Fuel Reversion from Carb - Enya 120 R Four Stroke

[Manish Chandrayan]

What (all) can I do to reduce the fuel reversion from the Enya 120 R carb. The engine is mounted inverted on a Seagull Chipmunk. Prop 15x8 APC, fuel 16% lube, 5% nitro. OS FS plug. Top end 8400 steady, idle set around 2000 and then I think further lowered with Tx trim till the model won't creep, did not bother to check the final idle rpm.

The engine was tested on bench (mounted straight up) with an Enya 3 plug and 7% nitro, and 15x8 MAS prop at 8800. The numbers look good to me when compared with the published reviews.

 

Both on the bench and in the model it was noticed that the carb spits out excessive amount of raw fuel. the carb is a metered air bleed and the engine transitions fine .

One may notice from the below picture that the carb intake is almost flat (with a very short lip to engage the bracket) and may not have any venturi effect to reduce the fuel reversion.

 

What is the considered opinion to reduce the reversion that is making a mess in the cowl and on the firewall of the model?

  

[J D 8]

 I run 2 Enya 90 4c's, one has the air bleed carb and other one has the twin needle carb and neither have that style of bracket holding the carb. It does look an odd set up.

[Manish Chandrayan]

JD, the Enya 120R and the 155 have that setup from Factory.  The 90 and the 120 (bored out 90) had different carb fixing as you rightly say.

Does your 90 have same issue?   

[Nigel R]

OS went with a short intake trumpet when they did the Alpha series. I'd guess that is the "right" answer. Obviously no factory parts are available for the Enya.

 

I'd be tempted to suggest some kind of moulded glass/epoxy creation. You might only need a very short 'tube' to prevent the reversion. No idea how you'd retain it with that bracket in the way, though.

[Mike T]

I have a slightly different problem with an inverted Saito 125.  It runs fine on the ground, but at speed in the air it seems to falter and surge.  I'm assuming its because air is chuffing around inside the cowl and 'spoiling' the airflow around the carb mouth.  The flat Saito carb has no means of directly mounting an intake 'trumpet' (the usual cure to manage air intake and/or reversion problems), so I machined up some al. tube with a groove for an 'O' ring at the bottom.  This sits on the carb (the 'O' ring preventing metal-to-metal galling) and is screwed to the engine mount.  Hopefully, being fixed on the engine mount will mean that the effects of vibration will be sympathetic.  Pic attached:

 

 

NB - as yet untested!

 

Edited September 11, 2023 by Mike T
typo

[Manish Chandrayan]

Hello Nigel, not only the 'a' even the standard FS series all have a slightly raised lip that I suppose helps.

People have made one even from heat shrink tube, but yes, fixing any venturi will be a major challenge.

[Jon H]

I have an enya 155 and it spat like crazy. It was also really hard to get running right as i think the carb was worn out so i modified a laser carb and fitted that. Less spitting now and running better but it does still spit. 

[Paul De Tourtoulon]

I have a 120 with that sort of funny carburetor, I never did get it to run properly, so I adapter an OS one to it. i posted it on Rcgroups a while ago.

[Engine Doctor]

A intake trumpet will sort it if you can find one to fit . The fuel spitting is due to the overlap on the valves timing . also if its running rich it can cause an exagerated spitting / mist from carb. looking at the bracket you could modify that to clamp a trumpet in place if it has a thin flange . You will probably need to find a friendly mate with a lathe to turn you one up as I doubt if they are commercially available.

Edited September 11, 2023 by Engine Doctor

[J D 8]

1 hour ago, Manish Chandrayan said:

JD, the Enya 120R and the 155 have that setup from Factory.  The 90 and the 120 (bored out 90) had different carb fixing as you rightly say.

Does your 90 have same issue?   

 No they do not. John.

[Paul De Tourtoulon]

You can try a piece of 'moulded' heat shrink tube here on a Perry carb.

[Manish Chandrayan]

I am very satisfied with the running, it is almost giving me numbers that was published in engine review (despite using lower nitro and being located in hot and humid tropics).

 

The engine idles and transitions reliably, it's just the mess it's making has me a bit bothered. In anticipation, all exposed wood was given a good thorough coat of epoxy before the engine was run on the model.   

 

Engine Doctor, yes I had read that the enlarged carb venturi and valve overlap on the 120 R created this issue.

Paul, I think I will try fashioning a trumpet out of heat shrink as it might be possible to anchor it under the carb retaining flange and see if that helps in solving my issue to some extent     

[Manish Chandrayan]

Having considered various options , the easiest to try was the heat shrink route. The carb intake has a slightly raised lip (around 1 -1.5mm) and an OD of 15mm that is surrounded by the bracket opening. I shrunk a suitable tube and tried to put it on, but there being hardly any clearance between the bracket and the raised lip the bracket would not seat. In order for the heat shrink tube to fit the bracket would have to be filed off so that the heat shrink tube is tightly held.

Second issue I encountered was that the bracket itself would foul the straight run of the tube, that is unless the bracket is cut off and made a two piece affair. (see the red lines on the picture). Even after being cut the raised round portion at the back of crank case (red arrow) would still create a sort of dent inside the heat shrink tube.

 

What is the considered opinion, regarding cutting off the bracket and making it a two piece affair and the resultant bump in the heat shrink intake trumpet ?    

[RottenRow]

I wouldn't cut the existing bracket, which would ruin it forever and you probably wouldn't find another one.

 

Better to remove the bracket and replace it with two L-shaped pieces of aluminium (or steel, or brass) each with with two holes for the mounting bolts. Perhaps making the carb. end of the heatshrink into two tabs and trapping those under the brackets will help to retain it in position. at least to see if the idea works in the first place.

 

The 'bump' that would be in the trumpet, caused by the back of the timing case, can't easily be changed, just make the trumpet fairly long and shallow.

 

If the improvised trumpet does reduce the carb spitting, perhaps a more permanent solution would then be to make a combined aluminium trumpet and bracket.

 

Brian.

[RottenRow]

Something like this perhaps... it would need longer screws of course. It's only a rough drawing, no dimensions.

 

It might even be possible to 3D print one, depending on the temperature around that part of the engine.

 

Brian.

 

[Nigel R]

3d printing seems tailor made for this problem. Clever suggestion by rottenrow fixes the issue of the bracket getting in the way, by also being the bracket.

[Paul De Tourtoulon]

Yes, keep the bracket and make a new one from an old aluminium car number plate.