1988 SB50 sputters high speed

[elitedio]

I would say that it is some variation of what wheelman is talking about. A vacuum petcock with a pin hole will do exactly as he says, or you say.

I assume that you have physically inspected the main jet with some magnification. A dirty MJ can make your engine run hot and slow after a few mins.

[smanke]

I haven't checked the main jet with magnification. What would I be looking for? What is the easiest way to test the fuel petcock? I appreciate all the help. Is it a good possibility the float needle is bad?

[Bear45-70]

On the jet and the tube above it you look for and obstructions in any holes. Also check the float level, a low fuel level will cause this too.

To check the petcock you will need a vacuum pump, Harbor Freight has them for about $25. Pull a vacuum on the valve and see if it leaks down. It should hold the vacuum, if it doesn't it is bad. If it was me, I just change all the hose first and use small tie straps as clamps.

[smanke]

I have a new float needle coming on Tuesday. The float level is ok. I have a vacuum pump to check the petcock. I will post the results of my tests tomorrow. Thank you for the help.

[smanke]

When I test the petcock I just need to pull the vacuum line from the intake? Can I leave gas in the tank and leave everything else hooked up?

[Bear45-70]

Well, you should pull the fuel line off the carb so you can see how much fuel actually flows. But yeah the tank needs to have fuel in it or how would you know if the petcock opens and closed.

[smanke]

What I meant to say is the vacuum read on the tester should remain constant and not go down. If it doesn't hold a constant pressure then the petcock is bad correct?

[Bear45-70]

Yes

[smanke]

I applied 5 inches of pressure and it held constant. After 15 seconds the fuel stopped flowing from the tank. So I would say the petcock functions normally. Or is that too long for fuel to keep flowing after vacuum has been stopped? I'm going to put it back together and see how it runs after cleaning the carb and honing the float needle seat. The float needle does create a solid closure so no air leaked through on that test.

[smanke]

So the new float needle has arrived and has been installed. The float needle seat has been cleaned. The carb was thoroughly cleaned once again. I put it back together and the same problem still exists. After running WOT for a couple minutes you start to lose some top end speed then the engine starts to cut in and out while holding WOT. Once releasing the throttle and the engine idles down it dies. It will start back up and idle fine. So I have pretty much met the brick wall. I've almost completely had it and ready to take it to the pros at the Honda dealer. Does anyone have another theory? I have heard something about a mystery whole in the carb that isn't visible with the naked eye. Could this be a possibility? I appreciate all the help.

[Bear45-70]

I applied 5 inches of pressure and it held constant. After 15 seconds the fuel stopped flowing from the tank. So I would say the petcock functions normally. Or is that too long for fuel to keep flowing after vacuum has been stopped? I'm going to put it back together and see how it runs after cleaning the carb and honing the float needle seat. The float needle does create a solid closure so no air leaked through on that test.

With vacuum still applied the fuel stopped flowing after 15 seconds or 15 seconds after you released the vacuum, the fuel stopped flowing?

[Bear45-70]

So the new float needle has arrived and has been installed. The float needle seat has been cleaned. The carb was thoroughly cleaned once again. I put it back together and the same problem still exists. After running WOT for a couple minutes you start to lose some top end speed then the engine starts to cut in and out while holding WOT. Once releasing the throttle and the engine idles down it dies. It will start back up and idle fine. So I have pretty much met the brick wall. I've almost completely had it and ready to take it to the pros at the Honda dealer. Does anyone have another theory? I have heard something about a mystery whole in the carb that isn't visible with the naked eye. Could this be a possibility? I appreciate all the help.

Every time I here a noobie claim their carb is spotless and it couldn't possibly be the carb, I could SCREAM, especially since the setup still presents a carb issue. I worked for many years as a professional 2 stoke mechanic and even with cleaning a 600+ carbs a year, I still missed something every once in a while. So for an amateur to claim their carb is spotless and it couldn't possibly be their carb, is ludicrous at best. It isn't how shiny the inside of the float bowl is, it the little tiny passages and holes in the carb that are the important thing and you can't see some of them. End of rant!

[smanke]

I may be a noob to working on this scooter but this isn't my first rodeo working on a mechanical problem. I appreciate the concern though. I have cleaned everything in the carb. I put a guitar string through every passage and carb cleaner sprayed out every hole like Ole Faithful. So I retested the fuel petcock. I left everything hooked up except I took the fuel and vacuum lines off the respective end for the test. I applied 5 IN of pressure and it held constant. After releasing the vacuum pressure the gas runs from the tank for almost 30 sec. This seems a little extreme but I'm not certain. Could this be my problem? I appreciate the continued support. Is it possible there could be a leak in the head gasket?

[Bear45-70]

I may be a noob to working on this scooter but this isn't my first rodeo working on a mechanical problem. I appreciate the concern though. I have cleaned everything in the carb. I put a guitar string through every passage and carb cleaner sprayed out every hole like Ole Faithful. So I retested the fuel petcock. I left everything hooked up except I took the fuel and vacuum lines off the respective end for the test. I applied 5 IN of pressure and it held constant. After releasing the vacuum pressure the gas runs from the tank for almost 30 sec. This seems a little extreme but I'm not certain. Could this be my problem? I appreciate the continued support. Is it possible there could be a leak in the head gasket?

Working with a baby carb for a 50 cc engine is not the same as working with a 780 CFM Holley 3310. They may both be carbs but the ports and passages are minuscule on these little carbs and it takes next to nothing to plug or partially plug one. Unless you followed the carb cleaning instructions and pictures and even if you did, chances are you missed.

That seems a little long for the fuel to flow after vacuum is gone. Weak spring maybe? How little vacuum does it take to get fuel to flow?

No, the head gasket can not cause your problem. It is either a fuel issue or a vacuum leak.

[toad772]

Greetings:

My first Aero - Flash Scootertrash 1.0 behaved as you describe in your first post. The issue was a vacuum line wasn't very tight on a fitting - wrong size. It developed enough vacuum at low throttle settings, but the vacuum decreases with WOT or close to it. As a result, the little leak was enough to allow the petcock to shut off after about 10 seconds at WOT. The fuel in the bowl burns off in about 10 more seconds at "High" speed, so it went chuff-chuff, closed throttle, refilled, re-fired and so on until I replaced the vacuum lines.

My aero is doing the same thing but my petcock does not work and fuel flows all the time.