You are right.
Some Magnetos have a rev limiter built into them.

If thats the case then it should turn the 18x6 the same rpm as the 14x8.

To me 7800rpm is plenty. I fly around at 1/3 to 1/2 throttle 80% of the time anyway.
But I want the power there if and when I need it.

it would barely fly at full throttle. i thought about a rev limit guys, but all the rev limiters i have seen i could hear them cut the fire and sputter, i guess maybe it could have been doing that and i just didn't hear it in this particular case because of the prop?

i can't get a 18 today, so i am going to try a friend of mine's 16 it is either that or a 20" and i don't think that would work with my landing gear.

ok i think i have figured out the problem!! the high speed needle was broken, instead of breaking the plastic i broke the needle itself so all this time i couldn't figure out why when i turned the high side nothing happened!! i pulled all the plastic out and the top portion of the needle, now all that is left to adjust is the little bolt that is deep down in the needle recess on the side of the carb, i can see it, but i can't figure out a way to adjust it..

today i tried a 15x8 prop at a max of 7200rpm and then a 16x6-10 at around 6000rpm max.. all of this was done by adjusting only the low end, if only i could figure out a way to turn the remaining piece of bolt down in the carb on the high side i think i would have a great engine! regardless, i got the big turd to fly here are some pictures of my first scratch built no plans spad with my first weedie conversion as well.

btw, if anyone can think of a way to turn the high speed needle please let me know asap! thanks!

got a different carb on today with an adjustable high and low needle, but the engine only made 6000rpm with a 15x10 prop.. does it need a new ring, or is that rpm about right for a stihl 30cc engine? it seems to be a little weak to me, where is a good place to get a ring for this engine for a decent price?

also the engine wasn't real reliable, it would run great on the table and then i would get it down on the runway and it would die half the time, the other half it wouldn't make full rpm, but every once in a while it would run up strong, it would fly slow, barely enough power to fly and run fine the entire flight and landing and then taxi back, then i would kill it, put it on the table, top off the tank and fire it up again only to find it not running right!! i spent about 5 hours working with this engine all day today, and it only was dependable about 3 or 4 times, the rest of the time i was working on it! i spent a very frustrated afternoon, could low compression cause this unreliability? the engine has to have some compression to turn the 15x10 6200 rpms and the 16x6-10 6000rpms, but this is barely enough power to pull this plane, i would be fine with that, what i am not fine with is the unreliability of the engine!!! any suggestions?

I would say you got a bad Carb.

A Stihl chainsaw should turn 10 to 12,000 rpm.

Does it feel like it has good compression?
Can you turn it over easily by hand, or does it get real hard to turn when the piston
comes up on the compression stroke?

I have a US 35, which is a low power engine.
It does run strong and I run an 18x8 and or an 18x6.

I still can't find my tach but I have tuned by ear for years and it turns those props with authority.

There are several things you have to keep in mind when you start swapping carbs. I always put in a gasket set so I start with a clean setup first. The diameter of the throat is pretty important. Too narrow and your engine wont put out all it can. Too big and it just bogs down (maybe, never had one too big). Would the needle on the second carb work in the first carb? Just a shot in the dark. What do you think Ralph?
Edwin

needles are not interchangeable, i swapped the diaphrams and tried the old carb's diaphram and the new carb's.. i guess i will go by the stihl dealer and see if i can get a carb rebuild kit. would that be where you guys would start or should i try to get a new ring first?

Just my opinion, but the ring would be the last thing I would try. Other opinions may vary. I get my carb kits from the local yard equipment dealers. My 10 year old wore out ryobi still did great at 6700rpm on TF 18x8. At least until it hit the power lines. I though it was shot out of the air. Bent the crank on that one, but I have 5 more in storage and one waiting for a plane.
Edwin

well unless it is the carb inlet size, i know i have gotten maximum power from this engine and the figures above are the maximum output i have gotten, the engine leaned out and ran smooth and at it's strongest at those rpms. so i don't think (without going to a larger carb) that i can get any more power than that from this engine. so i think it definitely needs a ring, and then maybe it needs a carb rebuild kit. that is just my opinion though, i don't think that more air and fuel is going to cure this engine from it's slouch.. that is if this engine is supposed to have more power than it is showing, or maybe you guys with other 30cc engines just have other brands that are better engines?? it either needs a new ring to increase the compression to make it turn an 18" prop effectively or it just will not turn one...?

To me it sounds like the diaphram in the Carb. is getting stiff.

Runs good one time and not the next.

The most you have got out of this engine is 7200 and now you are saying it's leaning out at 6000rpm.
By the way adjusting the low needle as little affect on the high needle byt the high needle does affect the low needle.

There is something wrong with the new carb or your tach.
What numbers are on your Carb? Especially the butterfly.

You did not address my question about the compression.
"Does it feel like it has good compression?
Can you turn it over easily by hand, or does it get real hard to turn when the piston
comes up on the compression stroke? "

A video would be nice. You can only judge so much with words.
Sight and sounds are really helpful.

oh im sorry ralph.. it feels like it has ok compression, however, my tower hobbies starter will spin it pretty fast when it is hooked up to a large 12v battery. it has to have decent compression to spin the props in that rpm range, also i just meant that when i lean the needle out slightly that is when i get the highest rpm.. and the 6000 figure is with a 15x10 the 7200 was with a 15x8 but it seems like a jump so little as 2 numbers in pitch shouldn't change that much, i think the engine is just wearing out and a new ring should help that.. what do you guys think about that much of a drop in rpm just bc of the pitch?

