Have had a ton of trouble with carb icing when it gets cold and damp. Tried all kinds of things to solve the problem but heat riser butterfly seems to stay open all the time. What causes it toclose? Vacume or temperature? What makes it open then? Anyway, while I was trouble shooting and trying to see if butterfly was working I took off the flexible snorkle hose that goes to the front of the truck. Made a world of difference. Must have cut down on the dirrect intake of cold, damp air coming straight in. Going to leave it off till spring comes! Hope this helps someone else whose truck runs like crap in these conditions!

86 F250 5.8 H.O. 4X

 

[font size="1" color="#FF0000"]LAST EDITED ON 14-Dec-00 AT 09:32 AM (EST)[/font][p]I've got the "snorkel" already off and this morning the carb still iced up. The vaccum air cleaner door holds vacuum but I haven't notice it closing. I'm picking up a new bi-metal switch (at top of air cleaner) Saturday and hoping that will cure the problem. I believe the vacuum source should be a ported source. Can anyone confirm this?
Also, I read an earlier post on same subject regarding the metal EGR tube/ pipe from rear of exhaust to EGR spacer as a carb heat source. Was led to believe this would not make much difference on carb icing. If yes, what did they do to prevent carb icing in the days prior to EGRs? We had a 65 Falcon that didn't have the pre-heater or an EGR and it never iced up.

 

[font size="1" color="#FF0000"]LAST EDITED ON 14-Dec-00 AT 01:56 PM (EST)[/font][p]Roscoe, I believe that the EGR tube, valve, and collar assembly is for emissions and combustion. Any heat from exhaust would be mixed with carborated fuel below the carb and drawn into the engine. Any heater or warmer air going into the top of the carb would be related to the air breather intake door. I believe you are on the right track with the vacuum problem to the air intake. Good luck.

Jakegypsum // Ford fleet, soup to nuts.

 

The vacuum source is not ported. I have an original setup
on a 351w with 2bbl and the vacuum line that is always a
nuisance when you want to take the air cleaner off goes to the
manifold vacuum tree fitting in the rear. There are guys
in the 351m/400 section complaining about the same problems
and I think that a properly functioning original air cleaner
will solve your problem. I don't know about older Fords but
have seen other brands of older vehicles with no snorkel pipe
and they had a exhaust passage which not only ran through the
intake like the newer vehicles but actually ran up to the carb
and the carbs had cast iron bases.

 

Is the bi-metal switch the circular disc on top of the snorkle? If so how do you replace it? It looks like mine is riveted on.
Pulling the flex tube to the grill sure helped but I'd like to fix it right to help it even more. My vacuum to it goes back behind the carb too. Thanks.

 

[font size="1" color="#FF0000"]LAST EDITED ON 14-Dec-00 AT 06:22 PM (EST)[/font][p]According to Alldata, "the valve in the air cleaner duct is in the closed position when the motor is not operating. When the engine is operating at below 75 degrees, manifold vacuum is routed thru the bi-metal switch (round, 2 vacuum ports, on top of air cleaner lid) to the vacuum motor (round, about 2" across, located top at end of air cleaner intake with 1 vacuum port) to close the duct valve allowing only heated air to enter the air cleaner. When the engine reaches normal operating temps, the bi-metal switch opens an air bleed which eliminates the vacuum and the duct valve opens, allowing only cold air to enter the air cleaner. During periods of acceleration the duct valve will open regardless of temps due to loss of manifold vacuum."
It goes on to say that a Cold Weather Modulator is used on some models to prevent air cleaner door/ duct from opening when outside air is below a predetermined temp. At higher ambient temps, the modulator is inoperative.The modulator is located in-line between the air temp control sensor and the vacuum duct motor. So, assuming the bi-metal switch and the vacuum motor is working, and if the bi-metal switch is supposed to "open" below 75 degrees, why the need for the CWM? For way extreme low temps? My truck will run all day perfectly without a functioning heat stove/ vacuum motor/ bi-metal switch when temps are zero or 10 or 20 degrees...and no precip. Sorry for the long post but I just got home and the truck stalled several times (still snowing). Can't wait to pick up that new switch and vacuum motor on Saturday. Hope this helps someone, anyway.

 

>Have had a ton of trouble
>with carb icing when it
>gets cold and damp. Tried
>all kinds of things

My dad drove half ton pickups for 30 years up here in North Dakota hauling mail to small towns. During the winter I remember him unhooking the snorkel that leads to the front of the truck and stuffing a rag in the inlet of the air cleaner. All air then had to come from the heat riser on the exhaust manifold. Sounds like a crude way of doing things, but it worked for my dad.



LeRoy Wolff, original owner of
1984 Ford F150
300 six 3 speed with overdrive
optional dual rusted out fenders
truck's nick name "Old Red"