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Jetting& Altitude #1644664
07/10/14 12:02 AM
07/10/14 12:02 AM
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pjc360 Offline OP
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Whats the basic rule of thumb for jetting and altitude with a holley style carb? I have been told you decrease 1 jet size for every 2000 feet above sea level and I have been told you decrease 1 size for every 1000 feet above sea level.
I live 4000 feet above sea level and I run a quick fuel super street series 680cfm vaccum secondary carburetor on a 300 hp crate 360 magnum. The other day I noticed the engine was a little shakey and felt different so I decided to check my pugs, I had 3 fouled plugs. And I think this happened because I forgot to install the primary vent on thhe primary metering block and it flooded the crap out of my engine. This carburetor came with 68 main jets and 78 secondary jets and .070 idle air bleeds and .031 idle feed restrictors and .049 power valve channel restrictors and a 6.5hg power valve and .028 high speed bleeds.
Since getting this carburetor last summer I had to go down to .025 idle feed restrictors because the .031's were fouling plugs at idle and I am running .072 idle air bleeds and I went down to 64 main jets and 74 secondary jets and I went down to .045 power valve channel restrictors and I have left the high speed bleeds alone at .028 and I switched to a 9.5hg power valve.
I was wondering if it is possible that the 64 main jets are too big? Like I said I'm pretty sure the reason the plugs fouled was due to the vent that I didn't put back in on the primary metering block. I have my floats set to half way up the sight glass. And all 4 of my idle mixture screws are out about 1.25 turns on all 4 screws.
I have some holley .063 main jets I installed them and ran them for a couple of days and I didn't hear any pinging or feel anything significantly different then how it ran with the .064 main jets. So today I put the .064 main jets back in it and am going to see if I can tell a difference between them and the .063's.

Re: Jetting& Altitude [Re: pjc360] #1644665
07/10/14 12:31 AM
07/10/14 12:31 AM
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RapidRobert Offline
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I'd not stray from the basic "one change at a time" rule. I'd get the vent taken care of & check if the float level should be at the bottom of the hole or where you have it halfway up. Wallace racing has excellent pics of plugs for jetting/heat range or an innovate LM1 is the best/pro way to get it spot on


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Re: Jetting& Altitude [Re: RapidRobert] #1644666
07/10/14 02:03 AM
07/10/14 02:03 AM
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yes I know a lm1 would take all the guess work out of this, but unforutently I cant afford one of those at the moment. Tonight I decided to richen up my idle air bleeds from .072's to .070's and I started the engine and the choke was on letting it warm up and the choke opened but my fast idle was still on and it is popping thru the exhaust sounding a little lean and when I looked under the hood I noticed my passenger side hedders were glowing red. What the hell is up with that? I'm running 64 main jets at 4000 feet above sea level. Which is what quick fuel suggested running it. I read this was most likely un-burnt fuel burning in the exhaust and that when I loaded the engine it would go away. My timing is 15 degrees initial and 33 degrees total. I have never noticed this before because I haven't really messed with it after dark. What do I do about this?

Re: Jetting& Altitude [Re: pjc360] #1644667
07/10/14 02:35 AM
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Well I just tried swapping him some .065 idle air bleeds and same thing, the hedders start to glow red right at about 2500 rpm when your holding the throttle open and steady. When I noticed them starting to glow red I gave the engine 3 or 4 shots of fuel with the accelerator pump and I noticed that made it go away. So are my main jets too lean at 64? I have some 65's I could try. This is strange to me because it runs great going down the road, no surging, no popping, no hesitations anywhere. I figured since they stopped glowing when I gave it a few shots of fuel with the accelerator pump then that means it's too lean. Correct?

Re: Jetting& Altitude [Re: pjc360] #1644668
07/10/14 02:44 AM
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Maybe holley saying 1 jet size for every 2000 feet above sea level is more accurate

Re: Jetting& Altitude [Re: pjc360] #1644669
07/10/14 07:13 AM
07/10/14 07:13 AM
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I've heard guys on here say that RPM with no load will cause the carb to go lean, thus red headers.

That said, before I knew anything about AFRs, rich/lean, etc. When I took my car out to Colorado, I followed the chart that Edelbrock provided with the carb, on what jets/rods to use at various elevations. The car ran and drove fine the 2 years I had it out there. ALthough, it didn't have any power, but what NA motor does at 7,000 feet ACTUAL elevation? I'd bet the DA on most days was up near 10-12k


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Re: Jetting& Altitude [Re: 70Cuda383] #1644670
07/10/14 07:58 AM
07/10/14 07:58 AM
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Mike P Offline
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One jet size per 2000 Feet has been my experience. I live at 4500 feet and usually make an annual trip to see family that live at 200 Feet. To tune the car back to "normal" would take 2 jet sizes and taking about 2 degrees out of the timing.


