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Petrol Newbie

Trying petrol for the first time

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Aero12016/06/2008 11:23:00
131 forum posts


I just wondered tif there is anyone who can give me a few pointers and general advice. I've flown glow engines for years and just decided to take the plunge into petrol with a 20cc Zenoah in  an Extra 260. I've got the appropiate bungs and tubing and converted the fuel tank, but wondered what peoples views were regarding carb installations and power loss by using or not using the intake trumpet and a straight versus a bent inlet manifold. I want to keep the cowl as neat as possible so I don't want the trumpet sticking out the side. If I take it off, the carb fits just within the cowl but I expect the airflow won't be very good. I'm planing to fit a 90 degree bent inlet manifold and trumpet so the carb and trumpet are full enclosed, however I expect there will be a power loss due to the manifold bend. Has anyone experimented to see how much bends and trumpets affect the power?

The carb instructions puzzle me. The Zenoah instructions state the throttle linkage should be set up to push against the return spring, surely it would be better engineering to pull against the spring as that would take any slack out of the linkage? I have also been advised to remove the spring, but surely that would allow the throttle buttlerfly to flutter is there is any slack. It is also specifically stated in the instructions not to remove it.

Surely fitting the throttle linkage to the existing drilled plate must be better than sticking on an additional arm as the instructions state.

I presume the large idle speed screw can be removed and the Throttle servo used to set the idle speed and allow throttle cut.

 I would also like to fit a smoke system and wondered if anyone could recommend a system and does the diesel tank need new bungs etc?

Does Agricultural diesel contain enough pigment to give red smoke?

I would appreciate any advise or comments

Ultymate16/06/2008 12:43:00
1700 forum posts
62 photos

Hi Paul, firstly if you can use the trumpet you get better airflow, you won't notice any loss of power when using the elbow manifold. Don't remove the throttle return spring as this leads to rapid wear of the throttle butterfly as you have said bin the the throttle stop screw altogether and use your servo to set idle and also to stop the motor by totally closing the butterfly. I use a bowden cable to (pull) open the the throttle and just use the aformentioned spring to close it.

               As for the smoke system personally I would'nt go there, you pay a big weight penalty both in the system and the oil the model has to carry, and in real terms this is a relatively small model your proposing (in petrol model terms). Secondly the mess the oil, usually diesel, makes is horrendous usually resulting in the covering lifting etc and no the red dye in Ag diesel will not give you red smoke.

Aero12016/06/2008 13:28:00
131 forum posts
Thanks for your comments Ultymate, Yes it would be a shame to loose the benefits of a cleaner petrol engine by adding  oil into the exhaust!
Simon Chaddock17/06/2008 01:57:00
5483 forum posts
2878 photos

Hi Paul

I think you will find that compared to glow the mixture with petrol is much more critical. Methanol (and even more so with nitro) burns over a relatively wide air fuel ratio and the glow is glowing all the time. Petrol only burns over a relatively narrow range and it must be correct just at the time the spark er sparks!

Removing the throttle stop screw does rely on the accuracy of your servo to maintain the idle speed and not to close the butterfly just that bit too much. A better engineering solution would be to retain the throttle stop and allow a bit of servo overtavel to actually cut out the ignition - just like my lawn mower!

I agree that pulling against the throttle spring is much safer. If anything breaks the throttle closes!

I hope this helps 

David Ashby - Moderator17/06/2008 05:14:00
10922 forum posts
1680 photos
610 articles

Paul - I've just posted a petrol aerobat review on the front page - there's a good pic of the motor set-up too.


Aero12017/06/2008 17:33:00
131 forum posts

Thanks again for the replies. I'm having a go this evening at making some alloy standoffs similar to those in the review pic to move the engine forward 24mm to where it belongs. Is it worth fitting a micro servo to operate the choke?

 As regards the throttle spring, the manual definately states to set it to push due to some harmonic effects, but I'm sure a pulled cable is less likely to resonate rather than a pushed one. If a trailer or caravan starts to resonate when being towed you accelerate to pull it out, not brake.

Doug Ireland18/06/2008 02:44:00
2088 forum posts
42 photos

Hi Paul,

Sounds like the carb on your Zenoah has the same configuration as the one on my MVVS (Evo) 45; push to open and pull to close. I use Bowden Cables for my throttle linkages and as long as the outer is as close to the throttle linkage as possible and thereby keeping the exposed inner cable as short as possible, I can't see a problem. It also appears you have the same ignition system as mine, I too use a Choke Servo (controlled by the ignition) but it is a standard size; I found that a micro would stall trying to overcome the ball/spring that keeps the choke butterfly open.

Aero12023/06/2008 11:30:00
131 forum posts
Thanks again for all the advice and information. Does anyone know if it is possible to rotate the black plastic fuel line connector? Mine points sideways and I would like to point it back towards the tank, but it seeems quite solid and I don't want to force it and break it!
Doug Ireland23/06/2008 12:31:00
2088 forum posts
42 photos
Hi Paul, Can you post a picture of the carb? It may be possible to remove the side plate on the carb and rotate the whole thing to the required position.
Aero12024/06/2008 00:26:00
131 forum posts

Here's pic of the carb

Doug Ireland24/06/2008 09:40:00
2088 forum posts
42 photos
Ah ok, its not quite the same as mine. I would not like to try and force that pipe round, looks like you're stuck with it as it is.
Aero12024/06/2008 10:31:00
131 forum posts
Thanks anyway Doug. It does seem to be set in that position and several photos I've seen show it in that position too. I'm reluctant to undo the side plate to investigate as I assume there will be delicate springs and diaphrams and associated witchcraft inside thats best left alone.
Aero12026/06/2008 11:26:00
131 forum posts
I can and have rotated the fuel inlet. If anyone wants to try it, then its best to get a spare one first just incase! I asked at the shop and was advised it was possible as long as you are prepared to replace the inlet pipe if it breaks. The pipe is a black plastic 90 degree elbow moulded over a brass tube. the brass tube is a push fit in the carb body, but is knurled at the other end over which the plastic moulded. I was lucky, got hold of the plastic with a large pair of pliers and it just swiveled to the angle I wanted!

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