AA5BY

The Ryobi was given a carb change to another with the same 20/64 intake but not having a prime tube sticking into the venturi a mm or so. It brought rpms up from 7300 to 7500 with the APC 18x6W prop so the limit was hit with that prop.

A Vess 19A was fitted with rpms of 6500 with the idea of now seeing if they can be increased by use of a larger carb. The 20/64 carb pulled from the junk box and now on the engine looks like it could be easily bored larger. Is doing so feasible or are the throats closely tuned to the fuel circuits?

I'm willing to sacrifice this carb in part because it was off a side carb engine and does not have a throttle linkage hookup. One would have to be brazed on. Do-able of course, but would be a pain. Do you suggest giving it a try at 24/64 and check for rpm gain or loss in control?

av8tor1977

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Duplicate post.

av8tor1977

My Feedback: (6)

Duplicate post.

av8tor1977

My Feedback: (6)

You could give it a try. The needles should be able to compensate for the mixture change if the change isn't too large. The midrange metering is fixed, but these engines tend to run rich in the midrange anyway. Right in the venturi where you would be boring it out is a fuel discharge orifice, so you'll have to be careful that your boring doesn't mess that up or accidentally close it off with a chip or burr.

All in all it would be best to just get an 11.1mm carb, but it never hurts to experiment if you have the time and inclination and are willing to accept the occasional failure.

Side note: There is no primer tube in the throat of a Walbro carb. Any brass that you might see in some Walbro carb throats would actually be a high speed check valve, which in reality is a good thing to have as long as the carb is suitable in other metrics.

For an explanation of how the primer system works, look in the "Gas Engine" forums at the "Sticky" for Newbies for my post on that. I believe it is post # 38.

Yeah, post # 38 here:

http://www.rcuniverse.com/forum/gas-...rmation-2.html

AV8TOR

AA5BY

Ahhh.... you jogged this aging memory about how a primer pump works... I knew that once... and should have remembered. The check valve is a new thing not seen previous.

I pulled a 24/64 ths carb off of an unused 28cc engine and have it installed for a test run tomorrow to see if it will crank out a few more rpms with the 19" prop.

Again... thanks for the tutorial.

AA5BY

RPMs dropped 150 with the 24 throat carb compared to the 20. The 24 has a choke plate whereas the 20 has a clean venturi, which might be the reason for the drop..

It tuned easy and ran well.. Hot restarts required one flip. Acceleration was smooth and the deceleration was the best of the three carbs... having a very smooth return to consistent idle from WOT. Full power was rock steady.

Of the three carbs so far, even though a few rpms less, this seems the best due to consistency and smoothness of all parameters of running. And... it has a needed choke.

I may have to invest in a 28 throat carb.

av8tor1977

My Feedback: (6)

Having a good quick drop to idle sometimes necessitates having the high speed mixture correct, definitely not lean, and the idle mixture just a touch rich. A slightly rich idle doesn't idle quite as smoothly as a near perfect mixture, but it is more reliable overall, aids in throttle transition, engine cooling after a high power run, easier starting especially cold, and a fast drop to idle. Many times guys attempting to tune out a midrange burble, (which CAN be annoying), will end up with the idle mix set too lean. This can not only cause a slow drop to idle, it can result in what we call a "high idle hang" whereby the engine refuses to idle down for a long period. There are other tuning tricks to get rid of a midrange burble, but probably the easiest and most effective is to use a higher prop loading. If you have ever heard a properly tuned chainsaw burble somewhat when revved, but then settle into a perfect clean two stroke sound when put to work cutting, this will make clear sense.

AV8TOR

av8tor1977

My Feedback: (6)

Forgot to mention this.... I don't know if you are working on a cowled engine installation or not. Some guys cut a small rubber ball in half, and glue it to a popsicle stick to use as a choke on Ryobis.

On anything of mine that is not cowled, I use a modified starting technique from what most people do with a choke. I hold my thumb over the carb throat with the ignition off, and flip the prop over until my thumb gets wet with fuel. Then I turn the switch on, and the engine starts on the very first flip nearly every time. What we are actually doing with the normal start procedure whereby we start it with the choke on, let it quit, open the choke and then flip till it starts, is flooding the engine. That is why it takes numerous flips after the choke sequence to get the engine started.

