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Surging at full throttle

#1

S

Skyesdad

I have a John Deere Z375 mower with a B&S 25hp V-twin M44 ELS motor that surges and backfires at full throttle after it warms up. It runs fine when the mower is engaged and at lower idle. Does anyone have a suggestion on where to start looking for what might be causing this?


#2

ILENGINE

ILENGINE

I would start with a carb cleaning and installing a repair kit.


#3

S

Skyesdad

Thanks, I figured that would be where I would start but was hoping someone might have seen this condition before and have some insight.


#4

B

bertsmobile1

We have all seen it before and the usual cause ( about 90%) is a blocked carburettor
After that is debris in the fuel tank cutting off the fuel supply
next is air leak between carb & engine
Then comes faulty solenoid.
Then comes blocked fuel filter
Then blown head gasket / crankcase pressure leak
Then failed fuel line
Then faulty fuel pump
Not hard & fast order but on average they go like this

But in any case you diagnose the problem and apply one fix at a time
I usually start by flooding the manifold region with WD 40 from a trigger sprayer and see if the engine changes speed or blows smoke
I work the governor arm while doing this because on the overrun you get more suction so manifold leaks become more apparent ( white smoke )

Next I pull the fuel solenoid off & replace it with a bolt

While both these are less likely to be the problem they are quicker & easier to rule out first than pulling the carb off & cleaning it .
Being a tech, time is money and fixing a surging engine is basically a fixed 1.5 hour charge + parts out regardless of what the cause of the problem actually was .


#5

S

Skyesdad

bertsmobile1 said:
We have all seen it before and the usual cause ( about 90%) is a blocked carburettor
After that is debris in the fuel tank cutting off the fuel supply
next is air leak between carb & engine
Then comes faulty solenoid.
Then comes blocked fuel filter
Then blown head gasket / crankcase pressure leak
Then failed fuel line
Then faulty fuel pump
Not hard & fast order but on average they go like this

But in any case you diagnose the problem and apply one fix at a time
I usually start by flooding the manifold region with WD 40 from a trigger sprayer and see if the engine changes speed or blows smoke
I work the governor arm while doing this because on the overrun you get more suction so manifold leaks become more apparent ( white smoke )

Next I pull the fuel solenoid off & replace it with a bolt

While both these are less likely to be the problem they are quicker & easier to rule out first than pulling the carb off & cleaning it .
Being a tech, time is money and fixing a surging engine is basically a fixed 1.5 hour charge + parts out regardless of what the cause of the problem actually was .
Click to expand...
Great info... thanks so much


#6

S

slomo

Have you ever pulled the metal engine shroud and cleaned the top of the block and cooling fins?

This is a yearly maintenance item in ALL engine manuals. Neglect this and you are looking at engine damage.


#7

G

Gord Baker

Yardmaster and possibly other engines have a small screen behind the 90° elbow that feeds fuel to the carb.
PULL out that fitting and check. If there is a tiny screen, blow it clean and hopefully out from Needle side.
In any event get/leave that screen out and put a serious filter on the fuel line. Donyboy Small Engine repair shows this operation.


#8

D

driz

With that carb bowl solenoid does it fail intermittently Sometimes? My engine is is surging steadily , doesn’t react to we 40 around the manifold and does shut off when the solenoid plug is pulled Ect.

whats the typical lifespan of the fuel pump, mine has 380 hours .
ive had mine apart twice already do clean the carb and blow out everything then started surging again in a few hours. Those plugged mains and debris were so small I couldn’t really find it when I cleared the jets which seems very odd for jets so large And finding nothing easily visible. My fuel pump puts out a decent bubbling steady stream which looks to be far more than that engine could possibly use. Still it shows nothing that I would call pressure as in your old style car. Should there be a certain PSI ?
Sorry if I’m hijacking the thread but my problem is very similar and from searching very common across models that use the Vanguard Twins. Thanks


#9

B

bertsmobile1

About 1/4 of all posts here are for surging .
1) yes & no, usually they fail intermittently because the wires are shorting or the ground wire is loose.
2) fuel pumps last almost forever I have replaced 1 in about 3000 services .
3) Fuel pump is good for around 3 to 7 psi, singles are higher than twins because the pressure pules are bigger
We need the full engine codes to make meaningful comments about the carb but you will find the proper cleaning sequence at outdoorpowerinfo
As there is cleaning and CLEANING


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