You can see my engine specs in my sig. The cam specs are
.560/.560 lift and 230/230 duration. The engine smooths out pretty good at about 1200-1500 rpm. With my L88 drop base and 3" filter, the air cleaner lid hits the front vent tube on my Holley 750 VS, so I know that is probably causing some airflow issues. However, when I try to tune the carb with the air cleaner off, the mixture screws do nothing, and you can tell the car is obviously running rich, just from the smell. I know I can drill the throttle blades, and whatnot, but I'd rather have a carb with more adjustability so I don't have to remove it every time it isn't running quite right. I was considering getting a demon carb with the idle-eze adjustment to help with my problem. Due to my air clearance issue, I would also like a chokeless carb, but the speed demon and mighty demon carbs look like they have a shorter choke horn than the Holley's; is this correct? I also can't decide whether to stay with a 750 and hope I can bleed in enough air with the idle-eze adjustment, or do I need to jump up to an 850? I was also planning on running a VS carb, but a couple people have been telling me to get a DP, but I was of the understanding that you didn't want to do this with an automatic car. Any guidance would be greatly appreciated.

Eric

 

I had a Demon carb on mine with thier idle eeze feature.....it does nothing. I now have a Holley HP, and the tunibility is much better than any Demon I've seen.

As a matter of fact, everyone around here that's tried a Demon carb has had the same problem with them running rich at idle. BTW, when I called the Demon tech line with my problem, they suggested I drill holes in my plates despite of the idle eeze feature that's already built in.....that tells me that they have no faith in them either. I'm through with Demon carbs myself. Just M.O.

 

So is carb sizing an issue, or what's my problem? Also, it's not a needle and seat blowby issue, as my fuel pump is just a stock mechanical pump.

 

Well honestly, I'm still learning to tune mine. I just got a Holley model 4150 780 cfm carb from Pro-systems, and I noticed immediatley that the 4 corner a/f screws make a difference on the Holley like it never had with my Speed Demon carb. When I adjusted the 4 corner a/f screwas on the Demon, idle quality just went from bad to worse. I just know a Demon carb isn't going to help you. Neither will thier tech line. They'll suggest you drill bigger holes where your 4 corner idle screws seat, drill holes in your butterflies, etc.. - before you know it, you have a POS carb that still runs like crap on your engine and you can't even sell it because it's been altered so much. So I decided to take the POS off, sell it to someone with a more stockish engine that can use it before I'd do all that.

-As you can tell, I don't care for Demon carbs..lol

But in all fairness, it could be the Demon carb I had was just a complete mismatch for my combo - after all in the fine print, the Speed Demon carbs are meant for cars running up to a 230 duration cam. A mighty Demon may have worked better for me, but after tinkering with the Speed Demon with no success I decided not to give them any more money.

The carb I have now came with custom metering blocks, baseplate, and is set up for an engine with a longer duration cam and lower idle vacuum and resonds much better when I adjust it.

 

If your car IDLES different with the air cleaner on, then you can't possibly tune it. Just think how much difference there will be once the engine is trying to move some air.

I suggest you do something about that before buying a carb. You might find that you don't need a carb at all. And similarly, you might just make it WORSE by changing to anotehr carb.

Demon carbs are a great design, but seem to have SERIOUS quality control issues. They must be one of those contract manufacturing situations, where they have basically no control over the actual physical plant output. There seems to be alot of complaints about them. When they work, they're GREAT; but most of them don't work according to the people that have them. People complain about finding chips in passages, passages not machined right, etc. etc. etc. besides just generally not working right like the way Confuzed's did him.

 

Might get flamed for this idea, but anyone ever use an Edelbrock Thunder AVS carb?

 

I agree with Sofa...

You can try the air cleaner I'm running. It's a MR GASKET part# 4339.

Drop base, 2" element, just clears the HEI dist. It's still a bit restrictive, but clears everthing on mine and most importantly, doesn't seem to effect the idle on my car at least.

I wish I could run a 3" element air cleaner on it, but I don't have enough room. But I'm fixing that....

I'm ordering a cowl hood so I can run a non-drop base with a normal 3" air filter element.

Also noticed that we have the same intake, so unless you're using a spacer more than 1/2" tall like me, the above ac will work.

