Tuning Advice

Hey all, after dealing with A/C, fan, and cooling fan problems, I’m back up and attempting to drive again. I had a few topics before about my car stumbling when it shifts and whatnot, but I decided to start again and mess with everything just to go through the checklist again.

The biggest issue I’ve had, is that when the car shifts up, most noticeably from 1st to 2nd, under light to medium throttle, it stumbles. It’ll stumble until I give it some time, or if I try to power through the stumble, sometimes it’ll pop, and afterward it’ll get to normal much quicker. Overall, 2nd gear is fairly unresponsive to throttle, especially around 20-30mph.

I’ve got a 351C, 2V, with a Quickfuel 680 4 barrel, Edelbrock Airgap intake, custom cam with .610 intake, and .617 exhaust, 225/233, 113 LSA. The heads were cut a bit to get a smaller chamber size, and the cam and pistons were made by Brent Lykins; he also recommended the carb. I swapped out my FMX for an AOD, that also has a shift kit.

I’ve messed with the carb, I thought the pop from the carb was from being too lean and so I swapped the jets from 70’s and 74’s to 72’s and 76’s, but it didn’t do anything. I swapped back, and then checked the plugs and found that they seemed like it was running quite rich, less than 100 miles and the plugs were black. I fiddled around with it a lot, and now I’ve ended up with 66’s and 70’s, which may still be too rich for it since the exhaust smells fairly rich. I’ve got the fuel level set at barely the bottom of the sightglass, and I went with advice from holley on how to tune the idle mixture screws, I had it turned out a bit and slowly turned it in until the vacuum level and rpm deteriorated, then went a little bit back.

I’ve been everywhere with the distributor, MSD Ready to Run unit, #8350. I’ve used the heavy springs, light springs, blue, black, and now silver bushings. I’ve had the vacuum on full manifold, and ported, and now I’ve got the vacuum advance disconnected completely. The current setup I’ve got is the silver bushing (25 degrees) with heavy silver springs, initial timing is ~10-11 degrees. Before this I had the black bushing, with a ported vacuum line on the vacuum advance, and I noticed what I thought was pinging sometimes when I would shift, so I went to the heavy springs to slow the mechanical advance, thinking that they’d be too slow but they seemed fine, however I’ll probably swap the springs since those heavy springs put the silver bushing all in at just shy of 5000 rpm, quite a bit later than I think is needed.

I considered fuel starvation, however if I just floor it, and let 1st go as long as it wants, and then it shifts, it has no stumble or problems that I’ve noticed, and I can keep goin with around 6 psi the whole time. I’ve swapped multiple spark plugs in and out of it, all have come back with black soot on them, I haven’t checked these recent ones but they were run with a richer setup before so I’m sure they’re black too.

The only other thing I’ve thought of is the transmission, since it’s occurring during shifts and immediately after. I’ve only got the stock torque converter in there, and that shift kit worries me. I think the shop that did the rebuild is fine, I just wasn’t looking for a shift kit but theirs was the closest available rebuilt AOD. I would think that if it was the torque converter that there would be problems at light throttle and at WOT, but only at light throttle makes me think its just a tuning issue, however torque converters are not something I’m too familiar with. I’ve had the transmission line pressure checked and set, and it did fix a problem I had previously with shifting into 3rd, however it didn’t seem to affect this problem at all.

Any ideas or help would be appreciated, I’ve just run out of ideas since nothing I’ve done has had any major change, and since everytime I try to test things out, once the car warms up a good but the problem gets slightly better, and so I can’t tell if what I’ve done has had a positive change or not. I’ll see in the morning if the mechanical only timing will fix anything, but I am doubtful. Thanks for any help.

After driving it to work this morning, mechanical advance only was not the fix. Stalled 3 times, was much harder to drive, even after it warmed up. Since it’s worse without the vacuum advance on, I assume for the mechanical to be better I’d need more advance, sooner, so I’m gonna look into swapping out those heavy springs that I mentioned might be too slow.

Another thought I had, the engine is much happier when I floor it. I stalled at an intersection under light throttle, then cranked it over and once it fired I just shifted and floored it and it peeled out and ran great all the way to the next light. I also sometimes hold the pedal down to open the blades when I start it, and it helps a good bit, so I’m wondering if I need a larger carb to get more airflow, or if I need to adjust the air bleeds, since it seems that it’s perhaps not getting enough air. I would think that would give me a very high vacuum reading, however I only get around 13-15".

You don’t mention anything about the timing. Typically these engines run best when total timing is around 35 - 38 degrees at 3500 RPM. If the timing is retarded then the engine is less efficient. That means it can’t burn all the fuel in the combustion chamber. Symptoms are similar to what you describe - big bog when the throttle is opened and lots of unburned fuel out the exhaust with sooty plugs.

