fj401968 Posted December 8, 2005 Share Posted December 8, 2005 My carbureted '87 GL wagon wasn't so happy last week driving to work. It acted like it was starving for fuel... would barely pull 2nd gear. I figured it was tank ice impeding gas flow at the filter/pump so I poured in some gas de-icer. Next day it was even worse. I wondered if maybe I was having a carb-icing-up-issue. It should be noted, my air stove isn't functional. The previous owner replaced the Y-pipe and the one that is now welded in there has no riser flange for the aluminum duct that goes to the airbox intake. So there is no duct coming from the Y-pipe. I figured I'd need to fab something up. It has been really cold here in Idaho (-15* F this morning). Last Friday the problem got to the point it wouldn't rev enough to even pull first gear. I realized it was probably more of a fuel delivery issue and possibly not the carb icing up at all. So, I replaced the fuel filter (near the fuel pump) a couple days ago and it was clogged. I could barely blow through it. I got a Hastings fuel filter at the local Parts Service. As an aside, I like that the Hastings fuel filter is clear. The car ran just fine after that. I've driven it the last couple days and it has had no problems. I drive 30 miles one way and like I said, it is coooold. The missing air stove duct doesn't seem to be adversly affecting the car... at least at these temps (-15*F). Should I bother fabbing an air stove at this point? It seems to be doing just fine without it. I checked and the diversion gate in the air box is working so in the extreme cold, it is pulling air from inside the engine bay but not actual warm ducted air from the Y-pipe. Tracy Quote Link to comment Share on other sites More sharing options...
Subarian Posted December 8, 2005 Share Posted December 8, 2005 If you've got the stock carb, it's passively heated by engine coolant. You don't really need the heat riser pipe, but it might help the engine warm up a little quicker. Quote Link to comment Share on other sites More sharing options...
NVBigBlue Posted December 8, 2005 Share Posted December 8, 2005 FJ, I have an '86 carbed, that I just replaced the pre heat hose on. I have noticed a slight loss of power in the mornings when it's sucking warm air off the manifold, but it does run smoother when it's warming up and at lower speeds on the hiways in the mornings. Do I really need it? I'm not sure....probably not, I've only ever had a carb ice problem one time many years ago in a suby wagon I had (with the same engine and carb that's in my hatch now!), but it was ubber cold (-17 as the daytime high for over a week), and I was out blasting through snow and such. In fact, most of the carbed cars I've owned were usually missing the pre heat hose, and I never replaced them, and they drove just fine. I would say if your happy with the car now, and it would kind of be a pain to fab something up, don't bother. Besides, it cost me $16 for the stupid hose at autozone! NV Quote Link to comment Share on other sites More sharing options...
fj401968 Posted December 8, 2005 Author Share Posted December 8, 2005 Cool... I do have the stock carb. That is an interesting feature. I'm glad to know carb icing won't be an issue even without the air stove. My Soobie seems to be working well in the cold temps. I just wish the plastic things like window cranks, blinker levers and such were a bit beefier. It felt dangerous rolling down the window at the McDonalds drive-thru window yesterday. Tracy If you've got the stock carb, it's passively heated by engine coolant. You don't really need the heat riser pipe, but it might help the engine warm up a little quicker. Quote Link to comment Share on other sites More sharing options...
fj401968 Posted December 8, 2005 Author Share Posted December 8, 2005 I've had two carbed and two injected Subarus. The last carbed Subie I had was a 1981 GL wagon. The carb seemed to ice up quite a bit (ran rough, sluggish and weak sometimes on really cold days). In "hind-sight", I think I might have just had a sticky choke. I appreciate the input. Tracy FJ,I have an '86 carbed, that I just replaced the pre heat hose on. I have noticed a slight loss of power in the mornings when it's sucking warm air off the manifold, but it does run smoother when it's warming up and at lower speeds on the hiways in the mornings. Do I really need it? I'm not sure....probably not, I've only ever had a carb ice problem one time many years ago in a suby wagon I had (with the same engine and carb that's in my hatch now!), but it was ubber cold (-17 as the daytime high for over a week), and I was out blasting through snow and such. In fact, most of the carbed cars I've owned were usually missing the pre heat hose, and I never replaced them, and they drove just fine. I would say if your happy with the car now, and it would kind of be a pain to fab something up, don't bother. Besides, it cost me $16 for the stupid hose at autozone! NV Quote Link to comment Share on other sites More sharing options...
cd45 Posted December 8, 2005 Share Posted December 8, 2005 My 87 GL wagon is carbed, and the exhaust has been redone from the Y back, no duct for the heat hose to the breather. Yesterday and today it was down to 7 degrees F, and the car ran great. I just wish there hadn't been so much ice, not nearly as much fun as snow. Quote Link to comment Share on other sites More sharing options...
