Stumped by Briggs engine's hard starting

[bchannell]

I'm soliciting some help here on a 6.75 Briggs lawnmower that is hard to start.
It's a new engine with the auto choke. I got it for a song as a store return, and it looks new.
When cold it takes 7-10 good pulls to get it running. When hot it almost won't start at all, and then sometimes when hot it starts on the first really good pull??? When it does start, it runs rough, surging a bit, for around 2 minutes then runs smooth and mows with good power.

I have:
put three different carbs on it, one of which is brand new.

it had a blown head gasket when I got it, and I replaced the head becasue it was blown out at the point where the gasket blew, but the block looked fine.

checked spark, intake leaks, adjusted the governor, replaced spark plugs, air filter, adjusted mag gap, new fuel. all the things I could think of maintenance wise.

found intake valve seal dislodged and seated it back into it's recess. The seat is not in the spring housing it's just below the valve's face in the intake air duct, so I thought that it might be getting in the way and changing air flow, but didn't help.

I'm wondering if it's possible I missed a burnt valve when I replaced the head, making it hard to start, but would think that would cause it to be underpowered and maybe backfire some.

Any thoughts?

 

[Quest]

always check for sparks, fuel+air (air filter clogged? any air leaks from the intake trac all the way, etc.) and so on.

then check ignition timing. you will need to undo the fan shroud and undo the nut to read that shear key. If it's off, replace it with fresh, new B&S or aftermarket ones made of pot metal, specifically for the newer ones (older, point type ignition key is slightly different).

Lastly: does the engine "pops" a bit when hot starting? if so, your valves maybe leaking. You said that it overheated and valve seat dislodged from it's position before? If that's the case, check again if it becomes loose. Check valve lash (for 125K series, both intake and exhaust should be around 6thou). and also lap the valves again to ensure proper seating.

Lastly: check to see if the thermo choke level is sticking or swinging properly. That unit is non-serviceable but can be lubed (so I've been told, but I never have one that sticks so far).

Good luck. I've done just about enough of these 6.75 series engines (from 6.0 and up, same headgasket BTW) to realise how painful for the auto-choke type to be.

Q.

 

[Cardenio327]

Originally Posted By: bchannell
I'm wondering if it's possible I missed a burnt valve when I replaced the head, making it hard to start, but would think that would cause it to be underpowered and maybe backfire some.


Have you checked it with a compression gauge?

If it is a push mower it only needs about 3 hp to mow, being a 6.75 hp it could have all kinds of engine maladies and still do a fine job cutting grass.

 

[crazyoildude]

every briggs i have with the so called auto choke and no prime button is hard to start (i use carb cleaner sprayed into the intake to start them all)

 

[Trav]

Originally Posted By: bchannell
got it for a song as a store return, and it looks new.

It probably had fuel in it from the original purchaser that drained it and now it has some gummy residue in the jets.
Try a few ounces of Berrymans Chemtool in the fuel, get it running so the treated fuel is in the jets and shut it off, let it sit a few days.
This would be my course of action before i started getting the toolbox out.

 

[Warstud]

Originally Posted By: Trav
It probably had fuel in it from the original purchaser that drained it and now it has some gummy residue in the jets.


Originally Posted By: bchannell
I have:
put three different carbs on it, one of which is brand new.

 

[JTK]

Silly question, but are positive the choke is on when cold and pulls off once the exhaust heats the button that yanks the linkage to pull the choke off?

 

[bchannell]

I'm trying to be methodical about this, and also do the simplest things first and prceed to more complicated remedies.
I've checked spark and it's good. Can't find any air leaks in the intake. I've done all the "tune up" type things, new plugs, new fuel, new air filter, etc.
Today I did a cylinder leak down test and it showed very good, so the compression, valve sealing etc. is fine.
This afternoon I'll check the key in the flywheel for damage, and also try changing the ignition timing. I have all the B&S manuals for every engine, and it does mention placing the ignition magnito all the way to the right when installing it. I did notice some side to side movement when installing it, but not much, and I didn't think it was enough to change the spark timing much, but who knows. I know in the past when I was a kid and my dad repaired engines there was not timing of the magnito, but we'll see.

 

[bchannell]

Well I got it. I just knew these engines aren't all that complicated, so it had to be found. It was the flywheel key had sheared completely in two and was waaaaay out of time, but I swear it ran pretty good once it warmed up. Anyway, a trip to Advance Auto and I made a key from an automobile shaft key, and it started on the first pull. It will now start the first pull every time.
I had gone thru all the things I could think of and timing was the only thing left. I just had never taken the recoil started flange off the flywheel to see the key.

