hey guys, first let me list my setup so we're on the same page:

9.2 comp 350
air gap manifold
headers
Hotcam w/1.6s
vortec heads
th350
10 bolt w/ 3.90s
stock HEI ignition
timing is 16 intial and 36 advanced with VA disconnected, VA hooked up to manifold and adds 19*
holley 650 4777 dp, no choke
jetting is 72/78, squirters are 31/32, 6.5 power valve
running 12.5 inches of vac at idle
holley electric blue fuel pump @ 6.5 psi


i recently adjusted my throttle cable after noticing that the secondaries we're barely opening. now the car revs MUCH quicker and i'm finding my self shifting much faster. :thumbsup:
only problem with the car is that when i step on the pedal and feel the secondaries open up, it'll cut out like its lean. does it slightywhen i shift and the rpms drop going into the next gear. it picks back up if i very lightly back off the throttle. wondering if the power valve is binding, it looks okay. its not a misfire or poping sound. i seem to believe its a fuel problem because like i said, it will stop if i lean the throttle out with my foot. but im not ruling things out like coils or ignition modules. the plugs and wires (new) look good, and the coil, cap, module and rotor are about a year old. bad part is that this is only at almost WOT, so it's not something i can be testing out between gas stations.
thanks for your help guys!

If I am reading this right, you are saying you have 55° of timing with the advance hooked up? Dood... you are going to melt that sucker down with that much advance. You are probably backing off of some of the vacuum advance when you let off a little... nothing will pull with 55° of timing. A good running small block can tolerate up to 42° - 44° of timing if it is set up for it, but for the most part 38° total is about where you will make the most power. Everything else you have looks ok, your total timing advance is the issue if I am seeing your explanation correctly. Try getting an advance curve kit for the HEI & disconnecting the vacuum adv all together. As long as you get 36° - 38° in at about 2500 RPM you should be good... Vacuum advance is good for economy while crusing @ freeway speeds, not for hot rodding.

larry, thanks so much for replying. i thought the same thing, and thats why i added it into my description. the only thing that made me think otherwise was that i thought the advance was non functional at the rpm due to the throttle plates being so far open (low vac = no advance?). i know thats opening up a can of worms because its a touchy subject to some. i actually tried running the car w/o the advance and it still wanted to cut up alittle and do the same, just not as bad (or maybe thats what i keep telling myself). i bought a mr. gasket recurve kit from napa, and after looking at the weights and center piece, i thought they were a joke. they looked really cheap and flimsy. they didnt even sit right on dist, so i just used my old weights and the springs from the kit. ended up with a black (lightest) and a silver (mid tension).

do you think that if i bought an adjustable VA unit, my problems would lighten up?

I had same problem on my 406 last spring.
problem was the fuel level on the secondary side was a bit low and fuel slosh on acceleration cause a lean condition.
I installed jet extensions on the secondary side and raised the float level up and problem is a thing of the past.:hurray:

thanks dawg, thats a great suggestion that i hadn't thought about. larrys theory of 55 degrees still sounds pretty advanced, but you may be on to something too. i think i'm going to be throwing a lot of parts and time at this thing.

Have you checked the float levels to see if either one is low? Even if the sec side was low, it would not react (or stumble) as soon as they were opened, but after pulling for awhile and draining the bowl. Another thing is if you use too light of advance springs, the weights can not keep up with the rapid RPM increase, and will actually fall behind (or retard) until the RPM stablizes. Even though the kit looks cheezy you bought, the amount of travel and the rate of how fast the weights get to full advance is way different that your stock units. Just because the springs are stiffer doesn't mean you will lose advance... just how the advance acts when hard on the throttle. The one you got will actually back off the amount of total mechanical advance you will get... thusly why they are for using without a vac advance. And yes, you can get an adjustable vac advance but you need to take into consideration how you will be driving the car. Lots of hiway miles and no going through the gears... then yes, ripping up through the gears I would not use one. That is why most hi-perf MSD dists have no vac advance, because they are for running hard going through the gears. Try the adv kit you got, it will work ok and be a good test... but do NOT use the light springs, maybe at the least one medium and one heavy to start, and if the problem is gone, change the heavy one to the medium and try that... :yes: And 55° of timing will never work...PERIOD!

And 55° of timing will never work...PERIOD!

