750 Top end rebuild

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750 Top end rebuild

Postby Freddy » 30 Sep 2018, 13:53

Been over 100,000 hard kilometers since the last top end rebuild so decided today to 'bit the bullet' and do it again. Compressions were starting to drop a bit and number 2 was 15 psi below the other 3 cylinders.

Spent the morning checking I had all the parts I needed, and kicked off stripping the engine about lunch time. By about 6 pm I had the head off, stripped and decoked. A soda blaster is the ONLY way to decoke a head. What would have taken hours by hand, took about 10 minutes with the gun. Going to fit all new valves so just need to now take it to the machine shop to get the seats cut. Head was dead flat, and valve guides fine.

Tomorrow its pull the barrels and fit another I have sitting on the shelf that has already been bored and honed for new 2 oversize pistons. Reckon I should have that finished easily in the day, and then its just a matter of waiting for the head to come back. Expect around a week turn around at the machine shop.

So another day to reassemble the head, and put it back one etc. So all up about 3 days (maybe 4) actual work time, and it should be running like brand new again. About the only possible holdup I anticipate will be the missing shim size to set the valves. I've got a fair collection, by Murphy's Law will dictate I won't have the size I want.
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Re: 750 Top end rebuild

Postby knight » 30 Sep 2018, 17:23

WoW Freddy,
Your a star,look forward to hearing your progress, wish i had your skills! took me weeks to sort my carbies,but i did have fun when you can enjoy the end results :D
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Re: 750 Top end rebuild

Postby Freddy » 30 Sep 2018, 19:52

It certainly helps to have done it previously. And some of the more difficult components e.g. the carbies, must have taken them off/on now 20+ times when playing around with different jet sizes. What the first time was an exercise in frustration is now a very simple task.

Like fitting the new pistons tomorrow. Never again, like the first time, would I attempt to do it without ring compressors.
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Re: 750 Top end rebuild

Postby David Richard » 30 Sep 2018, 21:16

hi fredy we know you have a manual cam chain tensioner ,what will the primary chain be like have you replaced that, it has been said it could of had a tensioner fitted to it look forward to reading about it and I hope it goes without any problems ,david
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Re: 750 Top end rebuild

Postby Freddy » 01 Oct 2018, 01:55

David Richard wrote:hi fredy we know you have a manual cam chain tensioner ,what will the primary chain be like have you replaced that, it has been said it could of had a tensioner fitted to it look forward to reading about it and I hope it goes without any problems ,david


Not touching the crank and con rod bearings etc. Measured a sample of them for wear about 40,000 Klm's ago when I replaced 4th and 5th gears in the gearbox, and they had little to no wear. So from that I'm confident the bottom end, if good lubrication is maintain, should outlast several top end rebuilds. I change oil every 5000 kilometers, and oil and filter every 10,000.

At the time of doing the gearbox (maybe it was when I did the starter clutch) I measured the primary chain deflection. The spec is somewhere in the manual. At around 100,000 kilometers the primary chain deflection was well withing the service limits. If I was rebuilding the bottom end, I'll probably take the opportunity to replace it. But as I don't every expect to have to do that, and the chain is showing no signs of wear I've no plans to touch it.

With a manual camchain tensioner, and new rubber dampeners in the primary chain hub replaced, when the starter clutch was done (old ones go as hard as steel), I find the engine to be pretty quite for an air cooled engine.
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Re: 750 Top end rebuild

Postby Freddy » 01 Oct 2018, 11:56

On track ..... new pistons fitted and cylinder block back on.

Most difficult task ...... fitting those tiny piston pin circlips to the two inside pistons. The Haynes manual has a suggestion to stick a screwdriver with a shaft with a larger diameter than the clip gap through the clip and pin so if it flies off when you are fitting it it'll get trapped on the screwdriver shaft. Sounded like a good idea so tried it. Has to be a phillips-head so the blade will fit through. Great theory, just makes it impossible to fit the clip because there is no room to work. Just roll them in with your thumb.

