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tuning HEI for 4spd 350

dyeager535 said:
Vacuum keeps the piston (and thus primary rods) pulled into the primary jets right? Won't any air bleeds affect that? How about changing the primary rods to a thinner tip profile?
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Bullseye, the richened idle smell was what got me to dial it back and look for another solution that was on the airside and not fuel side.

The QJet I have is OEM for a 160HP '77 automatic and the air bleeds are drilled to size by the factory. It is no longer bolted to a 160HP engine and automatic trans - it has to be somehow not quite right though close enough for sure.I need to reread Cliffs book but at some point tuning the QJet gets into drilling and adding adjustable orifices to reduce the idle air (increases vacuum). Before I do anything, I'd like to find someone that has had a similar tune issue with the QJet and solved it.

My father-in-law thinks it is timing but it does not sound like anyone here has had a lean buck due to timing. This evening if I go for a drive I will try disconnecting the vacuum advance and take the few no stop right turns around the house.

One concern I have is my idle screws are almost seated - I have maybe 3/4 turn of adjustment left. Anything I do to increase fuel through changing rods or reducing air bleeds will probably mean less fuel from the idle circuit and I don't have much room to adjust idle circuit fuel.

I am hoping this is timing related since I have more room to tune there.
 
I have the Roe book somewhere, but I recall all the air modifications had to do with trying to make wild cams idle, and/or deal with much larger displacement.

I went the wrong way with my rod suggestion, you'd want a thinner top portion of the taper, to provide a bit more fuel when the piston is pulled down. If lean is the problem, adding air would seem to be counterproductive right? Using the rods to richen up light throttle would seem to be a pretty easy approach. If the taper to do so is there.
 
dyeager535 said:
I have the Roe book somewhere, but I recall all the air modifications had to do with trying to make wild cams idle, and/or deal with much larger displacement. I went the wrong way with my rod suggestion, you'd want a thinner top portion of the taper, to provide a bit more fuel when the piston is pulled down.
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My cam is mild and idle is near perfect with 21 in-hg - it would idle at 500 rpm; crane 12353917 .420/.442 duration .270/.280 - basically same as the 2102 performer-plus. The guys that built the motor talked 16 year old me out of anything bigger - I think of them often.

I like your idea of a narrower upper rod - that makes sense and reducing the idle bypass makes less sense now because my problem is not at idle with steady high vacuum. As soon as the engine is under load the vacuum drops so the rod begins to lift and the fuel is increased to remove the lean spot. The rest of the rod stays the same. Before I start changing rods let me check the A/F curve on the analyzer.
 
I'm sure in one of your threads before I mentioned it, I removed EGR on my old 305, no other changes, and immediately ended up with a part throttle lean surge. Instead of spending all the time of trying to get the right rods (at the time the research resources weren't as handy as they are now) I threw in jets on the primary side that were one size larger. Problem gone. Oddly, didn't notice a downside occurring from doing so.

Somewhere, I think it was online and not a book, there was a listing of all the rods and all their tapers/measurements on the primary side. The secondary specs are easily found, primaries were a bit tougher.

I really started digging into some of those as I was dealing with an Olds 403 that was using a CCC Q-jet, but got to the point that fuel injection was an easier way to get to proper running than trying to mess with a carb. Not that it can't be done, but the fueling changes are a real pain on the Q-jet when you start dealing with primary rods and jets, springs, pistons, and everything else changed from application to application.
 
forest_peterson said:
I have a copy of the spreadsheet listing all the rods and p/n
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It was massive enough that I felt it wasn't worth the time to try and acquire all of them, then try one at a time to see where they too me lol.

I suppose if you figure you are pretty close as-is, going up or down a step it wouldn't be too tough.
 
forest_peterson said:
I agree. Is there a low speed adjustment other than idle fuel on the QJet? At that low speed both the power valve is not enriching quick enough and the accelerator pump shot is not enriching quick enough.
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Idle circuit stand alone with the idle orifice and down Chanel restriction with the throttle at idle are fixed other than the mix screws. Obviously has nothing to do with the power piston or acc pump.

