1984 mercruiser 470 backfiring

[badkins50]

I am looking for some suggestions. I am new to owning a boat although I have pick up a lot of great info from this forum. I have a 1984 Mercruiser 470, 170hp, 3.7 litre with a Mercarb 2brl carb. Things I have already done - replaced the spark plugs, distributor cap, points, rotor, fuel filter, fuel line, engine oil & filter (as well as 4" heat exchanger, new thermostat, anti-freeze, water pump assembly, seals and drive oil). You know the 470 yearly maintenance. I noticed that when opening the throttle fast, the boat would kind of hesitate before picking up then after running great for awhile then backing off for a minute and trying to opening it back up it backfires (does not stall out though). If you shut the motor off after backfiring, it will start back up perfectly. I should note that when changing the fuel sending unit, the gas in the tank smelled very weird.

It might be awhile before I can try some of your suggestions as I do live in northern Ohio and everything is winterized and ready our "lovely" winters.

Thanks in advance for any help you can provide.

 

[Don S]

Re: 1984 mercruiser 470 backfiring

How old is the gas ????

You listed a bunch of parts you put in, but did you set the points with a dwell meter or at least check them with a dwell meter to see if the were correct. After that, did you set the timing, and check the advance timing was correct and working. Did you adjust the idle mixture and speed? You may also need a carb rebuild if your gas is old and varnished.

 

[zbnutcase]

Re: 1984 mercruiser 470 backfiring

Way too many variables in your post, need more details, but probably do need to drain old fuel and rebuild the carb. Out here in Oregon (which has lower fuel testing standards than most any other state, and refineries dump their garbage here) I have seen fuel start to sour in as little as 3 months. 'nutcase

 

[stonyloam]

Re: 1984 mercruiser 470 backfiring

Mine had similar problem, turned out to be blown head gasket. You might want to do a compression check, just to be on the safe side.

 

[bruceb58]

Re: 1984 mercruiser 470 backfiring

Mine had similar problem, turned out to be blown head gasket. You might want to do a compression check, just to be on the safe side.

Way Way Way before you start suspecting a head gasket, you have to get your tune up configuration set up correctly. Also, the gas probably has to go if its age is unknown or run from a portable tank as a test.

 

[EddiePetty]

Re: 1984 mercruiser 470 backfiring

............?I noticed that when opening the throttle fast, the boat would kind of hesitate before picking up?


?then after running great for awhile then backing off for a minute and trying to opening it back up it backfires?


?I should note that when changing the fuel sending unit, the gas in the tank smelled very weird.?
........

First off: WELCOME to iBoats !!!!! It?s THE PLACE to be with boating questions. The corporate knowledge bank is full of wisdom.

When you say ?backfires? I am assuming you mean through the flame arrester.

A backfire condition usually exists for one or more of just a few reasons, though each of those reasons may have several attributes.
1) Lean fuel mixture: low fuel level, weak accelerator pump, vacuum leak, leaking valves,etc.
2) over-advanced ignition timing: high initial setting, stuck advance plate, vacuum leak, etc.
3) random ignition firing: defective ignition wiring, defective distributor cap/rotor, etc.

Based upon your initial descriptions of the backfiring condition, I would first suspect the accelerator pump in the carb.
I recommend you disconnect the main tank fuel supply line and temporarily connect an auxiliary, fresh/clean fuel supply in its place.
Run the engine on muffs a few minutes to ensure that the carb is full of fresh fuel. Secure the engine and remove the flame arrestor. Have an assistant at the helm throttle to full ahead (engine OFF)while you observe the accelerator pump?s squirt nozzles in the throat of the carb. You are looking for two healthy ?****? streams of fuel in the center of the venture. Lacking a full stream of fuel, there?s your main problem. ONLY a waterborne test can confirm the elimination of the back fire condition.
You will still have to verify/adjust your compression, dwell angle, basic ignition timing, idle mixtures and idle speed. And by all means, remove that ?weird? fuel before contaminating the fuel system again.
Fwiw....Ed in ?ol Virginny

 

[badkins50]

Re: 1984 mercruiser 470 backfiring

Thanks for all the avenues for me to look. It's getting close for the weather to break and I am looking forward to getting back out there and going over everything. I don't know exactly how old the gas was before I bought the boat and I should have known better and just replace it (I will replace before I start it back up). The only good thing about this past winter was I was able to get in a lot of research time on these forums to gather some really great ideas. Thanks again and I will keep you posted.

 

[badkins50]

Re: 1984 mercruiser 470 backfiring

I believe my problem is being caused by the nasty gas and/or the accelerator pump, so I think it would be best to rebuild it. I have a couple of questions for you guys, what rebuild kit should I get for my Mercarb two barrel #13898489 3Y02. The motor serial number 6750367. The boat is a 1984 Galaxy. Is there any other info you have about the carb and/or motor to help me along the way. I've done some research but why not ask the experts. Thanks again.

 

[badkins50]

Re: 1984 mercruiser 470 backfiring

All the old gas has been replaced with new and I was told my carb was going to be done tomorrow (4/9/2010). I ended up taking it to the local marine shop to get a complete rebuild. I figured it would be safer letting them do it than me fool around with it and screwing something up. After all if something goes wrong on the lake it's not like you can get out an walk. So now that the motor and the interior restore has been completed, the next nice day, It's off to the lake we go ! Thanks.

 

[badkins50]

Re: 1984 mercruiser 470 backfiring

I got the carb back form the marina Saturday. When I pick it up they asked me how long the boat had been sitting? I told we bought last October and ran it 3 times before winterizing. He told me that before that the boat has sat for years according to the rust and water collected in the carb. The filter has never been changed (1984) and when the carb was rebuilt the last time some one used sylicone instead of gaskets. The accelarator pump was not working. He also told me that it was a miracle the boat even ran in that condition. (NEVER TAKE SOMEONES WORD)!!!!! The gas tank has been washed out and refilled with new gas; new fuel line with new inline water separator, new fuel pump and new sending unit. Everything that can be replaced, has been now. I hooked everything up and primed the system and it started right up no problems. Can't wait to get it out on the lake. Thanks for the help.