Please help me dial in this Q-Jet

[67RedC10]

Hello everyone,

I have a Q-Jet p/n 17057229 (from a 1977 truck I believe) that is running rich at really light throttle.  The engine is a pretty much bone stock 1971 L48 270hp 350 from a '71 Chevelle (points and all) but is installed in a Toyota Land Cruiser with a 5-speed manual tranny.  The engine has about 8.5 to 1 compression and has cast iron ram horn exhaust manifolds with dual exhaust.  I have the mixture screws out 1.5 turns each and I'm right at 13:1 AF Ratio on my LM1 meter (at 650 rpm).  That's where I get my best vacuum reading (18 inches) and if I lean it out any from there it begins to pop a bit out of the exhaust.  13:1 is almost dead smooth. 

If I rev the engine in neutral from 1000 rpm to just below 2500 rpm the AF Ratio is around 12.5:1, then as soon as it get above 2500 is begins to lean out.  When I drive down a flat road under light throttle (such as 4th gear at 2000 rpm) my AF Ratio is rich at around 12.5:1 (the throttle is just barely cracked open).  If I maintain the same speed (2000 rpm) as I go up and over an interstate overpass my AF Ration goes to about 14.5:1. If I increase my speed on flat ground to 2300 rpm up through 3000 rpm and beyond my AF Ratio is around 14.5:1.  If I get into the throttle heavy without getting into the Secondary's my AF Ration drops to about 12.8:1.

I have the timing set at the factory specs (8 degree initial advance and stock spec vacuum advance canister connected to manifold vacuum).  I've tried advancing and retarding the ignition and it's happiest at 8 degrees initial.  I also tried ported vacuum advance and it prefers manifold.  I tried leaning out the idle mixture to 14.5:1 and increasing the ignition advance, retarding the advance and trying manifold or ported advance.  It prefers manifold advance and 8 degrees initial along with AF Ratio of 13:1.  If I disconnect the vacuum advance from manifold vacuum and plug the manifold vacuum port that the hose came off of the engine slows down and doesn't run as smooth.  If I unplug the vacuum advance from manifold vacuum and leave the vacuum port open to allow some additional air into the engine the engine actually idles quite nice and the AF Ratio registers about 14.5:1 on the meter.  But if I adjust the mixture screws to give me 14.5:1 idle AF Ratio then disconnect the vacuum advance and plug the port the engine doesn't run as well and pops a bit out the exhaust.

Okay, where should I go from here?  I'm thinking it's just getting too much idle fuel and maybe I should enlarge the Upper Idle Air Bleeds a bit?  Or should I decrease the Primary Jet size a bit?  My AF Ratio with heavy throttle (without Secondary's) I believe is very good so I hate to mess with the Primary Jets and mess that up.  Thanks for any suggestions.

Prior to all this I rebuilt the carb using a rebuild kit from Cliff and here's the specs after careful measurements:

Idle Tubes - .038"
Idle Down Channel Restrictions - .055"
Upper Idle Air Bleeds - .075"
Lower Idle Air Bleeds - .063"
Idle Bypass Air - .139" (currently plugged)
Mixture Screw Holes - Drilled to .086" (originally .060")
Main Primary Jets - .066" (originally .063")
Primary Rods - .026" and .042" (originally .026" and .039")
Secondary Rods - DG - .034" and .134"
Air Horn Primary Air Bleeds - .052"  (Single Main Air Bleed Carb)
Inlet Fuel Seat Diameter - .130"
Acc. Pump Discharge Holes - .026"
New Power Piston Spring installed that was recommended by Cliff.  I didn't write down the specs.
Installed Cliffs APT Screw in the Base Plate.  I haven't fooled with this much yet.  It's set to just start raising the rods.
I didn't include the Secondary Specs since that is working well.

[77cruiser]

If you give it some IAB I think it will lean out some. The IAB is just a controlled vacuum leak.

[67RedC10]

Do you recommend enlarging the upper or lower idle air bleeds (IAB)?

[77cruiser]

I'd leave the air bleeds as they are, & put the Idle Air Bypass IAB back to function.
How is it blocked off now? Maybe it could be opened up to about .05-.065.

[67RedC10]

Yeah, that sounds reasonable.  I drilled and tapped the top side of the base plate where the holes were and installed small set screws.  I could drill a small hole in each one.  I originally plugged them off because I couldn't get my idle speed down low enough.  I installed new primary shaft bushings and that helped a lot but still couldn't get the idle down below about 850 rpm.  With those plugged I now have a 650 rpm idle and my transfer slots are just barely exposed (like perfect square holes when against the throttle stop screw).

