To late I have already tested WT308. Actually it works fine. I mean the engine doesn't look like restricted and response is better. Now I need to do it for flying - I just tested it "quick and dirty" just to see if it works. I need to redo the throttle arm, fuel pump pulse supply, and the entire adapter (the white one is to much abused). I will also try to modify fuel pump cover where the priming bulb used to be so it will be smaller for better fit. This will take me few days between painting my house so be patient waiting for next report.

I would like to see how you made the intake for the carb. and What did you make it out off. In the pics you got there it looks like to did a good job. I think I am going to give my ST 75 a shot at this just for fun.

Paul

The adapter between the carb and the engine is made out of derlin (white plastic) but it is to soft for this type of mounting on the engine. I can turn the whole carb around in the engine. I'm going to use aluminium next time.

I used 1/4 inch aluminum to make mine for a magnum 1.8

I used aluminum for the one on my ST90. I'll take a pic of it and post as soon as posible.

I have finished all the conversion, made nice adapter and all works fine but...and here I need some help from somone who knows Walbro.

The engine runs great on idle and the Lo needle works fine. But when I get more than 1/4 throttle now the engine goes lean and dies. Turning Hi needle doesn't change a thing. Fuel pump works well. So I tried something else: I attached the silicone tubbng to the pressure mettering tube and I blew into it simulating increasing outside pressure. I was able to make such pressure (not much) so the engine was runnign well. The same effect was when I put the end of this silicon tube to the exit from the muffler. Exhaust blowing from the muffler created enough pressure in the tune for engine tu run smooth through entire throttle range.


So there is something wrong in the fuel mettering. The valve doesn't open. Is there some special passage that creates vacum on the gasoline side of the membrane to open the valve but in my carb this passage is blocked? Or should I simple play with the sping or the the lever? I know something is wrong on that side even everything visually os OK, so how to fix that?

The Walbro number is WT-308-1

Try lifting the metering lever just a tad normaly it's level with the carb body just raise it a little bit this will make the diaphragm open the metering valve more and letting more fuel into the carb

I did that - Based on the information from Walbro it should be about 1.72mm below the surface, I think it was just a bit below. now it is about 1.7 Maybe this will help.

In the meantime I checked that the main nozzle is working fine so the problem is that it just does not create enough vacum to overcome the spring. So if rising the lever doesn't help I guess I will have to modify the spring. For ther last 24 hours I learned a lot about Walbro carbs

well let us know how it worked out, how much oil are you using on these babies?

Did not try yet.

Because the metering system is working fine (I mean the valve closes and opens), fuel pump is giving a pressure, and as soon as the metering valve opens I see a fuel dripping of the main nozzle then I figured out there must be two possible reasons:

1. metering spring is to strong - very unlikely as this is original one not modified by anyone.
2. at the venturi there is no enough vacuum to suck the fuel of the metering chamber. This vacuum is the one that presses the spring and opens the valve. So the reason for not enough vacuum must be either not enough airflow to the engine (kind of unlikely) or intake disturbance by the prop wash. It is possible that prop wash is pushing the air into venturi creating pressure instead of expected vacuum, and then the metering valve never opens.

To fix the second possible issue I installed my ugly velocity stack. This additional 1 inch of tube in front of venturi should smooth out the air flow and fix any pressure wave problem. I'm going to run the engine soon.

The last mixture I made was 9% of Red-line synthetic oil, and it was plenty of oil inside the engine. Now I mixed next batch of fuel this time 1:16 (6% oil) with Amsoil 100. After I run a couple of tanks of that through this engine I will check it again. As long as I have heavy oil spill on the fuselage I think I am OK with lubrication.

Bummer, the same thing.[:@] It should work, but it does not. Now I'm ready for major surgery. The worse case scenario is I will replace the body of this carb with another one (I have 4 of these WT308).

Now I think it's time to to major surgery. I reduced the spring to minimum but the engine dies on more than 1/4 throttle. Manipulating with Hi needle doesn't change a thing. So I started to play with Lo needle till I got full throttle rpm. The engine was running only on one needle (Lo end) almost full range so it is not the issue with fuel delivery to the metering chamber. And it looks like this carb size work good on this engine. As soon as I figure out the fuel problem.

