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Pressure Equalization on petrol engines

If your engine is playing up, do this:

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Paul Marsh19/04/2019 20:10:52
3953 forum posts
1180 photos

It's happened again. A while back I found the reason why a petrol engine in my Su26 kept throwing props in my face, and couldn't tune the engine, although it ran fine with the cowl off. The same for my Black Horse T28 Trojan with a CRRC 26cc engine.

That was years ago, and still have those two aircraft, and still fly them sometimes, but managed to fix the problem.

Problem arises again, why trying to fly my new Yak 54, different make of engine, same type of radial cowl as the Trojan and SU-26. The fix: solder a brass tube onto the carb equalization hole, attach a piece of fuel tubing (petrol) and exit the cowl,
Problem fixed, engine runs fine.

So when you have a problem when your engine (petrol) won't run with the cowl on, this could be your solution...


Edited By Paul Marsh on 19/04/2019 20:11:36

cymaz19/04/2019 21:58:09
9177 forum posts
1180 photos

The diaphragm under the plate is acting the fuel metering lever. The the air hole is needed for atmospheric reasons, a Vacuum is on the fuel side so atmospheric pressure is needed to move the diaphragm. However, if the air pressure is disturbed in any way the diaphragm will move with this change, this in turn changes the metering needle causing poor running. Modification of the cover plate is easy...the other end of the tube should be put in the fuselage away from any fast moving air.


An excellent explanation with great pictures is HERE

HERE is how to make it, I didn’t use the film can though, just put it in the fuselage 

Edited By cymaz on 19/04/2019 22:05:19

Paul Marsh20/04/2019 11:16:44
3953 forum posts
1180 photos

Thats where I got the pic from. I'm really surprised on two things:

  • Very few people are aware of the problem;
  • And that engine manufacturers don't tell you or give you a nipple already fitted to the carb.

As said, it doesn't manifest itself in all installations, but for me radial cowls.

The other aircraft where either the carb is in the fast moving air or in a cowl are all fine.
For example, my Wot4 Extreme with a DLE40cc twin, the carb is below the engine, in the slipstream and runs perfectly, as do the others.

Looking now at my other airframes, these have been given the pipe treatment:

Pilot 60cc Yak54 with DLE 56 engine
Graupner Yak54 with RCGF 15cc
Black Horse 160 SU26 CRRC engine
T-28 Trojan, 60 and 120 size with RCGF 15cc and CRRC 26cc engine
2 Seagull Yak 54's one with CRRC 26 and the AGM 30cc engines.

Notice the trend: All radial cowls!!!surprise

Martin Harris20/04/2019 12:49:20
9260 forum posts
245 photos

Pressurisation of the cowl is the problem so radials and some rotary engined subjects (especially if not blanked off at the inlet to only admit cooling air past the cylinder(s) as required) are particularly prone to these problems. In all radial cowlings it is particularly important to ensure there's a good exit path for the otherwise stagnant hot air that needs to flow out but in-line engined subjects can suffer if these considerations aren't addressed.

Edited By Martin Harris on 20/04/2019 12:50:25

cymaz20/04/2019 17:41:30
9177 forum posts
1180 photos

My Mr Mulligan went immediately rich and rough on its first flight....radial cowl !

MaL22/04/2019 11:24:10
140 forum posts
13 photos

All the petrol heads at my old club soldered a nipple on the vent and ran a fuel tube back through the firewall into the fuselage where the atmospheric pressure varied the least and there would be no venturi effect on the end of the tube.

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