Trouble starting B&S Quantum when hot

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MI_Roger

Joined
Dec 31, 2007
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Location
Ypsilanti, MI
B&S Quantum engine, 6.5HP, maybe 15 years old. Engine receives new air filter, plug, and fresh oil at beginning of each mowing season. Gas is always treated with Sta-bil. Mower has always started on first or second pull, hot or cold, until now.

This season the mower still starts on first or second pull when cold, but when hot it is very reluctant to re-start. One time this reluctance was triggered by running the engine for less than two minutes, so the problem is really any re-start and not just hot re-start.

I thought it might be a plugged vent in the gas cap but my wife assures me she removed the gas cap to check fuel level during her battles with the mower on Friday. Engine would still not restart. Unless she flooded it by pushing the primer bulb too many times?

Recent solution to get it to re-start is to give the engine one more push of the primer button, or maybe two, and 10 or so pulls of the starter rope.

Any ideas?
 
Is there a possibility you are boiling the gas in the float bowl?
See if there is any reason that it may be running hotter than before.
The priming may just be getting cooler gas in there long enough to get it flowing before it boils.
 
O-ring gasket in the carburator......at junction where carburator meets hose wich goes to cylinder.

That O-ring is worn out and engine is getting too much air and less fuel....
 
Any ideas?? I was going to ask you for ideas... How do you get the wife to cut the lawn?

As for the mower problem i would first try running some carb cleaner through the system for a tank or two then use a small amount of mmo in the gas on a regular basis this cured the same problem with my snow blower. if this fails rebuild the carb the carb kits are cheap and they are easy to do.
 
TRy getting the model number from your engine stamp plate and perhaps a picture to go with it?

I know of 2 types of quantum engines, 1 is the older pulsa-ject type carb (on a 4.5hp quantum engined mower that I sold a few years ago), and then there's the transitioning to a walbro LMS sidedraft type carb.

If latter is your type, then check for debris and clogs from fuel bowl side, drain and compressed air blow all the holes, orifices, etc. clear and re-try again.

Lastly: check for flywheel key shifting. These sidedraft engines with low emissions carb are notoriously known for running very lean (lean as in very close to the stoiciometric ratio), as opposed to those dirty-rich running pulsajets), so any slight miss on the timing, spark plug dirty, etc. will make your engine very hard to start when hot.

This problem had me perplexed over a few sidedraft B&S over the years, with a couple of them ended up in need of a new carb (the 2nd one requires a retrofitting of a 90s LMS type carb with adjustable mixture screw)

Q.
 
Next time it doesn't start....check for spark...your coil may be shot. If it isn't that maybe it needs a valve adjustment.
 
Originally Posted By: JTK
Roger, is the gasoline you are using fresh/new?



That would be my first question. is the Gas fresh.
 
I had the same exact B&S 6.5HP engine on a Craftsman mower with the same exact hot restarting problem. Started fine when cold.

What I did to remedy the hot start issue was...
Pull off the top engine cover & flywheel to expose the coil.
Clean w/ wire brush, the area where the coil is lined up &
also adjust the coil...I use a matchbook or cereal boxtop.

Reinstall and give her a try!
 
Couple things to check.

1) Engine could be running hot. Take off covers and blow out any dirt/grass/leaves.

2) Carb and carb gaskets getting old. Could needs a rebuild or replace.

3) Spark plug gap too high. They will start fine when cold but not when hot. Make sure it is 0.030 on a factory Champion RJ19LM

4) Coil could be going bad. When they are on their way out they are very hard to start when hot. To test for a bad coil, reduce your plug gap from 0.030 to 0.020-0.025 to check for a bad coil. If it starts fine on the reduced gap, your coil is on the way out and needs replaced.

5) Check that the flywheel key is intact and not sheared in the flywheel. That will advance/retard your timing and make starting a bear.

Here are some videos on how to get to the innards of your engine for cleaning and to check the flywheel key. There are better ones on YouTube, I just found these really quickly.
https://www.youtube.com/watch?v=r28A5JJXKMw
https://www.youtube.com/watch?v=_EcZ4kXPjJA
 
Sorry for the delay, busy day yesterday.

Gas is fresh. Problem is slightly less severe since buying the new gas.

Valves have never been adjusted. Only mow a 5,000 sq ft subdivision lot so not a lot of use compared to landscapers and large lots.

Shroud has never been removed from engine. Not a dusty or extrmely dirty usage.

I will re-gap the plug. Remove the shroud and clean the engine top. Inspect key for any signs of shear displacement, replace key if any shear is visible.
 
Originally Posted By: MI_Roger
Valves have never been adjusted. Only mow a 5,000 sq ft subdivision lot so not a lot of use compared to landscapers and large lots.


Assuming you have the flathead Briggs engine, there is no (easy) valve adjustment. The only way to adjust the valves is to take the (usually exhaust) valve out and grind the top until it is back into spec. Not a very common repair, but does happen sometimes.
 
I've just gone through this issue with the 6.75hp Intek on my walk behind Ariens. After replacing all of the carb internal and mounting gaskets with no change, and also adjusting the valves three times, I finally knocked the gap down on the plug from .030" to .020" which was the lowest available measurement on the coin gap gage. Since then, I've only had to pull more than once when hot to get it to restart. Problem solved.
 
Originally Posted By: threeputtpar
I finally knocked the gap down on the plug from .030" to .020" which was the lowest available measurement on the coin gap gage. Since then, I've only had to pull more than once when hot to get it to restart. Problem solved.


Coil on the way out. Problem TEMPORARILY solved.
 
Problem solved!

When I bought my tune-up kit this spring the Hardware Store I stopped at did not have the packaged B&S Kits. They have a HUGE power equipment section with full time mechanics. The Mechanic asked about my engine, pointed to a hook of loose B&S filters, and a hook of loose sparkplugs displayed in small poly bags, said those were what I needed. Told me to go to the other side of the store and grab a 20oz bottle of straight 30 weight, what ever brand I like.

The plugs were not Champion or B%S brand but the Mechanic said they were as good or better and were pre-gapped. So I trusted him.

Tonight I bought the correct Champion plug, gapped it myself, and removed the nearly new plug from the engine. A very oily Oregon brand plug that was gapped far too tight!

The engine now starts on the first pull again, cold and hot!

Turns out that Oregon plugs are re-branded Bosch plugs, a very reputable brand, but it was the WRONG plug for my engine. A 77-301-1 plug rather than the right 77-313-1 to match the Champion specified for the Quantum engine.

Right plug, Right gap, problem solved!
 
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