yet another hot start question

Archimago

Archimago

Pre-takeoff checklist
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Archimago
Hot Starts.

I know this topic has been done to death so I will be brief.
I have the 0 320 E2G with 160hp STC with Slick mags in a Grumman Traveler


-The issue started to be more and more prevalent… ie I wasn’t having the issue before and now I have had to drag my plane from the fuel island to my parking space 3 times in the last week and a half.


-At first it seemed like a fuel issue. And in the beginning I could successfully start it by keeping the mixture at idle and introducing it when it fires off. But now even that doesn’t seem to work. I have since tried every combination found in various forum archives but nothing seems to work, even after an hour or so.


-BUT when starting it at the beginning of the flight, fires off magically within a few revolutions. Mag check at run up gives me 50rpm loss for each mag.


-I dug back through the logs and found that the mags were last serviced 498 hours ago.


So I need to address the mags at a minimum but I am wondering if a mag can work perfectly at start up and during flying and then fail miserably on the ground afterwards. And since I am dealing with Mags, does anyone recommend a prudent route to take. i.e overhaul vs purchase vs refurbished, vs new, vs Surefly.


Thanks!

Richard
 
Archimago said:
...So I need to address the mags at a minimum but I am wondering if a mag can work perfectly at start up and during flying and then fail miserably on the ground afterwards. And since I am dealing with Mags, does anyone recommend a prudent route to take. i.e overhaul vs purchase vs refurbished, vs new, vs Surefly....
Click to expand...

Yes.

I recommend troubleshooting before replacing parts.
Check the mag timing. Check the spark plugs (condition and resistance). Check the harness.

What is your cold start procedure?
 
I'd do the mags regardless which you are doing, but yes weak or misadjusted mags can be an issue for starting, parts such as coils can be fine cold and be degrade when heat soaked.

It's been a while since I started a hot carbed 360, but I seem to remember one squirt of prime and it would fire right up. I'm flying a 22 now with a fuel injected 550. When hot it's throttle a quarter inch, mixture full rich, fuel boost on, crank when you have fuel flow, starts perfectly every time. Find the method that works and stick with it.
 
Do you by chance have to start on one mag? or is there impulse on only one mag? If so, might be magneto P leads be reversed?
 
Install the Surefly. You will not be disappointed. Push the button, starts, cold or hot.
 
A gasoline engine should be easier to start when warm than cold since it is gasoline vapor that burns not liquid gasoline and more heat vaporizes fuel better.
So why often harder to start warm? Because a fuel pump cannot pump vapor, only liquid fuel, and when your hot engine sits a while the fuel in the system close to the engine vaporizes and not enough fuel mixture is getting to the cylinders. This is called vapor lock in autos and was once a common problem until the fuel pump was moved from the engine to the region of the cool fuel tank.

Fresh fuel cools the pump so once the engine starts no more vapor lock..
 
Snowmass said:
A gasoline engine should be easier to start when warm than cold since it is gasoline vapor that burns not liquid gasoline and more heat vaporizes fuel better.
So why often harder to start warm? Because a fuel pump cannot pump vapor, only liquid fuel, and when your hot engine sits a while the fuel in the system close to the engine vaporizes and not enough fuel mixture is getting to the cylinders. This is called vapor lock in autos and was once a common problem until the fuel pump was moved from the engine to the region of the cool fuel tank.

Fresh fuel cools the pump so once the engine starts no more vapor lock..
Click to expand...
There's more than enough fuel in the carb's float bowl to start the engine. It would die after a minute or so if the system was vapor-locked.
 
Since a schematic of the fuel system was not given I am left with general principles. I feel strongly that the problem is in the fuel system as ignition is not normally temperature sensitive. An overly rich mixture will prevent starting as will an overly lean mixture. I assume that the plane is low wing which mean a fuel pump even if carburetor is used. Once fuel begins to flow the vapor problem goes away as the new fuel cools the system. If the float level is too high or there is a leaking float valve the heated gasoline at shut down will expand and flood the engine. Not just theory as I had an old Honda Accord which had hot start problems due to this. It took a lot of cranking to expel the excess fuel.

One test is when restarting is to shut the fuel off from the fuel tank and push the throttle wide open while cranking which will quickly dry out the excess fuel. Of course if the engine starts it will quickly stop due to fuel starvation so you need to bring back the full throttle and open the fuel valve.

