Tuning a Demon 750...Grrrrr...
Thread Starter
Pro
Joined: Jun 2005
Posts: 725
Likes: 1
From: FL.
Tuning a Demon 750...Grrrrr...
The Engine is eccelerating fine it cruises pretty steady in any gear, its not rough at idle 700rpms. BUT it wont decelerate off Hi rpm without blam wham out both exhaust pipes!! I mean if I wind the motor up in any gear then let fully off the accelerator pedal..wham...wham..wham out both Pipes. I dont have any guages for testing vacuum. What needs a little turn or tweak, Anybody got any advice??. If I advance the dist a Little More could that end these explosions in the exhaust. Or maybe back the dist. off a hair..what would that do??
any feedback appreciated..Thanks in advance..
any feedback appreciated..Thanks in advance..
Le Mans Master
Joined: May 2003
Posts: 8,877
Likes: 3,556
From: Fernandina Beach FL
2023 Restomod of the Year finalist
2020 C3 of the Year Winner - Modified
After-fire is the correct term. Basically an incomplete burn and it causes the fumes to explode in the exhaust system. Could be running rich- the Timing is not correct or you have an exhaust leak. You might want to do a search- I think a while back there was a thread on it.
BUT make sure you've got the timing dialed in BEFORE you mess w/ the carb or you'll never get it right. Barry K did a great How to on setting timing-
http://forums.corvetteforum.com/show....php?t=1430339
Richard
BUT make sure you've got the timing dialed in BEFORE you mess w/ the carb or you'll never get it right. Barry K did a great How to on setting timing-
http://forums.corvetteforum.com/show....php?t=1430339
Richard
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
BUT make sure you've got the timing dialed in BEFORE you mess w/ the carb or you'll never get it right. Barry K did a great How to on setting timing-
http://forums.corvetteforum.com/show....php?t=1430339
Richard
http://forums.corvetteforum.com/show....php?t=1430339
Richard
Per Lars, most carb problems are ignition problems.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Afterfire is usually a sign of a lean condition. What happens is that the engine goes sudden lean after you've slammed the throttle closed at high rpm. This causes lean misfire in some cylinders, and a near-perfect unburned fuel/air mixture goes right into the exhaust where it explodes. Richen it up. First, back your idle mixture screws out a tad to see if that fixes it. If not, bump the jetting up a couple sizes and try it again.
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Lars is correct and you should try his advice first. Because the idle circuit influences part throttle and cruise. Turn your idle mixture screws out and see if the backfire goes away
Drifting
Joined: Jul 2005
Posts: 1,591
Likes: 0
From: Oakville Ontario
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3rd Gear
Joined: Aug 2007
Posts: 3
Likes: 0
From: new york new york
thanks, Lars.......i will try adjusting the idle screws! my car started backfiring after i connected an accel ignition box...i thought these boxes burned all the fuel with multiple sparks. car accelertes fine, but pops and bangs through the exhaust when i lift...especially after hard acceleration....i'm running a holley 3310 on a 355sbc and 5 speed...motor pulls 20" on idle...i have a small exhaust leak...i guess it could be any of these things...but you think it's 'cause of leaness?
3rd Gear
Joined: Aug 2007
Posts: 3
Likes: 0
From: new york new york
turns out to be an ignition timing issue! re-set the initial timing to 14* and she doesn't backfire anymore!....don't know how it happened, but the initial timing was set to 4*! i had taken the module out of the distributor and ran the magnetic trigger for my accel 300 box....perhaps the built-in timing and dwell of the box changed my original timing! anyway..it runs great now.....
