Jabiru Owners Group

Welcome to the world of Jabiru

carb heater

This is a read only forum and contains all the archived posts from jabiruowner.co.uk.
  • Advertisement

Re: carb heater

Postby Anonymous » Tue Feb 10, 2015 10:14 pm

Posted by: roger on 12:39:00 12/16/14 Tue

The original electric carb heater was designed to minimize the onset of carurettor icing in uncowled engines and in particular, engines fitted to flying school Thrusters. R&D identified how and where the icing was forming and the minimum amount of heat to prevent engine stoppage. A prototype was assesed by RAF engineers on one of their school Thruster with the end result being that the heater was approved by both the Laa and Bmaa and the concept adopted around the world.

The heater was never intended to assist cold starts but it was found that if the heater was switched on for a few minutes it could heat the carburettor to sufficiently to provide a start condition.

The R&D revealed some interesting data. At this time of the year icing would start forming within seconds after start up. It could be seen in the vapour swirl just before the butterfly. If the engine was allowed to run at idle, the drop in temperature would cause the ice to form on the butterfly and throat. The ice could build up to cause the engine to loose rmp and run rough. If the throttle is opened to regain rpm, the ice would break away causing a temporary blip and thereafter rpm would increase. This is worth remembering as icing can develop during a long taxi back after landind. If the engine is shut down without clearing the ice, the ice will melt and work its way into the bowl. The water can cause difficult start the next time or if left for a time, react with the fuel and aluminium bowl. Evidence of the corrosion can be seen as blackish stains in the bowl. Melting ice can also be trapped in the idle adjust screw (rough running at idle) and the choke jet located at the bottom of thd bowl. If the choke jet is blocked this will cause difficult starting.
Many owners have been persuaded to enlarge the choke jet but most cases cleaning out the jet is sufficient.

In consideration of the above, if fitted it is good practice to keep the electric heater on until shut down.
Anonymous
 

Re: carb heater

Postby Anonymous » Tue Feb 10, 2015 10:14 pm

Posted by: Keith Betteley on 16:46:42 12/16/14 Tue

There appears to be a certain amount of controversy regarding this issue, but I can only speak as I find. Went flying today with temp at 7 degrees cent, applied carb heat for a few minutes during checks and she fired up first time. This would never have happened previously. Perhaps my problem has been solved by having the choke drilled larger and applying carb heat, ie a combination of both has sorted it. Strangely though, I was talking to an acquaintance today who flys a 2200 engine Jab and has no problem starting in cold weather without ever having done anything to his engine???. Perhaps the jury is still out, who knows.
Anonymous
 

Re: carb heater

Postby Anonymous » Tue Feb 10, 2015 10:14 pm

Posted by: Steve on 16:53:58 12/16/14 Tue

I think this is very engine specific. My engine used to start first press of the button in any weather without the jet being drilled and without any sort of warming. I then had the engine overhauled and the 40mm card put on and now it is difficult to start and takes me 3 or 4 attempts. It still fires up without the jet being drilled or my electric carb heat on but doesn't start as well as it used to in cold weather.
Anonymous
 

Re: carb heater

Postby Anonymous » Tue Feb 10, 2015 10:14 pm

Posted by: DL on 17:57:22 12/16/14 Tue

Well, I used to have terrible problems starting my 2200 serial number #1157 in cold weather.

Anything below 8 degrees and it had to be jumped using my car battery. I even made up a special connector and cables to quickly start her on chilly mornings.

It was deffinitelly not moisture in the carb bowl as on more than one occasion I removed the bowl and checked it (very easy to do). One thing that did work (as recommended by PaulG) was warming up the engine with a 12V 500W fan heater.

Starting problems became history when on drilling the choke jet out 1.00 mm as recommended by Jabiru South Africa. Now, she start in any weather on a couple of turns.

As far as the carb heater goes, it is a dual element ( 2.5 amp X 2 ) heater connected to the side of the carb, closest to the butterfly and produces around 50 watts of heat (so around a 1/20th the heating capacity of a 1000 watt electric heater (single bar). Quite a bit of the heat that is produced is actually lost by convection.

The carb heater works by transferring heat by conduction. Although heat transfer by conduction is more efficient, there is a very small area that actually touches the carb body (the combined area of the retaining fasteners is probably bigger than the conact point) so transfer of heat is quite limited (a conservative guess is half of what is produced).

Considering the size of the carb and the amount of heat that is lost by convection and dissipatted by the carb body, I am at odds how it can possibly produce enough heat to significantly raise the temperature of the carb but that is something that can be measured using one of vic's temp gauges so perhaps someone could try that by measuring how long it takes to raise carb temperature by say 5 degrees before starting engine.

concluding and in my IMHO, the carb heater is pretty useless at:

1. stopping the formation of carb ice on the ground (better off using carb heat before departure for 10-15 seconds)

2. assisting cold starts
Anonymous
 

Re: carb heater

Postby Anonymous » Tue Feb 10, 2015 10:14 pm

Posted by: Roger on 18:29:25 12/16/14 Tue

It is very much engine specific. The engine has to be in good condition and the best way to assess the condition without instrumentation is to check the lowest rpm it can run smoothly when properly warmed up.. The rpm should be about 500-600 rpm for a 40mm carb and 400-500 for a 32 mm carb. Once you have confirmed this you can wind up the idle rpm to a more comfortable 750 rpm.

