Carb popping above 50mph when accelerating.-

[Joseph Knopf]

Hello Vintage auto owners,i have a 1972 Mustang hardtop with stock 62K mile 302,after market Eldelbrock performer intake and new Holley 600 Street Warrior carb.-I also removed the stock single exhaust from the car this past fall and installed long tube headers from Flowtech with the Purple Hornies header mufflers.-The vehicle runs very well /idles fine but when you drive it down the road and hit the gas above 50mph or below the carb pops which i believe might be a timing issue and too much advance.-I performed a full tune-up on car when i rescued the vehicle 2 years ago-Looking to restore the car back to better that staock with maybe adding a set of 351 2V Cleveland heads i had cleaned up @ the local machine here in Newcastle,De. -And freshen-up the engine of course like a street Boss 302.-

-But for now just want to get the engine running correctly with the right timing and no more popping when accelerating.-Thanks for the help- I only put about 100 miles on the car since buying it 2 years ago.-Fresh oil/filter again about 2 months ago.-

 

[Zach]

Sounds like too much timing. I believe you tap into vacuum advance once you get to highway speeds and that could be pushing you over. If you don’t have any vacuum leaks, 90% of carb issues tend to be your timing. Make sure it’s dialed in with a light, by red you might add too much initial advance. Vac cans can also be out of adjustment or not fit for some applications depending on if it’s stock or not.

 

[giantpune]

Could also be a fuel issue. Too much timing can cause backfiring. So can running lean.

I would check what the max vacuum advance is. Some of them are adjustable and right out of the box have a stupid amount of total vacuum advance. I checked mine with a mighty vac and a timing light. with the engine running, I was able to connect the vacuum pump to the advance and pump it up to get an insane amount of advance. Then put an allen key in the hole and adjust and check it again until I got to a more realistic value. That made my backfiring issue go away.

 

[71coop]

Just my experience, for what it's worth - It's not uncommon for an an engine to run lean after installing headers - you are effectively increasing the capacity for air through the system. I agree with post above, verify timing first. If that is OK, then try investigating your transition from cruise to power. You may need to tune the jetting, or a power valve that open sooner, or it could be as simple as a different accelerator pump cam or squirter.

Plugs might tell you something if they are light-colored, but if the idle is rich maybe not. Getting AFR right is harder now that almost all readily available pump gas has about 10% ethanol in it too - forces you to tune slightly richer. (stoic is 14.1 vs 14.7).

I recently went through this and found that Holley's stock jetting was "close enough" but I benefitted from tuning the transition from cruise to power circuits to be richer. For me it was both power valve and pump cam changes. I'm sure you know this, but every engine is a little different, so a little experimentation is usually needed. If

Do you have access to an O2 sensor/wideband to monitor AF mixture? That made it easy for me to see how the transition was behaving. If not, even a vacuum gauge can help give a place to start (monitoring cruise vacuum and how quickly it drops when you accelerate can help you judge whether or not the power valve needs to be changed out). Pay close attention to when the popping occurs too - is it immediately upon punching it, or is it after the pump has had time to add fuel and the squirt volume is trailing off? Just make one change at a time and evaluate/adjust.

 

[Joseph Knopf]

Sounds like too much timing. I believe you tap into vacuum advance once you get to highway speeds and that could be pushing you over. If you don’t have any vacuum leaks, 90% of carb issues tend to be your timing. Make sure it’s dialed in with a light, by red you might add too much initial advance. Vac cans can also be out of adjustment or not fit for some applications depending on if it’s stock or not.

Thanks very much for the helpful info/fast reply Zach and i willl adjust the timing when i have a day off this week.-Kindest regards,-Joe K.-

 

[Zach]

Thanks very much for the helpful info/fast reply Zach and i willl adjust the timing when i have a day off this week.-Kindest regards,-Joe K.-

Remember to disconnect vac line and plug the vacuum advance when setting initial. I am unsure how stock mustangs run vac but there is a difference between ported and manifold I normally run ported and is recommended on my Pontiac. I’d recommend looking into that or someone else might bring it up here. Good luck.

 

[Joseph Knopf]

Just my experience, for what it's worth - It's not uncommon for an an engine to run lean after installing headers - you are effectively increasing the capacity for air through the system. I agree with post above, verify timing first. If that is OK, then try investigating your transition from cruise to power. You may need to tune the jetting, or a power valve that open sooner, or it could be as simple as a different accelerator pump cam or squirter.

Plugs might tell you something if they are light-colored, but if the idle is rich maybe not. Getting AFR right is harder now that almost all readily available pump gas has about 10% ethanol in it too - forces you to tune slightly richer. (stoic is 14.1 vs 14.7).

