I need a carb for my 454 HO, the one thats on there has been highly "modified" and will not idle correctly. It is a 750 DP which runs great other than the idle problem (idles rough no matter what).

I'm thinking either another 750DP, or a Speed Demon 750 mechanical, or a 850 vac secondary. Any suggestions? I'm afraid the vac sec may lose the seat-of the pants feel.

Have you tried another carb to be sure the idle problem is not a vacuum leak? 750dp would be my choice.

remember idle problems all do not come from just the carb.alot of times the timing is a problem, vacuum leaks and vacuum advance canisters (specially the adjustable units with a low vacuum motor under 8" of hg or so)

your carb should be just fine to idle any combo ( ok within reason maybe not a top fuel car)

time should be taken to verify some details that maybe the problem.

timing should be the first thing that should be checked and correct even before you move to the carb. (this is just an example but usually a good starting point for a bbc street engine)timing should be set say at 16 degrees initial along with that make the corrections to get total in by 3000 rpms with 36 total. the curve should be smooth and this needs to be graphed to verify there is not obscure findings like timing spike that may cause detination. you need to make sure the springs that are used to acheive these setting do not cause idle problems (which most light springs do) verify that what you have at idle (16 degrees) does not move until about 1000 rpms. reason for this is because when your in park say idling at 900 rpms and you drop in gear to idle at 700 rpms the timming does not change.

most who use to light of a spring will find when they put there car in park it idles at 900 rpms just fine and pull it in drive it drops down to 500 rpms. why? because at 900 rpms they have it set at 16 degree initial but when they drop it in gear the timing falls back to 8 degrees or so and cause a rough idle that bounces up and down because the springs just are now able to pull the weights in at this rpm. this is a common problem when one goes and does this change with out verifing the timing graph.

also on a low vacuum engine that uses an adjustable timing canister can gives you idle problems as well. say you set your timing at 16 degrees initial and verified all is well with just the mechanical advance and then throw on the vacuum canister just to find out now it dont idle so well. with 16 degrees set at 900 rpms and the can hooked to full vacuum advance and its set to give 20 degrees more timing to give you a total of 36 degrees at 900 rpms. which is fine. but you pull it in gear the timing falls back or away to 16 degrees because theres not enough vacuum to operated the stiffer vacuum canister. and since you had the carb idle backed off some to lower the higher rpms once the the canister was installed sitting in park. now the engine doesnt idle to well with out that extra timing.

so you would have to verify that the canister you are using does not change the timing from park to in drive. so if you have 36 at 900 rpms you should have 36 all the way down to the in drive rpm. and really a good set up with a mild 2500 rpm convertor should only see about 150 to 200 rpm drop from idle to in gear. and the timing should read the same in either park or in gear.

once this is all done its time to move on to the carb and sort out any problems from there. which most would fine very little wrong with idling at this point other then to set the idle mixture screws. then put in the proper power valve and move on to jetting and if you really want to tune for 60 ft times then the pump cams. and squiter size. (some need alittle more fuel early on after the timing issues are sorted out and need to increase squiter to cover some rapid moves of the throttle to keep the engine from bogging of idle)

the end results that i find on my bbc's i can idle way down without stalling 300 to 400 rpms. i end up with about 150 rpm drop from idle (800 rpms) to in gear with 650 rpms. i can mash the peddle with no hessitation. and smooth street matters thru out the rpm range.

750 dp sounds ok for your 454...i just think everything has to be dialed in and corrected.

ddeennis is correct, the majority of idle problems can be traced to the ignition timing not being set properly, or the carburetor just being out of adjustment. Would need some more information about the combination to figure exactly what the problem is, but you should always make sure that everything is setup, and adjusted properly before installing a new product.

Vince,

Try not to associate poor idle quality directly with ignition timing. Everyone is giving some good ideas but your timing has to be off 6-10°(retarded) or more for it to TRULY affect idle quality.

I don't know what your total timing is, but you need to work backwards from there with a street driven vehicle. Total timing can be dictated by several factors. Elevation, head material, piston dome size, etc. (more timing in a domed piston motor, the flame has to travel over the dome)

Ideally you want about 12° initial timing to take advantage of the mechanical or vacuum advance. 16° will work like ddeennis said but it is also covering up a poor timing curve as he also stated.

