My Vette has given me nothing but problems ever since I started modifying it.  The car was a stock L 48 (350, 8.5:1, TH350, and 3.55's), I added a edelbrock air gap intake and a Demon 750  mechanical secondaries.  The engine was now more sluggish than before but had more top end.  Then I added a new holley fuel pump and a regulator.  This made no difference but solved my fuel starvation problem.  Last winter I added a pair of dart iron eagle 180cc cyl heads along with a comp hydraulic flat tappet 224 230 cam, and crane energizer 1.6 roller rockers.  I installed higher tension springs in the heads and manley swirl polished valves 2.02 and 1.6.  I used a thin head gasket that netted a 10.14:1 compression ratio.  Now the low end is even worse and I can barely get the 275 tires to spin, there is a huge bog off the line, I always have to ease into the throttle till about 3000, then I can pin it without a stumble.  The auto meter air fuel indicated a huge lean spot that I figured was the source of the bog, I also noted a lean/ bog spot when ever there was a quick change in throttle position.  The first thing that I looked into was the carb,  I added a little jet but this didn't help and decreased my fuel mileage further.  I thought that the slow increase in throttle meant that it needed more squirter, i bumpted it from the stock .031 to .037, this helped a little but didn't clear up the bog yet.  I then looked at the power valve and changed it to a 8.5 unit, and it also helped a minute amount but the problem still exists.  What else should I do?  Some people claim that the carb is too big but in the Jan 04 issue of CHP page 36 with the Vitamin C pickup, he sported a very similar combo of parts, the only difference was a small change in cam and comp ratio.  Thanks for the help!!  

sounds to me that you are still having a problem with not enough squirt. Have you made sure that the accelerator pumps are adjusted correctly? Try changing the accelerator pump cam(s) to one with a more aggressive profile. The 750 isn't to big.

What is your total ignition timing set at?

try a vacuum secondary carb

The intial is set at 22 degrees and total is about 38.  I am using a PerTronix HEI distrbutor with no ignition box.  I haven't strayed from the stock springs or weights yet.  I have 11 inches of vac at idle, so i figured I am getting sufficient vac advance for part throttle.  Is there any way that stock wires could have anything to do with this problem?  I haven't gotten around to replacing them quite yet.  Thanks again.  

Check the accelerator pump linkage. Check to see if there is gap between the pump linkage and the pump arm. If the linkage is sitting right on the pump arm, adjust the linkage until there is a slight gap between the linkage and the arm. This will delay the pump shot a little out of the hole and may help with your flat spot. If the linkage is not sitting right on the pump arm, adjust it until it is just resting on the pump arm and try that. Your problem could be the pump shot coming to soon, or not soon enough, this worked for me with a similar problem. Hope this helps?--Phil   

I tried earlier to add a small gap but it didn't help.  Then I changed it back so that the arm began to move as soon the throttle begins to move.  This did help a little but the off idle bog is still there.  I did readjust the butterflys to .020 which is where Demon suggests it to be set.  I noticed that I do have a small intake leak on the passenger side.  Could this have anything to do with it?  Thanks again

Jordan

Your over carbed, over cammed, over valved and under geared! This combined with a heavy car, which the 'vette is, reaps a bad combination.

There is no such thing as slightly different cam and compression i.e. your comparison to the pick-up project. The cam and compression reach to the heart of an engine's power capabilities, small changes can and do cause major differences.

To touch upon a specific, the mechanical secondary is a bad idea for a street driven vehicle. When you stomp on the go pedal the carb gives you full capacity whether the engine's capable of using it or not. Combine this with a healthy cam, long ratio rockers, and big valves,,,, even a 180 cc port velocity drops dead and the engine stumbles as mixture density goes south. especially with an automatic (a torque loving device) and the 3.55s.  

Now you can throw more fuel at it like your doing and this will hide the problem a bit, but you can't make it go away. Since you have a sizeable investment in this thing finding a way out without starting over is pretty important. First thing I'd do is convert the 750 to a vacuum secondary. This allows the carb to supply only the amount of air the engine demands regardless of what you do with your lead foot. This should improve both acceleration and gas mileage and just might make the problem go away. If not, the next thing is to either get control of the excessive induction capability or increase the rear end ratio. The least expensive move would be to ditch the 1.6 rockers for 1.5s. And or start shopping for some 3.7, 3.9, or 4.11 gears to get the RPMs up where the cam, compression and valve train are working.

