My Murray lawn mower idle oscillates up and down, I can see the spring
on the carb moving about 1.5".
I there a spring kit or some kind of throttle control that may fix
I rarely use this lawn mower and am about to put it up for sale, just
thought I'd fix this problem before advertsiing.
Its a Murray riding lawnmower, model 30552x12a w/a Tecumseh 10 HP
Thanks for any input..
I learned about this in my small engine repair course.
If the spring on the throttle gets a bit hardened (painted, rusty, etc) it
doesn't respond right. So the engine speed "hunts" for the right speed.
Replace the spring between the throttle and the carb.
Yup, could definitely be a spring. I read a little book on
repairing small engines from the library recently, and it
also suggested it could be any spring or linkage which
connects the governor system with the throttle.
You may have to remove the engine cover to fix this one, but
the good news is that if it's packed with grass or other
debris, cleaning it may resolve the issue for you (anything
that binds or blocks the spring(s) or linkages from proper
Buck Wheat wrote:
a spring won't fix it. you have junk in the carb. the idle feed
passage is clogged causing it to try and run off the main fuel
circuit. pull the bowl and clean thoroughly. up in the center of the
carb tube is a asmall passage running at a 45 degree angle towards the
top. this is where the restriction is.
hunt or oscillates can be the spring but in all my years of working and
teaching on small engines it is not a common failure. there is a list of
possible here is but a few, air filter plugged, worn throttle shaft carb,
intake manifold leaking, governor problems (worn or misadjust) linkage worn
bent or missing, fuel problems.
as I said the list is long but when one runs into a problem with a spring it
is more common it will either be stretched and the rpms are too low or
modified some how and has too much tension making rpms too high.
Actually, it would not start about a month ago, so I took the bowl off
and cleaned out the carb and hoses. I had to turn the bowl little by
little to get it started again, I think maybe the bowl is still not
I'll take it off and try to find the right position, I have not seen
any markings to line-up..if there are any(?) I'll check all other
parts for clogging, etc.
Thanks for taking the time to reply, it's a riding mower and I dread
trying to get it to the shop.
That's exactly what I needed to know, I tried several positions
yesterday w/the float bowl, all yielded the same result...no change,
I'll correct it today. I was able to cut the lawn as once it is in
full throttle it doesn't seem to oscillate.
What does the screw/adjustment at the bottom of the float bowl do?
The screw at the bottom is the main jet needle valve.
Tecumseh carbs have three adjustments: idle speed which will be a screw
possibly with a spring wrapped around it, that sets how far the throttle
plate can close off the fuel/air supply; a low speed needle valve which is
often located on top or on the side near the throttle shaft, and a high
speed or main needle valve located under the fuel bowl.
The low speed (often called idle mixture) needle valve is adjusted for
maximum RPM at idle. This is usually about 1-1/2 turns from fully tightened.
Turn it slowly each way, noting where the engine runs rougher or slows down,
then set it in the middle. Do this adjustment first.
Second would be to set the idle speed. This is often around 1800 for rotary
mowers but could be slower for tractors, i.e. 1200 - 1400 RPM. If you make a
significant adjustment here, go back and repeat the idle mixture adjustment.
Lastly is the high speed mixture. Set the engine at idle speed, then set it
to full throttle using the main speed control. Do NOT use a finger to
manually open the throttle at the carburetor as the engine is governed and
should limit its speed to around 3000 RPM. Adjust the mixture so the engine
accelerates without hesitation or backfiring. This will take several tries,
and is also usually around 1-1/2 turns from fully tightened. Usually it will
only take a change of 1/4 to 1/2 turn to get it to smoothly accelerate. You
can do this under load as well. Clockwise will result in a leaner mixture,
i.e. less fuel compared to the air going through the carburetor.
None of these adjustments should be way out unless someone pulled the valves
out to clean the carburetor and didn't pay attention to where they were set
beforehand. If you do need to pull them out, carefully turn each one in
(clockwise) and count how many turns it takes until it seats fully, but do
not exceed that position, then remove it and the spring. Some needle valves
also have a rubber O-ring and even a flat washer at the end of the spring in
the casting, so be careful.
These procedures are most likely outlined in the several good books on small
engine repair. A trip to a local library might give you more detailed
information. I'm going from memory as I usually make the adjustments without
even thinking about them.
I have seen two styles of water drains: spring-loaded types that you push up
with your finger and let stuff out, and an actual 12mm bolt with a gasket
that unscrews. Usually these are mounted off to the side of the bottom of
the bowl, not directly in the center, and it's hard to call them
adjustments. I'm pretty sure the original poster was referring to the
knurled screw and spring that screws up into the center bolt that also holds
the fuel bowl to the carburetor. This does go into the tube that feeds the
main jet, so if you stretch the previous poster's reply, he's got all the
words there to convey the message, but it could use just a few more. I
understood what he was saying, and it does indeed richen and lean the fuel
mixture once the throttle is off the idle position. I think the original
poster will find more than enough info here if he reads all of our replies.
Mine is the 12mm bolt w/gasket that unscrews. I've moved it around
and it does affect a change in the idle, but it does nothing about the
oscillation. I tend to think it is "hunting" and could believe the
carb is clogged.
My thought is that it was working, then I didn't use it for several
months and it was covered w/tarp, then I put a new battery in and it
would not start (it would start perfectly w/starter fluid, but when
fluid was gone it cut-out.) So
I took the bowl off and checked the gas lines and yes there was a lot
of "junk" in the bowl (and some each time I've cleaned it.) I have now
taken the bowl off and sprayed the entire system with carb-cleaner and
have not turned any screws. So I don't see how it can be anything
other than the bowl, and I have followed the advice of a poster about
the correct position, but it makes no difference, so it must be
clogged and/or a spring has failed.
I really did not want to remove this carb, it looks like a real bitch
because the screws are against the muffler and on and on...and I don't
want to screw up the linkage/springs. As I say, it does run when at
full throttle, but I'd rather have it running correctly at all speeds.
Thanks for the dialogue, I and others appreciate your responses.
On a completely different path, the reminder that a
tarp covering it forces moisture to be trapped, and
that can be even more of a problem as heat buildup
accelerates the deterioration process.
Ever consider just turning a plate upside down over
the rope-start portion?
Well, the only way you can completely clean it is to remove it. Those bolts
behind the muffler can be hard to get out, but an open end wrench and large
screwdriver should get it started, then use finger power. Chances are good
you'll need to replace the carb-to-engine gasket as well, as they tend to
stick to metal if left in place for more than a few years. You can pick up
the correct gasket at a small engine repair shop, or get some gasket
material that's the same thickness at a car parts store - they might even
give you a large scrap for free. Carefully cut the appropriate holes and
you're in business.
There shouldn't be too many things connected to the carb. The throttle
cable, maybe a spring, the choke linkage, but that's about it. The important
thing is to remember what holes everything went into. Taking or drawing a
few pictures is often a good idea. Get the shop manual for the engine - one
that tells you how to overhaul the carb. Buy a can of good spray carb
cleaner. An old toothbrush is also helpful for getting off massive
quantities of varnish and crud. I won't go into the details here, but a good
mechanical inclination is most helpful. Try to work in a clean area and be
careful of loose springs and other parts that may fall out of the carb as
you take it apart.
If it won't idle, it's probably the idle jet that's clogged, or the float is
way too low, or the throttle plate is closed too tightly. Of course, it
could also be something else that I didn't mention.
Just get a carburetor rebuilding kit and use it. Usually the problem is
the diaphragm in the center of the (B&S) carb is warn out. Part of this
runs the acceleration pump. That's what causes oscillating RPM.
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