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Idle Restrictors on 750DP Mighty Demon?

37K views 43 replies 8 participants last post by  CCK 
#1 ·
I'm still running WAY to fat on my idle circuit (eye burning fumes and fouling plugs). I have fine tuned everything I could externally on the carb with no luck.. My idle mixture screws on all 4 corners are 3/4 out (for maximum vacuum reading), both butterflies are open about .015 and I installed a 3.5hg power valve (as instructed by BG tech)..
Besides the fat idle, the carb runs perfect!! I dont have any fouling issues unless I'm idling.. I can mash the pedal from an idle and get a great launch (haven't been to the strip yet so I cant say what the 60ft times are yet).. The only other issue, which I assume is related to the rich idle condition is that after I go for a short spin my idle increases by about 200-300rpm so I am constantly having to adjust the curb idle.. Even if I start it and let it warm up to full temperature before driving, the idle changes (increases) dramatically after just a short 1/2 mile drive..
According to what I read, if your with-in one full revolution on the idle mixture screws and still running rich, you may need to alter the IFR?

Here is my motor combo

355 SBC
10.5-1 with aluminum 64cc heads
Performer RPM intake
Cam Dur- 243/257 @ .050 (.550 lift)
Vacuum at idle 8hg (verified no vacuum leaks)
Timing- Locked @ 36 degrees
Idle set at 1000rpm when hot (increases to 1300 after a short drive).
 
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#2 ·
My68Vette said:
I'm still running WAY to fat on my idle circuit (eye burning fumes and fouling plugs). I have fine tuned everything I could externally on the carb with no luck.. My idle mixture screws on all 4 corners are 3/4 out (for maximum vacuum reading), both butterflies are open about .015 and I installed a 3.5hg power valve (as instructed by BG tech)..
Besides the fat idle, the carb runs perfect!! I dont have any fouling issues unless I'm idling.. I can mash the pedal from an idle and get a great launch (haven't been to the strip yet so I cant say what the 60ft times are yet).. The only other issue, which I assume is related to the rich idle condition is that after I go for a short spin my idle increases by about 200-300rpm so I am constantly having to adjust the curb idle.. Even if I start it and let it warm up to full temperature before driving, the idle changes (increases) dramatically after just a short 1/2 mile drive..
According to what I read, if your with-in one full revolution on the idle mixture screws and still running rich, you may need to alter the IFR?

Here is my motor combo

355 SBC
10.5-1 with aluminum 64cc heads
Performer RPM intake
Cam Dur- 243/257 @ .050 (.550 lift)
Vacuum at idle 8hg (verified no vacuum leaks)
Timing- Locked @ 36 degrees
Idle set at 1000rpm when hot (increases to 1300 after a short drive).
Just a shot in the dark but do you think you could have to much Fuel pressure going to the carb and when your driving it draws just the right amount to not foul? I had a similar problem in my Super truck (race car) runs really high rpm at idle because the butterfly idle holes have been opened up a little
 
#3 ·
What kind of dist. do you have? and what initial timing?

Other things to check are,
float level.
sticky advance weights/springs to light.
fast idle cam screw.
not enough initial timing causing you to come off the idle circuit when you raise the idle speed.
BGTECH will probably jump on this thread if he's home this weekend.
 
#4 ·
Are you using a vacuum advance hooked to manifold vacuum? What is your idle timing at, as it sits now?

The reason I ask, is because if there's not enough initial timing, the incomplete combustion will cause that same eye-watering stink that you have and can also foul plugs. It seems counter intuitive that a lean idle mix and retarded timing would cause a fuel smell and/or foul plugs, I know- but it can happen.
 
#5 ·
I dont run a vacuum advance or springs in my distributor.. Timming is locked and was set at 36 degrees.. I have to use an ignition cut-off switch to start it so I doubt its not enough initial timing.
I have a Holley mechanical fuel pump and fuel pressure is 7.5psi at idle (not sure where its at when I'm driving but the pump is pre-set at 7.5psi so I assume its the same)
The distributor is a Mallory HEI with a high output accel coil on it.. I have tried both hotter and colder plugs with no luck and I dont think a different heat rating is going to totally cure eye burning fumes anyways... Float level is right in the middle of the glass sight window..

Its almost the same set-up I had in my previous drag car but with a BG Mighty Demon instead of a modified Holley.. I didnt have this issue with the Holley on my last car...
 
#7 ·
F-BIRD'88 said:
You want to avoid drilling idle feed restrictions etc at all cost.
The chances of the idle feed restrictions being wrong are 1 in 100.
This is a last ditch thing after confirming that the idle feed restriction is actually the cause of the over rich idle.
It is most likely not the cause.

