Hi Pressure Fuel System on 96 Speedster

[chansman43]

With the right engine cutting off at high power only, is there a general place to start looking at for this issue? Is it usually a filter clogged issue?, etc...It's only started doing it the last couple of times out. Was running fine before that and last summer. Any help greatly appreciated. I have the manual but it does not seem to give a good Description/Operation of how the fuel system works.

 

[Dr Honda]

I'm not exactly sure what you are asking, or what the issue is.

The title says "High pressure fuel system"... but there isn't any high pressure except internal of the carbs, or in the tube feeding the second carb. (in a dual carb system)

Then... you hint that the engine may be cutting out at high speed, but I'm also reading it like you are shutting it off.

Please tell us the exact issue of what is happening, and we will try to tell you where the problem may lie.

 

[chansman43]

OK, sorry for the flub. The problem is that the right engine dies at high power. Just like I hit the kill switch. It runs fine below 5000 rpm, just high power it will die on its own. No burp or hiccup, just a clean shut down. If it is starving for fuel at high power, I'm trying to see if there is a general consensus of what usually is the problem. I'm currently out of town so I'm trying to get some good ideas for troubleshooting when I get back.

I've read thru Chapter 6 of the Mtc. Manual but in my opinion it leaves a lot to be desired. It seems to be more removal/installation and adjustment/test. Those are great, but a description/operation section would help me to understand the basic operation. If I know how it works normally, I can maybe figure out what is abnormal about it.

My understanding of the fuel system is there is a low pressure circuit and a high pressure circuit. Maybe "higher" pressure circuit would be a better description. From what I've pieced together this "higher" pressure circuit is the most likely candidate causing the shut down if it's only happening at rpm's above 5k.

Again, with all the experience on this board, I figured someone would know a general place for checking or tests that would help to isolate this issue. Lot's of heavy hitters on here.

 

[Dr Honda]

OK...


A quick over view of the system is... all the black rubber lines you see are low pressure. Between the tank and the carbs... that is under a vacuum as the pump in the carb draws the fuel in. The return line has a very small amount of pressure, but only enough to return the fuel to the tank, in an unrestricted hose. (the pressure is just enough to overcome the rise in the hose)

Once the fuel enters the carb, there is a diaphragm pump. This pump will supply enough pressure to push the needle and seat open in the carbs. In your engine, that is about 40 psi. There is also a restriction in the return nipple of the carb, so any pressure above the "Pop-off" pressure becomes the return fuel.


After the pump, (this is all internal of the carb) the fuel's pressure opens the needle and seat, and enters the bowl diaphragm. (the same as a float bowl in a std carb) This pressure is known as the "Pop-off" Pressure. The Pop-off pressure is the equivalent of setting the float hight on a std carb. Now that you have fuel in the bowl, the pilot and high speed circuits feed from it. The pilot system has a jet, a idle adjust needle (low speed fuel) and multiple bypass holes in the venturi. All of that works together to have a smooth idle, and good throttle response to about half throttle. All the fuel goes though the jet, and it becomes the limiting factor in the system.

In the high speed system, you have a check valve, a jet, and the high speed needle. In this system, the jet and the high speed needle have their own supply of fuel, and can be adjusted separately. The check valve is there to keep the pilot system from sucking air back.


Sorry if that got long... but you asked for an explanation.


If your engine is cutting out above 5000 RPM... it could be a few things, but all of them are a fuel starvation situation.

1) the filters could be clogged.
2) the valves in the pump are going bad
3) the high speed jet is clogged
4) the check valve is leaking


All of the above can be taken care of with a carb rebuild with OEM Mikuni kits. (there are issues with the aftermarket kits right now)

 

[chansman43]

Damn Doc, reading your posts is like going to college. Exactly the info that I was looking for. Thanks for your response. Great troubleshooting info as well. If you ever make it down to Mississippi I'll let you work on my boat. Just kidding, but I will buy you a beer on Beale Street.

 

[Dr Honda]

Damn Doc, reading your posts is like going to college. Exactly the info that I was looking for. Thanks for your response. Great troubleshooting info as well. If you ever make it down to Mississippi I'll let you work on my boat. Just kidding, but I will buy you a beer on Beale Street.

Thanks for the props, and the offer. :cheers: