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O 320 Hard to Start

GHARBEN

Active Member
N12HR fly's about once a week. I have owned it about 2 years. It has always taken too much twisting to crank. I have disconnected the primer and it appears to be working. After a lot of cranking it suddenly bursts to life. Never a pop or crack! It feels like there is no gas or no spark. When you turn the propeller and as the impulse coupling spins ahead there is a weak spark about every 3rd try. I don't believe the mags have been overhauled in the last 800 hours. Should the boost pump be left on for starting?. I generally turn it off after I build up the fuel pressure. When first doing a condition check, it appeared that the electric pump flooded the carburetor. For that same reason, I do not use the boost pump for take off.
It also appears very ez to flood.
The engine is an O 320.
Any thoughts?
G Harben
N12HR
S/N 1664
 
Master on
Mixture full rich
Throttle wide open then boost pump till you hear the pump under pressure and about 2 seconds. Pump off
Throttle 1/4 inch
Crank it.
Mine usually hits on the 3rd revolution.
Cold weather way more prime.

That’s my method at least.
 
Slick recommends IRAN on their mags every 500 hrs. Consider swapping the impulse mag for an electronic ignition and having the other OH'd. The boost pump should not flood the engine; something else to correct.
 
O-320 Hard Starting.

I agree with these suggestions.
Also if the spark plug gap is excessive (due to normal wear); it will be very hard to start.
 
Could be mag, you can remove and check with spark tester. The spark will jump a significant gap (1/4" to as much as 1") on good mag spinning the impulse coupling by hand. Check the impulse mag, as it's the one doing the work during the start. It's definitely worth having the slick mags inspected every 500 hrs, and some recommend cracking them open for visual inspection every year...surprisingly fragile beasts inside

No reason to have the boost pump on during starting...certainly not on a carbureted 320.

Does your primer use the boost bump? What signs indicates its flooding during start?

I suspect only a little priming (if any) is needed in typical Georgia weather for a cold start. Prime requirement is dependent on fuel temp...not ambient temp. They are many times the same, but not when the engine is warm or has been preheated. That's why no prime is required when hot starting.

Remember, to get a combustible mixture you need the correct mix of air (containing the requisite O2) and fuel vapor...vapor is what's needed, not fuel. The vapor quantity is highly dependent on fuel temp. The colder the fuel, the more fuel quantity to achieve the required vapor.

FWIW...I live in WI, it's still snowing occasionally here. I don't have a primer on my aircraft. I preheat any less than 40F-50F. For cold starts I use the accelerator pump 1-2 times (dependent on fuel temp) during cranking, then I use the zero-throttle technique. Hot starts...just crack throttle 1/4" travel and crank. Starts, immediately, every time
 
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Perhaps unrelated to your starting problem but I would be concerned that your boost pump is flooding your carburetor, you should not have to leave it off during takeoff (probably the most important time to have it ON). In addition to a mag overhaul, you may be in need of a carb rebuild or inspection.

Chris
 
Also, the boost pump shouldn't flood a carb...if so, there's something wrong with the carb.

EDIT: Chris beat me to it!
Perhaps unrelated to your starting problem but I would be concerned that your boost pump is flooding your carburetor, you should not have to leave it off during takeoff (probably the most important time to have it ON). In addition to a mag overhaul, you may be in need of a carb rebuild or inspection.
 
TLC

Forgive me here for stepping into this....
Sounds to me the engine needs some TLC.
Maybe even have a fellow RV driver assess how abby-normal the engine is. Sometimes we live with things that aren't quite right and not realize they are seriously bad.
If I was you, I would ask a friend to look over your starting trouble.
Maybe even check compression, overhaul the mag and overhaul the carb. I say the carb because the boost pump flooding the engine could be a bad float or seat.

JMHO. Stay safe
 
Primer nozzles

How long has it been since the primer nozzles were cleaned.
I've seen them so carboned up that no fuel would pass through....
 
FWIW, we were having terrible starting problems on our O-320, and it turned out it was the out-of-spec high internal resistance on our Champion plugs. With the Tempest new plugs, we start up right away now.

~Doug
 
Suggest you first try a different method for starting before addressing engine problems beyond clogged primers. Try this and see what happens:

Skip the primer step (to eliminate issues with primer circuit)
boost pump for 5 seconds (to insure carb bowl is full)
Engage starter
Immediately pump the throttle lever once or twice and back to 1/4" from closed

When I had a carb, I found this method easier than the primer circuit. Note, that amount of pumping is directly related to the prime size (single or multiple full pumps, half pumps, etc.). The colder the engine, the more priming is necessary. 70* might need one pump and 30* might need 2 or 3 pumps. NEVER pump the throttle unless the engine is spinning.

