Losing my mind with an ATV

JDEM · Mar 2, 2018

I have a 4WD Suzuki there that is driving me nuts. Actually -I have two in my shop with problems - but I will stick to one that has me most perplexed.

1995 4WD Suzuki King Quad 250 cc. Four-stroke-cycle engine. Ran fine when I pulled it into my shop last Fall. I did a lot of mechanical drive-line work and NO engine work. I made one change to the fuel system by changing the fuel-valve under the tank.

Now - it will not start. At this point - I refuse to believe the fuel-valve is the problem. But like I said - I am beginning to lose my mind on this thing.

Here is how the basic fuel system operates. Fuel valve that I changed has two positions. "Run" and "Primer." It has a vacuum-gas-shut-off-valve. So when in "Run" position - no gas can flow without engine vacuum. When in "Primer" position, that valve is bypassed and gas runs out freely. I tested the valve and it works. From there - the fuel delivery line goes to what Suzuki calls a "vacuum fuel pump." My factory service manual gives NO info on it other then " if it fails, replace it."

I guess I do not understand how this pump is supposed to work. I have worked on similar looking pumps on two-stroke-cycle engines that worked off of crankcase impulses. This is however a 4-stroke-cycle. The port on the intake manifold it hooks to provides a steady 6-7 inches of vacuum and no pulsing I can perceive.

I took the fuel-pump apart and all looks fine. Diaphragms are soft with no holes. Two check valves look fine. Put it back together and tested it with a hand-held vacuum-gun. A steady-vacuum and it pumps no fuel. A on-and-off pulsing vacuum and it still pumps no fuel. BUT - I then tried blowing air into it, off and on with pulses. This way it pumps lots of fuel.

So I am almost at the end of my rope. Seems I never had to think this hard before about a pump like this. I am very temped to put on an electric pump but it is hard to find one that really runs at low pressure. I have tested a few rated at 2-4 PSI and they ran higher. Likely too much for this carb.

I stuck a gravity feed tank on top of the engine and it starts and runs fine. The built-in tank sits near at carb level and needs a pump. So - I am getting a mental fog here. Suzuki does call it a "vacuum" pump and not an "impulse" pump. It hooks to the intake manifold and I cannot figure how there should be any impulse there anyway. Just a steady vacuum.

Somebody here got an idea about what I am missing here?

495man · Mar 2, 2018

Got a 300 Zuk same set up, it would take a spell where it would just die, and you'd have to crank it with the throttle half open for about 5 minutes before it'd fire again, then be fine for a long time. I never "fixed" anything on it, by I suspected the fuel pump. Bet there is a vacuum leak somewhere?

ss55 · Mar 2, 2018

When was the last time the carburetors were rebuilt? With motorcycles I was lucky if they would make it ten years between overhauls (complete tear down, cleaning, new diaphragms, new gaskets, and re-tune). Carb overhauls usually made the old bikes run and start like new again. While it's apart, replace any o-rings in the fuel system, at 20 years they are ready to start leaking.

Ken Combs · Mar 2, 2018

I don't know how it works on a constant vacuum either. Every 4 cycle pulse pump I've worked on hooks to the crankcase, and the pulse from vacuum to pressure is what drives them.

However, I did find this old post on an ATV forum:"there is a fuel pump under the front left hand side on the front guard.

it is driven by the vacum from the motor.

most likely it is not broken! they have a good reputation.

with the 250 and 300 LTF kingquads made by suzuki, when ever you cannot get fuel to the carbi, just put the fuel tap on to prime, then blow in to the fuel cap vent hose (the one sticking out of the fuel cap)

once you clean the dirt off it, blow with your mouth for about 10 seconds with the same sort of pressure you would blow up a baloon.

while you are doing this... crank the bike over with the starter button (to activate the fuel pump)

this is what every motorcycle shop must do to the LTF 250/300 every time they get a new bike out of the crate and fuel it up for the first time.

The LTF seems to have a good pump as far as durability goes, but it is just a little weak when it comes to filling up the carbi when the bike is dry.

if you have any doubt about fuel getting to the carbi, pull off the fuel line (the hose that is going to the carbi with the metal spring protective cover) pull it off the carbi and see how much fuel comes out when you crank the bike over.

I say this because it is no good chasing a fuel pump problem if you only have a dirty carbi that needs cleaning"

Steve@Advance · Mar 2, 2018

I see your quandary, obviously it worked at one time, why not now?

Even though the vacuum gauge shows a steady vacuum, being a single cylinder I would think there is some pulsation, especially with an engine built for higher RPM.

Have you priced a new pump? If not something ridiculous, that would be my next move. Might find an aftermarket on Ebay.

