I have converted a st g90 to gas with a carb adapter from mr lambert, has a walbro carb, the problem i have is the engine idles and transition is great to mid where engine runs great but as soon as you make it to say 3/4 power it starts to surge and will just keep on surging till you bring the power back to bout half where it will run great again.

Any thoughts as to what this might be? Any advice on approx needle settings or potential problems with these gas conversions?

Sounds like it may be lean. My ST90 is somewhat sensitive on both needles but I started with them at 1 1/2 on the idle and 1 3/4 on the high. Adjust the idle mixture first and then move to the high needle. The idle mixture affects the high speed just slightly so any time you move it be sure to recheck the high speed needle. If the small screen inside the carb is dirty it can also cause surging so you might want to check that.

P.S. What is venturi size on the Walbro you are using? If the venturi is too large it can cause fuel delivery problems at higher power settings.

If tuning, especially richening up the high speed needle doesn't help, check the carb screen and then the fuel pump diaphram. I have seen the pump diaphrams get distended and then they won't pump enough fuel at high throttle. Also verify that your pulse line to the fuel pump is in good condition with no kinks nor air leaks. Ideally, that should be a somewhat stiff line, and as short as possible.

(All this is assuming you have no air leaks whatsoever in the fuel supply lines.)

Also, do you have a filter in the fuel tank? Could it be partially clogged?

AV8TOR

Will check all that has been Said and will return with results

How do you have your pump impulse line set up for your carb and what carb are you using?

I have it set up with the nipple at the back but not in the backplate, there is a moulding part half way up the cylinder wall that i have it in, will need to check carb but def a walbro one think it has a 9mm venturi

Its a 8mm venturi and the carb is a wt 160b/902

Sounds OK

Cleaned some debris out of the gauze, the diaphram looks exstended, should it be flat or ballooned. Prob 1.5-2mm

It should be flat, or very nearly so. You should check the other diapharm as well. (Metering diaphram.) It should be very supple/flexible. They often get stiff from being dry and/or from additives in the gasoline.

AV8TOR

Update, ordered a service kit, changed diaphragm and fuel pump side(blue acetene) thing. Also gaskets and now runs great after a bit of tuning

Great! Thanks for the update.

AV8TOR

Dear all,

I'm facing a big dificult to tune up my converted G90.

I'm using a WT391 walbro carb wich has 11.11mm venturi. What if I use another with smaller venturi?

Also, I'm using a chinese sparkplug, what if I use a rimfire or another branded sparkplug?


Thank you
Mauricio

carb is too big, I have found the smaller carbs work better on the .90's just richen them up.

What I get is low RPM at 3300 RPM, if I lower it by opening lower needle, when I accelerate, it gets soaked...
What would be an excelent low end RPM?
What would be an excelent high end RPM?

The acceleration must always be slow and progressive, if I accelerate it all of a sudden, it goes off.
is that a normal behavior?

Do you have a carb size recommendation?
Any model in specific?

How bad could the sparkplug influence?
Do you have any sparkplug recomendation?

Thank you!

Would anybody help me with the post above?
I don´t have any comercial interest on it, it is just for my own pleasure...

I just noticed the piston connection rod is now needding to have a needle bearing...

Does enyone know de needles beearing code to be used there?
I found the HK 0509, but the cranckshaft pin is 6.5mm .
I found the HK 0608, Same issue as above them, I would need to machinery the pin to 6mm and the rod to 10mm wide hole,

Question is, if I do thet will I weak the parts?

Thank you
Mauricio...

You have to use a lot of oil (like 18%) in a Super Tiger .90 when running on gasoline unless you change the rod bearing to a needle bearing. This is a very specialized process because the rod pin on the crankshaft is not hard enough to be used with a needle bearing and must be changed. This is very difficult for most people to do. It takes special equipment. Here is a website showing some of the process:

www.lambertsrc.com

A better carburetor would be one with a 9mm venturi. You are probably using a 1/4 x 28 spark plug, and that is fine. The spark plug is not causing your running problems. Your problem is caused by the carburetor being too big, and the low end being tuned too lean. You might also have the carb linkage backwards. There is a bevel on the carb butterfly. It must seat properly in the proper direction or you will have idle and transition problems.

AV8TOR

what direction is that?
do you have any pictures?
What is the Carb model(code) you use?

Thank you.

The carb linkage is the right way when you see the screw threads when looking into the carb from the choke side with the throttle closed. When the butterfly is closed, the screw head goes towards the engine. If this is incorrect, it can cause all kinds of idle and throttle up problems. Check yours. I use an 11mm carb on mine, and it works fine but I have it set up carefully. I use a long arm on the carb linkage to slow the movement down somewhat, and I use some exponential on my throttle to slow the movement down as well. I also tune my idle a touch rich to help with the throttle up transition. If everything is just right, an 11mm carb works ok.

A little less finicky and easy to set up, would be a bit smaller carb. Look for one that has either a "20" or "24" cast into the carb throat near the choke. Either should work fine on the .90. You can look up carb numbers and venturi sizes here: http://wem.walbro.com/walbro/partnum...anuItemNumber=

AV8TOR

been mounted up side down is that a matter for the carb??

If the carb throttle "butterfly" is "up side down" or backwards, it will definitely cause problems. High idle, mixture problems, and problems with throttling up. If yours was/is backwards, reverse it and try again.

Your bigger problem is that your connecting rod is now apparently worn out..... Install a new rod if the crank pin is undamaged, and run no less than 16% of good quality oil in your fuel. (160cc of oil per liter of gasoline)

AV8TOR

Av8tor,

I thank you very much for your support!
And you are right, the rod is worn out, I´ll send it to kenLambert on monday, since I don´t have such service in Brazil.
The biggest problem now, is that I left the piston drop on the ground and it broke! Damn bad luck guy!
Do you know where to get one on a very cheap price.
Maybe get a rod+piston+ring on a very cheap price?

Mauricio...

http://www3.towerhobbies.com/cgi-bin...?&I=LXFW53&P=7

http://www3.towerhobbies.com/cgi-bin...?&I=LXFY56&P=7

http://www3.towerhobbies.com/cgi-bin...?&I=LXFY73&P=7

When ordering the piston, you will need to know if yours is a G-90 or an S-90

AV8TOR

I have a used piston

I run my bushing rod ST90 at 16:1 oil using Redline. I also use the same fuel in my converted ST51 and Saito 91. No problems with any of them and the little 51 is turning just over 15,000 on the ground. By 16:1 I mean 8 oz of oil to a gallon of gas. I've run the two strokes at 22:1 which the four stroke tolerates just fine but the two strokes will eat the bushing if you get a real lean run. I just run the two stroke mix now in all of my conversions with bushings.

Ken Lambert,

A year ago, I sent my ST G90 to you, for servicing, after that, I got the engine, but I never flew again until last week.
I need your help, because I just cant make it to run, if i let the shoke on, it starts and runs ok, if I opend the shoke, it runs for another 15 seconds and go off.
Do you have any tip of what could be causing such problem?

Thank you!
Mauricio