The other thing to check would be float level which should be the first thing that is checked when installing a new carb.

I mentioned the timing in the first post, although it’s buried in with all the other info about the distributor. I ended up setting it last night around 10-11 BTDC, with the silver bushing which has 25 degrees of mechanical advance, however the springs are too heavy I believe, on the chart I’ve got it puts it at about 36 degrees at 4800.

I had the lighter springs on there before, but I thought I’d heard pinging so I backed it off a little and took off the vacuum advance to be sure that I wasn’t getting too much advance to quickly. Judging from my drive this morning and just now it needs those lighter springs back.

I’ve got the float level set so that the fuel level is just at the bottom of the sight glass on the bowl. I’ve heard that on carbs with plugs they need to be just below the hole, and carbs with the see through sight glass it should be halfway. I’ve got it somewhere in between, closer to the bottom of the hole.

The chart is guidance - you need to put a light on it and verify what it actually does. Typically the mechanical needs to be all in by 3000 RPM.

So I drove it today, with the 2 lightest springs and it was easier to start and didn’t stumble as bad, however if I gave it some throttle at lower speed or from low rpm it would ping, so I swapped it to 1 light spring and 1 medium spring.

I did verify that at 3000rpm it’s right at 38°, I still have the vacuum advance off of it right now.

It still smells a bit rich, I was wondering if I should swap the air bleeds to a larger size, or if I should put the fuel jets down again? I’ve got 66s in the primary and 70s in the secondary, mix screws are out less than a full turn.

I am running a 70 351C 4v cc with a custom hyd roller cam with simillar specs from Brent, a 680 Quick fuel headers ect. I had been fighting a simillar issue of it not running as good as it should, pinging and a very rich smell. It has an FMX with 3200 stall and 3.9 gears. I tried many different combos between carb and timing.

What has worked out good for me is that I locked out my mech advance at 12 Deg put one light and one med spring in so its all in by 2400 rpm put the timing light on it and ran it to lock out plus a bit (2800 to 3000 then set the dist up at 35 or 36deg total timing. This made the biggest difference for my motor. I also have bigger air bleeds in but I’d have to check for you tomorrow. I ended up running if I can remember 72 76 jets and I think a 5 power valve(I’ll check tomorrow not in the garage right now). I run an AFR gauge so I can see were the lean or rich spots are in the rpm range. My vacuum is around 12 I believe. My vacuum advance is on ported.

I had to reset my carb back to stock settings when I re-started tuning a different way than I was previously. Make sure those transition slots on the primarys are little squares and don’t adjust the main throttle screw too far off that. I’d rather open the secondaries slightly if needed to get more idle speed or bump the timing a degree or so.

Get your timing dialed in first then tweek the carb from stock settings.
Mine is running very strong and way more fun than it used to be without any pinging on 94 fuel.

I’ve definitely had a better time since I removed the vacuum advance and swapped to mechanical only timing. I’ve got a similar setup with the distributor springs, about 11 degrees initial timing and then 25 degrees in the bushing, with one light and one medium spring and it’s for sure all in at ~36 degrees by 3000, I think around 2800. I’ve got a 6.5 power valve, I tried running a 4.5 and an 8.5, they never really made much difference for me, at least not enough to fix the problem I was having.

I’ve gotten around 14" of vacuum pretty consistently, I do wish I had an AFR gauge to check, because from what I’ve seen with my spark plugs the carb has been pretty rich, and I’ve only got 66’s in the primaries and 70’s in the secondaries.

I’ll have to look more into tuning and the visibility of the transition slots, I’ve read about it but never checked and worked off of seeing how much of it is exposed at a certain time.

Did you end up messing with the secondary vacuum spring at all? I’ve had the screw pretty backed out and had a phone taking video of the carb and while driving, not seen the secondaries open up at all. The quickfuel comes with a purple spring stock, I was wondering if you’d had any trouble and if I should get a kit to move to a lighter spring.

I’ll probably swap the spark plugs for a new set tonight, or at least look at these ones and see if they’re still black. I’ll also check about hooking it back up to the ported vacuum port.

Just got all the spark plugs off.

Number 1 on the right, they all seem good except 4 and 7, which are both pitch black and very sooty. Would that be a common occurrence, since 4 and 7 are the two plugs that are opposite in the firing order compared to the plugs around them? I’ve not heard of that but that’s all that comes to mind.

All of them have a black ring, which I would think is still a bit rich, but a lot better than previously when the whole plug would be black. Number one also has some speckles, which is a sign of pre-ignition right? That would explain some of the pinging I heard.

