Does my Carburetter require overhaul?

Mercruiser, MCM 140, Serial number 5568367, Alpha 1, Serial number OB656591

I removed, overhauled and refitted the engine and sterndrive myself after a core plug blew out behind the flywheel. Fitted loads of new parts (including a new core plug ofcourse), incl. bottom end bearings, piston rings, overhauled the cylinder head, all new gaskets where disturbed, tune up kit (new plugs, points, condenser, rotor etc, water pump. Compression test after OH was good; 138, 135, 138 & 132 psi. It was hard too start but once warm was fine. Shifted ahead and astern fine. Launched boat and adjusted shift cables/shift cut out afloat, undertook a tune up, checked timing with timing gun, all seemed good.

I have since put 16 running hours on the engine. All pretty good but problematic running at low revs. Everything is fine over 1000 rpm and have had her up to 3000rpm for a short while all fine. She is difficult to start until warm. Once warm the idle speed is sometimes erratic. By that I mean sometimes when I shift to Neutral its at a steady 800rpm, at other times it idles at 450-500rpm in neutral, and often stalls when shifting, especially into astern. I cannot adjust the idle speed, adjusting the idle screws on the carb have little / no effect.

I have pulled and inspected the plugs again, and the H.T leads (which were renewed with Merc leads 2 years ago) and all are are good.

Fuel is fresh and watre free.

I am thinking that the carb needs an overhaul as it the only think I did nothing to it (other than clean the fine filter on the carb fuel intake, the spark arrester, and fitted a new gasket between Carb and manifold).

Any other ideas to check out?

Tks in advance, Paul

Re: Does my Carburetter require overhaul?

I would imagine you could have a rich float condition going on which would make sense why upper rpm's are not a problem. When the mixture is set at the factory, tamper plugs are usually installed. Did you remove them? Rochester carb would have had two, mercarb only one. Just wondering if the carb has been opened up before. If it is dumping even slightly, your mixture screws will not make a difference.

Re: Does my Carburetter require overhaul?

Hi mpdive,

No tamper plugs fitted. The two screws have slots for adjusting with a flat screwdriver, and a spring between the head and carb body. See photos. Carb has 2 such screws. Looks like it may have been opened before but not since I have owned the boat which is 2 and a half years. Engine is around 28 y.o.

Should I use a specialist to overhaul the carb? I have never done one.

Re: Does my Carburetter require overhaul?

Marine engines tend to idle higher that cars. should be set at 800. adjustment is made from the shift / throttle cable. not from the carb itself. return your screws to factory settings. try to run about 2 grand in neutral and spray an entire can of carb cleaner in. I try not to fool with the carbs until last resort. If all else fails go to autozone and buy a carb new. sell the other on craigs list. rebuilds can be costly, and dont always fix the problem.

Re: Does my Carburetter require overhaul?

All depends how much of a tinkerer you are. Carb kits are relatively cheap and if it doesn't turn out right your barely out anything. If you want to fly me out to England for fish and chips or bangers (wahtever those are) I'll come do it for you......

Re: Does my Carburetter require overhaul?

Have you adjusted the idle mixture to the new engine? The "factory settings" for the mixture screws are nothing more than preliminary adjustments, as all engines will run slightly different.
Your idle rpm should be 650 to 700 in fwd. On the 140's, it's best to keep them below 750 rpm in neutral or they tend to diesel when you shut them off.
The idle mixture should stabilize your idle and keep it repeatable each time you come back to idle.
If not, you may need a rebuild.
Do you have the dwell and timing set correctly? if not, adjusting the carb first won't help.

Re: Does my Carburetter require overhaul?

zerobalance said:

Marine engines tend to idle higher that cars. should be set at 800. adjustment is made from the shift / throttle cable. not from the carb itself. return your screws to factory settings. try to run about 2 grand in neutral and spray an entire can of carb cleaner in. I try not to fool with the carbs until last resort. If all else fails go to autozone and buy a carb new. sell the other on craigs list. rebuilds can be costly, and dont always fix the problem.

Click to expand...

Is this a serious reply. I sure hope not, maybe i just missed the sarcasm....

Re: Does my Carburetter require overhaul?

svxtech said:

zerobalance said:

Marine engines tend to idle higher that cars. should be set at 800. adjustment is made from the shift / throttle cable. not from the carb itself. return your screws to factory settings. try to run about 2 grand in neutral and spray an entire can of carb cleaner in. I try not to fool with the carbs until last resort. If all else fails go to autozone and buy a carb new. sell the other on craigs list. rebuilds can be costly, and dont always fix the problem.

Click to expand...

Is this a serious reply. I sure hope not, maybe i just missed the sarcasm....

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I was thinking exactly the same thing... The bit that got me was "adjustment is made from the shift / throttle cable. not from the carb itself." :facepalm: Please let me know where this guy works, and I'll put a 50mile exclusion zone around it!

Carbs are easy. If you have done the strip and rebuild yourself, the carb is a walk in the park. Just follow the book and you'll be right.

Re: Does my Carburetter require overhaul?

achris said:

I was thinking exactly the same thing... The bit that got me was "adjustment is made from the shift / throttle cable. not from the carb itself." :facepalm: Please let me know where this guy works, and I'll put a 50mile exclusion zone around it!

Carbs are easy. If you have done the strip and rebuild yourself, the carb is a walk in the park. Just follow the book and you'll be right.

