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Old 04-17-2013, 02:57 PM   #21
BCCHOPIT
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Default Re: not enough backpressure?

I hope I didn't miss it... But what size pipe and muffler
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Old 04-17-2013, 03:01 PM   #22
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Default Re: not enough backpressure?

The pipe is 2" so its not huge. Turns out I measured the dist shaft wrong so my old man brought me the wrong reamer so I will be finishing the dist a little later.

P.s. Bill I always run mine at a higher rpm than most people, I drive it like I stole it!
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Old 04-17-2013, 03:25 PM   #23
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Default Re: not enough backpressure?

Seems to me, now that you have less restrictive exhaust the "air pump" that is the engine is flowing more CFMs per given period of time increasing its volumetric efficiency. With no change in carburetion this would result in a lean mixture which could definitely give you a miss.

Try richening the mixture from the usual (GAV, larger jet).

Take a look:
http://mechdb.com/index.php/Exhaust_back_pressure_myth

Last edited by Smurkey; 04-17-2013 at 03:33 PM.
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Old 04-17-2013, 03:34 PM   #24
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Default Re: not enough backpressure?

My best all around set up is my 4 into 1 header 2" pipe 18" long no muffer

My open lake pipe header the sits under the car has no low end
But sounds good rolling into the shows

I used a 17/8 glass muffler and hated it no power on top end

The stock muffer to the header ran better
I us that when I drive with the with old guys with stock cars. They drive 50-55 tops so no need for top end.

When I go for real long drives I have 2" pipe bent that looks like the stock
EX. But no muffer runs nice and my wife can hear me talk in the car

This has real nice off the line but gives up a little on top end
I can't get past 68 with the long EX.


My 2 cents I play with my car all the time it's fun to see what I can
Get the old girl to do
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Old 04-17-2013, 03:42 PM   #25
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Default Re: not enough backpressure?

Quote:
Originally Posted by James Rogers View Post
If you want to prove this to yourself, get a 650 Yamaha twin and remove the stock mufflers and just run the header pipes. It will run like krap till you put the mufflers back on and introduce back pressure into the exhaust. Almost any twin cylinder motorcycle will react this way without back pressure as will the inline fours.
When I was in high school my friend removed the exhaust pipe right at the cylinder on his 250 Zundapp single cylinder 2 stroke motorcycle, and the engine wouldn't even start. Put the pipe back on and it ran fine.
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Old 04-17-2013, 03:51 PM   #26
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2 cycles do need back pressure. When I was racing snowmobiles, you could change your rpm range by the stinger dia and length. Also the chamber dia and length. The stinger is like a tail pipe on a car. 4 cycles seem to run better with no back pressure.
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Old 04-17-2013, 03:54 PM   #27
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Default Re: not enough backpressure?

Quote:
Originally Posted by James Rogers View Post
If you want to prove this to yourself, get a 650 Yamaha twin and remove the stock mufflers and just run the header pipes. It will run like krap till you put the mufflers back on and introduce back pressure into the exhaust. Almost any twin cylinder motorcycle will react this way without back pressure as will the inline fours.
Then why do the 4 cylinder Model A Peitenpol Aircraft conversions run so well with the straight 12 inch exhaust stacks ?
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Old 04-17-2013, 03:55 PM   #28
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Default Re: not enough backpressure?

2 strokes need back pressure to run or the fuel will go right out the exhaust and not be burned
4 strokes do not need back pressure
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Old 04-17-2013, 04:04 PM   #29
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Default Re: not enough backpressure?

in keeseys case the free flowing exhaust changed the airometric combustion ratio enough to show up a deficiency in possibly a worn component as this may be the dist shaft play that he has found
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Old 04-17-2013, 04:21 PM   #30
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Default Re: not enough backpressure?

Backpressure is needed on engines that have "tuned" exhaust systems. Often times with high performance motors the exhaust system is a significant part of getting the high performance from the motor, but, lets not regard the Model A standard motor as a "high performance" racing engine.....

Also, with the tuned motor the performance is limited to specific RPM values.

Smokey Y. wrote a great book in the past about the methods used to tune the motor, I really do not think that this is relevant for the Model A !

Marc
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Old 04-17-2013, 05:16 PM   #31
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Default Re: not enough backpressure?

Whoa, you're finding to many things to guess at. Stop, put the Meineke muffler back on and see if this corrects your problem. If it does then leave it or put the glass pack on and rejet the carb. Like James said above, less back pressure will alter the flow and my long gone Honda four proved it when I exchanged the stock exhaust for a Kerker system.
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Old 04-17-2013, 06:34 PM   #32
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Default Re: not enough backpressure?

