Timing...

[N95179]

Any of you have your mags set closer to top center than says so in the book for hand propping?

Had my annual done a few months ago and my mechanic moved it back to the book figure, now it back fires like crazy, especially on a warm start...big reason to wear gloves! I want to move it back to where it was...

[jgerard]

Re: Timing...

I think your mechanic timed it wrong, cuz a properly timed engine(or anywhere near proper) should not have those symptoms. Also check to make sure the mags are tight. They may have moved. And make sure the plug leads are on the correct cylinders.

The A65 should be 30deg on both mags. Perhaps the tic marks on the prop hub are not as legible as they should be.

[N96337]

Re: Timing...

I agree with Jason, and I'd also check the indexing of your prop... I see more and more props on at the wrong positions all the time.
JH

[1938BF50]

Re: Timing...

How do I know when the prop is indexed correctly?

[N95179]

Re: Timing...

I should probably clarify...it only fires back on starting, once it's going, it runs great. And it doesn't really POP, rather just springs back a good 3-4 blades. He said he set them to 29 degrees, but I'll have him check again. I wouldn't be too surprised if he set it wrong because he'd never seen my style mags before and had to do research on them before he even touched them.

Chris

[dedrekon]

Re: Timing...

Chris,
Due to my recent similar experience, I concur with John (always a good move, as he's usually right) on the proper prop "index". I got mine right and it fires on the 1st or 2nd pull every time now....while before it was doing exactly what you describe.


Vincent

[Armstrong]

Re: Timing...

I should probably clarify...it only fires back on starting, once it's going, it runs great. And it doesn't really POP, rather just springs back a good 3-4 blades. He said he set them to 29 degrees, but I'll have him check again. I wouldn't be too surprised if he set it wrong because he'd never seen my style mags before and had to do research on them before he even touched them.

Chris

It does not backfire out the exhast? The prop just spins back a few blades?
Mine does that alot when its hot,(when its slightly rich) and I think thats common with 65's.
My timing is slightly advanced( greater than 30).

[Karl Rigdon]

Re: Timing...

Chris,

What type mags do you have?? Does one, both or none have impulse couplings. If you have only one mag with a impulse coupling normaly it is on the left mag. With only one impulse coupling start on that mag and switch to both after the engine starts. If there are no impulse couplings you have to put a lot of force so there is enough inerta to get over top center when the spark plug fires at 30 deg. With impulse couplings the firing is delayed to nearly top center and when the engine starts the impulse coupling goes out of play and the engine will fire at 30 deg.

[lawheelock]

Re: Timing...

Any of you have your mags set closer to top center than says so in the book for hand propping?

Had my annual done a few months ago and my mechanic moved it back to the book figure, now it back fires like crazy, especially on a warm start...big reason to wear gloves! I want to move it back to where it was...

Karl may have hit the nail on the head! The question is: What magnetos are on your engine?
Some of the old Bendix, SF-4s did not have impulse couplings. If you have Eisemanns or Slicks or Case, then on most T-crafts both have impulse couplings. If you only have one, then start on that mag only. Perhaps if you have impulse couplings, one or both is not working. They should retard the spark around 30 deg for starting.
For maximum power, the timing for running needs to be 30 deg advanced.
Larry Wheelock

[alwaysoar]

Re: Timing...

I wonder if your A&P fiddled around with internal timing (in the mag). There really shouldn't be much if anything to research in timing mag to engine. The case of the mags (e.g. Eisemann's) have numbers on them so the leads are hooked up according to firing order. If you have a hub rather than a flange, you'd need a timing disc to make sure you had 30 degrees BTC because the hubs don't have marks on them.

Indexing the prop refers to how the prop is placed on the hub or flange of the engine and wouldn't have any bearing on backfiring unless it was way off and you couldn't get a good spin. #1 blade is aligned with #1 cylinder with angle adjusted for proping (10 o'clock position). Harry Fenton's site explains that as does the Sensenich prop site. Indexing is necessary to reduce vibration.

[jgerard]

Re: Timing...

Prop hubs have timing marks on them, just that most of them are so dang hard to read and are usually filled with paint.

[Karl Rigdon]

Re: Timing...

I have a taper shaft with a hub and it has timing marks. I also have at least 2 spare hubs and probably 2 more that are not airworthy. They all have timing marks. The problem is that when #1 is at TDC the marks are on the bottom. Once the mag is timed properly to #1 TDC on the compression stroke the engine could be rotated 180 deg and check the timing with the marks lined up with the top of the crankcase parting line. 4 cyl engine = 720 deg diveded by 4 = 180 deg spacing of the firing order.

Karl

[1948TCARTDRIVER]

Re: Timing...

On my Taylorcraft the index marks were hard to read on the taper shaft prop hub so I made an index plate out of aluminum, a protractor and plumb bob. Once I had identified the 30 before top dead center mark I painted it. Also with my old mags, neither of which had impulse couplings you had to really pull the prop though or it would kick back. I never had much of a problem with it but you couldn't just flip the prop like the guys who had impulse couplings.

[NC95842]

Re: Timing...

I was playing with the timing a few years past and shot some fire to the dripping needle carb and almost lost the plane!

[alwaysoar]

Re: Timing...

....that you can read. I still wonder what kind of mags, whether or not they have impulse, and what the A&P did to them.

Ed@BTV VT
TF 527