If the manifold heat riser passages are open, the manifold plenum will get too hot, especially after a short drive. These passages should be blocked or restricted to reduce the amount of heat that gets up to the manifold plenum.
This is a common issue with edelbrock (and other) aluminum intake manifolds.
When the manifold plenum gets too hot the fuel vapourizes way to easily, requiring constant re-adjustment of the idle mix screws.
The cure is to regulate the manifold plenum temp by blocking off both passages (hot summer) or block one side and restrict the other side so just a bit of heat gets up to the plenum.
This will likely fix the over rich mixture when hot and stop the plug fouling.

Your carb was designed for a functional PCV system. If you eliminate the PCV system, the whole carb idle circuit calibration is effected. (richer than you want)
Hook up the PCV to the carb so it functions as designed.
PCV is your friend. This will lean out the idle circuit back to where it was designed to do. No drilling of idle feed restrictions is necessary.

Beyond this:
Minor adjustment tuning of the idle air bleeds size may be necessary.
You will need a AFR gauge/meter to check the AFR at idle and just off idle (idle transition) in order to accurately adjust the idle air bleeds.
Does your carb have screw in swappable air bleeds?

Fix the manifold heat issue first. And the PCV.
PCV does not create a vacuum in the engine. The opposing valve cover requires a breather so the PCV can "breath" and vent the crank case vapours.

You'll probably have to pull off the intake manifold and replace the intake gaskets. Some gasket sets come with the heat riser passage block off shims included. If not you can make your own from flat thin sheet metal.

The original heat riser passages were designed for cast iron heads and cast iron intake manifold. Aluminum get hotter a lot easier than cast iron so the heat riser passage has to be blocked off or restricted.

Another cause is the Accel coil. There are two types of coils that work with two different types of HEI modules. If you have the wrong coil or the polarity wrong the spark will be weak, allowing the plugs to foul.
Accel coils are notorious for being junk. GM, MSD and Mallory make good HEI coils. Something to check out. Make sure the engine is grounded to the car body or the HEI distributor will have a weak inconsistent spark.

A wood carb spacer will help keep the carb cooler.

Excellent information, thank you!!
My intake gaskets do have the heat risers blocks so I'm glad I dont have to pull the intake again.
I did have the PVC hooked up but for some reason I removed it and currently run 2 breathers so I will intall the PVC again tomorrow.. I believe I removed it because of the limited vacuum ports on the BG.. The BG also blocks the only intake port, which is located right behind the carb... There is only one large port on the carb and I am currently using that for the PB so I will put a T in that line and branch off to the PVC too.. The other small constant vaccum port on the carb goes to the trans vacuum module..

The reason I have an accel coil is because the brand new Mallory I installed took a crap with-in the first 2 weeks so I lost my confidence in them. If the PVC doesnt reduce the fouling I'll look into another coil too..

When installing the new motor I did make sure I had good grounds.. The only downfall to the Vett eis that the body is all fiberglass but I did make sure there are good grounds to frame, motor and dash panel..

I'll report back after installing the PVC.. I will be running the car tomorrow eve and then again on Tuesday (local hot rod club) gathering.

Funny you mentioned a wood spacer, a friend just gave me one the other day so I'll try that too
Thanks again!!!
 
#8 ·
F-BIRD'88 said:
Ya funny how edelbrock saw fit to locate the vacuum port on the #8 intake port right where a holley carbs rear fuel bowl is. General Motors, Weiand and Professional Products and Holley all locate that vacuum port further down the runner towards the cylinder head (where it belongs) where it does not interfere with a Holley/Demon fuel bowl.
If "T"ing the vacuum port on the carb base results in faulty/troublesome PB's you will have to either remove the manifold and drill and tap your own vacuum port for the PB's either further down the #8 runner or on the #5 runner. or run a thick (wood) carb spacer with extra 3/8" vac source. To allow a vac fitting under the fuel bowl.
You can fab up your own if nessessary.

Then write Vic Edelbrock and nasty letter and tell him that some people like to run Holley/Demon carbs on his manifolds and to get his act together as to where the vacuum port go on a SBC Chevy intake manifold.
Your carb will function much better with the PCV system hooked up.
Edelbrock carbs have a extra vac spot on the front and rear of the carb body.

