Compression tested engine cylinders. 3 good, 1 bad. What to do next??

[ATM]

Hi

i have a 1979 suzuki gs750l (this has been a great motorcycle!). I recently wet and dry tested the 4 cylinders on the motorcycle. One of the cylinders (#3) went from 30 psi dry up to around 75 psi wet. The other three cylinders were good, all over 100 psi wet and dry.

What would be a possible explanation for three cylinders being good and one bad??

I do not want to do a complete engine rebuild and spend alot of cash unless it is deemed necessary.

Any help would be appreciated.


Bigcheese

 

[GSX1000E]

You, of course, already adjusted the valve lash and had the slides open when you tested correct?

 

[ATM]

Hi

thanks for the response. If you are talking about checking the valve clearances the answer is no. I do not know how to do this.

I can tell you that the valve stems on the motorcycle were replaced within the last 3 years when i had the carburetor taken apart and cleaned. The shop that did this work indicated that an engine rebuild would be necessary. Perhaps i could benefit from another opinion from a different repair shop.

Bigcheese

 

[60ratrod]

Sounds like a tight valve. adjusting the valves means measuring the space between the tappet bucket shim and the cam lobe. valves that are too tight will allow the valve to remain open a bit and not fully close. to adjust, all you do is see how far out of spec you are, remove the current shim that is there, and replace it with one that is either thicker or thinner depending on how far out of spec you are and in which direction, tight or loose. i don't know the specs off the top of my head, but if you go to bassclif's site, you can download your bike's service manual and all of your specs will be in there.

 

[Johno]

Sounds like a tight valve. adjusting the valves means measuring the space between the tappet bucket shim and the cam lobe. valves that are too tight will allow the valve to remain open a bit and not fully close. to adjust, all you do is see how far out of spec you are, remove the current shim that is there, and replace it with one that is either thicker or thinner depending on how far out of spec you are and in which direction, tight or loose. i don't know the specs off the top of my head, but if you go to bassclif's site, you can download your bike's service manual and all of your specs will be in there.

The above advice is the first step.

It is most unlikely you will need to get into the motor apart from the valve cover.

these motors are bullet proof but the most neglected issue is valve adjustment.

get them right as a first step and your motor will start more easily and get up to operating temperature as required.

Even modern day workshops including Suzuki dealerships overlook this simple issue and use low compression figures to recommend engine rebuilds.

i had the same issue recommissioning my bike. Asked to have the carbs cleaned and they did a cold compression test and declared the motor requiring a possible top end or full rebuild.

Luckily I had someone who could do the carbs for me and I have learnt how to do the valves from the good people on this site.

50,000kms later the motor is as sweet as a nut.

 

[ATM]

Thanks for the input. I will focus on the valves.

This website was incredibly helpful when I was working with the electrical system on the motorcycle and the end result of that hard work payed off bigtime. Hopefully the intervention necessary to improve the engines poor compression in cylinder #3 will prove to be far less time consuming and costly than what I was initially led to believe.


BIGCHEESE

 

[Redman]

.............................. One of the cylinders (#3) went from 30 psi dry up to around 75 psi wet. .
.
.
What would be a possible explanation for three cylinders being good and one bad??

.

I think you have two situations/questions stated. And you are getting answers to one but not the other.

One situation/question is:
Why could one be bad and others fairly good?
And then are discussions about the valve clearance.
And yes, vavles in one cyclinder caused that one cyclinder to show low compression.
But, so could a cracked or broken ring in one cyclinder.

Other situation/question is about the one being real low dry and not so low wet.
That is a symptom more related to rings/piston/cyclinder, and not so much valves.
Well, unless when you added some oil to the cyclinder you slopped in/around so much it got over the valves too.
Generally the idea is to put in just a little oil, just enough to go between the piston and the cyclinder walls, and get it down in through the sparkplug hole without it slopping against the sparkplug hole and running over the valve. The entire idea is to seal up any open clearance on the piston/rings/cyclinders only and not any of the valves, so can see if it made a difference, and if it did then you know it is piston/rings/cyclinder because that is all you changed.

But, yes, checking and adjusting valve clearance is something need to do on occasion. If they get "tight", then gets worse and are not closing all the way, then bad things happen. And checking/adjusting valves can be done with removing the cam cover (and tools..and...and...)

Takeing care of piston/ring/cyclinder problem is a larger project. The rebuild you mentioned.


I saw your other recent postings.
Where in Michigan are you?


.

 

[Redman]

.....

I can tell you that the valve stems on the motorcycle were replaced within the last 3 years when i had the carburetor taken apart and cleaned. The shop that did this work indicated that an engine rebuild would be necessary. Perhaps i could benefit from another opinion from a different repair shop.

Bigcheese

Valve stems? (that would be related to the wheel and tire)
Valve guides?

If you had valve guides replaced, that means they had the heads off, and when they said "engine rebuild" that would not be the valve adjustment, that would be the piston/rings/cyclinder we were talking about.

Having a shop rebuild your engine would probably cost twice what the bike could be worth.

.

 

[SVSooke]

Here's a tutorial http://members.dslextreme.com/users/bikecliff/images/gs850valve_adjust.pdf from BassCliff's site.

 

[Gorminrider]

I always thought adding oil to cylinders to increase compression is specific to differentiate between valve and cylinder issues as Redman said...but you could add headgasket to checklist too

 

[JTGS850GL]

Never answered the question: Did you do the compression test with the throttle wide open? If you don't do this then the readings will be very low.

 

[ATM]

If it is a cracked or broken ring in one cylinder, how labor intensive would this be to fix it??

Thanks for the input.

BIGCHEESE (from Kalamazoo, MI)

 

[cjanderson_90]

Depends if you got lucky and the broken ring slipped passed the exhaust valve or you have one the got jammed sideways and gouged the cylinder. The former would require maybe just a rehone and new rings.