'59 Popping out the Exhaust

[Brad K.]

I've been driving my Triple Crown winning '59 a lot more lately and a problem has come up. It's not a BIG problem but it's noticeable. It's a bone stock 270hp 283 with the typical Duntov cam, 2613 & 14 carbs rebuilt by Jerry Katter and an 891 centrifugal advance distributor recently degreed at 600 rpm at 0 degrees and 3700 rpm at 28 degrees total. When I'm cruising down the highway at constant throttle it "pops" out the exhaust about every 1/2 to 1 mile. Not multiple times...but only once. When I really get on it and then decelerate from about 5000 rpm....I hear the exhaust pop back about two or three times.

The car has about 1000 miles on it. Everything else is kosher....It starts consistently well even when hot and the carbs don't drip. There is a slight "fog" over the primary's however after stopping and both exhausts are burning slightly blackish. I'm running AC 45's and they need cleaning out occasionally. I'm thinking the carbs are dumping a little raw fuel into the exhausts.....What should be my plan of attack?

[Terry D.]

Re: '59 Popping out the Exhaust

I don't have my books available but it seems your advance is not right. I think timing should be around 10 to 12 degress at idle It should be all in by about 3000rpm. Check this to be sure of settings. I would then try a compression check to make sure all the valves are seated properly. Then try leaning out the carbs a little. Just a couple of things to try

Terry

[Brad K.]

Re: '59 Popping out the Exhaust

Terry.....After reading Duke Williams' article on "Tuning Vintage Corvette Engines" I interpreted from it that most 57-61 283's with only centrifugal advance should be set at 0 degrees @ 600 rpm with a total advance of 28 degrees @3700 rpm. Now....that's w/o the 097 cam, but the car certainly idles very smoothly anyway....so that's where I left it. Maybe Duke has other recommendations for the 2x4 270hp engine. I'll do a compression check to see where it's at, since I haven't done so in quite a while! Thanks....

[Joe C.]

Re: '59 Popping out the Exhaust

Your initial timing should be 7 BTDC @ idle. I would time it so that the engine sees 36 - 38 degrees spark advance at WOT, which translates to 7 - 9 degrees initial.

http://www.tpocr.com/corvette.html

[Steve G.]

Re: '59 Popping out the Exhaust

Could always use the Microsoft approach to dealing with the problem. "Restore" to the last known configuration that worked correctly. If all hardware remains the same, configure the tuning to what the engineers set it to when it left the factory and went down the road without popping back.

Do you have a properly operating heat riser?

Steve

[Frank D.]

Re: '59 Popping out the Exhaust

Popping in the exhaust is raw gas igniting in the pipe. Lean mixture, exhaust leak or timing are the usual causes...
If your car is a Triple Crown winner it probably doesn't have exhaust manifold gaskets. You might want to check the torque on the exhaust manifolds.

[Duke W.]

Re: '59 Popping out the Exhaust

Terry.....After reading Duke Williams' article on "Tuning Vintage Corvette Engines" I interpreted from it that most 57-61 283's with only centrifugal advance should be set at 0 degrees @ 600 rpm with a total advance of 28 degrees @3700 rpm. Now....that's w/o the 097 cam, but the car certainly idles very smoothly anyway....so that's where I left it. Maybe Duke has other recommendations for the 2x4 270hp engine. I'll do a compression check to see where it's at, since I haven't done so in quite a while! Thanks....

Those the the typical 283 centrifugal specs as listed on page 23 of the pdf. On page 28 under General Approach to the Project heading it says

1. Set initial to achieve 36-38 total WOT advance

Total WOT advance is the sum of initial and total centrifugal as explained in the slides.

It sounds like you set initial advance at zero. It should be 8-10. It may be tough to set the initial advance because if the centrifugal is as stated a Duntov cam may not idle below 600 stabily long enough to set initial. So use a dial back light to verify the centrifugal map, then set the total at 38 at an engine speed above where the centrifugal stops advancing, which would be over 3700 if the advance is OE.

That lazy curve can use some lighter springs, which should improve low end torque and allow you to set total WOT advance at less than 3700.

So verify that your initial is in the proper range, and if the problem continues, report back.

Duke

[Brad K.]

Re: '59 Popping out the Exhaust

Hey, Guys....Sorry it took so long to get back on this thread.......I have finally solved the problem (popping through the exhaust) but it wasn't what I thought the solution might be.

I'll quickly go through my steps....I followed ATS troubleshooting techniques and drove the car/tested for the fix after each adjustment so as not to confuse the results. As recommended by you guys, I did adjust my timing slightly to what Duke recommended...It didn't help. I then checked the tightness of my exhaust system manifolds and flanges....no help. I then properly checked the compression to be 140 +/- 5 psi....good check but no help. My thoughts then turned to the richness of the mixture....so I got the numbers off the metering rods and jets and started to write you guys back.....but I thought I might as well check the manifold vacuum at idle. When I pulled the air cleaner and upper ignition shield off, hooked up the vacuum gauge to the rear of the primary carb, started the engine and determined that I had 34 inches of hg at 750 rpm.

