r75 /7 starved for fuel or air??

[smitty]

hi folks, new to this forum and looking for some help. my r75/7 has been periodically losing power ( asif it was running out of gas) usually round 50 or so mph. i cant tell if it's running on one cylinder but i back off the throttle, limp on the shoulder and it comes back, get up to speed and it happens again. i had the carbs rebuilt, dyna III installed, new plugs , fuel lines and filters, and my mechanic found a field mouse's nest in the air cleaner, ( it spent a year in my shed) which we thought was the problem, but it's doing it again. it runs like a top when this isn't happening, i,m hoping it's not a compression problem . could something from this nest ( like a dead mouse! ) be obstructing the air flow further down the intake, or could there be some pieces of rust that periodically block the flow of fuel to one cylinder? are there any tests i can do to rule out one or the other, especially since my mechanic is over an hour away...thanks

[Rbike]

It will only take a screwdriver and a few minutes to be sure that the rest of the air intake system is mouse free. Just unscrew the clamps on the plastic tubes leading to the carbs, and the clamps that hold the carbs to the cylinder heads. Then wiggle the parts outward away from the bike and look inside.

If you lose power at speed then you are most likely running out of fuel. The next time you get the bike to act up: grab the clutch, quickly turn off the fuel petcock while the problem is happening and kill the engine, coasting to a stop. Pull off the carb bowls carefully and look to see how much fuel is in the bowl. You should be able to measure about 3/4" of fuel in the bowl. If it is low then you are starved for fuel.

If you have replaced the filters and lines as stated then the problem is most likely in the petcock. Remove the fuel line and turn the petcock on with the line in a jar to see if it looks like it is flowing enough fuel. I don't remember the spec, but if you ask around somone will know how much fuel should flow in an alotted time.

Good luck!

[Rbike]

A little math and some help from a cook book tells me that a bike getting 35mpg needs about a pint of fuel every 4 minutes (actually 1/7th of a gallon in 5 minutes). So check to see that you can flow a pint in 4 minutes.

[Rbike]

OR...to put this in Airhead terms, you should be able to fill an empty beer can/bottle in about 3 minutes:idea

[phoenixtexas]

smitty: I'm a former airhead wrench who has seen gas tank venting problems cause the exact symptoms you described. next time it happens, gently turn the fuel cap out far enough to make sure you're getting air and see if the problem suddenly clears up. as mentioned in another post, check your fuel flow from the petcock screen filter down, also make sure any inline filters installed between the tank and the carbs flow enough gas. hope this helps. ACS, Sherman TX

[sgborgstrom]

This sounds very similar to a problem I had on both a /6 and R100S. New air filter solved it both times.

Steve