# Briggs 404700 Twin hot starting problem



## ajar691 (Sep 13, 2011)

I have the above engine in a simplicity 6512 Tractor(1989) 650 Hrs. that does not start when it is hot. I have replaced the Ignition coil for the third time. The engine starts fine when it is cold, runs well. After running for 1/2 hour to 1 hour and shutting down it will not restart until everything is cold. It seems there is no spark from the hot coil. I read an article that said reducing the spark gap on the plugs (from .030" to .015") would help the over heating of the coil and allow the engine to start when hot. It didn't. Compression is 105 lbs hot on both cylinders and battery is good. I replaced ignition switch since it no longer functioned. All interlocks have been disconnected from the module. Help!!


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## Basementgeek (Feb 7, 2005)

Hi and welcome to TSF :wave:

When the engine will not start, when hot, does it spin about the same speed as when it is cold ?

BG


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## ajar691 (Sep 13, 2011)

Yes, it spins about the same. maybe a little faster since it does not try to fire.


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## Basementgeek (Feb 7, 2005)

Well, I had a possible theory, but that went out the window since it spins about the normal speed when hot.

By chance does it have points in it? I don't think it should from what I can see.

BG


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## K-B (Oct 24, 2005)

Have you verified for sure that there is no spark when the engine shuts down?


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## SABL (Jul 4, 2008)

Never heard of the gap mod to correct coil failure. .015" is kinda tight and expansion from heat may reduce the gap further, causing a 'no fire' situation.


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## MPR (Aug 28, 2010)

What is the gap between the flywheel magnets and the armature? I have an older mower that kept loosing spark and finally traced the problem to a too-wide gap between the flywheel and armature.


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## kbowley (Sep 23, 2009)

Hi, you have one of the best engines Briggs made, the 656cc opposed twin down-rated to 12.5 hp. It is a very smooth and strong engine that belays it's 12.5 hp rating. Those engines are solid 2k hr engine and I have seen them with well over 3k and going strong.

That said, the problem you describe is not foreign to that engine. It is nearly always caused by a defective coil and yet you have replaced it with a brand new one, not a used one I assume? The secondary causes are a weak fuel pump that is integrated with the carburetor and improperly adjusted valve clearance.

The first issue is a design flaw in which the coil is placed directly above the cylinder jug exposing it to extreme temperatures when the engine, and therefore the cooling fan, is stopped. Briggs improved the design to correct the problem a couple of years after the opposed twins came out to compete against the import engines. It is important that you purchase a new OEM coil to eliminate the issue for good. When replacing the coil, ensure that it is gaped correctly, .006-.008 is per the manual. 

To verify the coil is functioning correctly it should be tested with a multimeter per the manual. Alternately, check for spark when the engine is hot and will not start. If you do get a spark, ensure it is a very bright blue/white, a good strong spark, as a weak yellowish spark may be insufficient to provide ignition under compression. You can remove the coil and bring it to an authorized Briggs dealer where it can be load tested with a special Briggs tester.

After the coil is verified good, if the problem persists, move on to fuel delivery. Once again, because of the placement of the carburetor, it is subjected to high temperatures that will either cause vapor lock and/or the diaphragm to malfunction so that it fails to deliver fuel. Keep in mind, even if the above occur, the engine will usually fire and run for a few seconds as it consumes the fuel within the carburetor bowl. The best way to eliminate this as the cause is to remove the air cleaner cover and poor an ounce or two into the carburetor throat and attempt to start it. If it starts briefly than you will need to rebuild the fuel pump with a rebuild kit at a cost of 15 dollars.

Lastly, and equally important, is the correct valve clearance/adjustment. If the valves are not fully closing the engine will fail to start. The valves in a flat head cam in block engine are located within the cylinder and very close to the combustion chamber. This exposes the valve stems and valves to extreme temperatures, as high as 500 degrees on the exhaust valve, they need room to expand. As a general rule, the gap should be .018-.020 thousands for the exhaust valve and .012-.014 for the intake. Excessive clearance, say you find the exhaust valve is .022, is far better than to little, such as .012 as this will cause the valve to stay open a bit when hot causing extreme exhaust temperatures and low compression. Valve clearance should be checked every 100 hours of operation. It is a simple job, if a bit time consuming to remove anything obstructing access to the valve covers, and ensures long engine life and excellent performance.

I hope you find this helpful and do not get discouraged. It is a great engine that will run well for another 20 years if maintained. I recommend Mobile One 0w40 oil year round with 40 hour change intervals in those flat-heads.

Kevin Bowley
Kevin's Small Engine Service
South Berwick ME.


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## kbowley (Sep 23, 2009)

Also, I neglected to mention, check the fuel hose that goes between the engines' crankcase and the carburetor. If it is dry, stiff and cracked looking, replace it. That line is the "pulse" line that causes the fuel pump diaphragm to move in-out-in and deliver fuel. If the pulse line is leaking even slightly the fuel pump will be weak, exaggerated with heat, and fail to deliver fuel when hot.

Kevin Bowley
Kevin's Small Engine Service
South Berwick ME.


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## Stans Customs (Apr 22, 2014)

Hello..

I' pretty sure my mower needs the valves adjusted after trying everything else.

Could you explain how to do that on a flat head opposed cylinder twin.

Thanks...Stan


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