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Discussion Starter · #1 ·
Hey all, I've got an issue I've been working on a long time in the free time that I get, inbetween other small jobs on the project. I've got a 1970 Cougar, with a 351C. I had some pistons made for it, a custom cam ground, had it bored .020 over, and topped with the stock 2V heads, edelbrock rpm airgap, and a quickfuel 680, with an MSD Ready to Run distributor in it. I swapped my FMX for an AOD, and have the stock torque converter, stock ford 9 inch, with 3.00 gears. Most of the decisions were from an engine builder who got the pistons, and ground the cam. He suggested I cut the heads down a small bit, to 72cc chambers, to work better with the cam specs he had, and the pistons have a slight dome of 2cc I believe.

Right now, the main issue I have is that in 2nd gear (and I've not been able to test much above that), as soon as it shifts from 1st to 2nd, the engine stumbles real hard, and will sometimes pop out of the carb, and takes a while to recover, and then it seems to be fairly unresponsive in 2nd. This is only under light to medium throttle, I've not been able to test WOT in 2nd, but it's very responsive if you floor it in 1st. I've had the TV pressure set, and it seems to shift in the correct spots from what I've read of the AOD.

For the carb, It started with 70 jets in the primary, and 74 in the secondary. I've swapped those around richer, and leaner, and none seemed to really help it much, although it seemed that it was running rich since most of my plugs were darker than they should be. I had it at 66 and 70 up until a few days ago, now its back to the stock setting since I figured I'd restart. I've tried changing out the power valve, higher and lower, didn't seem to change much, so I stayed at the 6.5 it has in it now. I've not changed any of the accelerator pump things, as it doesn't have a bog off idle, it's only after the shift. I recently looked into the transfer slot size, and I got the square that it mentions, and opened the secondaries a bit since the slot wasn't exposed at all on those. I've gotten it to idle fine just with adjustments to the secondary, with about 14" of vacuum in park. In gear it drops to about 10-11".

I've done a lot of combinations with the distributor, changed the advance bushings and springs, higher initial with less centrifugal, vice versa, full manifold vacuum advance, ported, and currenlty I've got the vacuum advance disconnected. I've currently got it on about 16 initial, with the 21 degree advance bushing, and 2 blue springs, so all in at 37 degrees at 3000 rpm.

I've replaced the intake manifold gaskets twice, since I saw that my 4 and 7 were black and seemed to have a leak and were pulling in oil vapor, I've sprayed around and not found any leaks since. All the spark plugs have been swapped and checked for spark, no change.

I went on a test run recently with a tach and vacuum gauge, and in 1st under very light throttle it had around 10", and around 1500 rpm/17mph, it shifts to 2nd and the rpm drops to about 1000rpm, and the vacuum plummets to 5" and takes a while to recover. I'm out of ideas at this point and I've gotten help from others on forums, but so far to no avail.

I'm not particularly informed on the relation between engine load, gearing, rpm and vacuum, but from what I can assume, it seems like the shift is causing an increase in engine load, and just dropping the vacuum and causing the stumble, since it gets a bit better once it recovers. So I was wondering if I swapped out the rear gears to a 3.50 or 3.70, perhaps the increased rpm before the shift would help keep the engine in a higher rpm range, which is perhaps what it needs.

I'm up to any suggestions, kinda out of ideas so anything would be greatly appreciated. If there's any other info I can give out I'll try to dig it up, I've got the cam card below here.
Font Material property Paper Number Paper product
 

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Air quality is a factor this time of the year. Outside temperature, humidity, etc.
where do you live?

couple things pop up right away in your thread.
Air gap intake. Not a good intake for normal use.
Msd distributor. Poor quality stuff these days.
 

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1949 Ford Coupe RESURRECTION
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I have an Edelbrock air gap on "The Judge" (383) and MSD ignition module in my HEI with an Edelbrock carb and I've got zero issues with performance, regardless of the weather. Don't believe old wives tales...

It looks and sounds like you've done a lot of analysis but I don't see where you have put an air / fuel gauge in the mix. I would suspect you're not feeding enough fuel when that vacuum drops at moderate RPMs and load and it's starving out during a shift.

Gears and converter have nothing to do with it.
 