55° of timing is not unusual at all, there are lots of engines running this much advance. You need to realize that this is only under high vacuum conditions when the mixture very thin. As soon as you open the throttles, the intake vacuum goes away and the timing is reduced to the limitations of the mechanical curve. A 9.2:1 engine with a 'hot' cam will have no problems at all with this much vacuum advance.

Very true a Vacuum Distributor can reach 55 degrees, which he has. A mechanical distributer will never reach that much advance and maxes out, or say all in, at the top of the curve, say 38 degrees at 3200 rpm and basically suited for the strip. That's why a Vacuum distributer is ideal for cruising (no load) and the throttle plates are nearly closed = Learn Mixture and Hi-Vacuum = much easier on gas with that 55 degrees. When stepping on the throttle at say 3400 rpm the vacuum drops and the mechanical which is maxed out at 38 degrees takes over for acceleration (under load).

An adjustable vacuum advance canister is very handy to control the amount of vacuum advance above the mechanical.

One thing I noticed in your specs is that you have 12.5 Vacuum at Idle. I would think a 10Hg Power Valve may be more suited as that 6.5 could be causing your stumble or lean condition as you describe. Perhaps someone else might have an opinion on this.

I think you guys need to start charging for this stuff. Great info here. I've had a thousand thoughts as to what this might be. Z15, I was thinking that I may need to go up on the power valve too. The float levels on both sides look really good. I'm going to probably end up with the adjustable vac advance, throw on the advance kit and do the power valve. I'm thinking one of these are the culprit. I'll keep you all informed.

I think you guys need to start charging for this stuff. Great info here. I've had a thousand thoughts as to what this might be. Z15, I was thinking that I may need to go up on the power valve too. The float levels on both sides look really good. I'm going to probably end up with the adjustable vac advance, throw on the advance kit and do the power valve. I'm thinking one of these are the culprit. I'll keep you all informed.


When everything is working properly, your V/A would not be the problem at wot. A bad one can move around tho.
For troubleshooting..... I would just disconect it, make sure your total is set and does not move around in the upper rpms. Then concentrate on whats causing the lean condition.

Just a sugestion to keep you from chasing several things at once.

try a couple steps more jet in the secondary, your spread between primary and secondary will usually be 8-10 sizes. You may also need a larger squirter (35) in the secondary, if it is an acceleration problem that cleans up. If it is a constant throttle opening problem then it would be the jets.

I just wanted to let you know that I am enjoying this discussion and hopefully will learn something from it. I have a 1411 Edlebrock on a SBC engine, 383 Stroker with 9.5-1 compression from Blueprint. It bogs if I hit the throttle too fast. I was told it is a lean bog so I called Edlebrock and they sent me bigger set of metering rods and stiffer springs. it has helped a bit but I still hear a poping sound from time to time if I hit the throttle too hard. I have a 49 degree vacuum advance setting.

I too have a lean bog when hitting the throttle quickly. (383 with some Dar Iron Eagle heads, XE274 cam and a performer RPM intake). Tuning the accel pump cam/squirter has helped some but I've gone all the way up to a 40 squirter and haven't gotten rid of the lean backfire. I've read a lot on here about the issue but have yet to get to the bottom of it. Hopefully something new turns up in this thread : ).

My problem is alittle different then an "off idle" deal, but hopefully there is information for you guys that carries over. I'm really leaning towards power valve and/or jet sizing. I know it varies from setup to setup, but size 80+ jets seem on the high side for this engine, no? I'm certainly not ruling out the Dist, and I'm just going to bite the bullet and put this Mr. Gakset advance kit in. It's very interesting to me how such a problem could go either way; spark or fuel. It's also interesting how simmalar the symptoms are for both. Thanks again TC.

From my experiences, disregarding extreme altitudes, Carbs are generally jetted relatively right on for the applicate CFM rating by most Mfgr's. I believe the cam determines what Power Valve and Vacuum Distributer Pods to run. For instance; if you getting steady 12.5 Hg at idle then I would say the cam is not too extreme and you can use a Power Valve and Vacuum Pod to within 2.5 Hg's less; however, if the cam Idles with a fluctuating Vacuum between say 8 and 10 (providing there are no vacuum leaks) then I would recommend a 6.5 Hg Power Valve. The faster the Power Valve enables off Idle or Cruising Vacuum (without opening causing a rich condition) the less chance there is for leaning which starve the carbs metering circuits as designed by the Mfgr and improves throttle response.

