1988 SB50 sputters high speed

[smanke]

I received a 1988 Elite SB50 for free from my father. It only has 250 miles. It didn't idle at all so I took the carburetor apart and the idle jet was completely clogged. I cleaned it and put it back together. That problem is solved and a new one has emerged. Running it full throttle for 5 minutes it gradually loses a few miles per hour. It then starts to sputter like it's losing fuel. So it slows then full speed then slows. When you let off the throttle and the engine idles down it just dies. It will start back right up though. I don't know what is wrong with it. I know it's not the electric choke. Everything is clean in the carb. I've checked the float level and there are no vacuum leaks. The gas tank has been cleaned and all lines checked. I am reaching my wits end on what could be the problem. It's my first Honda scooter. I appreciate any help.

[Bear45-70]

First thing I would try is to loosen the cap (just not tight) on the fuel tank and see if it still does it. It sounds like you are developing a vacuum in the fuel tank and starving the engine for fuel (a bad thing as it can melt the piston. This is caused by a clogged vent in the gas cap.

[smanke]

I already tried the gas cap thing. I ran it with it not all the way tight and with it completely off. Any other suggestions? I am really getting frustrated at this point. I had bought a new carburetor with the choke. But the carburetor was the wrong one. I put the new choke in it to try it and it's not that. I know it seems like a fuel problem but I just can't figure it out. Could it possibly be a ignition issue? It's such a specific problem I would think someone out there has ran into this. Hold it open full throttle for 5 minutes or so and it loses a couple of miles per hour on the top end. Then it starts to cut in and out and when you let off the gas and the engine idles down it just dies. Then it will start right back up. Oh I'm so close to just throwing in the towel!

[Bear45-70]

I already tried the gas cap thing. I ran it with it not all the way tight and with it completely off. Any other suggestions? I am really getting frustrated at this point. I had bought a new carburetor with the choke. But the carburetor was the wrong one. I put the new choke in it to try it and it's not that. I know it seems like a fuel problem but I just can't figure it out. Could it possibly be a ignition issue? It's such a specific problem I would think someone out there has ran into this. Hold it open full throttle for 5 minutes or so and it loses a couple of miles per hour on the top end. Then it starts to cut in and out and when you let off the gas and the engine idles down it just dies. Then it will start right back up. Oh I'm so close to just throwing in the towel!

Sure it could be ignition. Heat related electrical component failure happens all the time. To test, run with the engine cover off so you can pull the plug wire off when it dies and check for spark. I would remove one side only at a time and if cover removal cures the problem, then at least you will know which side the failing part is on (maybe). But it does not act like an electrical failure.

[smanke]

Do you mean run it with the frame covers off? If so I have done that and it makes no difference. Or what covers are you talking about? I have the service manual so I can test the ignition components to rule them out. I feel like I'm getting close but yet so far from solving this mystery. It can't be anything to serious though.

[Bear45-70]

Do you mean run it with the frame covers off? If so I have done that and it makes no difference. Or what covers are you talking about? I have the service manual so I can test the ignition components to rule them out. I feel like I'm getting close but yet so far from solving this mystery. It can't be anything to serious though.

Yes I'm talking about the body panels. If it is a heat related problem you have to test each ignition parts TWICE. Once cold for a base line and then right after the engine dies when they are hot (and I mean RIGHT after) and probably only one each time it dies because they will cool off very fast.

[smanke]

So I will test the ignition components. Shouldn't the service manual tests suffice for testing the ignition components? Or is it imperative to test them after the engine dies? What am I really supposed to test if I do it this way? Is there any other ideas of what can be wrong?

[Bear45-70]

Heat related failures almost never show up with a cold component. The part works fine when cold. When they get hot the values change and that is when they check bad.

[smanke]

So I will test them when they are cold. Then I will run it til it malfunctions and systematically test each ignition component. I will post the results of my tests tomorrow. Until then are there any other theories to this mystery?

[vette66_00]

Running super lean? Running into soft seizing after five mins. Cause a soft seize will kinda act similar.

Could also be a fuel delivery issue, petcock kinda plugged feeding enough to fill the bowl at Idle but as the demand goes up it can not keep up with the demand so it runs out of fuel dieing. While trying to start it vacuum open the petcock again fills the bowl and the cycle starts over.

I dont know, I'm taking stabs, you asked for other theories.

I also have one about aliens but your not wearing your tin foil hat so I cant tell you..

[Bear45-70]

Is all the air cleaner system there an sealed up as it is suppose to be?

[smanke]

The air cleaner system is all there and in working order. I don't think the motor is seizing up at all. What is the easiest way to test the fuel delivery issue? I think though it's not a carb issue now.

[smanke]

So I tested the ignition components and they are not the issue. So I still think it's a fuel issue. I ran it again until it sputters in and out and then just let the engine quit. I opened the drain screw on the carb bowl and gas continuously pours out even though the engine isn't running. I know that the petcock is vacuum operated so this doesn't make sense. I pulled the gas line off the carburetor and the gas stopped flowing from the tank and the rest of the gas stopped flowing out of the carb bowl. So somehow it seems a vacuum is operating with the engine not running. I pulled both lines from the gas tank while it's not running and no leaks. So I think the fuel petcock is operating normally. Could it be the fuel needle in the carb? I have checked the float level and it's ok. If it's not any of this stuff then I don't know what is happening or where to go next. I appreciate all the help!

[Wheelman-111]

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.

[smanke]

The vacuum line is a tight fit. But gas shouldn't keep flowing to the carb with the engine off. So it's creating a vacuum somehow. It's quite the interesting problem. I will put on a new line for the vacuum and see what happens. Any other ideas anyone?