AV8TOR

AA5BY

Good tuning observations. What has gotten me several times is the effort to lean the LSN just short of spool up stumble and then a weather change has it stumbling. Consequently for much of the weather, the LSN has to be a little fat. It is really nice to find a sweet spot rich enough to avoid the weather fluctuations but yet not so rich that it wont restart when hot with a single flip, rather than several to clear it out.

A subsequent run session after my last post, has the RPMS within 50 of the carb that has no choke butterfly. I'm good with that... the intended plane is a Fly Baby so 50 rpms is not going to be much issue. If when flying, a slight bit more power is needed, a #28 throat carb can be considered.

I've two things left to do... at which the engine will be plane ready... soldering the choke hole and affixing a prop bolt into the hub for a nut, rather than the bolt screwing into the aluminum hub, where threads might go bad over time.

Again... thanks greatly for your good guidance in getting this engine air worthy.

av8tor1977

My Feedback: (6)

You're more than welcome. It's been good to see some action in this forum again. Please keep us updated on your nice project. And that is a VERY good idea about your prop bolt. Install a stud as you said, and I highly recommend using Loctite on it.

If in the future you do feel you need more power, there are some things we can do. I'll be glad to help. I think you will like the Ryobi. Frankly, I laugh at people that "look down their noses" at conversion engines.

AV8TOR

av8tor1977

My Feedback: (6)

Just happened to think just now as I was checking for new posts....

On some of my engine builds, I have done an internal "clean up" of the carb, using a Dremel. Sometimes there is a casting ridge in the venturi that can be carefully smoothed with a small sanding drum. Then I grind the staked part of the butterfly screws smooth, and round/smooth/blend the heads of those same screws. A drop of green, wicking/penetrating Loctite can be applied to the screws for peace of mind when doing this. This can make the carb flow some more air, without resorting to a larger venturi.

AV8TOR

AA5BY

Thanks for the heads up. When soldering the choke hole closed, a look will be given regarding the choke butterfly screw and for ridges.

av8tor1977

My Feedback: (6)

Actually, I pay even more attention to the throttle butterfly screw. Its location in an area of smaller area/higher velocity makes smooth airflow even more beneficial. When you take a close look at it, you will see that the head and exposed end of the staked screw actually occupy a fair percentage of area in the carb throat, especially on carbs this small to begin with. Smoothing both the throttle and choke screws should get you a measurable gain in your case. If it happens that it doesn't, I would then suspect that a low exhaust port timing would be limiting power increases. Some of the Ryobis had quite low exhaust port timing. The thinking most likely was, that by utilizing a quite small carb along with a low exhaust port timing, the engine would tend to rpm limit itself if/when the string broke on the weedeater, thus preventing engine damage.

AV8TOR

AA5BY

That makes sense about the RPM limiting. I'll clean up those screws perhaps this afternoon and run the engine again.

Not great luck on the stud... the bolt place said they could order some all-thread but I hate to buy a 3' piece, so settled for a 3" bolt that I'll try to chase threads on the other end and if successful it will work for the stud.

The morning job that has extended into the afternoon is replacing glow plugs on my old work truck which hasn't wanted to start lately. All but that proverbial impossible to get to back right are replaced (all of which were not working). I was getting some hits so it might be that back right one is the only one that was functioning... at any rate I'm not pulling the engine to get to it.

av8tor1977

My Feedback: (6)

It generally doesn't work well to make a stud out of a bolt by trying to thread it farther, as the shank of most bolts is a bit larger than the major o.d. of the threaded portion. Using all-thread might be ok, but some of that is quite inferior quality, and it's rarely very straight either. (Very evident when you chuck a piece in the lathe.) I've been lucky in finding studs at well equipped Ace Hardware stores. (Some carry a larger selection of hardware than others.) Another choice would be to find a bolt with a long enough threaded portion to use as a stud, and cut it. I suggest using a grade 5 bolt; not the "common" or grade 2 general purpose bolt.

Good luck with your truck!

AV8TOR

av8tor1977

My Feedback: (6)

Just happened to think about diesels... Ether is a bit hard on them, though sometimes necessary. Try this some cold morning. Just lay a gas soaked rag over the air intake. Works great!

AV8TOR

av8tor1977

My Feedback: (6)

I was going to dig out my spreadsheet for calculating prop thrust for you on the other computer, but then I found this one online:

http://godolloairport.hu/calc/strc_eng/index.htm

I took a guess at the Vess "A" prop of yours having about an 8" pitch. That may or may not be an accurate assumption.

