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View Full Version : Why won't my lawnmower re-start when it's hot?


berniesidney
08-08-2012, 10:37 PM
Starts right up when it's cold... but after mowing for a while... if I have to stop it to empty the bag or refuel it won't re-start until it has cooled off for a couple of hours.

I checked the spark plug, the air filter, fuel line... it all looks good.

Any suggestions?

hank57
08-08-2012, 10:42 PM
Guessing and I don't understand them gas motors al that well but, once the engine is too hot it might be creating the right air/fuel mixture to start or fuel filter might be just plugged a little and the pumping of the fuel gets a kind of negative pressure like suction so no fuel moves into the carb.

Like I said I don't much but I listen to Car Talk a lot....

aynirar27
08-08-2012, 10:44 PM
any idea what brand of motor it has? Briggs and Stratton's have a reputation for compression issues when hot.

Tuberattler
08-08-2012, 10:45 PM
Hot start problems are almost always valve related. As the engine heats the valve grows a few thousands thus not closing 100%. Here's a test next time it's cold pull it over to gage how strong your compression is, then when it's hot and you shut it down now pull it over and gage how strong your compression is.

My guess is that you have much more compression when cold than when hot... valve problem.

Solution; make sure valve lash is to spec when cold. This is a fairly technical ordeal but basically on the older style flat head engines you have to remove a small amount from the valve stem end to make sure your valve lash is to spec. You'll have to look up the spec and measure it.

I'd bet this is your problem.

Pedal Dan
08-08-2012, 10:50 PM
It's called vapor lock. Basically an air bubble from the heat. Put an electric fuel pump on my 6 bbl. Cuda to end it.

sunburst79
08-08-2012, 11:00 PM
It's most likely vapor lock or your ignition coil or magneto is going bad. If its vapor lock letting it cool down for 15 minutes to half an hour should let it restart. If its a coil or magneto winding that's getting hot it could take a couple of hours to cool down before it restart. If the mower is a B&S and it's 7-10 years old it quite common for the coil to go.

I used to use a Velcro cable tie to keep mine running. My latest one has a key start that makes me grin every time I use it.

I always let it cool down as I'm not really enamoured with accidentally spilling gasoline on a really hot engine.

berniesidney
08-08-2012, 11:21 PM
It's a Craftsman 7 HP Briggs & Stratton. We've had it for about 10 years... it usually takes a couple hours to cool down before it will re-start.

How do you test the ignition coil or magneto? Is there a specific resistance they should measure?

If the valve lash out is out of spec... after its hot and still running wouldn't there be excessive exhaust smoke from oil burning?

Tuberattler
08-08-2012, 11:52 PM
It's a Craftsman 7 HP Briggs & Stratton. We've had it for about 10 years... it usually takes a couple hours to cool down before it will re-start.

How do you test the ignition coil or magneto? Is there a specific resistance they should measure?

If the valve lash out is out of spec... after its hot and still running wouldn't there be excessive exhaust smoke from oil burning?


Ignition spark can be easily checked with either a spare known good spark plug or you can pull the plug out and turn it over with the plug grounded against the engine. Be careful as it can ignite gas vapors...

I'd bet it's still the valve lash as it won't make it smoke unless the valve guide is worn out which it'll be out of speck but not enough to do what you're illuding to. Ignition coils HARDLY ever go out.

BTW I owned an outdoor power equipment shop for almost 10 yrs and was certified Briggs, Onan, Wisconsin and specialized in two cycle engines including ultra lights before I got into Real Estate. :)

berniesidney
08-09-2012, 12:34 AM
Thanks Tuber... I'll start by checking the clearance on the rocker arms.

Travst
08-09-2012, 08:14 AM
This may also be related to the alcohol in the fuel due to its low boiling point. I've had that problem with an old Corvette and solved it by placing a non-metallic shim or riser between the carb and the engine.

Tuberattler
08-09-2012, 09:14 AM
If this is an overhead valve engine the valve lash should be adjustable via the rocker arm hold down bolts, refer to an appropriate repair manual for spec's and procedure.

