yamaha SR250 won't start, help is highly appreciated

Hi,

I am jeroen from Belgium and started my first build some time ago.
I am working on a yamaha SR250 from 1981, i will share some pictures when i get some real work done/result.
currently the frame ect are beeing powdercoated, so time for me to take a look at the engine.
unfortunatly I encounter some problems there and I guess i would need some help.
this is what I have already done:
first compression test showed me 75PSI which is to low i guess.
a leak down test showed me a leaking exaust valve, so i replaced and lapped that one. (I exchaged all cilinder gaskets while the head was off as well)
the compression got up to 125PSI, but the engine still won't start.
I cleaned the carb compleatly and it has the original setup, but the engine still won't start.
I have a spark, i tested that, and when I use startpilot it will run for a few seconds.
what else could it be?
Tomorrow i will try to refresh the cable and plug from the sparkplug because i felt a little shock when i tested the spark (held the sparkplug against the ground)
but besides that i am clueless. so any help is highly appreciated.

Many thanks in advance!

Did you do compression test before removing motor from frame? Did you have carburetor fitted? If so, did you have full throttle? It's very difficult to get an accurate compression reading without motor spinning pretty fast, electric start is best as it can spin motor around 300 rpm, kick-start , if your lucky will get around 30 rpm. Leak-down showed a problem, have you tried again with new exhaust valve? Cam timing could be out one tooth? If you can get it running with starter fluid or similar it points more to the carb, I don't remember Yamaha manuals being very good so look for a 4~5 yr earlier or much later model and see if you overlooked something. Do you have slide or diaphragm carb? (pretty sure it's diaphragm?) They run real lean, a larger pilot jet may cure things? Oh, do you have the 'pin' enrichener plunger or flat type?(both same OD but pin type looks like a mini carb slide with a needle) If it's the wrong one the 'choke' doesn't work We didn't sell many of the SR250's in Britain, 125 learner law was in place at the time.but I know I worked on a few. I'll see if I still have any information, the engine was in production for a very long time and with some modifications got fitted to a lot of different vehicles (trail bikes, , ATV, etc)

Thank you for thinking with me on this one!
I did the compression test first on the bike full trottle and without carb, both gave me 75 psi, changing the vavle brought that up to 125 measured off the bike both with carb (full trottle) and without carb. both measurements where done using the startmotor.
I have been able to do a leak down test since i removed the engine from the cycle itself because i can't hold the crankshaft at top of the compression stroke anymore.

so today I replaced the sparkplug (and cable ect) but the engine still doesn't start.
I cleaned out the carb again compleatly, but still nothing.
to answere your questions, I have a diaphram carb I added the partview in attachment.
what exactly do you mean with "do you have the 'pin' enrichener plunger or flat type?"
I am not sure i am understanding that right... sorry for that

part 32 pin type i would say

Yep, looks like it to me. Also, looks like later carb with some press in bits that need to be removed for cleaning but are F**%><£ impossible to remove easily. There are a couple of other people here with SR250's maybe someone will chime in who has worked on one recently? Working on so much stuff over so many years makes it impossible to remember everything without actually having the parts in hand (as soon as you can look at it 90% comes back almost immediately)

What exactly do you mean by press in bits? I think i might have found one from the floater chamber to the one that will feed the choke, but it is hard to see because it is deep in one of the holes, and it is not mentioned on the partlist...

That's a press in part, the manual doesn't mention parts that can't ( or shouldn't) be removed. They drill a hole and push a part in, jet, restrictor,, etc relying on interference fit. Yamaha were trying to compete with Honda so cut some corners to get bikes 'out the door'. (Yamaha is still number 2, bigger than Kawasaki and Suzuki combined but they kinda stopped trying to out-build Honda numbers-vast quality improvement) You need to strip carb body and find a good ultrasonic cleaner as the oxidation is almost impossible to remove inside drilling's (it was a 'fault' even back in the day, we changed carbs fairly regularly, under warranty) XV535 had a recall they were so bad. I've found many carbs need suspending at various angles and multiple 10~15 minute trips through ultrasonic cleaner. the 'dunk it for an hour' isn't very effective in my experience (after working on carbs that were 'professionally cleaned' : ) You could try a strand of copper wire but some of the drilling's are tiny, around 0.25~0.4mm
If you could find the original Yamaha carb cleaner that will work. (best carb cleaner ever made, even Honda said so -behind closed doors ;D ) Has to say 'Not for sale or use in California' , they get weak stuff