Low compression question

[GS750]

My good ole GS750B (1977) had an unknown service history until I purchased it 3 years ago and when i purchased it, the compression test showed that it had the following compresions :
1. 140psi,
2. 145psi,
3. 140psi,
4. 145psi

Now, recently it has been getting a little harder to start, and I noticed some popping in the exhaust on startup, so I decided to check the compression again with the following results:
1. 75psi,
2. 72psi,
3. 75psi,
4. 70psi,

and valve clearances of:
1. .14mm exhaust .03mm inlet
1. .14mm exhaust .07mm inlet
1. .11mm exhaust .11mm inlet
1. .07mm exhaust .04mm inlet

so, whilst there are some that are over the spec (specs say .03mm - .08mm) they should be still sealing.


The bike has been running very well otherwise (after the hard starting) and seems to have plenty of power still on the highway (Ive had 3 speeding tickets in the past 12mths, one was clocked at 168Kmh on a long, straight uphill run), and it still manages to lift the front wheel a few inches off the deck if I take off hard from the lights, and still gets 14Km per litre.


Ok, my questions are:
1. Why would the compression drop so much in 3 years and 40 thousand Km?

2. Is this just fair wear and tear?

3. Could it be that my 4 into 1 exhaust has a muffler that is too large and not allowing enough back pressure?

4. What should I do now? Remove cyl head and check valves for sealing, and view the cylinders at the same time to help make a better diagnosis?

thanks in advance,

[Nessism]

Do the compression test again by making sure the engine is warm and throttle is wide open. After getting the base line readings dump 1 teaspoon of engine oil in each cylinder and take the readings again. If the numbers jump you have bad rings. If the numbers stay the same you have valve problems.

Not sure why you have seen so much compression loss. Have you adjusted the valves regularly?

[GS750]

Hi Nessism, I will do the leak down test later today, I didnt even think about that!

The valves havent been adjusted regularly because it had good compression the last time I checked (3 years ago) and I have checked the clearances twice since then, they were all reasonably within tolerances so I didnt bother to adjust them since they didnt rattle.

[jetta90]

I would for sure adjust recheck valve clearances and adjust to get them all within specs then do another compression check.
There should be some wear and tear on the engine from your 40m KM but that is a significant change......................would also be good to know total kms on the bike.

[GS750]

Tomorrow morning I will do a compression leakdown test.

I am uncertain as to the actual total Km's on the bike, I only know that the odometer shows 80 thosand Km, and just over 40 thousand of that was from me.

Like I said earlier, I am totally unaware of its previous service history, or how it was treated. The only things that I do know is that it was laid down at least once because there was scratches on the sidecover and the front mudguard were slightly damaged too.

[Suzuki mad]

1st, get the valves within working tollerance, then do a full compression test.

Running a GS750, as will all bucket and shim engines, the cam wears the shim away slightly. Having not adjusted them for 3 years (and 40 thous km/mls) its time to do that.

If you don't there may well be burning on the edge of the valves/seats not making them seat properly/tightly causing the engine to be hard to start. Once warm it will still run well.

Suzuki mad.

[GS750]

First, Thanks to everyone for the helpful info.

Even though number one piston was riding the inlet valve ( I removed the shim and tested compression) I did the leak down test on the engine with the following results:

Dry compression ( as previously stated) was
1. 75psi,
2. 72psi,
3. 75psi,
4. 70psi,

Well, after playing arounda little, I managed to get it up to the following:
1. 110psi,
2. 85psi,
3. 85psi,
4. 85psi,

and after the wet test it showed as:
1. 140psi,
2. 125psi,
3. 110psi,
4. 125psi,


I have also had a chat to a guy that I know, he is a motorcycle wrecker and he told me that he would look at it, but that from his experience, it would most probably require a re-ring and valves to be re-ground, and posibly even need a bore and hone and oversize pistons.

He is very honest, and straight forward, and has worked on many many bikes over the years, as well as drag raced them.

I dont really have the patience, or the tools to do a re-ring (or even a strip and re-assembly) so i will probably just end up getting him to do it.

I do have some of the stuff here such as a new timing chain and the full engine gasket set, so Im guessing that it would really just cost me if it needed rings and or pistons, and the labour.

Thanks again for all the helpful advice.

[nabrams]

When you're talking about digging into the engine that deep, you need to think about whether it's worth the time/effort/money.
It will almost certainly be less headache (and much cheaper) to get a used, low-mileage engine that's never been opened. I've done this twice and it's worked out wonderfully - Not to mention that the price I paid in both instances was ridiculously low.

[GS750]

hi nabrams, I think it is worth the money etc because when compared to a new bike of around AU$15000 ++++ plus the fact that my bike is 30yo, and parts bikes are just not available here in Aus, I have tried wreckers from all over the country and they simply dont have GS750 parts, (or just wont sell them).

I was given a price a while back to do the top end on a mates GS850, they quoted AU$800 to re-ring, hone etcc etc, and that was with them supplying all the materials and labour, since I have all the gaskets and chain already, then I would imagine that it was going to be the costs of labour, plus a few sundry parts.

[GS750]

Forgot to mention that with buying another used motor you are also buying an unknown quantity again too, you wont know the service history, nor how it was treated by previous owner/s.