Carb Question
Printed From: Unofficial Allis
Category: Allis Chalmers
Forum Name: Farm Equipment
Forum Description: everything about Allis-Chalmers farm equipment
URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=209244
Printed Date: 25 Jan 2026 at 6:58am
Software Version: Web Wiz Forums 11.10 - http://www.webwizforums.com
Topic: Carb Question
Posted By: Gary Burnett
Subject: Carb Question
Date Posted: 06 Dec 2025 at 11:33am
| What causes the throat of the carb where it bolts to the intake to frost up? |
Replies:
Posted By: steve(ill)
Date Posted: 06 Dec 2025 at 12:21pm
|
the velosity of the air ( or expansion) going thru it... Like an air conditioner spraying freon thru an orifice.. as it EXPANDS it gets very cold..
------------- Like them all, but love the "B"s. |
Posted By: plummerscarin
Date Posted: 06 Dec 2025 at 1:53pm
|
Venturi effect. As air flows through the carburetor the reduced diameter causes the air velocity to increase which reduces pressure. This pressure drop has a cooling effect. Adding fuel to the mix cools it even. Moisture in the air condenses and shows as frost on the carb. This frost can build up on the inside reducing throttle opening and causing poor engine performance especially at WOT. Or something like that. I had that issue roading the 170 home in January a couple years ago |
Posted By: Gary Burnett
Date Posted: 07 Dec 2025 at 6:44am
| So is there a cure? |
Posted By: Les Kerf
Date Posted: 07 Dec 2025 at 7:42am
Our vintage tractors are not well equipped for dealing with carburetor icing. The cure is to introduce heated air into the air stream ahead of the carburetor. I have driven old Ford pickups that have the 300 cubic inch inline six engines for the past 40+ years, these reliable old workhorse engines are notorious for carburetor icing unless the thermostatically controlled hot air stove on the exhaust manifold is properly connected to the air cleaner and fully functional. I prefer the early 1980's 300-6 pickups because I can buy them cheap since almost everyone thinks they need a V8. Of the five I have owned, only one had a fully functional air cleaner because the previous owners (all named Bubba) thought they were smarter than the Ford engineers who designed them  Yup, they iced up until I could get the hot air stoves working.The worst conditions for icing are generally when the air temperature is just a few degrees above freezing with high humidity and cruising at fairly high intake manifold vacuum. |
Gary Burnett wrote:Posted By: steve(ill)
Date Posted: 07 Dec 2025 at 7:42am
|
some use sheet metal guards to cover the carb and exhaust manifold and get some heat in the area ... preheating the air into the air cleaner could help ... maybe under hood intake.. ------------- Like them all, but love the "B"s. |
Posted By: steve(ill)
Date Posted: 07 Dec 2025 at 7:54am
|
youi might be able to do something as simple as wrap a little aluminum foil around the manifold / carb ... but getting the air inlet below the hood is a great idea.. https://www.youtube.com/watch?v=RWaALhWCiIE&t=10s" rel="nofollow - https://www.youtube.com/watch?v=RWaALhWCiIE&t=10s ------------- Like them all, but love the "B"s. |
Posted By: Tbone95
Date Posted: 07 Dec 2025 at 7:59am
Had a 1980 F150, 300 6, 4 speed manual. Yes, the engine never gave one hint of trouble but had 3 various transmission issues in the time I owned it which was 5 1/2 years. About then I got my first big boy job and bought a new car. My father in law was on hard times so I gave him that truck (right after I replaced the radiator). He drove it a few years until it rusted out so bad he was dragging the gas tank down the road by the fill hose. Never an engine problem though! But holy cow did it rot away. Hard to believe there’s any of them on the road past 20 years old, but I do see them from time to time. |
Posted By: DrAllis
Date Posted: 07 Dec 2025 at 8:01am
| I had a Ford LN-800 medium duty truck decades ago, equipped with a Super-Duty 477 V-8 engine. I always would start the engine and give it a few minutes of warm-up time before heading down the road. Two or three times a year, if the ambient temp was just right, the humidity was just right, the barometric pressure was just right, the dew point was just right and the moon phase was just right, as it warmed up, it would slowly frost over under the big HOLLEY 4BBL carburetor, and literally choke itself dead. All you had to do was just wait a few minutes for the frost to dissipate and fire it up and awaaay we go !!! |