A lot of people don't realize that if you eliminate the PCV from any carb that has a 3/8" vac nipple on the base, the carb idle circuit gets way too rich as well as requiring further opening the throttle to compensate idle speed/idle air flow, without this metered extra air flow.
Thats about 99% of street carbs. "Stock" and otherwise.
Ok, I hooked up the PVC and the power brakes still worked great.. I took it for a spin, put about 15 miles on it and it really ran great!!!! The idle didnt seem to very at all from cold to hot, I did a few burnouts and then did a hard red-line pull through first and second gear and then parked it. This morning I pulled the #1 plug and took a picture, which I posted below.. I did a leakdown and compression check earlier this spring and everything is on par.. I believe its just down to fine tuning the fuel curve now.. The plug seems to have heavy carbon on the lower part of the porcelain and also on the flat part at the bottom of the threads? How does this plug look and is it telling me anything?



 
#10 ·
sqzbox said:
Hey F-bird, if that plug is new would that be wrong heat range? or too rich?
He does say it runs great. :confused:
It has too much idle and cruising time on it to make much of a read from it. To do it right, warm the vehicle up then change one easy access plug for a new one, fire it back up and make a hard 1/4(or 1/8) mile blast and kill the engine right at the end of the run, no idle time at all. Then pull the plug for reading. you will likely need a lighted magnifier to see what you need to, you are looking for the "fuel ring" coloring down inside the plug near where the porcelain meets the shell. Trying to read the tip of the plug will tell you very little.

The only thing out in the open easily readable is the "heat line" on the ground strap, it should be midway between the end of the strap and the plug shell, near the bend.

I'd say the pictured plug "might" be a touch hot in heat range, but would want to see one run correctly to really say. Can't tell anything about the mixture from it, it is too "dirty" with carbon.
 
#11 ·
These were brand new plugs that I just installed when I hooked up the PVC again. Thats why I was hoping we could get somewhat of a read from them. I'll be heading to my local Tuesady night car club meet later tonight so I'll see if the plugs get any worse.. Plugs are NGK BKR6ES-11 set at .045


Once I get this ironed out I have to work on traction.. I just installed a whole new rearend, including a 4.11 diff, new oversized half shafts, new trailing arms, yada, yada, yada and I can bust the tires lose without effort..

Thanks for all the help thus far!!
 
#12 ·
I brought up the detail, w/o altering the image enough to prevent "reading" it (as best as can be done over a monitor, that is):



With your cam, any amount of idling will soot it up at least some- even if the carb was spot on. That's why doing a "plug chop" is often recommended.
 
#13 ·
I thought it was new. I would check a couple of other plugs and compare with that one. If they are the same, might want to try 1 step colder or keep running them and see how they hold up.
A white or light gray is hard to achieve with a carbed engine but is possible. That plug is showing WOT condition and might improve over normal driving.
Another thing you can try is using another style plug that uses side gap technology AKA multi ground platinum electrode that will help over the wide open and normal street driving. They produce spark exposed to the cyl. opposed to the single ground electrode which in some cyl. shrouds the spark. In the olden days, indexing the plugs or filing the ground electrode to end directly over the center of the tip would expose more spark to the center of the cyl. Thats why they invented the multi ground electrode plug to keep from having to index or file the electrode for performance applications. :D
 
#14 ·
Ok, went to my local car club meet last night. After leaving I drove the long way home (about 5 miles) and ran it at 3000rpm most of the way before idleing into the driveway and into the garage.. I pulled a few plugs this morning and here is what they like (see below picture).. Obviously, they are getting worse and still fouling.. My biggest concern is that most of the events I will be attending in the near future are to far to drive so I will be trailering the car, which means alot of idleing, which means an even worse case sanario than the past few days..

According to BG, if the idle adjustments are less than 1 full turn out (mine are 1/2), further restriction of the idle feed restrictors are required.. Is that an option yet?

 
#15 ·
Have you considered going smaller on the jet sizes or trying a different heat range on the plugs?

I think that since you've got the idle quality tamed, that may be your next step being your carbon build up is happening beyond the idle circuit according to your plug read at hard and WOT passes. :cool:
 
#16 ·
sqzbox said:
Have you considered going smaller on the jet sizes or trying a different heat range on the plugs?

I think that since you've got the idle quality tamed, that may be your next step being your carbon build up is happening beyond the idle circuit according to your plug read at hard and WOT passes. :cool:
I did try different heat range plugs (hotter and colder) before re-installing the PVC but there wasnt a signitifcant difference.. I do have a complete holley jet kit here (64 through 99) so I could try dropping down.. My concern is that I'm nearly all the way closed on the 4 corner idle mixture screws yet still have a rich idle condition. I'm also still getting some pretty heavy fumes while idling too.

Edit: BTW, the car is still running great otherwise and launches hard from an idle and pulls good throughout.
 
#17 ·
Typically we consider anything from 3/4 of a turn out acceptable on the idle mixture screws.

