The XS250 From Hell

[Waddy92]

Hello all, 

This is my first bit of content on here, but I have been a silent voyeur for a while. Lurking in the shadows, trying for the life of me to understand why my bloody bike isn't working. 

I have taken on a lot of info and tried fruitlessly to solve the issues, but there's something not right. 

Its a 78' xs250, SOHC, of course. I completely took it to pieces, down to every last bolt. Did everything I could right, all the engine cases got vapour blasted, new stainless steen bolt kit, new gaskets, fresh everything. New coils, new, condensor, new timing chain. But my bike just doesn't want to go. 

I had an engine, that had a broken kickstart journal (laughably standard issue I've heard), took me ages to solve that kickstart issues, until i realised that I was missing a little shim, which apparently is the make or break between the splines engaging or not. Then my kickstart spring broke. So I've got a brand new one of those also. So after the usual bottom end problems, confusions (this is my first engine rebuild after all), I'm on the top end. Rebuilt the bike, got it all looking nice, kicking over well, and I'd spent a lot of time making the bike look how I wanted. Slaved over every largely irrelivant detail, until I was 95% finished. 

 

Went to breath some life into it, nothing; and my once perfect world came crashing around me. 

Checked spark. Yes

Checked fuel: Yes

Checked compression: No. Well 75psi

Checked carbs: Air Mixture Screw Snapped 

Life was bad. 

So what steps did I take to rectify this nightmare. Well, a mate of mine had a spare engine, also with a broken kickstart journal. It was a runner, I know, as I'd seen it running. So he kindly offered me the top end at mates rates. Yesterday, I took the barrel and pistons off my bike, took the barrel and pistons off his spare engine, and put them on mine. Bolted the head back down, timed up the cams (I hope correctly, LT mark on bottom, timing mark on end of camshaft at 12o'clock), and tried again. Compression was at 110psi. Checked it on his bike, which has a working engine and the Halfrauds guage read out 100psi. So even if 110 seems a little low, the reading is 10psi higher that his working engine. 

Here's my symptoms, and I'm hoping someone can help me get this show on the road. I timed the points with the rigged up light method, making sure the light comes on just before the LF mark and same on the right. Start kicking away like Billio (I'm running kickstart only), and I'm getting little to nothing, even with EasyStart squirted into the barrels. Once I open the throttle, opening the butterfly valves, I get two pops, and even at one point flames and smoke coming out of the cone filters (I must point out, I was using his carbs, as mine are goosed, took delivery of some practically new ones last night). The jets are #122.5, so should be about right for this set up. What is happening?!

Everyone I speak to who knows about bikes says timing. But which bit? Points? Valves? Why is there seemingly no life, and then loud bangs flying out the carbs? Is the head wrong? Do I have a valve problem? 

Any and all insight would be much appreciated. 

Thank you 

Waddy

[drewpy]

your timing is 180' out, swap over the coils

[anyoldiron]

l would start with making sure you have it timed correctly. Bike should run on 110psi, ive had bikes that have run on less, not to be confused with ''well'', but they have run.

[Waddy92]

Thanks for the replies. I manually wigggled the points to see if the left point sparked the left spark plug and it did. I'm pretty sure the coils are wired to the correct side. When I tried swapping them over i got the exact same issue.

Do you think it might be a valve issue?

[Oldfjman]

Hi Waddy - just my two-pennies worth, but I always take the simple approach, so here we go !

if you're getting spitting back through the carbs, then the ignited charge from the combustion chamber is getting back through your inlet valve.

in my opinion, there can be only 2 reasons for this :-

The inlet valves are open when the mixture ignites, or too shortly thereafter (either your valve timing is wrong, or your plugs are sparking at the wrong time, as Drewpy mentioned).

or

The inlet valves are not sealing properly and letting the ignited mixture blow back out of the inlet and into the carbs.

I'm not sure what your compression should be, but 110 psi does seem a bit low for a small engine with a high compression ratio.

Although you said the engine was running in his bike, that would be irrelevant as you dismantled it and then put the parts back on your engine. Again using the simple method, putting in parts that you know worked before and getting the same problem on your bike would indicate that it's either a ) nothing to do with the parts you've replaced, or b ) you've got something else installed or set up incorrectly.

If you're adamant the valve timing is ok, and the valves are sealing, it must be something to do with the ignition (firing at the wrong time).

The only thing you can do is check all of these again and eliminate them one by one by having someone else go through it with you to confirm everything is as it should be (checking everything off against the manufacturers settings).

[jimmy]

My solution is simple. Get some fresh petrol, not much, just maybe about half a pint. Make sure the bike is in a well ventilated area. Pour the fresh petrol all over the bike light a match, set the bike on fire....walk away whistling, and feeling as if a weight has been lifted from your shoulders........

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Not much help I know matey, but I hade a very bad experience with an XS250 back in the day as a spotty 17 yr old and I've never forgave it

[drewpy]

waddy update your info so we know where your from. may be a member nearby who can help