Honda-SOHC
SOHC.co.uk Forums => Other Bikes => Topic started by: gbjeppm on December 23, 2020, 12:11:45 AM
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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.
(https://i.imgur.com/wxnu5mA.jpg)
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
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Has it got points or electronic, points had to be spot on to fire, electronic needs a really good battery
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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.
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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
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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.
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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?
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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.
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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
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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
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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.
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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?
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Check the carb diaphragms.
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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...
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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?
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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.
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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.
I am sure that it is ok, and its firing correctly.
But i did swap the trigger wires over and it made no difference.
I also know that i am getting a signal from the points up the trigger wire.
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Strange that you get not even a pop from it.
If the plugs are wet enough though it probably wouldn't make a difference with starter fluid either.
Can you drain the carb float bowls and leave them empty, then crank it with throttle open so that it can draw no fuel at all, then give it a mist of starter fluid to judge if you get any response.
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Can you drain the carb float bowls and leave them empty, then crank it with throttle open so that it can draw no fuel at all, then give it a mist of starter fluid to judge if you get any response.
Yes i can, will do that tomorrow probably.
I did just manage to get a couple of backfires in the carb and a loud bang through the exhaust when i opened the thottle whilst cranking.
That is a major step forward at least. So back to looking at timing and fuel delivery i guess, but progress of sorts.
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Might sound daft but with that check that spark is on correct cylinder
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Might sound daft but with that check that spark is on correct cylinder
Yes i am sure i have done something stupid, i just need to find it :)
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Compression sounds OK for a cold motor, should test them when fully warmed up, maybe condensers, do a direct live to the coils to eliminate the rest of the electrics.
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Compression sounds OK for a cold motor, should test them when fully warmed up, maybe condensers, do a direct live to the coils to eliminate the rest of the electrics.
Thanks Kev, new condenser, and direct live to the coils is in place.
I have though bitten the bullet and ordered new points, its the only part of the ignition system that i have not changed, and I want to discount them, They are very worn.
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Don't leave the direct live permanently connected it will stuff the coils up.
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Don't leave the direct live permanently connected it will stuff the coils up.
Thanks Kev for the reminder, all is good.
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Well finally i got her running properly.
It turns out that the cam was two teeth out. I decided to pull the cam cover again as i was still feeling that timing/ignition was not correct.
When i took the cam cover off the first time to clean up the valves,I marked the chain and sprocket and put it back on in the same place and did not check it. On closer inspection it was wrong when i got the bike, whoever put it back together before me, has lined up everything at 12 o clock, perpendicular to the ground, not perpendicular to the head, which was two teeth out.
Put it back together and fired up no worries for the first time in 10+ years.
That also explains the lowish compression, now it is 125.
Thanks all for your thoughts and comments.
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Result. Bet that was a good feeling!!
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Nice one, dont expect it to be fast though
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Result. Bet that was a good feeling!!
Yes, i was really starting to doubt myself....
And of course it was my own fault :)
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Nice one, dont expect it to be fast though
i have heard that it can do 70 downhill with a following wind.... not sure i want to do more than 40 on it :)
Anyway it will be a while, as i am going to take it down to the frame and paint it now that i know it runs, and give it a slight modernisation, headlight, indicators, exhaust, seat etc. All need to be changed anyway, and stock parts are silly money.
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Quite a rare classic these days, popular enough back in the day but you never see them out and about these days. I see lots of jap classics at squires cafe, its a popular venue for classic bike enthusiasts but I don’t remember seeing any XS250’s... should be a bit of a celebrity when you finally get out on her.. good luck and more photo’s please.
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70 downhill, possibly if that hill is Beachy Head!!
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I don’t think any of the 4 stroke 250’s were exactly rapid back in the day... not compared to the RD’s and X7’s.. I remember lusting after a superdream when I was 17 (I had a CB100N at the time), when I finally got to have a go one one a few years later I remember it being almost moped like! At least they looked good, the XS was always a bit of an ugly duckling but I sort of quite like them these days... the late 70’s models.
https://www.motorcyclespecs.co.za/model/yamaha/yamaha_xs250%2078.htm (https://www.motorcyclespecs.co.za/model/yamaha/yamaha_xs250%2078.htm)
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I think Yamaha were still sulking about two strokes being phased out but deciding it was better to be seen in the game rather than be forgotten. I seem to remember that both the 250 and 500 twins tended to be a bit heavy and vibey?
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I think Yamaha were still sulking about two strokes being phased out but deciding it was better to be seen in the game rather than be forgotten. I seem to remember that both the 250 and 500 twins tended to be a bit heavy and vibey?