Anyway, using your posted numbers, I came up with 17.5 lbs. of static thrust for the APC (W) prop, and 15 lbs. for the Vess. Don't pay much attention to the speed numbers; mathmatical calculations don't reflect the "lift" vector of the prop and aircraft speed is always faster, (usually quite a lot), than the mathmatically predicted result based solely on pitch and rpms. (Besides the "lift" vector, another factor is that the prop unloads in flight, rpms rise, and the pitch speed increases, also throwing speed calcs "out the window")

I suspect the predicted thrust for the APC prop is somewhat close, while the result for the Vess might be a bit lower than reality. The Vess might quickly unload a bit in the air, and provide good performance. As always, only flight testing will "tell the tale."

For whatever it's worth....

Edit/Add: My guess at the Vess pitch might be close. I just ran your numbers for both props again, and in BOTH cases the required horsepower was the same, somewhat validating the results.

AV8TOR

AA5BY

The Truck jumped to life after installing the new glow plugs. Yeah, ether is a no no on the GM 6.5 though I don't know the reason. I don't use the truck often since retiring.

AA5BY

You were correct.

My plan was to cut the head off and turn the shank down to thread OD and thread that end enough for the prop nut, leaving the middle full 5/16 for the prop hub. Chucked it, cut the head off, and it spun it up and it is too far out... it is not suitable for a prop hub, my guess is it has about 3-5 thousandths wobble..

I'll look around a bit and if finding nothing, will use a piece of HSS, cut the threads on the lathe and heat treat. .

AA5BY

It was our first good cool day of fall here and hence why doing the truck project that has been awaiting such weather. In fact, two truck projects had been on hold waiting cooler weather. The other was my every day truck, which had an air handling mode actuator problem... and it is under the dash so not fun work. Turned out the actuator had a broken nylon tooth on a gear.

AA5BY

Per the advice to examine the carb throats for ridges...a discovery was made. I'd previously reported that two 20/64ths carbs had been compared and one produced 300 rpm greater performance. Well, I see now that that one with the better performance had been drilled out to 22/64ths. The drilling marks were clearly visible and that carb had no ridge. That carb came from a John Deere weed conversion I did many years ago without recalling.

I do see what you note as a casting ridge in the throat as well as screw shanks protruding on both choke and throttle butterflies.

Thus, the 22/64 without choke or throat ridge outperformed the 24/64 with choke and throat ridge by a very slight margin. I'll get the 24/64 carb cleaned up.

av8tor1977

My Feedback: (6)

Sounds good.

Ugggh, dash work. Hate it! I worked at a hot rod shop once, and I always breathed a sigh of relief when dash work would come in and the boss would say in a loud angry voice, "Get that thing out of here and take it to the DEALER!!!" "We DO NOT do dash work here!!"

AV8TOR

av8tor1977

My Feedback: (6)

Back to carbs.... I wonder just how far it would be feasible to bore a carb out. I have a bunch of small bore carbs lying around gathering dust. I only keep them in case I need to rob a part, but if they could be bored to 9.5 to 10.5, they would be usable on some of my smaller engine builds. Heck, a while back a friend of mine gave me 4 brand new 8mm carbs.

AV8TOR

AA5BY

I did get a stud made this morning using a 3" grade 8 bolt that ran true. It of course had threads on one end so required an inch worth on the other end which were cut on the lathe. Cutting the threads was a bit tedious in the cramped quarters but was successful.

av8tor1977

My Feedback: (6)

Nice. So, the engine is approaching being a "done deal." How's the plane coming along?

I went to the Tuscon Aerobatic Shootout today. Suzuki Yuhei, the Unlimited Freestyle pilot from Japan crashed his plane. I happened to be in the "pits" while he and his team were stripping out the broken bird of the engine and radio gear. Then he and the team all signed it, and GAVE it to me! I was just thinking of it being a "Man Cave" decoration/souvenir, but when I got it home I layed it all out, and HEY, it's definitely rebuildable! I've rebuilt much worse crashed birds, so I'm going to rebuild this one, and of course preserve all the autographs. How cool is that??

Then, later on the team from Mexico had a crash, and I scored two perfectly good wings, (also autographed), from them!!

Awesome flying, fun day! Some great videos here:

www.tucsonaerobaticshootout.com

AV8TOR