[nibsy342]

Hi guys, I am new to the forum and in need of some technical assistance. Introductions first however;-
my name is Pete, and I live in South Wales, UK.
I own a Yam DT250 'D' model, and a Kwak ZXR750H1.

I am nearing the completion of a 'ground up' restoration on my DT250MX 1977. This was originally a points ignition
engine which the previous owner has upgraded to a 'CDI' set-up. The engine is running rather erratically and I suspect the ignition timing is out. The problem being how to check it, as I don't have the necessary timing marks/pointers on my crankcases - as this was originally a points/magneto engine. Can anyone suggest a method
of checking/setting up the timing? either statically or dynamically, without the necessity of the crankcase
pointers.

The engine is fully rebuilt - new piston, rings, bearings, crankseals, reed valves etc, also new spark plug and cap, and the carburettor has been ultra-sonically cleaned and rebuild with a service kit. So I am fairly sure it is the timing that is at fault. Thanks in anticipation of any help received.

[oldgitonabike]

nibsy342, on 23 October 2009 - 02:55 PM, said:

Hi guys, I am new to the forum and in need of some technical assistance. Introductions first however;-
my name is Pete, and I live in South Wales, UK.
I own a Yam DT250 'D' model, and a Kwak ZXR750H1.

I am nearing the completion of a 'ground up' restoration on my DT250MX 1977. This was originally a points ignition
engine which the previous owner has upgraded to a 'CDI' set-up. The engine is running rather erratically and I suspect the ignition timing is out. The problem being how to check it, as I don't have the necessary timing marks/pointers on my crankcases - as this was originally a points/magneto engine. Can anyone suggest a method
of checking/setting up the timing? either statically or dynamically, without the necessity of the crankcase
pointers.

The engine is fully rebuilt - new piston, rings, bearings, crankseals, reed valves etc, also new spark plug and cap, and the carburettor has been ultra-sonically cleaned and rebuild with a service kit. So I am fairly sure it is the timing that is at fault. Thanks in anticipation of any help received.

Hi and welcome Pete, always good to see a new classic DT owner on here.
I take it that you know what the ignition timing should be set to, for examole 1.8mm BTDC.
This method does need some special tools, ie a dial gauge and stand.

1: Remove the magneto cover and the spark plug
2: Stick a pencil down the plug hole and slowly turn the flywheel until TDC is reached
3: Fit the stand and dial gauge, now find true TDC using the gauge and 'zero the gauge at this point
4: Turn the flywheel backwards (clockwise) until the gauge pointer has moved 1.8mm from zero
5: Hold the flywheel at this position and look for any marks on the flywheel, there are sometimes three lines, the central one being slightly longer than the others.
6: If there are lines then mark the crankcase adjacent to the central line, use a centre punch or something. You now have a 1.8mm BTDC referance.
7: now re-fit the plug but connect a strobe lamp, you may have to power the strobe from a 12v battery, with the engine warmed and idling you should be able to check the ignition timing versus your referance marks (Tip...use tipex correction fluid to accentuate the marks)

Job done

...Paul

EDIT, I believe the timing for your bike is 3.2mm BTDC, not sure if this is the fully advanced figure though, you would need to check in the manual

[nibsy342]

Thanks for a very informative reply. I shall implement this procedure when time allows and let you know the results.

[nibsy342]

Well the timing looks to be ok, but it's still running poorly - it's also putting out a lot of bluish smoke.
I'm running 40:1 premix at the mo just until I refit the pump and bleed/prime it etc.
Could I perhaps be pulling in gearbox oil past the crankseal and primary drive pinnion 'o' ring?
It is a new genuine Yamaha crank seal however, fitted with the little lugs facing inwards, which I believe is the correct fitting.

[oldgitonabike]

nibsy342, on 30 October 2009 - 05:19 PM, said:

Well the timing looks to be ok, but it's still running poorly - it's also putting out a lot of bluish smoke.
I'm running 40:1 premix at the mo just until I refit the pump and bleed/prime it etc.
Could I perhaps be pulling in gearbox oil past the crankseal and primary drive pinnion 'o' ring?
It is a new genuine Yamaha crank seal however, fitted with the little lugs facing inwards, which I believe is the correct fitting.

Not sure what you mean by those little lugs, however i assume you fitted the seals the same as the ones you replaced so all should be well there, anyhow worn of badly fitted seals would have an adverse affect on crankcase compression and this tends to make starting difficult. There is a quick test of the seals though (on a running engine)

1. First test on anything "old" is compression, if its below 125 your wasting your time.
Second test is crank seals, that model will run with the magneto cover off so do that, get it running and spray WD-40 thru the slots in the flywheel. If it runs different you know where to go to work.
Third test is the primary drive side crank seal. Start the engine, run the transmission vent hose into a glass of water, then rev the engine. If it blows bubbles or starts to suck up the water you get to split the cases

Other issues to consider are
Correct float height, too high and it will run badly, an easy check just run with fuel tap off to see if it improves,

Or maybe out of spec ignition coils, HT and magneto, check the ohms readings of these pulse coil Red/Whi to ground = 3.4 Ohms +- 10%, Source coil Brn to Gnd = 166 ohms +- 10%, Ignition coil Primaty = 1 Ohm +- 10% Secondary = 5.9 K Ohms +- 20%
I had rough running with mine and found a source coil measuring out of spec, on closer inspection found the laminated iron core was rusting and the insulation was beginning to break down, ran better after replacement.

