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Any small Engine guys on board??

General engine tech -- Drag Racing to Circle Track

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GLHS60
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Any small Engine guys on board??

Post by GLHS60 » Fri Jun 14, 2019 3:31 am

I have a typical riding mower with a 15.5 HP OHV Briggs and Stratton single cylinder Engine.

Its becoming harder and harder to start especially cold, it barely turns over, just grunts.

Reminds me of an overheated 396 Chevy on a hot day, only this one is worse when cold.

Everything electrical is fine so I googled the issue and it appears to be very common.

The general opinion is excessive valve lash, there are several videos on the exact issue.

Not only Briggs but many single cylinder OHV riding mower Engines are diagnosed the same.

The vids show a grunting Engine, the guy adjusts the valves and then the Engines spin normally.

In many cases the guy checks lash, they run from .012" to .020" and they get reset to about .005"

I'm trying to figure out how such a small amount of extra lash can cause this issue.

The vid guys say things like "the air can't get out so it makes the compression too high"

None of the guys seem able to explain just what's going on but the results appear positive.

Obviously I don't know either but I guess I'm more curious than some.

Any input is appreciated.

Thanks
Randy
Sherwood Park
Alberta,Canada

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Re: Any small Engine guys on board??

Post by Jarmo » Fri Jun 14, 2019 4:42 am

Many small engines have a small bump on intake cam for compression release and if valve lash is big enough then it does not open intake valve for compression release and starter can not turn it over.

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Re: Any small Engine guys on board??

Post by modok » Fri Jun 14, 2019 4:47 am

the famous BRIGGS and stratton, PATENTED "ez-spin starting system" was.....a cam that held the intake valve slightly open through the compression stroke, as compression release. Idle quality suffered but...who idles a mower?

More advanced designs had a stamped steel dingle arm on the cam, that was a compression release at low RPM but would swing out of the way when started. Eventually they ALL had something LIKE that, one form or another.

What "grunting" is I have no idea... but why valve lash could impact starting....that'd be my guess, is excessive intake valve lash could be bypassing whatever compression release feature they are using.
Of course tight lash would be a problem too, but for the usual reasons.

they start fine, or perhaps better without the compression release, but it takes a stronger arm to do it, or stronger starter motor. Flywheel has to spin up fast enough to make the ignition work.

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Re: Any small Engine guys on board??

Post by Dave Koehler » Fri Jun 14, 2019 8:13 am

Agreed that the loose lash and the compression release bump on some engines is a real thing.
Maybe not this time but certainly a suspect. I would verify the lash.

OP, you stated everything electrical is fine but have you tried a different battery?
Those baby batteries don't have a lot of life or grunt in them. I had the same thing with my old JD 318 this year.
Well, it began last year when it started that slow roll start up.
I found it odd since this Onan is one loose, noisy, ready to kick a rod out piece.
This year it got worse.
Throwing the battery charger on it did nothing.
Hmmm, tester read the battery as being next door to toast.
New battery and it started up quickly.
On a side note this battery was 1 yr and 1 day old. 1 day past free replacement.
Shortest life I ever got out of one of these.

I ran into the tight lash version of this last month.
Neighbor next door has a Kubota Lawn tractor. I noticed over time it had to crank longer and longer to fire. This year was the last straw. After Listening to it for the umpteenth millionth time I told him to bring it over.
Checked all the usual suspects. I don't recall the number but the lash was pretty tight. Reset it to the max factory setting. Bingo...back to starting in a normal manner.
I am guessing that when cold, hi hours, loose guides and some carbon build up was holding the valves up just enough to act as a compression release.
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Re: Any small Engine guys on board??

Post by GLHS60 » Fri Jun 14, 2019 1:33 pm

Excellent reply Jarmo!!

This makes sense to me especially as some vids claimed the compression release is on the intake.

I doubted this as I always thought it would be a centrifugal device on the exhaust lobe.

Many vids show this device but as it requires Engine disassembly replacement cost is probably high.

In one vid the guy was trying to show the rocker jumping as he slowly rotated the Engine by hand.

He said intake but I assumed he was mistaken as OHV Engines vary on valve placement.

Some OHV mower Engines have the exhaust above the intake and some are below.

There's probably a reason for this as well, possibly you can enlighten me.

I have to say this Engine has been a reliable workhorse for me since 1999, it owes me nothing.

Once it starts, it performs perfectly, not due to my rigid maintenance, I have not been dilligent.

Thanks
Randy




Jarmo wrote:
Fri Jun 14, 2019 4:42 am
Many small engines have a small bump on intake cam for compression release and if valve lash is big enough then it does not open intake valve for compression release and starter can not turn it over.
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Re: Any small Engine guys on board??

Post by GLHS60 » Fri Jun 14, 2019 1:52 pm

modok wrote:
Fri Jun 14, 2019 4:47 am
the famous BRIGGS and stratton, PATENTED "ez-spin starting system" was.....a cam that held the intake valve slightly open through the compression stroke, as compression release. Idle quality suffered but...who idles a mower?

More advanced designs had a stamped steel dingle arm on the cam, that was a compression release at low RPM but would swing out of the way when started. Eventually they ALL had something LIKE that, one form or another.