I recently went through this and found that Holley's stock jetting was "close enough" but I benefitted from tuning the transition from cruise to power circuits to be richer. For me it was both power valve and pump cam changes. I'm sure you know this, but every engine is a little different, so a little experimentation is usually needed. If

Do you have access to an O2 sensor/wideband to monitor AF mixture? That made it easy for me to see how the transition was behaving. If not, even a vacuum gauge can help give a place to start (monitoring cruise vacuum and how quickly it drops when you accelerate can help you judge whether or not the power valve needs to be changed out). Pay close attention to when the popping occurs too - is it immediately upon punching it, or is it after the pump has had time to add fuel and the squirt volume is trailing off? Just make one change at a time and evaluate/adjust.

Thanks for the helpful information,i do have a vacumn gauge and the popping occurs when you hit the throttle at really any speed,run fine with about 1/4 throttle just cruising up the road 30-40 mph until you punch it then the hesitation and stumbling,where the engine struggles to move.-I did adjust the timing a few degrees back yesterday morning when i arrived home from nightshift about 7am-The car does start up better and sounds much cleaner running.-When the rain stops here,maybe try the car tomorrow on a few mile drive up the road and see if the popping has stopped.-Have a safe and happy holiday everyone.-JK.-

 

[Stanglover]

I'm not sure if this will be any help as I only have experience with a 351C 4V, but back a while, I was having lots of problems with timing and carb issues. In short, I installed a new Holley 670 Street Avenger carb and taught myself to recurve the stock Autolite dist. to run with a Pertronix Ignitor II and matching Flamethrower coil, to run with 14 degrees of initial and all in at 34 deg. at 3000 rpm. Without going into too much detail, I still had this stumble when I hit the gas. I checked and rechecked everything, changed the PV, changed the cam a few times, but eventually swapped out the "squirter" from a #31 to a #35 and it all went away just like magic. Now, no matter what speed I'm driving, hit the gas and it pulls like a train.
What I'm saying is it could be as simple as that. Hope so and good luck.

 

[Joseph Knopf]

I'm not sure if this will be any help as I only have experience with a 351C 4V, but back a while, I was having lots of problems with timing and carb issues. In short, I installed a new Holley 670 Street Avenger carb and taught myself to recurve the stock Autolite dist. to run with a Pertronix Ignitor II and matching Flamethrower coil, to run with 14 degrees of initial and all in at 34 deg. at 3000 rpm. Without going into too much detail, I still had this stumble when I hit the gas. I checked and rechecked everything, changed the PV, changed the cam a few times, but eventually swapped out the "squirter" from a #31 to a #35 and it all went away just like magic. Now, no matter what speed I'm driving, hit the gas and it pulls like a train.
What I'm saying is it could be as simple as that. Hope so and good luck.

Thanks very much for the helpful information and will try to adjust the timing with a light,and get the vacumn meter again to see if i'm getting the most vacumn possible and look into the different size jets and other parts on the carb.-Best regards,-JK.-

 

[Stanglover]

Thanks very much for the helpful information and will try to adjust the timing with a light,and get the vacumn meter again to see if i'm getting the most vacumn possible and look into the different size jets and other parts on the carb.-Best regards,-JK.-

Jk, the 302 is slightly different with its needs for timing. Others can likely give a better idea of those requirements. That also may depend on what's been done to the motor, stock or otherwise. You've added a few trick items. Do you get any spark knock on acceleration? You may need to play with timing to find the sweet spot. I assume you have the stock distributor. Are you using points or an electronic upgrade?
As for swapping jets, I wouldn't rush into that just yet. You say "popping" and then you say "hesitation and stumble", which is worse? I think popping is a lean indicator and hesitation is a lack of fuel at the right time when you hit the gas peddle.
Again, I'm not the expert here, but do one thing at a time.

 

[Joseph Knopf]

Jk, the 302 is slightly different with its needs for timing. Others can likely give a better idea of those requirements. That also may depend on what's been done to the motor, stock or otherwise. You've added a few trick items. Do you get any spark knock on acceleration? You may need to play with timing to find the sweet spot. I assume you have the stock distributor. Are you using points or an electronic upgrade?
As for swapping jets, I wouldn't rush into that just yet. You say "popping" and then you say "hesitation and stumble", which is worse? I think popping is a lean indicator and hesitation is a lack of fuel at the right time when you hit the gas peddle.
Again, I'm not the expert here, but do one thing at a time.

