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1980 YAMAHA XT250 RUNNING PROBLEM – TIMING, NOT CARB

  • isetta
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I am putting this on here, not asking for advice, but in case someone who has the same problem as me comes across it and it might help them.A few months ago I bought a 1980 Yamaha XT250G (the first type of XT250 with 2 valves).I am in UK, the bike has come here from Australia. Frame and engine number begin 3Y4 but I believe it is exactly the same as USA bikes which begin 3Y1 other than USA bikes do not have a tachometer whereas mine and original UK bikes do.When riding the bike it would pull very well up to 4000rpm but above that it would struggle to gain revs, although standing still in neutral it would rev fine.I thought the mixture felt maybe weak but not supported by the sparkplug looking sooty/black. Putting the choke on whilst riding did not make it better, which I thought it would if weak mixture.I spent a long long time tinkering with float height and changing the slide needle position (the needle has five positions for the circlip) and trying different size main jets and cleaning the carb out.Anyone who has tried getting the carb off one of these bikes will know this is not a two minute job like it is on some bikes (my 1974 Yamaha DT250B being a bike where the carb is so easy to get at).  It was so frustrating. I had thought of igntion but it’s CDI and not adjustable and the advance is electronic built into the CDI unit.About 30 years ago I had a few rides on an old British 1940s motorbike with a lever on the handlebar to control the advance / retard. I thought the way the XT250 felt was like that old motorbike felt if you did not manually advance the ignition at higher revs.So I dug out my old neon strobe timing light, put spots of white paint on timing marks and started the engine. Up to 4000rpm the timing looked right, but above that the timing retarded slightly instead of advancing. How can it do that? It’s a sealed CDI unit and the pulser/pickup thing can’t even be moved.At least I now knew it was timing advance problem and not carb.
 But what to do about it? I thought I would check the wiring. Where the wires from the magneto connect to the main loom, there are 4 individual bullet connectors. The main loom colours of those wires are red/white, green/white, red, brown.  The red/white and green/white are for the pulser pickup, the red and brown are for the magneto power. The wires coming up from the magneto have a kind of coloured woven fabric on them but this had all decayed and fallen off, Underneath that the pvc insulation is the same colour on all wires. So I peeled back some of the black tape to show more of the woven fabric sleeving. I found that the red/white and red wires were around the wrong way. I changed these round and started the engine, checked with strobe light and it now advances at higher revs. Rode the bike, it is now excellent performance. I love it. I am surprised the engine would actually start before with those wires around the wrong way.So I have put this here in case someone else has the exact same problem and comes across this when searching the internet for advice.     
The following user(s) Liked this Post: MarkT, t-bone, Dirtboy, Andy L, TreasureState
05 Apr 2021 12:57 #1

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Good & interesting advice, thanks for posting. Back in the day when i worked for the local Yam dealer we had a problem with TT250's--same cdi setup--stuffing the cdi box & had no advance at all so although it started as normal it just stayed around tdc firing--like my "54 BSA B33 on retard--& if you tried to coax the revs up a little it'd make the header pipe glow red in the dark lol. We replaced a few of those boxes. Different problem to yours i know & top marks for finding what it was & very interesting. Sort of thing you'll probably never have again & get a lot of that with bikes when everyone's had a go at it before you.
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05 Apr 2021 15:58 #2

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'Hi fellas. I'm Garry in Australia. It was very interesting reading about these timing issues. Amongst my Yamaha collection is a pretty special XT 250. It's been rebuilt featuring circa 1980 YZ front and rear ends and a heap of special other features. But it's underpowered. Forgive me if it's a silly question but can I just bolt in a 500 motor of the same era? Any help is appreciated. Thanks,Gaz.
05 Apr 2021 23:00 #3

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Hi from NZ so not far away. Here's a link where someone's talking about fitting a 350 top end off a 350 big bear although cases need opening up for the cylinder liner overhang to fit in. Be a lot of work. Fitting another motor would likelu not be straight forward either. Your XT has the swinging arm hanging off the back of the motor where the Xt ot TT500 doesn't. They did do that on the XT550 TT600 so might be best to shelve that plan or get a cheapy running XT550 & shovel your YZ front & back end into that & carry on into the sunset. Project are alway great to have. Here's a link to Partzilla of the 3 crankcases.
thumpertalk.com/forums/topic/1302553-big-bore-for-1982-xt250/
www.partzilla.com/catalog/yamaha/motorcy...980/xt250g/crankcase
www.partzilla.com/catalog/yamaha/motorcy...980/xt500g/crankcase
www.partzilla.com/catalog/yamaha/motorcy...550-xt550j/crankcase
Last edit: 06 Apr 2021 01:29 by RT325.
06 Apr 2021 01:28 #4