Since this obviously is an OHV engine you will most likely have a carb with fuel bowl. Make sure you don't have any air leakage between the carb throat and where it mounts to the head. I've seen these setups get a little loose and leak when hot. Also check to make sure your muffler is free flowing as if it has a carbon screen it can get plugged and carbon tends to swell when hot.

Did you perform the before and after compression strength test?

cardamonfrost
08-09-2012, 09:22 AM
My guesses:

1) the head is warping when hot causeing a loss of compression
2) the valve(s) are not seating when the head is warped
3) your coill is shot
In either case, check the head bolt torque, may be loose. Do you see black gunk built up around the head gasket? I wouldnt think that it is vapor lock unless it stalls while running, but I could (and probably am) be wrong.

C

BarkingTree
08-09-2012, 09:32 AM
Im voting plug wire or coil..as well. Heat expansion to the ignitor and wire is
pretty common.

jumpnblues
08-09-2012, 10:29 AM
Are you using the choke to start it when it's hot/warm? If so, don't. Try starting it without the choke when it's hot/warm. The engine needs choked only when cold to add more fuel to the mix. In other words it needs "enriching". When the engine is warm it doesn't need enriching and if you use the choke you'll "flood" the carburetor with fuel (make it too rich) and the engine won't start for a few minutes until the excess fuel evaporates.


Tom

Dr. Tweedbucket
08-09-2012, 10:37 AM
I have same problem with my Toro. When cold, starts on first pull every time. When hot, no go. Got a little better after I cleaned the air filter (it was packed). Now I keep a piece of wire on the handle to I can keep it runinig when I need to let go.
:red reported :red

Whiskeyrebel
08-09-2012, 10:38 AM
We all change as we age. You just need to adjust and enjoy the time between.

I never heard any guy call it his " lawnmower" before.

art_z
08-09-2012, 11:08 AM
Put some NOS tubes in it.

Tuck
08-09-2012, 11:10 AM
Before you get too mechanical, check the fuel cap and make sure the vent hole isn't plugged up. This happened to my John Deere rider. Easy fix.

Structo
08-09-2012, 11:35 AM
Also after 10 years I'm sure the cooling fins and carberator are all gunked up with grass and residue.

I would clean the carberator being careful not to damage any springs and pull the cowling off of the engine to clean out the fins.

When is the last time you changed the spark plug?

You can tell a lot about an engine by looking at the plug.
It may be oil fouled or worn out.

berniesidney
08-09-2012, 02:52 PM
[QUOTE=Whiskeyrebel;13660908]We all change as we age. You just need to adjust and enjoy the time between.

I like the way you think.


Thanks for all the suggestions everyone. I'm wrenchin this morning and cuttin this afternoon... we'll see how it goes.

berniesidney
08-09-2012, 07:49 PM
We have a winner... Thanks Tuberattler

I adjusted the valve lash to specs and now my mower starts hot.

Runner up: art_z

NOS Tubes... unfortunately my mower is solid state, but if I ever come across a vintage tube driven Briggs & Stratton, I'll be all over it. Imagine the tone clipping through a mix of Kentucky Blue grass and Tall Fescue!

megatonic
08-09-2012, 10:20 PM
Because it's hot.

BluesForDan
08-09-2012, 10:35 PM
through a mix of Kentucky Blue grass and Tall Fescue!

yeah, but can you play 36 holes on it in the afternoon, take it inside and just get stoned to the bejeezus-belt that night on the stuff?

berniesidney
08-09-2012, 10:56 PM
You bet... so I got that going for me.

Tuberattler
08-09-2012, 10:59 PM
:banana:banana:banana:banana

I know those small engines as I've been fixing motors since I was about 12 as long I've played guitar

Glad your mower is back to running again.

