Carb heater ideas

[Wooky]

I have an old SJ410 with a nissan A15 motor fitted and to overcome the Steep hill issues with carbs I have fitted a CV type motorcycle carb to it.
In general it works like a champ for the gnarly off road stuff but at cruise throttle on the open road I am managing to ice up the carb under certain conditions - early morning with damp air.

I would like to heat the carb electrically so that I can switch it from the cab.
Has anyone ever done this before?

Thanks

W

[Saxj]

Gives new meaning to cold air intake

[Wooky]

Ya... I have heard of a few people warming their lunch on the manifold but I can use mine as a fridge.

[Saxj]

Strap a couple of warm beers to it. When you get to your destination they should be properly chilled.

More seriously, how about plumbing in a wrap of copper pipe from the cooling system.

[Wooky]

The A15 has a water heated intake manifold so eventually the carb does heat up a bit through conducted heat.
But still is markedly cooler than the water heated manifold even on a warm day which is good... nice cold air induction and increased air density.

I don't really want to warm the carb all the time, just when needed.

[Chris Stoffel]

Sirs, the SJ413 have a pipe from the exhaust manifold to the air intake. When the manifold heats up the air gets heated up and prevents frosting of the carb. It is the white pipe in the photo. I removed mine.

[Wooky]

Thanks Chris,

That is one of the alternatives I am looking at and is possibly the better way of doing it as it actually warms the incoming air.
I was just looking to see if anyone had come up with a sneaky electronic way to heat the carb so that I could switch it from the cab.

[Apocalypse]

have you got a pic of the set up?

It's quite unusual to ice up a carb that draws air from within the engine bay - have you got a fresh air inlet on the bonnet or something?

[Wooky]

Unfortunately no pics on hand.

The carb draws air from in the engine bay but it is a constant velocity carb working on the same principal as an SU.
As the air velocity through the carb remains constant you have max cooling right at the main jet almost constantly but is worst under part throttle cruise conditions. Ice then starts to build right on the main jet leaning out the mixture.

On saturday morning after about 40km at cruise the carb had actually iced on the outside

This is a fairly common problem on bikes in the UK and you used to be able to buy carb heaters for these carbs but nothing seems available any more with everything moving to FI.

[Apocalypse]

?Usually those carbs on bikes were facing the elements directly! The racing minis with the crossflow heads and SUs frequently had the problem , but only really in the arctic conditions of the UK

Interesting set up from the sound of it though. Really and truly the best way around it is to either dry the air more (ice can't form if there is no moisture!) or heat it slightly to prevent it cooling to below zero through the venturi

Is there a thermostat in the engine? maybe draw air through the radiator before the aircleaner?

[Wooky]

On a bike they are usually tucked behind the cylinder and bolted directly to the engine so stay fairly warm.
Also the volume of air being drawn through the carb is significantly less giving less cooling.

The Carb I have fitted was originally fed a 650cc single now it is feeding a 1500 4 pot so it is flowing considerably more air than on the bike.

yep engine does have a thermostat and engine temps run normal.
I don't think it would need much warming and would only need it under cold and damp conditions. so not very often at all.

The easiest thing would be to run an intake hose down from the aircleaner to close to the exhaust like Chris Stoffel mentioned but there is something fun about making things more complicated

[Wooky]

Only get to tinker with the SJ on weekends so will snap some pics next weekend

[ThysleRoux]

Thanks Chris,

That is one of the alternatives I am looking at and is possibly the better way of doing it as it actually warms the incoming air.
I was just looking to see if anyone had come up with a sneaky electronic way to heat the carb so that I could switch it from the cab.

The exhaust manifold ducting system used many moons ago usually had a crude automatic heat activated flap to close the warm airflow to the carb and open the cold air intake. This usually did not work though.

What I am actually trying to get to is that it should be possible to connect a cable to the flap and open and close it as required from the cab.

Some of the older Corollas had a vacuum operated mechanism which you may also be able to convert

Some older cars also had a spring loaded selector on the air cleaner nozzle for manual selection, which should be easy to convert to cable operation.

[Wooky]

The air cleaner box has a change over flap but only manually operated... so works very well but does require a stop and tinker under the bonnet to make the change.

Hence the interest with going electrical.

[Wooky]

So after burning some bandwidth on aviation forums it would seem carb icing can actually occur at air temperatures up to about 30 degC.
The relative humidity and dew point temps play a huge roll in in the occurrence as well. Ice formation will be most prevalent with high humidity and low dew point temps.

There also seems to be three ways to combat it:
1/ Fuel additives / winter fuel. This does not actually stop the ice forming but stops it sticking to the carb and so keeping the main jet and throttle plates clear.

2/ Heating the carb directly. This stops the ice formation but does not stop the water condensing as the air temperature drops due to the velocity increase through the venturi. This generates a lot of water vapour and leans out the mixture as the incoming air is to moisture laden to take on much fuel.

3/ This seems to be the preferred method - heating the incoming air stream. As the air temp is increased the moisture carrying capacity of the air is increased and the relative humidity is decreased allowing for a better "up take" of fuel giving a richer mixture. The increase in air temp also decreases the chances of reaching dew point and therefore keeps the moisture suspended in the air stream and limits ice / moisture formation. The down side to heating the air is the decrease in air density and the subsequent drop in engine power.

Looks like I will need to look at going the hot air pick-up route after all.....