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286707 runs rough when Crank Case Breather is attached.

#1

Y

yinmeout

Hi all - wondering if there are any of you who may have a good conceptual understanding of the elements in play for my engine problem.
The problem is that the machine struggles to start and runs rough. A quick check of the plug confirms rich too dark and not tan colour.

Today I went into it in more depth.
At first I thought it was the ignition so I got a spark tester and I can see a good spark all the time.
The plug was wet with fuel so I thought the bowl could be stale.
I swapped out all the fuel and replaced it with treated fuel since the machine doesn't get run very often - this is best anyway.
It then did start but still ran poorly - so I started to think I had a carb problem, since its about 10 mins to remove it I decided to do that.

Once I had the air cleaner housing off I decided to run it again and it ran and it ran nicely.
So thinking the air filter was clogged I cleaned it and re-oiled it and decided to re-assemble it with the engine running and this is how I know that attaching the crankcase breather is when the engine becomes upset and goes really rich. The revs drop off from 1200 rpm idle to about 700 and if I run it faster it runs lumpy as you would expect with too much fuel.
There is no pressure I can feel in the crankcase breather either out or in with the engine running so how that can be affecting the carb I don't know.
It makes no sense to me - the same jets are in play and the breather could be robbing or adding air [hypothetically] but in either case the fuel will still be proportional to the quantity of air so I am not sure why the crank case breather attachment is causing this issue ?

The engine is this
A B&S vertical 500cc air cooled side valve motor. 12.5HP.

I will say the engine generally runs well but has been starting to get progressively worse over the last couple of times I have used it.
On the parts diagram it isn't obvious whether the breather can be detached and cleaned.
The breathe pipe itself is sound and not oily.

Thanks in advance for any thoughts.


#2

R

Rivets

First, the foam air filter which goes over the paper filter should NOT be oiled. My guess is the oil is being sucked from the foam filter onto the paper filter, clogging it up, causing a rich running condition. I recommend you purchase a new paper and foam filter and install them DRY. If it solves your problem, case over.


#3

ILENGINE

ILENGINE

Also I would pull the dipstick and smell the oil. Gas in the oil will cause the exact symptoms since with the air cleaner removed it is pulling in fresh air but when the air filter is installed it will get the fumes of fuel from the oil breather.


#4

Y

yinmeout

Rivets said:
First, the foam air filter which goes over the paper filter should NOT be oiled. My guess is the oil is being sucked from the foam filter onto the paper filter, clogging it up, causing a rich running condition. I recommend you purchase a new paper and foam filter and install them DRY. If it solves your problem, case over.
Click to expand...
Oh! That would explain allot in that case.
ILENGINE said:
Also I would pull the dipstick and smell the oil. Gas in the oil will cause the exact symptoms since with the air cleaner removed it is pulling in fresh air but when the air filter is installed it will get the fumes of fuel from the oil breather.
Click to expand...

ILENGINE said:
Also I would pull the dipstick and smell the oil. Gas in the oil will cause the exact symptoms since with the air cleaner removed it is pulling in fresh air but when the air filter is installed it will get the fumes of fuel from the oil breather.
Click to expand...


#5

Y

yinmeout

I was re-reading the Briggs Engine Manual which covers my model and many others.
This is the section I was following and since my foam element does not have "DO NOT OIL" on it - I oiled it.
I am guessing now that its not a genuine foam element and so that was a mistake.
It makes sense not to oil a cleaner which has a paper element immediately upstream of its airflow.
"
Clean Oil-Foam Element or Foam Precleaner (Fig. 17)
1. WASH foam element in liquid detergent and water
to remove dirt.
2. Wrap foam in cloth and squeeze dry.
3. Saturate foam with engine oil. Squeeze to remove
excess oil.
DO NOT OIL PRE-CLEANERS THAT ARE IMPRINTED “DO NOT OIL.”
4. Reassemble.
"

Many thanks for your time and advice.


#6

ILENGINE

ILENGINE

yinmeout said:
I was re-reading the Briggs Engine Manual which covers my model and many others.
This is the section I was following and since my foam element does not have "DO NOT OIL" on it - I oiled it.
I am guessing now that its not a genuine foam element and so that was a mistake.
It makes sense not to oil a cleaner which has a paper element immediately upstream of its airflow.
"
Clean Oil-Foam Element or Foam Precleaner (Fig. 17)
1. WASH foam element in liquid detergent and water
to remove dirt.
2. Wrap foam in cloth and squeeze dry.
3. Saturate foam with engine oil. Squeeze to remove
excess oil.
DO NOT OIL PRE-CLEANERS THAT ARE IMPRINTED “DO NOT OIL.”
4. Reassemble.
"

Many thanks for your time and advice.
Click to expand...
Oiling the pre-cleaner is fine if done properly. The problem is the instructions basically set up a situation that leaves way too much oil in the filter. The entire filter shouldn't take more than a Tablespoon of oil. So if the precleaner is saturated would need to basically squeeze it out with a rolling pin.


#7

H

hlw49

Air filters that have a screen over the paper can be oiled if done properly. Filters that do not have the screen over the paper should never be oiled.


#8

Y

yinmeout

ILENGINE said:
Oiling the pre-cleaner is fine if done properly. The problem is the instructions basically set up a situation that leaves way too much oil in the filter. The entire filter shouldn't take more than a Tablespoon of oil. So if the precleaner is saturated would need to basically squeeze it out with a rolling pin.
Click to expand...


#9

Y

yinmeout

I think the proof is in the pudding as we say over here. I have now finished fixing the mower in my view. It starts as it always did with a single reasonably enthusiastic [but not herculean] pull on the cord. Even though it has a starter there is simply no need to use it. Runs like a charm and that is good enough for me. Plug is tan now again. As there is nothing inherently wrong with the unit I see no reason to dump it even though it is 32 years old.

The choke arm linkage had fallen off, I must have miss-fitted it on a previous occasion so that would not have helped the starting issues I was having whatsoever. It's not very dusty here at all, so I am happy to just clean the pre-filter with soap and water and allow to dry out at 25 hr intervals or so. The mower rarely gets used for actual grass cutting, mostly towing the trailer we have for it to move bulky or heavy items over a distance to husband our own energy reserves.

I must say the plastic pinion on the starter is very weak, I must have replaced that item alone three times in two years, but they are not costly so I don't particularly mind.


#10

S

slomo

You can oil it as the others said, if done correctly. Saturate it in SAE 30W then gently wring it out inside a new clean red shop rag. Filter inside the rag trying to remove all excess oil. That is how Briggs says to do.

Do what you can to stop all dirt and grit getting into the rings and bore.


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