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Ok, how can I fatten up the primary circuit?
Makes sense. Over 60mph there is just enough additional resistance that the main circuit is providing most if not all the fuel. The throttle is just slightly more open than mainting speed at 40 mph.
I thought we were talkin about that. lol
Main circuits aren't the problem. FWIW Holley's carb list shows 71, 76 is the jetting from the factory. Best to keep the spread in that range. 81 will be way too rich when you floor it. Focusing on the problem at hand 71 is a reasonable primary main jet.
Thanks for the input guys. Tomorrow morning I’ll do some more tweaking and testing and let you know how it goes.
Any reason you have the pump cam in the number 2 position? In my opinion sticks take a bigger pump shot. I start out at 37 and go up from there.
Trying to get the pump shot to come in hard and fast. It’s my understanding that #1 position gives a longer duration of shot while #2 position starts off already into the fat part of the shot. My plan of attack is to pull carb back off and verify t-slots are still set. Then back out idle mixture screws 1/8-1/4 turn and test. If it’s still there I will try the 37 shooter. If it’s still there I’ll back the timing off a degree or two. If that doesn’t get it I’ll feed it a bullet.
I experienced the same tip in at cruz flat spot problem with my 340, I got rid of it. Here's what it took to cure it. Initial timing was to high causing it to idle to high this closed the throttle plate to far exposing to much of the transfer slot. To much slot opening caused it to go lean on tip in. I backed down the timing to 18* this lowed the idle speed down which allowed me to reposition the plates for less transfer slot and bring the idle speed back up to 850rpm. Presto no more tip in flat spot. In the end I had to re curve the distributor to get the total timing back to 34* all in.
Not neccesarily. Duration is more the combination of shooter and pump size. But any generaliztion with pump shot is just that. The bend in the arm results in different contact points with different cams and positions. Pump is mostly to cover when the throttles move rapidly open from closed and nearly closed positions. To tune the carb plan on spending some time testing iteratively. If you're super lucky, each test will be progressivly better. Reality is there will be some changes that make the problem worse. Stuff is related together as 512Stroker wrote. Idle is the foundation. Steal from there is robbing from off-idle etc. Timing must match combustion conditions etc.
Update: Removed carb. T-port was at .045. Closed it up to .035. Lowered secondary jets to 77 (not related to the problem, just wanted to do it while carb was off). Installed white pump cams in #1 position (how it initially was before I started tinkering. Verified idle timing at 22* with can disconnected. Backed idle mix screws out 1/4 turn each. The idle was down to 650 rpm now. Went for a cruise. Stumble had improved slightly. Wasn’t happy with low idle so I turned curb idle screw in 1/4 turn to get back to 800rpm. Continued to cruise and it seemed to improve a little more. Came home and swapped in a 37 shooter in place of the 31. Haven’t tested yet.
Car ran better but still a little stumble. I’m going to back the timing off 2 or 3 degrees and retest. Will probably have to turn curb idle screw in to get the rpm back up.
Is it slow coming on the booster? That’s something I need to look at closer on some of the stuff I’m working on, and then figure out how soon (or delayed) it needs to be getting on the booster. Then, what happens when you decide it’s slow getting on the booster, so you drop .004-.006 on the high speed air bleed. Now you’re on the booster sooner, but it goes rich at the top of the RPM range? What’s the next move???? Add a few thou to the lowest emulsion hole? If you have one hole lower do you use it with a .024-.026 hole? This is stuff I need to work on and work out. So I’m curious what everyone else does when they need to get on the booster quicker but it goes rich up top.
Those are all good questions, but unfortunately those adjustments are beyond the capabilities of my old-ass carb. Without major surgery anyway. I’m gonna have to tune the lean stumble out and if it gets a tad rich somewhere else I’ll just live with it, until I come across a deal I can’t refuse on a newer, more adjustable carb.
Be a little more systematic. Change Transfer slots .045 to .035 ( a big jump) Tune 1: Adjust idle mix screws for best vac at 800 on the richer side (1/8 to 1/4 turn) Tune 1 a: If idle is not at 800, write that down. Road trial or not. Tune 1 b: Bleed in more air at idle. To avoid drilling the throttle plates for testing, crack the secondaries or put a restricted tee in the PCV line. Test Reduced Primary IABs Tune 2: Allow Transfer slots to be adjusted between .030 and .040 if needed. Tune 2: Adjust idle mix screws for best vac or highest rpm on the richer side. Trial that. if starting from 1b, (bleeding more air in), decide whether to keep or trial without the additional air at idle. Test effect of timing Already know that large changes in timing do affect the tip in due to previous trial with and without vac advance. Therefore measure the timing and figure the advances. Test of 2 or 4 degrees less timing at idle is fine to learn the effect. But know that it will may result in lower rpm, especially a 4* reduction. If in the previous tests, idle rpm came up (with overly opening the transfer slots), then timing can be used to get the rpm back down to 800 rpm or so. If rpm did not ever come up, then if anything, just try 2* reduction and repeat previous tests (stuff done in tune 1 and 2) to see if a stronger idle can be made with less timing. This would be tune3. That's all very informative because now you will know the effects in idle and very low rpm. But the issue encountered is in the 1500 to 2800 rpm range. It may be related to too much mechanical advance between 1400 - 2800 rpm. And it also may relate to too much or too soon contribution of vacuum advance. Once the timing curve is plotted, it should be pretty obvious. If you have an adjustable vacuum pump (mity vac or A/c pump) attach it to the vac advance and with the cap off observe the vacuum when the advance starts to move and when it stops moving.