Your 10+ to 1 compression is pretty wicked by today's standards even with the cam/valve train you have all wrapped neatly in cast iron heads. Too much ignition advance could be giving some detonation/preignition which will torpedo power pretty fast, these events are not necessarily audible.  Again, advance amount and rate as well as compression needs to be balanced with the type of transmission, overall gearing (that includes the effect or tire diameter as well as gears), and vehicle weight and weight distribution, i.e. there's things you can do on a tail light pick-up that won't work with a tail heavy car.

Bogie

I personally don't think your combonation is that far off. Are you sure a gasket change alone bumped your compression 1.6 points? Even if it did'nt, the car should not be a total dog like you described. I have heard a lot of horror stories about Demon carbs. All reports indicate they have fuel curve issues which are consistent with your lean conditions. I would beg, borrow or steal an HP750 or even a HP650 Holley carb and try it. Did you set the cam in the motor or just install it dot to dot? If it is retarded, it can cause the problems you describe, especially with low compression. You might want to degree it to check it out. Also, you should do a leakdown test on the motor if you have not as of yet. If the cylinders are not sealing correctly, parts that typically improve horsepower will just make it worse. One other area to investigate is the distributer and your timing curve. If it has a vacuum advance, make sure it is working correctly and is run to a ported vacuum source. If not, you may be relying on your centrifugal advance alone and if its stock, all the timing won't be in until 4000-4500 rpm which would leave the motor retarded down low, again causing a bog. Anyway, don't abandon that combo, its actually pretty darn close. I think you just need to get it tuned properly. Good luck.

Let me pick up some details from where I left off yesterday. I got a business call and had to steadily turn my focus from your Corvette problems to those my boss pays me to pay attention to.

You’ve got many issues with this car, in addition to what I previously said, this car is pulling a lot of cam for a stock converter, if you haven’t replaced the stock converter it could really use another 500 to 1000 rpm of stall speed. This will cost you some mileage but will really launch the car hard.

 You didn’t state which 750 Demon you have (Speed Demon(with or without choke), Race Demon). I’m not a big fan of the Demon for a street machine unless you make your living stoplight racing for pink slips. I also don’t care for air gap intakes unless you live in the desert southwest and race for pink slips. These unheated manifolds have serious problems with keeping fuel in suspension with the air, which is the reason the factory put a heated manifold in there in the first place. The lack of manifold heat is probably one reason why you keep richening the mixture with minimal results. The mixture flow from the butterflies to the plenum is subject to extreme shear forces. This separates the heavier fuel molecules from the air. The resultant lean mixture flows to the cylinders, the gasoline pooling at the bottom of the manifold reaches a point where there’s so much of it that it tears off and gushes into some in-taking cylinder and floods it. Of course this problem goes away with increasing engine RPM as the mixture velocities in the manifold reach a point where the precipitating fuel is continuously ripped off the plenum floor and put back into the air stream. So the overall mixture corrects itself with increasing engine speed. For example you might see this effect on the plugs, 7 showing lean and one all carboned-up. For a street driven engine I much prefer an induction system that picks up outside cool air, without ram, (ram’s a whole other engineering issue to get right) and feeds this air through an air cleaner of a factory type closed design. The factory aircleaner housing is perfectly acceptable as long as any external system for preheating the incoming air is terminated. Every 10 degree drop in induction air temp entering the carb or injection unit is worth 1% in power. This does not hold true to the same extent for heat applied after the throttle bore of an un-supercharged engine.

Clearly your having tuning problems with the Demon, I highly recommend you get SA Design’s book “How to Tune and Win with Demon Carburetors”. But let’s get into a little tweeking.

First thing is to readjust the throttle position for both the primary and secondary systems and change the accelerator nozzles. We’re, actually you're, going to remove the carb from the manifold and partially disassemble it, so get a gasket set for this carb. You should also have a set of round or wire feeler gages for this effort to measure the idle transfer slot opening and accelerator nozzle squirter size.

Do the usual the fuel, vacuum line, and linkage disconnect. I prefer to work on carbs in a raised edge cookie sheet, it’s great for catching fuel and small parts. Drain the float bowls; flip the carburetor up side-down. Check the transfer slot openings on both the primary and secondary throttles. The throttles should be adjusted so that .020" of the slot is exposed below the closed throttle.

Next right the carb and remove the accelerator discharge nozzles. These have squirters from .025 to .050 inch diameter. To save cost you’ve got to kind of box-the-problem. So you previously measured the squirter hole with you new wire feeler gages, and bought a pair of .050 squirters if those you have are smaller. Now these may prove to be too rich, but this is a place where you need to experiment to find the exact combination.

