My recent purchased 1967 cougar with a 289 seems to stall when I apply heavy throttle. I assume this is probably carb related but I thought I would ask anyway. At reasonable application of the gas it runs fine. Just when I really hammer it (not that I do that other then testing purposes). I don’t want to start messing with the carb if there are other likely culprits.
Thanks in advance for any opinions.
I had the same issue, for mine, it turned out to be running lean, and was leaning out to the point where it couldn’t stay running with the butterflies open. My fix was spark plug gap, it was open too far (not sure how I screwed that up) but after closing the gap on the plugs, no more problems. First thing to do is start yanking spark plugs and see what you find there, that will tell you a lot.
I did a little more trial and error. So in nuetral when I hit the gas hard it monentarily tries to stall and then revs beautifully. I was going to change the plugs and leads anyway since I don’t know how old they are.
Definitely sounds like a lean tip in. You didn’t say what carb you’re running, or what ignition setup. Like Pat said, the condition, color, gap, and heat range all can make a difference. I would inspect the plugs, wires, etc. I would also double check my ignition timing, as well as your total advance (initial timing added to your mechanical advance for total). Checking this is a great way to make sure that your mechanical advance is actually working.
If you’re satisfied with the above, then look at your accelerator pump shot, as well as jetting. You might also be sure you’re not sucking air around the intake and carb base.
My 289 did this with the old carb when I first got the car and found more than one carb port not covered and sucking air.
I have a similar problem with a 68 302 2v where even normal acceleration from a stop or from a left/right turn will stall it unless you first back off the accelerator and then slowly reapply pressure. I’ve tried plugs, timing and distributor but the carburetor may be the culprit. (Now if I can just get the guy at the repair shop who knows much about them to stop watching videos on his computer and actually work on the car.)
What about the accelerator pump? If it is shot this might happen
Before you delve deep into carb issues, I would pull the spark plugs first. If the ignition system has been changed to electronic, then the plug gap may not have been adjusted to compensate for that. Adjust and/ or check the ignition timing, then watch it change when the vacuum can is connected and also when the engine speed increases, as people have previously stated. For different carbs that are fitted, you will approach this issue in different ways…but the timing issue will be virtually the same for most applications. Mine runs a heck of a lot better with 12 degrees than it did with 4…which is what it was set at when I got it 
Plugs first, I sure wish I had looked into the plugs before I had started messing with the carb. Would have saved me a lot of headache.
Wow! thanks everyone for all the input. Im definitely going to delve into the plugs then since I know for a fact it was changed to an electronic ignition and when I pulled a plug wire the body of the plugs have rusted a bit… probably a sign they weren’t changed any time recently. I’ll post what I find out. I already changed the accelerator pump when I got the car as it was spewing gas everywhere.
When I first installed the four barrel on mine, I had the same issue. First problem was too much advance and too quick. This cured quite a bit, but still had a lean tip in. Different accelerator pump cam for quicker initial as well as longer total shot helped, but ended up jetting up one size. Keep in mind this was tuning new parts.
After going out and confirming, I am at 10 degrees initial timing BTDC, and have another 26 degrees of centrifugal advance for a total of 36 degrees total. All done by 2800 rpms.
If it’s the factory carb, you could try a different hole in the pump arm to quicken up your pump shot.
Quick note on the spark plugs… I changed them all and they are all very carbon crusted. Does this mean too rich? I’m wondering if my choke is stuck on or something. Is that possible? Could that cause my issue?
Carbon crusted…what color? Were they black? Oily? dry and white?
A misadjusted choke will cause a stumbling effect. Check and see what your pull off is set to. Do you have a factory service manual? Here’s a link with some color pics of plugs with different conditions to show the cause/effect. This might help a little.
http://www.hotrod.com/articles/mopp-0612-reading-spark-plugs/#mopp_0612_5_z-reading_spark_plugs-detonation There are 9 different photos.
The closest photo is number 5… very black. Maybe even worse then the photo. Haven’t got a manual yet. They are ridiculously hard to come by in Canada. And I have no idea what “pull” is. Sorry this is all a learning situation for me. But that’s half the fun right?
You can get both the paper and DVD rom factory service manual from Amazon.com
I checked the timing and it was alright I think. Was at about 8 degrees. Factory is 6 right? But with the electronic ignition it seems like a bit more is better. Still getting the same issue with the stalling.
Sounds like the check ball or check valve for the accelerator pump is not working. When you look down the throat of he carb and hit the throttle do you see gas squirting in two strong jets? If not that is where I would look first.
You don’t have to change the plug gap with an electronic ignition, you CAN change the gap. It takes more voltage “pressure” to jump a wider gap. The coil controls how much voltage is available, not the ignition switching device. So if you have a Pertronix or Dura Spark and a stock coil you are not getting any additional voltage…
I never really thought about this before as being the same issue but it might be. It can be hard to start. After it sits for a week or so I usually have to drop a little gas into the carb to get it going. Then it starts ok. I’ll try looking into the carb tomorrow when it’s running and report my findings.
Black plugs tell me that you are getting plenty of fuel. More precisely, too much fuel, too little spark, or too late of a spark.
I think Bill mentioned timing and advance, and for not then I will. On his big blocks he runs 12 degrees initial and 26 degrees mechanical advance, for a total of 38 degrees total advance and then he tunes his vacuum advance to not spark knock.also his mechanical us all done by 2500 rpm.
On my 289 (XE 256 comp cam, performer intake, summit 600dual feed carb, 1 5/8" full length headers, pertronix flamethrower III distributor) I am running 10 degrees initial timing, and 26 degrees mechanical for 36 total all done by 2800 rpms. Between the crappy gas and my being on the conservative side as far as tuning, this particular combo just runs better with those timing specs.
I would start at 8 degrees and advance it a little at a time, but 10-12 would probably run well. My car runs like a turd at 6. And pull off is the adjustment for how quickly the car comes off fast idle and the choke opens.