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satterley_sr
Samba Member
Joined: March 31, 2005
Posts: 650
Location: Belleville,MI
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 Posted: Thu May 14, 2015 1:29 pm Post subject: Popping on Decel |
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Have a customer build with a new 1776 cc, Weber progressive carb that back fires on decal from about 50 MPH, fairly loud pop then some lighter popping. Tried richer idle jets, less timing, checked for intake leaks, exhaust leaks. tried two svda distributors . Even tried a different carb but still there. Any suggestions?
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catahoula lou
Samba Member
Joined: August 30, 2008
Posts: 594
Location: south of Silver Springs, NV
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| Posted: Thu May 14, 2015 2:42 pm Post subject: |
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Not sure if you mean backfiring back up carb or out exhaust.
Had similar problem in my 1967 dune buggy, assuming yours backfires through exhaust. Header flange, even with the correct gasket, did not seat properly to head. Allowed air to be sucked in during engine deceleration resulting in a "re-mixing" of the exhaust, which typically has some unburned fuel in it, allowing re-ignition via short flames coming out corresponding head exhaust port.
Exhaust leaks an be hard to detect...
Took headers off, re-installed without gaskets, and re-tightened to seat flanges against heads to inspect seating (i.e., flat or angled).
Ended up grinding down subject flange for correct (flat) seating like the other three.
Did not pop at all on engine deceleration after re-installing with gaskets.
Best,
Thom
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1 great wife
2 Catahoula Leopard Dogs (Mahogany Star and Spartan) - RIP Lucy, Braveheart, & Dusty!
1 1959 mango green Ragtop Bug (the "Mango")
1 1958 pantina red Lowlight Ghia (the "Chili Pepper")
Still looking for:
(1) My Dad's 1955 356 (he raced it amateur-class at Riverside and other courses during 1950s),
(2) My parent's black 1955 hardtop bug (CA license plate FWC 201 or FWG 201), and
(3) My parent's agave green 1957 ragtop bug (CA license plate LFK 734). |
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udidwht
Samba Member
Joined: March 06, 2005
Posts: 3779
Location: Seattle, WA./ HB, Ca./ Shizuoka, Japan
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| Posted: Thu May 14, 2015 5:13 pm Post subject: |
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Check for exhaust leak/s using a length of hose one end near the ear. Notice I said near...not on the ear. The other end...probe. When you find the leak/s you'll know it.
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1972 Westy Hardtop/Type-4 2056cc
96mm Biral AA P/C's~7.8:1CR
Headflow Masters New AMC 42x36mm heads w/Porsche swivel adjusters
71mm Stroke
73 Web Cam w/Web solids
Dual 40mm IDF Webers - LM-2 - 47.5 idles/125 mains/190 air corr./F11 tubes/28mm Vents - Float height 10.45mm/Drop 32mm
Bosch SVDA w/Pertronix module (7.5 initial 28 total @ 3400rpm)
Bosch W8CC plugs
Pertronix Flamethrower 40K coil
S&S 4-1 w/Walker QP 17862
3 rib 002 Trans
185R14 Hankook tires |
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Bashr52
Samba Member
Joined: July 16, 2006
Posts: 5666
Location: On an island in VA
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| Posted: Fri May 15, 2015 5:39 am Post subject: |
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Get rid of the Pinto carb and see if that helps 
Popping out the carbs usually means lean condition, popping out the exhaust usually means too rich. |
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udidwht
Samba Member
Joined: March 06, 2005
Posts: 3779
Location: Seattle, WA./ HB, Ca./ Shizuoka, Japan
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| Posted: Fri May 15, 2015 9:36 am Post subject: |
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| Bashr52 wrote: |
Get rid of the Pinto carb and see if that helps
Popping out the carbs usually means lean condition, popping out the exhaust usually means too rich. |
Or an exhaust leak/s.
_________________
1972 Westy Hardtop/Type-4 2056cc
96mm Biral AA P/C's~7.8:1CR
Headflow Masters New AMC 42x36mm heads w/Porsche swivel adjusters
71mm Stroke
73 Web Cam w/Web solids
Dual 40mm IDF Webers - LM-2 - 47.5 idles/125 mains/190 air corr./F11 tubes/28mm Vents - Float height 10.45mm/Drop 32mm
Bosch SVDA w/Pertronix module (7.5 initial 28 total @ 3400rpm)
Bosch W8CC plugs
Pertronix Flamethrower 40K coil
S&S 4-1 w/Walker QP 17862
3 rib 002 Trans
185R14 Hankook tires |
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74 Thing
Samba Member
Joined: September 02, 2004
Posts: 7393
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| Posted: Fri May 15, 2015 9:50 am Post subject: |
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| The graphite exhaust gaskets CB Performance sells are supposed to be really good for stopping exhaust leaks. |
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satterley_sr
Samba Member
Joined: March 31, 2005
Posts: 650
Location: Belleville,MI
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| Posted: Sat May 23, 2015 1:33 pm Post subject: |
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After two carbs and an intake change due to suspected porosity, problem still existed. Popping out the exhaust on decel from about 50 mph. No exhaust leaks, intake leaks etc. Replaced the hot dog style muffler with a stock muffler and no more popping. Now it just seems to run a lot hotter. Frustration on top of frustration.
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modok
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Joined: October 30, 2009
Posts: 26789
Location: Colorado Springs
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| Posted: Sat May 23, 2015 1:46 pm Post subject: |
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Sorry you went through all that.
Backfiring on decel is normal when you have a tiny/short muffler on any engine.
If they guy doesn't like it, he should get a real muffler. |
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satterley_sr
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Joined: March 31, 2005
Posts: 650
Location: Belleville,MI
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satterley_sr
Samba Member
Joined: March 31, 2005
Posts: 650
Location: Belleville,MI
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| Posted: Sat May 23, 2015 2:03 pm Post subject: |
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Glen,
Do you think a stock muffler would cause the engine to run hotter than with the hot dog style muffler?
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modok
Samba Member
Joined: October 30, 2009
Posts: 26789
Location: Colorado Springs
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| Posted: Sat May 23, 2015 2:05 pm Post subject: |
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| Sometimes. The more radical the camshaft the more sensitive an engine is to the exhaust system. |
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