Happy Thanksgiving,and the motor is stock other than the Edelbrock performer intake,Holley Street Warrior 600 Carb,The new Flowtech headers and accel super coil. Stock distributor with points.-I do want to swap out the Holley carb for a another new Carb, a Demon 750 carb that i just purchased which has mixed reviews from the auto mod restoration community. Looks like it may be easier to adjust,better fuel economy and better throttle response.-I also plan to replace the stock points distributor from 1972 with an electronic distributor to help with starting/more spark and make the vehicle more reliable.-

 

[Joseph Knopf]

Happy Thanksgiving,and the motor is stock other than the Edelbrock performer intake,Holley Street Warrior 600 Carb,The new Flowtech headers and accel super coil. Stock distributor with points.-I do want to swap out the Holley carb for a another new Carb, a Demon 750 carb that i just purchased which has mixed reviews from the auto mod restoration community. Looks like it may be easier to adjust,better fuel economy and better throttle response.-I also plan to replace the stock points distributor from 1972 with an electronic distributor to help with starting/more spark and make the vehicle more reliable.-

Getting back to the engine stopping/studdering when you press down on the gas petal,,,,,anytime after 1/4 throttle or when the secondaries kick in,the engine instead of accelerating,it just make a horrible studdering noise and does not move like the engine is starving for fuel or the timing is off.-Funny the engine perfromed better this past summer before the new Holley Street Warrior was installed in teh place of the rebuilt Holley 4150 i beleive it was with manual choke.-Only reason i bought a new carb,was to have an electric choke that i have yet to connect because of the required 12-volt source.-I did connect the supplied red positive wire from the choke to the "Stator" stud connector on the back of the alternator which from what i read worked fine for Ford and many other auto makers in the past 30 years or so.-I'm sure everyone will answer with find a 12 volt ignition switched-on source like an unsed slot on the fuse panel.-I just dropped the car off @ a local repair shop yesterday to get the turn signals,wipers working,fuel gauge and hopefully the heater/radio which probably has not worked since the 90's when the car was taken off the road.-

 

[Stanglover]

Happy Thanksgiving,and the motor is stock other than the Edelbrock performer intake,Holley Street Warrior 600 Carb,The new Flowtech headers and accel super coil. Stock distributor with points.-I do want to swap out the Holley carb for a another new Carb, a Demon 750 carb that i just purchased which has mixed reviews from the auto mod restoration community. Looks like it may be easier to adjust,better fuel economy and better throttle response.-I also plan to replace the stock points distributor from 1972 with an electronic distributor to help with starting/more spark and make the vehicle more reliable.-

Joseph, I stress, I'm not the expert here, let alone on a 302. But before you start throwing stuff and money at it hoping for better power and performance, I would start by figuring out what will work best for what YOU want out of the car. Ask advice from knowledgeable people, maybe at your local speed shop and not just where they want to sell you stuff.
Personally, I think a 750 is way too big for a stock 302. It's pushing it for a 351C 4V. I only have a 670 cfm on mine and the AFR is pretty much spot on. My engine is stockish with just a mild cam increase and I actually lowered the compression ratio to 10:1 to accommodate the crappy fuel we are stuck with. My carb did need a bit of surgery right out of the box as Holley's tend to run a bit rich. I'll not go into details, but it needed more air at idle to be able to set the throttle plates correctly.
As for a distributor, there are newer options out there so again do your homework before buying. I had to do a major rebuild on my stock Autolite dist. but for me it was worth it as I wanted to keep the original factory look under the hood. I'm not into chrome and shiny stuff, let alone stuff that does not belong there. Maybe consider a DuraSpark set up. It is a Ford dist. and there is a company, name escapes me, who build to your engine specs.
I may get criticism for my comments, it's your car, your way. All I'm saying is don't throw stuff at it hoping to make it run better when all you might need is a proper tune and minor tweaking.

 

[Joseph Knopf]

Joseph, I stress, I'm not the expert here, let alone on a 302. But before you start throwing stuff and money at it hoping for better power and performance, I would start by figuring out what will work best for what YOU want out of the car. Ask advice from knowledgeable people, maybe at your local speed shop and not just where they want to sell you stuff.
Personally, I think a 750 is way too big for a stock 302. It's pushing it for a 351C 4V. I only have a 670 cfm on mine and the AFR is pretty much spot on. My engine is stockish with just a mild cam increase and I actually lowered the compression ratio to 10:1 to accommodate the crappy fuel we are stuck with. My carb did need a bit of surgery right out of the box as Holley's tend to run a bit rich. I'll not go into details, but it needed more air at idle to be able to set the throttle plates correctly.
As for a distributor, there are newer options out there so again do your homework before buying. I had to do a major rebuild on my stock Autolite dist. but for me it was worth it as I wanted to keep the original factory look under the hood. I'm not into chrome and shiny stuff, let alone stuff that does not belong there.
I may get criticism for my comments, it's your car, your way. All I'm saying is don't throw stuff at it hoping to make it run better when all you might need is a proper tune and minor tweaking.