The only time I have had plugs that black it was caused by intake gasket leaks.

Well I’ll check for that when I put the new plugs in, I figured I’d wait to see if anyone had any idea about the plugs.

Pretty sure I’ve checked for any leaks before but I’ll check again, perhaps the bolts came loose and they’re pulling in some air.

Typically leaks that cause plug fouling are pulling oil vapor in from beneath the intake. There’s no way to detect it from outside.

Yeah I’d read that, I was hoping that wouldn’t be the case but if my bolts are tight I’ll have to look into some new gaskets.

If it was pulling that vapor, would it be enough to make the plug wet? Because the 2 plugs were sooty and bone dry.

I pulled my intake off, and it does seem from what I can tell that any backfires through my carb were specifically through runners 4 and 7. Both runners were fairly sooty.

I had my intake on there with only the metal gasket pan, which I decided based upon reading a good few articles on what different gaskets to use. I have an edelbrock air gap, so I’m wondering if I don’t really need that pan, especially since it seems I had a leak from the oil galley, into the space between the intake and metal pan, and then perhaps into cylinders 4 and 7. They seemed like they had a bit of oil around the RTV, so they could have been pulling from there.

There was a surprising amount of sludge on the top of that pan, where I was expecting it to be clean, it was actually sludgy on the rear, from cylinders 3 and 7 back, and had a light layer of rust and some hardened fragile scale in there. Not sure if this is from some oil getting too hot but it was something I’d not seen before.

Unless someone here posts to the contrary, I think I’ll go ahead with just 2 regular gaskets, and go with a thick bead of RTV for the front and rear wall areas. The rubber that came with the metal pan squished and tore apart, which sealed but was not very sturdy.

You can see that runners 4 and 7 are pretty dark inside, in addition to the sludge leaving marks on the bottom of the intake.

All that black at the back was pretty caked on, and had chunks of a fragile scale like material inside. None of that in the engine, only in the gasket pan.

You need the pan. Success is from being meticulously clean. That pan needs lots of attention with razor blades, scotch brite, and a good solvent like lacquer thinner or MEK.

I would avoid the Fel Pro Print - O - Seal gaskets. The regular Fel Pro gaskets are better and cheaper. Or Victor or Mr. Gasket - anything but Print - O - Leaks.

Alright, would you recommend just the pan with some rtv on both sides? I’ve read that some people did the pan with rtv on the bottom, then did paper gaskets on top with no rtv. Everybody’s got a different take on what works from what I’ve seen.

I’ve never had a 351C but if it were mine I would use “The Right Stuff” and gaskets with the pan.

Alright, I’ll try that out tomorrow and we’ll see how it goes. Thanks for all the help so far!

Edit: Figured I’d just update this post.

I went to check and clean the valley pan metal gasket and I drilled some holes to help oil flow back down if it gets trapped, and I noticed how the heat and oil really wore down the metal and rusted it through in some small areas. Gonna get a new gasket and go with that, rather than have this deteriorate metal chunks into the oil galley.

I ended up replacing the intake gaskets and it seems to not have changed much. I went with RTV on the heads, then the thin paper gasket, then a small layer of rtv on that, a large bead of RTV on the front and rear wall, and then the metal valley pan and since it was a crush gasket I put no RTV on the top of that.

Not having RTV there may be a problem, I had just read that some people don’t bother since it’s a crush gasket, and I figured if it leaked I could just remove the intake, and try rtv afterward.

I still have the same vacuum reading, it idled differently and ran a bit rougher than before it seems, but now it’s settled and is fairly similar, unresponsive in 2nd gear and wants to die when it shifts.

It is still fine at WOT, I can go around a corner screeching the tires the whole way, and I noticed that if I sprayed any small amount of carb cleaner or starting fluid into the carb, it doesn’t up the rpm, it makes it want to die. I found that out when I was leak testing the outside of the intake to check for vacuum leaks. Would that be a possible sign that my idle circuit is perhaps leaking fuel or just giving too much at idle and light throttle?

I’m going to drive it around a bit more, perhaps get the timing light and adjust everything again to see if any leaks are fixed and now it just needs to be retuned. I did end up getting a set of gaskets that are meant to be used by themselves, so perhaps I’ll try those out if I find that the valley pan didn’t work out. I’m concerned that the small amount of work on the chamber size and the deck on the heads changed the geometry, and perhaps the crush gasket crushes fine on the top but on the bottom it may have an awkward angle, but that’s just a speculation at this point.

Just checked 4 of my plugs, 1,5 are darker than the previous ones and I’ve only run these plugs a few miles, 4 is already a bit dark and 7 is the same black sooty mess it was before unfortunately.