You said earlier at idle, now it's at low speed. 2 completely diff scenarios.

Just off idle brings in the main system along with the pump and Apt system(power piston).

If you have no lean surge at cruise speed or hesitation just off idle your
Apt setting is good or close as well as the pump is working fine.

You said you can tune out the bucking with a half a turn out on the mix screws, what's wrong with that unless the exhaust is offensive?
 
dyeager535 said:
It was massive enough that I felt it wasn't worth the time to try and acquire all of them, then try one at a time to see where they too me lol.
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Looking at the spreadsheet I don't have the primary rod profile like I do for the secondary rods.

My QJet (17056208) primary rods (assuming still factory) should be the #48K rod and the #77 jet; I have no idea what that means without looking it up.

Looking at the spreadsheet: The 77 jet is as big as they get on a small block and the 48 rod is also on the larger side; the area between the rod and jet is on the larger side side (0.00285 sq in). The manual transmission version of my carb has the same jet and rod. This jet and rod combination was only used on '76 small blocks (all models both fed and california emissions) and '73 truck big blocks. The next year in '77 they leaned the primary by increasing the rod to 53. In '75 the mix was leaner with smaller main jets 72 with a similar rod. The '74 and '72 had a similar mix. The 48/77 is 20% larger than the common 70/44 smaller ratio used in the late '60s but still not much larger (5%) than the average ratio used on all 4BBL quadrajets.

This Cliffs thread says my Qjet should have the smallest 0.026" power tip K http://cliffshighperformance.com/simplemachinesforum/index.php?topic=2006.0 that have comparisons here at quadrajetparts http://quadrajetparts.com/power-piston-primary-rods-a-2.html

So, to get a richer mix sooner with the power piston pulling up I would need a thinner primary rod. Online the only smaller rod I can find for sale is a #44K. Listed in the spreadsheets are #47, #46, & #45; if I look around I should be able to find these.
 
blazer74 said:
You said you can tune out the bucking with a half a turn out on the mix screws, what's wrong with that unless the exhaust is offensive?
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Yes, smells just bad enough at idle that I leaned it back out.

blazer74 said:
You said earlier at idle, now it's at low speed. 2 completely diff scenarios.
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Meant just off idle

blazer74 said:
If you have no lean surge at cruise speed or hesitation just off idle your Apt setting is good or close as well as the pump is working fine.
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All good here - have a pressure regulator that fixed surge/bog at cruise. off idle does not seem to have a bog but maybe. without doing a full wot I have never tried to detect a bog.

blazer74 said:
Just off idle brings in the main system along with the pump and Apt system(power piston).
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This is where dyeager535 is suggesting I try a different primary rod so the power valve comes into play with more effect. I could try a heavier power valve spring (I need to see what vac is doing). Maybe try giving power valve another turn up. I think the accelerator pump is working fine; the pump rod is on the large shot hole in pump arm.

Seems like there are a few possible causes
  • lugging (all I can do is slow and downshift)
  • timing
  • power valve spring
  • accelerator pump
  • APT adjustment
  • primary rods
It would be nice if I had the a/f profile to help rule that out; the local shop was going to check the curve last week but he is getting his shop shut down and has bigger problems.
 
77/48k. .026 tip rod jet combo is quite common for those year quads. They are also large main air bleed carbs which is why the larger jets to get the correct mix. Cliff himself does not like the K rods because they don't have enough taper for the Apt system.

The small main air bleed carbs tend to use the smaller jet rod combos.

Also the 53 rods you mentioned are .036 tips so that's another ball game.

I don't think a smaller rod is going to help. Best to stay with what came with the carb.

I assume your engine is basically stock.

That's what's nice about the adjustable part throttle allowing you to set the rods higher in the jet allowing more fuel in higher vac conditions(cruise).

Low speed is not going to cause your power piston to move up richening things up unless you have a very light spring under it.

Without knowing your ICR and DCR and Mix hole sizes which play a bigger part in your slow speeds.

If you can kill the engine with the mix screws or greatly slows the idle speed you can richening the idle circuit but I would not at this point.