Golden words of wisdon indeed,will work on getting the current carb tuned in and engine running as it should.-

 

[Stanglover]

Golden words of wisdon indeed,will work on getting the current carb tuned in and engine running as it should.-

Thanks, just trying to help save you some money.

 

[Zach]

You said the choke isn’t hooked up yet, is it possible the secondaries are locked up and you are only running on the primary bores? Slamming on it cold/locked secondaries will cause it to aggressively stutter if that’s the case. Choke is pretty critical, at least until it is warmed up ALL the way. If your choke is all the way open all the time it is 100% a lean condition that you cannot tune out. You need a choke in 99% of cases in order to operate a cold engine.

 

[Zach]

Getting back to the engine stopping/studdering when you press down on the gas petal,,,,,anytime after 1/4 throttle or when the secondaries kick in,the engine instead of accelerating,it just make a horrible studdering noise and does not move like the engine is starving for fuel or the timing is off.-Funny the engine perfromed better this past summer before the new Holley Street Warrior was installed in teh place of the rebuilt Holley 4150 i beleive it was with manual choke.-Only reason i bought a new carb,was to have an electric choke that i have yet to connect because of the required 12-volt source.-I did connect the supplied red positive wire from the choke to the "Stator" stud connector on the back of the alternator which from what i read worked fine for Ford and many other auto makers in the past 30 years or so.-I'm sure everyone will answer with find a 12 volt ignition switched-on source like an unsed slot on the fuse panel.-I just dropped the car off @ a local repair shop yesterday to get the turn signals,wipers working,fuel gauge and hopefully the heater/radio which probably has not worked since the 90's when the car was taken off the road.-

Rereading this post not half asleep, gonna have to say the choke is the culprit here. I’m unsure how Holley does it but either your choke is completely closed or completely opening during all operation, which will either make it run rich (closed) or lean (open). The reason the first one worked better was cause you had a manual choke installed which allowed you to manipulate the choke instead of it either being completely on or off like your current situation. I’d get your choke figured out before going down the timing rabbit hole and buying parts.

 

[Stanglover]

Rereading this post not half asleep, gonna have to say the choke is the culprit here. I’m unsure how Holley does it but either your choke is completely closed or completely opening during all operation, which will either make it run rich (closed) or lean (open). The reason the first one worked better was cause you had a manual choke installed which allowed you to manipulate the choke instead of it either being completely on or off like your current situation. I’d get your choke figured out before going down the timing rabbit hole and buying parts.

I must have been half asleep too, I missed the choke bit. On a Holley electric choke, it will be closed cold and open hot and is adjustable depending on the climate you're in. Set in the middle is stock. If it's closed it will run like a bucket of bolts. Mine does anyway until it's warmed up fully.

 

[Zach]

I must have been half asleep too, I missed the choke bit. On a Holley electric choke, it will be closed cold and open hot and is adjustable depending on the climate you're in. Set in the middle is stock. If it's closed it will run like a bucket of bolts. Mine does anyway until it's warmed up fully.

My guess is his choke is permanently closed during operation if his choke isn’t hooked up, the spring in the housing cant heat up with the 12v source in order to open up the choke butterfly over time, although the radiant engine heat might very slowly heat it up. Meaning if he slams on the throttle he’s asking for air that doesn’t exist since the choke is closed. I’m sure his car starts up great when cold though lol.

If you look at your carb after a long drive / car is at operating temp and the choke is still closed that is your issue. You can always jam a scredriver down the primary to keep it open and drive it around to see how it feels, but it would be easier to just set up the choke.

 

[71coop]

A few things to call out & keep in mind - a 750 is a bit large for a 302. it can be tuned to work just fine, but Stanglover is right that its larger than necessary. When you put a larger-than-needed carb on a car, you can expect to adjust the fuel mixture.

With a carb, intake and header change, I'm sure you are lean. However - first definitely verify your timing (Stanglovers timing specs are a good guide), make sure your choke is open when warm.

The .pdf attached is a guide I used when learning my way around a Holley. It was written by Lars Grimsrud out of Colorado, and he is well known in corvette and muscle car circles for helping with both timing and carb tuning issues. He even has a Wikipedia entry, if you can believe that... It's a good easy-to-follow guide for anyone.

I think the stator should work OK with the choke, it'll just open slower, from what I've observed. Not a bad thing since I think Holley electric chokes open too fast anyway...