Timboguitar
08-09-2012, 11:37 PM
Many moons ago, I had a 4 cyl Dodge Colt that did the same thing--esp on a really hot day. I'd go into a store for ice cold refreshment, come out a moment later--and the car wouldn't start. I assumed a vapor lock would form in the fuel line. The solution I thought would be to somehow initiate combustion that would pull the fuel through and eliminate the vapor lock. Since it was an on going problem, I needed a combustable that I could keep in the car that was relatively safe. I tried regular rubbing alcohol. Worked like a charm. I just popped the hood, spun the wingnut on top of the air cleaner---dropped less than a capful, then fired up the engine. Worked every time!! On a really hot day, after I drove a while, if I had to turn the engine off--you know, gas station, fast food, quick run in and out of a store, I would't even bother trying to start it without putting in a capful of alcohol in the carburetor. Remember those days?? Changing and gapping your own points and replacing your own distributor cap? Rebuilding your own carburetor? I don't even change my own oil anymore. It got more and more difficult to find a place that would take the old oil-so I thought it wasn't worth it anymore. Isn't it amazing how many guitarists work (or use to work) on their own cars?
As far as my lawnmower, pressure washer, weedwacker and chainsaws---- there's a fellow right up the street, Brent, that does phenomenal work and is very reasonable. Too bad there isn't one like him in every neighborhood.

Tim

Dr. Tweedbucket
08-10-2012, 06:15 AM
We have a winner... Thanks Tuberattler

I adjusted the valve lash to specs and now my mower starts hot.

Runner up: art_z

NOS Tubes... unfortunately my mower is solid state, but if I ever come across a vintage tube driven Briggs & Stratton, I'll be all over it. Imagine the tone clipping through a mix of Kentucky Blue grass and Tall Fescue!



I had a Honda minitrail that did that.... started cold but wouldn't start hot. Valves were too tight.

tommy321
05-14-2013, 09:00 AM
Starts right up when it's cold... but after mowing for a while... if I have to stop it to empty the bag or refuel it won't re-start until it has cooled off for a couple of hours.

I checked the spark plug, the air filter, fuel line... it all looks good.

Any suggestions?


If it has a metal shroud over the spark plug try removing it. This happened to me. My local lawnmower guy said that it may be shorting out. It worked for me:)

tommy321
05-14-2013, 09:07 AM
If your spark plug has a metal shroud on it try removing it. This fix came from my lawnmore repair guy. It worked for me. Apparently it could short out.

mark norwine
05-14-2013, 09:07 AM
Your first post is to dredge up an old thread from last summer? :huh

Your second post is to immediately repeat yourself....???

Besides...did you see that the problem was solved in post # 21 back last August?

Welcome to TGP.....I think.

jscooper
08-24-2014, 01:30 PM
Hi,

The tips here are great except what if you're like me and have no idea what valves you mean, or where they are or what they look like, or what you mean by compression or anything else? My mower (Toro with Tecumseh motor) won't start when hot. Thanks!

Jeff

catpeople
08-24-2014, 01:34 PM
Give it a couple of cold beers and it'll start right back up.

ModdersAnon
08-24-2014, 02:11 PM
Hi,

The tips here are great except what if you're like me and have no idea what valves you mean, or where they are or what they look like, or what you mean by compression or anything else? My mower (Toro with Tecumseh motor) won't start when hot. Thanks!

Jeff

So you joined today, to post in a thread completely non-gear related and defunct for one year, which had also had been previously dug up by another poster who joined only to post in the same thread
:nuts:nuts

http://i59.tinypic.com/21oub75.jpg

EightySix
08-24-2014, 02:36 PM
Give it a couple of cold beers and it'll start right back up.
Yeah, it's just tired. It may not cut as straight after the beer though.

Sidney Vicious
08-24-2014, 05:59 PM
So you joined today, to post in a thread completely non-gear related and defunct for one year, which had also had been previously dug up by another poster who joined only to post in the same thread
:nuts:nuts

http://i59.tinypic.com/21oub75.jpg

Ruffians from some lawn mower forum coming over here to mess with us!:fisticuffs

The_Whale
08-25-2014, 03:48 PM
So you joined today, to post in a thread completely non-gear related and defunct for one year, which had also had been previously dug up by another poster who joined only to post in the same thread
:nuts:nuts


I look forward to another suspicious post in the summer if 2015 in this same thread.

ModdersAnon
08-25-2014, 03:51 PM
I look forward to another suspicious post in the summer if 2015 in this same thread.
Perhaps then it will be a Prince from Nigeria who needs us to cash a check for him :anon