Quote:
Originally Posted by marc hildebrant View Post
Backpressure is needed on engines that have "tuned" exhaust systems. Often times with high performance motors the exhaust system is a significant part of getting the high performance from the motor, but, lets not regard the Model A standard motor as a "high performance" racing engine.....

Also, with the tuned motor the performance is limited to specific RPM values.

Smokey Y. wrote a great book in the past about the methods used to tune the motor, I really do not think that this is relevant for the Model A !

Marc
Geez Marc, you just dashed all my hopes!
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Old 04-17-2013, 07:45 PM   #33
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Default Re: not enough backpressure?

When you say miss.
Are you getting a popping?
Popping out the tail pipe = rich
Popping out the carb = lean
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Old 04-17-2013, 07:54 PM   #34
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Default Re: not enough backpressure?

First, Tom and George, the 2 strokes do not use exhaust back pressure they use sound waves to control the cylinder scavenge. This is why the muffler is shaped like 2 cones welded together at the big end. I found out how to tune 2 strokes running them back in the day. We used to make our own stingers to get the best performance from the 2 stroke racers.


Second, the reason a pietenpol runs good is the carb is tuned to the engine without exhaust and the engine is mostly run at a steady speed not up and down like a car. If you run the throttle up on any motor without a muffler or with straight pipes slowly it will respond without problem. It is when you stab the throttle that it hesitates and splutters.


I don't think the problem with this motor is back pressure because even a straight pipe with a bend would present enough to make it run OK.
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Old 04-17-2013, 08:16 PM   #35
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Default Re: not enough backpressure?

Quote:
Originally Posted by James Rogers View Post
First, Tom and George, the 2 strokes do not use exhaust back pressure they use sound waves to control the cylinder scavenge. This is why the muffler is shaped like 2 cones welded together at the big end. I found out how to tune 2 strokes running them back in the day. We used to make our own stingers to get the best performance from the 2 stroke racers.


Second, the reason a pietenpol runs good is the carb is tuned to the engine without exhaust and the engine is mostly run at a steady speed not up and down like a car. If you run the throttle up on any motor without a muffler or with straight pipes slowly it will respond without problem. It is when you stab the throttle that it hesitates and splutters.


I don't think the problem with this motor is back pressure because even a straight pipe with a bend would present enough to make it run OK.
What ever you say James I'm just a dum country boy.
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Old 04-17-2013, 08:37 PM   #36
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Default Re: not enough backpressure?

Well it wasn't the distributor shaft. The engine now runs smoother but still the same general problem as the first post. I changed back to a meineke muffler and a smaller pipe diameter and still the same thing. I'm fed up with it right now.
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Old 04-17-2013, 08:43 PM   #37
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Default Re: not enough backpressure?

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What ever you say James I'm just a dum country boy.
YOU RANG??---"Country road take me home, to the place-----where Momma's working up Beans & Hamhocks & Cornbread & Poke Salad Greens & Buttermilk, cooled in the well, & Chief & the Kid's finishing up that ooold Green Tudor"! HARK! I hear it runnin' We goin' to the "SHOW" when dark comes! Bill W.
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Old 04-17-2013, 09:08 PM   #38
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Default Re: not enough backpressure?

I quit.
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Old 04-18-2013, 01:19 AM   #39
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Default Re: not enough backpressure?

Quote:
Originally Posted by James Rogers View Post
If you want to prove this to yourself, get a 650 Yamaha twin and remove the stock mufflers and just run the header pipes. It will run like krap till you put the mufflers back on and introduce back pressure into the exhaust. Almost any twin cylinder motorcycle will react this way without back pressure as will the inline fours.
Yamaha XS750 SE, I have owned two of them and countless other bikes. Tuned many a Harley after converting from stock exhaust. When you change the exhaust you need to adjust or change the jets. This is due to changing the flow characteristics of the exhaust. If you want to call muffler restriction the same as back pressure then I am with you. But the muffler does not create pressure, it alters flow. The same effect would occur if you changed the intake to the same degree. Or allowed the Air Filter to clog, or removed the air filter.
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Old 04-18-2013, 04:44 AM   #40
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Default Re: not enough backpressure?

Okay, what would be a good jet size? The carb started with a 42 and ran very poorly so I tried a 52 I had on hand and it ran better. The next size up I had was 64 and it ran worse. Does anyone have a comparable setup? The carb is a later Holey 94.
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