Well after 3 years I’ve finally got to drive my cougar around. 351C, AOD transmission, all new stock suspension and 67 disc brakes. I’ve been just driving it around the block a few times and making adjustments, and I was hoping to get some advice on getting it running right.
I’ve got a Quick Fuel 680 on it, and it’s worked great the whole time, however I suspect it’s the culprit here. Every time it shifts to 2nd, the engine stumbles pretty bad, can’t even touch the throttle or it gets worse and will backfire out of the carb. I thought that the secondaries were opening early and it was going lean when it would shift, so I richened the secondary air fuel screws and I adjusted the spring with the screw on the diaphragm. After that it was much better, definitely a little down on power after a shift but a major improvement.
Then today I adjusted my alignment a little bit and did a few things, nothing pertaining to the carb or timing, and now it’s doing the same thing. I adjusted the spring rate a little more and it was better, however I’m thinking now that it just might be the engine warming up thats fixing the issue.
I’ve got initial timing at ~14 BTDC, and total at ~36, and it idles and runs great in 1st gear, and if the engine catches up from the stumble it runs fine in 2nd, but theres a long time after the shift where it just hardly wants to run.
Best bet would be to run it on a chassis Dyno with a wide band air fuel meter. It could be timing or carburetor and you really don’t know which way to go until you know where you are.
Alright, I figured I’d be doing that at some point, I may have to tweak it before since it’s not currently registered to drive, just non-op right now, and I’d rather not tow it just for tuning purposes. I’ll probably see if it’s any better tomorrow to find out if it is the secondaries or just the engine not being warmed up, however it’s a big difference for just not being warmed up. Thanks.
I have the QR 680 (HR) carb also but on a 390. Your comment about warming up reminded me of an issue I am facing. Assuming electric choke. These newer carbs really wants a solid 12VDC. On mine, previous owners have installed two wires from the fuse box. Both however are in the 10V to 11V range when the battery is reading 12.7V. Sometimes they go to 0V for no obvious reason.
Are you running with choke wire to the stator of the alternator? It’s a long shot but when your engine goes to lower RPM from shifting 1st to 2nd the charging system might not be outputting enough to keep the choke open. Nothing spoils a good time like choking the carb. The stator only outputs 6-7V but AC. Could not get a reading on my volt meter using VDC but could using the VAC setting. Technically a RMS valve for the electrical savvy. I will run a dedicated relay to switch a solid 12V (10A fused) to the choke using the stator voltage to power the relay coil.
Yes I do have the electric choke, and it is hooked up to the stator post. I never would have considered that just in the time of a shift that the choke could close and mess with it but I’ll look into it today. I have a random wire that I couldn’t identify that has 12V in run so perhaps I could hook that up to my choke and see how it goes. Thanks.
I think there is a misunderstanding of how an electric choke works. The choke uses a bi-metalic heating strip. When voltage is applied it gets hot and unwinds a bit moving the choke butterfly. Higher voltage will move it a little quicker but it is intentionally slow moving and even at that it moves in small increments. As it gets how its resistance rises so it won’t burn up. This makes it self regulating. It is really easy to verify that the choke is not the problem. Just unplug it and rotate it so the choke is never set.
Just went around the block twice and the secondary adjustment, and the choke being wide open the whole time didn’t change it, I’ll check the timing and perhaps mess with that a little bit, or check the other springs for the distributor since the curve could be off. Not sure what springs come on a stock MSD but I’ll look and see what spare springs I’ve got.
Thanks for the clarification. I wonder though if the engine vacuum could slam it shut somehow under some weird condition? I don’t plan on taking any chances next season.
No, it can’t. You should re - adjust it every time the weather changes from hot to cold or from cold to hot. You want it to be barely closed when the car has been sitting overnight for best results.
I’ve checked my timing multiple times now, I thought I was at 14 BTDC but I must have had the dial incorrect a while ago and it says its at 22 now. I don’t believe my vacuum advance is turning on at all, I checked it with the line disconnected and plugged, and with it attached and both were the same, and I can take off the line and plug it and the idle does not change. I’ve got 12-13" for vacuum, very steady, if I advance it goes up a bit and if I retard the timing pretty much at all it drops to below 10".
I swapped the advance springs to the lighter blue ones and I believe that was incorrect, I get an odd noise now pretty much all times in 2nd gear, so I’ll go back to the heavier silver springs.
I’m a novice at this stuff as this is the first time I’ve gotten to drive this thing since I got it years ago, but just from reading around it seems that since my vacuum advance is not kicking in, the initial timing is having to be set higher, and since that is higher, at higher rpm it has too much total advance. I might change the advance bushing on it, it has the blue right now but I could change to purple or black for less mechanical advance.
