adverseyaw

IO-360 misfires at idle and low MP

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1 hour ago, jetdriven said:

replace the intake gaskets. you cant accurately determine if they are leaking in place, and it can be a huge leak. use permatex aviation sealer on them. 

That's a good idea. We pressurized the intake already and it held pretty well. Would the gaskets still be likely even if we've preformed this test?

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10 hours ago, adverseyaw said:

That's a good idea. We pressurized the intake already and it held pretty well. Would the gaskets still be likely even if we've preformed this test?

Yes. I suspected  a vacuum leak and I messed with it for a year. pressurized the intake.  sprayed soapy water. ran the engine and sprayed carb cleaner on the intake flanges. norhing. then once i had to remove both copilot side intake tubes for unrelated maintenance.  when i started it up to check the work out, pulling the throttle to idle smooth killed it. I mean I had to bump the throttle 1/8" to keep it running.  Ended up I had to turn the idle stop screw in about a full turn which is a lot. We accidentally fixed that big of a vacuum leak.

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my IA timed my engine to 20 degrees one time without me knowing it.  It ran like complete crap.  After months of trying to figure out what was wrong, I figured out the timing was changed, so I got it back to 25 degrees and have been purring along ever since.   That SB is such a crock.  If you’re too hot with 25 degrees, then it’s a problem with baffles and flying too slow.  IMHO. ;)    

Make sure low speed mixture is set for 25-50rpm rise.  Tempest finewires help too

Edited by Browncbr1

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Before you mess with mag timing - if you have an impulse coupled mag understand what the lag for the impulse coupling is / means and what implications it has for starting. 

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3 hours ago, bradp said:

Before you mess with mag timing - if you have an impulse coupled mag understand what the lag for the impulse coupling is / means and what implications it has for starting. 

Is the impulse coupling lag adjustable?  I thought it was just a fixed angle from the normal timing?

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1 hour ago, jaylw314 said:

Is the impulse coupling lag adjustable?  I thought it was just a fixed angle from the normal timing?

The lag is specific to the impulse coupling - ie it can't be adjusted. However (as I understand the principle), if you have a mag timed to 20-deg with a 15-deg lag, setting that mag to 25-deg BTDC will result in the impulse coupling firing at 10-degrees BTDC instead of 5-deg (nearly at TDC).  The result is a risk of kick-back during starts with an impulse coupling that is set too far advanced. 

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I've been chasing the induction leak theory and have had some success so far. After changing the intake gaskets and o-rings, and cleaning up the Precise Flight standby vacuum line and valve, we got the popping to stop at much lower RPMs and get the mixture to richen way up (so much that it barely ran). Clearly, I had big leak(s).

I'll be testing the sniffle valve again this weekend to see if that's leaking further air. We only have a few more possibilities.

One thing that tipped me off is that I can only pull down to 12-13" MP with the throttle at idle and 600rpm. I found a few YouTube videos showing IO-360s pulling more vacuum at idle. It may be I can only get it this low but high numbers indicate a leak is possible (not guaranteed, but worth investigating).

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On 4/23/2018 at 11:56 PM, bradp said:

The lag is specific to the impulse coupling - ie it can't be adjusted. However (as I understand the principle), if you have a mag timed to 20-deg with a 15-deg lag, setting that mag to 25-deg BTDC will result in the impulse coupling firing at 10-degrees BTDC instead of 5-deg (nearly at TDC).  The result is a risk of kick-back during starts with an impulse coupling that is set too far advanced. 

Yes, to undo the service letter you need to change the magneto or the impulse coupler, effectively changing the part number of the magneto. 

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On 4/16/2018 at 8:20 AM, jetdriven said:

Yes. I suspected  a vacuum leak and I messed with it for a year. pressurized the intake.  sprayed soapy water. ran the engine and sprayed carb cleaner on the intake flanges. norhing. then once i had to remove both copilot side intake tubes for unrelated maintenance.  when i started it up to check the work out, pulling the throttle to idle smooth killed it. I mean I had to bump the throttle 1/8" to keep it running.  Ended up I had to turn the idle stop screw in about a full turn which is a lot. We accidentally fixed that big of a vacuum leak.

Winner!

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On 4/27/2018 at 12:26 AM, adverseyaw said:

I've been chasing the induction leak theory and have had some success so far. After changing the intake gaskets and o-rings, and cleaning up the Precise Flight standby vacuum line and valve, we got the popping to stop at much lower RPMs and get the mixture to richen way up (so much that it barely ran). Clearly, I had big leak(s).

I'll be testing the sniffle valve again this weekend to see if that's leaking further air. We only have a few more possibilities.

One thing that tipped me off is that I can only pull down to 12-13" MP with the throttle at idle and 600rpm. I found a few YouTube videos showing IO-360s pulling more vacuum at idle. It may be I can only get it this low but high numbers indicate a leak is possible (not guaranteed, but worth investigating).

12” is not the lowest MP available, but for a good quick descent from the TPA this will work... throttle closed/engine idle is even lower...

Typical NA engines....

If you can’t get that, your MP instrument may be out to lunch too...

Which MP gauge are you using?

The mechanical one with a calibrated air hole in the line?  Also Another source for air entering the induction system....

Whats your lowest MP at idle?

PP thoughts only... not a mechanic.

Best regards,

-a-

 

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