2000 40hp mercury; No compression in top cylinder under load.

[Chris Cole]

Hello forum,

Today I ran my mercury and again I had no greater than about 3500 RPM's. I took the cowling off while I was running the motor. It sounded like the motor was running on two cylinders, and sure enough when I put my hand over the throat of the top carburator there was no vacuum. The motor kept running just like it was while I held my hand over the top carb. I put my hand over the middle carb and the motor acted like it was gonna die. I put my hand over the bottom carb and the motor acted like it was gonna die. There was also a bit of racquet coming from the top cylinder when the motor was running. When I Idle I can get the top cylinder to start pulling some kind of vacuum, but not wide open. Can someone offer an opinion on what the problem might be???

Before you offer your opinions I have just changed the trigger( not the problem), disconnected the black/yellow from rev limiter while running
(not the problem), checked battery charge while WOT ( 13.9V), and checked spark. Compression on each cylinder is 120. Please help????

 

[redjmp]

Re: 2000 40hp mercury; No compression in top cylinder under load.

If you get good compression but no vacuum at the carb then most likely you have a bad reed.

 

[Chris Cole]

Re: 2000 40hp mercury; No compression in top cylinder under load.

redjmp,

I guess the reed is on the bottom side of the piston, below the ring. Is that why the compression still checks out?

 

[redjmp]

Re: 2000 40hp mercury; No compression in top cylinder under load.

That's right. I'm not sure about your model, but usually the reeds are actually mounted around the crankshaft much like a main bearing.
Go to mercurypartsexpress.com and enter your serial # and then you can find diagrams showing an exploded view of the various components.

 

[WillyBWright]

Re: 2000 40hp mercury; No compression in top cylinder under load.

Your reeds are right behind the carburetors, not under the pistons. It's unlikely to have a broken one on a 2000, but I suppose it isn't impossible. It would be far more likely to have some foreign object blocking one open. That's assuming it's a 2-stroke in the first place. Does the oil go in a plastic tank under the hood or into a hole on the cylinder head with a yellow cap?

You can't lose compression the way you describe. If you have it testing, you have more running. I think you have a problem with the upper carburetor. Either the float valve is stuck and not letting gas in or you have a main jet blockage preventing gas from getting out. That wouldn't matter if it's 2-stroke or 4.

 

[achris]

Re: 2000 40hp mercury; No compression in top cylinder under load.

Willy be right, he be... 3 carb/3 cylinders=2 stroke... If the top cylinder is not getting fuel it's also not getting oil!!! Fix it quick! As Willy said, likely the top carb not right...

Chris...........

 

[Chris Cole]

Re: 2000 40hp mercury; No compression in top cylinder under load.

Thank you for all the responses guys. I appreciate the help.

The top carburetor is getting gas, I do not have a carb cover, and can look directly down the throat of the carbs when I am running the motor. The carb is not the problem, as it has not been setting up with gas at all. The top cylinder is getting gas just like the other two cylinder, I am positive.

To give you guys a little history, I overheated the motor one day. The motor has been running on two cylinders during WOT ever since. I have eliminated all electrical problems.

Question for you guys?
If my motor had overheated, would a reed valve be harmed by temperature? What is the reed valve made out of? (Steel, Aluminum,Fiberglass)

 

[Chris Cole]

Re: 2000 40hp mercury; No compression in top cylinder under load.

WillbWright, aChris,:

I have a 3 cylinder 2 stroke 40hp. It is not oil injected, it is a pre-mix deal.

 

[achris]

Re: 2000 40hp mercury; No compression in top cylinder under load.

I'd looking at all the things that can be damaged by overheating, and I'd start with the top crankshaft seal. That was a fairly significant piece of information.... Glad we got it before we had you pull the carbs and reeds on a goose-chase....

An quick check to see that it's not a reed is to put you hand in front of the offending carb and see if you feel any 'blow-back' or spitting from the carb. If you do, then you have a broken/stuck reed.

The other thing that sticks in my mind is that you say you have no vacuum from that carb. If that's the case, then that cylinder is not being lubricated. Even if there is petrol in the carb, it's not being pulled into the crankcase where the oil it's carrying is needed.

