ts250 woes

[dollydog]

i've tried everything i can think of [bar one], to get this sob running right, but it will not rev over 3000 revs. it just starts sort of blubbering, gives and almighty backfire and then stops. i rebuilt the vm28 carb as a normal one, with the main jet where it should be, instead of in the floatbowl i shortened my homemade header pipe by 100mm to the length i eventually worked out from jenning's book. i put 2 new pulser and primary coils on it, a cdi off a good running ts185, at the advice from a ts400 owner i went down from 180 main jet to 170, it was still flooding terribly. i lowered the needle to its lowest position and even dropped the float height by 2mm, from 15mm to 13mm. it's now stopped flooding and sounds a lot crisper - up to 3000 revs. i changed coil to a twin spark, it made starting a lot easier, but still 3000. engine has new mains and seals in it - i built the bottom end, then left it overnight with the crankcases full of parafin. in the morning, no leaks anywhere, so tipped it out and rebuilt the top end [new piston kit and bore is fine, measured the ring gaps and they were well below max]. the only thing i haven't changed is the supposedly good cdi - because i haven't got a spare the engine will idle when it's warm at about 1500 revs and starts usually first kick. i've just run out of ideas why it won't rev over 3000 any suggestions gratefully received.
cheers, dd.
p.s. i've ordered a new 160 main jet to try and it already has a new #25 pilot jet in it.

[tz375]

DD, when you say you dropped float height, do you mean that fuel level is now higher than it was? If possible set fuel height to say 3mm below the gasket surface as a starting point.

The problem does sound like it's way rich or there's a problem with the high speed coil in the mag. One thing you could do is to hook up an auto timing light and see if the timing is moving or if sparks disappear at 3k. I don't know about the ignition in that bike but Yamaha use two different charge coils. One for low speed and one for higher speed. They have very different windings and resistances. On a Yamaha, if it won't spark at low revs replace the low speed coil, and if it drops off at higher revs replace the high speed coil. Other manufacturers use a single charge coil and manage the curve. I would guess that Suzuki had one lighting coil and one ignition coil.

That reminds me, when Lucas used a version of their alternator to fire the ignition without battery they had a very narrow timing window. The pulses from the charge coils had to be timed to arrive at the right point in the ignition cycle. Too far off and the spark was too weak to fire. I wonder if your replacement charge coils might be slightly out of time.

And of course all electrical problems are really jetting, so you could drop the needle all the way and or fit a smaller main and see if that cleans things up at all. I suspect the issue is in the ignition though. Is it possible that teh coil is breaking down or arcing or might the ignition switch be grounding out as revs and vibration rise?

I'd try a new coil, cap and HT lead next because that's quite cheap.

[rngdng]

My DT400 was not revving up correctly and one of the things Chuck Q found was that the resistor in my plug cap was resisting WAY TOO MUCH. Instead of 5K, it was reading about 100K!! Just that one change helped a lot. Then he worked on the jetting quite a bit.

[dollydog]

hmm, where do i start? try tz first:
i lowered the float height, which lowered the fuel height in the floatbowl. now although it's stopped flooding, it now runs for about 20 seconds and then stops, so i'm going to lift the float level up 1mm and try again. glad this carb is easy enough to get off.
this mag has pei ignition and has 4 coils. 2 for charging and the other 2 [sitting on top of each other] are for the ignition. works something like: one sends a signal to the capacitor to charge up and the other tells it to discharge and fire the coil - that's much simplified.
i've tried 3 different coils, one new. i've tried plug caps with resistors in and without them in. although it starts very easily with the non-resistor type caps it still won't rev over 3000. oh yes, the ignition switch and most of the wiring is new.
ok, rngdng: think i answered that one with trying non resistor caps - which are still on
many thanks for the suggestions, i'll keep working at it and when i sort it i'll let you know what simple thing it was
failing that, i do have a plan b - it's actually plan d, but hey ho
cheers, dd.

[rngdng]

I believe that the ignition coils are "low speed" and "high speed". Not sure, but maybe the "high speed" one isn't doing it's job...I'm running out of ideas.

[dollydog]

after buying and fitting a stator off 'a good running engine', i fitted it and expected all my earlier woes to come to an end - BUT, i tested the 2 coils in the mag as per the clymer manual and black/red wire to earth should be 220 ohms, mine tested at 219 - that's the pulser coil. the red/white wire to earth should be 75 ohms, mine tested at 183 that's the primary coil. so, we know what to do now. just wished i'd tested it before fitting it, instead of believing somebody's word. i always was too trusting. anyway, thanks for the help and hopefully i can now get the thing sorted.
cheers, dd.