Basket Case Yamaha XS 250

[gbjeppm]

Hi All,

I put this on the XS400 forum, but it seems pretty quiet, so i am posting it here as well.

I know it is not a Honda, but it is an SOHC twin.

I picked up a 1980 XS250 at auction a month or so (sight unseen), no history, no idea when it last ran, a total punt to be honest, but its lockdown and I need something to keep me occupied.

Anyway it turned up, more or less as expected, tired, filthy (lots of dried clay on the frame), but only 14k miles.



Anyway I have worked through it, and done the following.

New plugs
New coils
New condenser
New leads
New caps

I have spark ( it seems ok, i say ok as I am not convinced its that strong, but its there)

I have static timed it with a test light, and all seems good

I have fully stripped and rebuilt the carbs, I have an ultrasonic cleaner, and I am comfortable that they are good enough to start the bike at least.

Compression is a bit on the low side, around 100 psi on both cylinders. So i took the head off, cleaned and lapped the valves, Pistons look good, i covered them in oil to see if it would leak past the rings, and 48 hours later it had not moved. So i am calling the rings good for now. Compression did not change after clean up, but I still think its enough to start. It easily blows my finger off the spark plug hole on the compression stroke if I hold it there.

The starter motor is very sluggish, cleaning up all the wiring I have managed to make it run a little better, but a replacement is on order

The problem though, using either the kick start or the starter motor is that it cranks, but it has never even popped or coughed, the plugs are wet from fuel from the carbs.

I have also tried starter fluid in the carb, and even direct in the combustion chamber, absolutely nothing at all.

So to date, it has not created a combustion event at all.

My gut is telling me it is spark, and that under compression it is not firing. I have an inline spark plug tester and it does look weak in comparison to my other bikes.

I am now at a loss, i have spent hours checking wiring, coil specs, etc.

Everything checks out.

One other thing is that the kick starter only seems to engage in the second half of its travel, is that normal?

Perhaps the combination of my weak starter, or poor kick starter, it is not getting enough oomph, but again my gut says poor spark.

So please, any ideas, no matter how off the wall are welcome.

Cheers
Matt

[Bryanj]

Has it got points or electronic, points had to be spot on to fire, electronic needs a really good battery

[gbjeppm]

Has it got points or electronic, points had to be spot on to fire, electronic needs a really good battery

Yes points, very similar setup to the CB550K, but on the cam. Points are gapped and set correctly, Battery is new, full and on a tender.

But i will triple check the points again tomorrow if they need to be spot on.

[Bryanj]

You say new coils are they pattern and if so are they same ohm rating as original, no i dont know from memory what that is

[gbjeppm]

You say new coils are they pattern and if so are they same ohm rating as original, no i dont know from memory what that is

The new ones are pattern from Yambits, they are the same Ohm rating.
I have the original ones as well, the primary and secondary resistance readings of all of them are in spec.

No difference in behaviour between the two sets.

[Laverda Dave]

If you fitted new plugs make sure they are not a resistance type (marked with a 'R' after the plug code such as B79 R) because if you have a resistor plug cap as well as a resistor plug the spark will be weak. It has to be one or the other, not both.
Are you definitely getting fuel through the carbs?

[gbjeppm]

If you fitted new plugs make sure they are not a resistance type (marked with a 'R' after the plug code such as B79 R) because if you have a resistor plug cap as well as a resistor plug the spark will be weak. It has to be one or the other, not both.
Are you definitely getting fuel through the carbs?

New Plugs are NGK BP7ES, which are correct according to the Haynes manual. (they are the same as the old ones)
The original caps have no continuity according to my multimeter, so i have tried two other types of NGK caps that do have full continuity. The secondary coil readings are in spec with these caps.

As for the fuel in the carbs, they are wet and smell of fuel, so something is getting through. They may not be perfect, but I would expect at least a pop. And I dont even get that with starter fluid directly.

Thanks for all the responses so far.

[hairygit]

You could try running a separate live to the positive side of the coils to eliminate the possibility of excessive resistance in the wiring. As standard the Live goes from battery to ignition switch, through the kill switch then to the coils. That involves a lot of places where poor or dirty connections are possible, which can seriously affect the coils performance

Sent from my POT-LX1 using Tapatalk

[Bryanj]

Be worth checking carb slides are all way down and that butterflies can close, i am working with a very old memory here but i think the slides are handed so can be wrong if mixed and a bit like bench sync on the 500 you can have the butterflys too open to get a start.
Compression is low but that could be the gauge type and its the same after as before, i do know those carbs are a swine to clean, bit like the 250 N superdream----could take 4 or 5 goes to get clean and sometimes even that didnt work

[gbjeppm]

You could try running a separate live to the positive side of the coils to eliminate the possibility of excessive resistance in the wiring. As standard the Live goes from battery to ignition switch, through the kill switch then to the coils. That involves a lot of places where poor or dirty connections are possible, which can seriously affect the coils performance

Sent from my POT-LX1 using Tapatalk

I will try that today, and report back.

[gbjeppm]

Be worth checking carb slides are all way down and that butterflies can close, i am working with a very old memory here but i think the slides are handed so can be wrong if mixed and a bit like bench sync on the 500 you can have the butterflys too open to get a start.
Compression is low but that could be the gauge type and its the same after as before, i do know those carbs are a swine to clean, bit like the 250 N superdream----could take 4 or 5 goes to get clean and sometimes even that didnt work

Thanks Bryan, i have heard the same on the carbs, but i think they are good enough at this point to at least get a pop.

For the moment though i feel that i want to take the carbs out of the equation.

I don't get a pop/backfire or anything else with either fuel or starter fluid down the intake without the carbs on.

If i could get it to pop i would then be looking at the carbs. Does that make sense?

 

[Skoti]

Check the carb diaphragms.

[Skoti]

Just remembered back in the 70's when working on those bikes that the strange thing about them was that they would not start
from cold if you gave them throttle.

I think the cold start procedure was fuel tap on prime position, full choke and no throttle.

Maybe somebody who also remembers this will be along soon...

[K2-K6]

Can the timing cam (advance and retard mech) be assembled on these 180 degree out?

If so,  and unlike the four cylinder system, there's no "spare" spark type facility to work.

In effect, can you assemble it so that it's firing exhaust stroke on each cylinder?

[K2-K6]

If the points are, as I think, on the camshaft? Then swap the coil trigger wires over so that the "wrong" coil is connected and try that.