Reinstall the carb on the engine, hook everything up and prepare to adjust the idle mixture. Turn the idle mixture screws out a turn and fire the engine up. Adjust the screws toward lean till the engine pops out the carb, then open them till the engine smoothes out.fficeffice" />>>

Now there are other adjustments that can be made with the power valve, float height, and main jets but I’ll make this the subject of another epistle if this doesn’t help you.

Bogie

I also changed the heads that originally had 76cc ports and now are 64cc.  The distributor is a Pertronix unit but has the stock springs and weights in it.  Should I put in the smaller springs to get a quicker advance curve.  It is a vacume advance unit and hooked up to a ported vacume source but I thought that vacume advance in only uder part throttel anyhow.  How would this affect the wot bog?  Thanks again  

Yea, that would up it the 1.6 pts. If the vacuum advance is not working properly, it can affect the rate at which the advance comes in, but you are right in that it mainly affects part throttle drivability. The lighter springs will allow the mechanical advance weights to advance the timing quicker, which may help your problem. I would give them a shot. Also, what is your total timing? I would guess it should be around 36 total for best performance, but I've seen small blocks that like as much as 41. It usually has to do with how efficient the combustion chamber is. The iron eagle chambers are pretty good, so it shouldnt want more than about 36-38 total and it should be all in by 3000 rpm. The lighter springs should allow this. Also, the post above is correct about the torque converter. I am assuming it has more stall than stock. While your cam is not huge, it will cause the motor to be lazy with the stock converter. You need about 2000-2500 stall.  

I have a Matco adjustable timing light that I use to read the ignition advance.  At idle it reads 22 degrees. when checking the total I slowly increase the throttle while adjusting the dial on the light as to keep the balancer mark on zero on the marks.  The mark continues to go up until the lgiht maxes out when the engine is a at about 4000 RPM and the light reads 60 degrees.  I don't know what in the heck this is about becasue I verified the mark on the balancer with the timing marks, I used a piston stop to find each side and then calculated the center by dividing the degrees by two.  This could be something to look into but I just thought it must have been some kind of a fluke because I couldn't hear the sligihtest knock under any acceleration, not even on Nitrous. What could this mean?  The car has about 150 pounds of wegiht reductions from removing different things,  would you recomend a stall of about 3000, or is that too much?  And would the 3.70 gears be enough help cure this problem or would youy go with the 3.90's?  Thanks again for all your help

The exact carb is a 750 Speed Demon mechanical sec 30cc pumps, the carb didn't come with a choke nor am i using one, but i do have one for the carb in the shop if i ever need it.  The air cleaner that i'm using is a k&n 14" x 3", it is also a drop base in effort to clear the hood, there is about a 1/4 of an inch between the two, which i'm thinking isn't that good of an idea but it's the best that I can do unil i can afford a cowl and new paint for it.  I reajusted the transfer slot before writting any of these things to you, i did set it at .020 of an inch using a round feeler gauge.  I also ajusted the mixture screws before hand as well.  As for the squirters, isn't the size labeled on the front of each squirter, or is that just a number for the squirter like jets are numbered?  The stock size was .031  and I changed them to .037, if the number on the front of each squirter is the size as you were talking about.  Thanks for all the help so far, anyother suggestions would be greatly appreciated.  One other thing, I'm thinking that a set of 3.90 gears and a 3000 rpm stall would be a good addition to this combo, do you think this is too much gear or converter for a street car?  It's not a daily driver but I do cruise with it in the summer, but I do want to spank just about anything that comes my way.  I know it is a tall order, i wish I had a T56 to put in the car but that would be a major project in itself.  Thanks again for all your help with this matter.

If you're running nitrous, I would go with a 2500-2800 stall and the 3.90 gears. Nitrous cars don't need as much gear or stall as non-nitrous'd cars do. I reread your earlier post and you said your cam has 224-230 degrees duration, I'll assume this is at .050 valve lift. It seems you may be under cammed. You've got a big carb, and at 10.14:1 comp the cam may be too small.  I think a 3000 stall would be to much for your combo, this is based off of cam duration. This is just my two cents worth.--Phil

Are you unplugging the vacuum advance when you are checking the timing? If you are not, unplug it and recheck the timing. Make sure you plug the end of the vacuum hose. I agree with the other answer on the converter. 3000 is too much if it is truly a 3000 stall. Converter selection is one of the most important and most often overlooked things when putting a combonation together. The advertised stall speeds of most of the street/strip converters are just ballparks. I would call ATI and get their recomendation on a converter for your specific application. Have all your specs at the ready as they will ask you a bunch of questions. It is well worth it to spend a few extra bucks to get the right converter for your car the first time. Take it from someone who has made this mistake before (only once, lol).