Idle Mixture

[MikeK2B]

Hey, i have a ford 302 omc with a 2bbl holley. I'm having some trouble setting the idle mixture, according to clymers, i am supposed to set it to make the highest idle speed possible. Well the rpms don't move at all, its either 450rpms or Orpms. Not much destinction in between. I also tryed increasing my idle speed to 800rpms and set the mixture there, same thing. Any hints? I did a search on the forums about this and came up with using a vacuum gauge to do it. how do you do it like that? THANKS!!! Mike

 

[Lou C]

Re: Idle Mixture

When setting the mixture, you have to turn the needles very slowly to see changes, you can turn it in until the engine starts to slow down (lean roll) and then back it out until it starts to slow down (rich roll). Usually the correct setting will be between these 2 points. Now if you get no change until you screw it in all the way (be careful not to damange the mixture needle) the idle curcuit is too dirty to adjust the carb, it needs to be cleaned first.

 

[merc 140 pontoon]

Re: Idle Mixture

Lou C> Now if you get no change until you screw it in all the way (be careful not to damange the mixture needle) the idle curcuit is too dirty to adjust the carb, it needs to be cleaned first.<br /><br />Lou - Could you expand on that statement a bit? Assume I know nothing about carburetors (which is pretty much true). My Rochester carb on my 1974 Merc 140 has two idle mixture screws. I have trouble getting any rpm drop when turning in the aft screw--it goes all the way to the stop. I've thought about a carb rebuild over the winter, but I've never done one before.<br /><br />Thanks.<br /><br />Steve

 

[Lou C]

Re: Idle Mixture

Well if you get no change when screwing in the mixture screws, the idle circuit is just too dirty to be properly adjusted. Take a look at the shop manual so you can make sure of what screw is the mixture screw and which one is the idle speed screw. A 1-bbl carb will have one mixture screw, a 2bbl or 4bbl has 2 one for each throat. I have heard you can try to clean it on the engine by carefully unscrewing and removing the needle (dont' lose the spring behind it!!), squirting some carb cleaner into the idle passage, and then (wear goggles!!) blow it out with compressed air. Then you have to know a basic setting for the mix needle (for ex some you screw in all the way lightly, then back out a certain # of turns so you can get it to start, then you try to adjust. But I have never cleaned a carb this short cut way. Plus other problems can cause bad running too (float bad or adjustment off, etc) <br />Carb rebuilds on these old carbs is not hard, but you really need a good manual with a re-build kit, carb cleaner, compressed air, and goggles for your eyes!!

 

[merc 140 pontoon]

Re: Idle Mixture

Thanks, Lou. My Rochester carb has 2 idle mixture screws, and of course 1 idle speed screw. About how many hours does it take an amateur to rebuild a carb (in rough numbers)? And, if I pay a pro, about how many hours should it take them? Is that better than buying a rebuilt carb?<br /><br />I'm not having horrible problems with the engine running. It just idles rough, and the aft idle mixture screw does almost nothing.<br /><br />Steve

 

[Lou C]

Re: Idle Mixture

I have not done one in years (like 20 or more,last one was on a Honda in 1983 or so) but if you can get it all apart, a couple of hours should to it. The main thing is the cleaning, have to blow out passages and jets with comp air. You could try putting Chevron Techron in the gas tank and see if that helps clean out the carb (Geo on Boater ed said to use it at 1/3 normal strength) but you will probably wind up with a re-build to get it right. Make sure you get a marine rebuild kit, not an auto one since there are safety differences as you know for inboard boats.

 

[merc 140 pontoon]

Re: Idle Mixture

Hi Lou -<br /><br />I just bought a bottle of Techron yesterday. Haven't decided yet if I'll get any time on the lake to run it through this year yet, or if it'll have to wait until spring. I'd sure like to try it this fall yet, so I can decide if I should rebuild the carb this winter. I've also seen George's recommendation of 1/3 strength (still not sure why, though).<br /><br />Steve

 

[Buttanic]

Re: Idle Mixture

Rebuilding a Holly 2 barrel is a pretty simple job. The rebuild kit will contain instructions and a exploded drawing. Once it is dissambled use a spray can of carburator cleaner to blow through all the passages. After putting it back together turn the idle mixture screws all the way in than back out 1 1/2 turns. Start the engine and set the idle speed you want. Find a place on the intake manifold where you can connect a vacuum gauge. You may have to take out a plug and install a fitting to connect the gauge. Once the gauge is installed start the engine and slowly turn one mixture screw in and out while watching the gauge untill you get the highest vacuum. The idle speed will most likely change. Once you get the first mixture screw set to highest vacuum ajust the idle speed back to what you want. Now do the same thing with the other mixture screw.<br /><br />Captain

 

[MikeK2B]

Re: Idle Mixture

So the majority of you think i need a carb rebuild?

 

[merc 140 pontoon]

Re: Idle Mixture

Actually, I think I hijacked your post.<br /><br />Sorry...

 

[pecothern]

Re: Idle Mixture

Don't just assume it's you carb because it "idles rough" pull off each spark plug wire when it's running and see if one of the cylinders are missing I would change spark plugs,check cap and rotor button, timing and plug wires and finally do a compression test to rule out valve problems. Remember allways think of the simplest things first when troubleshooting