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What should I get as a compression reading 78 750 EC

  • Thread starter Thread starter pnedac
  • Start date Start date
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pnedac

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I have the manual but it does not tell me what I should be getting as a compression reading and I am curious. I just picked up a GS750 EC 1978 and I feel like the bike is sluggish. If I did the compression reading correctly then each cylinder is about 58-60 which seems awfully low to me, but I have never had one of these before. It starts and drives alright. No smoking or anything like that. The bike seems to have been very well taken care of.

Any ideas?

Thanks, Pnedac
 
that almost seems to low to even run. Are you sure your gage is good , and when you did it , did you hold the throttle wide open?
 
Aha! Yeah so I wasn't holding open the throttle. Thank you and you can now make fun of me.

Well now that I got that out of the way I got a good read of 128 on cylinders 1,2 and 4, but I got only about 110 on cylinder 3. Is that too far out of tolerance? Another major reason this causes angst is that I see on the third carb that the idle screw is, well, screwed up looking. Looks like a mash of metal and the housing of the carb has been cut on both sides at the idle screw like someone was trying to screw around.

Anyway the symptom of the bike is that it feels like it just isn't quite all there. Starts fine, idols fine, but when you are riding it just feels sluggish and I think it is a not so good thing. I have had a 750 before and a 83 1100 that is one of the BOTM, but I know a goofy feeling. Is this usually a pretty serious deal to get into the engine and make a major correction on this 3rd cylinder if needed?

Thank you for any and all help.

Pnedac
 
Close enough for now. Have you adjusted the valves ?? How long has the motor run ??
Run it for awhile and you will probably find it goes up..
 
Yes as stated above, Has the bike been sitting before you got it? What service has been done to it? has it got a pipe on it?, does it have pods, or an air box?Also add what model and year to your signature, it will help us help you. Plus everyone won't be asking you what bike you have.As for your new readings yes a little low but not totally horrible, especially if, there has not been any maintenance done like a valve adjustment. Also what you are describing almost sounds like the bike is running a tad on the lean side.
 
as above, have you done the maintenance on it?

Does it still have points?

The lower reading on #3 could be the result of a bad petcock

How's the oil level?

Does the oil smell like gas?
 
Thanks for all the responses. Very comforting to hear that it may not be the end of the world here. As for maintenance, this is the beginning for me on this bike as I just picked it up on Friday night and I noticed that the bike was not playing with a full deck. I figured I would start with a compression test and go from there. I do know this:

It is not running points: Dyna S ignition with an Accel coil and wires. Looks all to be in good order. Looks to have been professionally installed (professionally wired and such, very tidy). The PO gave me all the receipts in a big binder so I guess I ought to go through it and do a little investigating. He also told me that the top end was rebuilt at 25K miles and it now has 30K miles.

The bike is running pod filters and the PO "told me" that it was rejetted to stage one for the pods (stock exhaust).

That one carb has now become more curious to me because the PO also gave me a big bix of well organized parts and within it was one carb housing. Something is telling me this guy knew that the one carb was funky and wanted to replace it but did not know what to do so decided to sell it.

Well more to follow as I zero in on the issue. I appreciate the help with this 1978 GS750 EC
 
Sometimes the mixture screws get buggered up and people cut down so they can cut a slot in the recessed mixture screw to get it out. Don't be too sure that you have a bad carb until you adjust the valves, etc. Also the compression test is best done with the engine warm although that probably wouldn't affect the discrepancy in the one cylinder. You might see what the condition of the mixture screw is. Does it have a new slot cut in it like the original one was buggered up? "Mixture" screw might not be the appropriate term but it's the recessed one that controls how rich or lean your bike is at idle. There is only on idle screw that controls all 4 carbs as far as how the idle rpm is set.
 
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