Compression?

Eddie · Post by **Eddie** » Sun Dec 21, 2014 9:37 am

I have asked this before but I do not recall the answers you guys gave me, what is normal compression for a gt750? I think mine has low compression but it runs really good , it has 110,100, and 105 across, is this to low? She runs good and pulls good but I have read some guys on here that have around 150 when they check so I'm thinking mine is low and I'm wondering how much better it would run if I gave it a fresh bore and Pistons ? But I'll probably wait until she really starts running bad but it just bothers me that it low or is that low?

Coyote · Post by **Coyote** » Sun Dec 21, 2014 11:57 am

In new, stock form, the compression spec is 6.7 to 1. That's pretty low compression to start with. 2 strokes run that way. I don't know how anyone could get 150 without some serious modifications.
100 is getting down there, but I wouldn't myself over it - especially if the bike runs good. A fresh top end wouldn't gain you anything really noticeable IMO.Also it is advisable to borrow or rent another compression gauge and do a comparison. They can vary a lot.

Cyzygy · Post by **Cyzygy** » Sun Dec 21, 2014 3:39 pm

Eddie,
Check if your compression gauge has the schrader valve (one way valve) fitted where the gauge is inserted into the spark plug hole or at the gauge head. If the valve isn't fitted at the plug hole the volume in the stem or hose that leads to the valve fitted at the gauge head becomes "combustion chamber volume" thus lowering your "indicated compression" dramatically.
Bob

Eddie · Post by **Eddie** » Sun Dec 21, 2014 5:19 pm

The valve is at the end of the hose where it screws into the cylinder head , not at the gage . I m not sure why it checks so low , when I read a post about 150 on compression check I thought that was really high or mine is really low, but even if it should be around 130 mine is low but I am going to just ride it until it dies then I will do a total rebuild . I'm just hoping it doesn't die anytime soon.

Craig380 · Post by **Craig380** » Mon Dec 22, 2014 3:03 am

Sorry if this is telling you stuff you already know, but are you doing the comp check with the engine fully warmed up (that is, pretty much straight after a ride) and with wide-open throttle?

Eddie · Post by **Eddie** » Mon Dec 22, 2014 9:01 am

The last time I checked it the motor was cold, and wide open throttle I don't recall if I have checked it after a good ride, but I have checked it when its warm, I had it idling for a while then checked it, maybe I will take it for a good ride then check it, and yes I have it full throttle when checking, thanks for the idea I will try that and see if theres any difference ...

Craig380 · Post by **Craig380** » Tue Dec 23, 2014 3:35 am

Again, ignore me if you're already doing this, but only take the reading after 5 or 6 kicks / turns on the starter in quick succession.

When I've done comp tests on my 380, the engine is still hot after a ride (at least 5 miles), and I kick the engine over 5 or 6 times real quick, with no pause inbetween kicks. After the first kick the gauge reads maybe 105/110psi then climbs after every kick until it peaks on the 5th or 6th.

Eddie · Post by **Eddie** » Tue Dec 23, 2014 8:42 am

craig, I appreciate all the help I can get, thanks for the help, I usually hold the throttle wide open and hit the starter until the needle on the compression gauge wont climb anymore. I have not been able to take her for a ride yet but as soon as I can I will check it and post my results, hopefully it will be higher. I had taken the exhaust headers off at one point because I was working on it and I could see in the middle or right hand cylinder don't remember now which one it was but the cylinder looked pretty scored , and the piston skirt looked the same, not sure if they were deep or just superficial lines caused by the oil and exhaust, but ever since then I have been weary of it, but it might just be me being paranoid......

T350guy · Post by **T350guy** » Tue Dec 23, 2014 11:10 am

My old 74 had 135 and my 77 has 140. My buddy just check his with a fair amount of miles at 120.

Craig380 · Post by **Craig380** » Tue Dec 23, 2014 11:31 am

See what difference it makes when the motor is good and warm ... if you get readings in the 120psi range (with no more than around 10% difference in the readings) then that's OK and I'd suggest leaving it alone.

