Anyone know how to test a vacume fuel pump

Rio_Grande · Apr 27, 2011

I dont understand why suzuki finds it necessary to make things so dang complicated but I have a vacume operated petcock and a vacume operated fuel pump.

I need to figure out how to test these components. I am not getting fuel to the carb on my 99 ltf300f. "king Quad"

I was thinking pulling the out line on either would show fuel coming out but no dice.

Any ideas?

oldnslo · Apr 27, 2011

Rio_Grande said:
I dont understand why suzuki finds it necessary to make things so dang complicated but I have a vacume operated petcock and a vacume operated fuel pump.

I need to figure out how to test these components. I am not getting fuel to the carb on my 99 ltf300f. "king Quad"

I was thinking pulling the out line on either would show fuel coming out but no dice.

Any ideas?

On the vacuum operated pet cock there should be a vacuum line that leads to one of the intake ports. Remove this line and draw a vacuum on it. The pet cock should open and allow fuel to flow.

Suzuki motorcycles had the same setup and it was not uncommon to have one of the valves in that cylinder be to tight and this will cause the petcock to malfunction. This Caused more than one cussing contest at Suzuki....

The fuel pump actually works off of the vacuum - pressure pulses created in the crankcase. best way I have found the best way to check these are:
1) pull the line off that goes to the carb and stick it in a container to catch the fuel.
2) Stick the inlet line in a container of gas so that you know you have an unrestricted inlet.

Crank the engine over, does it pump fuel yes or no. If no make sure the vacuum line line is properly connected.

Roy

Rio_Grande · Apr 27, 2011

Thanks Roy I also found some information on you tube, not my same application but it mimmicked what you said. The pump on this looks new, I am leaning towards that petcock but have to do some testing to find out. I was also thinking about putting a vacume guage inline on the vacume side to check the pulsing.

So would a tight valve cause a sudden no run no fuel output?

I will give this a shot when I get home tomorrow.

oldnslo · Apr 27, 2011

Rio_Grande said:
Thanks Roy I also found some information on you tube, not my same application but it mimmicked what you said. The pump on this looks new, I am leaning towards that petcock but have to do some testing to find out. I was also thinking about putting a vacume guage inline on the vacume side to check the pulsing.

So would a tight valve cause a sudden no run no fuel output?

I will give this a shot when I get home tomorrow.

Yes a tight valve or one that has carbon stuck under could cause this problem. On the motorcycles it was typically they would not start unless the petcock was put in the prime position and then they would start and run.

If your petcock has a "prime" position this overrides the vacuum opening feature. It was meant for the times you forgot your whre on reserve and ran out of gas etc... may want to check this and see if you get gas through it if it does have the prime position. Be one more variable that could be easily checked and eliminated.

A vacuum gauge should follow the pulses if they are not to quick. These fuel pumps are simply rubber diaphrams being pulsed to act as fuel pumps. Over time the diaphram can stretch and cause loss of pumping ability. If it looks new did some one try a rebuild and not get it assembled correctly?

Good luck

Roy

Rio_Grande · Apr 28, 2011

Thanks for the info Roy,

I will give it a shot. the quad had sat for a while before I bought it but ran off and on all winter before I finall got the oppritunity to take it for an extended open throttle ride which I had to push it home from. After it sat for a half hour it would start up and run for a short time then back to a no run condition.

It about has to be in the fuel system, the plug seems to have a good spark, but I am gonna toss in a new plug to make me feel better.

oldnslo · Apr 28, 2011

Have you got the equipment to do a compression test on the cylinder. This also may help point you in a general direction.

I find random failures are the hardest to diagnose so patience is a virtue....

Roy

Rio_Grande · May 9, 2011

I have no idea,,,,,, Pulled the vacume hose to the petcock and it has measurable vacume (more than I expected) and plugged it back up. Checked the petcock for fuel flow and it definately has fuel flow now. Pulled the fuel line after the pump and while it diddnt have any flow initally it pumped up and started pumping. Started the quad ran it around a bit let it idle for about 45 min and never a problem.

What the heck!!

I even plugged off the fuel line and it went to spraying so it has pressure.

I do not have a compression tester anymore as I dropped my last one LOl.

oldnslo · May 9, 2011

Sounds like the pump wasn't priming. May want to take a good hard look at the hose between the petcock and the pump. Does it show any signs of wether checking of cracks? If yes replace it. I have had a couple of lawn mowers that where sucking air through the hose but did not allow gas to seep out. They where a real SOB to diagnose since they would run fine for period of time and then start missing sputtering etc. NOTE: On both of these the fuel tank was remote and may have had gravity fead under some circumstances.

Roy

Rio_Grande · May 29, 2011

Hey Roy any idea how much fuel pressure these pumps should be putting out?

I thought I had this licked but yesterday in the woods it did the same thing. Got it home after pushing it up hill to the trailer and it started and drove off the trailer???

I am going to replace EVERY vacume hose I can find but one thing I noticed while cranking the fuel comes in spirts as expected but it has almost no pressure??