobe clipped on the HT, and with the engine at 3000rpm, I started with the stator mark aligned with the timing mark on the casing. With the genuine Yamaha CDI, the timing was bang on the timing mark. Swap to the Taiwan CDI's, the timing was too retarded, around 1.0mm btdc. I slid the stator round to the most advanced position in its slots and the timing is still too retarded at around 1.2mm btdc.

In a nutshell, the CDI's are behaving differently to each other.

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  • Moderator

So what happens with the yam cdi and a little more advance. Go to you max spec limit.

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With the genuine Yamaha CDI, if I advance the ignition timing it doesn't improve the non running over 3000rpm. At max advance it starts to kick back during starting.

I thought I would be creative and extend the slots in the stator to allow enough movement for the Taiwan CDI's to be timed to 1.8mm btdc. Unfortunately, it still only revs cleanly till 5000rpm and then only fires occasionally after that.

I don't know if it will be connected, but with all the starting and stopping I have noticed I can hear the battery fizzing after the engine stops.

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  • Moderator

Right, different cdi's seem to be able to have an effect, but nothing else.

The cdi issues are repeatable with the same brand cdi. That says ig problem. Nothing else is affecting the engine. You have swapped most of it.

Not got a sneaky red wire peeking out of the loom/cdi somewhere.

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  • 3 years later...
  • Moderator

Thread is 3 years old, start a thread of your own to get a clear timeline and proper replies. Good idea to post an intro in the new members area too.

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