 

The car doesn't idle differently with the air cleaner on, but I figured the low clearance couldn't be helping me any. I swear though, with this setup, it rubs the hood insulation. Any higher above the carb and I'd be out of luck. Also, somebody recommended that I lock out the timing; what does that mean, and how do I do it?

I've screwed the mixture screws in all the way, backed them out 1.25 turns and tried to adjust from there, and it just doesn't work. I've also replaced the power valve due to backfires I've had. Something I've forgot to mention is that sometimes when I try to start it, it loads up and will actually backfire through the carb. If this is a timing problem as has been suggested to me, how do I lock out the timing? I've got a Mallory 85 series HEI with vacuum advance. Do I need to remove the vacuum advance? If so, what do I put in its place?

Thanks,
Eric

 

You do not need a new carb. You do not need to drill holes in the throttle blades.

The problem is the throttle blades are too far open at idle.
The cause is not enough initial timing at idle.
that cam needs lots of initial timing at idle. like 24deg+.
The more the marrier. Recurve the distributor to shorten the mechanical advance travel from stock 20-24deg to around 12deg. Then re-install and set the iintial timing to 24deg and check for total timing 36deg at 3500rpm +.
You have to limit the travel of the advance pin on the slot. Swapping the weights and springs will not do this alone.
Then remove the carb and flip it over. Look at the throttle blade opening relative to the idle fuel transfer slots. Readjust the idle stop screws on both the primary and secondary sides to allow .030" of slot to be visable under the throttle blades on both the primary and secondary barrels. Install a 4.5" power valve in the primary metering block. replace the metering block gaskets.
Shorten the bowl vents to clear the air cleaner lid. cut shorter and on an angle. Remove the choke flap.
Better yet cut off the whole choke horn. (L88 air cleaner mod)
reinstall the carb with a new carb/ manifold gasket. Be sure the PCV is hooked up and functional. The other valve cover must have a vented cap.
Start the car up and warm it up and reset the timing to 24 inital and 36 total with the vacuum advance disconnected.
readjust your now functional idle mixture screws for best idle quality. the motor should idle nicely at 800-900rpm.
it will now not burn your eyes out at idle and there should be no flat spot in the throttle response.
Drilling the carbs thrttle blades is not the fix. Even with much bigger cams than yours.
Getting the carbs throttle blades opening in the "sweet spot" and the ignition timing curve right , is.

 

Thanks man. That sounds like the answer I needed. I am obviously not very good at the tuning issues though, so just to clarify, I don't really need to mess with the weights and springs in the dist., but just adjust it so the advance pin only travels about half as far as it currently does? What is the best way to do this? Also, is it for sure okay to leave the vacuum advance hooked up. I know some people say not to.

 

I dissassemble the distributor and weld up the end of the pin slot to reduce the travel. The bushing included in the advance curve kits are not enough. then use a medium tension spring pair from the kit and reuse then stock HEI weights.

be sure to look and see which end of the advance pin slot is the start point and which is the end point and mark it before disassembleing the distributor. weld up the end (fully advanced side) of the slot that the pin slides in.

Until you are able to get this done you can temperairly Lock out the advance curve to allow you to retune your carb and get 'er running better.
Remove the distributor cap and rotor. Remove the weights and springs. Tie wrap the advance mechanism fully advanced. orientate the knots of the the wraps down beneith so the rotor will go back on. Now there is no mechanical advance. Reinstall and set the timing to 36dBTC at idle.
It will now not advance with rpm. now set up, the carb.

When you n get the chance tear down the distributor and modify it properly.

You want to retain the vacuum advance at all cost on a street driven car. The problem is not the vaccum advance but the fact that it is tailored to a stock motor, not your modified motor. You need to limit its travel (amount) as well and tweek the rate (diaphram spring tention) Crane sells adjustable ones.
around 10-12 deg travel and experiment with the rate spring tension)

You want 24deg+ at idle 34-36 deg at high rpm and an additional 10-12deg of part throttle cruise vacuum advance
do not exceed 51deg combined total advance.
Some people advocate eliminating the vacuum advance cause they can't be bothered to dial it in for a modified motor.

Do all this and you'll think ya bought the best carb in the world.

 

Thanks so much. I'm entering unchartered territory, but it'll give me something to keep myself occupied for a while this winter.

Eric