SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

Spent some time searching for a similar post, but no luck. Doing my part to avoid forum duplication. I'm involved in a project to convert a Saito 1.5 four stroke glow to gas. Here are some pictures. The rod has been converted to needle bearings to run less oil in the gas than otherwise would have been required. The pics showing the crankcase being machined was to groove out the ID of the case to accept a larger rod journal, that resulted from the needle bearing installation. By the way, I'm aware that the crank pin is not sufficiently hard to last long as an inner bearing race. This will be dealt with after initial runs, probably by carbeurizing (case hardenning).

The big issue is the carb. Since the lower end of these 4 stroke engines are lubricated basically through calibrated piston blow-by, the pulse tube can become clogged with oil. This in turn fouls the diaphragm on the Walbro. To deal with this, Walbro has developed some carbs with a built-in diaphragm spring that replaces the positive pulse. With this setup, the pulse tube can be moved to tap directly off the intake port. Now, I know that some of you experts out there will respond that some Honda GX 4 strokes don't have the helper spring. We don't need to clog up this forum with that debate.

Here's the core question. Does ANYONE out there know how to interpret the WALBRO website to find out WHICH carbs have the helper springs? I find their website to be both informative, and lacking in any helpful details. It seems like they take special customer requests, engineer a carb for them out of their existing series (such as WT), give it a part number, and add it to the list without actually describing what is special about that particular carb. That is why there is such a wide variation of WT's, etc. Their parts list is more like a log book than anything else. Maybe I'm wrong...hope someone out there can show me where this detailed information is available. Or at least a list of carbs that have the helper spring in the pump. Thanks! I appreciate any info.

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

The Walbro carb numbering system is well, there is no system. They just assign a number every time they make a new one for a customer. Finding a type with a certain feature is impossible. You have to have a part number.

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

It almost seems like everytime the workers at Walbro come back after coffee break, the forgot what they were doing and start a new model. There are more Walbro model numbers than Carter had little pills?

The WT-456 is a carb like you are referring to. It comes from the old original Ryobi 26cc four stroke. It is a butterfly carb with both a high and low needle and it has the fuel pump spring you mention.

The spring itself is part number Walbro 98-3225-7. You might could try this spring in other carbs?

Walbro carbs with an accelerator pump and the high speed nozzle check valve would also be nice like is on this Walbro WT-456.

http://wem.walbro.com/walbro/product...&Series=WT

The Saito 150 most likely would not need a "28" size carb like a 25cc two stroke engine because the Saito is a four stroke? I say this because the WT-456 is small (#16) when you start looking at it. But at least give it a try?

The Honda GX engines all have barrel type Walbros which is no big deal but none of them have high speed needles which is bad.

The Saito FG-36 gas engine has a Walbro WYD carb which is a barrel type with the pump and regulator 90 degrees apart whereas most WY barrel type carbs have the regulator mounted underneath the pump.

The WYD has a pretty good sized venturi and might be too big for a 150? I have no clue as to the carb on the new Saito FG-30. It appears to be a WY style but that's all I know.

There is little information available on the Walbro WYD-1-1 and WYD-2-1 carbs.

The barrel spring on the Walbro WYD carb works backwards compared with the barrel spring in a model airplane engine carb. The spring is on the front of the barrel and pushes the barrel towards the carb body rather than away from it. The low speed mixture screw is in the center of the barrel in a Walbro WYD just like on a model airplane engine carb. But the spring tries to close the barrel and not open it like on a model airplane carb. To choke this type carb, close the barrel and pull out on the throttle lever which will pull the low speed needle out of the barrel to the high speed position effectively allowing the engine to suck a lot of fuel through the carb. The Walbro guy that designed this carb is a model builder and generally seems to like war birds.

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

The ones on Honda's GX series have the diaphragm spring. I can't recall right now what model is on my Honda, WYB?

