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GS750 Upper Motor Rebuild

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

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Just completed a 'ground/up' rebuild on a 78 GS750E. Full rebuild on the carbs using new carb kits and setting the rebuilt carbs to original settings. Attempted to sync the carbs using a carb sync tool with little luck. No matter how I attempted to adjust number 4 carb, nothing worked to bring the mercury in line with the other carbs. Number 4 cylinder reading on the sync tool is significantly higher than 1 through 3. Ran a compression test on all 4 cylinders. My understanding is anything under 100 psi means trouble. Here's my results.

1 = 90 psi
2 = 90 psi
3 = 90 psi
4 = 35 psi

Obviously, we're now in a top end motor rebuild situation.

I'm familiar with boring car engines but this will be my first motorcycle motor. What are the applicable boring diameters, example 30 over? Would anyone be able to point me to a vendor that supplies the appropriate pistons, rings and gaskets.

Also, is there any 'special knowledge' needed to lap and reseat the valves?

Any help would certainly be appreciated.

donmcw
 
You may be aswell getting a set of barrels/pistons from ebay. I know a set of pistons & rings here in the UK are in excess of ?200 (approx. $350).

I bought a complete enging for spares for about $120.
 
Don, There is no point in trying to sync the carbs when you compression is bad. Did you measure compression with bike warm and throttle cracked open? If not, then the readings may be bogus. IF your readings are correct, the next step is to pour a teaspoon of oil down each cylinder in turn and measure the compression again... (warm engine/throttle all the way open). When you do, then you MAY see a significant jump in compression. This indicates the oil is sealing the gap between piston and ring and you may have worn pistons/bores OR bad rings. If this is the case, you will need to buy some measuring tools to measure the pistons and the bores. Too big a gap and you will have to buy new pistons and go to an overbore. Most manufacturers have oversized pistons or a "big bore" kit will be in order. If the gap between your pistons and bores is within the service spec, then you can just put on a set of standard sized rings after doing a crosshatch/hone on the bores so the new rings will seat. A lack of compression can also mean problems in your cylinder head. Valves can stick open a tad due to accumulated crud. Very worn springs can cause poor compression and valve lash not being set properly can cause poor compression. Once you get your top end rebuilt, go back and sync your carbs... there will be more vacuum in the cylinder where there was poor compression. Good luck!
 
Check Ebay there have been several 850 kits for your 750 lately Also check the valve clearances.
There is no way to tell what is needed until you take it appart and measure everything. You may be able to rering and do the valves
 
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