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PostPosted: Tue Dec 29, 2015 9:51 pm 
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Joined: Tue May 26, 2015 3:07 am
Posts: 3
I just recently picked up a CJ750 in non running order and need some help with it. The Bike was in running condition but was blowing some white smoke. It went to a gentlemen (if you can call him that) that claimed he knew how to work on the bike. It got the improved pistons/rings installed along with new cylinders. The guy also claims to have done a full tune up but could never get it running again. It went from that owner to other and now to me, and it hasn't ran since that menace put his hands on it.

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I'll also just go out to say that I am not a mechanic however after hours/days of research and tinkering The bike is closer to starting/Running again now more then it has been in a long time. I've got new spark plugs, a new battery, replaced all the old fuel, oil, pulled the heads/cylinders off and doubled checked the ring gaps. She's got all new gaskets, I re-gapped the valve clearances, and retimed the bike according to the manual, and rebuild the carburetors to the best of my knowledge and ability.

So I pretty sure the bike is getting fuel, I know its getting air and spark, maybe its just lacking compression? So before I get some oversized piston rings or something, I was wondering what kind of compression (PSI) should it get? I got a tester and she's getting 35PSI in both cylinders. That seems low to me, but on 24HP 750CC engine designed in the 1930's maybe that's fine? Again I'm not a mechanic so if there's anyone else out there with a CJ750 could you test your compression for me, or if there's anyone else out there that knows a lot more then me with engines I would really appreciate the help, Thanks!


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PostPosted: Fri Jan 01, 2016 9:44 pm 
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Joined: Sun Dec 19, 2010 8:20 am
Posts: 910
Location: surrey
type in chang jiang google search someone there should be able to help, also try old timers garage (Poland) he speaks good English and is fast with his deliveries
graham


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PostPosted: Sun Jan 03, 2016 10:29 am 
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Joined: Wed May 23, 2012 9:00 pm
Posts: 134
Location: UK
Also suggest you try Ben at Sidecarpro who is an Aussie living in Beijing and is extremely helpful and provides a great service. I've bought loads from him including many major engine parts:

http://sidecarpro.com
ben@sidecarpro.com

There's also John Lawes at BEMW (Derby, UK) who is always helpful and knowledgeable on the phone and I've had some smaller parts from him - but have heard mixed reports on him from others for major works:
http://www.bemw.co.uk

Good luck and keep us posted....


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PostPosted: Mon Jan 04, 2016 9:14 pm 
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Joined: Sun Apr 08, 2007 7:23 pm
Posts: 846
try tow starting it in second gear

sometimes old bikes just need a bump and when they have run a bit they will work off the kick start


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PostPosted: Mon Jan 04, 2016 9:21 pm 
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Joined: Wed Apr 08, 2015 8:51 pm
Posts: 23
You say you have it near running, and a spark, it would be a matter of balancing the carbs up and getting the mixture right, and tweek the advance timing a fraction if need be. Also remember that the needles in the throttle can be adjusted up and down. You are most probably thinking the worst of things that may not be wrong with it. My CJ starts on the second kick and straight away on the starter. Its running a fraction rich but they don't like running lean and hot anyway, I thought I have a go at getting it lean but had no play in the adjustment screws so I adjusted the needles down and it didn't like it one bit, it wouldn't start, so I adjusted it back, starts perfect and runs well. Try adjusting the mixture screw on one side in and out as you crank it over with the starter motor and find the spot, then try the other carb. Then balance both carbs using a carb balancing tool. If there was something major wrong with it you would hear it and in some cases it would start but not very well. if no joy then make sure your carbs are squeaky clean. I was lucky having a brand new squeaky clean spare carb that I tried out when I first got the bike when the originals wouldn't fire up, the bike worked with the new so the old needed an extra clean, you will be surprised how much micro crap can clog up a carb also use spray tin of carb cleaner, that helps.
Let me know how you get on.


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PostPosted: Mon Jan 04, 2016 10:11 pm 
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Joined: Sun Apr 08, 2007 7:23 pm
Posts: 846
i put my BMW R12 back together after 35 years of it sitting on a shelf

it would not start everything was correct but it just would not have it, so i towed it with a tractor and it started and after it had been warm it started off the kick start, i have had it running now for ten years and ride it like i stole it most of the time and apart from the clutch making a clonking noise last year (it was the one from the war) she is on the money (i done the main bits like the crankshaft, rings, timing gears and chains but she still has the wartime pistons and until last year the mag was original-the points still are)

Tow it it will at least start to fire and you will be able to work it out from there yes the needles etc might need adjusting but it will run if its too lean or too rich and as its a four stroke it wont detonate, balancing the carbs should be easy enough just make sure the slides go up together and stop at the same time if not you can adjust the cables to rectify it

tow it!


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PostPosted: Tue Jan 05, 2016 5:54 am 
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Joined: Tue May 26, 2015 3:07 am
Posts: 3
Thanks for the responses, found just a few posts stating it needs around 100 PSI for compression, so I'm about 70PSI sigh :( Its been towed before and was the only way it would run, but Before I try that myself, I'm going to make sure Everything else is good to go. On top of all of that while I'm waiting for parts to come in the mail, I ended up gutting all the old electronics and wiring (waaaay to many nasty splices and rusty/corroded components) This way I'll get to know/understand the bike, and I'll know everything was done correctly.


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PostPosted: Tue Jan 05, 2016 11:13 am 
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Joined: Wed Apr 08, 2015 8:51 pm
Posts: 23
Whether high or low compression the bike should still start just means you will have less power. Still work on the carbs, I reckon at a guess you have the needles set wrong or your carbs still have crap in them.


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