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

Hondas have WYL and WYB carbs the 31's have a 8.5 mm bore in the barrel which is about a "21" if you are comparing sizes

Most have the spring some do not

Non of them have a high speed needle valve

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

W8ye,

thanks for the info. Some questions, and then comments. What is a "28" size carb? And what is the (#16)?

Glad it wasn't my imagination that Walbro's site is deficient. Maybe that guy you know at Walbro could put in a word that a new site is needed?

I'll try to get a WT-456 to start with. Another idea occured to me. Besides having oil clog the pulse tube and eventually the pump diaphragm, I've noticed that the engine crank rotation is severely restricted when the pulse tube is connected. This will rob horsepower due to piston backpressure. It also would inhibit blowby, possibly leading to bearing failure. So.....here's the idea: Add a tee to the line, with a tiny bleed screw. Adjust the screw slightly open just enough to get about 4-6psi (measure this) at the regulator, and the rest is vented to ambient. This is a compromise solution, but might relieve the crankcase backpressure enough to prevent performance loss. Comments?

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

The WT-456 just uses the vacuum pulse from the intake manifold. There is no connection to the crankcase vent


For years, Walbro butterfly carbs have had a little number cast into the venturi. This number relates to the number of 64ths of an inch that is the diameter of the smallest part of the venturi.

Zama carbs never had this but many times people relate to this number when they are looking through a box of old carbs

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

I have put many an hour on my converted Saito 91. The leanest oil ratio I could get away with was 22:1 and not because of the rod bushings. There is plenty of oil in the case to keep the crankpin and wristpin bushings lubed. The problem is the exhaust valve guide. There is no oil traveling up to the rocker boxes from the crankcase, all of the oil for the exhaust valve lube comes from the exhaust gases. If you cut the oil ratio too much there isn't enough and the exhaust valve will stick in the guide. Been there done that. The exhaust runs much much hotter on the gassers and it tends to dry the oil off the stem and guide pretty quickly. I made it a habit to pull the two screws and take the rocker cover off after each flying day and drop about 5 drops of high quality synthetic on the valve train and put the cover back on. Never had any more trouble after going back to 22:1 and lubing after flying. I was trying to run at 32:1 when the valve kept sticking. There are really no drawbacks to running the extra oil except the cost. Plugs (I was running the RimFires) lasted an easy 30 to 40 hrs and sometimes more. Good luck with yours.

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

Its not a debate, its fact. Honda used only 6 different carb model numbers, its easy enough to look up which carbs and then check the Walbro website.

If you had a different attitude I would tell you which ones.:eek:

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

<table cellspacing="1" cellpadding="3" border="0" class="background"> <tbody> <tr> <td align="center" class="smalltabletitle">Engine</td> <td align="center" class="smalltabletitle">Walbro Part Number</td> <td align="center" class="smalltabletitle">Description</td> </tr> <tr> <td class="menudetail">GX 22</td> <td class="menudetail"><u>WYL-73A</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX 22</td> <td class="menudetail"><u>WYL-96</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX 22</td> <td class="menudetail"><u>WYL-62B</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX 22</td> <td class="menudetail"><u>WYL-126A</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX 22</td> <td class="menudetail"><u>WYL-129B</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX 22</td> <td class="menudetail"><u>WYL-131A</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX 22</td> <td class="menudetail"><u>WYL-67A</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX 31</td> <td class="menudetail"><u>WYL-68A</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX 31</td> <td class="menudetail"><u>WYL-97</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX 31</td> <td class="menudetail"><u>WYL-75A</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX 31</td> <td class="menudetail"><u>WYL-74A</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX 31</td> <td class="menudetail"><u>WYL-127A</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX 31</td> <td class="menudetail"><u>WYL-133B</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX 31</td> <td class="menudetail"><u>WYL-63C</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX 31</td> <td class="menudetail"><u>WYL-135A</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX25</td> <td class="menudetail"><u>WYB-8A</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX25</td> <td class="menudetail"><u>WYB-7C</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX25</td> <td class="menudetail"><u>WYB-6-1</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX-35</td> <td class="menudetail"><u>WYB-15B</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX-35</td> <td class="menudetail"><u>WYB-16C</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">GX-35</td> <td class="menudetail"><u>WYB-14-1</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">ZM 3A</td> <td class="menudetail"><u>WYL-62</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">ZM 5A</td> <td class="menudetail"><u>WYL-74</u></td> <td class="menudetail">Carburetor</td> </tr> <tr> <td class="menudetail">ZM 5A</td> <td class="menudetail"><u>WYL-75</u></td> <td class="menudetail">Carburetor</td> </tr> </tbody></table>

