# 6hp B&S won't start after hot



## jytrbug (Aug 20, 2009)

I've got an '03 B&S that won't start after being run 10min. It has spark,checked the valve adj.,seat etc.,hooked the air hose and checked for valve leaking with very little seeping from exhaust valve,Compression cold is 60psi,turned it off after running 5-10min (while it was shutting down alot of blue smoke coming out of the exhuast) did a comp test and got 30psi when hot. Am I right to think the rings are shot. Also when checking for a leaky valve how much air psi should I be using.


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## LowRider (Dec 23, 2008)

what size engine is this? i would think 85psi or so would be minimal if not normal. i would definitely say rings/piston/housing if not the coil. maybe I'm wrong but how did you even get it started at such low PSI? wouldn't think the engine would start.... also did you use this type of compression tester http://www.northerntool.com/images/product/images/800507_lg.jpg?? 

the blueish smoke means oil burning which ain't good. i think your on the right track. might want to check oil levels.


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## jytrbug (Aug 20, 2009)

It's a 6hp. Year 2003. It's on a Craftsman leaf shredder/brush chipper. The motor will start 1st pull when cold at 60psi but you can pull all day at 30 psi after its up to temp. That is the type comp gauge I have. The only thing I see is it must have a comp release on the crank lobe because when the ex. valve opens completely then closes it will open for a short duration before the compression stroke.


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## dj722000 (Oct 29, 2008)

I know on alot of engines with compression relief, the low readings on compression is typical because the ACR on the cam is popping the exhaust open briefly so the reading doesnt really meen nothing. (The ACR will release fully once higher RPM is achieved, Mechanical flyweight) However, if your puffing blue smoke during shut off im guessing your rings are shot or really close to being shot. When you put the engine under load do you notice any blue smoke, a quick puff is all you will see. Should maybe notice a lack of power the longer you run it?


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## bc11 (Jul 14, 2009)

I'd think it would have to have been run a lot of hours by a tree trimming company to be burning oil already. There is a lot of old B & S mowers out there with a lot of hours that don't burn oil yet.


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## penguineman (Apr 15, 2009)

have you check the valve lash when hot?? what is it at? when you pressured it up with air was you using a leak down tester?


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## jytrbug (Aug 20, 2009)

penguineman said:


> have you check the valve lash when hot?? what is it at? when you pressured it up with air was you using a leak down tester?


Going to check lash when hot today. Don't have a leak down tester,I was just using my ears and heard very little air escape from the exhaust valve when cold. Going to check for possible plugged crankcase breather as well.


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## jytrbug (Aug 20, 2009)

Relapped the ex. valve today which came out good. Ran my fingernail on the cylinder and it has a small groove, just enough to catch the nail in one spot. While pumping air into the cylinder I pulled the oil cap and can hear air in the crankcase as well as the breather which to me says rings are shot. Right? Can I just replace the piston,rings,etc. without honing/boring the cylinder?


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## Al Bunzel (Apr 20, 2009)

jytrbug said:


> Relapped the ex. valve today which came out good. Ran my fingernail on the cylinder and it has a small groove, just enough to catch the nail in one spot. While pumping air into the cylinder I pulled the oil cap and can hear air in the crankcase as well as the breather which to me says rings are shot. Right? Can I just replace the piston,rings,etc. without honing/boring the cylinder?


Hi,

You may want to do the following checks before boring and honing the cylinder:

Have you measured the bore diameter using an internal micrometer?
You will need to measure it in various places in the cylinder and check if it is within tolerances as specified by the manufacturer.
If this is not within spec, piston rings alone won't necessarily fix the problem and the cylinder may require boring and honing. If scratches are extremely deep or the cylinder is way out shape, boring may not be possible.

You will also need to check the piston for roundness. You can do this by taking measurements using an outside micrometer at various points around the piston and checking it is within manufacturers specifications.
Again, if this is not within spec, piston rings alone won't necessarily fix the problem.

Check the piston ring side clearance.

Before taking the piston rings off, ensure you label which is the first ring and which is the second ring. (Oil ring is distinctly different.) Ensure you note which side is up.
Why? one of the rings has chamfer, the other has a step. If piston rings are fitted wrongly, some weird stuff can happen.

You also want to check the piston ring gap. 
This is done by placing the ring at the bottom of the cylinder and measuring the gap using a feeler guage. 
If they are not within manufacturers spec, then new piston rings may be required.

Kind Regards
AlBunzel


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## kodiakyoshi (Aug 24, 2009)

I had one do that and it was as simple as replacing the coil. They can get hot and crack. Even if it is a minute crack it is open when the engine is at operating temp. Replace the coil and you will be good to go.


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## Al Bunzel (Apr 20, 2009)

jytrbug said:


> Ran my fingernail on the cylinder and it has a small groove, just enough to catch the nail in one spot.


Hi,

I forgot to mention, you should also measure the cylinder at various points to ensure the diameter is within spec.
If you have a small groove, you may be lucky and get away with honing the cylinder, but that depends how deep the groove is.

Kind Regards
AlBunzel


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## jytrbug (Aug 20, 2009)

Al Bunzel said:


> Hi,
> 
> I forgot to mention, you should also measure the cylinder at various points to ensure the diameter is within spec.
> If you have a small groove, you may be lucky and get away with honing the cylinder, but that depends how deep the groove is.
> ...


The scratch will just barely catch my thumbnail when slid around the bore. I don't have a mic for inside dia. Any recommendations to a decent but not to expensive one?


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## jytrbug (Aug 20, 2009)

kodiakyoshi said:


> I had one do that and it was as simple as replacing the coil. They can get hot and crack. Even if it is a minute crack it is open when the engine is at operating temp. Replace the coil and you will be good to go.


I thought about that but it has the same type spark cold as when hot. Not a real big blue spark but enough so it will start first pull when cold. Relapped the ex. valve yesterday and I can get it to restart when hot by choking it, I still think the rings are shot.


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## Al Bunzel (Apr 20, 2009)

jytrbug said:


> The scratch will just barely catch my thumbnail when slid around the bore. I don't have a mic for inside dia. Any recommendations to a decent but not to expensive one?


Hi,

when I measure inside bores, I use a telescopic gage (guage) like in this picture:
http://www.nitt.edu/home/academics/departments/mech/facilitiesandservices/metrologylab/telescopic/

Then, I measure the telescope bits with a micrometer,
like in this picture: http://www2.tech.purdue.edu/at/courses/aeml/powerplantimages/micrometer&telegage.jpg

I'm not sure what brands to recommend.
I've been lucky to be able to borrow these measuring tools when I need them.

Kind Regards
AlBunzel


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## jytrbug (Aug 20, 2009)

Al Bunzel said:


> Hi,
> 
> when I measure inside bores, I use a telescopic gage (guage) like in this picture:
> http://www.nitt.edu/home/academics/departments/mech/facilitiesandservices/metrologylab/telescopic/
> ...


Thanks for the tips and reply. I have a place I may be able to borrow the tool for this project.


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## hd4ou (Aug 25, 2009)

I think valves are supposed to be checked and set when cold and not hot. if set when hot, after it cools the clearances are too close and could hold a valve open.


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