robm Posted December 8, 2005 Share Posted December 8, 2005 Carburettor icing occurs when the air is very damp (saturated), and fairly cool. Around 40 deg. F is where I have encountered it. What happens, the air going through the carb venturi actually drops in temperature. This is a weird effect, has to do with the flow of compressible fluids (air) near sonic velocity. This causes the moisture in the air to condense out. If the temp. drop gets to the freezing point of water, ice forms in the carb throat, restricting the air flow and killing the engine power. The car stops, you wait 5 minutes, and drive away, as the engine heat melts the ice, and all is good again. Until it happens again, 5 miles down the road.... It rarely happens below freezing, as there is very little moisture in the air anyway. It doesn't have to be heated up very much, and it is no longer saturated. It is just above freezing where it is a problem, as that is where there is lots of moisture in the air, and the temp. drop through the carb can cause it to condense and change to ice. No air stove may give you driveability problems, as some of the ultra-lean carbs from the mid 80's were really fussy about the air temperature. My sister's Civic wouldn't run worth a darn when the hot/cold air mixing valve fell apart. It liked the air at a nice steady 100 deg. F. I don't have any experience with carbed Subarus, so they may be different. Quote Link to comment Share on other sites More sharing options...
NVBigBlue Posted December 8, 2005 Share Posted December 8, 2005 Carburettor icing occurs when the air is very damp (saturated), and fairly cool. Around 40 deg. F is where I have encountered it. Most pilots can tell you, that if the air temp and the humidity are within 2 points of each other, you can get carb ice....... and you are spot on about what happens when you get it.... go...slow...stop.....melt....go....slow....stop....melt... Quote Link to comment Share on other sites More sharing options...
robm Posted December 8, 2005 Share Posted December 8, 2005 Most pilots can tell you, that if the air temp and the humidity are within 2 points of each other, Can you define "within 2 points"? Wet bulb/dry bulb temperatures, in deg F or what? There has to be a dry bulb temperature where this occurs, as well. I can't see it happening at 90 deg. F, no matter how moist the air is! Temp.drop through the carb can't be much more than a few degrees. It is pretty common in low lying areas, where the cool wet air collects. You can be running along just fine, then the road drops into a bit of a valley, and the car craps out. Once you are through the valley, the air warms a bit and dries a bit, and all is good again. Quote Link to comment Share on other sites More sharing options...
fj401968 Posted December 8, 2005 Author Share Posted December 8, 2005 I was wondering about that... as I think back, the only time my Landcruiser carb iced up (I remember spinning of the wing-nut on the air box and looking down to see lots of ice formed around the inside walls of the carb) was on a fairly warm, wet-tish day. It makes perfect sense. It leaves me wondering if I'm back to trying to fab something up so I'm ready to travel when that combination of conditions exist. A co-worker who also owns a Subaru has had trouble sucking up water through his air stove tube when he's been out splashing through the mud. It makes for a low and unprotected point on the vehicle to be pulling air from. I suppose a guy could run with it disconnected most of the time and just have it there for that rare time when you need it. Thoughts? ...and thanks for the input. Tracy Carburettor icing occurs when the air is very damp (saturated), and fairly cool. Around 40 deg. F is where I have encountered it. <snip>. Quote Link to comment Share on other sites More sharing options...
robm Posted December 8, 2005 Share Posted December 8, 2005 Old Datsuns used to have a summer/winter valve on the air cleaner inlet. Winter took the air off the stove, summer took it "straight". Early 80's, this feature was automated, due to emission controls. Maybe old Subarus had a similar rig? Or the valve from another vehicle could be scabbed onto your intake. Quote Link to comment Share on other sites More sharing options...
NVBigBlue Posted December 8, 2005 Share Posted December 8, 2005 Sorry, my mistake..... I should have said if the temp and the dew point are within a couple of degress apart. The drop in air temp through the carb condenses the water back out of the air, and walla.....you can get ice. And you'd be surprised how much air temp can drop through a venturi... you can read a pretty good article about it here: http://www.sacskyranch.com/crbice.htm NV Quote Link to comment Share on other sites More sharing options...
robm Posted December 8, 2005 Share Posted December 8, 2005 Great article! They mention outside air temp. as a factor as well . I never thought about evaporation as part of the problem, and I didn't realize the flow over the throttle was a source of Joule-Thomson either. Good discussion on the effect of carb heating, too. Adding heat to the air results in a richer mixture, which is a good thing in a lean-burn carb. It is curious that the SPFI's don't ever seem to ice up. They have evaporation, they are throttled, and they don't get heated air. Or is the throttle body connected to the cooling system, to provide heat? Quote Link to comment Share on other sites More sharing options...
robm Posted December 8, 2005 Share Posted December 8, 2005 http://www.chevron.com/products/prodserv/fuels/bulletin/motorgas/gallery.asp?img=46 Great little piece on carb icing. It flat-out states that it doesn't happen below freezing, the air is to dry. Can happen as warm as 55 deg. F, which is like 12 deg. C! Quote Link to comment Share on other sites More sharing options...
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