1970 LT1 bog and backfire WOT
Thread Starter
Pro
Joined: May 2011
Posts: 606
Likes: 100
From: Gilbert Arizona
1970 LT1 bog and backfire WOT
My lt1 runs great at idle and also when you quick hit the throttle is super responsive however at WOT the thing bogs out and backfires thru the carb. I believe it's a fuel issue as I have great steady vacuum and the initial timing has been set correct. I will try the total timing this weekend but I really think when the secondaries open it's not getting any fuel as the butterflies and veturies seem bone dry. I did rebuild the carb and adjusted the best I could but how can I check if fuel is actually being released from the secondary side?
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,098
Likes: 9,229
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
I don't think a backfire is indicative of no fuel. I think you're on the right track with the timing. I would guess that you're either get too much vacuum advance or your not getting any. I would start with the easy things first then venture into the carb. A backfire through the carb to me means your igniting your mixture and its coming back through the valves versus pushing the piston down (timing).
If it was backfiring thought the exhaust I would think lean condition then. ( carb like you were thinking)
If it was backfiring thought the exhaust I would think lean condition then. ( carb like you were thinking)
Race Director
Joined: May 2006
Posts: 18,648
Likes: 2,557
if you have the original 780 or any Holley with a float bowl with removable screws to check the float level, have you checked the float level when the engine is running?
When you rebuilt the carburetor, did you use the original sliver colored secondary spring or change it to something else?
When you rebuilt the carburetor, did you use the original sliver colored secondary spring or change it to something else?
Thread Starter
Pro
Joined: May 2011
Posts: 606
Likes: 100
From: Gilbert Arizona
So I think I have another issue. I tried setting advance timing and I can't get it anywhere near 38 degrees. It only wants to run halfway decent when it's in the 40s. Valve timing issue?
Thread Starter
Pro
Joined: May 2011
Posts: 606
Likes: 100
From: Gilbert Arizona
if you have the original 780 or any Holley with a float bowl with removable screws to check the float level, have you checked the float level when the engine is running?
When you rebuilt the carburetor, did you use the original sliver colored secondary spring or change it to something else?
When you rebuilt the carburetor, did you use the original sliver colored secondary spring or change it to something else?
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
5 MOST and 5 LEAST Popular Corvette Model Years in History!
Joe Kucinski
2027 Corvette Buyer's Guide: Everything You Need to Know!
Joe Kucinski
Thread Starter
Pro
Joined: May 2011
Posts: 606
Likes: 100
From: Gilbert Arizona
Another thing I noticed is that it has headers that don't seal very well as they are very loud and some of the plugs are turning black in the outside from exhaust leaking past. Could it possibly be a back pressure issue as well? I have the stock manifolds and I ordered exhaust for it last week. Will be installing on the holiday weekend but was wondering if I should just pull the front cover while I'm at it and inspect the timing chain alignment?
Race Director
Joined: May 2006
Posts: 18,648
Likes: 2,557
Is your distributor stock or modified, softer advance springs?
where did you come up with the number 38, 36 is usually the total number with vacuum disconnected?
Are you using a dial back timing light or have a marked damper?
there are easier ways to check for chain stretch than opening up the cover, it involves rotating the crank one way by hand and then rotating the other way by hand, can provide more detail if necessary.
where did you come up with the number 38, 36 is usually the total number with vacuum disconnected?
Are you using a dial back timing light or have a marked damper?
there are easier ways to check for chain stretch than opening up the cover, it involves rotating the crank one way by hand and then rotating the other way by hand, can provide more detail if necessary.
Thread Starter
Pro
Joined: May 2011
Posts: 606
Likes: 100
From: Gilbert Arizona
Is your distributor stock or modified, softer advance springs?
where did you come up with the number 38, 36 is usually the total number with vacuum disconnected?
Are you using a dial back timing light or have a marked damper?
there are easier ways to check for chain stretch than opening up the cover, it involves rotating the crank one way by hand and then rotating the other way by hand, can provide more detail if necessary.
where did you come up with the number 38, 36 is usually the total number with vacuum disconnected?
Are you using a dial back timing light or have a marked damper?
there are easier ways to check for chain stretch than opening up the cover, it involves rotating the crank one way by hand and then rotating the other way by hand, can provide more detail if necessary.
Le Mans Master
Joined: Aug 2005
Posts: 6,506
Likes: 1,941
From: Temple Terrace Florida
If you got a few backfires through the Holley carb, check the power valve. They will often time blow out after a carb backfire. I would also check the vacuum advance can on the distributor. You can use a hand held pump from HF. Jerry
Race Director
Joined: May 2006
Posts: 18,648
Likes: 2,557
Stock distributor
You cant set maximum timing on a stock distributor with the original springs, they are too stiff. Set your initial timing at idle to the stock number until you modify the distributor.
Thread Starter
Pro
Joined: May 2011
Posts: 606
Likes: 100
From: Gilbert Arizona
I removed the springs so I could achieve maximum advance. I used the techniques listed in Lar's timing paper
Old Pro Solo Guy
Joined: Dec 2017
Posts: 8,001
Likes: 4,357
From: Marlton NJ
There are lot's of possibilities.
Are you checking timing at 3000 rpm with advance disconnected?
If yes and it still likes 40 I would check to see if the balancer has slipped. Find TDC with a piston stop, and then degree in the balancer. With out a degree wheel you could mark sharpie lines at say 3 inches either side of zero, etc.
Are you checking timing at 3000 rpm with advance disconnected?
If yes and it still likes 40 I would check to see if the balancer has slipped. Find TDC with a piston stop, and then degree in the balancer. With out a degree wheel you could mark sharpie lines at say 3 inches either side of zero, etc.
Race Director
Joined: May 2006
Posts: 18,648
Likes: 2,557
Thread Starter
Pro
Joined: May 2011
Posts: 606
Likes: 100
From: Gilbert Arizona
Race Director
Joined: Nov 2006
Posts: 13,143
Likes: 7,753
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
There are lot's of possibilities.
Are you checking timing at 3000 rpm with advance disconnected?
If yes and it still likes 40 I would check to see if the balancer has slipped. Find TDC with a piston stop, and then degree in the balancer. With out a degree wheel you could mark sharpie lines at say 3 inches either side of zero, etc.
Are you checking timing at 3000 rpm with advance disconnected?
If yes and it still likes 40 I would check to see if the balancer has slipped. Find TDC with a piston stop, and then degree in the balancer. With out a degree wheel you could mark sharpie lines at say 3 inches either side of zero, etc.
A few years back I found my factory balancer had slipped. Making the timing mark useless not to mention other problems. I got a performance balancer from summit that has timing marks stamped on it. No need for a dial back or timing tape now.
Something is definitely wrong if it only runs O.K. in the 40's
Melting Slicks
Joined: Dec 2009
Posts: 3,220
Likes: 651
From: Thunder Bay
There are lot's of possibilities.
Are you checking timing at 3000 rpm with advance disconnected?
If yes and it still likes 40 I would check to see if the balancer has slipped. Find TDC with a piston stop, and then degree in the balancer. With out a degree wheel you could mark sharpie lines at say 3 inches either side of zero, etc.
Are you checking timing at 3000 rpm with advance disconnected?
If yes and it still likes 40 I would check to see if the balancer has slipped. Find TDC with a piston stop, and then degree in the balancer. With out a degree wheel you could mark sharpie lines at say 3 inches either side of zero, etc.
You can't really do anything until you confirm that your timing marks are correct.