Find the problem before you run that engine any longer, or you won't have a top cylinder to diagnose.

Chris.................

 

[Chris Cole]

Re: 2000 40hp mercury; No compression in top cylinder under load.

achris,

Thank you for your response. When I put my palm over the top carb I do get a significant amount of gas on my palm. The carb seems like it is spitting gas everywhere, the gas going into that carb does spray all over the place. In the other carbs the gas coming in has a consistent path.

Do you know what that reed assembly is made of in my model? Is it prone to melt?

Thanks again achris.

 

[achris]

Re: 2000 40hp mercury; No compression in top cylinder under load.

Chris,

With your engine serial number you can go to http://www.mercruiserparts.com and look up the parts breakdown for your engine. As I'm currently in Burma I don't have my manuals in front of me. (and before DonS, Bond-o or any others of that ilk chime in and express shock that I don't carry them with me, 'No I don't')....

The reed blocks and reeds are usually made of steel, but an overheat can easily distort a petal, or warp the reed block. Depends on how hot the engine got.

Good luck,

Chris...........

 

[Chris Cole]

Re: 2000 40hp mercury; No compression in top cylinder under load.

Thanks for the help achris and others,

My parts diagram shows the reed assemble is right behind the carb, It looks like I can get away with just taking the carb out to change the assembly. I will change the reed assembly and let you guys know the results.

The saga with this motor continues.........

 

[achris]

Re: 2000 40hp mercury; No compression in top cylinder under load.

Hi Chris,

Glad to be some help. When you get the reed block out, don't just 'put a new one in', have a good look at it and see that you have actually found the problem. You may just need to replace a set of petal, rather than the complete assembly....

Good luck, keep us informed. (maybe take a picture of the fault and post it so we can see...)

Chris..........

 

[Chris Cole]

Re: 2000 40hp mercury; No compression in top cylinder under load.

Well I took apart my intake manifold to look at my reeds, they look fine to me. No broken petals, gaps look good...

I think I figured out the problem though If you look at these pictures you will see the very end of the piston broke. The piston is broke like this, both on the left and the right side. What the heck happened???? How did it chip like this??? Someone please help me on this one. What do I need to do next? a chris, Willybwright, laddies...

 

[Chris Cole]

Re: 2000 40hp mercury; No compression in top cylinder under load.

Bump..... Help anyone?

 

[Chris Cole]

Re: 2000 40hp mercury; No compression in top cylinder under load.

Right now Iam just trying to figure out what happened. Can anyone tell.

 

[achris]

Re: 2000 40hp mercury; No compression in top cylinder under load.

Hey Chris,

The detail in the photo is not too good. I have no idea exactly what I'm looking at. Some more photos might help.

Chris................
(I see you've started a new thread. That is just going to make things a bit more confusing too)

 

[Chris Cole]

Re: 2000 40hp mercury; No compression in top cylinder under load.

Click on the thumbnail for an enlarged image achris...

I am going to post some more pics right now that are a little more descriptive

 

[Chris Cole]

Re: 2000 40hp mercury; No compression in top cylinder under load.

Okay guys check out these pics.....

The first two are the left side of the piston, note the chip. The last one is the chip on the right side of the piston. ( On the second pic, note the burnt cylinder wall behind the piston chip.

 

[redjmp]

Re: 2000 40hp mercury; No compression in top cylinder under load.

That looks like the sleeve and not the piston. My guess is that the con rod did the damage. Maybe the cylinder sleeve slipped down a bit? Then again it might have been done like that at the factory?
How does it compare with the other sleeves in terms of height in the block?
Can you post more pics of the other holes?
I can't see this as being a direct cause of your problem though.
It really shouldn't make any difference if this was just ground away to make clearance for the con rods.
If this was indeed caused by the conrods or maybe improperly installed rod bolts (backwards?) then some other damage might be lurking. Have a good look for cracks etc. Double check your reed assemblies for missing bolts etc.
If something worked its way loose and got caught it might account for what you see.