Tuning help
10-06-2013, 09:42 AM
#1
1st Gear Member
Thread Starter
Join Date: Nov 2012
Location: Wisconsin
Posts: 55
Tuning help
After my car started running so bad that I couldn't even drive it I decided to have the carb rebuilt. I knew it was junk and bought a crate Engine for it, but want to enjoy it some more before taking it all apart.
Now the car starts right up, idles and drives great as long as you stay below 3/4 throttle. Once you go past that it backfires horribly! It doesn't matter if your driving and floor it or off the line and floor it.
I checked the timing, dwell, plug gap and advance seems to hold. It's a 71 302 with a 2bl carb. HELP!!! lol
Now the car starts right up, idles and drives great as long as you stay below 3/4 throttle. Once you go past that it backfires horribly! It doesn't matter if your driving and floor it or off the line and floor it.
I checked the timing, dwell, plug gap and advance seems to hold. It's a 71 302 with a 2bl carb. HELP!!! lol
10-06-2013, 10:28 AM
#2
Moderator
Join Date: Dec 2005
Location: Hills of California
Posts: 5,208
It sounds like it could be running out of fuel.
If you floor it from a rolling start does it run great for 3 seconds then fall on its face or does it backfire and run poor the very instant the pedal goes to the floor?
What fuel pump are you using and what carb?
If you floor it from a rolling start does it run great for 3 seconds then fall on its face or does it backfire and run poor the very instant the pedal goes to the floor?
What fuel pump are you using and what carb?
10-06-2013, 10:38 AM
#3
1st Gear Member
Thread Starter
Join Date: Nov 2012
Location: Wisconsin
Posts: 55
Thanks for the response.
It happens immediately anytime the throttle is past 3/4. Carb tag is gone, but rebuilder said it was a factory ford 2bbl from a 71 302. pump is mechanical and I believe stock.
Before I had it rebuilt, the car would smoke like hell and go fine at WOT. It just wouldn't run at a steady throttle or start worth a ......
It happens immediately anytime the throttle is past 3/4. Carb tag is gone, but rebuilder said it was a factory ford 2bbl from a 71 302. pump is mechanical and I believe stock.
Before I had it rebuilt, the car would smoke like hell and go fine at WOT. It just wouldn't run at a steady throttle or start worth a ......
10-06-2013, 07:01 PM
#4
Moderator
Join Date: Dec 2005
Location: Hills of California
Posts: 5,208
It sounds like its leaning out.
If past 3/4 throttle and you keep your foot into it was happens? Is it blowing black smoke out the back?
Does it clear up after a bit?
If you roll the throttle on nice a slow does it run perfect and then go to hell instantly at 3/4 throttle or does it get progressively worse as you approach 3/4 throttle?
If you roll on power fast to WOT and keep it pinned what happens? Black smoke?
If past 3/4 throttle and you keep your foot into it was happens? Is it blowing black smoke out the back?
Does it clear up after a bit?
If you roll the throttle on nice a slow does it run perfect and then go to hell instantly at 3/4 throttle or does it get progressively worse as you approach 3/4 throttle?
If you roll on power fast to WOT and keep it pinned what happens? Black smoke?
10-06-2013, 09:14 PM
#5
1st Gear Member
Thread Starter
Join Date: Nov 2012
Location: Wisconsin
Posts: 55
Thanks Gun for your time and nice gun by the way...
It's pretty much anytime it's 3/4 throttle or past any which way you do it. I don't know if it would "work through it" or not. No real progression, either it's fine or not. I got out of it as soon as it started backfiring. As far as smoke, I don't know as I only drove it the one time and it was at night.
Question I have is could this in anyway be related to an electrical gremlin. The electric is totally rigged by the previous owner. Also noticed when I turned the headlights on, the car stumbled at an idle.
It's pretty much anytime it's 3/4 throttle or past any which way you do it. I don't know if it would "work through it" or not. No real progression, either it's fine or not. I got out of it as soon as it started backfiring. As far as smoke, I don't know as I only drove it the one time and it was at night.
Question I have is could this in anyway be related to an electrical gremlin. The electric is totally rigged by the previous owner. Also noticed when I turned the headlights on, the car stumbled at an idle.
10-06-2013, 09:34 PM
#6
Moderator
Join Date: Dec 2005
Location: Hills of California
Posts: 5,208
It could be electrical...
For a carb to run cherry at 70% throttle and then fall on its face straight away at 75% throttle isn't a 'tuning" issue as far as carb adjustments are concerned.
There is a significant issue at hand that's causing this misfire but I have no real idea what it could be.
Its highly suspicious that head lights cause it to stumble at idle. With the headlights on does it run okay below 75% power or does the issue occur sooner?
Could be weak power to the coil....Lets say maybe the coil power checks out (I actually dont know what it should read) but one of the wires is bad and at voltage meter level it can flow enough current to prevent voltage sag so meters says its good. But as load increase with higher RPM (Coil needs to charged quicker) V sag increases and it cant charge the coil.
What about high revs in N does it still spin nice at 4 or 4.5k? what about in 1st gear can you spin it up to 4500 and it still runs fine?
For a carb to run cherry at 70% throttle and then fall on its face straight away at 75% throttle isn't a 'tuning" issue as far as carb adjustments are concerned.
There is a significant issue at hand that's causing this misfire but I have no real idea what it could be.
Its highly suspicious that head lights cause it to stumble at idle. With the headlights on does it run okay below 75% power or does the issue occur sooner?
