hamaf

I'm deeply sorry about a long post, but if anyone bothers to read and have some advise or suggestions I'd be very thankful.

What confuses my mind is the fact that I had almost perfect tune last year when my car had auto transmission. I have Innovate LC-1 so I can acurately measure AFR. When my car had auto transmission(700r), my AFR's were:

Cruise/idle: Almost exactly 14,7 at any speed/rpm

Power run: From a stand 0mph, slammed the throttle wide open and changed through all gears manually (semi manual valvebody) at 5500 RPM. LogWorks showed AFR 12.6 through the whole run, topping 125mph at the end of third gear. Exept a big dip down to about 10's(AFR) because of accelerator pump. But it lasted "only" about one second after throttle was slammed open. I cant remember the exact time. Obviously tires turned in to smoke in first and alittle in second. The carb setup was:

4160 Holley that came with the engine gmpp 350HO
65 jets
6.5 power valve
70 rear "jets" (metering plate)
31 squirter
Yellow long secondary spring
BLUE accelerator pump cam in pos 2
about 6 psi fuel pressure
Timing 12 initial, 32 total, all in @ 2800RPM +10 full manifold vacuum

Blue accelerator pump cam was the only cam that got rid of the lean spike when throttle was quickly opened. And also the only one that managed to brake tires loose on dry asphalt :-D It squirts alot of fuel early but overall too much. And this annoys me.

The problem is, now that I have 6 speed(T56 Magnum) AFR's are all over the place! Everytime I change gear, accelerator pump pumps massive amounts of fuel into my engine, and it never get's a change to burn it, because it pulls so fast through the gear and then after each gear change there's another shot.

So basically AFR's dip into about 10.5 with every gear and stay there. I probably shouldn't be even using this carb with a manual transmission, but I still want to tune it before I change to double pumper. Cruise AFR is also a lot leaner, probably because of lower rpm cruise. About 1500RPM. Cannot richen without richening idle.

So what is the plan? I assume this is the time to swap to a higher number power valve and get rid of the huge pump shot. I have already changed the secondary spring to yellow short spring. I probably have to change jet's to ~62-64

Then there's about 0.8 sec lean spike in the middle of every gear when accelerating hard. what is this? Power valve openig too late? or running out of squirtter? Maybe this did not happen before because I had stiffer secondary spring.

Accelerator pump tuning is stupidly hard because I want instant response, but not too much fuel overall. Maybe I'm too picky. It might be that the pump shot as it is now, is so big that it never clears. Fuel economy is also a consern because fuel costs 6,5 dollars per gallon here in Finland :-O

What shoud I do first? I probably need to buy jets and squirtters and powervalves. I would like to get it tuned fast, before I burn hundereds gallons of fuel.

jwfirebird

iTrader: (2)

i think your initial timing is a little low. decent cam engines like 15-20. i would start there then put in a 105 PV and see where you are at.

you have a distributor that adds timing at cruise? go through the tuning sticky here, the idle, vacuum advance, and adjusting the mains like he says all helps get good mileage.

sofakingdom

iTrader: (1)

There's a "sticky" at the top of this forum, where one of the mods took advice I had given repeatedly, and condensed it down into a single post.

Follow it. Start to finish, IN ORDER, not, "oh I think I don't need this I'll just go work on this other instead". DON'T. Start at the beginning, and follow through each step, in order, skipping none, not moving on until each is complete. Set the fuel level, find the correct main jet size, select the correct power valve, establish the correct transition slot relationship, set the idle, re-test for main jet size, repeat until no further improvement can be made; then select sec jets for best 1320' mph and sec spring for best ET. Put the pump back stock and LEAVE IT ALONE until ALL else is dialed in; even if you THINK it needs more, DON'T. LEAVE IT ALONE. Holley pumps are already too big, and thinking you need a bigger one, is a 100% SURE sign the PV is too low of a number. The PV usually needs to be about 1 - 2" below cruise vacuum, NOT connected to idle vacuum in any manner way shape or form whatsoever. Ignore Holley's instruction for choosing it, it is WRONG. Allow me to repeat, the "half of idle vacuum" thing is WRONG WRONG WRONG WRONG WRONG. That, plus the transition slot error, is where most Holley carb tip-in flat spots come from.

You don't say what carb you ACTUALLY have; all we can tell (4160) is it's a 4-bbl w sec metering plate instead of a block and interchangeable jets, most likely vac sec but you don't even tell us that. 1850?

For a timing curve, establish "total" (static plus centrifugal) of 36°, with static at around 16 - 18° and only 18 - 20° of "centrifugal; then about 12 - 15° of vac adv on top of that.

Looking at what you posted, assuming your timing #s are correct (highly doubtful), you need about 4° more static AT LEAST. Ignore the # however, and set it to where the engine runs the best. Your jets are too large, your PV is too small, most likely the transition slot is too far exposed at idle meaning it's getting completely drained at idle and has no fuel left in it to make the "transition" (the change from idle to power... the malfunction you're trying cover up by dinking with the pump), and all the pump stuff is just a hack job to cover up what's REALLY wrong with the carb's tuning.

I'd also suggest not being quite so hung up on trying to graunch it into running at stoich all the time. It probably needs to be richer than that at idle and full-power (probably around 12.8 or so), and leaner than that at cruise (probably 16 or maybe even higher). There's not many circumstances where it should constantly run at 14.7.

hamaf

I had written a long reply, but lost it, pressed "preview post" and something weird happened. Anyway, I will do the "sticky" when I get the parts and I know it will probably work. But I'm also trying to learn what affects what, and why.

Here are the numbers from the choke tower: 80457-2 and 3076. Yes it is a vacuum secondary 600cfm, 4 barrel.

Timing numbers are exactly what I said. IF my timing light and marks are acurate. I'll definitely try more initial timing like you guys suggested.

I'm still waiting for parts so I can't do the sticky completely yet. I'm currently just testing jets, VS springs and PV's that I have, to see what does what and trying to learn why. I check'd the transition slot today and it is square, then adjusted floats, then I also put 8.5 power valve in. It already made the car much more responsive :-) Dont have a 10.5 yet. I still have no idea how to get that momentary lean spot away. I think it happens when secondaries open and it lasts about 1 sec. I was just wondering how bad it becomes when I lean out my main jets. 6.5 to 8.5 power valve did not seem to help this problem. So I assume this is something else. Thanks for your help sofakingdom! and jwfirebird. When I get the parts(may take a while) I'll post some log files after I've done the "sticky" and we'll see how it looks. And in the end it probably does not even matter, as long as it performs well. It's easy to get caught on some exact numbers.

sofakingdom

iTrader: (1)

If it goes lean when the secondaries open, it's because they're opening too soon; spring is too weak. Put in the next stiffer one.

But until the primaries are right, it's not possible to tune the secondaries. First things first. In order. Note that in the order of things, the sec spring is one of the very last; can't jump over all the rest of that stuff and plow into the end of the process and expect it to work. At best, even if you do get it to work right, it stands a good chance of being wrong once everything else that should have been done before it, is done.

80457 is the "list" number; the carb assembly part number, if you will. That's the one you need.