RPM

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zach520
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RPM

Post by zach520 » Tue Dec 01, 2015 1:18 pm

Ok I gaped my points to 18 on my 20 ser.mags , should say this its on a fuel injected 520 , motor starts real good at this setting , So the question is i'm pretty sure the motor was turning 2750 with a 72 Q 2 blade set around the 2 1/2 or 3/4 mark , seems to me it should be able to turn this prop at this setting around 2850 , what's everyone's thoughts on this .
Also need the plastic plugs for the mag's where the timing marks are at if someone has a couple laying around .

GMAC 76
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Re: RPM

Post by GMAC 76 » Tue Dec 01, 2015 4:08 pm

what did you set the e-gap at?....that's more important than the points gap...if it turned 2750 before you set the points are you thinking that it should now turn 2850....also I got the plastic plugs (used) but good....someone with that engine prop combo would be able to answer the rpm question best

zach520
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Re: RPM

Post by zach520 » Tue Dec 01, 2015 5:36 pm

I haven't touched the E gap just the points , was having a issue with it not starting after a rain , wouldn't run so I pulled the mages and reset the point to 18 and got a good spark , going from 16 to 18 on the gap I lost hundred rpm , motor runs strong just thought someone could say with the prop pitched at 2 3/4 it should be able to turn it 2850 to get the full out put of the motor . yes I need the plugs how much .

broncobill
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Re: RPM

Post by broncobill » Tue Dec 01, 2015 6:21 pm

I am sure someone will correct me if i,m wrong. The E Gap is the moment when the points begin to open. You need a timing box to achieve that result. With mag removed and mag lock in place of the red gear you need to hook up your timing box. You can move the gear ever so slightly the gap point is where in begins to open, With box hook up you will hear the buzzer and see the light go on and off as you move gear back and forth. If you removed the mags how did you time it? It might have been a little more advanced before.

GMAC 76
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Re: RPM

Post by GMAC 76 » Tue Dec 01, 2015 6:50 pm

the e-gap is the number of degrees from the rotor neutral position to the point where the contacts just start to open........ it is 10 degrees plus or minus 4 degrees...it is set with a buzz box and e-gap timing kit......the point gap is the distance between the contacts when they are on the highest point of the points cam...It is .018 plus or minus .006...it is set with a feeler gauge....oh I forgot ...give me your address on a pm and I will send ya a couple plugs in the morning...no big deal

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getitdone
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Re: RPM

Post by getitdone » Tue Dec 01, 2015 6:57 pm

broncobill wrote:I am sure someone will correct me if i,m wrong. The E Gap is the moment when the points begin to open. You need a timing box to achieve that result. With mag removed and mag lock in place of the red gear you need to hook up your timing box. You can move the gear ever so slightly the gap point is where in begins to open, With box hook up you will hear the buzzer and see the light go on and off as you move gear back and forth. If you removed the mags how did you time it? It might have been a little more advanced before.
you are right the e-gap should be 10+-4 degrees and is more accurate than setting just the point gap to .018+-.006 inch. you can make it like this one with paperclip and solder to washer and glue the cutout to a piece of metal/aluminum etc

I also attached the diagram here Click on the SB 474A attachment to open the templates.
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SB 474A.pdf
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"Poachers Dream"
13'x7'6" Hamant Ultralight With 0540 And 72"Q

zach520
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Re: RPM

Post by zach520 » Tue Dec 01, 2015 7:28 pm

didn't have a buzz box or don't have one , so I bring up the red gear and mark it ,re set both mags to only drop 100 rpm when shut off . its interesting working on a/c engines , should be reset with a buzz box .

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Re: RPM

Post by RickMc » Tue Dec 01, 2015 10:12 pm

If your mags have been set up by a pro. and you have installed them as accurately as you can statically . Is it not better to check them each and fine tune them with a inductive timing light, while running? That way you know they are both firing together dynamically at the correct time .
The glass is always half full....til I get a hold of it

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Planeguy
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Re: RPM

Post by Planeguy » Sat Dec 05, 2015 9:15 am

Mags fire when the points open. It really doesn't matter how fast the engine is turning because magneto timing is fixed. Unlike automotive engines with vacuum advance, the spark happens at the at the same time no matter what the RPM. Add to that the safety factor... some timing marks are next to the prop. Do you really want to be monkeying around next to a spinning prop that is trying to suck you and your tools into it?

The buzz box is a continuity tester that gives the user sound and light to check when the points are opening. The same check can be done with any multi-meter. It is just not as convenient.

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