83 Mercury 70 hp backfire

[dakota25]

Ok where do I start. I'm having problems with a very rough idle and backfiring. I cleaned out the carbs new gaskets and needle valves. Did a spark test at 7/16 in. and that was ok. Compression readings are from top to bottom 126, 137, 135. I thought it might be something wrong with the timing because the guy who I bought it from got it all messed up trying to tune it, but at idle the only thing I can move is the spark arm and that doesn't help the problem. It'll start right away on first turn of key and I push the choke and it'll die almost instantly. Almost like it is getting to much gas. Thanks a lot for any help I could get, this is the first boat i've had and so far it has been a pain. Oh, i almost forgot, i did a decarb and ran it WOT for 5 minutes when it was able to go WOT, now it just kind of sputters along.

 

[Laddies]

Re: 83 Mercury 70 hp backfire

If sounds like two problems, the first was the synchronization was screwed up by the first owner, that needs to be corrected. The second your decarbing may have caused by loosening gunk in the fuel system and plugging the high speed jets. If you hope to correct these problems yourself you will need a OEM service manual.

 

[dakota25]

Re: 83 Mercury 70 hp backfire

Thanks for the reply. So it could be the high speed jets that is causing the backfire? I thought that they only came into play when you were faster than idling? I will definitely check this out though. I have a Seloc manual and it doesnt really go into detail about the timing being there is so many motors that it covers. Does anything else factor into the timing besides the throttle arm and spark arm? I would like to solve the backfire problem before I bring my boat to a shop with a test tank to do timing so I dont pay to much extra money having it fixed. thanks.

 

[dakota25]

Re: 83 Mercury 70 hp backfire

I cleaned out the high speed jets, and got the timing back where it was and still have the problem. I should say it is more of a really fast pop, pop, pop rather than an actual backfire. My eyes hurt from reading all these posts and am surprised i have not seen anyone else with this problem.

 

[dakota25]

Re: 83 Mercury 70 hp backfire

did a decarb today, thought there might be some gunk stopping the exhaust, didnt work.

 

[j_martin]

Re: 83 Mercury 70 hp backfire

Bad reed?

 

[dakota25]

Re: 83 Mercury 70 hp backfire

how would you go about checking that?

 

[dakota25]

Re: 83 Mercury 70 hp backfire

I took the carbs apart this morning to look over everything again. It all seems fine but I noticed a ton of black gunk under the propeller through the exhaust. So I thought I would take a look at the pistons and they were covered in carbon. I forgot to mention also that the plugs are really wet. So what would cause it to run so rich? When I first got the boat the guy who owned it before me had cracked the carb overtightening, so he gave me a replacement carb that matches it perfectly. I took it on the lake and gas was flooding into the intake, i didnt know any better at that time but obviously the engine was having trouble starting and running. So I got the manual and started looking at these forums and it really helped alot with pretty much everything i know now. In one shot i took the carb apart, noticed that the needle wasn't even in the seat.lol, rebuilt the carb and changed the spark plugs. Thats when the backfire popping started. Sorry for not being so specific before but havent had much time to do anything but work..lol. any help is appreciated.

 

[CharlieB]

Re: 83 Mercury 70 hp backfire

Is this 'backfire' only occuring at idle and very low engine speeds?

If yes, then you still have a lean carb, additional cleaning/adjustment is required

Clogged air tube, float level too low, something.

 

[dakota25]

Re: 83 Mercury 70 hp backfire

I got the carbs out so I'm going to clean them again but it seems like it's running to rich. If i spray premix or push the choke it kills it pretty quick and runs really bad.

 

[dakota25]

Re: 83 Mercury 70 hp backfire

So I took the fuel line off the motor and it kept running for a good 2 min. with no fuel, and slowly ran better and better until it finally ran out of fuel. Also the popping backfire just disappeared completely. Does this mean its running to rich? Also what would cause this? Thanks

 

[thefairlaneman]

Re: 83 Mercury 70 hp backfire

I would think a COMPLETE cleaning of the carbs with new kits and proper adjustment of the floats might solve it. good luck

 

[gatormeat]

Re: 83 Mercury 70 hp backfire

sounds just like my problem! that black partiacles in the float in my case was the fuel pump diapham that was dryrotted. ordered a 15.00 fuel pump kit to take care of it. oh btw I did the same thing put a cut off on the fuel line and shut it off and it ran great for a minute until it ran out of gas.

 

[dakota25]

Re: 83 Mercury 70 hp backfire

ok, thanks for the replies i think i will take out the carbs (again..lol), and see if the needle is sticking or the float set to high, clean them again... then I will buy a fuel diaghram kit and install...i set the lines on the float even with the gasket surface, is this right? I will keep you all posted on progress.

 

[dakota25]

Re: 83 Mercury 70 hp backfire

while im waiting for my fuel pump kit and new carb kit I got a question. I have a seperate electric fuel pump and was wondering if you...say.. held the button in to pump for to long would this excess pressure damage anything? I checked the diaphram before i ordered the parts and it had no pin holes or any tears in it. I have two extra fuel lines going into the lower part of the engine what are these for?

 

[dakota25]

Re: 83 Mercury 70 hp backfire

anybody?