Back Fire Rough Idle? Bad Carb?

[nazz2005]

00 mercruiser 5.7. I got a a back fire coming from the carb intermittenly not all the time and mostly when trying to be pushed hard (WOT). It will rev up fine in neutral when sitting, but then when in gear does not want to move. Also when it does start to pick up i cant get it passed 3K RPM. I was wondering if bad plug wires would have any effect on this? I coated them with WD-40 but no difference. Also it has a rough idle, and runs rough running while moving. Fuel filter new plugs within last year oil etc.. Just trying to figure if its a carb problem before sending it for rebuild..

 

[mkast]

Re: Back Fire Rough Idle? Bad Carb?

I coated them with WD-40 but no difference/QUOTE]

Why would you do that?
When was the last time the engine was tuned up?

 

[rbh]

Re: Back Fire Rough Idle? Bad Carb?

sounds like its time for a tune up for sure!
as well could your timing be out, or your plug wires are on the wrong plugs?
if you have a leak on any of your base gaskets, carb, manifold spray them with the wd40 and see if that picks up your idle, dont spray your wires as the spray breaks down the insulation, and if there is a hole in the insulation it now has an easier path to ground
rob

 

[nazz2005]

Re: Back Fire Rough Idle? Bad Carb?

I did that beacuse if any of the wires were arched/cracked and leaking out a spark the oil will temporary subside that which would let me know notice a difference in rpm change/idle. Almost like spraying carb cleaner around manifold looking for a vacum leak...

 

[180shabah]

Re: Back Fire Rough Idle? Bad Carb?

Checking ignition and fuel delivery are good to check, but don't forget the fuel itself. Water in the gas will cause exactly what you are experiencing. Plumb in an outboard tank (after the filter), fill it with fresh gas and see if it runs better.

 

[mkast]

Re: Back Fire Rough Idle? Bad Carb?

I did that beacuse if any of the wires were arched/cracked and leaking out a spark the oil will temporary subside that which would let me know notice a difference in rpm change/idle.

I thought checking resistance in the plug wires with a multi meter was the accepted practice.

 

[FreeBeeTony]

Re: Back Fire Rough Idle? Bad Carb?

Sounds like water in the fuel to me..........

 

[bigskiohio]

Re: Back Fire Rough Idle? Bad Carb?

plugs or plug wires if not good wiil miss under load and pressure

 

[cr2k]

Re: Back Fire Rough Idle? Bad Carb?

Pull your distributor cap and look inside for any corrosion or carbon tracking.

If it is not very very clean inside and out replace it.


90% of carburetor problems are electrical.

 

[carterinmo]

Re: Back Fire Rough Idle? Bad Carb?

I had similar problem this year, boat started and idled great, even reved good in neutral, but in gear under load, sputtered and back fired, tried to die, etc. Checked out electrical, fuel presure, timing, etc. all fine. Added can of Sea Foam and still ran like crap next time out. Back to shop and took for test run and now perfect. Decided must have been bad gas issue and Sea Foam took longer to get things cleaned up. Put in more fresh gas and another can of Sea Foam even added some Stabil for ethanol. Running perfect now...knock on wood.

96 Chris Craft 21 Concept BR
5.0L FI Volvo Penta

 

[nazz2005]

Re: Back Fire Rough Idle? Bad Carb?

Humm, yea i put boat in water after cooling system overhaul, ran it home semi-fine by this i mean it was as such....idle up to near say 3600 rpm held there and every few seconds like 10-15 seconds engine woudl serge up by itself to 3800-3900 rpm and then back down and so on by itself..woudlnt hold steady...im a little confused by this i dont know if its plugs wires fouling out or so, or possibly bad gas like you guys say. went home changed oil and fuel filter added can sea foam and 10 gals fresh gas. NOW within two weeks of that (keep in mind i start it everyday) i had the problems described above barely any power wanting to stal wen throttle up bogging down, back fire, etc..Prior to last year half boat sat w/ oveheat prob..it ran perfect 4200 rpm WOT.

Im going to do as you guys mention. Check plugs..Cap..try add more sea foam new gas. Maybe one can wasnt enough.

 

[mycirus]

Re: Back Fire Rough Idle? Bad Carb?

My boat was doing the same thing and it turns out I have a cyl with no compression. Its at the shop getting torn apart now. I would check your compression.

 

[nazz2005]

Re: Back Fire Rough Idle? Bad Carb?

hey guys just in. Today i changed the plugs and ton better!. all the plugs that came out were basicly fouled. all ends were dry jet black. its 90% better. i also added another fuel stablizer to mix in with old gas. During test run i found the carb to be minor backfire "coming out of the hole" I have to accerlate semi-slow up to crusing. I max WOT at 3800 my manuel says up to 44-4800 for my boat/engine...I would like to at least get to 4..I still have to change cap-rotor(no one at parts today) see how much difference i get after that...

 

[180shabah]

Re: Back Fire Rough Idle? Bad Carb?

I would like to at least get to 4

You need to get to 4400 to prevent damage.
Sounds like the rest of the tuneup will probably help, but stilll need to burn off that old gas, and see what it does when you have some fresh new gas in it.

 

[nazz2005]

Re: Back Fire Rough Idle? Bad Carb?

4400 to prevent damage? Now someone was telling me something about the pitch of prop? My settings could be tweaked more for "out hole" perfomance rather than top end? I dont know of anybody changing this thou..But if i dont get to 4400 rpm..?

 

[180shabah]

Re: Back Fire Rough Idle? Bad Carb?

Should have been a little clearer.

The WOT rating from merc is given as a range, 4400-4800. we all know what can happen if you spin an engine too fast. But lugging can cause damage as well, increased bearing stress, tuliped valves etc. When running properly, your WOT needs to fall in the "safe" range listed to prevent damage.

 

[nazz2005]

Re: Back Fire Rough Idle? Bad Carb?

Wow, i did not know that. Had boat few years always serviced outside now that i am starting to do it myself im really learning alot. So as of right now when ever i go out is it best not to run at at WOT since its only getting to 38? instead just cruise at say 3K ? And after the change of cap and rotor if im not near safe range wat should i consider doing prop change? I also wanted to add that yesterday when running at wot 3800 It was running pretty smooth no hesitaiton or sputtering...just looking at scenery go by it didnt feel fast felt like maybe 30 mph max...stats said i should get my boat to about 40-42mph
24ft BR 4325 pounds half tank 2 people no gear..

 

[180shabah]

Re: Back Fire Rough Idle? Bad Carb?

If your boat was previously able to reach the proper WOT range with the installed propeller, then most of the above can be ignored, but you still need to resolve the lack of power you are experiencing.

 

[nazz2005]

Re: Back Fire Rough Idle? Bad Carb?

Ok just back from a test run....I installed new Cap and rotor..cleaned flame arrestor. Went out and gained few more rpm. Now im maxing at 4000 exactly, 34mph gps. No backfire today or sputtering. I also trimed up slightly to maximize perfomace of top speed/rpm. I still do not know my current prop/pitch set up but do know its a 3 blade 13" diameter. will try find out soon and post. I have learned probably will need to drop one pitch down and boat store guy said possibly increase diameter to 14". Sound right?
Thanks mn

 

[nazz2005]

Re: Back Fire Rough Idle? Bad Carb?

I did change thoese plug wires but didnt get the 4400.