Sure, it could be higher, but it's not so bad that it demands an immediate tear-down and rebuild, you'll get another few summers riding. Just my 10 cents / 6.4 pence

Eddie · Post by **Eddie** » Sun Dec 28, 2014 4:22 pm

ok I took the gt750 out for a 10 mile ride came back and did compression check , as I was doing the 1st cylinder the neighbor came by and we started talking, the bike cooled down so not sure how accurate this is...also I used my gauge and my dads old gauge, here goes the results with my gauge ... left cyl then middle then right , 1st row is left cyl. reading top to bottom so 96,105,100 and so on......
left cyl. middle cyl. right cyl.
96 100 105
105 100 102
100 98 100

dads gauge
105 105 110
103 102 109
100 100 110

I checked each cylinder three times , with each gauge, the motor was still warm to the touch when I checked, but the temp gauge showed cold, or slightly above cold, I even started it and let it idle for a while then checked again and the numbers were about the same no big difference, so what do you guys think? the left cylinder is the one that smokes the most until warm and when I checked it the first time it showed 96, which freaked me out a little , but then I checked it again and got 105 then 100? who knows, but does this compression look to low?

Eddie · Post by **Eddie** » Sun Dec 28, 2014 8:02 pm

I know that the compression on a stock gt750 was 6.7.1 , I have been trying to figure out how to convert the gauge reading to compression ratio, my barometric psi is like 30.09 which is like 14.78 , so if you divide the gauge reading of say 100psi by 14.78 it comes out to 6.765 if I did that right?

so I guess that's pretty close to what the factory called for . I looked this stuff up on the net to figure out how to do this, but still not sure if I did it right, if I did it comes out about right, so what type of reading do you guys get that have a fresh rebuilt motor? sea level is 14.5 , if you divide 100pis(gauge reading) by 14.5 it comes to 6.896 so should a stock fresh motor have a gauge reading of 100 to 105 psi reading???? 105 on gauge comes to 7.2 compression ratio when divided by 14.5 (sea level) I have no idea if I did this right or not but just going by a site that I found....which I think might and probably is wrong

Craig380 · Post by **Craig380** » Mon Dec 29, 2014 3:16 am

Good call on using two different gauges to eliminate instrument errors.

My 10 cents is this: unfortunately, those numbers are on the low side, so you have worn bores or piston rings, or both. One of the 750 guys will be able to advise better on compression readings, but I believe you should be seeing 120 - 130psi on a healthy top end.

On the upside, however, the bike runs and rides OK so it doesn't require urgent attention. You're not going to do any harm by waiting a little to do the job.

Personally, I'd just start gathering the bits you'd need to refresh the top end and then do the refresh at a time of your choosing. Parts usually come a little cheaper if you're not in a hurry to get hold of them

yeadon_m · Post by **yeadon_m** » Mon Dec 29, 2014 3:29 am

Eddie,

On the gauge I own, I get 120-123psi on a freshly built GT motor (380/550/750).

As gauges vary, its a good idea to buy and keep the same one so you can track any changes. For now, using any working gauge, I would worry if I had more than 5psi difference between two pots. I can't really make much sense from your numbers, the number on any cylinder shouldn't vary when you measure it repeatedly, unless the technique used differs - it is what it is.

If it runs well, and isn't noisy, I'd also be inclined to leave it alone unless for other reasons you planned to take it apart!

Cheers,
Mike

Eddie · Post by **Eddie** » Mon Dec 29, 2014 8:44 am

thanks guys, I thought it was a little weird that in 3 different tries I got different numbers, but they are all about the same. I did a check after I bought the bike a few years ago and the numbers are pretty much the same, (I just found the book that I wrote them in), and I have put about 4,000 miles on the bike since I bought it, I had to redo a lot of stuff on it(gas tank, oil pump and lines, put electronic ignition, new water pump and so on) plus my health wasn't so good so I didn't get much riding time on it. I guess im just going to ride it until she wont run anymore, then rebuild time. I have a couple of questions on that 1st I have a set of 1st over Suzuki pistons for it already but im wondering if I should opt for something different like wossner's? , 2nd should I just rebuild the whole thing (crank seals) or just try the top end? and 3rd if I do rebuild the bottom end , I just put a brand new water pump in , so will this have to come out? or can it be left in and cases split without taking it out? I have never rebuilt a gt750 motor this will be my 1st.....also I checked on the wossner pistons and notice they come with 2 left and 1-right, I take it one left goes in the middle? also is the 1st over 70.5mm?

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