Large Model Association Forum

Hi, can anyone please tell what the adverse effect would be if I remove the trumpet from a rear carb 3w80? I've got to rebuild the carb and getting the carb on and off with the trumpet fitted is a pain.

Stewart.

The trumpet not only acts to collect back spill of fuel from the carb but primarily serves the purpose of "speeding" up the airflow through the carb which enables better atomisation of the fuel air mix & a more complete combustion. Removing the trumpet will gain more air "volume" but causes the fuel air mix to become lumpy and poor combustion & less power.
Cary

Thanks for that Cary, that means I'll have to put the awkward little sod back on.

Cheers

Yep!

hope you dont mind me butting in on this thread, how much difference does a trumpet make on a zenoah 38 as these do not have a trumpet when supplied new ?

Thinking back to when I was cart racing we only fitted trumpets to increase bottom end power and smoother acceleration i.e. for in the wet, otherwise we ran without for max power, using tiloson carbs on 100cc.

Dave

David Brown wrote:Thinking back to when I was cart racing we only fitted trumpets to increase bottom end power and smoother acceleration i.e. for in the wet, otherwise we ran without for max power, using tiloson carbs on 100cc.

Dave

Dave, I also raced karts in the 250E/Formula E class using Rotax & Yamaha engines. The carbs we used were either the Aprilia/Dellorto or powerjet units of the Kehein variety. The trumpets were used as I previously described for atomising the fuel air mixture in these performance engines especially when the rpm were getting into the regions of 13,000rpm. RC engines such as Zenoah have a lower stroke length & hence lower rpm, but poor fuel atomisation will still cause poor performance, uneven running and occasional "glugs" of fuel entering the crankcase (after the carb) which when not atomised can cause the engine to misfire or even stop. 3W engines need the trumpets fitted to keep their fuel atomising as best as possible therefore giving the best performance (Tillotson carb). I have a Zenoah 38 that does not have a trumpet fitted but after a recent dead stick that as a result of getting her down quickly caused wing damage I am considering all areas of prevention to ensure the engine does not suffer this problem again.
Cary

So I presume then the smaller the engine the less of an advantage is gained from a trumpet. Would it make a noticeable difference on a 1.20 two stroke glow engine?

James Stewart wrote:So I presume then the smaller the engine the less of an advantage is gained from a trumpet. Would it make a noticeable difference on a 1.20 two stroke glow engine?

Hi James, trumpets do make a difference it just depends on what you want to get out of the engine. Atomising fuel in any engine is a plus but in many cases the performance/power output of the engine is not critical whereas the reliability is. Most rc engines these days are pretty reliable but engines designed solely for outright operformance will have trumpets already fitted & supplied or are available as an optional extra!
Cary

Just thought I would mention something that I found by accident when adjusting the jets on my old Zenoah 62. With the engine at full revs I noticed when my hand was just behind the carb intake the revs increased. With a bit of experimentation I came up with the mod in the photo, this increases the rpm by about 400 and brings this old '62 back up to the performance of my newer Zenoah 62's.

This not my area of expertise but I think we are missing a trick here. If an engine is designed to have a trumpet, and then is not fitted with one, there will be less than the intended airflow into the carburettor. This due to a phenomenon called the vena contracta. There is plenty on the web about it so I recommend a browse for those interested. A sharp-edged entry into a fluid channel is very rare so a trumpet seems to have an important function. For model engines I suspect that lack of a trumpet is only noticed at high power settings when the engine will want all the breath it can get.

Chris Lane

Chris Lane wrote:This not my area of expertise but I think we are missing a trick here. If an engine is designed to have a trumpet, and then is not fitted with one, there will be less than the intended airflow into the carburettor. This due to a phenomenon called the vena contracta. There is plenty on the web about it so I recommend a browse for those interested. A sharp-edged entry into a fluid channel is very rare so a trumpet seems to have an important function. For model engines I suspect that lack of a trumpet is only noticed at high power settings when the engine will want all the breath it can get.

Chris Lane

Bang on Chris! Looking at Dave's "vane" what is happening here is that the air is being smoothed out by virtue of the metal fin acting to stop the turbulence being caused around the carburettor, allowing the carb to draw in the air without a negative pressure (vacuum) occuring that can occur within turbulent air movement.
Cary

Very interesting lads, thanks =)

I used a Zenoah 23, firstly for some time without a trumpet, then with one. Two differences: (a)without the trumpet there was fuel staining down the fuselage side from liquid fuel runs, and (b) the running time on a full tank was at least 20% greater with the trumpet.
I have had similar effects with a trumpet on a Zenoah 38.