96 115 Merc factory compression level?

What should my 96 115hp carb/2 stroke motor have for compression? I contacted Merc and they said they dont give specifics. My manual confirms this, no more than 15 psi between cyls. Hypothetically if it were rebuilt what would it have? Approximate number? Mine starts and idles fine, I'm having high speed issues. I'm thinking fuel related, also stator possibly bad. Trying to determine whether its worth investing the money into this one. BTW top to bottom my compression is 102, 103, 103 and 105.
Thanks in advance for the input!

Re: 96 115 Merc factory compression level?

Your compression is a little bit low but still ok...heck yeah it's worth fixing..This motor is worth $2000 in good running condition....New compression on the bigger motors stock is around 120 or so....You will need to do a check of the entire ignition system which will take time, patience and a DVA meter along with a peak voltage adapter..Only then will you be able to rule out what it is and what it isn't. Parts are very expensive and many peole on this site throw parts at their motors only to find out that it was something small and they spent upwards of $1000 in parts...If you are not up to the task, find a good marine mechanic(good luck). I got tired of bad mechanics some 8 years ago and have spent more money on tools and time on Web sites but have learned so much that my motor's never go to the shop. There are good mechanics but in the marine business far and few between...

Re: 96 115 Merc factory compression level?

The compression will vary due to the guage.As long as it's within a few # it's ok.Yours looks good.
Different guages will give different readings.They can vary as much as 30#.The way U take the reading can affect the outcome.
Usually the more U spend the better the reading.J

Re: 96 115 Merc factory compression level?

Thanks for the responses. I did not realize the guages could be that far off. Mine is an older Harbour Freight/China special so I guess that makes sense!
I did a search and some reading and as far as the compression goes, correct me if I'm wrong, if you have issues they would typically show up low end and kind of self correct at higher RPM's. The only problem I may have is lower power levels from the reduced compression?
Lastly I think I will take your advice and have a mechanic take a look at it rather than throw money at it. The reason I asked if its worthwhile is a local shop has a 94 150hp carb that they checked out, 430 hrs, good compression etc, they took on trade. He would swap motors keeping my 115, for 1500. So I was trying to figure if its worth spending the money on mine or swap. I'm guessing by the time a mechanic looks at it, does the maintenance stuff it will be 500 to 1000... I dont think it would be wise to invest that into a block that was toast.

Re: 96 115 Merc factory compression level?

I have the same engine. My numbers were right there with yours on the compression test too. I realized after I did the compression test that I hadn't opened the carbs up to WOT before doing the test(something I was taught to do with auto engines) so I thought that might be the reason for the lower looking numbers.

Funny you should mention it but I am chasing a high speed issue too. I just rebuilt the entire fuel system including rebuilding the carbs--it was pretty cheap to do and I wanted to try my hand at it and learn---but I haven't been back on the water to see how it runs now. It idles great but that doesn't mean anything. If my problem turns out to be electrical, which I suspect it will, I will probably have the mechanic hook it up to his diagnositc machines and run it in the test tank to at least diagnose it for me.

I'll be watching your threads to see what you come up with on yours.

Re: 96 115 Merc factory compression level?

Now you have me scared, I did my test at WOT... What did you come up to after your retest?
I will post after the mechanic looks at it. It will be a week or two before I can get to it.

Re: 96 115 Merc factory compression level?

That motor is really touchy about sync and link and if misadjusted it will have low end probems such as : bogging,hestitation, transition from 2 to 4 cylinders.

Re: 96 115 Merc factory compression level?

fyrdriver said:

Now you have me scared, I did my test at WOT... What did you come up to after your retest?
I will post after the mechanic looks at it. It will be a week or two before I can get to it.

Click to expand...

I didn't retest mine. With my numbers being all basicly the same I wasn't worried about it. And I don't think you should be worried about anything either IMO.

Re: 96 115 Merc factory compression level?

