Fishndude

Ok guys, I have been pulling my hair out trying to get my rich idle condition ironed out with no success.. I’m no stranger to Holley or BG carbs but I’m not that knowledgable when it comes to internally modifying them and I think that’s where I may be at (idle feed restrictors?) Below are my motor specs and a list of what I have done but my idle condition is still overly rich and even eye burning..

Motor specs
355 SBC
less than 300 miles
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- no vacuum advance and weights tied (ignition cut-off switch installed)
Mallory HEI dist with high output coil
Idle set at 1000rpm
PVC is hooked up

I have the motor currently idleing smooth at 1000rpm (well smooth with a big lope )
4 corner idle mixture screws set at ½ turn “out” (gives me highest vacuum reading and smoothest idle)
Primary and secondary butterflies open to expose .015 of the transfer slot (this also cured the off-idle hesitation)
Power valve reduced to 3.5
Float levels are perfect
I tried both hotter and colder plugs with no success

The car launches hard from an idle and pulls good throughout.. In other words, its runs fantastic, I just cant get the overly rich idle condition ironed out. I can get the plugs to clean up by going for a long drive but that’s not always an option. I just installed another new set of plugs and after taking it for a 5 mile spin, 2 days in a row, the plugs are already black.

I have a BG tuning book and it says if your idle mixture screws are less than one turn "out", you may need to restrict the Idle Feed Restrictors (IFR's) but I,m not sure how to do that..

Thanks for any help

Scott Marzahl

Does BG have a call in tech line?

capevettes

What size jets are you running? It sounds like it may be over-jetted and getting too much fuel. What size carb?

lars

Jets won't affect idle mixture -idle mixture is controlled by the IFR and the low speed air bleeds.

If you have a big lumpy cam, it is not uncommon for idle mixtures to be in the 13 - 13.5 AFR range. With only .015" transition exposed, the idle mixture screws should have good usable range control over the mixture. Are you saying that the mixture screws are not effective and you can't lean it out? Or are you saying that the engine won't idle if you lean it out further? Or are you running rich in the transition/light throttle cruise range?

Lars

Fishndude

Jets play a role in the idle circuit? I believe thats only the case if you expose to much of the transfer slots, which starts the transition from the idle circuit to the main circuit.

I called BG the other day and they told me to drop the PV down to 3.5, which I did but that didnt do anything.. I had a feeling it wouldnt because the car pulls 8hg at idle and had a 6.5 PV installed from the factory so it wasnt opening at idle anyways..

gerry72

I wonder how much more helpful it would have been to a response had you bothered to include what carburetor you were working with and what, besides the plug sooting, leads you to believe you have a rich mixture?

If all you're going by is plug sooting under the conditions you describe, that doesn't indicate anything other than the engine spends a lot of time at idle and low speed. If they soot up with an engine that is driven like a daily driver, that's a different story. It's kind of hard to do much with sooting under those conditions and with a cam that big. There is a lot of cyclic variation occuring.

As far as the idle fuel channel restrictors -or idle jets- it would be difficult to say where you should go with that without knowing what carb you're using or if you really are fat on the idle circuit and by how much. If it's around a 750 cfm, then you're probably at a low to middle 30s on the jet and in the middle 70s on the idle air bleed. Small or larger carb won't make too much difference on the jet since this circuit is activated by manifold vacuum and cfm has no huge affect here at idle. It does have a fuel timing component and it does contribute to metering well into the main circuit but idle metering is somewhat isolated.

And pay no attention to the nonse about changing your main jets and powervalve. These do nothing until the booster start flowing and if your boosters are flowing at idle, you are using a lawnmover carb. Main jets and the powervalve circuits have no affect whatsoever on an engine's idle circuit.

Fishndude

The idle mixture screws obtain the highest idle speed and highest vacuum reading at 1/2 turn out.. If I turn them in farther, the engine begins to die and I cant get a good idle.. I think I'm running a little rich across the board but wanted to get the rich idle condition sorted out first so I can get a better reading off a cleaner plug.. However, the car still runs great at cruise and WOT.. Sorry, I dont have any gauges to check air/fuel ratios..

Fishndude

Sorry, its a BG Mighty demon 750dp and it fouls a set of "new" plugs by just starting it one time and letting it get up to temperature. After a 3rd start you can bascially toss the plugs in the garbage if you dont take it for a long drive to clean them up..

capevettes

No the jets won't cause it to run rich on the idle circuit. I was focused on your plugs turning black, assuming it was after being driven for a period.

lars

You might want to run a hotter set of plugs. If you can get the engine to die using the idle mixture screws, then you have full idle mixture control and you can lean it as far as you want. You're indicating that the engine doesn't want to idle if you lean it further.

My Demon carbs idle best between 1/2 turn and 3/4 turn on most engines, so that's not an uncommon setting. If you want to try playing with the IFRs, the Mighty Demon has replaceable IFRs in the metering block. But you can try this first: Take a staple from a common office stapler - these are usually .017" in diameter. Straighten one leg out andleave the other one at the 90-degree angle. Pop the metering block off the carb - you will be able to see the 2 screw-in brass IFR inserts - they look just like the air bleed inserts in the venturi area. Stick the short leg of a staple into each of the 2 IFRs in the metering block and let the long leg of the stable get sandwiched by the gasket for retention. This will lean out your idle and transition circuit by up to 2 full points and will require re-setting the idle mixture screws.