John Hinckley has written an excellent article on Timing Vs the Distributor's Mechanical and Vacuum functions entitled "TIMING AND VACUUM ADVANCE 101" here: http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/Timing101Article.pdf (http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/Timing101Article.pdf)

With an automatic trans, check the idle vac with the engine warmed up and trans in gear.(use a helper) Use a PV rated a couple below that.

Can you replicate this stumble with the car just idling in the driveway in neutral by stabbing the throttle to WOT? If you can, a trick I've had some success with on double pumpers is to remove the rod that connects the primaries and secondaries. It's on the driver's side of the carb. With this rod temporarily removed, you can figure out if it's the front half or the back half of the carb causing the stumble and tune accordingly.

On squtiter size - too much OR too little can cause a stumble. Don't be afraid to try smaller squirters.

On most vortec head motors 30-36 degree's total works best with the Vac adv. adding 12-16 more.
With that cam 16 or more initial is a good start.
As for the bog don't neglect the acc pump cams-They are often the simplest fix,also raising the rear bowl level a bit and making sure the there is no major slack at the pump cam arm/pump housing lever helps.

All of you are giving me great suggestions as far as what to look for, thanks again. BPOS, this problem seems to be one that is only "under load", so to answer your question.. no, if I netrual rev it all I get is screaming motor. Thats why I feel it is least likely an acclerator pump, pump cam, or squirter. Kinda making me lean towards vacuum controlled objects or anything thats acting under load. Power valve is on the top of my list. I'm just not sure whether to go by the vac reading I get at idle or in gear now. Gonna dig around alittle. In the mean time, tommorow I'm going to throw that Mr. Gakset advance kit on it. I'll report back.

Here it is from Holley. I was actually off by a little. You can certainly disconnect the secondaries and drive it and see if the problem comes from the fronts. If it runs fine on just the fronts, look to the rears for your problem. I also like the idea of raising your rear float level a touch. And for anyone else reading this, have you ever noticed than on a Holley DP (maybe vac sec as well?) that the rear float level sight plug is slightly higher up on the fuel bowl than the front? Interesting........

http://www.holley.com/data/TechService/Technical/power_valves.pdf

Just an update for everyone. Today I put the Mr. gasket advance kit in the car. Adjusted the timing back to 36* advance, didn't fix my problem. so I went on a road trip and unpluged the vacuum advance just to be sure that the vacuum canister wasn't sticking or anything. Still, same thing. I took a trip up to Napa to see what size power valves and the highest they had was a 7.5. I went home with it and replaced my 6.5 with it. I can tell a noticeable difference but as a nit-picker, I know that there is still some bogging. I'm thinking a 8.5 should solve my problem. Thanks to all.

Thanks for the update. Do let us know how the 8.5 works out. I've been curious what effect a larger PV would have on mine but with my idle vac in gear at around 8-10 I've been reluctant to try anything higher than 5.5. I've read numerous things.. some say the PV can open at idle, others say it can't. Anyone know for sure?

yes they can open at idle

Just found this: http://www.carcraft.com/howto/116_0501_carbs_tips_tricks_cheap/index.html

Exerpt:

For years, magazines warned the world that running a power valve with anopening point below the idle vacuum would dump all kinds of fuel intothe engine at idle, foul plugs, and waste fuel. The whole idea of apower valve is to allow you to run leaner jets in the main circuit andstill add fuel when load increases. The truth is that the power-valvecircuit is completely separate from the idle circuit, so if your engineidles at 7 inches of manifold vacuum and the power valve opens at 8inches or lower, the power valve will open at idle, but the engine willnot receive any additional fuel until the main metering circuit beginsflowing fuel out of the boosters.
However, if your carb is set up this way, with a low idle manifoldvacuum and high power-valve opening point, it will run rich atpart-throttle since it's likely that the power valve will be open muchof the time. So, the smart move is to install a power valve that opensat a lower point, such as 4 or 5 inches of manifold vacuum instead of 8.Your engine will be much happier and run much crisper under lightacceleration. There are 11 Holley power valves available with openingpoints ranging from 1 to 10.5 inches of manifold vacuum.


SO... I would guess that unless you have your carb tuned incorrectly to where its running on the main system at idle, the PV won't be dumping any fuel at that point?

Sorry.. didn't mean to hijack the thread, just trying to clear this up for myself.