TOPIC: 1972 Sea Ray Mercruiser 165

I have a thread on this boat under DIY Projects but this is a separate issue. It is a problem with the Mercruiser 165 (GM 250)

Took the boat out to cruise around the lake. Engine ran fine, idled ok, rough when cold but smoothed out. At about 2000 rpm seemed to be surging, speed up a little, slow done a little. Then at about 2100-2200 rpm it backfired though the carb. (that ting when it hits the flame arrester scares the crap out of me) Then it began to backfire more often. I suspect intake valves or lifters, but it could be other things, bad timing, carb not adjusted right and so on. I am going to do a compression check but just haven't had the time yet. This weekend maybe.

Any thoughts, ideas, experiences, would be appreciated.

Some info. replaced ignition system with a Mallory distributor (still has the points) and a mallory coil, new ignition wires, new plugs. But engine has run fine until now. I haven't done anything to the head or block or carb. Did clean the carb. It's all original as far as I know. Completely new cooling system. New alternator.

Well, today I got out my tools and went at it. I did a compression test and all were ok... sort of. 120 to 130. A little low but considering how old this engine is not bad. Adding oil to each cylinder raised it to about 130 except on one cylinder which raised from 130 to 150. But no big surprises.

However, the good news. The distributor cap was loose. One of the clamps had come loose. That would throw the timing off and cause the backfiring. So I buttoned it all up. It ran good. A little stumbling at low rpms (carb needs adjusting) but it ran at 3000-3100 rpm at WOT and was smooth. It accelerate smoothly and ran steady. It still dies at low rpms when shifting into reverse. Carb adjustment, and rechecking dwell and timing should bring it up to the rated rpms (3900-4000)

If anyone knows how to hook up a vacuum gauge to a merc 165 with a 2GV Rochester carb I would appreciate knowing. I cannot find anything to access the manifold vacuum.

Now for the bad news. Tuned up checked and set timing, dwell, tuned carb. Idles nice and smooth. Doesn't accelerate well and still backfires though cqarb under acceleration and especially above 2000 rpm. Made the carb a little richer, no difference.

Hi Peter,
You might want to check into the carburetor's accelerator pump. The adjustment and/or a new one may be in order. A fuel flow problem, such as dirty filters or a weaken fuel pump, are also possible issues.

Regards,
Joe
www.fergusonpoolemarine.com
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Thanks. That's what is next on my list. Replace the fuel pump and rebuild the carb.

Update: I finally decided to let a pro look at it. Took it to them in mid August. They had it for a month. I won't list everything that was done but the major ones were, replace distributor with fully electronic, and Carb rebuild. A lot of other stuff was done that had nothing to do wit misfire/backfire but needed doing. Got boat back on Sept 11. Lousy weather prevented me for doing anything until 13th. Still doesn't run right, back fires, stalls. Arrgh!

I keep a log on everything I do or have done on the boat. Went back through it. The last thing I did was replace the coil with a new mallory coil with ballast resistor. So what the hell. I took it out and put the old one back in. Now it runs almost perfect. Had it up to max rpm. No misfire, no stalls no backfires. This one fooled us all. People usually don't suspect a new part. But problem solved for now.