jack1933

My Feedback: (2)

Hello all You giant scalers. Would any of you be interested in the fact that your 100-210cc engines have the wrong carb. on them from the mfgs? I posted this on another forum, and there was NO response. Let me know, please.
Jack

aussiesteve

Go on - tell us more - I have a good reason to want to hear it.

I am currently involved in optimising the design of some engine lines for a company (No - not RCGF).

rmh

what carbs do you recommend ?
and why?
FWIW if there were any superior carbs on the market -why have they not been made known?

Tired Old Man

My Feedback: (1)

That plus the reed assembly was the demise of the first round of the BME 115. Took some time to get it corrected. Definately interested in hearing more on the carb subject Jack. Also would like to know what the grade of steel is that you are using on your reeds.

altavillan

My Feedback: (1)

I would like to hear more. I've managed to find better carbs for most of my engines by trial and error.

jack1933

My Feedback: (2)

Dick Its not about superior carbs. Walbro makes carbs for all the engines we use, as you know they are quality. The problem is the mfgrs. are not taking the time to check with Walbro to see if the carb they have selected is calibrated for the cc engine they are selling. I am refering primarily to the 100 to 210 cc engines. The mfgrs. are supplying a carb that is calibrated for a 100cc engine on 150-200cc engines. this is a fact, and anyone can get the same info from Walbro. The problem with a carb that is too small is this: One cannot get the engine to run rich at wide open throttle. Have fought this problem for years, with a 150cc engine with a Walbro wga-7, which in stock form is in fact calibrated for a 100cc engine. Now my question is : Why is it so hard for the engine experts to do a little research and install the right carb???Why is it so important to be able to richen the mixture at w.o.t.? the only way you can be certain that your mixture is not too lean, is to have this reference point when tuning your carb, and I think this would be very important to folks using 100-1 oil mix. the correct carb for 150cc engines is the Walbro wb-25, this may be ok for a 170cc engine too, the 200 cc engines should be ok with a walbro wb 45, of course this is just an opinion abiout the 170-200cc engines. The wb-25 is in fact the correct carb. for the 150cc engines

jack1933

My Feedback: (2)

Hi Pat the spring steel I got from M.S.C. is: type 1095 high carbon, rockwell hardness 48/51.
Jack

Ram Jet

I never had a Walbro carb apart but I probably will have soon. I picked up a 30cc Homelite line trimmer out of the garbage this summer. Your carb may be too small for your engine but I can see no reason why the high speed mixture can't be balanced. The small carb venturi may restrict the amount of income air but a simple increase in main jet size will balance your fuel mixture. You can achieve a balanced fuel mixture but maybe not at the volume you think the engine can handle. There are some advantages to an undersized carb in that the fuel will more than likely be better atomized by the increased airflow, the engine will be less sensitive to atmospheric changes and it will be easier to tune.

Bill

altavillan

My Feedback: (1)

There is a simple trick to get your engine to draw plenty of fuel at or near WOT. Just close the choke an amount needed to increase fuel draw. By use of a servo or restrict it's movment to not go all the way open. Perfect solution for the BME 115 engine that tends to run out of draw at WOT.

tkg

My Feedback: (1)

A carb that is to small will have a BETTER fuel draw. Because it will have a higher velocity at the venturi. It will have a better throttle response for the same reason. It MAY cause a power loss at WOT. The only reason that a smaller carb would cause a lean mix at WOT is if the jets are way undersized. With a new welch plug and my trusty micro jet reamers that wont be a problem.
P.S. Hogging out the jet under the needle won't make it rich. The JET (S) are in the venturi.
P.P.S. Hogging out the venturi is a bad idea too.

RTK

My Feedback: (1)

tkg,on some carbs all you need is to hog out the needle area (no welch plug), direct feed from under the diaphragm to needle and very large orifice to the venturi.

Jack,I agree the wb-25 is a poor choice for the 100cc class engine. I bet if you bought enough carbs from Walbro they would taylor one to your specific engine, or you could just pay to have them do that.

Truckracer

My Feedback: (19)

I would think carbs on our engines would be a bit small compared to ones on similar sized engines in an industrial application. First, we're running considerably lower RPMs and this by itself would reduce the fuel / air requirements. Also smaller carbs should produce better throttle response through the full travel of the butterfly. I've flown 50 sized engines with both large and small bore carbs and much prefer the smaller ones ..... as have become fairly standard on most of the engines these days. I'm looking for a smaller carb for a 4.2 Sachs based engine at this time for the same reason. And no .... I don't have any experience with the big 150cc and larger engines.

I know on a racetrack, a smaller carb is much more friendly to the right foot than a huge one.

rmh

As noted by many - a carb that is seemingly undersized is prefferable for most model use.
Unless one is running WOT all the time , the bigger carbs can be a real problem for trying to get smooth power band -and I am not talking "torque". Just getting a good blend of low to high needle transition under prop load suited to the engine
The BME 115 appeared to me to have an oversized carb which did not have sufficient flow to draw thru HS needle .
I did not use their engines so it the question was merely of casual interest.
However - using tuned exhausts and rotary valve engines and propping for max power , I found that a smaller carb ,operating at higher vacuum (most flow into engine) is a better overall setup.
In switching carb types n sizes on a 50cc setup, I saw that once the DEMAND of the engine was maximized, larger carbs simply added nothing to performance.
A well tuned pipe setup maximizes mixture flow -that's where the power comes from
We saw the same thing 50 years ago ,running small block Chev's.
We ran faster than the competition using 3 Stromberg 48 carbs as opposed to the then popular dual four barrels setups
In driving watching etc., it was obvious we had more power in lower rpm and no flat spots . At full throttle the dual 4's did put out more but overall we had more tractable, usable power.
Our models only need a carb which puts out max power BUT- transition from low speed has to be usable.

