Mercarb tuning

[sirchad]

I am looking for a procedure for tuning my mercarb. 4.3L 2002 I just rebuilt it following the videos from Mikes carburetor on youtube. It ran descent prior to the rebuild with the exception of a occasional bog at high speed and some dieseling. I cleaned it in simple green in a harbor freight ultrasonic cleaner. I adjusted everything per the spec sheet. After reinstall I can get it started but it sounds like crap. Played with the choke/mixture screw. Can't get to near idle, it dies below a 1K RPM. A simple step by step procedure would be great. Also I do not have great understanding of how carbs work.
I do realize it has to be in the water for final tuning but was hoping to get it running a little more reliable on the hose.

 

[wellcraft-classic210]

Hi -- What your describing sounds like a lack of fuel at idle - As in no fuel being delivered via the idle circuit

Likely causes are -- idle mixture screws not open far enough, clogged passage , something not assembled quite right



Some important things to understand for carburetors are:

That the Venturi's deliver fuel as air flow picks up ( works similar to airplane wing creating lift but instead it pulls fuel in )

When the throttle plates close ( at low rpm) the Venturi's cannot pull the fuel / The vacuum also increases under the throttle plate(s) // This is why the idle circuit is designed to deliver fuel using that increased vacuum under the throttle plates

Fuel burns very inefficiently at lower temperatures ( before the motor warms up ) so the choke increases the vacuum on both the idle circuit and venturi's to richen the mixture

Any dirt or debris in those passages can block or reduce fuel delivery ( common issue for carbs and justification for a good fuel filter )


Personally I have had good luck using compressed air with a rubber tip blow gum -- blowing both ways and verifying that the air is exiting as expected.


Good luck

 

[sirchad]

Thanks for this information. That aligns with what I was thinking. I was thinking about the following options that I read online:
1. removing the mixture screw and blowing air through it. Maybe canned air or from my compressor. Or maybe carb cleaner???
2. Doing a back flush. Getting RPMs to around 2500 and covering the air intake until just before it dies and let it recover. Repeat several times. This is supposed to flush out the idle circuits.
thoughts?
Also I see fuel dribbling from the venturis. Not sure if that is normal or not

 

[wellcraft-classic210]

Fuel dribbling at higher rpm's is normal and shows the venturi's are working

The Back flush seems like long shot // same for doing the idle screw while assembled -- it could blow debris back into the float bowl that will cause issues again if not removed and or it may not work at all -- But you never know unless you try.

Persistence is the key -- If needed a second tear down & cleaning may be needed -- You would not be the first to need to to do that. Its likely to be more effective than when assembled. ( The second time is usually much faster and easier due to familiarity )


Gumout jet spay and compressed air -- Perhaps a small wire to clean ( rod out ) any passage that's suspect are my preferred methods.

--Compressed air with pressure on the fuel passages and flow both ways is essential for debris removal.

 

[Bondo]

I cleaned it in simple green in a harbor freight ultrasonic cleaner.

Ayuh,..... Did ya Completely disassemble it 1st,..??

Did ya blow out ALL the passages with compressed air, Before ya put it back together,..??

If the answer to either question is, No,..... Ya didn't rebuild the carb at all,... ya just shoveled the junk around,....

 

[sirchad]

Yes I did completely disassemble it the first time. I don't use compressed air, I know it contains a fair amount of water unless a dryer is installed(which I don't have).
Update: So i tore the carb back apart. I ran wire through the passages followed by carb cleaner(lots of it). I also noticed how the accelerator pump mechanism was rubbing causing additional friction near the maximum end of the travel range. Adjusted that and now it operates to it full range of motion. Put it all back together and it runs good now. Still on the hose but I will fine tune it once I can get on the lake.
And just to admit my mistake, for the first time in 7/8 yrs of owning a boat I forgot to turn on the water hose before starting the engine. I ran it for about 5 minutes so now I get to replace the impeller now. The engine didn't get above normal operating temperature so there wont be any serious damage. The worst part is that I just replaced the impeller a month ago(it was just time). Thanks for you help wellcraft-classic210. I appreciate it.

 

[wellcraft-classic210]

So Glad it worked out & appreciate the feedback !

P.S.. -- A little moisture will not usually cause any problems unless its below 32 deg.and of sufficient QTY to freeze & blocks fuel flow in the the passage. Otherwise it just passes through.

I am guessing you wont ever forget the muff water again after this -- .

 

[sirchad]

Wow this has been a hell of a project.
Replaced part of the deck. Got to learn how to do fiberglass work. Hope to never do it again.
New carpet
New shift and throttle cables.
Redid the outdrive bellows/hoses/gimble bearing. Wow what a pain in the back side.
Rebuilt the back bench and engine surround and gas tank cover. Marine grade plywood and carpet covered.
Built a new transom cover and sewing our own vinyl for it. Again a first for me.
Rebuilt carb.

 

[Bondo]

Ayuh,..... Boatin' is Fun,..... Ain't it,..??..??