1903 Won't Respond to Tip-In Procedure

[bry593]

I would proceed adjusting the choke pull-off to open the choke blade a little less, about .160", measured between upper end of blade and rear wall of choke housing and increase fast idle speed some to keep it running.
For off-idle respond at least .052" idle DCR´s will help.

I did consider closing the flap a bit more and that was probably the right thing to do.  I had previously set up the choke flap with a 1/4" bit per typical Quad specs (and an 1/8" inside the coil housing).  But, opening the IT from .031 to .033 did do the trick.  But now, wouldn't you know it, got a new problem.

Now, when the engine is warm and I stop at a light, the idle is fine for about 10 seconds.  Then suddenly, like someone is flipping a switch, the idle stumbles and tries to die.  I can pump it a few times, bring the rpm to 2000 and it clears out.  But then once it returns to idle, it runs great for about 10 seconds, then like a switch tries to die.  My first thought is that the air horn gasket has become fuzzy around the power piston from the many times I've had to remove/reinstall.  Maybe some of this fuzz has circled the drain (the jets) and is plugging up the feed to the idle tubes?  I can't think of any other reason it would act like a switch (unless my needle has gotten damaged, but I don't think so). 

Tonight, I'll pull the jets and idle tubes and blow out the channel.  Might as well solder the tubes back to .031, reinstall my .041 UIAB, and bend the tang to put the flap @ .160.  I'll check the gasket and replace if it looks fuzzed up. 

[Kenth]

Are you setting the mixture screws with hot engine, in gear at 700 rpm´s.
How far out are the screws at best lean idle setting?

[bry593]

Yes, I take it for a drive, get the stat open, then adjust the screws for highest rpm but on edge of lean.  When I had IT .031, UIAB .041, screws were out 3.0-3.5.  Now that IT .033, UIAB .052, screws are 4.0-4.5.  I thought maybe it would go the opposite direction with larger orifices, but the screws are farther out now.

After adjustment, I set idle screw to about 750 in park and it drops about 100 when put in gear.  Nothing odd about that. 

On a side note, this carb has some casting irregularities down near the jets.  No obvious porosity, just some lumpy looking areas.  I might see if I can lower my air pressure as low as possible, fill the bowl with water and blow thru the idle tube channel with my finger over the jet hole.  Just to help retain my sanity a bit....

[Kenth]

I would have stayed with .031" IT and .041" UIAB.
Enlarging the UIAB´s to .052" and IT´s only to .033" will make the blend leaner than the previous. You may need at least .035" IT´s for .052" UIAB´s.
Also, .052" or thereabout DCR´s will make for better control of mixture screws and cure any off-idle issues.

JMHO

[bry593]

I measured my fuel pressure and found it to flutter per pulse, between 5 psi (must be a checkball in the pump?) and 9 psi.  I then noticed the airhorn gasket was soaked with fuel.  It was flooding at idle and was the cause of my on-off switch at a stop light.

The carb was removed and I went to work figuring out what was going on with the float.  First off, the air regulator was set to 9 psi dead head.  Note that I double checked the pressure using the same gauge I used to measure the fuel pump pressure.  Then the bowl was filled with gas up to about 1/4" from the top of the casting.  Air was applied to the fuel port and bubbles came out under the needle.  This was with the float set to 9/32".

The needle was removed and the seat peened with a checkball.  The needle and float were reassembled and 9 psi applied.  Still bubbled.  I put my finger over the needle and it stopped bubbling.  Apparently, not enough force on the needle.

I then removed the float and started bending and reassembling until it quit bubbling at 9 psi and the fuel level was about 3/8" from top.  I thought maybe I should add a little safety factor, so I bumped the pressure to 10 psi.  It quit bubbling at about 5/16" from the top.  This float level ended up being 3/8". 

Edelbrock says the 1910 should use a .300" setting.  I looked at all the fuel pumps on RockAuto and most say pump pressure is 5-9psi.  So, unless you have a regulator, I'd recommend a lower float level, around 3/8".

To speed the process of removing and installing the float for test purposes, I removed the little wire clip from the needle.  Turns out, you don't need it.  Just an extra part.  Works fine without it and the needle ain't going anywhere so long as the float is installed.

The idle tubes were removed, the collars tapped back in place and then the tips soldered and drilled .031.  I reinstalled the previous UIAB setscrews, which actually measured about .042.

The APT did have about .015" difference in the arms.  I measured this using the depth side of my dial calipers.  To do this, I stood the APT on the piston end on a steel plate.  Then I put the caliper tail on top of the arm and measured down to the plate.  Bent them until they were within a few thousandths using my crude method.  For now, I've set the APT at 3.5 turns out and will lower a bit at a time until I get lean surge or off idle hesitation.  Not today though...