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That’s a lot of cam; I‘d say you have neither the final rear axle ratio nor sufficient stall on the converter. The engine on the 1 to 2 gear change is dropping too many RPMs. The load on the crankshaft is going up because mechanical advantage is lost as the gears go up toward high. This drops RPM which then moves the engine lower on the power curve, drops ignition advance further exacerbating the loss of mechanical gearing advantage with a power drop your compensation is to add more throttle which in turn causes a velocity drop in the carb’s venturies that messes up metering.

You need either a much milder cam, or get the engine revs up. To the latter end a converter over 3000 rpm stall and a rear axle depending on street use probably like 3.89 to 4.11 or if you wanna race something around 4.56.

Bogie
 

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That’s a lot of cam; I‘d say you have neither the final rear axle ratio nor sufficient stall on the converter.

You need either a much milder cam, or get the engine revs up. To the latter end a converter over 3000 rpm stall and a rear axle depending on street use probably like 3.89 to 4.11 or if you wanna race something around 4.56.

Bogie
Thank you Bogie
 

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1949 Ford Coupe RESURRECTION
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Amateur hour.
For what it's worth I'm running a jaguar xjs 2.88 rear end with a 700 r4 and a fairly radical cam and when I lock up in second third and fourth regardless of the throttle position some of those shifts bring the RPM down to anywhere from 900 to 1100 RPM and although it lugs down I never get a backfire or loss of power. What I posted does not conflict with anything that bogie said.

My 700R4 drops the RPM by a factor of 0.533, and the AOD drops RPM by a factor of 0.6125 from first to second gear. There are thousands of cars out there with this same setup that don't have the problem that op described. Bogey is correct but it points to a problem with his carburetor. Backfiring is a common symptom of a very lean mixture. And no I'm not an amateur. That just shows what you are to everybody else on this forum.
 

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Discussion Starter · #9 ·
Thanks for all the replies, I'm glad to get a few other opinions on it, and some confirmation of a few things I'd thought.

RWENUTS, I'm in Southern California, and while definitely not my favorite place, I've test run it over the past year since I got it running and it's had this problem year round. In between I'd been fixing brakes, electrical, fan and A/C, and power steering, so I've gotten to test it a lot and this problem has been on the backburner while I fixed small stuff, but neither temperature or weather seemed to have an effect. As for the Air gap and MSD, I know a lot of guys with cougars who run them, and I hadn't had a problem with it so far, but I'm a novice when it comes to airflow and it seemed like a safe bet, and pretty much same with the distributor, easy to adjust, and I've had no issues so far.

'49 Ford Coupe, I do wish I had an A/F gauge, if I didn't have the gear idea and I was more convinced it was the carb still I'd probably be goin out to get one. I thought that the lack of fuel could be the case there, and even though the vacuum was already dropping below the threshold of the power valve I had in there, I raised it to an 8.5 once to see if I could get more fuel in there faster, but didn't help much. I'd also tried to let off the throttle, let it recover, and then give it more gas and hope that the accelerator pump would give it the extra gas that it needed but it would still be stumbling and popping at that low rpm. If the rear gear doesn't fix it, I'll probably look into a gauge to really dial it in.

Bogie and bigmac, I'm not well versed in gears, and I had plans to put a trutrac and shorter gears in there since it still has the A/C 3.00 ratio gears in the rear, so it's good to hear that could possibly fix this problem, as it was already coming down the pipeline. The torque converter I was never too sure of, they are also not my strongest subject, however I had considered it a while ago, I just never read anything that jumped out and said "yeah that's your problem". As for the cam, this is my first build and a lot of the specs and suggestions were from a guy who builds these quite a bit, and after looking at the specs and talking to the machinist who worked on the engine, I did notice that the lift is pretty up there, where as the duration, and LSA seemed pretty standard stuff. I have no real intention of drag racing it, maybe a few quick romps every once in a while on an empty street but it's just intended to be a fun daily, hence the AOD and fixing the A/C. I'll look into gears around 3.70 or higher, since those were already on the list, and if those don't fix it completely I'll look into a torque converter.

Thanks for the more in depth explanation as well Bogie, those are exactly my thoughts once I started piecing together some of the info I gathered, I just have very little experience on that so it's good to see that my assumptions were on the right track.
 

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1949 Ford Coupe RESURRECTION
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For what it's worth, an A/F gauge might be a lot less trouble to install than changing out gears, and even after you change out the gears or torque converter or cam or whatever you do you still won't know what your carb is doing without a gauge. And reading the plugs isn't going to tell you what's happening when you hit second gear. I'll shut up now.
 

