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Ryobi 31 carb mismatch - 11/25/2012 1:16 PM   
brad59


 

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Status: offline 		I have a converted Ryobi 31 that I bought from a man who did conversions a few years ago. Can't remember his name but it was at the West Coast IMAA meet at Castle Airport. He told me that it was test run and had lot of mods to make it better, including battrery ignition. The price was good so I snatched it up. It only had a single mixture screw on the carb so I asked him if there was a better carb and he told me yes, and he would send it to me. And so he did, and I put it in a bag until last week, but now it's in a plane. It wouldn't draw fuel so I checked the tank, lines, etc. then pulled the carb and checked the screen and pump diaphragm. What I found was the the transfer port for the pump wasn't drilled though and that this carb was designed for an external pressure hose for pump pressure pulse. Sooooo, I have the wrong type of carb apparently.
My question is.....what are my options? The original carb is in storage in the USA and I can't get it because I'm overseas. If I have to replace it, what carb should I buy and from who?
This carb is a WT-340 with a 14mm throat at the butterfly. From what I have read, 11mm is best.
Can an external pressure tap be added on the plastic adapter at the rear of the engine?
Any help will be greatly appreciated!
Brad

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RE: Ryobi 31 carb mismatch - 11/25/2012 1:35 PM   
earlwb


 

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Status: offline 		I have added a fitting to several carbs to allow for a external pulse pressure feed to the pump. A 8x32 Dubro fitting works well, but other fittings would work too. Depending on the fitting you might have to file a little off of the threaded part to have it come out flush on the inside and not touch the pump diaphragm.  If the carburetor mount block permits it, you can drill and tap a hole there someplace to match a carb's pressure hole in the base of the carb too.

Here is one where I used a 8x32 Dubro fitting on the pump cover.


Here is one setup with a right angle fitting modified from a boat engine fitting.


Here is a SV engine setup with a external pump pressure feed line. It is the black rubber tube running from the back of the engine to the front of the carb where the pump cover is.


Here is a example on a engine of using a external pulse feed to the base of the carb to match the pulse feed hole on the base of the carb. Quite a few engines do something like that too.





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RE: Ryobi 31 carb mismatch - 11/25/2012 1:39 PM   
brad59


 

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Status: offline 		Hi Earl(?)
I should have mentioned that this wt-340 does have a pressure fitting on the side of the carb that is vented to the top of the pump diaphragm. I would need to pick up the air pulse form the case somewhere. This might be more simple than shipping a carb from the USA
Brad

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RE: Ryobi 31 carb mismatch - 11/25/2012 3:08 PM   
earlwb


 

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Status: offline 		Ok, that is easy to do. You just need to locate a convenient place on the engine, then drill and tap a hole for a pressure fitting.
Granted in some cases it may not be as easy as removing a backplate to drill and tap a hole in it. But as long as the hole leads to the crankcase bottom inside, then you are in business.  Your engine likely has the heat isolation block on it and that block should have a hole in the crankcase matching it. You can drill a small hole to meet up with the hole in the block and then drill and tap that smaller hole for a fitting. You could likely do a light press fit of a length of brass tubing into the hole and use it for a fitting too.




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RE: Ryobi 31 carb mismatch - 11/25/2012 3:37 PM   
flyinwalenda


 

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Status: offline 		The WT-340 is a stock carb for ryobi's in trimmer form and  it should have close to an 8mm venturi and it does have both  high and low speed needles but they are typically covered with caps. If it's not drawing fuel the float needle could be stuck and the main needles and passages could be gunked-up. May want to completely disassemble the carb , soak it in cleaner, blow out all of the openings and reassemble it. Careful with the float so as not to bend the tang(changing the float level) when handling it.

I have used larger carbs with excellent results. If you can find a WT-324  or look for a WT-891 as they both have 11.1mm venturies and there are others out there as well but don't go too large on the venturi size.

If you want to use a remote pulse port you can drill and tap the case of the engine and install a fitting.


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RE: Ryobi 31 carb mismatch - 11/25/2012 5:22 PM  1 votes
av8tor1977



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quote:

ORIGINAL: brad59

Hi Earl(?)
I should have mentioned that this wt-340 does have a pressure fitting on the side of the carb that is vented to the top of the pump diaphragm. I would need to pick up the air pulse form the case somewhere. This might be more simple than shipping a carb from the USA
Brad


Slow down a minute guys. I have never seen a Walbro carb that had a pulse fitting on the SIDE of the carb. I think he is talking about the primer bulb fitting which of course, won't work. If his carb has a pulse fitting, it should look something like the second picture above; a fitting in the fuel pump cover, not the side of the carburetor.

Can you send us a picture of the carb?

Other than that, yes, you can drill and tap anywhere on the crankcase or back plate for a pressure pulse fitting. Pick a thicker part of the crankcase to do so, and of course make sure it won't hit/interfere with the crank, reed valve assembly, etc. I sometimes make a pulse hole in the base of the cylinder into a transfer port. Good thick location, and not in the way of anything usually.

Another option would be to just go ahead and drill the carb passage out and use the original Ryobi pulse hole...

And finally, as mentioned, the WT-340 carb is really too small. It has an 8mm venturi and the Ryobi will do best with an 11mm venturi. The size of the carb is determined by measuring the smallest diameter of the carb throat. Walbro carbs also have a number cast into them near the choke butterfly location. That number is the size of the venturi in 64's of an inch. For example a "28" would be 28/64" which equals .4375" which equals 11.1 mm, which is what you want.

AV8TOR

< Message edited by av8tor1977 -- 11/25/2012 6:49 PM >

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