I’ve got the choke adjusted so that it has just a little slit when it has sat overnight, southern california doesn’t get particularly cold anyway.
I’m far from a master at timing and I’ve always set it around 12-16 initial, with the vacuum advance in mind, so if I had it disconnected or in this case not engaging at all, is 22 too much? Thanks for all replies.
Either my timing light is messing with me or something, I swapped the advance bushing, now I have the black (18°) and I swapped 1 spring so I had the heavy silver and light blue, and somehow got 32° initial and 50° total.
Just now replaced the light blue for the heavy silver and now I’m 14° initial and 32° total. Going back to the original blue advance bushing would put me at 37° total, however I had the problem with that setup of bushing and springs so I’ll try this setup first.
P.S. Got it to run better, we’ll see if she holds till the morning. Definitely odd that the timing light gave me very different readings while the car ran very similarly in idle and at 3k on the tach, but I’ll just chalk it up to my own incompetence.
Take the distributor cap off. While looking at the advance plate, suck on the hose leading to the vacuum advance. You should see the plate move. If it doesn’t move the timing light is not your problem.
My vacuum advance does work, engages around 5 inches it seems, I did find out that I had my vacuum line on the wrong carb port, I had it on ‘Spark’ instead of the full manifold vacuum, so thats sorted out.
Took it around the block twice, going to wait a bit as it got very hot just from that, but it still stumbles once it shifts, not nearly as bad but enough to still get a backfire out of the carb. I’ve gotta get a better radiator hose since the one I’ve got is kinked, as my radiator has the outlet on the wrong side.
I’m around 16° initial, 34° with mechanical, steady 13" vacuum. Not sure what to mess with next but once it can cool itself better it may be time for the dyno to help tune it.
I had a similar issue when I completed the AOD swap I my car , once the car shifted into 3rd or 4th it would stumble at part throttle, I put a vacuum gauge on it and seen that the vacuum would drop to about 4 inches when the throttle was opened, I thought it might be the power valve (6.5) opening to soon, swapped it to a 3.5, had the same problem.
After seeing the result of that I figured I better start looking at the ignition, I found a cracked plug and some pretty good corrosion at the plug wires on the distributor cap, plus the majority of the wires on the plug side came apart in pieces as I checked them.
Installed new plugs, wires , cap/rotor and re installed the 6.5 power valve, stumble is gone car is running great.
I’ll look at my vacuum at throttle to see how low it goes, I’ve never swapped a power valve but I’ll look into it.
The only issue I could see with my plugs would be if there was some issue with signals being crossed over but all my plug wires are fairly separated, but they’re all new, all new distributor, I’ve even changed the terminals and boots on the ends since I had the wrong ones. I’ll take it around today and see if the lower timing helps, now that the vacuum advance is actually working.
Just as a side note, I also fixed my float bowl level as it was a little bit high, about 3/4 up the sight glass instead of half like the manual says. I also checked my fuel pressure, 7psi out of the pump, 5.5 where it splits to go to both fuel bowls. Not sure if that’s too low, however I’ve never seen the float bowls empty, it always stumbles at the exact same spot, and it’s hardly at quarter throttle. Thanks for the replies.
My interest in cars started with electronics that eventually turned into a career. Despite all that I found it tricky to make a good solid set of spark plug wires. If you can measure the resistance per length and confirm they are all good it may eliminate them as a source of trouble. My Ford Racing 9mm wires are spec’s to be 1000 Ohms per foot. Each brand may have a different spec.
I’ll check that out today, I did check continuity but not resistance, and I’d imagine it would be weird it would only give me an issue right after it shifts, but its good to know down the line.
Well, I didn’t get a chance to check out the resistance on the spark plug wires, took most of today trying to get alignment and brakes done, but I did take it around the block a few times and I’m still having the same issue. I richened up the air fuel mix, I had the primaries about 1.5 turns out and the secondaries 1 turn out originally, I went up to 2.5 turns out on the primary and 1.5 on the secondaries. Too rich at idle, but it seemed to help the shift, so I’m thinking I’m starving at the shift, however at no other point does it starve, even at WOT.
I was wondering how vacuum changes when the shifts happen? Is there a chance that at the shift, the vacuum is going above the power valve threshold and closing it off? I don’t have a way to check, at least not currently, but I’m going to try to get the things I need to read fuel pressure and vacuum at that shift point, and see what’s going on.