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

If we could all be perfect like you... Ah maybe someday...

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

GX31 SA (1997-) 1.5 hp, WYL-68 NO SPRING, WYL-68A no info avail., WYL-97 W/SPRING, WYL-127 W/SPRING

GX31 TA (1997-) 1.5 hp, WYL-79 NO SPRING, WYL-79A no info avail, WYL-99W/SPRING, WYL-99A no info avail, WYL-115 W/SPRING

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

Thanks again for the info. The carb on this Saito right now came off a Featherlite 25cc. I measured the carb venturi and it's 11 mm. So, If I got this right, a "28" would be 28/64=.437". .437x25.4mm=11.09mm. Just what you recommend. Nice to know what that number means.

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

Jezmo, thanks for the info. My plan, though I did put in needle bearings on the bottom conrod, was to still run fairly rich on the oil. Coincidentally enough, right around the ratio you mentioned. It's nice to get the info about the valve guide. Got me thinking how to run an oiler up there. I've been thinking how to rig a dry sump oiler system from a tank. Perhaps using check valves and a spare little tank. One could use the up & down pressure pulses to drive the oil thru the engine similar to how Walbros use check valves to keep the primary pump resevoir filled with gas.

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

Thanks for those carb numbers. Now it's a breeze to click and verify springs are there. I did pull up the WT456 and looked at the exploded view. The part number for what I THOUGHT was the return spring (surge diaphragm spring) was one number off from the one you quoted as the spring # in your first reply. Do I have the right spring? It's on the side opposite the diaphragm [that one side vents to ambient, the other side contains the gas one it's way to the tiny venturi orifices]. Sorry, I don't know the proper name for it. Hope that description ID's which side of the carb I'm referring.

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

To whom are you referring?

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

Thank you very much for your help. Much appreciated. Sorry if anyone was offended by my "debate" comment. To explain, it arose out of my frustration searching forums for just the type of info now being provided in this thread, and having those discussions veer far afield of the orignal topic. I was trying to innoculate this one I guess.

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

you are looking at the demand regulator and not the fuel pump

The demand regulator has a vent to the atmosphere.

If you are looking at the barrel type WY carbs, it is very confusing to seperate pump and regulator functions

The fact remains that the Honda carbs do not have High Speed Needle Valves.

In a model airplane engine, a high speed needle valve is a must have

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

Any comments on the above idea? I'm open to suggestions on this as a way to use carbs with traditional positive/negative pulse surge diaphragms (no spring).

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

One guy routed the crankcase vent through the pump pressure signal chamber of the carb (in - out) to try to purge the oil. The crankcase vent ultimately vented to the atmosphere

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

The fuel pump pulse chamber of the WT- 456 is not unlike any other WA or WT Walbro carb. It uses the same cover and diaphragm. The only difference is the spring and source of the pressure signal.

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

neat

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

this should be interesting when finished

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

Can one add this pulse pump chamber spring to pretty much any such Walbro and turn it into a "4 stroke" Walbro?

RE: SMALL DISPLACEMENT FOUR STROKE GLOW TO GAS CONVERSION
 

I would at least try it

You will want your pulse pick up to be from the intake manifold and not the crankcase.