lean backfiring??

[mh11]

Hello there, i have the merc 260 with the 350 chevy crate motor.

Question, my plugs when I take them out look like they are brand new.

I didn't notice the backfiring/popping until I ran the boat with the engine cover open trying to find a water leak which I did on the exhaust bellows, the 3-4 inch rubber bellows.

Doesn't seem that I am ingesting water, just seems lean. I was told to play with the secondary metering jets, is this correct? All ignition is set to stock, fuel delivery seems to be fine.

Should I just play with those jets? Seems easy to change them out.

Thanks!!!

 

[Bondo]

Re: lean backfiring??

Donno,... What Carb are We talking about,..??

 

[mthieme]

Re: lean backfiring??

With the engine running, slowly turn the mixture screws in (one at a time) until the engine starts to loose rpm or run rough, slowly back out until running good. You can back out 1/4 turn from here which should put you away from the lean side.

I suspect there is more to the story though.

 

[JustJason]

Re: lean backfiring??

Is this engine stock, a homebuild, or built by a reputable remanufactorer of marine engines.
Are you using a stock carb and a stock ignition system?
What carb is it... a rochester or a weber.......

Are you aboslutely sure that your base timing is correct, and that it is advancing correctly?

 

[mh11]

Re: lean backfiring??

I am not sure, the last owner just told me that it was a brand new crate motor that has 100 hrs on it.

Rochester 4 barrel carb.

Where are the mixture screws? I was told to change the secondary metering rods, that are hung with the hangers that you see when you take the flame arrestor off correct? This will affect the top end performance.

I had the local boat shop set all ignition to stock specs, replace the points and condenser. Made a big difference smoothing out the idle and bottom end. Was well worth it.

Any more help will be greatly appreciated!!!

 

[dan t.]

Re: lean backfiring??

turning the idle mixture screws will affect nothing but the idle, a low float level will give you a high speed lean out. the specs for the q jet on my merc 350 magnum 17080562 are
primary jets 69
primary mrtering rods 35
secondary metering rods CH
secondary rod hanger G
hope this helps

 

[chiefalen]

Re: lean backfiring??

Not knowing the specs of the motor and what they used internally, if it were my motor, i would retard it it bit the timing, till you hit the sweet spot.

Was i ok that i posted here Jason i don't want you getting out of wack or anything.

 

[JustJason]

Re: lean backfiring??

Was i ok that i posted here Jason i don't want you getting out of wack or anything.

I don't care what you write when you post as long as it's correct information. I only correct you when your information is incorrect.
I don't have a problem with you as a person at all Chief.
Please stop acting like a child.

 

[thedillybar]

Re: lean backfiring??

I have been dealing w/ a similar problem (noise at high rpm under load). It goes away when I retard the timing. http://forums.iboats.com/showthread.php?t=286177

I just got done cleaning the carb..will get it back on the boat tomorrow and hopefully in the water next week. I have done a lot of research on the Quadrajet and know quite a few people who have worked on them. A lean condition at WOT could be a number of things. Bad gasket, dirt/debris in the carb, secondary rods (too big)/hanger (set too low), primary jets (too small), float level (too low). You can check for a bad gasket by spraying carb cleaner at the gasket while engine is running at idle...if RPMs increase you have a leak.

My plan is to rebuild since it's been a while and go from there. Also going to pickup a vacuum gauge and see what it tells me.

 

[chiefalen]

Re: lean backfiring??

How to use a vacuum gauge.

http://www.secondchancegarage.com/public/186.cfm

 

[mh11]

Re: lean backfiring??

Ok, this is where I am confused. I just spoke with the local boat guy today, and he says to fiddle with the two mixture screws at the base of the Q-jet.

Bottom line to me is, one, I am lean, two, probably too much timing, even though the local boat shop just set all the ignition stuff to stock specs.

Other people tell me to change (richen) the secondary metering rods.

Should I just go buy a rebuilt carb, set a baseline jetting spec (stock) then adjust from there?

I am going to replace the water seperating fuel filter, and the filter at the line at the carb, when I get the boat out for spring.

The plugs look brand new when I pulled them the last time indicating a lean condition.

Sorry, but which way do I adjust the timing to retard it a little bit?

I know from tuning motorcycles and cars in the past, you need to do one thing at a time. What order should I tackle things in?

Thanks for all the help!!