If you get better results this way, you can go back and address issues with your piming system. If this doesn't net an improvement in starting after trying various prime sizes, you can look at your ignition system.

Larry
 
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Suggest you first try a different method for starting before addressing engine problems beyond clogged primers. Try this and see what happens:

Skip the primer step (to eliminate issues with primer circuit)
boost pump for 5 seconds (to insure carb bowl is full)
Engage starter
Immediately pump the throttle lever twice and back to 1/4" from closed

When I had a carb, I found this method easier than the primer circuit. Note, that amount of pumping is directly related to the prime size (multiple full pumps, half pumps, etc.). The colder the engine, the more priming is necessary. NEVER pump the throttle unless the engine is spinning.

If you get better results this way, you can go back and address issues with your piming system. If this doesn't net an improvement in starting, you can look at your ignition system.

Larry

+1 on Larry's technique, works every time!

The only variations I have...
  • I bring the throttle to idle to act as a choke and then bring it up to 1000 rpm right away after it starts. Though it fires off so fast I don't get to full idle on my last pump.
  • I turn the boost pump on while I sump the gascolator during preflight (otherwise it comes out slow)...this ensures the carb bowl is full

Same thing as pumping the gas peddle in a carbed car, though you need to do it while cranking on a Lycoming because of the carb location and threat of induction fire.
 
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It works

I tried Larry's method for a 40 degree OAT start: Boost pump ON for 5 seconds, Boost pump OFF, Starter engaged, Throttle full throw pump. Engine started as I was beginning the third pump. I just about wore out my battery yesterday before finding this thread. Thanks guys!
 
I tried Larry's method for a 40 degree OAT start: Boost pump ON for 5 seconds, Boost pump OFF, Starter engaged, Throttle full throw pump. Engine started as I was beginning the third pump. I just about wore out my battery yesterday before finding this thread. Thanks guys!

Glad it helped someone. About 30* today and a guy on the ramp was just about to burn up his starter. 30 seconds of tips/education and it fired right up. Most don't realize the inverse, exponential relationship between air temp and fuel required to start an engine.

Larry
 
+
[*]I bring the throttle to idle to act as a choke ...

FYI, for a plate to act as a true choke, it must be upstream of the jets. Closing the throttle plate only reduces airflow. It does not increase the amount of fuel flowing relative to that air. Only a blockage upstream of a jet can do that. You may have seen an improvement, but only because the reduced airflow required less total fuel than the higher airflow to reach the correct mixture for starting. A common mistake I see in cold wx starting is having the throttle open too far, which further increases the demand for fuel which most tend to underestimate.

Larry
 
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For what it's worth:

Here's what the throttle pump on an MS-4SPA carb does when you pump the throttle once.
https://photos.app.goo.gl/DWfykmmRDA6ZsXVu6

If the engine isn't turning and sucking induction air when you do that, all that fuel is going to fall back under the influence of gravity, through the throttle plate and venturi and pool in the bottom of your FAB.

If you have a backfire, you'll start a fire, and very likely have to find a new RV with the resulting insurance claim.

Don't pump the throttle unless the engine is cranking, and even then only do it once. If an O-320 doesn't start after slurping in that much fuel, you very likely have a proper problem which needs investigating, and pumping it more probably isn't going to help.

- mark
 
For what it's worth:

Here's what the throttle pump on an MS-4SPA carb does when you pump the throttle once.
https://photos.app.goo.gl/DWfykmmRDA6ZsXVu6

If the engine isn't turning and sucking induction air when you do that, all that fuel is going to fall back under the influence of gravity, through the throttle plate and venturi and pool in the bottom of your FAB.

If you have a backfire, you'll start a fire, and very likely have to find a new RV with the resulting insurance claim.

Don't pump the throttle unless the engine is cranking, and even then only do it once. If an O-320 doesn't start after slurping in that much fuel, you very likely have a proper problem which needs investigating, and pumping it more probably isn't going to help.

- mark

Possibly you don't live in a cold climate. Your point about cranking is very valid and needs to be heeded by all without exception. However, there is simply no way you are going to get a lycoming started at 10 degrees ambient with one pump of the throttle. Lots of cold wx experience to back that up.

Larry
 
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