Middle cyl. burn down, what am I missing

M

motodave289

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Dec 1, 2005
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N.E. Indiana
Put crank seals, rering,& serviced carbs before start of season on 00 srx. Got 20 miles on first ride of year and burned center cyl. down. PTO and MAG cylinders don't look lean. Was at part throttle around 6-7k rpm NOT wot around 1/4 - 1/2 throttle.
Took carbs back apart can't find anything wrong. Pressure tested crank case had a small leak at fuel pump vent. Plugged vacuum hose then would hold 5psi for a long time.

Is it normal for a leak at vent of fuel pump? Hooked up vacuum pump to fuel pump and it pumps fuel, but test in manual shows nothing about volume or a time frame to pump. Took pump apart didn't find anything torn or plugged. Drained bowls and measured before taking carbs apart all were at the same level.

Thanks in advance for any ideas.
 
Check those plastic rings around the main jet. if they are
loose, replace them. mine got loose and slid up that shaft blocking off the floats. took two meltdowns to figure that out. they should fit tightly.
 
Few???

Take a real close look at that carb boot/reedblock & the exhaust gasket/donut...your looking for a tear or separation...If carbs are good...can you tell us to what extent you cleaned your carbs, were they all to specs...can you post pics of that plug & the other two & all three heads
 
Also, if you haven't already, check out your intake manifolds and carb boots for cracks. I'm guessing your jetting is stock? You are either starving for fuel or have an air leak in the center cylinder if you're sure it's a lean burn down. Crank seals are normally the outside cylinders to go... Check out the manifolds and carb boots, if you can check out the seal between the case and the reed blocks aswell.
 
change_up said:
I"M ALWAYS ONE STEP BEHIND!!! this time by 10 mins LOL
Click to expand...
LoL...I just want to get it in everyone’s head that it is important to clean them carbs, especially the needle & seat, screen…
 
i don't think there should be a leak at the fuel pump vent, maybe a pin hole coming in the diafram . not sure if that could cause a burn down. when you preasure tested the engine did you do a vacuum test also . some times a air leak won't leak both ways.
 
Jetting is stock except mains 1 size up so middle is 147.5. Carb cleaning consisted of R&I pilot jet, pilot screw, slide & needle, main & spacer, enrich plunger, air jet, float, needle & seat. Inspected inlet screen, needle & seat, blew through all passages. Just looked at main jet baffle is tight on spacer.
Doughnut gasket & pipe to cyl. gasket are new 20 miles ago and look good.
Reed cage gasket new, boot looks good and pressure test with boot on engine. Didn't think to vacuum check crankcase before tear down. Can't get pics to load.
 
Is 2000 srx, all stock including air box.
 

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"Pressure tested crank case had a small leak at fuel pump vent. Plugged vacuum hose then would hold 5psi for a long time"

yor talking about the pulse line for the fuel pump? if so theres your answer as it is drawn from the center cylinder, a airleak.
 
Has to be somthing in that carb or the way fuel was getting delivered to it. You check the fuel line to that center carb? What is the needle set at? I have to ask but is the center head the same thickness as the pto and mag??

If anybody knows and I am asking because I dont know.. Will a stuck or in-operable power valve cause a lean condition???
 
mrviper700 said:
"Pressure tested crank case had a small leak at fuel pump vent. Plugged vacuum hose then would hold 5psi for a long time"

yor talking about the pulse line for the fuel pump? if so theres your answer as it is drawn from the center cylinder, a airleak.
Click to expand...
Just for future reference...what was the cause exactly & what would be the fix, mrviper...I was gonna say that the centre plug fouled & created a super hot environment which leads to a melt down...
 
mrviper700 said:
"Pressure tested crank case had a small leak at fuel pump vent. Plugged vacuum hose then would hold 5psi for a long time"

yor talking about the pulse line for the fuel pump? if so theres your answer as it is drawn from the center cylinder, a airleak.
Click to expand...

good answer Don, SO would this mean all you have to replace is a pulse line or fuel pump if this happens to anybody?
 
Yes plugged pulse line right at pump. Would leak from 5psi to 4psi in 1 min. Hose should be good since it held pressure. Have pump on order along with the rest of the stuff. But with that damage and that small of leak not sure.
For the water, had tank out and drained preseason, but you might be on to something. Are you thinking it's freezing in the carb hose. But when drain bowls there was nothing.

Thanks for all the replies.
 
while the engine is running is it producing pressure and pulling vacum both on the pulse line, so it would be more then what you observed. Also looking at your piston it did this over the time it was run, not all at once, thats how things got to be so exposed, it burnt alot of the piston away before it went, also thats how the hole was put into it.
 
Finally got a chance to check fuel pump. There is a .025" hole which is vent in same chamber as pulse hose hooks to. Didn't look like there was ever any kind of check valve in it either. Put pump back together and put 25" vacuum on vacuum port no leaks. Put 10psi no leaks with my thumb over vent both times. Only thing left to check is volume. Have a 01 srx in shop. Will hook up my pump to it and run it with 1 delivery line off and run in measured container.
Looked in tank for water didn't see any but going to take off tank and pour out to double check.
 
When I look at the pictures of your piston I am shocked, it looks almost exactly like my viper piston that melted.

I have found the following issues on my sled:
- probably air in oil pump after having the engine out when rebuilding the waterpump, used 200ml of oil on 30 liters of gas

- found a hole in the gas hose to the center cylinder carb (the one that burned down)

Some say the hole in my hose is the reason for sure and some say it cant be the hose. Hard to know what to believe.

Hope you figure out what caused your burndown!
 
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