Now I think that the Hi end just doesn't work. Maybe it is clogged or check valve doesn't work. Anyway this piece is going to have a major surgery. But before that I will try another body - maybe it will be better.

Of course any tips and comments are welcome.

Some carbs have a fine screen under a welsh plug. Sometimes that welsh plug has got to come out and screen cleaned. You will need a new welsh plug also. This may not be the problem with yours though. Good luck Capt,n

“Some carbs have a fine screen under a welsh plug.�

Interesting,I've never seen this.Please list an example or two.
Thanks.

OK.I see were the hidden screen is in the metering circuit.

http://www.scotiabladerunners.ca/carbs.htm

RysiuM Some thoughts on pop-off pressures that may give you some ideas. http://www.rcuniverse.com/forum/m_41...%2Csite/tm.htm

I think now it is not the pop-off but the lack of pressure. I did not have time yesterday to play with it but now I'm convinced that the fuel is not going through Hi-end passages. I mean it is soaking, but not enough to create a vacuum under the membrane. The only fuel that is going through is via Lo-end passages and it does activate the pop-off valve.

Capt'n you are genius.

I looked through my carb and there was a brass cap. It was on the metering site and looks like above the main jet. I popped it out (it was easier than I thought using some metal screw (about 4.5mm dia) and there was a fine mesh screen under it. The screen looked clean but I couldn't see a light through it (I used flashlight inside the carb throat). So I took the screen out - it was clean, but I still couldn't see the light through the main jet. 0.3mm wire fit very nicely into it and using it i pushed out something that was inside the jet. After that I was able to the light (I mean the flashlight, not the "light in the tunnel"). Then I assembled everything together and try to run the engine.

Now there is no problem - engine is running great. There is a weired transition that is, when I kick the throttle from idle to wide open then engine hesitates for a while. Then running full throttle it looks like it takes to much fuel from the idle jet as when i close it rapidly it gets slightly rich and after a second or two it smooths it's idle. Moving throttle slowly doesn't create an issue. I'm afraid my "modification to the metering spring is showing up. I'm going to replace the spring with the new one and try it again.

Anyway the engine is running fine with this carb. I'm getting close to the point where I want to be.

That's great news. Glad to hear it's getting close to being right.

I replaced the spring with the original one and set the lever "by the book". There is no problem running the engine through entire throttle range. Mixture is fine. Still there is a problem of two kinds:

1. From idle slam the throttle wide open and the engine hesitates. Doesn't die but it runs very rough. If I move the throttle slowly (about 1/2 second from idle to WOT) engine accelerates without problem.

2. If I set the idle to quite low but stable then run the engine at WOT for a while. Then I suddenly close the throttle to idle the engine dies. When move throttle slower (about 1/2 sec from WOT to idle) the engine deaccelerates without problem and stays at idle. If the idle is setup higer and I shut down the throttle from WOT the engine doesn't die. Just slows down to the high idle.

Any tips for that?

LSN is to lean.

LSN ??? - what's that?

Low Side Needle ?

If you mean Lo needle it is set up to keep the engine at reasonable idle. If I unscrew it more, the engine starts sputtering on idle.

. Also pressed into the carb are two welsh plugs which respectively each cover the high and low speed fuel jet chambers. Underneath the high speed welsh plug is another filter screen most people do not know about. It is retained to the chamber floor with a small ring and the welsh plug has to be removed to gain access. Dirt and crud can get past the first screen located at the front section before the metering diaphragm needle. Rys....is this where you found blockage? Now on the transtion...some engines will hesitate when slamed open. Just do not slam it open. You might try and richen up the low speed needle a tiny bit. Also if a engines idles to long...they will load up and have to clear up on the quick opening. I think you are very close. The only thing I wonder how is that engine going to get good cooling air with all that carb and adapter up front. You may have to make air control metal-work!!! Capt,n