One other cause of starting problems is very low compression. Cold, thick oil will seal the piston rings but once the oil gets hot blowby reduces compression at low cranking speeds so the engine won't start. A running engine has much better compression due to the quantity of the cylinder gas vs. leakage rate. Again not just theory. I had an airport car which would start cold and run fine but not easily restart hot. However, it's doubtful an aircraft engine would be allowed to have compression this low. Also my John Deere tractor will not start if I over choke which washes sealing oil of of the cylinder walls. I then need to pull the plugs and squirt oil into the combustion chamber to start.
 
In an aircraft engine, we do differential compression tests to determine ring and valve condition. The compression numbers are always higher when the engine is hot, and that's because the piston and rings have expanded and closed up the clearances. Aircooled engines are like that. I never once encountered compressions being better in a cold engine.

If the engine is flooded we pull the mixture to idle cutoff and crank. With a carburetor, as in the OPs airplane, failing to pull the mixture will let the engine suck fuel from the main and idle nozzles. Shutting the fuel valve off achieves nothing. In an injected engine we also have to pull the mixture to clear a flooded engine.

If the OP is experiencing problems he didn't have not too long ago, and the mags have 500 hours on them, a weak spark is quite possible. When the mixture isn't ideal, spark strength is really important. He also said that he usually started hot with the mixture at cutoff, similar to a hot injected engine, but that doesn't work anymore. That implies that fuel has little to do with it, as long as there's no fuel dripping on the ground from a flooded carb bowl caused by a bad float or float valve.

And I'm assuming he's shutting down by pulling the mixture, not by shutting the mags off.
 
My Slick mags have over 4000 hours on them and the engine starts fine. And why would one need a hotter spark to ignite a warm fuel mixture than a cold one? I doubt its a spark issue. I feel something is wrong with the fuel mixture but, anyway, it's not my problem so I will go to more important things. In 8000 ours I have never had a hot start issue. But I have had a few at zero degrees F.
 
Snowmass said:
Since a schematic of the fuel system was not given I am left with general principles. I feel strongly that the problem is in the fuel system as ignition is not normally temperature sensitive. An overly rich mixture will prevent starting as will an overly lean mixture. I assume that the plane is low wing which mean a fuel pump even if carburetor is used. Once fuel begins to flow the vapor problem goes away as the new fuel cools the system. If the float level is too high or there is a leaking float valve the heated gasoline at shut down will expand and flood the engine. Not just theory as I had an old Honda Accord which had hot start problems due to this. It took a lot of cranking to expel the excess fuel.

One test is when restarting is to shut the fuel off from the fuel tank and push the throttle wide open while cranking which will quickly dry out the excess fuel. Of course if the engine starts it will quickly stop due to fuel starvation so you need to bring back the full throttle and open the fuel valve.

One other cause of starting problems is very low compression. Cold, thick oil will seal the piston rings but once the oil gets hot blowby reduces compression at low cranking speeds so the engine won't start. A running engine has much better compression due to the quantity of the cylinder gas vs. leakage rate. Again not just theory. I had an airport car which would start cold and run fine but not easily restart hot. However, it's doubtful an aircraft engine would be allowed to have compression this low. Also my John Deere tractor will not start if I over choke which washes sealing oil of of the cylinder walls. I then need to pull the plugs and squirt oil into the combustion chamber to start.
Click to expand...

You’ll find free advice is usually dispensed for the benefit of making the giver feel brilliant, without regards to what the recipient really wants to know and is ready to integrate.
 
Snowmass said:
My Slick mags have over 4000 hours on them and the engine starts fine. And why would one need a hotter spark to ignite a warm fuel mixture than a cold one? I doubt its a spark issue. I feel something is wrong with the fuel mixture but, anyway, it's not my problem so I will go to more important things. In 8000 ours I have never had a hot start issue. But I have had a few at zero degrees F.
Click to expand...
You need a hotter spark to ignite a leaner or richer mixture.

Lots of Slicks can go 4000 hours, but not without periodic maintenance.
 
Thanks again to all who gave advice. Turns out it was a failing impulse coupler. Not sure why the temperature effected it initially but it finally failed for good last week. Pulled the prop through and it missed every other cylinder. Pulled the Mag and coupler and you can see the worn pin not catching one side and if wiggled, neither side would catch. Put a friends mag in, changed the plugs and it started right up, and even when hot.
I ordered a Surefly mag and it got here today so we will be installing it this Friday.
 