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Re: Jetting& Altitude [Re: Mike P] #1644671
07/10/14 08:38 AM
07/10/14 08:38 AM
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RapidRobert Offline
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Yes too lean will make em glow red as will way too rich & it's burning in the ex but for it to go away when you pumped it tells you which one you are dealing with. I'm assuming there is no vac leak that is skewing this and the float levels are correct. I'd k.i.s.s. richen those jets ASAP before something fries.


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Re: Jetting& Altitude [Re: pjc360] #1644672
07/10/14 09:03 AM
07/10/14 09:03 AM
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It appears from what you have written that you are lean from idle to 2500rpms which would point to the idle/transfer circuit, not the main jetting. The idle and transfer consist of the Idle Feed Resisters, Idle Air Bleeds, and the metering screws. You noted that your metering screws are 1-1/4 turns out which sounds like a lot for a QF carb. If you richened up your IFR's, you might be able to get the metering screws back down under 1 turn (Z at QF suggests under one turn out each.)

My guess is that your .025 IFR's are too lean and you are making up for that by turning your metering screws outward to 1-1/4 turns. Once you are off idle, the metering screw setting still affect the A/F during the transition into the main circtuit. But, above idle, the IFR's and the IAB's are also contributing to your A/F ratio. Since you richened it up by adding 3-4 pump shots, that is telling you that you are lean and could be the real reason for the red headers. I would suggest going back up to the .031 IFR's to richen up your transition circuit.


(REF NOTES ONLY: On my Super Street QF carb, my mains start in around 2500-2700rpms. In four years, I have never had my metering screws in or out more or less than 1/2 to 7/8 turn outward each. I currently run .033IFR's and .050 front and .060 rear IAB's and I get 14.0-14.2 at 45mph which is 2400rpms but I am at a lower elevation than you. I looked back at my tuning notes and I found that when I first started tuning with an A/F meter, I was running with .028 IFR's and my 1700rpm cruise was WAY LEAN at 17.0 and my 2500rpm cruise was still lean at 15.6. [E10 fuel]. QF also suggests floats set at 1/2 up the glass. My SS also came with 68 main jets and I am now running 67's. I have tried 58-67 main jets with many other changes while testing A/F ratio's. The 67's give me mid 13 A/F readings at cruising speeds at 55-65mph [2700-3300rpms] which is my current goal) and they give me the most throttle response when my PV is open [along with .078 PVCR's])


1970 YO7 A66 [Canadian Export] F8 Challenger
340 (Currently in shop for stroker assy.)
Re: Jetting& Altitude [Re: YO7_A66] #1644673
07/10/14 08:51 PM
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When I was running .031 idle feed restrictors my plugs were fouling at idle it was so rich. I have 3 .028 idle feed restrictors but I broke the 4th one so I need a new .028. What size of idle air bleeds should I run when I swap to the .028 idle feed restrictors? .070's? or .072's? This has me confused. I even dropped the idle air bleeds down to .065 from .070 and it didn't change the lean condition with the hedders.

Re: Jetting& Altitude [Re: pjc360] #1644674
07/10/14 08:53 PM
07/10/14 08:53 PM
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And another thing that is confusing me is that it doesn't act like it's lean when your driving it, it acts like it's perfect. But when it's sitting in park idling with the fast idle on it pops out the exhaust and the hedders start to glow red. Why doesnt act lean while driving it?

Re: Jetting& Altitude [Re: pjc360] #1644675
07/10/14 09:17 PM
07/10/14 09:17 PM
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I'm assuming your timing is in the ballpark at the particular fast idle RPM that makes em glow red. This might be too simple but I'd think it could be way too lean at (fast) idle (the idle circuit) but the transition/mains are in the normal range & it richens up enough to straighten out.


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Re: Jetting& Altitude [Re: RapidRobert] #1644676
07/10/14 09:42 PM
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Yes timing is set at 15 degrees initial and 33 degrees total. I have some .028 idle feed restrictors, but I only have 3. So I will put some .028 idle feed restrictors in the primary metering block and see what that does. If it helps then I will order another .028 idle feed restrictor and put them in the secondary metering block as well.