Wasn't there a 400 too?
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Yes there was a 400, the 250 was actually a sleeved down 400 (same as the Superdream) but both still carried the weight of the 400 hence the relatively slow speed. The best 250 four strokes back then were the Kawasaki Z250 (especially in the Kork Ballington colour) as it was designed as a 250. Shame about the cam bearings though or rather the lack of oil to them. I still think the best four stroke 250 was the Honda CB250RSA, 90mph, light, fantastic handling and small. But I'm biased because I've got one 😁
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And the 2 strokes still had more than a decade of astonishing development and exciting biking in them before finally being phased out... starting with the iconic LC and then it’s YPVS derivatives before finally bowing out with the parallel twin TZR’s in the UK... By that time the R1 was in the pipeline so not much reason for Yamaha to be sulking as far as producing exciting performance motorcycles is concerned ;) ;) ;)
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The Yam 500 twin was a real lemon, the Kwak Z 250 had a bad rep for big ends in the trade but i liked the Honda 250 single as it looked good, ran like stink, handled and was cheap to keep on the road unless like the nearly written off insurance job i did some scroate undid the headlamp mount bolts and cut every wire coming out of the shell, that was a long expensive repair and some parts just wernt stocked like the nuts indide with earth wires on
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And lets not forget Honda's finest 250, the CB72, it's a pity that Honda did not develop it a little bit more with a five speed box
https://www.google.co.uk/url?sa=i&url=https%3A%2F%2Fwww.carandclassic.co.uk%2Fcar%2FC1097137&psig=AOvVaw37ZAf47H_ID0_RvxVO6iAT&ust=1609593657261000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCLiD1O3p-u0CFQAAAAAdAAAAABAI
Sorry about the hijacking of this thread ::)
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And the 2 strokes still had more than a decade of astonishing development and exciting biking in them before finally being phased out... starting with the iconic LC and then it’s YPVS derivatives before finally bowing out with the parallel twin TZR’s in the UK... By that time the R1 was in the pipeline so not much reason for Yamaha to be sulking as far as producing exciting performance motorcycles is concerned ;) ;) ;)
Of course the LC Yamaha were impressive machines and even had their own race series, but water cooling was only a stop gap to prevent buyers switching allegiance to other brands after expensively wooing them with the Fizzie. Unfortunately Honda yet again then pulled a rabbit out of the hat with the Fireblade and forced Yamaha's hand to come up with the R1. Both fine machine of their type and catering to the allegiances of their loyal camp followers.
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I don't see the R1 as a catch up to Fireblade as Yamaha were years ahead in concept that the Fireblade copied, effectively at larger capacity from this
[attachimg=1]
Which, more than anything, switched on changes to multi valve, liquid cooling, tilted cylinders, downward inlet tracts, etc. all resolved in one step. These are fabulous engines.
The R1 is just logical development of their own concept.
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Mine wasn't as posh as that! FZX 750 but quite a nice machine.
[attach=1]
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It's one of those bikes that seems to be missed in terms of appreciation as game changer.
The motor is absolutely jewel like inside, a real masterpiece. It's not generally known that Yamaha consulted on 4 stroke cylinder head design for quite a few of the mainstream motor manufacturers in bringing aluminium multi valve heads through r&d stages.
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It's one of those bikes that seems to be missed in terms of appreciation as game changer.
The motor is absolutely jewel like inside, a real masterpiece. It's not generally known that Yamaha consulted on 4 stroke cylinder head design for quite a few of the mainstream motor manufacturers in bringing aluminium multi valve heads through r&d stages.
They were involved in F1 motors too. I had an original 1000 Genesis pre exup. Rocketship. I really like the 750. Especially with top fairing only. Always fancied one.
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The 550 Yam head with odd induction was designed in Ireland
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" I had an original 1000 Genesis pre exup. Rocketship"
They really did didn't they, pulled like a train and just kept going. Another nice piece of engineering.
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Quick update, coming together quite nicely.
Few final things to do, new bars, new chain and mirrors on order.
Not for the purists, but i am quite liking it.
(https://i.imgur.com/zXD6b5a.jpg)
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I like it, well done for keeping it on the road. I bet the exhaust will sound good :)
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I bet the exhaust will sound good :)
Thanks Dave, first time i have cut the rear off a frame and welded in a hoop, which was slightly disconcerting.
I decided to mod this bike as putting it back to original was just too expensive with the cost of yamaha stock parts. It needed a lot.
At idle its quite quiet, but throaty when being opened up, so far seems good. Not ridden it much yet though (thanks covid)!!!
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That looks like an "SE" model which had high bars standard