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Thanking you RT325. Here in Australia there's  a VMX class known as "Evolution" which for instance includes the likes of my YZ  465 H 1981 at the limit. e.g not the 1982 490 with linkage suspension. Yours is fantastic advice. I had hoped to man up my XT. I'm going to investigate the lineage of the TT600 and XT550 and also the 500 Honda etc stressed member motors of the era, i.e pre1982 for qualification purposes. Sorry not to be a Yammy purist! But I expect little joy in the endeavour for what it's worth! Many thanks again. Garry.
07 Apr 2021 23:31 #5

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TT600 is linkage of course but motor might drop in a 550 non linkage frame if that's allowed. If "82 is the cutoff do they stretch it a bit for some model that don't change much. We have Evo & all the other vmx classes here but i've not been involved for a while mostly due to crash recovery & old age lol. Had a YZ465G new in "80. Actually i'm forgetting that the pic is in my Avatar 'if that's the word'. Was a good day 'that'.
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Last edit: 08 Apr 2021 04:52 by RT325.
08 Apr 2021 04:50 #6

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Replied by TreasureState on topic 1980 YAMAHA XT250 RUNNING PROBLEM – TIMING, NOT CARB

I am still chasing gremlins in my XT250 - thanks for the scoop.  Would you please post a picture(s) if convenient?  I too have been looking at the bewildering fabric wrapped wires that do not pull the same color through the junction.  Not sure if these attachments help you or anyone else: 


Cheers, 

TS

isetta wrote: I am putting this on here, not asking for advice, but in case someone who has the same problem as me comes across it and it might help them.A few months ago I bought a 1980 Yamaha XT250G (the first type of XT250 with 2 valves).I am in UK, the bike has come here from Australia. Frame and engine number begin 3Y4 but I believe it is exactly the same as USA bikes which begin 3Y1 other than USA bikes do not have a tachometer whereas mine and original UK bikes do.When riding the bike it would pull very well up to 4000rpm but above that it would struggle to gain revs, although standing still in neutral it would rev fine.I thought the mixture felt maybe weak but not supported by the sparkplug looking sooty/black. Putting the choke on whilst riding did not make it better, which I thought it would if weak mixture.I spent a long long time tinkering with float height and changing the slide needle position (the needle has five positions for the circlip) and trying different size main jets and cleaning the carb out.Anyone who has tried getting the carb off one of these bikes will know this is not a two minute job like it is on some bikes (my 1974 Yamaha DT250B being a bike where the carb is so easy to get at).  It was so frustrating. I had thought of igntion but it’s CDI and not adjustable and the advance is electronic built into the CDI unit.About 30 years ago I had a few rides on an old British 1940s motorbike with a lever on the handlebar to control the advance / retard. I thought the way the XT250 felt was like that old motorbike felt if you did not manually advance the ignition at higher revs.So I dug out my old neon strobe timing light, put spots of white paint on timing marks and started the engine. Up to 4000rpm the timing looked right, but above that the timing retarded slightly instead of advancing. How can it do that? It’s a sealed CDI unit and the pulser/pickup thing can’t even be moved.At least I now knew it was timing advance problem and not carb.
 But what to do about it? I thought I would check the wiring. Where the wires from the magneto connect to the main loom, there are 4 individual bullet connectors. The main loom colours of those wires are red/white, green/white, red, brown.  The red/white and green/white are for the pulser pickup, the red and brown are for the magneto power. The wires coming up from the magneto have a kind of coloured woven fabric on them but this had all decayed and fallen off, Underneath that the pvc insulation is the same colour on all wires. So I peeled back some of the black tape to show more of the woven fabric sleeving. I found that the red/white and red wires were around the wrong way. I changed these round and started the engine, checked with strobe light and it now advances at higher revs. Rode the bike, it is now excellent performance. I love it. I am surprised the engine would actually start before with those wires around the wrong way.So I have put this here in case someone else has the exact same problem and comes across this when searching the internet for advice.     

2000 XR70R
2005 XR100R
2008 TT-R230
1982 XT250G
2003 DR-Z400
2008 DR650
2020 DL650 V-Strom Adventure XT
. . . looking for a CRF230F!
10 Apr 2021 21:38 #7

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