jetfire40

Has anyone had to change to a smaller carb on their diesel conversions to get the engine to run well across the full rpm range?

Ralphbf

My Feedback: (27)

It is not nessary to use a smaller carb.
Just don't let them open all the way.


Ralph

Hobbsy

My Feedback: (102)

Here are a couple of examples where I put smaller carbs on converted engines. The first picture is a friends Fox Eagle II .60 with an MVVS 7mm carb wide open turning a 12x8 Graupner three blade. The second is my MDS 1.48 conversion with a SuperTigre .60/.75 sized carb and my Fox Eagle IV .60 with a SuperTigre .40 sized carb with the fat spraybar in it.

Ralphbf

My Feedback: (27)

So Hobbsy, are you saying you get more RPM with a smaller carb, and if so how much more?

Bob Davis told me that stock carbs work great, but when you hit max RPM just don't open them any further.


Ralph

Motorboy

Dieselfuel are difference from Glow engine fuel. Dieselfuel has high calorie and do not need much air, also "more Energy" in fuel.

Stochiometric ratio in dieselfuel are about 14.7:1 and Glowengine fuel has 5:1, also Glow engine are drinking more fuel than diesel engine in same engine size.

Jens Eirik

Hobbsy

My Feedback: (102)

Ralph, it doesn't always work that way, when I put the MVVS 7mm carb on my MDS .78 the rpm went down from the stock carb rpm. I lost 400 rpm when going to the smaller carb, Mr. Davis says, as a general rule if you can run the stock carb at full throttle you prop is not big enough yet. So some more "sperimentin" is in order.

AMB

Most of the traditional diesels use a small bore carb and no muffer pressure which gave them a strong fuel suction from the tank my first conversions for mvvs and irvine using the stock carbs using the muffler pressure taps they do indeed run max at 75 to 80% open see the post on the irvine davis/irvine 53 conversion on his site paul landels saw this and flew the plane with this set up this
changed his opinion on pressure to the tank this was with the irvine jetstream stock carb my mvvs 40 and 61 are now using a small bore and run wide open with them draw your own conclusions Mr davis is of the opinion you do not have to spend the extra bucks
on an other carb running them at 80% or so.it may be easiler to set the needle with the smaller carb but I have had no issues with it
with either set up I do not see any difference, my stock irvine 40 diesels I added a pressure tap to the carb and the throttle response
seems improved I have no load up problems from idle
the small carb thing goes back almost 50 yrs on the old diesels that were non rc for control line and free flight ( which did and still works well) for this,Hobbsy is running pumped carbs on his larger engined ( see his post)

AndyW

My Feedback: (1)

I've been converting Norvels to diesel along with an Enya11CX. Some of my Norvels, (.06 and .075) had the barrels bored out a bit for more power as glow. No problems, I think, because I alter the engines to mount bolt-on sealed mufflers and this gives good pressure to the tank. All of these engines, when converted to diesel, run just fine on the next larger prop from glow even those with bored out barrels. The exception is the .06 conversion when I tried an 8 X 4. Ran well, idled nicely and transition was good. However, tilting the engine back, simulating a climb, caused instant flame out. The cure was to add an exhaust extension. This extra back pressure increased the pressure to the tank and improved fuel draw for nose up condition. Only a couple hundred rpm lost and no sign of overheating.

Pictured is the .06 in question sporting a custom made head.
Also, the Enya 11CX and a Norvel .15. Both run well with the stock throttle.
The Enya at 10.5K to 4K on a 10 X 3 Graupner
The Norvel at 12K to 4K on the Graupner.
Both idle solid with excellent transition.

Well, the durn pics won't load. Will try later.

Note that I'm using the stock throttles and am opening the throttles all the way and response is linear. All of these are modern ported engines with modern timing. I suspect that is why they run on the stock throttles. My fuel may also contribute with 40x40x20oil. Occasionally I mix with even more ether.