Posted By: Lars(wi)
Date Posted: 07 Dec 2025 at 9:27am
Yep, back in High School days, worked for the local BTO(by those days standards), they had a Farmall 560 that was notorious for carb ice. Any cloudy day, below 50 degrees, humid. That thing would ice over on its first startup for the day. Would choke itself off, let set 5-10 minutes, restart then good for the day. ------------- I tried to follow the science, but it was not there. I then followed the money, and that’s where I found the science. |
Posted By: NEVER green
Date Posted: 07 Dec 2025 at 10:00am
Very strange to have issues with Ford manuals, usually bullet proof. The 300 has been voted the most reliable engine on many polls. Chevy ran Ford Top loader trannys (not a pronoun) for yrs in NASCAR, their rock crusher Muncies did not hold up. ------------- 2-8050 1-7080 6080 D-19 modelE & A 7040 R50 |
Posted By: Tbone95
Date Posted: 08 Dec 2025 at 7:33am
Well, I don’t know what the deal was. Maybe a lemon. The guy who fixed it a couple of the times told me that the tranny was built in Mexico and everything about it was cheapened up. When I bought it it was 4-5 years old but I don’t recall the mileage. Not a lot. I also don’t know what the original owner used it for to know if it was abused, but from what I know about him I doubt it was used hard. I used it pretty much like a car. Very rarely hauled any more than a few suitcases in the back. Most miles were highway. It was so under carburetor-ed it didn’t have enough power to hold speed up a hill. Anyway, first time it was howling awful. Carrier bearing. I suppose I could have lived with that for a while but it was pretty annoying on a 3 1/2 hour drive. This actually happened twice. It would pop out of 4th gear while going down a hill. I never had that addressed. One night I was almost to my place, in second gear, pushed in clutch and pulled shifter to neutral, and “bang”! Wtf was that? Hmm, stuck in 2nd. Turns out a pin broke and came out and whirled around inside and screwed a bunch of stuff up. Then 1st gear went out, like you’d try to take off and it wouldn’t stay in gear. That was near the end. |
Posted By: Les Kerf
Date Posted: 08 Dec 2025 at 10:33am
That must have been the aluminum case 3-speed + Overdrive unit, definitely not their best transmission. My T-18 granny trannys are pretty durable, as are the New Process 435.
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Posted By: ORBill
Date Posted: 08 Dec 2025 at 12:14pm
Must be a regional issue. Growing up in Arizona I didn't know ice, frost, humidity and rust were in the English language!  My second pickup was a '66 Ford 1/2 ton 4wd, moved up from a '62, it had the 300 six and the NP 435 trans, granny low. With out a doubt the most bullet proof vehicle I ever owned. It was used, abused and even given a bath occasionally. Never let me down, never iced up, refused to die even when I wanted it to. After 330,000 miles of no power steering or power brakes I started thinking about a replacement. Started a family about that time and needed to move up to a Suburban with ore seating capacity. Kids are all grown up and gone but blessed us with grandchildren so the need for a multiple seated vehicle still exists. Still miss the old Ford, open the hood, climb inside, park my rear on the inner fender and change the spark plugs. Crazy pickup also got very good gas mileage for its time. My comment to the OP's question = move to Arizona! You won't have a carb icing problem. Just watch our for the rattlesnakes and scorpions!! Bill.... Formerly from Arizona, now in the rain drenched Pacific North West.
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Posted By: Steve in NJ
Date Posted: 11 Dec 2025 at 11:10am
|
Those 300 inch Ford sixes were one tough ass engine! When tuned right, they feel like a V8 under the hood. I know a few fellers that Drag Raced those engines. Man, would they fly with the right combination! Steve@B&B ------------- 39'RC, 43'WC, 48'B, 49'G, 50'WF, 65 Big 10, 67'B-110, 75'716H, 2-620's, & a Motorhead wife |
Posted By: Dennis(IA)
Date Posted: 13 Dec 2025 at 10:26am
| I had a IH Scout with a 6 cyl and it would ice up. Had to be the right temperature and humidity. Only did it a couple of times all the while I owned it. |