What would happen if I drill the lower idle air bleeds a tiny bit bigger?  How does that compare to drilling the upper idle air bleeds a tiny bit bigger?  But first, I'd rather drill those set screws since those are easy to change.  I have a small drawer full of those.

[77cruiser]

Try the IAB. Going back & looking the idle tubes being that big is a major factor, if you have a way to get them down to about .033-.035 it would help. Carb must have been modded for a more radical cam at some point.

[67RedC10]

I believe the carb came off a 1977 GM truck that had a 454, but it's possible too that it was modified. 

[67RedC10]

Okay, I've tried various changes and I'm finally about there.  I pulled the IAB plugs and installed different plugs with holes drilled in them. I first tried one with a .077" hole and the other with a .107" hole and that helped quite a bit except it increased my idle a bit too much and the throttle plates won't close any tighter.  I was still a bit rich at really light throttle so I tried drilling my upper idle air bleeds from .075" to .080" and that helped also.  I then drilled my lower air bleeds from .063" to .068" and I was getting really close.  I then drilled the lower air bleeds to .072" and that was pretty much the ticket.  I also ended up very lightly stretching the power piston spring a little (probably a sin to do that) and that cured my off idle lean stumble.  I tried a next smaller primary jet change and that was a wrong move so back in went the .066"' ones. 

The last move I'm planning to make is change the .107" IAB plug to a .077" like the other one so I can drop the idle speed 50 rpm or so.  My AF ratios are now really good at all throttle areas and loads and it's driving very nice.  We'll see how much better it is this winter compared to last! 

[Cliff Ruggles]

I'm slightly confused.  What are IAB plugs?

Use the APT system to correct an off idle lean condition by raising the .042" metering rods a little higher.  Stretching out the PP spring is the WRONG way to tune light throttle "tip-in".  The power piston should be down at idle, and when accelerating lightly from idle to the main system.  It's sole purpose is to richen things up and heavy throttle openings, or when vacuum falls low enough to call for fuel enrichment.  At the same time the vacuum advance timing falls out, to keep from pinging the engine.

So tune all light throttle situations with jets/metering rods/APT system.

The idle tubes are HUGE for what you are doing, with .075"/.063 IAB's, it would have been fine with much smaller idle tubes.  A slight amount of bypass air would most like help it as well, based on the comment in one response about removing a vacuum hose.

That engine should not want or need a lot of timing at idle speed either, and be fine without MVA.  I've tuned a LOT of those engines over the years, and haven't found one yet that needed any additional timing at idle beyond the base setting......Cliff

[67RedC10]

I had plugged the idle bypass air by installing 1/4-20 set screws in the idle bypass air passages that go through the base plate.  I tapped the holes on the top side of the plate and screwed in the set screw "plugs".  That was the only way I could get my idle to drop down.  It used to be really bad until I installed new bushings for the primary throttle shaft.  Even still though with the new bushings and my throttle plates fitting real nice with no daylight showing around the edges when closed tight I still had a high idle speed.  With those "plugs" installed my idle came right down perfect with the throttle plates just barely cracked open.

It sounds like what I really needed to do was to pull the idle tubes and install replacements that were quite a bit smaller., but I must say its running really nice at the moment.  Those idle bypass air "plugs" in there now are really short pieces of 1/4-20 bolts (without heads) that have small holes drilled through them and the top slotted for a screwdriver to install them.  I need to replace the one that has the .107" hole with the new one I have that has a .077" hole the same as the other one.  That should bring my idle back down an additional 50 or so rpm that I want.

If I have a lean off idle condition I'll raise my power piston with the APT screw.  This thing has always run best with the timing set at factory specs but using manifold vacuum advance.  Possibly with the advance curve altered and the total amount of vacuum advance limited ported would be the better option.  Cliff I wish I lived close to you and you could help me perfect the tune on this thing.  I know I'd learn a ton and have a ball doing it.  How are you with Holleys?  I need to tackle my old truck next.

[Cliff Ruggles]

Manifold vacuum advance works poorly for all of the early low performance 350 Chevy engines with stock or near stock cams in them that I have tuned here.

They make PLENTY of vacuum with a normal base timing setting, and adding more timing lowers the throttle angle and often the tuner looses control of idle speed and has to start blocking off idle bypass air. 

Doing all of this has less transfer slot exposed, and can induce transition issues as well, which is aggravated by the lean idle mixture used with the additional timing.

The best news is that the engine will run equally as well everyplace on ported vacuum, as ALL the timing is in with the slightest movement of the throttle, or full vacuum right off idle......Cliff