Trojan Boat Forum

As some may remember, I ended up rebuilding the uppers on both my Merc 225's last summer.

When I did the starboard engine last year, I noticed some rust right at the bottom of the #7 cyl exhaust manifold port. I bought new manifolds, risers, end plates, gaskets for both cyl banks, but have not installed them. Couldn't find one section of the exhaust in between the manifold & the riser-a piece that has a hose fitting on one side, and another hose fitting on the side 90 deg. to the 1st fitting. Engine seemed to run fine the rest of the season (about 3, 3 1/2 months).

I did a tune up on my engines last Sunday in preparation for taking the boat out to the islands Tuesday. Both points had closed up considerably. Regapped & both engines fired up right away. I took off my flame arrestors & cleaned them. Port engine OK, starboard engine sooty. I then proceeded to set my idle RPM's with my tach/rpm/dwell meter & that's when I noticed trouble.

The port engine was sweet-tach needle rock steady. BUUUTTT...the starboard engine was running high on RPM's & the tach needle swinging several hundred RPM's in rythmic fashion. Oh no...I've seen this before!!! I got my compression test kit & check all cylinders. #'s 3 & 7 were low on compression, 78# & 102# respectively. All other cylinders were between 150 & 170. The post on #3 cyl spark plug was blown away. #'s 6 & 8 cyl spark pluggs looked like carbon build up, probably due to timing issues. Needless to say, I didn't take the big boat out yesterday, but my buddies & I went over to Kelley's in my little boat. We all ate perch lunch at the Village Pump.

I'm thinking I've got a blown head gasket on the outboard bank of my starboard engine. Now the question becomes, why am I seeing the same problem on this engine that I saw last year before I rebuilt it? (Remember that I had a chunk of my #1 cyl exhaust valve broke off-0 compression on that cyl.) Could it be that if the exhaust water jacket is leaking, the water getting into the exhaust gas side is creating an over-pressure situation resulting in blowing the head gasket?

Anybody else experience something similar to what I am? What was the fix? I do plan on tracking down the 2 pieces I'm missing, build up & install 2 new exhausts for both cyl banks of my starboard engine. Anybody got a lead on where I can find the piece I'm missing? Although I'm going to replace the exhaust manifolds, anything else that I may be missing that could be causing my problem?

usually when the electrode is gone, its running hot in that cylinder, as far as the rpm fluctuating, I dont see a relation, unless the spark plug is kicking in and out, , hignly unlikely if the electrode is gone. if you had water entering the cylinder, it would show up as a build up or some type of smoke. are the valves adjusted properly in that bank? I Would most certainly do a leak down test, rather than a compression test, it would be more conclusive in regards to a diag.

IF (big IF) water got into the cylinder, the compression therof might have blow the gasket. I wouldn't say this is likely in a relatively low (8.5:1?) compression engine (highly likely in diesels with 10:1 or more compression).

That same water might ALSO have simply rusted the rings a bit - and they need to be freed up.

Or - the valve guides may be shot. Did you do a valve job when you replaced the broken valve? Heat may have ruined the valve guides, allowing blow-by ...

Humm ...

I bought brand new (not used) heads assemblies, complete with new valves & guides.

When I rebuilt the engine, I did not see any sign of water in the cylinders proper-only rust at the bottom of #7 cyl exhaust port. If I have a crack in the water jacket, I'm wondering if that's where it is.

A crack in the water jacket would be very obvious - external would squirt - internal would fill oil with white liquid, and in cylinder would put out white smoke.

So, if it's not an exhaust manifold problem, then would bad rings be a likely cuprit? There was a lot of soot on the inside of my flame arrester. That may be from burned oil due to bad rings. If that's true, then am I looking at having to pull my block? Yuk!!!

As stated earlier, I would do a leak down test. It will give you much for info campared to doing a compression test. A little more involved but easy to do.

Agreed - the leak-down test si the next step.

Also:
Check your oil - clean?
Closed (FW) cooled? If so - are you losing water?
White smoke?
Timing right?


You may be able to free the rings with Marvel Mystery Oil ... no guarantee, but it might work. Seen it work on a few older engines. Rislone is similar ... If these don;t work - out she comes.

Go back to your BASICS!

First, assume that your issues started with the last step you did- tune-up.
The points are adjusted first, then timing, then idle speed. I have seen the breaker plate come loose and cause your RPM flucuations. The point dwell (the amount of time the points are closed) will be off. Be sure to inspect and repair.

The low cylinder compressions may be a result of cylinder washout, where unburnt fuel is in the cylinders and past the compression rings...
Double check all, then proceed.

Thanks, guys, for all your input!

OK - will check timing 1st, then dwell. Spark plugs out right now, will put in a fresh set. Once timing & dwell are OK, or ruled out, will address idle speed. If I'm still wavering on tachometer, will do leak down test.

Oil in this engine is a little dirtier than in the other engine: will change, along with filter. I'm raw water cooled. Don't know about white smoke in exhaust.

If timing is out, and I know it was last August, maybe I have a timing chain issue. SOmething to consider.

No, no! Point set first, then timing check and adjustment! If your point set is incorrect- your timing will be incorrect!

Stripermann2 wrote:No, no! Point set first, then timing check and adjustment! If your point set is incorrect- your timing will be incorrect!

Correct ! dwell will changethe timing . Timing will not change dwell always set the points first

I've already gapped the points to .017. Did that Sunday. When my mechanic tuned up the engine after we rebuilt it, he found it liked the high side of the range best.

Auto Zone gave me a free replacement of the AC R45TS spark plug that the post snapped off (#3 cyl). I'll put the spark plugs back in. That bad plug may well have been why I am seeing a lot of soot.

Then I'll go on to timing. Should be 8 deg. BTDC. Once I set that, I believe dwell should be in the 31-35 range if timing is OK.

If after that I'm still having problems with the RPM's not holding steady, I'll recheck my compressions.

If I'm still getting low readings, I'll try the Marvel Mystery Oil. If that doesn't work, I'll get my mechanic to do a leak down test (He's got the air compressor) and/or check the timing chain.

If I step through this one variable at a time, hopefully I can determine the root cause & be the wiser for it.

David the timing can be way off but the dwell can be ok . check the dwell first . if you set it afterwards you change the timming .

setting the timing & dwell will not change the compression issue.
.
The killer of these cooling systems is in many cases the age issue moreso than an "hours run" issue.

Determining what failed and what caused thatfailure is your task.

Look at the clues, How many hours did you run that engine to get that much MORE blowby than in the other engine?

If you assume the heads are O.K. then we have to assume that te blowby is being caused by compression getting past the rings and into the crankcase.

Are these engines "tilted" back?

Would the low compression cylinders be a place where water might first show up due to a problem in the cooling system on the exhaust side?

Just some thoughts.

Good luck,

RWS