Then is when I noticed the coil and its ignition shield bracket where very loose (the bolt is very difficult to tighten anyway) To my surprise all the aluminum intake manifold bolts required at least an additional 1/4 turn to return to the proper torque....with the rear two bolts on the passenger side (under the coil) took nearly one turn to tighten. This condition existed even without any "obvious" intake manifold leaks under low speed conditions. I was anxious to test drive the car.....the issue was solved.
What I re-learned from this experience....After building the engine 10 years ago...never assume things that were once tight, remain tight....duh!

[Ed S.]

Re: '59 Popping out the Exhaust

Brad,
Glad to hear you solved the problem. I have a question for anyone that would be kind enough to respond. I read the possible reasons posted in response to your question. I clearly understand how any number of them (especially incorrect timing) could cause popping in the exhaust - except one, that is "a lean mixture". I don't understand the physics or mechanics of how a lean mixture could cause popping (detonation) in the exhaust. I can understand how a too rich mixture - raw gas or a healthy flame front in the exhaust - could cause popping but not a lean mixture. Can a lean mixture actually cause popping or is this incorrect. If valid, can someone explain how this could happen? Thanks in advance for the tutorial.

[Joe C.]

Re: '59 Popping out the Exhaust

Brad,
Glad to hear you solved the problem. I have a question for anyone that would be kind enough to respond. I read the possible reasons posted in response to your question. I clearly understand how any number of them (especially incorrect timing) could cause popping in the exhaust - except one, that is "a lean mixture". I don't understand the physics or mechanics of how a lean mixture could cause popping (detonation) in the exhaust. I can understand how a too rich mixture - raw gas or a healthy flame front in the exhaust - could cause popping but not a lean mixture. Can a lean mixture actually cause popping or is this incorrect. If valid, can someone explain how this could happen? Thanks in advance for the tutorial.

Ed,
It is not detonation, but combustion in the exhaust that causes popping. Lean mixtures can cause this because of "lean misfire" which causes unburned fuel to flow into the exhaust pipe, just as with over rich mixtures.

[Joe C.]

Re: '59 Popping out the Exhaust

Brad,
Thanks for mentioning that I found your correct timing spec before anyone else did.
Nah.....nevermind.

[Duke W.]

Re: '59 Popping out the Exhaust

started the engine and determined that I had 34 inches of hg at 750 rpm.

...glad you solved the problem, but 34" Hg manifold vacuum is not possible. If you could pull 30" Hg vacuum at sea level, you essentially have a pure vacuum. Air conditioning service machines can typically pull about 29" when evacuating the system.

If you have a real Duntov cam idle vacuum should be about 12", but I doubt if it would idle very well at 750; 850-900 would be a bit smoother, but still with plenty of lope.

From your description of idle behavior it doesn't sound like you have a Duntov cam.

Duke

[Brad K.]

Re: '59 Popping out the Exhaust

Duke...I still have the cardboard tube the cam came in when I purchased it from GM a number of years ago....The numbers on the sticker are somewhat smudged, but as best I can read them they say "4261FO GM 143736097 GR.0?5.19 CAMS 283 MADE IN USA" So.....what kind of cam is it? All these years I thought it was a Duntov The engine sounds mighty pretty with plenty of lope and the mechanical lifters are clicking away. I am also running the off-road, straight-through mufflers. I would be happy to send you an audible recording of the idle......if I can figure out how to do it electronically through the mail.

You are correct about the vacuum....I should have said 34 cm of hg or 13.5 in of hg.....My gauge has both numbers on it and I read the wrong side of the line and I too hastily reported it. I've got a 4.11" (270hp) diameter generator pulley on the direct mechanical drive of the tach (which is what I am reading...not electronically) with a 3/8' wide drive belt from the engine which is riding a little low in the groove so the tach may not be reading accurately. I'll get an electronic measurement of the idle speed if I can get my dwell meter to work.

[Duke W.]

Re: '59 Popping out the Exhaust

I recall you said earlier that it idled smooth, which is why I questioned that it's a Duntov cam, but ...097 is the part number, so you have the real McCoy and idle vacuum is in the ballpark.

One should always use a test tach with a low scale to set idle speed as the in-car tach is not granular enough to read within 25 RPM, and it may not be that accurate at idle speed.

Duke

[William F.]

Re: '59 Popping out the Exhaust

Think one person is talking about when CENTRIFICAL advance starts and what it does. Other person is referring to INITIAL advance. The 38* is the sum of the initial and centrifugal degrees.