1990 3.4L 250HP cranking compression

[Capt Randy]

I have a 1990 3.4L 250 HP Carb motor and an ongoing performance problem. As a side note, my day job consists of being an ASE Master Auto Tech, and I run a 5 tech shop. I am fairly knowlegeble in regards to mechanics, and have owned several outboards and do all my own service, including powerhead and lower unit rebuilds for the last 15 years. <br /> That being said, this motor runs funky upon hot re-starts, and sometimes stalls @ idle. To re-start hot, I must choke and open the throttles. It starts and clears and runs great at 4,000+, and will push my 24ft CC 40kts @ 5800. As I decelerate, you can hear a cylinder (or 2) quit firing and usually it (they) will kick back in, though a little rough. If I slow to idle, it may slow-troll for a minute or two, then eventually stall. Choke, throttle, re-start, over again. <br /> When this problem started I went thru the carbs, with needles and seats. New Boysen reeds. 6 new coils. New wires. Pull powerhead and check exhaust housing gaskets and housing. Fine. Bleed/balance hoses and check valve seem in order. Again, from 4,000 up it runs perfect. 3,800 and you can hear a little missing. <br /> I check the compression, with all plugs removed and WOT, and I got 30psi all holes!!! What is up with that? How can I have great power, and such low #'s? Should the throttles not be opened, as on a 4 cycle automotive engine? <br /> After reveiwing all the symtoms, can any Merc guy give me any direction, or diag??<br /> Curious in Cocoa,<br /> Randy

 

[ronmold]

Re: 1990 3.4L 250HP cranking compression

I wouldn't think the motor would run at all with 30psi /cyl! It should be around 120, where is that extra 90 psi going? Past the rings?, head gasket? Weird.

 

[Capt Randy]

Re: 1990 3.4L 250HP cranking compression

Pulls 5800-6000 r's, depending on seas and load!!!

 

[Capt Randy]

Re: 1990 3.4L 250HP cranking compression

Anyone else??

 

[Kenny Bush]

Re: 1990 3.4L 250HP cranking compression

Glad to see another Big 6 Merc owner have the same problems as me.....(I am NOT glad your having trouble) That 3.4 should be checking 120 lbs or more on all cylinders. It won't run with 30. You need a new guage. I test mine the same way as you, with the throttle wide open. Make sure your cylinders are within 10% of each other. Anything less than 110 is going to be a problem. Check compression and post back. It is a ***** (B+itch) to get these motors to idle smoothly, but they scream wide open. Also your RPM's sound a little high, like you need to prop up a size.

 

[Kenny Bush]

Re: 1990 3.4L 250HP cranking compression

I have also replaced those parts also... plus a trigger plate, stator, ECU's, and rectifier. Could not find one single improvement. I eventually got it worked out, but It was an experience........

 

[Capt Randy]

Re: 1990 3.4L 250HP cranking compression

Just got 6 new O-rings for the heads. Will re-install heads (barring a slowdown from the TWO hurricanes coming!) and fire up this weekend. Will post up a report after I do a heavy de-carbon, new plugs, timing and synch adjusts.

 

[CFronzek]

Re: 1990 3.4L 250HP cranking compression

Dear Auto Tech,<br />Every tire has a Schrader valve. Every compression tester has a Schrader valve. The core in these valves look alike. <br />They ain't! <br />If you put a tire core in a compression tester you can't read cylinder pressers because the spring is too heavy. It will pass about 30 lbs. and no more. <br />A buck or so at the auto parts store will get you the proper core. <br />If this doesn't work you'll have to chuck the whole thing.<br />Posted this same message a few months ago and got a favorable reply. Will I get another?<br />Charlie

 

[KCLOST]

Re: 1990 3.4L 250HP cranking compression

I just don't believe your compression numbers... However, with heat the cylinders will expand and create more compression....<br /><br />BUT... You do need to confirm them before tearing into the motor...<br /><br />If they are all 30psi... You've either got a serious problem, or a simple one (like head gaskets not seated properly)...<br /><br />Re-check compression, and let us know...<br />Get a new guage with a peak reading indication also....

 

[Capt Randy]

Re: 1990 3.4L 250HP cranking compression

CFronz,<br /> You are correct. I did use a regular valve core from our tire department in my newish, quality comp tester. I will obtain the correct core and re-test compression after I re-assemble and de-carbon this weekend. Yes, another favorable reply!!

 

[Kenny Bush]

Re: 1990 3.4L 250HP cranking compression

Take your time on the synchronization. Everytime I sent mine to the dealer, they set a little different (I think in an effort to make it idle better). Come to find out, I had the only 3.4 they had ever seen. They did not even have a Merc manual for it. I eventually bought my own Merc manual, and started doing all the work myself. Things got better. My Carbs were the biggest problem, trying to get them all flowing the same is tricky. A guy gave me some old motorcyle vacuum guages (4 in a row), and I was able to get all carbs pretty close. Mine also always ran rich, and I ended up leaning the gas out a little.