Walbro experts needed
 

I have a saito 180 converted to gas using I believe a Walbro wt-542? (I will have to check).

Once I have the needles set for optimum high and low, the midrange suffers. It runs a little rich at 30-50% throttle, it's loading up. If I bang the throttle back and forth it cleans out, but if I slowly run up or hold at mid position it pulsates and stumbles.

Is it possible to modify the carb or pump to adjust the midrange? I have heard you can bend the fork inside to acomplish this?

Thanks for your thoughts

RE: Walbro experts needed
 

Are you using a velocity stack? Without one it is not uncommon to get erratic operation.

I wouldn't mess with the pump/metering needle fork but someone with more knowledge may say otherwise.

What are your high and low needle settings?

RE: Walbro experts needed
 

This is not for the faint hearted, nor someone that is worried about possibly ruining their carb butterfly.

You can file a tiny notch in the butterfly, located exactly where the idle holes are in the carb throat. This gives a better idle mixture and quality usually, and also tends to lean out the midrange. If you go too far, you won't be able to get it to idle low enough, even with the butterfly closed. You also might not be able to kill the engine by closing the throttle all the way with this mod.

It is tough to do, (but not impossible), without removing the butterfly. If you do decide to remove the butterfly, you need to grind the staked part of the screw off so it doesn't ruin the shaft when the screw comes out. Upon reassembly, use Loctite, and be sure the butterfly is both located exactly as it was before, and is centered in the carb bore.

You might want to experiment with carb regulator lever height adjustments first. Lowering the regulator lever a very small amount can sometimes help a rich midrange. You will have to readjust your high and low needles after doing this of course, just as you would with the above mod as well.

AV8TOR

RE: Walbro experts needed
 

AV8TOR's advice is great and one thing to add, the little butterfly plates are cheap from Walbro. Like around 2 bucks each. I bought several when I modified my last one.

RE: Walbro experts needed
 

+1
It is good to know whether your midrange is lean or rich. To that end, tune idle for best rpm (forget about throttle acceleration!!!!) Then slowly go to 30% throttle. Re-adjust for best rpm. If you needed to close the idle needle, midrange is rich, and vice versa!
A  rich mid range can be cured using the valve cut out method.
A lean mid range needs adjusting the middle idle jet to a  larger size (most times there are three idle jets)<br type="_moz"/>

RE: Walbro experts needed
 

I can not say I am an "expert" but, I have ruined a lot of walbro carbs playing with them.

The WT-542 is a 13.49 mm carb with an accelerator pump. It is a great carb. When you "bang it back and forth" you are shooting extra gas in to the engine. I suggest it might be leaning out when it stumbles. The 542 is a largish carb for a 30cc engine. Perhaps a smaller one? It will run smoother over all and be less fussy to adjust. You will trade a few hundred RPM a the top end to make it run smoother. Buy NOS carbs off EBAY.

Pop off levers.........
I am assuming that you are already in the area of 1-2 turn out on both needels. If you are not you have bigger problems. The needles should not be way out or closed to make the engine run. If they are you need to clean the carb and replace the rubber parts and or the pop off lever is not working right. Say the carb seems close to right, then you would bend the pop off lever up to richen the midrange and vice versa. Only bend it a tiny amount. It should be close to level with the casting . To richen bend about the thickness of the lever to start.

When this fails buy a NOS carb off EBAY. Here are some carbs with accelerator pumps:
11.11 mm
WT 29-1
WT 356-1
WT 358-1
WT 464-1
WT 516-1
WT 517-1
WT 527-1
WT 613-1

12.7 mm
Wt 513-1
Wt 537-1

13.49mm
Wt 171-1
Wt 172-1
Wt 228-1
Wt 285-1
WT 529-1
Wt 451-1
Wt 480-1
Wt 540-1
Wt 542-1
Wt 555-1
Wt 562
Wt 610-1

I would try to get an 11 and a 13mm carb and compare them.

RE: Walbro experts needed
 

Thanks guys for the great info.

The problem I have is when the low end is tuned right it will not die at idle, even when the butterfly is fully closed. I like the idea of trimming the butterfly though. I will have to run it again to make sure it's rich.

This was a conversion kit I had purchased and it did come with a constricting velocity stack, but I removed it and performance seems better. I have many engines, but I am new to a gas 4 stroke. I'm open to trying different mods because I am doing this just for fun anyway.

Great idea on trying different carbs. Yes it is a pumper and the stumble is when I hold it at about 30% power. I will run it again and tune out the stumble to make sure if I am lean or rich. The needles are about right but I can make a count on the turns as well. Perhaps my 4500 ft elevation has an effect?

RE: Walbro experts needed
 

13mm would probably be a bit large for a 30cc four stroke.... But yeah, I love the carbs with accelerator pumps.

AV8TOR

RE: Walbro experts needed
 

Fileing the butterfly is a wast of time right now. It sounds like you have an air leak and/or the carb is too large. I assume the idle stop screw is removed. If it won't die with the butterfly closed you probably have an air leak, or the carb shaft is badly worn.

You could have a lot of time and unnecessary motion by installing a smaller carb, really!


He had a 13.49 mm on it right now.

RE: Walbro experts needed
 

Elevation has the effect of richening the mixture, reducing effective compression ratio and reducing effective power.
Prop rpm will be the same, because it operates in thinner air.