If you want to test your jet rod combo in relation to your bucking problem screw out your apt a turn or so and see if it helps as you mentioned. If course that's going to richen up other things also.
I assume you have the external plug installed.

With all that said it may just be too much load at low speeds causing you issue.
 
@blazer74, you got me on these, ICR, DCR, Mix hole sizes; I googled them
I have the APT plug mod so it is easy access. The engine is more or less stock or at least what should have been stock. Cliff sent me the APT spring for my carburetor after I set him the engine and vehicle specs.

Can kill engine with idle screws.
 
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Sorry about that, ICR idle channel restriction is the first calibrated orifice in the idle system, DCR is the down channel restriction for the idle system and the second restriction before the mix holes.

You would have to measure the mix holes with drill bits to find the size, they can vary and could be modified previous along with the other restrictions mentioned.

If you have a good idle and clean accelleration your should be good, give your Apt a turn or so to test.
 
Something new to add - it turns out my cam has a fairly long duration (for a low rpm cam) that reduces the bottom end and I think that could be contributing to the lugging/lean buck; the long duration cam means some of the cylinder is pushed back into the intake manifold and that could push back to the carb where it creates a stall (no vacuum) in the air velocity and I think that could explain what I am finding where the throttle becomes very sensitive to any input and too much throttle (drops manifold vacuum further) puts me into the lugging/bucking zone. Why richening the idle fixes this might be another question to understand.
 
Vac controls the carb, longer duration cams reduce the vac signal to the carb therefore the carb does not supply sufficient fuel at idle.

So we increase the idle passage size to compensate.

Vac your engine produces at idle dictates

You have control of your engine with your mix screws so you should be fine.
.
 
I had some issues with lean bucking with the Q-jet that I put on my 302 in the Ford, fattening up the fuel mix solved it for me too, but I hated the idea of running what felt to me like stupid-rich to fix a low rpm buck.

I put a wideband O2 sensor in the car so that I could keep an eye on fuel mix at WOT (because in the car I run WOT all the time) and I found that I was cruising between 16:5 and 17:1 at those low RPMs. I fattened up the idle mix to get me into the 14.7:1 range at idle and low rpm cruise and the lean buck vanished. I get a little fuel smell at idle now, but I feel much safer. Again, different situations, but I already burned up two valves due to running too lean, and I feel that the wideband O2 sensor was worth it's weight in gold.

For comparisons sake, my 302 is running a 219/226 @.050, with a 112 lobe separation and 108 centerline. Intake closes at 61° ABDC. Static compression is 9:1 and dynamic is 7.4:1. I pull around 10-12 inches of vacuum at idle, and around 13 or so at low load, low rpm cruise (I am at 6000 ft). The car is a stick.

As for my truck, it's still rocking the Q-jet too, and is 100% stock. No lean buck whatsoever, even at very low RPM (though it is a auto)
 
NotLaw said:
was cruising between 16:5 and 17:1 at those low RPMs. I fattened up the idle mix to get me into the 14.7:1 range at idle and low rpm cruise and the lean buck vanished. I get a little fuel smell at idle now, but I feel much safer.
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sounds like confirming that it is lean - did you try changing the timing curve?
 
I didn't really mess with the timing all that much. I think I tried advancing the timing, as well as switching between ported, manifold, and no vacuum advance. If I recall correctly, advancing the timing did help a little bit, as did running manifold vacuum advance. This was all last summer that I did this, and when I was doing all this testing, I was not doing so in an attempt to fix the low RPM buck, but rather a high rpm hesitation I had (which turned out to be the aforementioned burned intake valves)

I would go out and test some of this stuff now, since I do have the wideband, but sadly piston met chunks of exhaust valve as the valve was disassembling itself at 6000 rpm a few weeks ago, and left a nice window in the side of cylinder #2, so the car is down at the moment. When I get it all back together, and if this is still ongoing, I can do some testing.
 
NotLaw said:
If I recall correctly, advancing the timing did help a little bit, as did running manifold vacuum advance.
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No need to go test anything; I just got back from a test run. Removing the vacuum advance made it worse so I think you are correct that advancing the timing helps. Later today I will upload the videos.
 
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