What do you know, there is a screw on the 170 series choke pulloff.  I applied vacuum to it and stuck a .140" drill bit between the front edge of the choke flap and airhorn.  Then I turned the screw until the flap just contacted the bit.  I still need to fine tune it in the morning when it is cold.

Reinstalled the carb and started the engine.  Turned the idle mix screws in until the engine tried to die then backed out a half-turn.  Let it warm up and then readjusted for verge of lean.  Idle is smooth, very little engine vibration.  At this point, I put a vacuum gauge on it and found this cam is pulling 19.5 in Hg!

Took it for a several mile cruise.  No problems dying at the stop light.  No run-on when I turn off the engine.  Restart is instant, no having to fiddle with the pedal.

I'll fine tune the choke tomorrow morning when it is cold.

Plan to drive it like this for a few weeks to see how I like it.

[bry593]

Carb Assy Tip - Spray your gaskets with Aerosol Silicone Lubricant.  This will help prevent sticking and ripping when disassembling for multiple tuning changes. 

I have disassembled this carb at least 10 times and still using all three gaskets:  airhorn, throttle plate and manifold. 

[Cliff Ruggles]

I breezed back over the thread and noticed a few common denominators with not having a big enough camshaft in an engine based on the CID and static compression ratio, or the LSA is a little too tight, etc.

Some run-on after shut down.

Pinging in certain conditions without a lot of timing.

Making a LOT of vacuum at idle.

Difficulty tuning the idle system.

Not a deal breaker but for sure this engine would have loved about 10-15 degrees more seat timing, duration @ .050" and pushed out of a wider LSA.....IMHO

I tune couple of Saturdays a month and ran into a very similar scenario with a 454 "LS5" engine last summer.

Long story but it follows yours pretty close.  The short version is that I did the OEM carb for it based on the owner supplying me specs that the engine was built to "stock".

This was a "high end" restoration so the carb was restored and built to stock specs with the only change being the addition of an external APT screw and slightly smaller primary metering rods.

The engine didn't run for chit from day one and of course the carb guy was getting ALL the blame from the engine builder.  After at least a year of correspondence and trying to help out I simply had the owner bring the car here so I could see what was going on.

Immediately when he fired it up and backed it off the trailer I knew the engine was off someplace.  I very quickly found out that the timing was retarded pretty far, it had pretty high cranking pressure, and idling very smooth and making a butt-ton of vacuum despite not much initial timing.

Any attempts to increase initial timing resulted in "bucking" the starter pretty hard when it was hot and ping right off idle and at heavy/WOT.

I tried few things with the distributor and timing and it didn't like any of it, but taking away timing everyplace helped some.

So I went to the owner and asked him once again what the engine specs were?  I told him that there was no way this was a stock or even close to stock cam in the engine.  So he contacted the engine builder via his cell phone and had him send a copy of the cam card.

Turns out the engine "builder" and "guru" had Howards supply the cam.  It was supposed to be a "modern" version of the original LS5 camshaft but turns out it had about 30-40 degrees LESS seat timing and ground on a much tighter LSA.  The .050" numbers were close but even slightly less than the factory camshaft.

We were pretty much done at that point.  I recommended removing the cam and tossing it down over the hill and making a better selection since the engine had pretty close to the stock compression ratio up near 10.75 to 1.

Of course that went over like a fart in Church and the owner ended up leaving with the car and not overly happy about it.  I did "de-tune" the carb to help out some as it had WAY too much idle fuel since the engine was making up near 20" vacuum if you put any timing in it.

So more or less a "crutch" fix for a fundamental issue not really related to the carburetor or distributor.

As for your set-up I think it can be effectively tuned to correct all the issues.  I personally believe that the real fix is to install a bigger camshaft in it on a wider LSA.......FWIW......Cliff

[bry593]

I have unintentionally installed an A/F alarm to the truck.  What is that you ask?  Well, I had my mufflers changed from short to long last Friday, and one has a loose baffle.  Any slight backfire will vibrate like a snare drum.

This morning I fired it up with one pump on the throttle.  At first I heard some snare drum, but it faded within a few seconds and smoothed out at 1400 rpm.  The next step on the choke, same thing with a bit of drum followed by quiet.  By the time it hit warm idle, no backfires and smooth. 

A slight drum during transition, but is gone instantly.

Cruise seems good, no drum noises from the muffler. 

Acceleration gets a bit rattly at higher rpms, but I suppose that's just due to a more powerful exhaust pulse.

Tomorrow, a replacement muffler will be installed and I plan to run it through all four barrels.  Right now it is just too embarrassing sounding with the loose baffle to open her up. 