Re: Jetting& Altitude [Re: pjc360] #1644677
07/10/14 09:53 PM
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""When I was running .031 idle feed restrictors my plugs were fouling at idle it was so rich.""

Then you compensate with the metering screws. If I remember correctly, you have an auto. The next time you have the engine running and if you can take it for a short drive to get it up to temp, then put the car in D (with emergency brake or someone behind the wheel) and tune the metering screws evenly until you get the best vacuum reading at the idle rpm of your choosing.
The goal is you have the highest vacuum reading at your chosen rpm and the metering screws are all outward even. This vacuum number becomes your goal after any future idle adjustments. Always meet this number or get higher at the same rpms.
You will need to readjust the metering screws every time you change the IFR's or the IAB's.
I would still suggest that you retry the 031's and then readjust your metering screws for the highest vacuum reading. Then if your metering screws are still outward more than 1 full turn, then drop down (richen up IAB's).005 sizes (or what ever size you have) and then readjust your metering screws once again. You are super lean at high idle (apx 1200-1500rpms or so?) and this is the IFR's/IAB's or the metering screws. Since your metering screws are out 1-1/4 turns, I think your IFR's are tooooooo small. You need to fix the glowing headers first and then lean out your idle in D. Fix the leaking baffle that you wrote about too.
If you don't want to go larger on your existing .025 IFR's, then put some small wire down inside of your IAB's to test. This will make the IAB's smaller/richer but I am not sure it will be enough.


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340 (Currently in shop for stroker assy.)
Re: Jetting& Altitude [Re: pjc360] #1644678
07/10/14 09:56 PM
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What seems strange to me is, there was no symptom of a lean condition while driving it. Only when it was sitting in park. Why is that? With the hedders glowing red you'd think there would be a lean stumble or hesitation at lower speeds but there isn't, in fact it runs great when your driving it around. The only reason I noticed this is because I had the fast idle going with the hood popped when it was dark outside.

Re: Jetting& Altitude [Re: pjc360] #1644679
07/10/14 10:13 PM
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""there was no symptom of a lean condition while driving it. Only when it was sitting in park. Why is that?""

When you are sitting in P at high idle, you are idling on the transition circuit. When you are tooling around town, you have help from the front pump circuit to help on acceleration until your mains start in. The pump circuit is a designed-in "bandaid" to help cover up any lean spikes during the transition into the mains. It sounds like your current pump setup (squirters/pump cam) is doing its job along with the .028/64 main setup.

Since you are using 64 jets and .028" MAB's (fairly rich which starts the main circuit sooner than a .033" for reference), this may be allowing the main circuit (jets and MAB's) to start fairly early and the main circuit may be tuned fine. This might be why you do not have any known issues while cruising.
Please concentrate on richening the IFR's and the IAB's.


1970 YO7 A66 [Canadian Export] F8 Challenger
340 (Currently in shop for stroker assy.)
Re: Jetting& Altitude [Re: pjc360] #1644680
07/10/14 11:23 PM
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I really need a wideband so I can take all the guess work out. Starting today I'm setting some money aside for one.

Re: Jetting& Altitude [Re: pjc360] #1644681
07/10/14 11:33 PM
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They are not only a tuning tool but a teaching tool as well! You can have a gauge/O2 sensor kit for apx $190.


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340 (Currently in shop for stroker assy.)
Re: Jetting& Altitude [Re: YO7_A66] #1644682
07/11/14 12:30 AM
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The only reason I am scared to run the .031 idle feed restrictors is because last time I had them in the carb 4 of my plugs fouled at idle and I was setting my idle mixture screws to the best vaccum reading in drive with the brake applied. That didn't matter plugs stil fouled.
I have noticed my engine fouls plugs super easy, and I did take care of the vent, I didn't break the vent I just simpily forgot to put it back in.
You also have to remember I live 4000 feet above sea level, so since .031's work fine on your 340 right now, if you came up here you might notice they are too big. I don't want to foul any more plugs, I am tired of removing wires and plugs all the time. I have 8 fresh plugs in there right now and I am about 80 percent sure if I changed back to the .031 idle feed restrictors some of my plugs would be fouled regardless of where my idle mixture screws were. And I just double checked my idle mixture screws and they are all out about 1 turn.

Re: Jetting& Altitude [Re: pjc360] #1644683
07/11/14 12:37 AM
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Plus the biggest idle air bleeds I have right now are .072's. Running .031 idle feed restrictors the .072 idle air bleeds probly wouldn't be lean enough to keep my plugs from fouling.

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