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Discussion Starter · #12 ·
Installing gears is something I already had planned, I just was hoping to fix this issue before I'd do that, since I originally thought they were unrelated. I probably will still end up getting an A/F gauge since it can't hurt to have, and would help since I've learned how to do a lot on this carb, but obviously I'm not sure of what is best since the problem was possibly unrelated to the carb, so whenever I'd do a fix that I thought would work I never got any results. That bit about the lean condition is interesting, and I probably have seen evidence of that, as sometimes I've seen that the bottom of the plenum has been wet with fuel. I've also noticed that the problem gets slightly better as the car warms up, it definitely is still down on power and stumbles, however once the engine has warmed up, it won't pop out of the carb or die. I hadn't considered the possibility of a plenum being too cold, especially out here in southern california since it's always pretty warm, but I'll keep that in mind. I would imagine that would cause a problem in all gears and at all times, not just in 2nd, but perhaps it's a factor contributing to the problem along with the gearing, and it's most noticeable when the shift is happening.
 

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I don’t know anything about the Ford carburetors, but on a Quadrajet it’s important to check the transfer slot exposure at idle. When you turn the carb upside down you will see the throttle bore at curb idle has a narrow transfer slot in it that is partially covered by the throttle plate. This slot allows more fuel to be pulled in from the idle circuit at light throttle, which can help resolve a light throttle stumble.

The slot should look approximately square when the plates return to idle. If you have been playing around with idle speed and mixture, you might have the transfer slot exposure way off. Set the idle speed screw so you have the right transfer slot exposure, and then tune mixture screws for best idle after it’s mounted on the vehicle. I also have a wideband AFR gauge, and have found it works best for my engine to be a little rich at idle (about 12-12.5). In cold weather I may still get a slight stumble right after the choke comes off, if the engine is not really warm yet.
 

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Discussion Starter · #16 ·
I recently did the transfer slot adjustment, it was a bit overexposed before, but I got it to just a square and then adjusted the secondaries to get the idle speed correct, as adjusting the primary idle speed screw would have exposed the transfer slots more. Unfortunately it did nothing to really fix the problem, however I did leave the primary and secondary idle screws in those positions since it seems to be what most people do.

Currently working on getting the rear gear changed out, I was looking forward to it anyway so I may as well get to it. Just got the axles out today, hopefully the center out when I've got another set of hands to help out, as I've never messed with a differential before.
 

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Quick Fuel is generally too rich right out of the box, and if your ignition is not strong enough to light the fire on an overly rich mix the plugs will be black. The puke thu the carb says ignition in most cases. Plug gap no more than .032” - .035” to insure spark under compression without a CD ignition set up Is necessary to insure it fires off. Understand older production muscle cars had less CFM carb’s because the points ignitions were not up to the task of burning more fuel. Maybe an exception would be the Chevy transistor ignition. But a well tuned HEI should be adequate.

I’ve had best results leaning out almost all circuits in the Quick Fuel while ignition with HEI that’s been hopped up a bit, now it performs nice. The transition slot & IFR also needs restriction as compared to vintage Holley’s size passage. You will notice better idle screws (4 corner) sensitivity as adjusted & longer effective idle circuit as you drive. Understand this is for a street application, not track.
 

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I don't think it's the cam or intake either. I mean, it's easier to adjust the carb rather than work backwards and adjust the engine so the carb works better.
My guess is it's going lean on the idle to mid range fuel curve. When it shifts, RPM lowers, the load goes up, the vacuum signal of the boosters is lessened and the boosters stop flowing but the idle circuits aren't enough to cover the hole.
To cover that up, either enrichen the idle OR Midrange or most likely both but if you do that, it's going to be rich on both of those.
Imma say it needs some metering block and air bleed work to be more responsive to midrange.
You can try raising the floats so the boosters pull earlier but I can't say that's going to fix it all.
You can also use a paper clip or small wire and plug the air bleeds. If it's better or worse, you'll know what direction you gotta go in.
 

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You could try smaller idle air bleeds to extend the idle circuit a few hundred RPM’s. But a good ignition fire is necessary to light the richer fire. It’s not just lean conditions that benefit from HEI. But honestly Quick Fuel‘s are not set up lean out of the box. Quite the opposite. Most thought to be carburetor problems are ignition.
 

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FWIW, Clevelands like to operate warm. I think the stock thermostat is 190-195*, AND, it is a special Robertshaw type thermostat to work correctly. Those are getting very hard to find. (this relates to the vaporization comments)
 
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