Valve Clearance/Lash

S

Skippydiesel

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B & S Model 305447, Type 0116-B1, Code 11030711

What if any effect, does valve clearance/lash have on engine idle speed? 😈
 
I would think it affect all speeds as the engine just don't breathe as freely with wider clearances as it should but it not something that is normally noticeable on small engines. IE the open duration timing changes.

A good example would be my 79 Chevy. It came from the factory getting 15 mpg highway. After I installed a mild racing/mileage pro camshaft, variable duration lifters, and a turbo exhaust muffler it was getting 32 mpg highway with a combined mpg of 22.6. Yet it had plenty of get up and go power. Btw drafting behind a 18 wheeler got it up to 45 mpg but that is very dangerous to do normally, I was in constant contact with that driver at that time and on an open interstate.
 
The average person probably wouldn't notice any idle speed changes, but valves out of spec will have an effect on either advancing or retarding valve timing to the point can effect runnablity.

But was checking on the Briggs EFI engines since valve clearance has been listed as possible cause of a P0336 crank position sensor fault. I have heard in mentions in one of my update schools that if you are running live data with the engine software one of the sensor readings will tell you how far out of specs the valves are adjusted by thousands of an inch. I believe if was a MAP sensor reading.
 
ILENGINE said:
one of the sensor readings will tell you how far out of specs the valves are adjusted by thousands of an inch. I believe if was a MAP sensor reading.
Click to expand...
MAP is all about pressure as in atmospheric. Not really anything to do with valve clearances.

I highly doubt, Briggs, has an OPTICAL sensor (high precision) on the cam or rocker arm/s that talks to the ECU like this. Maybe they do, I do NOT know.

Valve clearance and crank POSITION are different potatoes. Is Briggs looking at valve clearance to monitor crank position now? They better have solid steel cam gears and optical sensors. Why does that sound backwards? I'm willing to learn.........
 
slomo said:
MAP is all about pressure as in atmospheric. Not really anything to do with valve clearances.

I highly doubt, Briggs, has an OPTICAL sensor (high precision) on the cam or rocker arm/s that talks to the ECU like this. Maybe they do, I do NOT know.

Valve clearance and crank POSITION are different potatoes. Is Briggs looking at valve clearance to monitor crank position now? They better have solid steel cam gears and optical sensors. Why does that sound backwards? I'm willing to learn.........
Click to expand...
It is kind of a weird situation. There is no direct sensor for the valve clearances but the valve clearances out of spec changes the readings on the MAP sensor that if you know what you are looking at can tell the valve clearances. It is not a direct read system, more a deviation that results from changes in valve clearances.
 
ILENGINE said:
It is kind of a weird situation. There is no direct sensor for the valve clearances but the valve clearances out of spec changes the readings on the MAP sensor that if you know what you are looking at can tell the valve clearances. It is not a direct read system, more a deviation that results from changes in valve clearances.
Click to expand...
But the MAP pressure changes nearly by the minute with the weather and altitude on said engine. MAP is not static. I'm sure there is more to the story. :unsure:
 
a tight valve would be more noticable at idle because the compression has longer to leak out it if the valve is open. Also it makes the camshaft a longer duration, which alone favors higher rpm. At low rpm, a valve that is tight might allow reversion into the intake manifold,and open the exhaust sooner, wasting part of the compression stroke. One has to think of how it affects the river of air and fuel flowing into the combustion chamber.
 
I like your drift into MAP (inlet MAnifold Pressure)- As I understand it, MAP is always relative to pressure altitude, which is the combination of atmospheric pressure and temperature, at a given altitude (above sea level).

MAP is primarily used by pilots flying IC engines from sea level to ?? ' altitude as an indicator of power/throttle setting. Pilot will have charts/tables giving optimum MAP at a given Cruising level, so that best power/econamy Cruise can be set.

BUT

The old B&S (listed above) I am working on, is never likly to go above 1,000 ft AMSL and although temperatures in my part of the World can vary from near 0C to 45C, which will effect MAP, I doubt MAP is going to be something I need to be concerned with, in such a basic engine. 😈
 
The small planes I flew all we did was to adjust the mixture for best speed, no charts. Of course these never get above 10,000. And whenever we had engine problems we went to full rich setting and turn on the carb heater.
 
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