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77' DT400 timing question

16 Feb 2020 18:08
Art
Mini Enduro
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77' DT400 timing question #1
I accurately found TDC and 2.9mm BTDC with a dial indicator on the piston, I made a degree tape with Auto-Cad based on the diameter of the flywheel. Turns out 2.9mm BTDC is 21degree. I found a Yamaha document that said Static 2.9mm BTDC & Dynamic "Centerline @ 4000 RPM". This is a weird term "Centerline @ 4000RPM" and I have decided that it means to Center the timing at 2.9mm BTDC @ 4000 RPM. Anyone think my thoughts are correct on the meaning of "Centerline @ 4000RPM"?
With a timing light I found that I don't have any advance changes with RPM and I am not sure if the CDI unit has any advance designed into it?. With the marks lined up as per the manual, coil plate to case and flywheel. The dynamic timing is at 18' BTDC. @ 4000rpm my conclusion from this is that my timing is retarded by 3'. Is there any way to tell when the CDI actually fires other than using a timing light? This is on a stock CDI.

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16 Feb 2020 18:59
MarkT
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77' DT400 timing question #2
I don't have my 77 apart right now but some CDI flywheels had 3 marks. One longer one flanked by a slightly shorter one on each side. It's the longer one in the center that, when aligned with the mark on the stator, is supposed to be when the plug fires. That's probably why they said "center line @ 4000 rpm"... means the center line of the three lines.

There should be a timing mark on the crankcase at the edge of the stator, often very hard to see on that model.... then there's a timing mark on the stator plate that is lined up with the mark on the case during assembly... then the third mark is on the flywheel (the center mark if there are three). When piston is at the specified distance BTDC, all three marks should be lined up. If not, you need to rotate the stator to align the mark with the flywheel mark.

On the advance, two strokes actually like to be retarded at higher RPM... and yes, the early CDI's really didn't have a "timing curve" except that the timing should be retarded at low rpm... very low rpm... to prevent kickback when starting. At idle, the ignition is already fully advanced.

One last thing to try is clean all the CDI connectors... and even then sometimes the CDI on those models seems to be"out of time" when checking with a timing light. So it wouldn't surprise me if you are off. Not sure why this happens but my guess is CDI is failing or defective in some way. Some have rotated stator so timing is correct with a timing light... sometimes even having to drill new holes/slots to rotate stator plate far enough. Others replace the CDI with a Powerdynamo or 250 points ignition which also solves the problem.

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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16 Feb 2020 20:13
Art
Mini Enduro
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77' DT400 timing question #3
I only found 1 mark on the flywheel and it did line up at 2.9mm BTDC. I am surprised that the timing is only 21' BTDC and 3' retarded could be significant in the performance. It runs fine but I am trying to figure out why I have had to reduce the jets from stock a couple of sizes ( I'm @ sea-level ). I've read a lot about jetting on this site and have applied a lot of changes. Run it with and without a filter, haven't tried to clean the pipe yet, pressure tested to 5psi, reeds are tight, carb is clean. My thoughts are that with retarded timing I have had to reduce the jet sizes to get the engine running correctly, crisp midrange and revs to redline in 5th. CHT runs around 300'f, 325'f if I run it hard so I think I've got it jetted correctly. This is why I am questioning the timing, and maybe 3' retarded could be the culprit.
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16 Feb 2020 21:23
MarkT
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77' DT400 timing question #4
Could be... one thing to keep in mind is most timing lights are naturally inaccurate at higher rpm due to the latency in the electronics and bulb. Many engines have been blown by setting "max advance" at high RPM using a timing light because even a slight delay in the light flashing will make the timing appear retarded several degrees.

4000 RPM = 67 Revs per second = 24,000 degrees of crank rotation per second!

If I did the math right, a 3 degree error at 4,000 RPM would only take a small fraction of a second delay from the pickup sensing the spark to the light flashing... if I did the math right, at 4000 RPM it takes the engine only 0.000125 seconds to rotate 3 degrees.... a blink of an eye takes about 0.1 seconds...

Two things I would check... one is since there shouldn't be any "timing curve", what is the timing at lower RPM? It sounds like you already answered this.

And are the marks on the stator and the mark on the flywheel aligned when the light flashes? Those two marks should be when it sparks. If those marks look "off" then I'd guess you need to make a new mark on the stator where the spark is occurring and align that mark to the case mark. If you do this and retest, then you should find the timing is corrected. Hope that makes sense.

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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17 Feb 2020 00:42
RT325
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77' DT400 timing question #5
I'll 'try' keep out of it & MarkT has it more than sorted. But i thought [wine talking on empty tummy] that center mark is set to 2.9 then at kick start speed the retard side of the line would line up [although you'd likely never see it with strobe] but immediately it starts it'd be on the advance mark the other side of the long line, possibly about 3.2, then move back to the long line with revs. If it was tweaked to rev super high it might retard off to 'that' retard mark past the long line. Ignore me if i'm woffling rubbish!!, had a bugger of a day.
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