Learning ..... tried fitting the 4 pistons in one go. Too hard and didn't work. Rotate the engine till the inner 2 are at the top of their stroke, when they are partially inside the block remove the ring compressors, and then rotate the engine to bring the outer 2 up. I also found I had to put stick through the back wheel to stop it and the engine turning as I bumped the block down.
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Re: 750 Top end rebuild

Postby Fishrider » 01 Oct 2018, 22:18

Nice thread with a couple very good pointers to refer back to when I get there. Thanks Freddy
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Re: 750 Top end rebuild

Postby Freddy » 02 Oct 2018, 02:13

Fishrider wrote:Nice thread with a couple very good pointers to refer back to when I get there. Thanks Freddy


Thanks,
The main thing I wanted to see is just how long it would take if I was really prepared parts wise, and had a good method for tackling the bits I found challenging the previous time, and had to spend time figuring out.

The two main problems last time were .....

(1) removing and refitting the carburetors. With plenty of practice since then doing jet swaps, with the right method they are actually a 'piece of cake'.

(2) Getting the pistons up into the block. Don't even think about doing it without ring compressors, well not unless you want to spend a couple days of total frustration as one end jumps back out as you're trying to get the other end in. 4 VERY large cable ties, and a $3 piece of 68mm ID PVC plumbing pipe joiner is all you need to make 4 ring compressors. Just wide enough to cover the rings, and grind a 45 degree taper on the top edge so they go up inside the taper at the bottom of the cylinder sleeves. Fit them to the pistons before you put the pistons on the conrods. With ring compressors and doing two at a time, pistons went straight in.
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Re: 750 Top end rebuild

Postby Freddy » 02 Oct 2018, 08:41

Took the head to the machine shop to get the valve seats cut. Going to take a week so had time to clean up, etc. First chance I'd had to check the old cylinders and pistons.

Basically the center 2 cylinders and pistons were in near perfect condition, BUT number's 1 and 4 pistons and bores were badly scored on the back (inlet) side. In the picture below the left piston is from a center cylinder, one on the right is from the outer cylinder. It's only damaged on one side.

Image

Why? What's the chance of the problem being say oil related in the exact same place on the exact same pistons. Next to none. So that leaves overheating. Why would both number 1 and 4 cylinders overheat, and not 2 and 3 which you would expect to be hotter being in the center of the block with less surrounding cooling fins. The hot spot corresponds to the back of the cylinder, exactly where you'd expect (out of the wind).

What's common to number 1 and 4 cylinders, and different to 2 and 3 that influences cylinder temperature? How about smaller main jet, needle jet, and needle. Stock both the outer cylinders run a leaner AFR. About the first 90,000 kilometers of these pistons were run with stock jetting. About 10,000 kilometers ago I changed to one size larger jest in the middle and two sizes larger on the outer cylinders (making all 4 the same size).

Anyhow I reckon the outer piston damage relates to the leaner AFR run stock in those cylinders. Probably never going to worry someone who runs the engine conservatively in modest climates. However worked hard in hot climates like Australia and this may be the result. Also these pistons are 2 oversize, so compression ratios and hence combustion temperatures would be slightly elevated compared to stock.

No certainty to this conclusion, just my best educated assessment.
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Re: 750 Top end rebuild

Postby Freddy » 13 Oct 2018, 03:10

Cylinder head back from the machine shop yesterday morning. A$180 to cut the seats. All new valves so no need to face the old ones. Been faced before so didn't want to do it again as each time (together with cut seats) sinks them slightly further into the head (reduced compression). Reassembled, head fitted to bike, carbies back on, valves adjusted.

This morning fitted exhaust, oil cooler, and sundry items e.g horn. Just got to oil up the air cleaner, spin her over without the plugs to build up oil pressure, fit the tank ..... and hopefully BRUM.

Just stopped for a cup of tea. Expected possible holdup of setting the valve clearances through not having a correct shim size didn't happen. Over the years I've built up a modest size collection.

Anyhow, back to work in about 30 minutes we'll see if it starts. I'm confident it will. Probably have to go to the shop to get some more oil. Just had enough to fill it to the top sight line but by the time it refills the new oil filter, oil cooler, oil lines, etc its bound to be low.
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