'01 DS drivability issue when hot

[FrankenCub]

We've been driving our DS everywhere and usually never have any problems. A few times I noticed it starving for fuel so I replaced both fuel filters with better results. A couple times, including today when the ambient temperatures have been pretty hot it's had the same problem. I put my vacuum gauge on it today after nursing it home and it has a terrible vacuum pulse. Runs great until it get hot, then only when it's in the upper 80's or so.
I'm not seeing oil leaks to lead me to believe there's a bad seal.
Any pointers as to where to start with this?

[FrankenCub]

So I'm thinking of either adding an outside body air intake to be ducted to the fan grills, or an electric fuel pump.
The engine runs perfect unless the ambient temps are high making the engine run hotter after a long drive then loosing a consistent vacuum pulse.

[BobInOK]

Off the top without really knowing what I'm talking about. If that pump pulse has anything to do with crank case pressure could your oil be thinning out at high temperature? Might be just a matter of going to a higher viscosity oil in hot weather.

[FrankenCub]

The reciprocating piston makes the pulse. Oil viscosity shouldn't effect the pulse. I could see bad rings doing it but there's no lack in performance other than the lack of fuel flow.
I'm about ready to find a low pressure electric fuel pump lol.

[CP241]

The intake is already outside the body in 2001. It sucks in air from under the front cowl.

Are the filters full of fuel when it suffers? The fuel.pump needs very little actual pulse to work. You cant measure it with a vacuum gauge because it alternates "suck-blow" so quickly. Does choking it have any affect when it starts to stumble?

For giggles, carry a spare spark plug with you. When it starts running bad, quickly lift the seat and plug the spare plug into the wire, hold.it against a valve cover bolt, and crank it over. What color is the spark? Yellow? White? Blue? Ia it steady and consistent?

I'd lean towards a failing coil or rev limiter before I would assume issues with the pulse line.

[FrankenCub]

Originally Posted by CP241

The intake is already outside the body in 2001. It sucks in air from under the front cowl.

Are the filters full of fuel when it suffers? The fuel.pump needs very little actual pulse to work. You cant measure it with a vacuum gauge because it alternates "suck-blow" so quickly. Does choking it have any affect when it starts to stumble?

For giggles, carry a spare spark plug with you. When it starts running bad, quickly lift the seat and plug the spare plug into the wire, hold.it against a valve cover bolt, and crank it over. What color is the spark? Yellow? White? Blue? Ia it steady and consistent?

I'd lean towards a failing coil or rev limiter before I would assume issues with the pulse line.

I meant an external air intake for the cooling intake, so it isn't pulling in heated air underneath the seat.
I'm positive it's a fuel issue. When this problem shows itself the filters are drained. One day on the way home I put it in maintenance mode to watch the filters, fuel flow looks diminished. Flow can't keep the engine happy. Only does this when the ambient temps are high though. On an 80 degree day it's fine. Start getting around 85+ and it acts up during a long ride.
If I use less throttle it's able to run without starving for fuel so bad.

[FrankenCub]

Originally Posted by Fairtax4me

These will blow the front case gasket completely out, blast oil all over the side of the motor, and still make enough pulse to operate the fuel pump just fine.
The hose or port on the crankcase is clogged, or the hose is drooping down and collecting oil. The hose has to go up-hill all the way to the pump. If it's old, it's a good idea to replace it. Make sure the nipple on the engine isn't full of sludge and caked up old oil.

It's possible the pump has a problem, but I've never had to replace one that wasn't leaking fuel.

The hose doesn't droop down, just looked to be sure. I don't think the hose or nipple would be clogged as it runs fine any other time. It's only during high ambient temps. Maybe I should bring along my compression guage to check it when it acts up.

[CP241]

Try checking spark. Just to rule it out of nothing else.

I had a DS I had installed.a FE350 in do a similar thing. On hot days (in GA, way hotter than 85°) fuel pump would give up after about 5-10 miles of constant riding. I replaced hoses, filters, ran it with the cap off, etc. Didnt change the fact that only when it got really hot I could pull the fuel line off the carb and nothing came out. Let it sit for 20 minutes and it would run again.

For me, it was the fuel pump. I swapped pumps with my carryall and the DS worked fine. Weird thing is, the "bad" fuel pump worked fine in my carryall too. Perhaps because it had a 6" lift, substantial larger engine compartment, and better air flow? I dont really know. But I ordered a new fuel pump anyway so I could put my carryall pump back where it goes :) I don't like stealing parts from one cart to fix another. a $25 mikuni briggs and Stratton fuel pump from amazon fixed it. It's the same fuel pump as OEM but has a Briggs logo on it.