Newbie from South East UK with a Basket Case XS250

[gbjeppm]

Hi All,

My first post, and I apologise upfront that I am also after your experience and assistance.

A little about me, I am no stranger to rebuilding motorcycles, and in my time have rebuilt a number of motorcycles, in fact i have a nice 1979 HondaCB550k SOHC in my stable right now, which from a technology perspective is quite similar.

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

 

[amc49]

Once you wet the plugs good with fuel it won't start as the spark shorts down side of the porcelain instead of going through the airgap. Fouled plugs.

The point gaps must be right or coils do not saturate and then spark intensity is low.

At 14K miles compression should be good unless really abused but suspect the 100 psi number which is for junk engine. They are considered worn out that low. I'd suspect your gauge, if like many they won't read accurate on smaller cylinders like 125 (half 250) because they have too much internal volume in them. Even moreso if a car tester.

Compression testing gives utter fits with results due to the exact demand and the need for a good quality device and most are garbage to begin with.

I have a CB550F '77 and 22K miles when I got it for free and mostly sat outside. I changed virtually nothing in parts but did have to redrill coils for new wires since the coil type has integrated coils. After major tuneup the bike has run at over 100 mph way more than once and running fine since '04. I did add pods to it and blocked off the front opening of airbox and converted it to a nice little tool holder or baby trunk to hold stuff in. It held fuses for a bit until I understood a weird vibration issue that had fuses shatter at the very ends of the filaments where you couldn't see it, then refusing using a different holder and fuse type stopped all that. The single muffler was long gone and the ratty one on bike got replaced with a Vance/Hines megaphone I had lying around and it looked much better that way.

 

[gbjeppm]

Ah interesting, i also thought my gauge, which is a cheap one was playing up. The finger test says all is good.

So basically this engine is very unlikely to fire at all with petrol or starter fluid direct into the intake, it needs dry plugs with a properly working carb?

Is that a correct assumption?

 

[amc49]

What, are you asking in terms of absolutes? Even normal 'running perfect' engines misfire part of the time. You should get a pop every once in a while but not guaranteed, Nature always sides with the hidden flaw. You've never seen a wet plug foul spark down the side of it to not ignite correctly?

You'll get a good finger compression test even on a 4/1 compression lawn mower engine, that idea is crap. I've seen people swear to that with blown head gaskets.

You apparently overlooked my first sentence, and possibly the most important one to you.

 

[drewpy]

try heating the plugs up too, helps a lot.

turn off fuel and shoot the easy start instead and it should at least cough.

If that doesn't work go back to the timing as you could have switched the coil connections around (I've done that before)

 

[gbjeppm]

Your advice was sound, i ensured the plugs were dry, and finally i managed to get it to at least backfire a couple of time.

I have ordered new points, it is the only part of the ignition system that I have not changed, and on close inspection they are very worn.

We shall see, i appreciate your assistance,

 

[amc49]

Set the point gaps with a dwell meter if you have access to one, it's virtually impossible to set a pair of points the same using gap alone. I'm still running the OEM points on the 550, the engine revs to 9000 rpm all day long. I tend to not buy new parts until 100% sure I need them, I won't hesitate at all but the need has to prove itself out. How I save thousands working on computer cars. I never throw parts at them, almost always fixing with one part only. Ignition points if the pivot point is still tight are simply renewed by filing them back flat for free. I tend to lower cost as well by using like small block Chevy condensers instead of more expensive OEM stuff, the mfd is the same. And the car ones work better.

Make sure your hits of ether have either run out or dissipated before shooting more, I've seen an explosion from built up ether blow rods out the side of an engine before. Also saw somebody play utterly stupid to burn half his face off when the ether and fuel would not light the engine. Not a good or noble tale. Use the 2 second rule, no more than a 2 second shot and make sure it's gone to not accumulate before shooting more. Stacking shots of ether together is bomb building.

Dry plugs are better but still can be fouled, the fuel coats the surface to provide a shorting path. Best is to degrease the plug with like alcohol or acetone. Once dry they are almost like new if they haven't colored yet. If black though you likely need new ones unless you can get engine lit off to then burn them back clean. 50/50 odds of that.

 

[motoTrooper]

I've been watching some Allen Millyard videos and he showed how he takes fouled plugs and puts them in his barbecue to burn off the carbon build up. Smart fellow.

 

[drewpy]

I've met Allen a few times, great bloke

 

[amc49]

Maybe to post #8. If you get them TOO hot too quick then the fuel layer glazes to a ceramic similar to the porcelain and makes them worse, the glaze absolutely can conduct electricity. That glazing is commonly what makes plugs misfire under load. Why I said 50/50 chance.

You can take a point file to sharpen the wire electrode edges up as well as dress the center tip back flat and the new sharp edges will point spark like a gun, Ignition spark loves to jump off sharp edges.

Remove all resistance out of plug caps and wires and you can open the plug gap up another few thousandths to actually do something useful.

One trick I used all the time way back on Kaw triples was if a plug fouled out then remove the plug lead and then gently set it back down to not be snapped in place, the extra airspace increases gap to make coil go all the way hot and often the the plug comes back to firing when it wasn't. Drive around for a minute or two and then snap the lead back down and go on with your business. Instant plug change.

 

[gbjeppm]

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.

 

[xschris]

125 is better than the 100 but still the bottom end of a good motor. 155-160 is the upper end for these. I would run a few hundred miles on it at highway speeds and do another cold compression test. If it's still low, you might be on barrowed time and want to at least start collecting rebuild parts.

 

[amc49]

FYI, a smaller 125cc. cylinder generally will not register quite the same compression as a bigger 200cc. one. I'd allow up to 10 psi less just for that. A general rule of small engines if the advertised compression ratio is the same. How you get the whopping compression numbers out of car engines in relation to smaller bike which may have higher ratios.

 

[xschris]

The xs250 has a 9.6:1 compression and the 400 has 9.3:1

 

[amc49]

So? I've seen it be more different than that. The cam timing has a whopping effect in there too. Bigger cylinders almost always use more cam timing. The compression numbers they give are never actual, rather, they are computed with no regard for say intake closing points, they are purely mechanical only using swept displacement vs. compressed area. A car engine of say 2000 cc. can hit 200 psi at only 8.5/1.

Part of it I think is the time to charging thing there, the smaller cylinders don't charge as well at slow compression testing speeds. The bigger ones having a bigger volume can 'soak' more density in as the engine approaches intake closing point to trap a bit more air in at the closed intake. The smaller cylinder instead has already started backflowing back out. Part of that comes from more weight in a bigger air column once it has started moving, bigger cylinder then goes a bit longer before the air density saturates to then reverse to go backwards. It seems odd since smaller cylinders are more 'air active' than big ones but that is what makes that happen. My take on it anyway.

Hope that makes some sense.