3.7l 165hp Merc Back-firing - Running Poorly

[hallsy2]

Hello Gents,

I have an in-line 4 merc, a 1987 165 hp, that is running pretty poorly. I have installed new fuel filters, complete new ignition w/ wires, and have checked it all twice. It runs OK for a little, then starts back-firing, with squeaking sounds coming out of the flame arrestor. It idles pretty good, once in awhile it back-fires, then if I put it in drive, it realy goes to hell quick, quite a bit of back-firing, won't run. A friend of mine mentioned the shift interrupt, since it goes to hell quick when I put it in drive. The firing order is 1-3-4-2 - does it matter where 1 starts on the cap? I think timing errors usually start the back-firing, but I am pretty sure I have that ignition set-up properly. I guess I can't be too sure, since it sounds like crap. I also hate to admit it but the gas in the tank may not be the best. Does bad gas cause back-firing?

Thanks Guys - Steve

 

[cooter2506]

Re: 3.7l 165hp Merc Back-firing - Running Poorly

Get rid of the bad gas. Also backfiring is a lean fuel condition. Run it on an auxillary tank to see if problem is with tank or engine itself. If it is tank related it could be tank pick up screen, bad gas, water in the fuel or antisiphon valve. If it is engine related it could be fuel pump, carb needs rebuilt, or carb filter. Up in the adults only section there is a great post on how to check your boats fuel system read over that and do evry test dont skip any of them it will find your problem

 

[P 0 P E Y E]

Re: 3.7l 165hp Merc Back-firing - Running Poorly

You might want to check your firing order and distributor timing.

First try this: since you are hearing popping in the induction system, take the coil wire off and crank the engine and listen to the noise it makes.

If the engine sound like DE DE DE DE DE DE DE your compression is OK move on and check your timing and for fuel problems

If the engine sounds like de de de daaa de de de daa you have a dead cylinder and moist likely a bad intake valve that is making a popping sound in the intake.

It does matter where the rotor is pointed for the no. 1. It must be pointed at number one spark plug post on the cap at the right moment of engine rotation.

You must have the number 1 piston on the compression stroke and a few degrees before top dead center, the rotor will now be pointed at no. 1. follow the correct rotation of the rotor and do the 1342

Get this out of the way just to sure.

 

[wateroverroad]

Re: 3.7l 165hp Merc Back-firing - Running Poorly

Ok i had same problem but boat away for winter gt it all set to run this summer and lo and behold no compression on cly 3-4 pulled head was corroded and valve seats cracked (lovely 470) found head it was also bad decided to do a engine swap decided it was much better then paying close to 500$ for new head and engine swap done by marine 900 that includes a 4.3 Good luck.

 

[hallsy2]

Re: 3.7l 165hp Merc Back-firing - Running Poorly

Thanks for the input guys. I am starting to zero in on the head. I have smoke coming through the flame arrestor so I think a valve must be the culprit, since there is no where for the exhaust to go but back through the carb. It is random, so I hope it is just sticking, and not a full blown disaster. Ill let you know.

 

[stonyloam]

Re: 3.7l 165hp Merc Back-firing - Running Poorly

Agree, looks like a head problem. Do a compression check, If if 3 or 4 (especially 4) is low, probably a water leak from the head gasket (common problem, corrodes the valve seat) into those cyls., will probably need a valve job. Good luck

 

[hallsy2]

Re: 3.7l 165hp Merc Back-firing - Running Poorly

Good Day Gentlemen. I've got an 87 searay cuddy - got her on ebay with a nice trailer for 1500 bucks. I installed a waterpump in the drive, wirebrushed every connection, installed a couple new gauges, did a complete tune-up, cleaned the carb. Do you think I could put the wires on the cap in the right firing order? Hell No. I had them going counter clockwise - instead of clockwise. What a dumb ***. I did enjoy my first boat ride yesterday, on my birthday. Thanks for your help guys.

Steve

 

[stonyloam]

Re: 3.7l 165hp Merc Back-firing - Running Poorly

Well at least you had two of them right. Just a suggestion, you might want to keep an eye on the compression in the cylinders, especially 3 & 4. Maybe check it at the end of the season as part of the winterization.