togatoga

My Feedback: (42)

I'm just wondering if anyone knows what would be the max carb bore for a chinese 26cc engine e.g crrc 26 or spe 26 without it becoming an issue. I have a walbro WT-793 with a 9.5mm bore on a CRRC26. I'm thinking of going for a larger bore for more power if thats possible. Thanks

Ram Jet

You can put nearly any size carb on an engine as long as you have the ability to change High speed, mid-range and low speed jetting or fuel metering. An over-carburated engine will be very sensitive to atmospheric changes such as temperature, barometric pressure and humidity and may be a nightmare to set a nice idle on. An excessively large carb with also have a negative affect on throttle response and may require an accelerator pump - good luck with that one. 9.5mm? I would try a 10.5mm and quit. Be sure you have the ability to change fuel jetting up or down.

Bill

SRimer

my SPE 25 came with 9.5mm carb, I have swapped it for 11.1mm and gain 600-800 rpm, respond is very good and adjustments are easy, I have checked all sizes carbs up to 13.5mm with no rpm gain.
I know G26 comes with 12.7mm carb and crrc 26 is a g26 clone. I hope it help some.

Super08

My Feedback: (2)

I put a 11.1mm on my BCMA SPE 26 as well. Runs great. I ordered mine from Tower. They use it on the Fuji 24. It was rather costly. A WT433 would be a cheaper alternative.

pe reivers

The carb choice depends on intended engine use.
For moderate applications and good low rpm response, a 26cc side ported engine could use up to 11mm venturi size.
For racing, at rpm 8000+ use 14mm max. Idle still ok, but transition suffers a lot
for all out tune of 10,000+ rpm, a 16mm carb would do it, but you loose low rpm response (big time) in the process. The engine will idle OK. This is not carb size dependant, But when throttle is applied, you better not load the engine too much until the carb can provide the right fuel/air ratios again. (I would like to insert the word "capiche" here, but it seems to have a bad ring in the US) It's all about understanding the nature of the two stroke engine, and combine that with the limitations of a carb to provide the required air/fuel mixture.
larger carbs in general improve power over the total throttle range.
Smaller carbs are much sweeter to set up, but you pay the price powerwise.
Each engine has it's own sweet spot, where carb size and power curve look best. That is the carb the engine is happy with, but are you??
It's all up to you and your intended engine use.

To change the sweet spot of an engine, you need to change port timings and exhaust systems and crankcase compression ratios. It is complicated, but humans can do it.
Good luck.

togatoga

My Feedback: (42)

Got a couple of big bore carbs on the way.I'll see if they are what the doctor ordered.Thanks everyone for the feedback.

Rcpilot

My Feedback: (78)

Not exactly true. Walbro main jets are a fixed size and you can't change them without major modifications and access to a machine shop.

Not exactly true either. They will idle fine with jst about any carb.

Also not really true. There is no "accelerator" pump like on a Holley carb. But I agree, it will have horrible throttle response.

Agreed on the venturi sizes, but again, no way to change jet sizes on a Walbro.

When the carb gets too big the engine will run up to about 1/2 throttle just fine. After that, the engine begins to draw enough vacuum that the high speed circuit starts to come into play. Once the high speed circuit opens, the engine will be almost instantly flooded with too much fuel and it will die. It happens suddenly and there's pretty much no way to keep the engine running once you flood the snot out of the crankcase.

No amount of fiddling or adjusting on the needles will cure the problem. The high speed circuit is just too large. Most carbs will properly meter fuel out of the low speed circuit just fine for any number of engines. It's right at the point where the engine really starts cranking up and sucking strongly on the venturi that the problem rears it's ugly head. The high speed circuit opens up and dumps large amounts of fuel right down the venturi. Unless the engine is already turning very high RPM's (small prop) and can use that fuel instantly, it will most certainly load up the crankcase with too much fuel and die.