Thanks to all for the suggestions and helping me get on the right track.

 

[wvrailroader]

Is the key you made steel? If so, you may want to buy a regular soft flywheel key at Lowe's or TSC, etc. as soon as possible and replace it. If you hit something solid with your mower, you are risking severe engine damage due to the flywheel key not being able to shear.

 

[Quest]

flywheel key on all mowers (incl. B&S) must be made of soft potmetal in order to shear upon blade impact (e.g. hit rocks, stump, etc.) and the flywheel still moving. It's design as a safety feature.

replacing it with non potmetal varieties will ended up with a twisted or even broken crankshaft upon impact.

Q.

 

[JTK]

Guess it's just my luck, but I've never had nor seen a flywheel key sacrifice itself to protect the crankshaft.

 

[TFB1]

Originally Posted By: JTK
Guess it's just my luck, but I've never had nor seen a flywheel key sacrifice itself to protect the crankshaft.


Me neither, I have seen a couple keys that were a bit "off set" but the crank was still bent... Most flywheels on rotary mowers are so light I doubt the type of key makes much difference...

The key on this one was still fine(but then I don't think the crank was actually bent)...

 

[JTK]

LOL! Oh man.. that's a beauty of a Snapper disk drive too!! I wish mine had the aluminum deck.

 

[bchannell]

You guys brought up a great point, which I had actually thought of. Small engine keys are VERY soft, and I did replace it with a harder key until I can get the right one. I don't have anything in my yard to cause problems, but I do have soft keys on order and they should be here this week and I will replace it. Those keys definitely do save engines when an object is hit, I've seen several the were saved by the key.

 

[bchannell]

One final note, I'v read all the bashing of the newest B&S engines and their auto choke, but I must say, that I'm pretty impressed with them. When correctly setup, they start with gusto, and very easily. I bought several returned lawnmowers from a hardware store and, so far they have all paid off with lots of power and easy starting. Hopefully, it'll pay off next spring when I resell them.

 

[Dean85]

Originally Posted By: TFB1
Originally Posted By: JTK
Guess it's just my luck, but I've never had nor seen a flywheel key sacrifice itself to protect the crankshaft.


Me neither, I have seen a couple keys that were a bit "off set" but the crank was still bent... Most flywheels on rotary mowers are so light I doubt the type of key makes much difference...

The key on this one was still fine(but then I don't think the crank was actually bent)...

Holy cow!!! What did it hit??

 

[old1]

Regarding crankshaft keys, I have seen keys sheared when hitting a large3 clump of dirt or a loose blade, and I have seen keys not sheared after hitting something hard enough to bend the crankshaft badly. If I remember correctly in B+S school we were taught that the soft key was to protect the flywheel from splitting in half, which I have seen happen with a steel key

 

[wag123]

Originally Posted By: old1
Regarding crankshaft keys, I have seen keys sheared when hitting a large3 clump of dirt or a loose blade, and I have seen keys not sheared after hitting something hard enough to bend the crankshaft badly. If I remember correctly in B+S school we were taught that the soft key was to protect the flywheel from splitting in half, which I have seen happen with a steel key

Yep.
The soft flywheel key is a holdover from back in the day when ignition points were used. This was a safety feature. When the key sheared (even partially) the engine would loose spark and wouldn't restart. If this happened, most people would bring their machine to the shop for repairs. This allowed for a professional inspection to determine if anything else had been damaged which could cause a safety issue, B&S was adamant about technicians doing this. Replacing the flywheel key with a steel one is a major no-no. Like old1 mentioned, it can cause breakage of the flywheel. When/if a broken flywheel comes apart, pieces of it can fly out like shrapnel and cause serious injury. Besides, the correct flywheel keys are VERY CHEAP! Also, after replacing a sheared key, make sure that the flywheel nut is torqued to factory spec (55-60 ft/lb, more than this can also break a flywheel).

 

[wag123]

Originally Posted By: TFB1
Originally Posted By: JTK
Guess it's just my luck, but I've never had nor seen a flywheel key sacrifice itself to protect the crankshaft.


Me neither, I have seen a couple keys that were a bit "off set" but the crank was still bent... Most flywheels on rotary mowers are so light I doubt the type of key makes much difference...

The key on this one was still fine(but then I don't think the crank was actually bent)...

This likely happened because of previous damage from hitting something. When you hit something, the crankshaft can (and usually does) twist and may not actually be bent. Most small engine crankshafts are cast iron. Cast iron is brittle. Twisting the crankshaft can cause microscopic hairline cracks and it is only a matter of time before it will break (like you see here).