What "grunting" is I have no idea... but why valve lash could impact starting....that'd be my guess, is excessive intake valve lash could be bypassing whatever compression release feature they are using.
Of course tight lash would be a problem too, but for the usual reasons.

they start fine, or perhaps better without the compression release, but it takes a stronger arm to do it, or stronger starter motor. Flywheel has to spin up fast enough to make the ignition work.
Now that you mention it I always idle for a while before turning it off and the idle is very erratic.

I have a newfound respect for this and compression releases now as they obviously aid in starting.

As to "grunting" you likely missed my hot 396 comment, the starter struggling against compression.

I could recite the 396 firing order as the Engine grunted over, one compression stroke at a time.

The Briggs ignition impresses me, I'm continually amazed how well it starts, barely turning over.

The Engine durability also impresses me, I never thought it could work so hard for so long.

Thanks
Randy
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Re: Any small Engine guys on board??

Post by treyrags » Fri Jun 14, 2019 2:41 pm

I have sprained my wrist bad a couple of times on 14 horsepower pull start engines like this that we're out of adjustment and the compression release did not work and kicked back on me. Not fun

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Re: Any small Engine guys on board??

Post by modok » Fri Jun 14, 2019 8:07 pm

GLHS60 wrote:
Fri Jun 14, 2019 1:52 pm

I could recite the 396 firing order as the Engine grunted over, one compression stroke at a time.
Put a longer duration cam in it, and the starter will be able to spin it a lot easier :wink:

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Re: Any small Engine guys on board??

Post by Dan Timberlake » Sat Jun 15, 2019 7:59 am

I like to do a voltage drop test while cranking. It is surprising (at least to me) how machinery seems to conspire and continually create new ways to fail.

Just sitting here I recall I've been bit twice by simple spade connectors when the female wore to cause a lose fit with the male.
The last time was just last weekend when a neighbor dragged her unconscious John Deere riding mower over to our house.
Turn the key to on, the electric carb main jet clicked on, but the hour meter remained blank, and of course no crank.
Checking voltage all over the place produced some crazy voltages between 5 and 10 volts at some locations where there should have been full battery 12.X DC volts.

It turns out this JD joins nearly all the grounds together eventually to a single wire that crosses over the chassis to the same point where the fat black battery ground cable bolts to the chassis. About 8 inches upstream, well hidden in the dark tight nether regions below the battery and behind the engine, is a single spade connector. THE spade connector. Snip, crimp, hopefully good for another 10 years.

The volt meter sniffed out the bad connector like a blood hound.
It would have taken a "shot gun" replacement of the $100 front wiring harness to inadvertently fix it.

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Re: Any small Engine guys on board??

Post by GLHS60 » Sat Jun 15, 2019 2:26 pm

Appreciate the suggestion but I'm not interested in modifying the mower with a different cam.

Just trying to get it to start like it used to.

Thanks
Randy

modok wrote:
Fri Jun 14, 2019 8:07 pm
GLHS60 wrote:
Fri Jun 14, 2019 1:52 pm

I could recite the 396 firing order as the Engine grunted over, one compression stroke at a time.
Put a longer duration cam in it, and the starter will be able to spin it a lot easier :wink:
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Re: Any small Engine guys on board??

Post by GLHS60 » Sat Jun 15, 2019 2:30 pm

I appreciate the suggestions Dan but I've been through all the electrical.

Jarmo and others have expertly explained the valve lash issue so thats my next step.

Thanks
Randy
Dan Timberlake wrote:
Sat Jun 15, 2019 7:59 am
I like to do a voltage drop test while cranking. It is surprising (at least to me) how machinery seems to conspire and continually create new ways to fail.
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GLHS60
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Re: Any small Engine guys on board??

Post by GLHS60 » Thu Jun 27, 2019 5:12 pm

UPDATE!!

I finally finished the mower valve lash today and it starts like new.

It's taken me awhile to get the job done as I only have 1 hand functioning after a stroke.

Valve lash was about .007" exhaust and .013" intake cold, both were set to .006"

I rotated the Engine by hand many times watching the rockers move and caught the "bump".

Surprisingly, like Jarmo mentioned it was on the intake, not the exhaust.

This has been a very educational and and enlightening experience for me!!

I really appreciate all the input.

Thanks
Randy
Sherwood Park
Alberta,Canada

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Re: Any small Engine guys on board??

Post by Dave Koehler » Thu Jun 27, 2019 5:18 pm

Congrats.
Amazing little detail isn't it?
I just got done convincing my son in law of this. He was ready to trash a perfectly good engine.
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Re: Any small Engine guys on board??

Post by GLHS60 » Thu Jun 27, 2019 5:30 pm

Appreciate the reply Dave!!

This old Briggs continues to amaze me over its 20 yrs of loyal service.

Still clean as a whistle under the valve cover running # 30 oil and no real oil leaks.

With the top cover off the Engine there is what looks like a plastic starter drive gear??

How this has held up is beyond me but it's never been an issue, even with hard cranking.

Thanks
Randy
Sherwood Park
Alberta,Canada

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