If that can be achieved then the carb choke system should be checked to ensure that the choke is fully open. This means that the bing linkage meets the stop. %0% of the carbs I see do not meet this citeria.

Drilling out the choke jet is controversial. A lot of owners have reported better starting after the mod, but they could have achieved the same results by just clearing a blocked jet. If the jet is increased in size then the overall mixture will increase and this has to be taken into account. If in doubt as how to proceed, next time the carb is removed for servicing, inspect the choke fuel route. You will notice that the choke fuel chamber is filled to the level of the fuel in the main bowl. The fuel bowl is also vented to the main bowl by a small gallery located above the choke fuel chamber. There is a pick up tube that sucks neat fuel from the bowl to the mixing chamber on the side of the carb. Half way up the pick up tube is a small hole which means that neat fuel will be drawn only when the fuel is above this hole. By that stage a well maintained engine should be running. If you are still with me then you will realise that increasing the jet size does not do much as the choke will be drawing air at this stage. Better to make sure that the whole choke system is clean out first and properly adjusted If the engine is difficult to start remove the jet completely to see if it makes a difference. If it does then I am wrong and go ahead and drill it out.

Regret to say Steve that the 32 mm carb is better suited for the 2200. You can get a better starter, lower smoother idle, the same max static and overall better economy.
Anonymous
 

Re: carb heater

Postby Anonymous » Tue Feb 10, 2015 10:14 pm

Posted by: Peter Knight on 18:48:07 12/16/14 Tue

With only one element on (20 - 30 watts of power), I get a 5 deg C temperature rise in about 120 - 150 seconds.

This is with the carb heater attached in the standard position but with a bit of thermal paste to aid conduction and the temperature measured at the small metal fillet on the top of the carb between the bowl and the throttle body.

30 watts is a respectable amount of power (try holding an old 60 watt incandescent light bulb for very long) and the thermal mass of the carb and fuel is not too great. I may do a more exact measurement if I remember next time it's cold.

Incidentally the energy of a the fully warmed up hot air carb heat supplied to a 3300 is of the order of 3 - 4 kilowatts. That is seriously hot.

Peter
Anonymous
 

Re: carb heater

Postby Anonymous » Tue Feb 10, 2015 10:14 pm

Posted by: RogerLewis on 09:58:44 12/17/14 Wed

Peter,

that is useful information and the effectiveness of the heater, similar to Keith's and many others I know, is a good indicator that your engine induction system is properly set.

Thermal paste is a good idea and one i did not think of at the time. Also some owners have chosen to wrap a fire proof insulator around the body of the heater so that more heat is directed to the carb body.

We have heard extreme opinions on this subject and I am sure that your data will help those in the decision making process. Alas the Ice Eliminator is no longer being manufactured but there are other similar heaters in the field which should be as good.
Anonymous
 

Re: carb heater

Postby Anonymous » Tue Feb 10, 2015 10:14 pm

Posted by: Steve on 20:05:46 12/16/14 Tue

I agree, my idle is rubbish now the 40mm carb has been installed. I cannot achieve below about 900rpm as it stands, but it does have all new parts such as rings and has only done 2 hours on them.

But, I certainly do notice more power, especially in the climb. I was averaging about 800fpm before and now can touch 1000-1100fpm. My static rpm has gone from 2600rpm to 2750rpm.
Anonymous
 

Re: carb heater

Postby Anonymous » Tue Feb 10, 2015 10:14 pm

Posted by: Gary on 19:19:53 12/16/14 Tue

@ Steve, sounds like your maint engineer has either not set up the carb correctly, assume it was fully clean before fitting, or your overhaul was a pants job
Or possibly an inlet rubber issue
If all well was before. Too many changes at once to put a finger on it, but wind the clock back on the work, go back to basics and you should find the issue.
The motor should start just as well with either carb on it, its a bad set up or something not right on the overhaul.
All the 40mm set ups I know start with no issues. I think they have all been drilled, certainly worked ok for mine 3 years ago
Anonymous
 

Re: carb heater

Postby Anonymous » Tue Feb 10, 2015 10:14 pm

Posted by: Steve on 20:40:30 12/16/14 Tue

There was a massive load of changes done at the time including Pistons and rings, re-hone, flywheel bolts, through bolts, new sump, new induction system, induction pipes along with other maintenance such as under carriage bolts and a whole host of other things. Basically a top end overhaul with a few extra bits

All done by a very respected, well known Jabiru engineer. He has told me to let it all bed in before I do anything to it. I am supposed to wait 25 hours. It's running on Aeroshell 80 oil at present.

Engine runs perfect in general but does not like to idle below 900rpm. Dean and myself checked everything over at the weekend including float level, jets, idle mixture and air leaks and cannot find anything wrong.

To be fair, I have not managed to get the oil temperature much above 40-50 degrees with ground running at the moment so it has not really been a fair test. When I flew it back after maintenance I did notice a bit of extra power and higher static but not sure whether that was down to the carb or just the fact that the engine is in better condition now.
Anonymous
 

PreviousNext

Return to Archive

Who is online

Users browsing this forum: No registered users and 1 guest