Thats what happens with a standard muffler and normal sized prop on a Ryobi 31, Zenoah G62, Poulan 54, Craftsman 25, Fox 3.2, ZDZ60, Zenoah GT-80 and several other engines that I can't remember right now. Been down that road. Ended up turning around and going back the way I came. Living at high altitude means we fly with less power than most of you. I've tried all kinds of things to make engines run better at 6000' altitude. Best thing to do is buy a bigger engine.

Don't get me wrong, a larger carb can and does make more power.......but only to a point. Untill you find the carb thats too big. Then you got a nice bench engine, because all it will do is idle.

Bob Cimarusti

I've found the WT-433 carb to be a very good fit on both the SPE-26 & the CRRC -26. I believe it's an 11 mm bore carb.
Have had no problems tuning either high or low ends and transitions have been great. Looks like about a 400 rpm gain
on the high end. The CRRC is definitely the more powerful of the two engines.

togatoga

My Feedback: (42)

OK did some experimenting. I tried a 15mm carb and the engine wasn't behaving. I then went for the walbro wt811 which I had handy and found on a G20.It upped the rpm on a 16x8 apc from 8000 to about 8250. I think the carb bore is about 10.5mm. I am trying to locate a wt433 as recommended.Anyone tried a wt645 found on a g26.Is the wt433 and wt645 of the same bore.
How come there's so many walbro models? Do they all have different bores or different size jets?

SRimer

Pe, what in your opinion should be the carb size for a 40cc?

travwhit

Just bought a CRGF 26cc engine and after breaking it in by the book (2 Gallons at 2500 with occasional short burst of WOT) I am getting 9030 RPMs with an APC 16X8 prop, high octane with 5 OZ of Klotz 100 per gallon. Problem is with the high side turned rich enough to get that kind of RPMs and the low side set to get a good idle and transition at just above 4000 RPMs it 4 strokes and I have to hold the throttle at half till it picks up enough to take full throttle. I can turn it in till I go down to 8450 RPMs and it does fine transitioning from low to high RPMs, does that sound like something is not right with the carb? I am happy with the 8450 and I think that when I get the JFX 18X8 prop on it the engine will load up just enough to bring the RPMs down to 7900 or so.

Rcpilot

My Feedback: (78)

It doesn't matter what the RPM's are. As long as you aren't exceeding the max rated RPM don't worry how many RPM's it's turning.

Just lean the high speed needle to peak RPM and then richen about 200RPM.

Now lean your low speed needle until you find peak RPM at idle. Richen 100RPM.

Now adjust your throttle servo endpoints to get the idle speed you want. Typically fast enough to idle smooth but not enough to move the plane. 1700-2100RPM.

Antique

My Feedback: (4)

Some Walbros DO have accelerator pumps...A small spring loaded brass plunger operated by a flat on the throttle shaft..You can see an extra hole in the carb base...If you take the shaft out the plunger sometimes springs out and gets lost...
Other Walbros have a hole in the shaft corresponding to an extra hole in the carb body that lets fuel in when the holes meet...
Walbros don't have fixed high speed jets...The fuel nozzle in the body IS fixed diameter..Fuel flow through the nozzle is controlled by the high speed needle...the nozzle is quite a bit larger than the hole at the end of the needle....

Aussie Maddog

My Feedback: (16)

My SPE 26 came with a WT 419 (Vent. 9.5 & Throat. 14.29). WT 645 and WT 257 (Vent. 12.7 & Throat 15.8) improved RPM to 8300 with APC 16x8.
Lost some transition control but was manageable (645 probably the best). I am going to try a WT 600 (Vent 11.1 & Throat 15.8). If max RPM is maintained then this should give me the best transition.

To those going for larger throat carbys, be sure to enlarge the spacer hole if needed. You can see my original throat was 14.29 and now I'm using 15.8 throat carbs. I've enlarged the hole to suit.
This cut into the pulse line hole and I have filled it in with sealant and run an external line.