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Discussion Starter #1
Hello,
just did a top end job and now it doesn't seem to be getting fuel to the carbs? if i prime the carbs it starts right up and runs for about 3 seconds. when i take the main fuel line off the carb it's dry? when i crank the engine with the main fuel line disconnected, no fuel comes out? It ran before i tore it apart. i can't find the answer in the manual. doesn't the fuel pump operate with the starter button and pump fuel? I know there's also vacuum to consider. did i hook up a vacuum line wrong. i have a piece of tube about 18 inches left over that i haven't found where that goes yet. it's either a vent or vacuum? any suggestions to test the fuel pump is operating correctly?

thanks,
BL
 

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The "i"s have the same vacuum pulse pump as the 750s. There is a vacuum line that runs from one head to the pump...and it is about 18"s long. Once hooked up and there is fuel, then you pull the actuator controler...that's the one behind to the CDI...and you will see the fuel valve. It has to be on PRI to prime the first time. Then turned to ON to run. I usually just pull the fuel line off the carb and suck fuel to the end. That primes the fuel pump without vacuum or wearing the starter out.
 

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Discussion Starter #3
Great. thanks Man. I also looked up your previous responses to guys having the same issue. it worked!
I found the hidden vac line, looked at my pre-tear down pics, and connected the vac line to the front cyl nipple. Then as suggested, sucked up the gas line to prime it, connected the other line to the rear cyl nipple and boom, started right up!
after warming it up, i took cyl compression measurements: both cylinders approx 90 - 100 psi on a fully charged battery.
That looks a little high? I had the valves rebuilt and I used 0.010 over rings ground to lower spec on lightly honed cylinders.
it runs strong with no smoke. But! while running, i removed the dipstick and it's still blowing mist out the hole?
whatdya think?
Thanks again for help!
 

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Great. thanks Man. I also looked up your previous responses to guys having the same issue. it worked!
I found the hidden vac line, looked at my pre-tear down pics, and connected the vac line to the front cyl nipple. Then as suggested, sucked up the gas line to prime it, connected the other line to the rear cyl nipple and boom, started right up!
after warming it up, i took cyl compression measurements: both cylinders approx 90 - 100 psi on a fully charged battery.
That looks a little high? I had the valves rebuilt and I used 0.010 over rings ground to lower spec on lightly honed cylinders.
it runs strong with no smoke. But! while running, i removed the dipstick and it's still blowing mist out the hole?
whatdya think?
Thanks again for help!
Compression's OK. Most are over what they say in the book and the mist from the stick hole is also OK..very normal. Sounds like she's back. :)
 

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Discussion Starter #5
Dang it, she's not back..... After rebuild i only drove it around short trips to break it in and make sure everything is cool. recently, i smelled gas, and actually saw some leaking but i couldn't see where it was coming from?
Then it started to run like crap when it got hot, and backfiring too? heavy bog, no power and backfire on decel.
I also noticed that i have a line i forgot to install it's 18 1/4 inches long and 3/16 bore? i have no recollection where it goes.
Any ideas on where to start my diagnoses? i'm bummed about having to pull it apart again....
Any help appreciated!!
 

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Dang it, she's not back..... After rebuild i only drove it around short trips to break it in and make sure everything is cool. recently, i smelled gas, and actually saw some leaking but i couldn't see where it was coming from?
Then it started to run like crap when it got hot, and backfiring too? heavy bog, no power and backfire on decel.
I also noticed that i have a line i forgot to install it's 18 1/4 inches long and 3/16 bore? i have no recollection where it goes.
Any ideas on where to start my diagnoses? i'm bummed about having to pull it apart again....
Any help appreciated!!
Well, first things first. It was running so the pump was working but...strange..the only piece of rubber line about 18"s long is the vacuum line to the fuel pump....and if it was left off, it should never have run. But, check- there is only one vacuum port on I think it's the rear head on top right in the casting. And if that like was on there and just fell off, that would explain the running issues...to the most part.


After that and if it doesn't fix it test the tank vent check valve, then fuel pressure and volume. Look at the bottom of the carbs at the bowels- there you will see two ports or nipples. The one closest to the drain screw is...the drain port. The other one should have lines on them that T together and go down the left side to a large piece of round plastic. That is your overflow ports. If the float levels get too high it comes out those so it doesn't go down the engine. If the lines are off and you were on some rough trail, that may kick some gas out them. Another thing you best check is the carb boots. They all crack..and usually where you can't see them..under or at the head-side clamp.
 

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Discussion Starter #7
thanks for the advice. I'll check the carb boots. do you think the carb boots behave differently when hot? Funny, it ran great when hot after the top end job then only recently (approx. 50 miles after re-build) started to act up only when hot?
Do you think the ignition could be affected when hot? Right now it starts up and runs great, then suddenly bogs down and backfires after 10 mins of hard riding. I can't imagine dirty jets in the carbs would only affect performance when hot? The electric fan works but isn't coming on so i'm guessing the cooling system is doing it's job? The backfiring leads me to fuel or ignition? Thanks again.
 

