Distributor off a tooth?
Thread Starter
Racer
Joined: Apr 2005
Posts: 494
Likes: 0
From: Virginia
Distributor off a tooth?
My 73 – 350 has misfired and backfired since it has been running, after a rebuild. Set the dwell and set the timing at 36* at 3000, still have basically the same misfiring and backfiring. Does it sound like the distributor is off a tooth? Any help will be appreciated. 
Racer
Joined: Dec 2006
Posts: 417
Likes: 40
From: Swansea IL
Vacuum Leak?
If you know for a fact that you are getting all your advance in at 3000 rpm, then my next questions are:
How does it idle?
Does is only backfire when you open the throttle?
Is it backfiring when the throttle is held at a steady speed?
First: If the idle is high and you can't get it to idle down, you may have a vacuum leak. Use WD-40 or something like that around the base of the carb and along the intake runners. If the engine speeds up, you have a vacuum leak.
If the backfire occurs mostly when you open the throttle, you need to check you accelerator pump on the carb. Might not be working and causing a lean condition.
If the backfire occurs at a steady speed, have the spark plug wires been off lately. There may be a crossed ignition lead.
Give those a try and see what works.
Bob
How does it idle?
Does is only backfire when you open the throttle?
Is it backfiring when the throttle is held at a steady speed?
First: If the idle is high and you can't get it to idle down, you may have a vacuum leak. Use WD-40 or something like that around the base of the carb and along the intake runners. If the engine speeds up, you have a vacuum leak.
If the backfire occurs mostly when you open the throttle, you need to check you accelerator pump on the carb. Might not be working and causing a lean condition.
If the backfire occurs at a steady speed, have the spark plug wires been off lately. There may be a crossed ignition lead.
Give those a try and see what works.
Bob
Thread Starter
Racer
Joined: Apr 2005
Posts: 494
Likes: 0
From: Virginia
How does it idle?
Won’t idle below 1700 RPMs
Does is only backfire when you open the throttle?
Backfires below 1700 RPMS, misfires at higher RPMS
Is it backfiring when the throttle is held at a steady speed?
Depends on the RPM, usually yes.
First: If the idle is high and you can't get it to idle down, you may have a vacuum leak. Use WD-40 or something like that around the base of the carb and along the intake runners. If the engine speeds up, you have a vacuum leak.
Don’t think this is the problem.
If the backfire occurs mostly when you open the throttle, you need to check you accelerator pump on the carb. Might not be working and causing a lean condition.
Running rich.
If the backfire occurs at a steady speed, have the spark plug wires been off lately. There may be a crossed ignition lead.
New Accel 8mm wires, connected properly.
Thanks!
Ed
Won’t idle below 1700 RPMs
Does is only backfire when you open the throttle?
Backfires below 1700 RPMS, misfires at higher RPMS
Is it backfiring when the throttle is held at a steady speed?
Depends on the RPM, usually yes.
First: If the idle is high and you can't get it to idle down, you may have a vacuum leak. Use WD-40 or something like that around the base of the carb and along the intake runners. If the engine speeds up, you have a vacuum leak.
Don’t think this is the problem.
If the backfire occurs mostly when you open the throttle, you need to check you accelerator pump on the carb. Might not be working and causing a lean condition.
Running rich.
If the backfire occurs at a steady speed, have the spark plug wires been off lately. There may be a crossed ignition lead.
New Accel 8mm wires, connected properly.
Thanks!
Ed
Le Mans Master
Joined: Nov 2006
Posts: 5,262
Likes: 283
From: I'd like to propose a toast... to internal combustion and wind in the face.
You stated the engnine has not been running properly since a rebuild so I'll assume the distributor has been removed and then re-installed. Did you re-install the distributor with the number one cylinder at top-dead-center on the compression stroke? If it was then the distributor should not be your problem. Quite possibly you've got the engine "cross-wired".
Get your engine's number one cyliner at t.d.c. on the compression stroke and locate your number one plug wire on the distributor cap. Mark the wire's location on the distributor base and remove the distributor cap. If the distributor was installed properly (and if the engine timing is set properly) then the rotor button tip should be just beyond or on the other side of the mark you made on the distributor base as the rotor rotates clockwise. If the rotor tip is not where I've described then where the rotor tip is pointing is your new number one spark plug wire location. Install your number one plug wire at the new location and move the remaining spark plug wires accordingly (firing order: 1-8-4-3-6-5-7-2).
I hope have not been too elementary or too confusing. If I've given you any incorrect information I'm sure someone on the forum will correct me.
Get your engine's number one cyliner at t.d.c. on the compression stroke and locate your number one plug wire on the distributor cap. Mark the wire's location on the distributor base and remove the distributor cap. If the distributor was installed properly (and if the engine timing is set properly) then the rotor button tip should be just beyond or on the other side of the mark you made on the distributor base as the rotor rotates clockwise. If the rotor tip is not where I've described then where the rotor tip is pointing is your new number one spark plug wire location. Install your number one plug wire at the new location and move the remaining spark plug wires accordingly (firing order: 1-8-4-3-6-5-7-2).
