Compression test didn't go so well

[Rob_K]

Just done a compression test on My GS750 and all looks low:

95, 100, 105, 85 from left to right with all plugs out. It runs ok, but it's really low on power and feels strained. So I'm looking at top end rebuild. Now what I understand is that you can put an 850 top on straight onto this, is that true as I've found 850 barrels pistons and slide carb head for the price of a new piston kti. Assuming this is possible, will I need to change that carbs also. From what I've been reading there are so many interchangable parts going on between these two?

I'd much rather have all the bits ready to go than strip it to work out if it is piston rings or valves, hence thinking about the 850 conversion, that way I can get the parts all nicely sorted and then simply swap it all over in one day. 

Am I mad, or can this work? I waas fully aware this was going to be a money pit from the off, this was the cheapest gs750 I found at the time. I love it, so want to rebuild it anyway, but if I can get a little extra umph out of it, then that would be swell.

Edited August 15 by Rob_K

[Gixer1460]

You are assuming worst case scenario! The rings could be fine and you may have defective valves or valve seats! A comp test just tells you that but not the where or why - a leak down test is a better test to determine those questions. As regards the 850 conversion - i've no experience other than reading what others have done - you 'may' have to open up the crankcases to take the larger dia barrels? or they could be the same 750 / 850 OD's ? You could use the 750 carbs with some tuning, they may be down at the top end but the bottom end would be much improved. I think i'd do some more investigation before shelling out the coin!

[Swiss Toni]

Yes. The 850 top end will fit fine! But, as 1460 says, it could be badly seating valves, among other things.Make airline connection up for the plug hole, bring to tdc, and stick some psi into it. Inlet & exhaust valves leaking ... self explanatory. Hissing in the oil filler ... rings. Air outa the head/barrel interface ... head gasket. 

Edited August 15 by Swiss Toni

[gsxwill]

As others have said, could be sticky rings, leaky valves, better to check out before you spend money,, 

[Rob_K]

Hi all, thanks for advice. As far as I can work out, I'm fairly certain that it is burnt or leaking valves, main symptoms are wont' tick over when at full running temp, missing and poor acceleration that picks up at about 4000 rpm, a bit of spitting back when first starting and running on choke. When pulling plugs, I get two that are a nice colour and then I get one running rich and I get the right hand pot (that one had a  closed valve clearance too) is always wet when i  pull it out. I would expect worn rings to have much more consistent running but decline in power and increase in smoke over time. 

Regarding the 850 barrels. I did a ton of research yesterday on the GSResources page to find that the slide carb 750's and the slide carb 850's use the same heads, it's the same part number, when the 850 went to CV carbs, they opened up the ports and valves a little. It turns out that any barrels will do, but if they come from a cv 850, then there is some dremel work to do. Regarding cams, the 850 cams are a little more tame than the 750, so it turns out, just swapping pistons and barrels is actually quite a nice quiet upgrade. 

As I have to rebuild the top end anyway, once the head is off, I'll definitely look at doing the upgrade, I'll document it with pics too as I haven't actually found any documentation of this being done, yet it seems loads of people used to do it as everyone likes a kickstart, so manly.  

[Reinhoud]

Good excuse to stuff a GS1000 engine in it

[TonyGee]

4 minutes ago, Reinhoud said:

Good excuse to stuff a GS1000 engine in it

Or even a gsx1100 engine 

[Reinhoud]

Just now, TonyGee said:

Or even a gsx1100 engine 

Even better!