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thanks for the advice. I'll check the carb boots. do you think the carb boots behave differently when hot? Funny, it ran great when hot after the top end job then only recently (approx. 50 miles after re-build) started to act up only when hot?
Do you think the ignition could be affected when hot? Right now it starts up and runs great, then suddenly bogs down and backfires after 10 mins of hard riding. I can't imagine dirty jets in the carbs would only affect performance when hot? The electric fan works but isn't coming on so i'm guessing the cooling system is doing it's job? The backfiring leads me to fuel or ignition? Thanks again.

Just go through it..you will find it. These carbed machines are pretty basic and easy.
 

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Discussion Starter #9
OK, i finally started tearing into it. it would run great for 20 mins and then lose all power. it would run a little rough but it would idle. the symptoms repeated time and time again. i rode it hard for 20 mins, then it started losing power again so i took off the air box and tried revving it. it continued to bog, so i squirted gas into the carbs while hitting the throttle. well the motor started to scream again, so that tells me i'm running lean when it gets hot? i disconnected the vacuum fuel pump (not electric) and checked that it was drawing and pumping gas and it is. once i connected everything back up, it only runs now for 5 seconds, wait 10 mins and it runs for only 5 secs again and again. I disconnected the fuel supply hose to the carbs and installed a clear tube to that nipple and filled it with fuel like a gravity fed line. once the fuel level in the tube settled, i started it, watched the fuel level drop for 5 seconds and it stalls again. it will not start again unless you wait 5-10 mins?????? I then hooked up my gravity fed tube to the back of the fuel pump and the exact same thing happens. I'm now trying to see if it's an electrical cut off by checking spark after it dies? Any help appreciated. I'm stuck.
 

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OK, i finally started tearing into it. it would run great for 20 mins and then lose all power. it would run a little rough but it would idle. the symptoms repeated time and time again. i rode it hard for 20 mins, then it started losing power again so i took off the air box and tried revving it. it continued to bog, so i squirted gas into the carbs while hitting the throttle. well the motor started to scream again, so that tells me i'm running lean when it gets hot? i disconnected the vacuum fuel pump (not electric) and checked that it was drawing and pumping gas and it is. once i connected everything back up, it only runs now for 5 seconds, wait 10 mins and it runs for only 5 secs again and again. I disconnected the fuel supply hose to the carbs and installed a clear tube to that nipple and filled it with fuel like a gravity fed line. once the fuel level in the tube settled, i started it, watched the fuel level drop for 5 seconds and it stalls again. it will not start again unless you wait 5-10 mins?????? I then hooked up my gravity fed tube to the back of the fuel pump and the exact same thing happens. I'm now trying to see if it's an electrical cut off by checking spark after it dies? Any help appreciated. I'm stuck.

All that ...and the one thing you didn't mention was the #1 thing that happens to most all of these and that's the fuel tank's vent check valve. They all get stuck-shut at some point because of how they are made and that people like over filling the tanks getting the disk wet...then it dries out and sticks to the valve seat. And it takes about 20 minutes or a little less for enough vacuum to build up on the tank to slow and stop the pump from pulling fuel. Float levels drop and it runs leaner and leaner until it just won't any more. Let it sit a while and the vacuum on the tank equalizes and the process starts all over again. So, take it out and run it until it starts acting up. Then loosen the gas cap to relieve the vacuum. If it runs right again as soon as it re-fills the carb bowls, you have your answer. Just replace that check valve...we all have had to do it at least once.

The only other thing that makes one act that way is low voltage on the coils. If there is a battery or charging problem..that's the reason.
 

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Discussion Starter #11
OK, so the 5 second shut off was the rollover sensor - DOH! disconnected it when i took everything off to get to the fuel pump and petcock. I also saw the the rear spark boot was not tight on the plug? not sure it that is running hot problem or not. I havent taken off the bottom carb boots yet but now i can get to them so i'm going to run it hot and when it bogs i'll squirt some carb cleaner on the boots to see if a crack as opened up hot?
 

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Discussion Starter #12
I had it bogging and removed the gas cap to see if there was a clogged check valve. that didn't stop the bogging. i also looked over all electricals in the pitch dark while it was bogging and couldn't see any arcing. I have an aftermarket CDI. while it was bogging, i replaced it with the original CDI and it was maybe a little worse with bogging and backfire? I replaced the charging system a couple years ago. The voltage is good at battery. I'm going to get it to bog again and see if the charging system is still working.
 

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Discussion Starter #13
Well, i checked out everything with fuel delivery, pump, petcock...etc. They all checked out.
in the end, i found the rear plug wire was not seated fully, it was over the plug but could bounce around.
that was it. no more bog. But now i have a clear exhaust leak on the rear cylinder! i'm really hoping it's the exhaust pipe gasket and not a head gasket. I'll report back later.....why don't i buy a new quad.....
 
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