Hey Faztbullet.......interesting you talked about transitioning between 2 and 4 cylinders is misadjusted. AFA marine has a 115hp Merc on there that they said runs on 2 cylinders at idle and changes to 4 at WOT. Which had me suspicious since the only engine I have seen do that is the the 5.7L chevy engine.....not a 2-stroke.

Re: 96 115 Merc factory compression level?

The 100/115/125hp inline 4 cylinder idle on 2 cylinders and around 1200-1500rpm the other two become active. The lower 2 carbs have restricted idle passages and feed just enough fuel to cylinder to keep them from siezing. It also has a accellerator pump and check ball injector to make it transition. Sync & link misadjusted,clogged injectors,bad accel pump,weak fuel pump will cause low end problems...

Re: 96 115 Merc factory compression level?

Is this a recent CHANGE in performance or are you sorting a a boat/motor that is still relatively new to you?

What boat/prop/WOT RPM?

What was it before?

I'm looking to find out what the change is in the performance.

Re: 96 115 Merc factory compression level?

Yeah its new to me so I dont know the history other than it supposedly had a tune up within a year of me purchasing it.
As far as the performance changes, it ran OK when I got it but I did start to notice a surging at high speed and it seemed to be getting worse. So I started with a compression test to get a baseline. As stated before it starts right up and idles great, its about 3000 rpm where it starts to surge intermittantly. I assumed a fuel pump on this one.
As far as WOT RPM, I dont know because I try not to run the heck out of it, I'm still trying to gain confidence that the entire assy wont grenade itself.
The other problem I just got the last time out was my tach started working intermittantly then died. My volts started dropping to under 11 and the motor started "breaking up" or missing. Assuming stator on this one.

Re: 96 115 Merc factory compression level?

In regards to the compression test. I just got off the phone with my mechanic who is doing the link and sync for me after my carb rebuild. He did a compression test on my motor and said the compression is fine. Hopefully that makes you feel better about the readings you got.

Re: 96 115 Merc factory compression level?

Yeah thanks, I'm gonna try to get this one running better. Hopefully it wont cost too much.... I'll still post up when I get it looked at.

Re: 96 115 Merc factory compression level?

fyrdriver said:

Yeah its new to me so I dont know the history other than it supposedly had a tune up within a year of me purchasing it.
As far as the performance changes, it ran OK when I got it but I did start to notice a surging at high speed and it seemed to be getting worse. So I started with a compression test to get a baseline. As stated before it starts right up and idles great, its about 3000 rpm where it starts to surge intermittantly. I assumed a fuel pump on this one.
As far as WOT RPM, I dont know because I try not to run the heck out of it, I'm still trying to gain confidence that the entire assy wont grenade itself.
The other problem I just got the last time out was my tach started working intermittantly then died. My volts started dropping to under 11 and the motor started "breaking up" or missing. Assuming stator on this one.

Click to expand...

Now we have something to go on.

Your right, surging in the midrange is often a sign of a faulty fuel supply, checking the fuel pump, lines and fittings as a god first step to be followed by checking fuel FLOW, disconnect the fuel line just after the fuel pump, remove the spark plugs, ground all the wires/pull the lanyard and crank the motor to observe for a strong fuel flow.

Also check into the voltage regulator, a faulty regulator will not only kill a battery and tach BUT can also interfer with the ignition system at midrange and above.

Disconnect the yellow stator wires from the regulator and retest performance, any change?

Inspect the potting on the backside of the reg for burning, test continuity or each yellow to the red then reverse your test leads and retest, you should have continuity in only one direction on each yellow to red. Any difference then replace the part. Regulators are more common failure. I highly recommend a CDI replacement.

WOT RPM is critical knowledge to guage motor performance, too high or too low, either will shorten motor life, the occassional short WOT run is a good indicator of fuel/ignition systems condition, often helpful in troubleshooting and insuring correct prop selection.

Re: 96 115 Merc factory compression level?

Thanks alot Charlie. Thats some good info and will get me started. I'll definitely try those tests first to see if I can isolate it before taking it in.