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!!!!!!!!!!!!