I have a 1991 Mazda B2200 with a manual transmission. When it is cold outside, I like to warm it up. When I first start it up, the automatic choke engages, and it idles at 1100 rpm. But after 5 minutes of idling, it works its way up to over 2000 rpm. Is there any way to fix this?

 

3 ideals came to mind right off but it is only GUESSING

GUESSING but that sounds more like an intake leak somewhere?
Maybe time to start spraying starting fluid around the intake manifold and carburetor?
GUESSING it may be the choke linage on the carburetor that is dirty and hanging the choke up?
GUESSING check all the hoses connect to the carburetor and intake manifold

 

Yeah - start it up and drive off. Idling is a terrible way to "warm up" an engine - one wants it to get to operating temperature as quickly as possible to minimize wear. Look that up in the owner's manual - ALL vehicles !!! When you push on the pedal after the choke engages, the idle should return to normal.

Stock carburetor or aftermarket Weber?

 

it has a stock carb on it. I live in cold weather and when I do not let it warm up it does not run right and is dangerously underpowered so not letting it warm up is not a option.

after hitting the gas pedal, when it is 2,000 rpm it will return to 700-800 but will shortly return to over 2,000 rpm.

 

I always like. appreciate and even respect the responses of members that their hobbies and experience level is on one specific model vehcle but maybe it my age.... there are some things that are just empirical in nature and the laws of physics not to mention Thermodyne's and basic precibal of expansion rates of similar and dissimilar materials or metals as opposed to what is written for more or less consumers regarding some operational aspects of an engine .
Maybe some will remember in the days of carbureted engines some use to place cardboard in the front of the radiator to not only help the engine warm up faster but to also better control the environment temperature. Or better worded the passenger compartments of a vehcle. There was also many that converted back to manual chokes for the same reason. On some platforms the automatic chokes were somewhat adjustable for the different clients of engine operations.
Then there is the argument or debate of the type of gasoline fuel formulas now available and very different back when some carbureted engines were manufactured and placed in various auto manufactured vehicles. Yes gasoline type has a direct effect on the cold start of carbureted engines. And most are not in the higher percentage of positive running from cold start to operating temperatures.

At any rate while there is great suggestion for what is happening to your engine, I would keep an open mind and explore all the suggests and add to your own experience and resolving the problem.

Now speaking as a Performance Engine builder and needing at times to build by way blue printing and engine that often including building an engine in a clean room and running it up on an engine DYNO recording all aspects of it operation finding like many other Performance engine machine shop extensive testing we all seem to agree that allowing the engine to warm up to operating temperatures insure less damage or wear on all the reciprocating parts of an engine.

BTW in Aviation the first rule when starting an engine before even taxing out to the run way is to achieve all the necessary temperatures and pressures as per the operating manual and also required in some FAA regulation. it is also part of all training to get a basic and other piolets license.

 

Yes, the stock carb has a high idle adjustment screw that will raise or lower the idle speed when the choke linkage is engaged. To lower the idle you should "loosen" the adjustment screw......but only turn it about a quarter of a turn at a time. If you go too much, you may need to start the truck several times when it is cold.