Advice from folk cleverer than me needed! (about compression) GB won't run right

[stogadog]

As you may have guessed I have a problem wity my GB250 that is driving me to distraction. when I got the bike it was making a fairly horrible rattle that seemed to go away and then start again. I managed to find another engine that I swapped over (and found out that the camchain guide in my original one has snapped somewhere inside). My new engine starts and runs nicely and sounds healthy but the bike bike has no power above around 6000rpm and will only do around 65mph no matter what I do to it. I'm fairly sure that it revved better with the original engine but not 100% because I was scared of the noise in case it blew up.
I've cleaned out the carb with my ultrasonic machine and it has made no difference, also a plug chop from 8000 rpm showed a healthy colour.
My next port of call was compression. The bike should have 200psi and when I test it I'm getting 150psi whether hot or cold. If I put a squirt of oil into the bore I get 180 psi. I take it this means that the rings are not sealing properly? What i don't know is whether losing 50psi on a 30 year old engine is normal and whether the compression would go up with a squirt of oil even if the rings are OK. I know some of you might think this a daft couple of questions but I don't know the answers!
Also , would worn rings or bore cause this kind of problem? As I have the other engine which I think has a decent top end I can get it swapped over if needs be. I'll have to pay someone I reckon , it's a bit beyond me as it's a complex little thing but I'm a bit scared to spend more money on it needlessly so all advice welcome!

[johnr]

200psi is a bit ott, id consider an aircooled engine to be healthy with a compression reading of 150 to 180psi, so id say that your compression figure seems ok, have you checked that the valve clearances are ok? before stripping anything, do a compression test on your old engine to see what its like.
i dont think its carb relaed, after all its the same carb as you had on the old engine, so, check what youve changed first,  ignition timing? spark plug? valve timing and clearances? does the motor have automatic ignition advance retard or manual using bobweights? if so have they seized? the bobweights on jap stuff can seize either fully advanced or fully retarded?

[themoudie]

Aye stogadog,

I would agree with johnr about compression testing and the readings that you might obtain. Also his comments about the "Vital Spark" are very pertinent, especially the timing, I believe that the GB250's ignition is CDI, so not sure if it either goes PHUT entirely, or partly!?! 

GB250_restoration

Further, I understand the carb to be similar to that fitted to Yamaha SRX singles , in that it is a two stage job, with one carb only coming int operation at larger throttle openings. When cleaning and reassembling your carbs did you use a new gasket, 'O' ring and diaphragm set and also renew the needle and needle jet?    Cleaning carbs and then using worn needles and needle jets can cause to much fuel to be available and also holes in diaphragms, however small, can prove disastrous (no inuendo intended, but I know what some correspondents are like    ).

David Silver appear to be the best port of call for parts for these beasties.

I hope this helps sort the problem.

My regards, Bill

[stogadog]

Thanks chaps

Sometimes it just needs a second pair of eyes so to speak. I realised earlier that I hadn't checked the valve clearances on the new engine. They were so far out it was like I had set them in the first place! Once set to the proper clearances I took for a spin up the road and it has a full rev range now. Only went for a little spin but it feels loads better now so more miles to come but hopefully we are sorted.
For the record and for anyone else who might read this with a GB in the future , it has cdi ignition so there are no bob weights etc to play with regards timing and mine has a single carb because it's later one.

Thanks again chaps - I really was banging my head against a brick wall for a while there. Time to get it prepared for the Thunderfest at darley moor next month now.

[themoudie]

Enjoy Darley Moor!    I hope the snow has melted.

my regards, Bill

[stogadog]

Hopefully by May all the snow will be gone! I shall report on our progress 

[johnr]

oh, and long tim no see! you still at lancaster?

[stogadog]

Hey John! Yes it's been a while. I did mean to say hello but side tracked by the fact i had fixed something! Yep, still in Lancaster but I've now got a bigger shed! We are trying to mover though but still in the same area. Do you know about Wray bike night tomorrow? Check out my next post in the events bit if you don't.