MOTOBRICK.COM

TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: kennybobby on May 22, 2016, 10:39:48 PM

Title: [Solved] Lack of Power above 4krpm, stalling, backfire, poor performance
Post by: kennybobby on May 22, 2016, 10:39:48 PM

i’ve read thru lots of posts with these sort of issues, but haven’t read of anybody finding the problem and fixing it, or at least they don’t post the solution if it was found.

So the quick answer here is that i had a combination problem:  water in the tank due to a defective seal under the fuel cap, and a cracked vacuum line at the fuel pressure regulator.

Here’s the story:
i bought an ’88 75S with 18k miles several years back as the third owner.  Put some fresh tires and rode her hard and fast for 5k miles. Got busy and let it set up for a winter outside, then in the spring it ran like crap—bogging and stalling and no power.  i always buy pure gas, so unlike my restraining orders--i couldn’t blame it on the alky-hawl.

After checking thru the fuel system i found a couple of cracked fuel lines (original, >25 years old), and a cracked vacuum tube at the throttle body (that runs to the fuel pressure regulator).  i  replaced the fuel lines and cut a fresh end for the vacuum hose--thought that would fix it but it was stalling even more.

So with further investigation i found that the fuel cap was facing backwards and the little vent slot in the rim was on the wrong side.  Under the cap was a pile of blue rtv compound filling the drain hole and slathered all over the place.  Got the drain cleared and put the cap back in the correct orientation—didn’t solve it but i felt better…

Now i’m feeling bad and getting pissed at this crappy running machine, so i drained the tank to take a look around inside.  That’s when i found nearly 1/2 gallon of water in the bottom of the tank.  Cleaned and dried it out and replaced the fuel filter.  It ran better for awhile but still didn’t want to rev like it should. It is parked outside all the time, and after a few short months it was stalling again.  Checked the tank and there was a quart of water in the bottom!—wtf?  Oh now i see why it had the rtv job, something is not sealing and letting water in the tank when it rains.

So i got a plastic cover for the tank and no rain has hit the fuel cap in the last year, but going for a ride this spring and it doesn’t like to take any throttle or run much more than 4k rpm, stalls at random times, backfires, etc.

Finally fixed it today and got my screaming machine back, it pulls great like it should—i replaced the vacuum tube from the throttle body to the fuel pressure regulator.  It was cracked at the base of the FPR where it makes the hard bend.  It can’t be seen or changed without removing the base of the air cleaner housing.  i was focused on the water in the tank thinking that was the problem, but it was a combination of issues.

So here’s what i think was happening:
the FPR return valve stays closed unless the vacuum-level pressure from beneath the throttle butterfly valve is high enough to open the regulator valve.  So at idle the vac pressure is the greatest and the return valve is opened to it’s fullest.  This reduces the pressure in the fuel rail as the return path has less restriction.  At idle the butterfly is closed, and at 60mph at 4krpm the throttle is barely open—so the vac pressure is high and the fuel pressure is low, which is fine for that power level.  When you crack the throttle open to accelerate the vacuum pressure under the butterfly is reduced, so the FPR valve closes somewhat to momentarilly increase the rail pressure, which gives you an extra squirt of fuel to go with the extra air and give you that kick in the butt.  With a wide open throttle the butterfly vacuum pressure regulates the FPR valve for maximum fuel pressure.

When the vacuum line is cracked and broken the FPR valve is stuck closed, so the fuel pump is deadheaded with little to no return flow to the tank.  If water gets pumped down into the fuel rail the bike will stall hard on the bad mixture.  If the FPR was working as normal then water in the tank wouldn’t matter so much—some would get squirt to the engine and the rest returned to the tank, and over time it would pump all that water thru the fuel rail and eventually ‘burn’ it off.  But the bike would stall and run crappy during that time when water diluted the mixture.

Title: Re: [Solved] Lack of Power above 4krpm, stalling, backfire, poor performance
Post by: mystic red on May 22, 2016, 10:59:21 PM

So here's the deal Ken, in your spare time, now replace all of the rubber bits connecting the engine to anything. They are all going to bite you eventually. :bmwsmile

Title: Re: [Solved] Lack of Power above 4krpm, stalling, backfire, poor performance
Post by: K1300S on May 23, 2016, 06:19:28 AM

Yup.   Every piece of rubber in engine, brake and cooling system needs to be replaced.   These things are old.   Rubber does not last.