Honda Elite Carb/Diaphragm Issue. Dies if throttle is given.

[hotcuppachuck]

The other day I went to get my Honda Elite 80 started for the first time since last season. Put a brand new battery and fresh gas in it, started right up. and off I went. About a mile down the road she died. It would start back up no problem. If I hit the throttle ever so slightly it would move a bit, but anything more than that and she tapered off again. 

Figured first place to check was the carb, it was clean. So I put the carb top back on and sure enough it started right up and ran great even giving it full throttle. Another mile down the road and the same thing happened. 

Clearly this has something to do with the rubber vacuum/piston apparatus. It seemed like it would get sucked down in the idle position and not want to come back up in order to go full throttle. Is it the pin or the rubber slide/gasket that is the issue? 

Any information is greatly appreciated!!


(I do not know a whole lot about these engines for the most part all I have ever done was change the oil)

Pictures of the part I am referring to: 

[GrumpyUnk]

Have you checked the tiny holes in #26 and #27? They are the 'jets' that meter a specific amount of fuel to the various parts and passages of the carburetor.
Does the rubber/plastic diaphragm on the 'piston slide' have any tears or punctures? If so, it may not pull the slide up out of the venturi when the engine develops vacuum. When that happens, the engine is starved for fuel:air mixture and may very well sputter and stall.
Can you run the engine at less than full throttle? IOW, can you back off when it starts to fail, and it 'catches up' and runs normally at less than full fuel flow? That might indicate that you are getting less than FULL flow through the filter/fuel outlet. There could be some 'stuff' built up in the tank the gathers and clogs the screened tube outlet from the tank, and then sort of 'falls away' as the flow stops, and allows fuel to flow once again.
You could have a sticking float bowl or needle valve that allows fuel to slowly fill the bowl, the engine to run nicely until the fuel in the bowl is used up, and the engine starves again until you slow down and the bowl trickle fills once again. You could also have a fuel filter that is partially blocked, causing the same limited flow of fuel.
To test, you can remove the fuel line from the side of the carburetor, directing it into a catch vessel(soup can?) and applying suction to the vacuum line on the fuel petcock. Fuel should flow at full diameter of the fuel line without any interruption until the tank is emptied or you cut the flow. If it does not, check backwards towards the tank, including filter, petcock, and outlet screen.
You might want to check flow before doing anything with the carb as would prove/disprove that you have fuel available to the carb.
tom

[mikehailwood]

hotcuppa, I'd do what Grumpy first suggested. Pull the carb, remove the jets and make sure you can see daylight through them. Or maybe you did that? You said the carb was clean.

[pinkscoot]

As Grumpyunk said. I would also make sure that the diaphragm is seated properly. Its easy to screw that up. Make sure when you put it in that its sitting all the way around the edge in the groove. Here is a video:


A way to test that its seated properly is here:


You also want to be sure not to get any cleaner on this diaphragm because it will make it so it won't fit right.

[GrumpyUnk]

I have one carb that seems to have been fitted with a shrunken diaphragm. The slide fits fine, but the diaphragm does not want to stretch out to fit into the outer groove. I tried stretching it by hand, much as you do a dixie cup sailor hat if you ever served, to expand the perimeter. I finally used some tape to hold the outer rim into the groove, and pulled the tape pieces as I assembled the cover. Well, that didn't work. The diaphragm was too 'small' to allow the slide to move properly in the bore, keeping it raised a bit too much from the at rest position.
I measured the slide diameter, and it is not one of the two sizes I could find. The carb is labeled Keihin, but I have suspicion it may be something else in disguise.
Any way. Putting the edge of the diaprhagm into the groove, and making sure the slide/diaphragm are oriented properly (tab in the appropriate slot) should allow the slide to seat properly and rise out of the way properly when vacuum is applied.

You should be able to lift the slide, and let it fall after the thing is assembled. You should hear 'vacuumy-like' noises as the slide rises and falls, and it should move freely in both directions.
tom