to rebuild or not to rebuild

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graciesdadpaul
Posts: 33
Joined: Fri May 14, 2021 4:22 pm

to rebuild or not to rebuild

Post by graciesdadpaul »

Hi all,

i am in the north Buckinghamshire area and i am new to this forum. i have been riding for 40 years now and own a few modern and classic bike. I recently completed the restoration of a 1981 Suzuki Gs250T and having gained a lot of confidence on this I finally took the plunge and purchased an imported (usa) GT750J with 12k on the clocks. it has been standing for a very long time and will need a bit of work to re-commission. The engine turns over nicely and i managed to get the head off after a bit of a struggle to check the condition of the bores. All looks good, you can still see the honing marks on the barrels. Now I am aware that these engines may need new crank seals etc, but that means a lots of work and expense just to get the stubborn barrels off just to look inside the crankcase.

As the bores look ok and the engine turns over would it be worth trying to run it and see if the seals are blown before i tear it down further, or would this be a foolish waste of time and maybe catastrophic to the engine. Any advice on this would be gratefully appreciated.

paul :D
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Gary66
Posts: 710
Joined: Sun Jan 05, 2014 10:53 am

Re: to rebuild or not to rebuild

Post by Gary66 »

Mmmmm, well i did just what you are thinking of doing 7 years ago with my GT380. Basically I hadn't got the reg or v5 so I didn't want to invest masses of money into the bike and fall foul with the v5. So fast forward 7 years, and the bike still runs well without having the crank rebuilt etc. Alan H suggests soaking the crank whilst in the motor in diesel which I did do on a 1975 yamaha rs100 and again got away with it l. Might be worth a shot but only you will know whether to chance it or not, others will probably tell you not to chance it , but for me it was a no brainer. Good luck either way
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Alan H
Posts: 12104
Joined: Fri Aug 05, 2011 6:38 am
Location: Wombwell, Republic of South Yorkshire

Re: to rebuild or not to rebuild

Post by Alan H »

I did the diesel trick with 2 engines and ran them for over 4 years each.
Proof that four strokes are over complicated
teazer
Posts: 1035
Joined: Mon Mar 07, 2011 8:10 pm
Location: Chicago

Re: to rebuild or not to rebuild

Post by teazer »

Two ways to look at that.

One is to do the job "properly" and rebuild the crank and know that it's solid. That's what I always do because it gives me the chance to clean all the cases and make sure everything is sound. And it gives me a good excuse to open the ports and raise compression ....If the barrels have to come off for other reasons, I'd fully strip it.

Or, try the diesel trick. It costs next to nothing to try, though it can be messy, and see if you can get it running. There's not a huge investment in time or cash and if it works you can run the bike for a few years until it really has to be done. Worst case, the crank is shot and has to come out. All you wasted was a few days of letting the seals soak.

There is no "one way" to bring an old bike back from the dead. We have different budgets, time scales and skill sets. Do what's right for you and yours.
graciesdadpaul
Posts: 33
Joined: Fri May 14, 2021 4:22 pm

Re: to rebuild or not to rebuild

Post by graciesdadpaul »

Hi all,

thanks everyone that's really helpful. I suspect being a bit tight i will try the diesel trick. Sorry to be thick but how is that done? where does the diesel oil go, in the crankcase i assume?
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Alan H
Posts: 12104
Joined: Fri Aug 05, 2011 6:38 am
Location: Wombwell, Republic of South Yorkshire

Re: to rebuild or not to rebuild

Post by Alan H »

The 3 550s I have restored had been stood a long time, so I took the head and barrels off them and filled each crankcase with diesel and turned the engines over slowly - each day for a week or more - drain the diesel out and repeat - (via SRIS valves in the front of the cases.)
This should wash all the bearings and soften the crank seals. It's worked on both my engines on the fully restored bikes, but unfortunately the J crank was completely knackered, so just wasted a pint or so of tractor blood was all.
It might be worth trying on yours before you actually go out riding it. Oil the bearings before starting it again - lots of smoke but the engines seem to benefit from it. 
You might need 'easy start' to get it going! I used it before I fitted the airboxes and squirt directly into the carbs, a bit at a time until the smoke cleared and the engine runs without spluttering.
Unfortunately, if the seals are knackered, then it's game over, but always worth a try for the sake of a couple of quids worth of tractor blood.
Proof that four strokes are over complicated
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zapo
Posts: 691
Joined: Fri Mar 29, 2013 5:07 pm
Location: Near Derby

Re: to rebuild or not to rebuild

Post by zapo »

Tractor blood, eh, Alan.
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I had two stroke's and survived.
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