Same as me, as I stated before. The surefly is worth it when the impulse mag goes out. Better investment than repairing the old one. You will not be disappointed.
 
Archimago said:
Thanks again to all who gave advice. Turns out it was a failing impulse coupler. Not sure why the temperature effected it initially but it finally failed for good last week. Pulled the prop through and it missed every other cylinder. Pulled the Mag and coupler and you can see the worn pin not catching one side and if wiggled, neither side would catch. Put a friends mag in, changed the plugs and it started right up, and even when hot.
I ordered a Surefly mag and it got here today so we will be installing it this Friday.
Click to expand...

Good job!
 
90% of engine performance troubles are electrical, and that means ignition on an aircraft engine. It's so tempting to blame fuel issues, but that's often because ignition, and the importance of a good hot spark at the right time, is so misunderstood. Much time is lost in fixing all the wrong stuff.


It's also the main reason we have two ignition systems. Failure of ignition is much more likely than failure of carburetion or fuel injection. (Aside from carb ice, of course, but that's not a system failure. It's a pilot failure.)
 
How are you trying to start the engine hot and cold? Both my previous and new engine will start with two primes cold, and hot with no prime and one pump of the throttle during cranking. I crack the throttle beyond idle. I currently have the HC STC, and Skytec starter. '74 AA5.

If you are getting hard starts in general, I'd check plugs and the impulse couplings. Otherwise are you over priming when hot? It should start hot with no prime with a throttle pump. It is easy to flood when hot.

At 500 hr, I would inspect and refurbish your Slick mags as required. They are not known for longevity.
 
Absolutely a bad impulse coupling on a mag can cause this. Had it happen on a 172.
Flew into a small deserted airport one night, shut down briefly to plan the next leg of the Night training flight. 10 minutes later it would not start, would just crank. I got out and tried hand propping it (with a qualified pilot at the controls), it fired on the 1st pull. Run up was fine, flew it home. Mechanic the next day determined the impulse coupling had failed. My best guess that cranking it with the key was grounding one of the mags. Hand propping it allowed it to fire on the other mag. But I am not a mechanic, so just speculation on my part.

Brian
 
chemgeek said:
I currently have the HC STC, and Skytec starter. '74 AA5.
.
Click to expand...
Same here. I also have HC STC and Skytec starter.
I don’t introduce any fuel when hot. In fact it starts easier if I enrich the mixture from idle cutoff as the engine catches.
 
nrpetersen said:
Do you by chance have to start on one mag? or is there impulse on only one mag? If so, might be magneto P leads be reversed?
Click to expand...
I haven’t been, but I am thinking of starting on left mag to avoid potential kickback as I have a button start
 
brcase said:
Absolutely a bad impulse coupling on a mag can cause this. Had it happen on a 172.
Flew into a small deserted airport one night, shut down briefly to plan the next leg of the Night training flight. 10 minutes later it would not start, would just crank. I got out and tried hand propping it (with a qualified pilot at the controls), it fired on the 1st pull. Run up was fine, flew it home. Mechanic the next day determined the impulse coupling had failed. My best guess that cranking it with the key was grounding one of the mags. Hand propping it allowed it to fire on the other mag. But I am not a mechanic, so just speculation on my part.

Brian
Click to expand...
The non-impulse mage fires at its advance point, often causing kickback, which is why the switch normally grounds it during start. The impulse on a mag both retards the spark to near top dead center, and snaps the mag forward sharply to generate a good spark.
 
chemgeek said:
At 500 hr, I would inspect and refurbish your Slick mags as required. They are not known for longevity.
Click to expand...
Slick recommends a 500-hour inspection interval. Bendix recommends a 400-hour interval. Both have mechanical breaker points that gradually erode and burn away over the hours, and my experience was that the points at 500 hours weren't too bad and at 1000 they were getting poor. I also checked the condenser for its capacity and any indication of internal shorting. An open condenser--not working at all--will result in no spark. It's part of the arresting circuitry, absorbing the initial surge when the points open to get a nice steep collapse of the magnetic field. An open condenser won't absorb anything and the arc that results at the points as they open represents continued current flow and a slower collapse of the field.

In old cars, points were the rule, and they needed replacing yearly, usually, after around 12,000 miles.
 
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