I hope have not been too elementary or too confusing. If I've given you any incorrect information I'm sure someone on the forum will correct me.
Drifting
Joined: Apr 2002
Posts: 1,716
Likes: 4
From: New Baden Illinois
One hint to determine if you are off a tooth is to see if it runs better with the dist. cap turned all the way to one side. When we installed a friend's engine in his 80 a few years ago, we actually had it off by two teeth. It took us forever to figure it out. Eventually, we turned the cap all the way to one side, and the engine was trying to catch. We took the distributor out, moved it one tooth in that direction and tried again. This time it would only run if we had the cap turned all the way to the one direction. So, we rotated it one more tooth and it worked perfect.
Safety Car
Joined: Oct 2004
Posts: 3,725
Likes: 5
From: Minnesota
You could be off six teeth and make it run great as long as you turn the cap to the proper timing. I time by ear. Just keep advancing until you get some detonation under acceleration with a warm engine and then back it off a little.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Drifting
Joined: Apr 2002
Posts: 1,716
Likes: 4
From: New Baden Illinois
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
the only issue with being off a tooth is if the vacuum cannister is hitting something preventing you from getting the proper timing...almost sounds like the plug wires are in the wrong order.....i am always amazed at how well these cars run even if some plug wires are mixed up
Race Director
Joined: Aug 1999
Posts: 18,126
Likes: 174
From: WI
except that TDC on the balancer could be #1 or #6 ...you need to pull the #1 spark plug and have someone click the key while you hold a finger over the hole...when the compression blows your finger off the hole thats #1
...redvetracr
...redvetracr
Pro
Joined: Dec 2006
Posts: 665
Likes: 0
From: wilmington nc
I dont think your dist. is off, if you set timeing to 36*! reason being when my dist. was off a tooth so was the timeing mark trying to use the light. chilton showes 74 and earlier with points, the #1 on the dist should point to the # 2 cylinder (what you refering to as 7 o"clock) for 74 and later with hei the #1 on the dist should point towards the #1 cylinder (what you"d call 5 o"clock) if you moved it to 6 o"clock i dont think your going to see the timeing mark with a light.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
This really says it all. Won’t idle below 1700 RPMs That is a massive vacuum leak. Like the rear 3/8th under the rear bowl of a double pumper.
I would check and plug every carb ported and unported vac. connection. Plug every manifold vacuum port. Just to eliminate down stream hose leaks. If spraying flamable liquids around the base of the carb and manifold to head does not increase the idle speed, you can actually have a vacuum leak from the cam valley to the lower intake runners
Tech Contributor
Joined: Aug 1999
Posts: 14,379
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
If you cannot get it to idle below 1700 and you're getting popping and missing, you have a massive vacuum leak. If timing is right, backfiring through the carb upon throttle opening is indicative of a lean condition -associated with a vacuum leak.
Le Mans Master
Joined: Nov 2006
Posts: 5,262
Likes: 283
From: I'd like to propose a toast... to internal combustion and wind in the face.
[QUOTE=lars;1559348013]There is no such thing as being "off one tooth." If your timing is set right, installed distributor orientation is irrelevant. If you're off a tooth, it only changes the clocking that the distributor ends up in once you time it. On some installations, you cannot get it timed because the vacuum advance will hit the back of the manifold. But if you're timed at 36 total, the installed orientation is completely irrelevant to performance - you can install the distributor in any position and still make the timing right.
I wanted to say this but didn't have the nerve!
I wanted to say this but didn't have the nerve!
Thread Starter
Racer
Joined: Apr 2005
Posts: 494
Likes: 0
From: Virginia
I’ll just leave it at 6 o’clock, the distributor vacuum advance was close to the firewall.
Thanks for all the advice! It’s starting to sound like a vacuum leak. The Holley 650 DP carb is from Ebay, but was completely rebuilt by me. The air horn is cut off. Everything is removed from the main body that can be removed, plugged all the holes that appeared to be potential vacuum leaks, but not all of them. I did notice when it was first started that some gas seeped out from one of the throttle base plate shafts.
Was going to replace the throttle base plate and main body with Proform parts later, but sounds like I should do it now…
Thanks for all the advice! It’s starting to sound like a vacuum leak. The Holley 650 DP carb is from Ebay, but was completely rebuilt by me. The air horn is cut off. Everything is removed from the main body that can be removed, plugged all the holes that appeared to be potential vacuum leaks, but not all of them. I did notice when it was first started that some gas seeped out from one of the throttle base plate shafts.
Was going to replace the throttle base plate and main body with Proform parts later, but sounds like I should do it now…
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I've got a 750 carb at home sitting on the shelf. It looks beautiful, but all I can figure is that it must have got water condensation while parked and then froze. The body must have internal cracks or something because it leaks gas down the throttle bores. I have also considered buying a new center body for it.