Set your initial timing at 12°, rev the engine up until the timing stops moving and that will give you your and vacumn advace amount. Hopefully it's somewhere between 20-26° and total timing is done between 2800-3200 rpm (low side for auto trans)12° + 24° = 36° total timing.
If your initial timing is too high, say 18-20° then you're not taking advantage of off idle response.

Make sure the car is in drive when you check your initial timing. This will help the poor idle problem that ddeennis talked about that many people fight when the put their cars in drive. Stick cars don't matter since you can push the clutch in at a stop light. Your vacumm advance may have the amount of vacumm it's supposed to have stamped on the bottom of it also, but if its worn out it may not be acurate.

Now it sounds like the real problem IS your carb. If it is highly modified like you said it may be something internal that you can't fix with out some disassembly. Could be an internal idle circuit or an airbleed or a booster. Get the "Holley carb" book and read, read, read. If it's too much for you to handle then either a new factory Holley DP or HP would be the best.

Good Luck, Rob

Good advice here..double check your timing.
The 750 double pumper is a good choice,and FYI, you won't lose any seat of the pants with a vacuum carb..my 3310 lays enough rubber rolling @ 35-40 MPH that would make most people jealous :D

[QUOTE]Originally posted by mfsr:
[QB] Vince,

Try not to associate poor idle quality directly with ignition timing. Everyone is giving some good ideas but your timing has to be off 6-10°(retarded) or more for it to TRULY affect idle quality.

Rob,

We’ll respectfully disagree with you on this point. As little as two or three degrees of initial timing can have a major affect on the idle quality of an engine, and drivability. It can cause the individual to tune the engine incorrectly to compensate for the timing issue. We’ve got a 66 Chevelle that will smoke at 11 degrees initial timing, and barely break the tires loose, yet with 14 degrees initial timing it idles completely clean, and will spin the tires at will.

Without know what type of modifications have been done to the 750 that is on the engine it would be impossible to say what may or may not be the problem with it.

I have toggles under my dash for timing retards for cruising - have one degree in one and two in the other. (normally on). It helps when trying to drop the coolant temps.

I will set total timing so that it is 29 deg with the retards on (it likes 29 total N/A) and then when cruising I will add 1, 2, or 3 deg of timing for 30, 31, 32 degrees total. At an idle clicking 1 deg makes a noticeable difference on my engine! If I add 3 I have to turn down the idle speed or it will run at 1600!

Hi guys.

I tried a 1850 600cfm carb and it idles like a champ now!

I knew the carb was the problem - I am skeptical of anything "modified". There were internal passages and other things that were tweaked. To the untrained eye (or even trained eye for that matter)its hard to see what was done to a carb.

BTW, my timing was set @ 17* and ran awesome. I'm wondering once I get another carb on there if now I can lower the timing and get more low-end (4* is recommended if you can believe that).

On second thought - I don't want anymore low-end? I'm smoking the tires as it is!

I do not have the amount of knowledge that some of the sharp guys here do but I will tell you this.
I think not enough Street Driving enthusiasts spend enough time on their distributors setup.
Here's why

As cam duration increases the need for more initial advance increases because of this increased overlap.
Increased overlap kills vacuum wich the carb needs to idle correctly.As you know the vacuum is what pulls the air/fuel into the engine.
Now how's it going to do that if you have just killed the vacuum signal with a radical cam???

Here is a strong point.

My friends little 289 had a cam that had 230 at.050 It idled in gear without problems and had smooth responce from idle to full throttle.
We swapped in a cam with 256 at .050 and reset his timing for the same total advance of 40.
The initial worked out to be 12 degrees.

He adjusted the idle mixture screws and noted that they didn't respond well at all.

Well we wanted to take it for a cruse but it refused to idle in gear without loading up the plugs and giving sluggish responce.

I told him that we needed to decrease the distributors centrifugal advance.
This way we could dial in more initial and it would bring the idle vacuum back up.

We argued about it for about two weeks.
Finally he agreed to change it.I reset it so it had 20 in the distributor and another 20 initial.
Whalla, the idle mixture screws responded MUCH BETTER, it idles smoothly in gear, and has the responce back smile.gif

Go figure ;)

Lots of people who run big cams just lock out their timing out and don't have to worry about it. On the street locking out the timing is not recommended.