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View Full Version : Briggs 6.5 Quantum won't start when hot


bluedakotasky
03-22-2012, 08:29 PM
I have a Briggs Quantum 6.5 hp pushmower that will not restart when hot. It's for a customer. Prime the mower (cold) 2-3 times, and it starts on the first pull. Mow for 30 minutes, shut down to empty the bag. Empty the bag, put it back on (5 minutes, max) and it will not start. Leave it for 15 minutes, and it will start right up.

Checked: kill switch - disengages properly, engages properly.
Kill switch wire - no bare spots, routed correctly.
Air filter - clean.
Fuel system clean - this mower is garage kept. Very clean.

Replaced: carburetor and ignition coil.

Customer took the mower home. He mowed 30 minutes, repeat problem.

Help??

dutch1
03-22-2012, 08:59 PM
Check compression when hot and cold. If there is a lack of compression when hot you could have valve problems--valve clearances not sufficient when hot.

piston slapper
03-22-2012, 09:04 PM
Ditto Dutch.....might also want to make sure the head bolts are tight.
Make sure the plastic intake tube isn't broken or cracked.
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dutch1
03-22-2012, 09:54 PM
Good point on the crossover tube, Slapper. I've seen a few of those over the years along with a defective crossover tube intake gasket where it is retained to the block.

cgaengineer
03-22-2012, 09:59 PM
Valves not seating...
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bluedakotasky
03-23-2012, 10:11 AM
Thanks everyone, I'll look at that stuff today. I'll just pull the head off and check everything. One other thing I haven't done is change the spark plug. I haven't seen a bad spark plug probably in 20 years. But you never know.

piston slapper
03-23-2012, 10:28 AM
Thanks everyone, I'll look at that stuff today. I'll just pull the head off and check everything. One other thing I haven't done is change the spark plug. I haven't seen a bad spark plug probably in 20 years. But you never know.

Hmmmm.....Where have you been for 20 years??????
Sparkplug would have been at the top of the list....:hammerhead:

Here we go....Its springtime again.......:dizzy:

bluedakotasky
03-23-2012, 11:03 AM
Sorry,
I was working on General Electric turbines for American Airlines, lol.... : )
We called them "ignitors" there.

piston slapper
03-23-2012, 11:50 AM
Sorry,
I was working on General Electric turbines for American Airlines, lol.... : )
We called them "ignitors" there.

Nothing to be sorry about....They dont call me " Slapper " for nothing.
I've been doin this stuff since Reagan was in office and I still aint got it all figgered out.

Nothin personal......Slapper

bluedakotasky
03-23-2012, 04:06 PM
No problem! I appreciate you guys immensely! I certainly don't know it all, and I don't trust someone who claims that they do : )


I pulled the head and valves today. There was so much carbon build-up, that I'm not sure the intake valve was closing very well. It was really caked in there. That's crazy to me. This is a very new engine. Very low time. I don't know if it's from fuel leaking in after shut-down, or if it's oil leaking in after shutdown or while it's running. It's a mystery. There is just slight movement of the piston up and down in the cylinder (cold) but there has to be some clearance there. I will media-blast the cylinder head and valves on Monday. I won't have the head gasket until probably Tuesday. We'll try this and see if cleaning all the carbon out fixes the problem. I will put in a new spark plug, too. Although I know that when troubleshooting, we should only change one thing at a time : )

piston slapper
03-24-2012, 09:57 AM
Sounds like incomplete combustion....poor compression....spark....or weak fuel.
With your background on turbines.....this will be childs play.......
If for some odd reason you don't get it right.....at least you won't startle 400 passengers, and a few pilots...........
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Techno
03-24-2012, 05:03 PM
Things iv found on 6.5 quantum engines. The carburetors are junk first off. They have a rubber seat and solid float valve, which is a big problem with ethanol gas. As soon as the ethanol hit the seat it swells and makes the float 10 degrees lower than it should be. Best solution tho is not to replace the carb. Get an aftermarket seat from rotory and replace the original seat. When these are installed they actually make the float 10 degrees higher than needed. When they swell from the ethanol they will make the float near level then.

Another thing is the flywheels. Iv had many come in that were leaking. Take a screw driver and put it next to the magnet on the flywheel and see how much it pulls. If it pulls too hard its throwing the timing off from the magnet leaking over onto the flywheel itself.

Had many come in with leaking valves, either from loose valve seats, valve itself damaged or carbon buildup from the bad seat in the carburetor. Also make sure the valve lash is set properly. A little off on this engine and it wont start right.

I also take the compression releases out of these engines when rebuilding. They start better. Its not needed anyway.

dutch1
03-24-2012, 06:58 PM
Techno, your experience is entirely different than mine regarding 120000 series carburetors. The only carburetors that were junk were one that came in with water in the carb bowl--badly eroded barrels. By the way, the OEM seats are viton, not rubber and I haven't found their capacity to swell any worse than any other seat. I've tried several aftermarket replacements, from a variety of sources, for the Briggs 498260 kit and have found a 60-70 percent failure rate when pressure testing the needle/seat prior to reassembly. The orfice in most of the aftermarket seats are much larger than the OEM, which is why the float doesn't set level when installed and most likely the reason they do not consistently hold pressure.

Never experienced a leaky flywheel. The ignition current is created IN the CDI pack(coil) by the magnet in the flywheel passing the posts of the CDI. The air gap(distance between the flywheel magnet and CDI) is the only thing, external to the engine, that will change timing.

Maybe I haven't seen all versions of the 120000 series engine, and I've replaced numerous cranks, but can't recall seeing one with a compression release. On the other hand, maybe it's because I'm an old fart with a short memory.

piston slapper
03-24-2012, 08:21 PM
Dutch.......Teckno's experience is ...........well .....you know....
Who knows????......he is not an authority on anything other than his own opinion.
We all know (those of us that have been inside thousands of briggs carbs ) that his findings are pure bunk.

Was waiting for you to check in on the school me and Rob held this morning. ..
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dutch1
03-24-2012, 09:07 PM
I didn't get an invite, Slapper. If it had involved a plate of biscuits and gravy I'da shown up. But it appears, as I just read, that school is out, according to the Robster.

Matter of fact, I was busy making a little quick "walking around" money on what else, a Quantam--carb rebuild and a PM service. Need to go back out and check for the leaky flywheel though.

piston slapper
03-24-2012, 09:19 PM
Those leaky flywheels can be elusive.....been hanging around with hundreds of those engines for years.....and ain't heard a peep from one yet.....

Nothin wrong with a little spendin cabbage....comes in handy round here.
I may have to reopen the school....those guys sure make me feel smart.....
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