If you feel everything else is correct and within spec but want more of a range on the idle mixture screws you have a couple of options. The idle air bleeds can be made larger but you will see very little from doing this. If less fuel is needed then the ifr's in the block can be stepped down 2 thousand which will decrease idle fuel and increase the range of adjustment on the screws. Leave the pcv system hooked upas others have advised.

Something else you may wish to verify is the voltage going to the ignition as well as some of the older GM stuff uses a resistance style of wire that shows 12 volts with the key on yet drops in the 8.5 or so once running.
 
#18 ·
Tech @ BG said:
Typically we consider anything from 3/4 of a turn out acceptable on the idle mixture screws.

If you feel everything else is correct and within spec but want more of a range on the idle mixture screws you have a couple of options. The idle air bleeds can be made larger but you will see very little from doing this. If less fuel is needed then the ifr's in the block can be stepped down 2 thousand which will decrease idle fuel and increase the range of adjustment on the screws. Leave the pcv system hooked upas others have advised.

Something else you may wish to verify is the voltage going to the ignition as well as some of the older GM stuff uses a resistance style of wire that shows 12 volts with the key on yet drops in the 8.5 or so once running.
Are the IFR's removable and if so do you sell the smaller sizes? Also, do you have any links or information on where the IFR's are located on the block and/or how to change them out or restrict them (tech article?)?

Meanwhile I'll check voltage at the distrbutor too.

Thanks!!
 
#19 ·
#2 in this picture is the ifr. Your block will look a little different in that it doesn't have the other adj bleeds but will have a screw in ifr where #2 is shown.

These are blank in 10 packs through our dealers and then you drill to size with apin drill set as needed. Best to use the same pin drill set to measure the ones currently in place and then step them down accordingly.
 

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#20 ·
Tech @ BG said:
#2 in this picture is the ifr. Your block will look a little different in that it doesn't have the other adj bleeds but will have a screw in ifr where #2 is shown.

These are blank in 10 packs through our dealers and then you drill to size with apin drill set as needed. Best to use the same pin drill set to measure the ones currently in place and then step them down accordingly.
The default IFR's are .033, what size would you recommend dropping down to?
 
#21 ·
My68Vette said:
The default IFR's are .033, what size would you recommend dropping down to?
Would verify their size first but we normally only drop them 2 thousands at a time . They are real sensitive to size changes and if you go too far you will create an off idle stumble problem.
 
#22 ·
The ifr is a simple screw them out and screw the new ones in . If it doesn't help or correct the issue the carb can easily be returned to stock with no harm done. Most cases the gasket can be re-used and if he wants to test it prior to buying bleeds he can restrict those bleeds with a small piece of wire which will have the same effect.

Have to disagree on the iab's though as .002 on the ifr can have a greater effect then .004 - .006 on the iab's . You can go out of the range of what the iab will correct long before the ifr. If one is just slightly rich then the iab can help but not in a case of "pig rich".

If the car runs right everywhere else other then idle and everything is correct then the ifr would be the next logical choice.

We would not change jetting for a rich idle unless it was a case where you are at altitude but then it would be rich at wot whereas that is not the complaint here.
 
#23 ·
Do you have a recommended AFR Gauge (brand or type)?
Meanwhile I will verify full voltage at the dist and then try installing a temporary .017 staple in the IFR to see what effect it has and/or if it resolves the rich idle issue (provided I find full voltage at the dist).. So far, in the past few days, I only made a few short trips and the plugs lool like hell already. I understand that it would be good to drive it longer but what If I haul it to the dragstrip 2 weekends in a row, isnt that bascially the same thing I just simulated (a bunch of ideling, a few burnouts and a few hard pulls)? I also had a 73 camaro that had a similar motor set-up that ran 11.40's and it never fouled plugs this easy. I even had a modfied Holley 800dp on it with twin 30cc acc pumps and #36 squirters.

This has been very interesting and educational for me.

Thanks again
 
#24 ·
If this is a new engine, it can take a considerable amount of time and miles for the rings to be fully seated. Depending on the ring type, material and cylinder finish this can be upwards of 500 or more miles.

If that's the case, I would tend to want to keep the heat range and carb tuning safe, not overly rich but not set to "KILL", either. Get some miles on it and check the plugs along the way.

I'm not saying the IFR's won't need resizing smaller or the IAB's larger- I'm just suggesting there may be other circumstances preventing a good plug read at this time.

I'd suggest you look at all the plugs to get a reading of what is happening, not just the same one or two- this may be deceiving unless they're all seen together, and kept in order to determine if there's a cylinder outside the norm.
 
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