...But it won't allow your engine to idle any leaner than what it wants to idle. If the "fix" works, order some air bleeds from BG and increase the size of your low speed bleeds - this will lean you out at idle and transition.

For info, a power valve that is open at idle won't affect idle mixture as long as the throttle blade opening is not excessive (less than .020" transition exposed). But if it's open during transition and light throttle cruise, it will certainly screw things up.

Lars

corvettedave383

Ive got the same problem, I had to drill holes in the throttle blades to get more air in there to lean it up at idle, ive got the afr o2 sensor which proves it has worked, the holes have made a big improvment, but with a big cam you can only go so far.

I think if you start playing with the idle air bleeds, you can upset the transition from idle to wot, etc

dave

gerry72

Okay, first call up BG tech and ask them: IFR and IAB size (again, you're probably in the low 30s on the IFR and middle 70s on the IAB but you need to know for sure). If you don't already know this, ask if the IFRs and IABs are screw in and what thread size they are. IFRs are usually pretty small 6-32 and IABs are larger at 10-32. But you do need to know. Also, ask their tech folks what sizes they recommend to address your issue. Don't be surprised if the tech person knows almost nothing.

So now you have a starting point. Now see if your idle circuit is really rich based upon how the engine responds to this test: Set your mixture based only upon the highest vacuum reading. Now raise the idle speed up to around 1,200 rpm. Turn the idle mixture screws in a small equal amount at a time. Does the engine speed rise? if it does, your idle circuit is fat. Now turn the mixture screws out from you baseline. Does the engine speed rise? If so, the circuit is lean. If the engine speed faulters when you go in or out from your baseline of highest vacuum at normal idle, the circuit is fine.

Once you have your information, go to Quick Fuel Technologies and order the jets and bleeds. Quick fuel has the cheapest prices around for these. IFRs come in packs of four so you'd order one pack. IABs come in packs of two so you'd order two of those. You're trying to make the whole circuit smaller if you really are fat. I'd go down two on the IFR and five on the IAB. That's just a starting point. Your engine will tell you if it is running too rich.

Can you go lower? Maybe but you may find your engine just won't run as lean an idle as you'd like. If the plugs are blowing out, get a hotter plug and/or put an MSD on the thing.

lars

Drilling the throttle plates is a technique that is used to allow adequate air into the engine to maintain a desired idle speed without opening the throttle blades far enough to expose more than .020" of the transition slots. If more than .020 of the slots is exposed, the engine will run rich due to fuel being discharged from both the idle fuel, transition slots, and in some cases, from the main discharge nozzles. The drilled holes do not lean out the carb by themselves - they allow a smaller throttle opening so the carb can function as it should. Fishndude only has .015" transition slot exposure, so drilling the plates will serve no purpose in his case.

Lars

Fishndude

Now thats what I'm talkin about!!!!
I read soemthing a while ago about using wire to restrict the IFR's but couldnt find the article. I'll try that with the staple and see how it works and your right, I just put a mic on a common staple and it was exactly .017

BG says they have blank IFR's that you have to drill out yourself so if this works, I'll pick up some blanks.

Fishndude

Lars, while I have you on the rope, what would be a resolution for a rich condition at part throttle should I discover I have one? I figure, why not ask now plus I really enjoy carb tech

lars

At light-load, part throttle, the carb actually operates on the transition circuit - not on the main metering circuit. Curing a rich condition is the same as fixing the rich idle - reduce IFR or increase low speed air bleeds. If you install the staple as a temporary troubleshooting technique, you will find that your cruise mixture will go lean.

Lars

Fishndude

Excellent, thank you!!
I'm heading to the garage right now to install my staple

gerry72

Don't be surprised if it won't idle at all. A four-corner idle has IFRs on both the primary and secondary metering blocks. It also has symetrical IABs at all four corners at the throat entry. A two-corner system has a fuel side only on the primary block. On a two-corner 750-sized carb, you'd be looking at an idle jet around .045 (feeds both primary and secondary throttle plate) which has an area of .0015. If you put a .017 wire (.00022 area) into that restrictor you knock the area down to .00128. Or an equivalent jet of .040 and that's a lot. A four-corner, which you do have, has four IFRs (two in each metering block) in the low to middle 30s, so lets go with a .033 which gives you an area of .009. That .017 wire will reduce your jet area down to .009 which come out to a .011 jet -and that pretty much closes up that hole and if it does, it will pretty much be running off the idle circuit still untouched on the secondary side. And that won't tell you very much other than such an imbalance is not a good tuneup.

On a two-corner Holley, I find it hard to get a rich enough (no cruise lean surge) idle circuit going more .013 on a 350-sized engine. But that is of no value to you since you have a four-corner system with IFRs in the primary and secondary blocks.

Ironcross

Check the floats, and I don't mean the drop measurement. Look for one that would have gas in it and therefore not controlling the needle and seatl

Fishndude

I found that out the first time
They want me to drop down 2 thousands on the IFR but I have to drill them out because they come blank..

There is no doubt its fat since I cant even breath, it smells like raw fuel and my eye are watering


I'll try the reduction in IFR first and then see what happens because its something I can do without having to buy or wait for parts

Thanks for your help too. I dont want anyone to think I dont read their post, I read them all