1972 Sea Ray Mercruiser 165

[Ike-110722]

I have a thread on the rebuild of this boat 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.

 

[achris]

Re: 1972 Sea Ray Mercruiser 165

Backfire up through the carb is usually a sign of a lean mixture. Couple of things could cause that. Weak fuel pump or a dirty (on the inside)/out of adjustment carb... Occasionally if the timing is not advancing ...

I think you are doing the right thing though, start will the basics, then move on to the more complex things.

Chris.........

 

[Ike-110722]

Re: 1972 Sea Ray Mercruiser 165

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.

 

[achris]

Re: 1972 Sea Ray Mercruiser 165

Unfortunately there is no vacuum port to hook a gauge...

Chris.....
Just for the record. The loose cap wouldn't change the timing, just make the spark weak by the time it reached the plug, and cause a misfire/backfire....

 

[Ike-110722]

Re: 1972 Sea Ray Mercruiser 165

Unfortunately there is no vacuum port to hook a gauge...

So what method do you use to adjust the carb?

 

[achris]

Re: 1972 Sea Ray Mercruiser 165

So what method do you use to adjust the carb?

Mark-I calibrated ear-hole....

Set the points gap, then set the ignition timing as specified in the book (which for your engine is 6? BTDC)...
Then set the idle speed and idle mixture for the smoothest idle at the specified revs. If the engine hesitates on acceleration, just richen 1/4 turn... That's it....

Chris.....

 

[Ike-110722]

Re: 1972 Sea Ray Mercruiser 165

The carb is a Rochester 2GV ser no. 7043183 See pic. (by the way that is a photo of it when I first got this boat. I haven't rebuilt the carb yet but I have cleaned it up and run cleaner through it. It works fairly well. It is next on my list to rebuild. Also that fuel line, filter and fittings have all been replaced (got rid of that filter) with USCG Type A fuel hose.

What is a good starting point to set the idle mixture. I have seen posts that say 2 1/2 turns out. What is the black circular thing for? it says rich-lean with a 2 headed arrow on it as well as the label for type of carburetor? I find no mention of this in the manual, or maybe I just missed something? On the parts list it says Choke thermostat housing. Does this adjust the choke? Sorry for all the questions but the last carb I worked on was a Solex 2bbl in 1972 and it's been a long time.

 

[Fishermark]

Re: 1972 Sea Ray Mercruiser 165

What is the black circular thing for? .... Does this adjust the choke?

Yes - that's where you can adjust when the choke opens and how tight it closes. With the engine cold, loosen the nuts holding the black ring and turn the ring till the choke is at the point between closed and just opening. Tighten the screws holding the ring and you should be good to go.

 

[Ike-110722]

Re: 1972 Sea Ray Mercruiser 165

Thanks. For now I will just leave that alone.

 

[Oshkosh1]

Re: 1972 Sea Ray Mercruiser 165

On my Inline 6 of roughly the same vintage...I have a vacuum port just aft of the carb on the intake manifold. I've used it several times to adjust the carb.

 

[Ike-110722]

Re: 1972 Sea Ray Mercruiser 165

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 through carb under acceleration and especially above 2000 rpm. Made the carb a little richer, no difference.

 

[achris]

Re: 1972 Sea Ray Mercruiser 165

Did you check the timing was advancing correctly. Those older distributors have weights and springs in under the breaker plate. They can get a bit rusty and stop working...

Chris....

 

[Ike-110722]

Re: 1972 Sea Ray Mercruiser 165

Yes. This does not have the old distributor. It has a nice brand new Mallory electronic distributor.

 

[achris]

Re: 1972 Sea Ray Mercruiser 165

Did you check the timing was advancing correctly. Those older distributors have weights and springs in under the breaker plate. They can get a bit rusty and stop working...

Chris....

Yes. This does not have the old distributor. It has a nice brand new Mallory electronic distributor.

Hmmmmm... You got me there... :embarassed: :facepalm:

I would still check out the timing advance... Your problem does sound a bit like a timing problem.

You have the curve?

Chris....

 

[Ike-110722]

Re: 1972 Sea Ray Mercruiser 165

No. I don't think I have seen a curve in the manual. I am looking into other probable causes. It could be starving for fuel. So I was going t replace the fuel pump anyway (the only component of the fuel system I haven't replaced), and check out the accelerator pump too.

 

[achris]

Re: 1972 Sea Ray Mercruiser 165

Here's a curve for you... Just an easy check (and can help rule things in or out) is to put a small section of clear hose in the incoming fuel line (just before the carb). If should see a clear stream of fuel, no bubbles...

 

[bruceb58]

Re: 1972 Sea Ray Mercruiser 165

Just so you know, adjusting the idle mixture does squat for anything other than idle. If you are having issues at 2000 RPM, forget about your idle mixture.

 

[achris]

Re: 1972 Sea Ray Mercruiser 165

Just so you know, adjusting the idle mixture does squat for anything other than idle. If you are having issues at 2000 RPM, forget about your idle mixture.

Hmmmm.... I wonder if that's why it's called an IDLE mixture screw..... :bolt:

 

[bruceb58]

Re: 1972 Sea Ray Mercruiser 165

Hmmmm.... I wonder if that's why it's called an IDLE mixture screw..... :bolt:

Apparently the OP doesn't understand that since he is trying to adjust his idle mixture to cure a 2000 RPM stumble and backfire through his carb.

 

[achris]

Re: 1972 Sea Ray Mercruiser 165

Apparently the OP doesn't understand that since he is trying to adjust his idle mixture to cure a 2000 RPM stumble and backfire through his carb.

o....k... Yeah, that'll work...