BTW I noticed that the static timing should be 2.9mm BTDC for the CDI version DT250E, and dynamic timing should be checked at 4000 RPM
These figures were taken trom the service data section of the DT250E parts book Here

Perhaps a coked up exhaust?, if it is then your engine wont breath properly. I once read of the 'Vacuum cleaner test', I could see it made sense but have never had the need to try it out. The theory is, remove the exhaust start the cleaner and make a good air tight connection to a cylinder type vacuum cleaner, if the motor note changes a lot (like when you cover the end of the suction pipe) this is an indication of a clogged exhaust and it will no doubt be barbeque time...

Another point is to give it a good thrashing (within reason)just to see if it clears up, it may be residual oil in the cases.

You say the carb has been ultrasonic cleaned, did you remove the component parts for this. here is a jet from a DT175MX, this carb had been previously ultrasonic cleaned before this was removed...enough said about that!



...Paul

[nibsy342]

Many thanks Paul, plenty of 'food for thought' there to keep me going for a bit!! I will get the multi-meter out this week and start checking those readings. With regard to the carb, I cleaned it myself in our sonic bath in work after completely dismantling it, and all the jets are new from a Keyster kit - so I'm pretty sure it's like a new pin in there. I may however, have an issue with float height, as I have noticed that I am getting overflow when the bike is on the sidestand, but it's ok on the paddock stand - I did set it to the book though at 16mm, measured from the casting with the gasket removed.

Great link to the service data Pdf's too - very handy information. Will let you know how I progress.

[nibsy342]

ok. have now checked the electrical readings and all were in spec apart from the ignition coil - I have fitted a brand new coil but this has made no difference. My compression check however, is only giving me about 114 PSI (8Kg/cm2). I have already had the barrell rebored and fitted a new piston and rings so I am somewhat concerned at this, as I would have thought all would be well in this department? Are you sure the 250 engine should be higher than 125PSI?

[oldgitonabike]

nibsy342, on 05 November 2009 - 01:49 PM, said:

ok. have now checked the electrical readings and all were in spec apart from the ignition coil - I have fitted a brand new coil but this has made no difference. My compression check however, is only giving me about 114 PSI (8Kg/cm2). I have already had the barrell rebored and fitted a new piston and rings so I am somewhat concerned at this, as I would have thought all would be well in this department? Are you sure the 250 engine should be higher than 125PSI?

Hmm, its a bit low for a re bored barrel I expect, but I doubt if its your problem and it may improve when run in (by the way those are not my words but something I found on the web once). Did you test with full throttle? What exactly is the problem as running erratic doesnt give much away?. Did you do the strobe test in the ignition? is the sdvance working ok? is the exhaust clear or is it an old one that may be coked up?

[nibsy342]

oldgitonabike, on 05 November 2009 - 05:01 PM, said:

Hmm, its a bit low for a re bored barrel I expect, but I doubt if its your problem and it may improve when run in (by the way those are not my words but something I found on the web once). Did you test with full throttle? What exactly is the problem as running erratic doesnt give much away?. Did you do the strobe test in the ignition? is the sdvance working ok? is the exhaust clear or is it an old one that may be coked up?

Hi Paul,

Regarding the running, it starts easily, no probs there. But the tick over is poor, it wont maintain an even idling speed, 'hunting' etc, and when I rev the engine it just isn't nice and crisp - it is spluttery and choked. I tried to cover every base with the rebuild so I'm a bit miffed. I did the crank seal test with the water and no bubbles. I've got the exhaust off the bike now and soaking in caustic, although it's in such good condition (original paint) that I doubt if it's had much use. I'm wondering whether I should try reverting it back to flywheel magneto/points ignition and ditch the CDI. The service data sheet quotes the timing as being the centre scribed line on the magneto @4000rpm, which I presume is the advance setting?

[oldgitonabike]

nibsy342, on 08 November 2009 - 04:09 PM, said:

Hi Paul,

Regarding the running, it starts easily, no probs there. But the tick over is poor, it wont maintain an even idling speed, 'hunting' etc, and when I rev the engine it just isn't nice and crisp - it is spluttery and choked. I tried to cover every base with the rebuild so I'm a bit miffed. I did the crank seal test with the water and no bubbles. I've got the exhaust off the bike now and soaking in caustic, although it's in such good condition (original paint) that I doubt if it's had much use. I'm wondering whether I should try reverting it back to flywheel magneto/points ignition and ditch the CDI. The service data sheet quotes the timing as being the centre scribed line on the magneto @4000rpm, which I presume is the advance setting?

Have a look at the service data of the other DT250's and you will see that DT250E uses a CDI and has an advance of only 1.75mm BTDC. It could be that your setting is way too advanced at 3.2mm as this is the figure for the points versions. If this is the case your engine is trying to compress rapidly expanding gasses and will lose power accordingly, furthermore with use riding it may cause overheating and possible damage...having seen that, if it were mine I would be trying that setting now.

...Paul