Could be weak power to the coil....Lets say maybe the coil power checks out (I actually dont know what it should read) but one of the wires is bad and at voltage meter level it can flow enough current to prevent voltage sag so meters says its good. But as load increase with higher RPM (Coil needs to charged quicker) V sag increases and it cant charge the coil.
What about high revs in N does it still spin nice at 4 or 4.5k? what about in 1st gear can you spin it up to 4500 and it still runs fine?
10-06-2013, 10:10 PM
#7
1st Gear Member
Thread Starter
Join Date: Nov 2012
Location: Wisconsin
Posts: 55
It's definitely a head scratcher! While my Father and I were adjusting the carb, it ran great. Put it in gear, warmed up, reved up, everything. Then I went to pull it out for a test and had it idling in gear. Then I switched the headlights on and immediately it stumbled and ran for a few seconds before dying. Started right back up and did the same thing once I turned the headlights on.
I gassed it and went for a cruise. Ran good, until ya get to 75% throttle. Didn't matter what gear your in, how fast you were going or if you rolled in or stabbed it. 75% and she backfired like hell!
We had originally thought coil too. But when I tried from a dead stop and stabbed it, it did the same thing at low rpm's.
This is definitely a PITA... I figured between myself (growing up in the EFI era) and my Father (carb/points guy) we could figure it out.
I gassed it and went for a cruise. Ran good, until ya get to 75% throttle. Didn't matter what gear your in, how fast you were going or if you rolled in or stabbed it. 75% and she backfired like hell!
We had originally thought coil too. But when I tried from a dead stop and stabbed it, it did the same thing at low rpm's.
This is definitely a PITA... I figured between myself (growing up in the EFI era) and my Father (carb/points guy) we could figure it out.
10-06-2013, 10:54 PM
#8
Moderator
Join Date: Dec 2005
Location: Hills of California
Posts: 5,208
So you're sure it would run great in N at 4000+ rpm OR 4000+ RPM in 1st (say 50% throttle) ??
Low RPM from a dead stop and then stab it would be a high load situation with a rich fuel mix and initially low RPM ...could still be weak spark
I would figure out how to jump past all the old wiring for the coil and run it direct off of a temporary test harness. Someone else will have to help out here because I dont know if that system runs off of 12v or something like 6 or 8v via a resistor.
Being able to bypass all the old system and verify solid power to the coil would be the ave that I would work hard at exploring.
So check this out
http://averagejoerestoration.com/res...rams/1965c.jpg
It says
wire 16 is the + coil side it run to a multi plug and becomes 16a which runs to a resistor and finally to the ignition switch.
So my guess would be its an 8v system? How much V do you see with the ignition on? Test from + coil wire to battery ground ground.
The trick would be to run + coil wire to an 8v (or whatever) source and bypass all that mess and see if it runs right (say a jumper wire with correct resistor to + battery). Also you'll want to check the - coil wire which should run to the points and make sure its in good shape.
This is apparently correct as far as I can tell from that drawing. Hopefully someone who's actually done this can help.
I think im on to something though.
-Gun
Low RPM from a dead stop and then stab it would be a high load situation with a rich fuel mix and initially low RPM ...could still be weak spark
I would figure out how to jump past all the old wiring for the coil and run it direct off of a temporary test harness. Someone else will have to help out here because I dont know if that system runs off of 12v or something like 6 or 8v via a resistor.
Being able to bypass all the old system and verify solid power to the coil would be the ave that I would work hard at exploring.
So check this out
http://averagejoerestoration.com/res...rams/1965c.jpg
It says
wire 16 is the + coil side it run to a multi plug and becomes 16a which runs to a resistor and finally to the ignition switch.
So my guess would be its an 8v system? How much V do you see with the ignition on? Test from + coil wire to battery ground ground.
The trick would be to run + coil wire to an 8v (or whatever) source and bypass all that mess and see if it runs right (say a jumper wire with correct resistor to + battery). Also you'll want to check the - coil wire which should run to the points and make sure its in good shape.
This is apparently correct as far as I can tell from that drawing. Hopefully someone who's actually done this can help.
I think im on to something though.
-Gun
10-06-2013, 11:07 PM
#9
1st Gear Member
Thread Starter
Join Date: Nov 2012
Location: Wisconsin
Posts: 55
Thanks for the reply again. Its just weird that this didnt happen before the rebuild. I will have to test voltage and look at my wiring diagram as well as test drive again. Might be a bit as I have to leave for work tomorrow. But it gives me a place to start when I get back.
10-06-2013, 11:27 PM
#10
Moderator
Join Date: Dec 2005
Location: Hills of California
Posts: 5,208
Well if its idling fine no problem and you turn on the headlights and it falls on its face and dies and it does that each time you turn on the headlights...That is pretty hard to ignore even if you try to tell yourself "its just because the idle is a tad low and the extra load on the alternator lugs the engine down and causes it to drop low enough to die I'll just tune that out with a idle mix adjust and higher curb idle speed"....yep keep telling yourself that.
Good luck! Isolating the issue should be pretty easy there are only a few things it could be. I would start with trying to bypass as much of the ignition system as possible at least then you can say its NOT a coil power issue.
-Gun
Good luck! Isolating the issue should be pretty easy there are only a few things it could be. I would start with trying to bypass as much of the ignition system as possible at least then you can say its NOT a coil power issue.
-Gun