RC Extreme power

My Feedback: (24)

There is one thing that most people do not even think about, Walbro carbs are made for chain saws, Not model airplanes.

A chainsaw does not worry about a mid range, They are either setting and running at idle or being run wide open.

I think most of the engine manufactures have done a pretty good job using something that is made for one application and making them work for another.

So far I have not had a engine that i could not get to run rich enough on the high end, Getting good throttle response and mid range is the hard part.

Milton

captinjohn

There is one thing you did not think about. Walbro carbs are made for a lot of power products besides chainsaws! Capt,n

RC Extreme power

My Feedback: (24)

Very true, Weed whackers, trimmers, edgers, Blowers and so on, But they still were not made for RC airplane engines.

Now Walbro will design and manufacture one for any application if you want to spend the money to get it done.

Milton

altavillan

My Feedback: (1)

My 80+cc chainsaws will run for 30 to 40 minutes of hard, steady wood cutting and use about 14 oz of fuel (guessing but it is a small amount).
Same size engine in a plane will drain a 24oz tank in about 18 minutes. Our plane engines are working much harder than the carbs are designed for. Any throttle setting above 1/2 is working the engine pretty hard and everything possible to help the engine stay cool and survive should be concidered. It would be good for me if I knew more ways to mod up a carb to treat my engines better.

rmh

The other part of getting good midrange and top end power is th exhaust systems used
Many exhaust setups are wretched, offering no pressure as needed at low/ idle rpm and then restricting badly at high rpm
The port design on most industrial setups are also not good for best power rand good low/ midrange.
The setup as used on the new DA/ZDZ/ 3M / 3W are much better as they utilize very large smooth intake ports and well directed small exhaust ports .
This design is easy to spot - there are large rounded ports on either side of the exhaust port.
Casting this design requires a different technique and generally costs more to accomplish.
For most sport flying, the effort is of little value but on all out power setups it is worthwhile.
The old open ports and small vertical ports commonly used for many years wil still run well as will non reed bank and non rotary valve setups
The BUT is that they simply don't flow as much .
The carb selection is nowhere as critical as the exhaust selection.
the problem for many is that a really good exhaust setup on a two stroke requires more space and and costs more than fiddlin with carbs.
So it's easier to play with carbs in a search for power - but nowhere as good in the results
This probably is not what some want to hear but it is the real world on power .

Truckracer

My Feedback: (19)

Wow Dick, Stromberg 48 carbs. This is going back a bit. These days, many circle track orgainzations require a single small spec. 4bbl or 2bbl carb. It is a challenge to get the HP with these small carbs but they do drive well. Well over 500 HP is still possible using a 500 cfm 2bbl on a big cube small block. You'd be amazed at how flat and broad the torque curve can be! These things can pull like a tractor coming off a corner.

It would be interesting to see some real dyno tests on our gassers with various carbs and other setups.

jack1933

My Feedback: (2)

Was n ot aware the BME engine now was "running out of fuel" thought the wga-7/9 was the perfect carb for the engine. I tried not to mention brands here Tom, will you please jump in here and explain. Thanks
Jack

jack1933

My Feedback: (2)

Hi Bill,,, Thanks for the response. I dont think the venturi has much to do with the jetting in the carb. Actual case in point. I could not get the high speed needle to richen at wot, on a 150cc engine with a wga-7 walbro, this was before any mods to the carb. I installed a wb-25 and got the high needle to richen the engine at wot, was testing for air leaks around the intake system, however there was no noticible increase in rpm, and the wb-25 has a larger venturi than the wga-7/9 at that point I started enlarging jets in the carb, it is responding well enough, so its still on the engine. Then I called walbro and was informed the wga-7/9 was calibrated for a 100 cc engine. The wb-25 is going to be installed on the engine, and I know it will work, because its already been on the engine.
Jack

Tired Old Man

My Feedback: (1)

I was referring to the original relaease of the 115 where the carb was a bit too much (wb 25) and the reed bank design needed some work. The current arrangement on the 115 seems to be working out just fine.

jack1933

My Feedback: (2)

The wb-25 is , in my opinion, too large for a 100cc engine, and if one would give Walbro an order for 10,000 carbs, they would indeed calibrate a carb for you. this market is too small compared to the industrial market, all we are is just a little pain in the arse to them, Have been in touch with walbro and Homelite engineers in the past, 30 years ago, and learned a lot about this subject. All I can relate to anyone is, IF YOU CANNOT RICHEN THE MIXTURE AT WOT THEN THE WRONG CARB IS ON YOUR ENGINE. This is not my opinion, engineers opinions.

rmh

A dirty inlet screen will do the same thing

jack1933

My Feedback: (2)

I assumes you were, they are now using the wga-7/9 which is responding very well. The wb-25 was calibrated for around 140-16cc, and is going on my 150cc engine,for sure.