[tayto]

it's interesting how the exhaust note can change as the tune gets better. i built a Qjet for a friend of mine for his 80 firebird a few years ago. he had (at the time) owned the car for 12+ years, never really ran right, but did run. One of cliff's kits, some major straightening of the air horn & main body, major tune up and fixing an exhaust leak it had had for sometime. the note totally changed from idle to part throttle and wot.

[bry593]

Yes, I noticed a substantial decrease in exhaust noise going from the 800cfm 170 to a 750cfm 70 series.  However I assume it was due to the calibration.  The 70 series was calibrated to a '71 350 and had the IABP plugged to work on a 305.

How did you straighten the main body (zinc die casting) of a quadrajet?  The two front corners are typically mashed due to an overzealous mechanic.  I've heard that you can bolt the body solidly to a thick piece of steel plate, bake it in the oven at ?F for awhile, loosen and re-tighten the screws, then repeat the process.  After ? cycles, let it cool while bolted to the plate and voila, the corners magically return to almost flat.  Not sure how this works exactly, but that's what I heard.

Of course I also heard that you might as well toss that mashed quad body in the scrap pile.

[bry593]

I would have stayed with .031" IT and .041" UIAB.
Enlarging the UIAB´s to .052" and IT´s only to .033" will make the blend leaner than the previous. You may need at least .035" IT´s for .052" UIAB´s.

I get what you are saying and for grins, calculated the ratio of IT to UIAB areas and assumed a constant .047 DCR.
IT .031, UIAB .042 -> UIAB/IT = 1.8356
IT .033, UIAB .052 -> UIAB/IT = 2.4830, This means the air bleed area is 2.5x larger than the idle tube.  So it will pull more air under the same vacuum, but isn't it pulling a greater volume of fuel through the bigger idle tube?  I think it results in more fuel and air, but the mix is leaner.
IT .036, UIAB .050 -> UIAB/IT = 1.929, This is Cliff's book recipe and is close to the ratio of .031/.042.  Surely this has to pull a larger quantity of both fuel and air through a larger hole (even with the DCR constant).  This would suggest idle fuel is greater than .031/.042. 

I could be wrong, I have been before....

Carb tip:  To set your power piston arms even:
1.  Assemble your pp with needles into the carb, but leave out the power piston spring.
2.  Set your dial caliper tail on the top of the needle where it bend and enters the arm.  Then measure down to the top of the jet.
3. Measure the other side and bend the arm so it matches the previous arm dimension.

This should be darn accurate since it is measuring distance from the needle to the jet.

[Pav8427]

Staightening the main body the way you descibed is the way I do it. I built 2 plates that bolt through the carb. Heat till plates are 300ish degrees, snug, cool down, repeat if necessary. Only reason I did was because it was a numbers original carb. Have used it on others and really helped seal things up. I also do the airhorn. Always requires some flat filing to get gooder. The plates I made are machined so I dont have to remove any tubes or parts from the airhorn.  Of the few I have done, seems the warp is concentrated at the point where the divider?? plate is in the secondary airvalve bores are. Possibly because the main casting is 'thinest' there.

Doug.

[Kenth]

Use of .031" idle tubes with .042" upper idle air bleeds OR .038" idle tubes with .052" upper idle air bleeds, the A/F mixture will be about the same. Your choice.
.033"-.035" with .052" upper idle air bleeds will obviously be leaner.
What counts for you now is to get this idle A/F mixture down to the lower air bleeds and mixture screw needles though the idle down channel restrictions in proper amounts, the A/F blend is fixed.
Larger lower idle air bleeds provides larger amounts of idle/low speed A/F mixture that cures off-idle and low speed issues.

And, even if you measure and bend the arms on power piston there is no guarantee the drilled hole for the rods are at the same spot. Most times they´re not.
Only way to know the rods hangs the same distance is to mount them on the power piston and bend the rod ends towards eachother until they meet at center of power piston end and then adjust the arms if required.

[bry593]

I thought it would be best to take the hole in the piston arm completely out of the equation.  That's why I suggested measuring from top of the needle to top of the jet.  Only two items in this measurement, needle and jet.  Being that we are trying to control needle relative to jet, woudn't this be accurate?  Maybe I'm missing something?

[Cliff Ruggles]

I spend a LOT of time on every carb that I build here to insure that the metering rod hanger arms are level and metering rods are EXACTLY even.  I do it like Kenth suggests above, pull them together under the bottom of the piston and make sure the tips are even.

Another good bit of advice is here is to make sure the rods are the same length and that they are the correct length.  Not uncommon at all to find the WRONG rods in a carburetor as most have been out there 40-50 years and who knows what has been done to them.

I also recommend putting new jets in them during a rebuild.  I find jets all the time that have been drilled out at some point and WAY bigger than they should be.......