Or, temporary the source to crankcase pulse, see if that works.

Or, change it over to a B&S impulse pump, connected to crankcase impulse.

Electric would be my last resort, especially if this is a customers ATV.

Crazy Horse · Mar 2, 2018

There are always brand specific forums on the internet, here's one below here. Join up (just like you did at the YT site here) and I'll bet you'd get a lot of good help. Here ya go ....
Untitled URL Link

Crazy Horse · Mar 2, 2018

Oops ..... try this link, it should work ....
Suzuki ATV Forum link ....

JDEM · Mar 2, 2018

No customers. These are my machines. I am too cheap to buy good running machines. I tend to buy non-runners or running and very beat-up and then fix up for my family.

There fuel pumps are $80 if buy the one made specific for this machine. I already bought a new one from China for $15 and found out it cannot work. It was sold as a "vacuum fuel pump" but looks to be for impulse-hookup only.

I have another Suzuki in my shop that pumps fuel very well. Near the same machine as my non-runner. So I took the pump off of that and put it on this one - and it does not work. So I guess that pretty much proves the fuel-pump is not the issue.

I have NO idea what the theory of operation is. The vacuum source for the pump is a tap on the front of the carburetor by the intake-manifold. So maybe - somehow some part of the carb takes steady vacuum and turns it into a pulse? If so, mine is not doing that since I get a pretty steady 5-6" vacuum reading.

One possibility that is messing with my "cause and effect" diagnosis it this. Maybe the level of gas in the tank was higher last Fall when it ran. If so, maybe the fuel pump never worked and it ran because the tank was fuller. Once the tank is over half-full. gas can gravity feed to the carb. I wish the tank was a little higher and I'd just forget having any fuel pump.

I am usually pretty stubborn and like complex problems. NOT this one. I spend near the entire day on this thing and got nowhere. Unless I wake up tomorrow with a better brain - I think I am going to opt for an electric fuel pump. It is that or mount a gravity-feed tank on the handlebars?

JDEM · Mar 2, 2018

I have been to many of those forums and got nowhere. Lots of people post problems and rarely does anyone ever repost with a fix that makes any sense. That happens here too, but to a lesser degree. At least here - once in awhile when somebody rectifies a problem - they post and update. Not often, but it DOES happen now and then.

MARK ROBKE · Mar 2, 2018

On the other bike,what was the vacuum on it ? Is it the same or way different ? Maybe you can diagnose this one by figuring how it works on the other bike. You have proved it will run, but is not pumping gas to the carb,and you know the other one runs.Good luck,Mark.

Louis A · Mar 2, 2018

I have a 4WD Suzuki there that is driving me nuts. Actually -I have two in my shop with problems - but I will stick to one that has me most perplexed.

1995 4WD Suzuki King Quad 250 cc. Four-stroke-cycle engine. Ran fine when I pulled it into my shop last Fall. I did a lot of mechanical drive-line work and NO engine work. I made one change to the fuel system by changing the fuel-valve under the tank.

Now - it will not start. At this point - I refuse to believe the fuel-valve is the problem. But like I said - I am beginning to lose my mind on this thing.

Here is how the basic fuel system operates. Fuel valve that I changed has two positions. "Run" and "Primer." It has a vacuum-gas-shut-off-valve. So when in "Run" position - no gas can flow without engine vacuum. When in "Primer" position, that valve is bypassed and gas runs out freely. I tested the valve and it works. From there - the fuel delivery line goes to what Suzuki calls a "vacuum fuel pump." My factory service manual gives NO info on it other then " if it fails, replace it."

I guess I do not understand how this pump is supposed to work. I have worked on similar looking pumps on two-stroke-cycle engines that worked off of crankcase impulses. This is however a 4-stroke-cycle. The port on the intake manifold it hooks to provides a steady 6-7 inches of vacuum and no pulsing I can perceive.

I took the fuel-pump apart and all looks fine. Diaphragms are soft with no holes. Two check valves look fine. Put it back together and tested it with a hand-held vacuum-gun. A steady-vacuum and it pumps no fuel. A on-and-off pulsing vacuum and it still pumps no fuel. BUT - I then tried blowing air into it, off and on with pulses. This way it pumps lots of fuel.

So I am almost at the end of my rope. Seems I never had to think this hard before about a pump like this. I am very temped to put on an electric pump but it is hard to find one that really runs at low pressure. I have tested a few rated at 2-4 PSI and they ran higher. Likely too much for this carb.

I stuck a gravity feed tank on top of the engine and it starts and runs fine. The built-in tank sits near at carb level and needs a pump. So - I am getting a mental fog here. Suzuki does call it a "vacuum" pump and not an "impulse" pump. It hooks to the intake manifold and I cannot figure how there should be any impulse there anyway. Just a steady vacuum.