Click to expand...

Couple of cold ones and a screwdriver. Life don't get no better. No high rev carb clean needed here.

Re: Does my Carburetter require overhaul?

Someone MUST teach me this high rev carb clean trick! i've been tearing them apart for nothing...?!

Re: Does my Carburetter require overhaul?

svxtech said:

Someone MUST teach me this high rev carb clean trick! i've been tearing them apart for nothing...?!

Click to expand...

Hey, I do it all the time. I tell the customer I have cleaned the carb, bill 'em $200 for the privilege and walk away giggling...

Here's how it's done. Rev the engine to about 4000prm, hold it there for about 20 seconds. Without moving the throttle turn the key off and just before the engine stops completely, turn it back on. The engine should backfire through the carb, blowing out all the crap. Easy.... All those shops selling carb cleaner and overhaul kits are ripping us off....

Have fun... Oh, and you get an engine and leg rebuild later down the track...

Re: Does my Carburetter require overhaul?

achris said:

Hey, I do it all the time. I tell the customer I have cleaned the carb, bill 'em $200 for the privilege and walk away giggling...

Here's how it's done. Rev the engine to about 4000prm, hold it there for about 20 seconds. Without moving the throttle turn the key off and just before the engine stops completely, turn it back on. The engine should backfire through the carb, blowing out all the crap. Easy.... All those shops selling carb cleaner and overhaul kits are ripping us off....



Have fun... Oh, and you get an engine and leg rebuild later down the track...

Click to expand...

Damn. I have been doing it wrong all this time! I would stuff a rag down the throat just as it dies so the backfire would wipe it clean on the way out.

Re: Does my Carburetter require overhaul?

Are you sure all the linkage is hooked up correctly? From that pic it almost looks like something should be hooked up to that lower little hole on the bottom rear of the carb and it looks like a little rod in front of IT. Kinda hard to tell from those pics, Is the carb bolted down tightly ? Have you done the spray test with carb cleaner around the carb base tio see if there is any vacuum leaks. With the engine running ? The rpms will change if it sucks in carb cleaner indicating an air leak

Re: Does my Carburetter require overhaul?

Check throttle cable pre-load also. Set the cable end so you almost have to force it back onto the carb linkage...not too much but enough

Re: Does my Carburetter require overhaul?

In response to the quote about backfiring the carb to clean it out:


Is this dangerous? Is it potentially bad for the engine? I am ready to go try it. My carb is running at about 80% of WOT right now. First I will spray an entire can of carb cleaner at 2000 and will wait for replies to this before I try the big backfire. Oh and is that backfire with the flame arrestor off?

Thanks, don't want to blow meself up

Re: Does my Carburetter require overhaul?

iskiglass said:

In response to the quote about backfiring the carb to clean it out:


Is this dangerous? Is it potentially bad for the engine? I am ready to go try it. My carb is running at about 80% of WOT right now. First I will spray an entire can of carb cleaner at 2000 and will wait for replies to this before I try the big backfire. Oh and is that backfire with the flame arrestor off?

Thanks, don't want to blow meself up

Click to expand...

It was a joke. Do not do either, both can result in engine damage and loss of eyebrows.

Re: Does my Carburetter require overhaul?

svxtech said:

iskiglass said:

In response to the quote about backfiring the carb to clean it out:


Is this dangerous? Is it potentially bad for the engine? I am ready to go try it. My carb is running at about 80% of WOT right now. First I will spray an entire can of carb cleaner at 2000 and will wait for replies to this before I try the big backfire. Oh and is that backfire with the flame arrestor off?

Thanks, don't want to blow meself up

Click to expand...

It was a joke. Do not do either, both can result in engine damage and loss of eyebrows.

Click to expand...

Did someone actually take us seriously? That'd be a first for me.

On a serious note. If you want to clean a carburettor then buy a gasket/overhaul kit and a few can of carby cleaner, make sure you have the PROPER manual and a supply of clean DRY compressed air and follow the instructions. Take your time and be clean and thorough and you should have no problems, even with a quadrajunk.

Re: Does my Carburetter require overhaul?

achris said:

Did someone actually take us seriously? That'd be a first for me.

On a serious note. If you want to clean a carburettor then buy a gasket/overhaul kit and a few can of carby cleaner, make sure you have the PROPER manual and a supply of clean DRY compressed air and follow the instructions. Take your time and be clean and thorough and you should have no problems, even with a quadrajunk.

Click to expand...

What!!!??? I went out and did it before I checked back. AND WOW -it worked like a charm! Big backfire and now I'm running like a champ now with full power!!! Maybe even more power than before this all started. Flames shot and it looked sick! Luckily I yelled, "CLEAR THE HOLE!!!" before I did it to make sure no one was looking down the open throat of my carb for the big bang.

LOL - did I get ya? Just kidding, no I did not try it...

Re: Does my Carburetter require overhaul?

iskiglass said:

what!!!??? I went out and did it before i checked back. And wow -it worked like a charm! Big backfire and now i'm running like a champ now with full power!!! Maybe even more power than before this all started. Flames shot and it looked sick! Luckily i yelled, "clear the hole!!!" before i did it to make sure no one was looking down the open throat of my carb for the big bang.

Lol - did i get ya? Just kidding, no i did not try it...

Click to expand...

roflmao

Re: Does my Carburetter require overhaul?

He had me too...:facepalm: