Interesting Geo Carb Info... Possibly More fuel Mod.

[MrTea]

Sat here twiddling my thumbs...

I thought I would re investigate the Geo engine..
We all know that they are pretty miserable due to lack of any adjustment on the carb...

On the Walbro barrel carbs...
The fuel is drawn up the small white plastic tube, and is metered by the needle...
I assumed that the needle was tapered and as the needle lifted it allowed more fuel past the reducing taper..
WRONG....
The needle is parallel...
The fuel is sprayed from a slot cut in to the side of the white plastic tube..
This slot is tapered.. Thin at the bottom, wide at the top..
As the needle rises, it uncovers more of the slot, allowing more fuel out.

Upon further investigation...
I found the Geo carb WYA 5, has a brass jet underneath the white plastic fuel delivery tube...
WARNING... Never attempt to remove the white plastic tube..................
The brass jet can be found if you remove the 4 screws holding all the gubbins on to the base of the carb..
The jet is held in by the base plate, and has a tiny rubber O ring underneath it...
Removal of the jet is easy.. small screwdriver, and piggle it out..

The jet has a number on it 375.
Looking on the Walbro site... it is .375mm ..

I took a few more Walbro barrel carbs to pieces...
They all have the brass jet....
BUT... different sizes.
Big Foot standard carb WYJ 138.
Jet size .40mm

Unknown application WYJ 111.
Jet size .41mm

In theory.....
The problem with the geo carb is engine speed pick up...
They rev out beautifully, but not under load...
I think this problem is mass fuel flow... Jet size...

The other issue is a generally lean mixture...
So......
A while back I realised that possibly adding a shim under the main barrel mounting would lift the needle a smidge..
This would have the effect of more fuel... At the needle..

I have not been able to try these mods because I have no way of mounting the engine..

What I need is someone with a running Geo to try these out..
First change the jet to a bigger one...
There must be thousands of small sized Walbro barrel carbs kicking about..
The jets all seem to be marked..
Do this mod first and try it....
It should allow the engine to pick up quicker...
If it works, then move on to the Barrel shimming modification...
Get a .005" feeler gauge and loosen the 2 barrel retaining screws...
Slip a piece in between the main carb body and the black plastic barrel plate..
Nip the screws but don't over tighten them..
In the absence of shim material... Food packaging plastic will suffice, but you need to know the thickness..
What you are trying to achieve, is a lifting of the whole barrel assembly..
It may only need a couple of thou thickness to get the required result....

Caveat Emptor-
All the words above are theory....
BUT, they should not do any damage...
You can always go back to stock..
... J.

[MrTea]

Photo 1-
Brass fuel jet in situ.

Photo 2-
Jet and O ring removed.

Photo 3-
Geo twin flow carb barrel.
... J.

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DSCF2291.jpg

[blt113]

As much as I want to be the guinea pig, I have other engines I must get sorted first.

I have a decent gz25n23 shoved deep into my closet. I haven't started it since I bought it, but I have been saving it to see what can be done with it.

Give me a week or so. I will have an open weekend to finally get everything squared away.

[MrTea]

Good lad... BLT.

One thing I meant to write down........
The Geo engines, like the G43L, will all be suffering from old and dry crank seals..
The engine I have is from 2004..
12 years old... the seals will be shot.

If anyone is planning on having a go at a Geo re-birth...
Order of works-
- Change the crank seals, and run it up..
Has it made a difference.
- Change the brass jet for a slightly bigger one.
Run it again.......
- Shim the carb barrel.
Try again..

With a 25cc engine it should pull reasonably well..
The Standard G23LH with an adjustable carb is not a bad engine..
SO, with a little love... The GEO could sparkle.

The 45cc Strato charged engine has more BHP than a standard 45cc engine...
SO, in theory.. it should be the same case for the Geo lump..
The 45cc Strato does have an adjustable carb, but the screws had restrictors fitted.
These restrictors only allowed 1/2 a turn on both high and low screws..
The restrictors are easily popped off the screws.. Then you can add as much fuel as you like..
AND IT LOVES IT..
... J.

[qwerty]

Great work, about time the Geo's get the love they deserved and which their nontunable lean carburator, high unit price and additional complexity in part number and built instead make a stilborn out of that engine series.

Not sure if the 26.4cc geo would benefirt the same way as a 43, having ahd several of them open and comparing the memory with the recent posting of an opened GP290 there is more than the 2 piston rings in difference, the missing 5th port and lag of cowling would likely not make up for any benefit the backpressure release ( a pressure based alternative to a mechanical Atkinson cycle) brings in, the 1.8L in the Prius is a scuh and the engine performance is not that great for a 1.8 despite toyota running high compression on their motors. Vauhall/Opel used Stratified scaveningin as well during the early 2000nds, the motors where economic and runned lean but not more powerful per cc than otherwise to be expected.

However reliability should forther improve and of course the well needed power.

It would be interresting to know if pickup power could be raised alternatively by expanding the upper jet oulet only, starting them was a biatch and i ended up flooding them until i found that starting with the throttle slightly engaged and the choke fully closed remedied the cold start issues and may be even easier to try if having a spare Geo carb needle to mess with ?

[qwerty]

Another theory on why the Geo may be slower in picking up revs under load is its tight tolerances and 2 piston rings mated to a average crankcase bearing and hihgh-torsion srive outputs: It has more friction to overcome and this amplifies with loads.

However and on occasion of the Piston friction, the single-ring piston of a RC260 will fit the "25 cc"/ 26.4cc Geo as well.

[qwerty]

Another angle of attack to increase fuel flow on a ported upper jet nozzle/ outlet could be to increase airflow on closed choke lever itself by expanding the hole drilled in it.

Makes one wonder if a different type of sparkplug, a ignition timing adjustment or possible a deletion of the top nozzle oulet entirely would make a difference ?

With top nozzle delete and possilby a 0.4 - 0.45 single nozzle only the issue may be sufficiently adressed spare for the inactivation of the stock chocke and airfilter box, this may be a minor problem in case the screw distance of the Geo airboxes is not exactely matched a standard airfilter box - but its weak thinly plastic so all you'd need is a drill to match the mounting holes of a standard airfilter box to the geo carb. This may work as reed rerouter, still, and improve the piston turns from around 70 to 300-whatever-the-ignition-timing-is degrees ( if at 0 degrees it may help with momentum up to at least 180+ degrees if not the full 360 degrees))

[MrTea]

My original thoughts on my carb mods....

Just make the engine run better...
Not thinking of more power..
Of course, more fuel should yield more power.. But this is not the main aim..
There are thousands of Geo Peds out there..
When they are running right, they are more than adequate for paddock use, kids etc.
Also, the Geo sports cannot run any other engine.. Making them unusable and worthless.

The upper venturi of the carb barrel, is air only..
Deleting it or opening it up would cause many issues.

Blocking it up would make the engine breathless as it uses the air to clean out the cylinder prior to the main fuel/air charge entering.
Opening it up would serve no purpose, as the air entering is controlled by the little reed valves either side of the barrel.
... J.

[MrTea]

Some More Info.......

Looking at a WYK barrel carb with adjustable main jet.......
The high jet screw is very long.. and screws all the way in to where the brass jet would be...
BUT, they do not have the brass jet...
So adjusting the high screw is effectively the same as putting a bigger brass base jet in a non adjustable carb..

Jet surface area-
Walbro WYA-5 .375 jet =.110mm
.40 jet =.125mm
.41 jet =.132mm

The Walbro WYA-44 is a GEO carb off a bigger machine..
Venturi is 12.2mm as opposed to 9.5 in the WYA-5 carb.
Base fuel jet is .590mm.

There are many base jet sizes to choose from...
First stop would be a BigFoot jet to get a idea of where to head..
... J.

[displayking]

Very interesting MrTea, I have a running Geo that I would be interested in working on some more.

The first thing I did was get the cylinder and piston ported with the Geo Madness specs. I also had the piston mod done. This helped very much from mid to high rpm ranges where the low and lower/mid rpm ranges weren't affected.

I then replaced the crank seals which helped tons with starting mostly.

I believe I have an LH carb that I can try and switch the jets on.

[MrTea]

Very interesting MrTea, I have a running Geo that I would be interested in working on some more.

The first thing I did was get the cylinder and piston ported with the Geo Madness specs. I also had the piston mod done. This helped very much from mid to high rpm ranges where the low and lower/mid rpm ranges weren't affected.

I then replaced the crank seals which helped tons with starting mostly.

I believe I have an LH carb that I can try and switch the jets on.

Perfect... More the merrier..

Keep the results flowing.
... J.

[MrTea]

The Jets are not pressed in....
They are only held in by the base module and rubber ring...
No force is needed to remove it.
It might be a bit tight on the rubber ring if it has dried out.
... J.

[pedguyman]

Also, the Geo sports cannot run any other engine.. Making them unusable and worthless.

The all tube geo frames can be converted to gsr with new mount tabs. I have done 2.
My buddy actually mounted a 54mm mount on stock geo tabs with a lock down.



I read your thread and am intrigued to see the out come.

[displayking]

The Jets are not pressed in....
They are only held in by the base module and rubber ring...
No force is needed to remove it.
It might be a bit tight on the rubber ring if it has dried out.
... J.

Yes, the jet came out very easily with a small amount of force because of the o ring. The carb I extracted the jet from is a WYJ-138 model Walbro.

The jet had the 41 marking on it indicating that it is biggest jet that you have listed.
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Correct me if im wrong, as I am trying to learn more about these carbs, the changing of the stock jet to a bigger one will help with the lean condition of the geo engine because there is more fuel flowing through the jet?

[MrTea]

Found the part numbers of some main jets.......

.385mm P/No 112-796
.390mm P/No 112-797
.400mm P/No 112-807
.410mm P/No 112-827

... J.