Tip. Premeasure the number of turns of the idle speed screw to reveal .20 through .040 of the t-slots. Tip. Use a note book. I'm sure you already do. Use a notebook of some sort because its way too much to remember. Not sure why pump cams were changed. Too many variables. Don't really care about the secondary side for this. But primary side, stay with whatever. Does the white work OK for more rapid throttle opens, like leaving a traffic light briskly? (not spinning tires, just in slight hurry brisk, LOL). If so, stay with it, if not go back to blue. 'Cause if not, that will annoy the heck out of you while driving,
Thanks Matt, that’s all very good info. I do keep a notebook with different settings/tunes and corresponding results, although I should do a better job of keeping it neat. My chicken scratch can be hard to decipher sometimes! I think you may be on to something regarding the timing curve. Currently I have 2 lightweight springs in there and the timing comes in hard and fast. I do believe that the carb is very close and with a closer look at the timing I think I can get this narrowed down. I may try some different spring combos to try and flatten the curve a hair.
Go to one light, one medium spring. Put the blue cam up front #1 hole. White cam in back #1 hole. What size IAB and MABs are in the carb?
Air bleeds are non adjustable in this carb so they are whatever Holley made them. I don’t have any pin gauges to measure.
The IABs are in the .070 to .78 range. If you have a numbered drill set, with no burrs on the shanks, they can serve as pin gages. Or just shove some wires in there and measure later. LOL But that was how we did it when I first was learning. We had drilled the blades so they could close further and then it had no off idle power - it wouldn't make it up the hills at 30-35 mph. Came back to my coworkers house and stripped some wire down and stuck two strands in bleed. Worked great.
If you recall when I got those Holly Street demons I had twice the trouble LOL same problem. Couldn't get a big enough squirter in there to cover the problem. It ended up being in the transition rods for the primary circuit. They had to be offset from one carb to the other. of course absolutely none of this helps you but my question becomes about the vacuum advance. Right at the beginning you said that when you took it off it ran fine. Which took away basically a vacuum leak.. it was always my understanding that vacuum advance was for Economy cars. Are you trying to get some economy out of this car?... When I had one on my car I plugged it but now my MSD doesn't even have one on it.. I was wondering what benefits you were looking for having it?.. Besides this stumble you're having lol......
I don't know what distributor you started with. A 2 bbl 318 is pretty efficient combustionwise at low rpms. Your engine obviously gives that up, but probably dvelopes good efficiencies by 2000 rpm. Any time using the vacuum advance, the mechanical advance has to be darn close. For drag racing, the quick in is OK with no vac advance. The engines don't get heat soaked and actually may work better; and at WOT the vac advance is just one more thing not needed. Anyway. here's a couple of timing plots using the specs from the plymouth shop manuals. Compare with We can see the 318 needed around 16* lead at 1400 rpm where as the 273 power pak needed arond 24* at 1400. More important for your issue, notice how in both cases the rate of advance is reduced above 1400 rpm. Why? Because these Chrysler engines (and others too) combustion speeds up as engine rpm hits a range where it has better efficiency. The second spring should have a loop on one side. That length of the loop and the spring perches determine how many degrees into the advance before it starts to restrain the weight. While we can't do this with many non-chrysler distributors, its not such a big deal when initial timing is around 20*. the mechanical advance handles lead time needed for rpm. Vacuum advance handles lead time needed for low density (leaner and slower burning) situations/ My interpretation of the 273 4 bbl vac advance going as low as 5" Hg is that it the engine had good part throttle fuel distribution. I would generally like to see a vacuum advance that is out of the picture about the same vacuum as the power valve opens. The logic here is that the power valve is enrichment and vacuum advance is for lean conditions.
What I'm saying is something like this would almost certainly be OK with vac advance. And something like this almost certainly will not work well with vacuum advance.
You said, "I unhooked the vacuum pod and plugged it and the stumble was totally gone, but I would like to keep the vacuum advance." Make sure the vacuum can is not leaking.
Ok sounds good since you are 1.5 turns on the idle mixture screws (IAB and IFR are probably good as is). I assume this is a 2 screw idle mix carb?
OP where are you pulling the vacuum from for the vac advance? Is it from the full manifold vacuum carb port or the timed port on the carb?