Author Topic: Model I.C. Carb with Throttle  (Read 17611 times)

Offline Johnmcc69

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Re: Model I.C. Carb with Throttle
« Reply #60 on: March 20, 2014, 05:33:50 PM »
AWESOME!! Congrats Brian & Chuck! You two need to team up & create a Rupnow-Fellows or Fellows-Rupnow design engine.

 John

Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #61 on: March 20, 2014, 06:24:27 PM »
If I have done this correctly, this link should get you all of the drawings as .pdf files.---Brian

http://www.mediafire.com/download/cjgmg9kj2vcgy70/RUPNOW-BRITNELL_CARB.zip

Offline cfellows

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Re: Model I.C. Carb with Throttle
« Reply #62 on: March 20, 2014, 09:03:13 PM »
Great test, Brian!  Needless to say, I'm pretty tickled with the results.  While watching your progress on the Britnell carb, I've also been doing further testing with my original design, although I haven't posted anything.

I learned a couple of things. 

First, the rotational position of the needle valve assembly is very touchy.  The fuel aperture into the air passage should point directly at or slightly in front of (away from the engine) the throttle screw.  This makes a considerable difference in the way the carb performs. 

Second, it turns out the problem I was having wasn't the fault of the carburetor, it was the ignition I was using.  As you recall, I was testing the carburetor on my vertical single and I hadn't really shaken all the bugs out of the engine itself.  So, with a proper working ignition installed, the results I'm getting are much better.  I built another carburetor for the vertical single, sticking to my original design, but I made the venturi 5/32" instead of the original 1/8".  I also used a 10-32 throttle screw instead of the 8-32.  This carburetor works quite well and I'm happy with it.

I'm still using my first carburetor with the 1/8" venturi on my plumbing parts engine and it runs flawlessly.  It will idle down to a very slow speed and never miss a lick.

So, in summary, I was a bit quick on the trigger to abandon my original design and your test seems to confirm my later results.  I think both your carb and mine will satisfy most needs for slow running model engines.

Thanks for all the work you put in on this, particularly in documenting your build and in running the comparative tests!

Chuck
So many projects, so little time...

Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #63 on: March 20, 2014, 10:55:47 PM »
Chuck--It's been fun. In the overall scheme of things, you and I seem to approach these things in similar fashions. I hope that between us we have provided the small i.c. engine community an alternate to model airplane carbs. Many thanks to you for your input, and many thanks to George Britnell as well. I also fall into the trap of "My engine isn't running right. How do I figure out if it is carburation or ignition." That was the main reason I went to electronic ignition on my Rupnow hit and miss engine. Inconsistent ignition that I simply couldn't track down.

Offline gerritv

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Re: Model I.C. Carb with Throttle
« Reply #64 on: March 20, 2014, 11:17:04 PM »
Excellent conclusion to a journey.

I read in a 1900's book on carburettors that a blunter angle on the needle results in a more linear increase in passage size. Given the overall size (tiny) of the components, a finer thread to control the needleis not in the cards, perhaps a slower taper will do the needful?

Gerrit
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Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #65 on: March 21, 2014, 12:07:29 AM »
My carburetor WAS made with a finer thread. 40 threads per inch. I'm not really sure if that makes any significant difference or not. Other carbs I have built used a 32 threads per inch pitch on the needle.

Offline gerritv

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Re: Model I.C. Carb with Throttle
« Reply #66 on: March 21, 2014, 12:56:35 AM »
Hi Brian
I acknowledged that, which is why I suggested using a slower taper, it will give finer control over the increase/decrease in the open area of the orifice.
What I am quoting is from http://books.google.com/books?id=0dY3AAAAMAAJ, page 73. with a straight taper "the increase in the area of an orifice is not proportional to the movement of the pin". E.g. IIRC Weber DCNF idle adjustment needles are a quite shallow taper, resulting in finer control over idle mixture (I had 8 to adjust on my car :-( )

Gerrit
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Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #67 on: March 21, 2014, 12:22:04 PM »
Alright Gerritv--I know what you are talking about, and I am sure that is why many carburetor builders use sewing needles when they build carburetors, because of the much more gradual taper. The carb with the 20 degree included angle taper IS very sensitive.

Offline cfellows

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Re: Model I.C. Carb with Throttle
« Reply #68 on: March 21, 2014, 03:37:35 PM »
FYI, I have uploaded an updated version of my carburetor drawings to the plans and drawings section.  The latest design eliminates the air bleed and increases the venturi diameter to 5/32" and the throttle screw size to 10-32.  I've also increased the OD of the needle jet assembly to 5/32", making it less likely to break during assembly.  Finally, I included a simpler way to make the through holes in the jet tube. 

Based on further testing that I've done, I'm very pleased with his carburetor design and plan to use it for all future engines.

Chuck
So many projects, so little time...

Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #69 on: March 22, 2014, 10:39:24 PM »
Okay--Small things amuse small minds. (Nah, that isn't really true). We've got 6" of new snow here and its still snowing, so I decided to play indoors today. I wanted a throttle handle on Chucks carburetor, so today I built one. Now if you immediately ask "Why not just drill a cross hole through the head of the #8 shcs that acts as a throttle?", there is method in my madness. We don't really know what rotational position the head of the screw will be in at the desired low rpm. I wanted to put a stop pin in for the throttle lever to stop against, so I had to be able to adjust the position of the lever in relationship to the position of the #8 capscrew.



Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #70 on: March 22, 2014, 10:44:44 PM »
If you look close, you can see that I just had room to drill one corner of the carb body for a 1/16" diameter "stop pin" that I Loctited into place.



Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #71 on: March 22, 2014, 10:46:45 PM »
And here are the sordid details of how I made it----





Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #72 on: March 22, 2014, 11:02:51 PM »
This is a view of the carb from the far side, which shows the "stop pin" for the throttle arm. I just had enough room to mount it in the corner of the carbs rectangular body. The throttle arm is in the "fully open" position in this picture, but when it swings up against the pin and stops, that is the idle position.



Offline gerritv

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Re: Model I.C. Carb with Throttle
« Reply #73 on: March 22, 2014, 11:34:32 PM »
Classy industrial design....

Gerrit
Don't confuse activity with progress

Offline cfellows

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Re: Model I.C. Carb with Throttle
« Reply #74 on: March 22, 2014, 11:51:19 PM »
Nice, Brian.  As it turns out you really don't need more than about a third of a turn to go from idle to scary fast.

Chuck
So many projects, so little time...