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Old 04-16-2013, 07:53 PM   #1
jkeesey
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Default not enough backpressure?

Okay, so I recently put a glasspack muffler on my A with a short straight pipe after it. The entire exhaust is about 3ft total. After doing so it seems to have a miss under load. I went through the distributor and the entire fuel system and the only thing I can think at this point is there isn't enough backpressure. Does this sound right to anyone or am I just pissing in the wind.
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Old 04-16-2013, 08:49 PM   #2
H. L. Chauvin
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Default Re: not enough backpressure?

In the late 1950's my exhaust manifold broke off at the muffler clamp.

Ran the car with no muffler & no tailpipe for a couple of years.

Loud noise but ran fine. Had to idle while going through the Drive-in theater.

Could see the exhaust flames through the home made floor board openings while running at night.

Don't forget to check the wind direction before starting.
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Old 04-16-2013, 08:55 PM   #3
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Default Re: not enough backpressure?

Lack of exhaust back pressure will not make your engine miss. The need for back pressure is one of those old myths that just won't go away. My first concern for a "miss" under load would be detonation. How is your spark advance?
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Old 04-16-2013, 09:17 PM   #4
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Default Re: not enough backpressure?

I didn't think it was the exhaust but it ran like a top before I changed the muffler and immediately ran bad after I changed it. I checked and replaced the coil, points, condenser, dist body and cap and rotor. Non of them were evidently bad but I figured what could it hurt to do a tune up. The dizzy is a B, I just pulled it apart to check the flyweights and they seemed okay so I cleaned and reassembled it but haven't ran it sense.
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Old 04-16-2013, 10:01 PM   #5
H. L. Chauvin
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Default Re: not enough backpressure?

This happens to all of us with Model A's & other vintage vehicles; however, before radar, electronic ignition, x-ray, electronic engine dianostics, laser technology, & CAT scans, if one walked into an old time mechanic's shop & said,

"My car ran perfectly until I tried performing a tune up, & now it runs horribly -- why?"

The mechanics response would be, "No matter how insignificant it may appear, tell me "everything" you touched because therein lies the problem."

As parochial as this appears, with pencil & paper write down every item you touched, (mainly those you know did not cause this problem), & re-check same.

It will always be something so simple.

Hope this helps.
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Old 04-16-2013, 10:44 PM   #6
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Old 04-16-2013, 11:25 PM   #7
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Default Re: not enough backpressure?

Now that the engine breathes and scavenges a bit different the compressed fuel mix has changed. Perhaps you have one spark plug that doesn't like it. Try a different set.
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Old 04-16-2013, 11:39 PM   #8
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Default Re: not enough backpressure?

I'm with Mike on this one. You have altered the flow properties. An increase in exaust flow will require an increase in intake flow. Run it and read the plugs. I have a glass pack and headers on mine but it also has a pair of 94s at the other end.
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Old 04-17-2013, 07:29 AM   #9
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Default Re: not enough backpressure?

Milton, mine has a 94 and a Burns intake on top and the header and glasspack on bottom. I tried changing out the champion w14 plugs to a motorcraft set and it seemed to run worse. The only new plugs Ihave left to try in the shop are a set of 3x's and I don't want to use $100 worth of plugs if there's not going to be a difference.
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Old 04-17-2013, 07:31 AM   #10
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Default Re: not enough backpressure?

The rear 2 plugs have been coming up oily, not wet with fuel but oil. The front 2 seem perfectly normal.
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Old 04-17-2013, 09:58 AM   #11
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Hey jkeesey,
Watch out for that uric acid, may burn your skin !
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Old 04-17-2013, 10:07 AM   #12
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Default Re: not enough backpressure?

Quote:
Originally Posted by PC/SR View Post
The need for back pressure is one of those old myths that just won't go away.
Agreed! I have fallen for old auto myths before, we hear them when we are young from someone older, sometimes in a passing conversation, and we just take it as the truth and file it away. But this is one I have challenged folks to prove for years.
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Old 04-17-2013, 10:13 AM   #13
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With all due respect, you're committing the "post hoc ergo propter hoc" fallacy--translated that means, "after this, therefore because of this." Of course you might be right!!
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Old 04-17-2013, 10:35 AM   #14
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Default Re: not enough backpressure?

I bit the bullet and installed brand new 3x's with no change, they seem to be burning cleaner though. Also found a faulty ignition switch and replaced it with no effect on the power. My next step is throwing that old meineke muffler back on and seeing if the problem persists.
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Old 04-17-2013, 01:13 PM   #15
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Default Re: not enough backpressure?

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Originally Posted by QGolden View Post
Agreed! I have fallen for old auto myths before, we hear them when we are young from someone older, sometimes in a passing conversation, and we just take it as the truth and file it away. But this is one I have challenged folks to prove for years.
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.
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Old 04-17-2013, 01:15 PM   #16
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Default Re: not enough backpressure?

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Originally Posted by jkeesey View Post
Okay, so I recently put a glasspack muffler on my A with a short straight pipe after it. The entire exhaust is about 3ft total. After doing so it seems to have a miss under load. I went through the distributor and the entire fuel system and the only thing I can think at this point is there isn't enough backpressure. Does this sound right to anyone or am I just pissing in the wind.
Try a longer pipe with a bend in it.
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Old 04-17-2013, 01:24 PM   #17
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Default Re: not enough backpressure?

I just went to check the points and found the dist bushings to be very worn, it looks like it was causing the gap to change at higher rpms or load. It also looks like the drive end was very worn and causing a lot of slack between it and the lowed shaft. I replaced the bushing and found a good original dist shaft, now I'm waiting in my old man with the right reamer to get the bushings cut right. I will keep everyone posted on whether this solves the problem.
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Old 04-17-2013, 02:11 PM   #18
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Default Re: not enough backpressure?

With the new noisy pipe, bet you drove it higher RPM's than with the quiet pipe!!! NOW, the gas mileage will go DOWN too!!!!!!!!!!!! RUDNNN, RUDNNN!!!! Bill W.
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Old 04-17-2013, 02:38 PM   #19
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Default Re: not enough backpressure?

Quote:
Originally Posted by H. L. Chauvin View Post
This happens to all of us with Model A's & other vintage vehicles; however, before radar, electronic ignition, x-ray, electronic engine dianostics, laser technology, & CAT scans, if one walked into an old time mechanic's shop & said,

"My car ran perfectly until I tried performing a tune up, & now it runs horribly -- why?"

The mechanics response would be, "No matter how insignificant it may appear, tell me "everything" you touched because therein lies the problem."

As parochial as this appears, with pencil & paper write down every item you touched, (mainly those you know did not cause this problem), & re-check same.

It will always be something so simple.

Hope this helps.

O how true. It happened all the time when I was working on cars for a living. Bad thing is they would not want to look bad, so many times they would not tell all the story.
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Old 04-17-2013, 02:42 PM   #20
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Default Re: not enough backpressure?

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Originally Posted by BILL WILLIAMSON View Post
With the new noisy pipe, bet you drove it higher RPM's than with the quiet pipe!!! NOW, the gas mileage will go DOWN too!!!!!!!!!!!! RUDNNN, RUDNNN!!!! Bill W.

I think you are right on. Tell the dog hi.
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