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Old 04-12-2010, 03:20 PM   post #1 of 10
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Default why do some carbureted engines run rough when they're cold

why do carbureted engins run like crap when they're started cold? my 4-wheeler won't get out of its own way and backfires untill it warms up for a few minutes. i had an '84 silverado that wouldn't run worth a darn untill it was warmed up. my briggs mower won't idle for the first 10-15 seconds.
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Old 04-12-2010, 04:35 PM   post #2 of 10
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Default Re: why do some carbureted engines run rough when they're cold

Because cooler air is more dense with oxygen. A carburetor measures air volume but can't measure the amount of oxygen in that volume of air. To a carburetor a cubic foot of air at 100 degrees looks the same as a cubic foot of air at 32 degrees even though there will be more oxygen in the 32 degree air. and of course it mixes the same amount of fuel with each cubic foot of air regardless of the temperature. So the air/fuel ratio changes as the temperature changes.
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Old 04-12-2010, 07:09 PM   post #3 of 10
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Default Re: why do some carbureted engines run rough when they're cold

Also, the fuel 'mist' droplets are coarser upon their arrival in the cylinder due to the air & metal parts' lower temp (less vaporization). Bigger droplets = less surface in contact with the O2 molecules = leaner perceived mixture. Kinda analogous to a coal or grain dust scenario.
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Old 04-12-2010, 07:16 PM   post #4 of 10
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Default Re: why do some carbureted engines run rough when they're cold

I perfer cold blooded engines, Make you wait to place a load on them. Unlike the electronic controlled ones that get thrown into drive, and are screaming down the road before the oil has made it from the pan to the valves. Manual choaks are great.
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Old 04-15-2010, 10:17 PM   post #5 of 10
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Default Re: why do some carbureted engines run rough when they're cold

Going a long with "coarser" fuel droplets, the fuel droplets also have a tendency to condense and stick to the cold walls of the carb and intake rather then go into the engine. This becomes more of a problem for bigger engines with big carbs. At the relatively low cranking speed, there might not even be enough air velocity to suck up meaningful amounts of fuel through the jets into the bowl. Which is why a choke is often needed.

Modern car engines can build oil pressure before the engine even lights off during cranking. By the time you even manage to get the shifter into drive (or 1st gear) the top end is already being covered in oil. Letting an engine sit and idle to warm up with the choke on is actually worse for it. The rich fuel mixture and cold engine lead to incomplete combustion. This means you are washing your cylinders down and filling up your crankcase with unburned gas.
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Old 04-16-2010, 07:43 PM   post #6 of 10
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Default Re: why do some carbureted engines run rough when they're cold

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Originally Posted by Lead Head View Post
Going a long with "coarser" fuel droplets, the fuel droplets also have a tendency to condense and stick to the cold walls of the carb and intake rather then go into the engine. This becomes more of a problem for bigger engines with big carbs. At the relatively low cranking speed, there might not even be enough air velocity to suck up meaningful amounts of fuel through the jets into the bowl. Which is why a choke is often needed.

Modern car engines can build oil pressure before the engine even lights off during cranking. By the time you even manage to get the shifter into drive (or 1st gear) the top end is already being covered in oil. Letting an engine sit and idle to warm up with the choke on is actually worse for it. The rich fuel mixture and cold engine lead to incomplete combustion. This means you are washing your cylinders down and filling up your crankcase with unburned gas.
You don't know my kids Literly have the car in gear before the key is out of start. full throttle 6k rpm in first within the first 3 seconds.
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Old 04-16-2010, 08:48 PM   post #7 of 10
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Default Re: why do some carbureted engines run rough when they're cold

LeadHead raised a good point. One of the reasons modern engines last so long is that between the EFI and modern ignition systems, the engine is always running properly, right from the start. No more mis-adjusted chokes, carburetor icing, or flooded engines that were common in the past. Modern oils help a lot also.
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Old 04-16-2010, 09:16 PM   post #8 of 10
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Default Re: why do some carbureted engines run rough when they're cold

The newer EFI controlled engines, in whatever, can adjust the air/fuel mixture faster than we could ever had imagined just a few years ago.
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Old 04-16-2010, 11:39 PM   post #9 of 10
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Default Re: why do some carbureted engines run rough when they're cold

Quote:
Originally Posted by tpolley View Post
why do carbureted engins run like crap when they're started cold? my 4-wheeler won't get out of its own way and backfires untill it warms up for a few minutes. i had an '84 silverado that wouldn't run worth a darn untill it was warmed up. my briggs mower won't idle for the first 10-15 seconds.
they dont. WHEN used and adjusted right. mondern engines last so long miles or years? 200,000 plus in ten years is common now. usually junk before then though and rebuilding isnt cheap. but 200,000 in 40 years is a good engine. just sharing my thoughts.
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Old 04-16-2010, 11:53 PM   post #10 of 10
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Default Re: why do some carbureted engines run rough when they're cold

Most cars have means of preheating the air/fuel mixture for better fuel vaporization--they have heat riser passages in the intake manifold that exhaust gases are routed to when the engine is cold,to prevent carb icing and beeter vaporization...many had "stoves" built around the exhaust manifolds too,that sent hot air to the air cleaner housing to further increase the fuel temparature for better vaporization..

I have found most engines that are "cold blodded" had insifficient heat being delivered to the base of the carb, or the hot air pipe from the manifold to air cleaner was not connected,or the "doors" in the air cleaner weren't shutting when cold...or the carb was jetted too lean for the size of the engine,and then the choke had to be left closed longer, to enrich the fuel/air mix..
Most of my older vehicles had hand chokes I installed,and I found the engines would warm up faster with the choke wide open as soon as possible after it started,only used the choke for 15-20 seconds or as long as it took to keep it froim stalling..once you get past the first 3 minutes after a cold start,the engine will usually pull a load without faltering..
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