4.3 Mercruiser high rpm missfire - popping

[newc048]

Got the boat around a month ago and everything has been wonder, no problems starting, or anything of that nature. Just been experiencing this high RPM miss-fire while running WOT lately. I have not had any chance to check out anything so just looking for some advice on what/where to start and what to look for.

It is when I'm cruising WOT at around 35-45 mph... Its a 1989 Rinker 186 Captiva with a carb 4.3 mercruiser. prop is a 21p stainless steel. The sound it almost like a loud pop in the engine compartment and yesterday i could feel it cutting out while the pop was occurring.

I'm not sure where to start, so looking for any advice here, if anymore information is needed I will provide as much as I can..


Thanks so much!!

 

[wes89gt]

Re: 4.3 Mercruiser high rpm missfire - popping

I just had this exact problem, mine needed a tune up. sorry can't be more specific to what caused mine, I just decided I needed to do everything anyways. I did the cap, button, plugs, and wires.

 

[NHGuy]

Re: 4.3 Mercruiser high rpm missfire - popping

My 5.7 carb did that when the wires were dying.

 

[Alpheus]

Re: 4.3 Mercruiser high rpm missfire - popping

It very well could be a lack of fuel issue. (lean) Do not run it like that or you will fry your engine. Have your fuel pressure checked first. Lean conditions kill engines quick. If everything checks out on the fuel side, then move on to the electrical side...

Read This:
http://forums.iboats.com/showthread.php?t=283269

and this:

http://forums.iboats.com/showthread.php?t=295070

All this info is available in the Adults Only Section...

 

[newc048]

Re: 4.3 Mercruiser high rpm missfire - popping

I read in that first thread a possibility could be a clogged fuel tank vent, one thing i have noticed on my boat is it take a LONG LONG time to fill the tank and you have to pump the gas very very slowly otherwise the gas will overflow back out even when it is no where near full. The line going from the gas cap to the tank itself is i believe maybe 1.5-2" ID which is somewhat small but bigger than the gas pump hose so that shouldnt cause the restriction problem. So in my opinion the tank is not vented enough. I have heard from one other person so far with a Rinker that there boat has a similar problem pumping gas into the tank.

You guys think this could be a possible problem relating to this?

 

[mastercylinder63]

Re: 4.3 Mercruiser high rpm missfire - popping

Greetings, i've had this problem a few times before. Complete tune-up may be do. Also check or replace coil. Things that won't cost you any money: check timing. A degree or two off can make a difference. What octane gas are you using?? 87 octane can cause ping in this motor. 89 is a better choice. Lastly, you said you just got the boat. Is the pinging or missfiring occurring @ 5,000 rpm? If so, your probably hitting the revv limiter which some people may think it's missfiring. In that case your prop is probably not the correct one. Hope this helps.

 

[newc048]

Re: 4.3 Mercruiser high rpm missfire - popping

Greetings, i've had this problem a few times before. Complete tune-up may be do. Also check or replace coil. Things that won't cost you any money: check timing. A degree or two off can make a difference. What octane gas are you using?? 87 octane can cause ping in this motor. 89 is a better choice. Lastly, you said you just got the boat. Is the pinging or missfiring occurring @ 5,000 rpm? If so, your probably hitting the revv limiter which some people may think it's missfiring. In that case your prop is probably not the correct one. Hope this helps.

Anyone have any good walk-throughs on how to check the timing, i've never messed much with a carb'd motor, I'm a car guy but i only mess with fuel injected stuff (ls1's etc) I have never used less than 89, and most of the time its BP 89octance. The missfire is definetly only at high RPM, I did not know older motors had rev limiters. I know the stock pitch prop for the boat was a 19 and there is a 21 stainless steel on there now...

 

[jsmigielski]

Re: 4.3 Mercruiser high rpm missfire - popping

Had a similar problem with my 5.7. I started with changing the plugs, distributor cap and rotar. She runs fine now. The cap was in bad shape

 

[Alpheus]

Re: 4.3 Mercruiser high rpm missfire - popping

I would start with your vent line. If its clogged it will mimic the symptoms you are describing. Make sure its clear and no critters made a nest in there. Don't waste your time with throwing parts at it. Make sure your fuel system is working as is supposed to...

 

[newc048]

Re: 4.3 Mercruiser high rpm missfire - popping

I will locate and check the vent line tomorrow. I looked quick around the boat for the vent line today and could not locate it, its a 1989 Rinker 186 Captiva if anyone knows off hand where the vent would be?

 

[Alpheus]

Re: 4.3 Mercruiser high rpm missfire - popping

You can diagnose a clogged vent line issue pretty easy out on the water. Just loosen or remove your gas cap and run her wide open. Make sure its a calm day so no water can spray into your tank opening. If the problem goes away you then know with certainty its a clogged vent line...

 

[newc048]

Re: 4.3 Mercruiser high rpm missfire - popping

You can diagnose a clogged vent line issue pretty easy out on the water. Just loosen or remove your gas cap and run her wide open. Make sure its a calm day so no water can spray into your tank opening. If the problem goes away you then know with certainty its a clogged vent line...

Thank you for the idea! Tonight i am replacing, the spark plugs, wires, cap and rotor, then going out for a cruise to see if anything cleared up. Also going to stay out a bit past dark to see if any plug wires are arching. Hopefully get this solved tonight, or atleast narrowed down to what it is! The weekend is quickly approaching and the girlfriend and her friends will not appreciate a broken boat!

 

[newc048]

Re: 4.3 Mercruiser high rpm missfire - popping

Got everything figured out! Previous owner must have known about this and tried to do a cheap bandaid fix. The plug wire boot was ripped and he taped it up with electrical tape and put a new plug in just that cylinder. So he obviously knew about this and didnt want to spend the 65 bucks on OEM wires and 20 bucks on plugs to have it running perfect again lol. Got it all taken care of and improved my top speed to 48 today better than my previous 45!