Somebody here got an idea about what I am missing here?

The first step should be figuring out what you actually have because there is no such thing as a King Quad 250! If it actually is a King Quad then it's a King Quad 300 LT-F4WDX. If it actually is a 250 then it's a QuadRunner 250 LT-4WD. I hope this info helps!

Steve@Advance · Mar 2, 2018

Could you drill a hole in the oil cap, thread in a fitting, connect it to the pump for cc pulse?

Nothing to loose, plug the hole if it didn't work.

JDEM · Mar 2, 2018

It is a 1995 250 cc Quad Runner LT-4WD. Next to it I have a 1992 280 cc King Quad LT-4WDX. The two machines are nearly identical.
The machine I having the trouble with is the 1992 LT-4WD. Only difference between the two fuel systems is the 1995 has a vacuum-fuel shut-
off in the gas-tank fuel-valve. The 1992 does not. Both use the same - what Suzuki calls -a "vacuum" fuel pump. I still do not know how
it is supposed to work. Steady vacuum doesn't make it work. If it takes a crankcase pulse - I have no idea how it gets that from a tap near
the intake manifold on the carb.

I am tempted to make a crankcase tap somewhere for a pulse-line. But these things worked once. I'd like to know how. No description of it
in the Suzuki manual.

JDEM · Mar 2, 2018

I wish the other ATV was together so I could test it. That one pumped fuel fine but I found out it had the wrong carb on it. Right now the carb is off and all apart. So with no carb on there -there is no way for me to test the vacuum or pulse port (not sure what to call it). I wish I had it together so I could stick a vacuum/pressure gauge on it and see if it reads a steady 5"-6" like the other ATV does.

I asked around at several Suzuki forums and found lots of guys who swap parts trying to fix things. I have not found a single person yet that knows how this system is supposed to work.

4play · Mar 3, 2018

Those pumps do work on pulse vacuum. A single cylinder engine would not produce a steady vacuum signal, maybe would look steady at higher RPM on some gauges. Anyway, I'm sure you checked and compared the vacuum line routing with the other machine but I think you might have the vacuum lines on the wrong ports at the carb.

If donor pump works on donor bike but does not work on other bike then compare vacuum on both, double check lines for obstruction and routing. By design these need a fuel pump due to low mounted fuel tank and fuel has to travel uphill. If you decide you need a fuel pump there are cheap options, many small engines use this type of pump and they are all very similar. An example of a pump that would fit is used on Honda GX series engines, they are cheap on amazon/ebay etc

JDEM · Mar 3, 2018

250 cc engine make 5"-6" of steady vacuum with the engine at idle speed. I have no idea how a steady vacuum source gets turned into a
pulse. I am guessing

I bought a new fuel pump for a Honda for $9, from China. Just like the original, it does not work.

I might try to find a place to drill and tap the engine crankcase or valve-cover and hook-up there instead.

4-wheelers are not as fun as I thought to work on. Finding parts is not as easy for some and the tech manuals from Japan not very
thorough.

I need a carb for the other Suzuki. Nearly $500 which is crazy. 1992 King Quad uses a kind of rare 32 MM Mikuni BST31SS carb that I have
not able to find used. I can get a new Honda 32 mm carb from China for $25 but I have been warned there is no way to make it fit.

JDEM · Mar 3, 2018

Just a side-note. I got thinking that Briggs & Stratton used to have lawn-mower engines with fuel-pumps built into the carbs that worked on steady-vacuum (not crankcase pulse). I never gave it much thought until now. Might of been called "Pulsa-Jet." I wonder how the heck they worked? I know there was a metal cup and a spring. I wonder if Briggs converted steady vacuum into a pulse?

If so, maybe the carb on this cursed Suzuki is supposed to do the same.

Ken Combs · Mar 3, 2018

Here's a good Pulsajet explaination. It does use the vacuum pulse created by the intake stroke of a single cylinder. I'll bet yours does to if it is a single. Most, if not all, automotive vacuum gauges are too slow to react and just show the average intake pressure.

did you try the priming procedure in my earlier post?

Ken Combs · Mar 3, 2018

I just read this and realized that I didn't paste the link:

http://www.compgoparts.com/TechnicalResources/BriggsCarburetors.asp

chuckinnc · Mar 6, 2018

Good luck, I had the same problem and only able to fix it by adding a aux tank on the rear rack, gravity feed fuel line to carb
it actually good like that, the grandkids took the front plastic off, removed a mile of breather hoses, now it turns much easier.
They also made fat,long handles for the 3 levers above the tank & called it a "rat rod atv" They use it all the time now.

Losing my mind with an ATV

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