Just be sure about somthing,are you sure this is a 30cc.The numbers your stating are what my echo 22ccs run with a 15x8.Also,on page 2 you said something about a clicking noise when the piston comes up,this might be a cracked or broken ring hitting the intake or exhaust port.One other thing you mentioned was crankshaft play,was this end play or was it play at TDC when you rock the hub or flywheel back and forth at TDC.If you have more than 1/4 inch of play,rocking motion at TDC,and the piston isnt moving,then you may have bad rod bearings top or bottom.End play on a crankshaft is fine and most all engines have a little most of the time from the roller bearings.I bought a new tach and ill put a CF 18x8 on my 28cc stihl and see what it does.

im sorry for that confusion.. the clicking i was hearing: the flywheel was loose on the shaft and when the magnets on the flywheel met the ignition coil, magneta whatever it is actually called they clicked against it making it feel unnatural, i didn't investigate at all and i just thought about it while i was typing so i wrote it without looking into the matter.. anyway, there is no play in the crankshaft whatsoever, the engine sounds solid and smooth while running so i think the only problem is just the compression and/or the fuel delivery.

OK,thanks for clearing that part up with me.Now,are you sure its a 30cc?The only reason im asking is i bought an engine from a guy that evryone told him it was a 27-31cc engine.I guess he thought he couldnt use it.I bought it after i asked him a few questions.I was the only one that bid on it.It was a BNIB sachs engine made in germany.2 rings,B&B specialties muffler,ect..production engine.Turned out to be a 61cc.The guy never checked for himself.

it is a 30cc, it seems i have fixed it, at least the reliability part of the equation! changed the diaphragm from the old carb to the new one that is on the plane now and viola, she holds a dependable 6300 rpm on a 15x10, i can't wait to get a 16 or 17 inch prop on there with a lesser pitch, i would love to try a 17x6 or 7.. if my landing gear would stand up high enough to allow for it i would love to try a 18x6.. if the engine is turning a 15x10 only 6200rpm, what would you guys say would be the best prop for this engine? i added about 2 ounces of 15% nitromethane to 1/2 gallon of gas tonight and plan to try that out tomorrow to see if maybe that will increase the power output.. it should do so in my opinion, and safely, because of the castor oil that the nitromethane will bring to the gasoline. what do you guys think about that? will it be as safe as i imagine?

On props the basic rule is drop one add one.
It's a ball park method.


15x10 = 16x9 = 17x8 =18x7

You never did answer my compression question.

try this 1 part glow fuel to 2 parts Hi test gas.
Start off with 10/10 glow and go from there

Just out of curiosity what muffler are you using ? A restrictive muffler will greatly reduce rpm's. Stock mufflers are very restrictive................

i am just using the stock muffler, but i pulled the screens out and now all that is left is the hollow baffle with holes all in it, couldn't be too restrictive.

Ralph, i don't know which compression question you are speaking of, could you please ask it again because i can't find a question that i did not answer.

if this is the question you are referring to:

"Does it feel like it has good compression?
Can you turn it over easily by hand, or does it get real hard to turn when the piston
comes up on the compression stroke? "

i answered it in post #61 yes it gets hard to turn when the piston compresses. i would guess from experience around 60-70psi.. i don't know what it is supposed to be, but i would wager that this engine is a little low on compression.

OK,just checking.Its true about the muffler that antique stated.Even the baffles reduce flow.Thats why pitts are hollow,also why there louder.The baffles mix and churn the air to reduce the poping effect.Just for a reference,i just converted a shindaiwa 27,used,but it said 140lbs? compression on case.Im at 3000 MSL,i used a G23 carb rotator that fits ok.I used a 11mm carb,2 needles,stock muffler.I am spinning an APC [email protected] ill do tomorrow is put my G23 muffler on it and see if it helps any.[Its hollow].60 -70 lbs is a little low.

well guys, yesterday was extremely fruitful! with the 2 ounces of 10% nitromethane in the 1/2 gallon of gas/oil premix i gained a whole 500 rpm's! then i pulled the crimp off of one side of the muffler and pried it open and pulled out the baffle to the tune of 300 rpm gain so i am now sitting at 7000 rpm's wtih a 15x10.. i thought i was going to have to deal with this engine being weak so i went ahead and ordered some props for it about three days ago, and since i thought it would continue to be weak i ordered 16x6's! so if the engine is at 7000rpm with a 15x10 what is it going to do with a 16x6 add into that the fact that the 15x10 was a k-series prop by MA and the 16x6 i ordered is an apc.. i don't think i will be able to use the 16x6's they are going to allow the engine to over rev probably.. does anyone know what the max rev is on a stihl 30cc? or i guess i could go by any 30cc trimmer engine's max rev.

I have seen engines that started well, idled well, and in general run quite well, but still gain almost a thousand rpms with a ring change....

Pretty much every used conversion engine I have ever taken apart needed a new ring for optimum performance.

AV8TOR

Av8tor,
Are those bowman rings you're using? Sure cant argue that point, mine are all near wore out by the time I get them. Never delt with bowman's rings. Will have to do that sometime.
Edwin

Yep, I only use Bowman rings. They work great.

AV8TOR

link me to the bowmans please.

what is the max rpm this engine should run?

Franks e-mail is: [email protected]