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4cylndrfury
4cylndrfury MegaDork
11/8/22 7:42 a.m.

Its been a while since I posted here... like several years- life got in the way and car fun wasnt in my cards for about 6 or 7 years. Hoping i can return humbly to the hive for some sweet sweet car knowledge.

This summer, SWMBO and I purchased a 1972 Monte Carlo - pretty much a factory stripper except for power steering and a TH350 automatic. Crank windows, no vinyl top, bench seat etc etc. Originally she was a 2b 350/350, though now she sports a Holley 4b 670cfm carb. Outside, the Monte is in incredible condition for a 50 year old car. No rust, body is great, new paint, original interior. 

But...

It suffers a bit from what I think is a restoration by body guys - it has some tuning issues that are beyond me, and the (very professional, and great to meet and get to know) guys who did the resto are convinced thats just how old cars are. I beg to differ.

I know my way around most EFI cars - enough to be dangerous. Ive gotten pretty deep into a few of my cars, and came out the other end with a car that was better off for the effort. Im out of my depth with Black Sabbath era tech unfortunately, and finding competent mechanics in the 2020s is proving a challenge. 

So, im returning to the maddness of the forum - help me diagnose my issue, plz. Here are the symptoms

  1. The car stumbles off idle
    1. She idles a little high, but when I go to ease into the throttle from a stop, or even from coasting, she will stumble, like shes too rich, or timing is off. If i blip the throttle during the bog, she gets right again, but if i keep the pedal there, it will backfire/miss. Feels like too much fuel is loading up at idle, but adjustments to the accel pump make no difference. 
    2. in true shadtree mechanic form, Ive tried to floor it because that usually fixes everything. She opens right up, and makes good power, but the acceleration isn't as smooth as it should be - i get random hesitation just for a moment here or there as she accelerates. Its not a smooth arc up to shift, but more of a bumpy staircase.
  2. trans leaks like last moments of Bonnie and Clyde (too soon?)
    1. ive narrowed it down to all of the trans seals except for the pan gasket...because i replaced that one. Theres fluid coming out from the front of the bell housing, from the speedo cable, and the tailshaft seal
    2. The reason in include this trans issue in my tuning question is that when I dropped the pan, the fluid was grey-red, and full of fine metal (no chunks thank goodness). Not burnt smelling either - It was honestly surprising given how well it shifts.  But, I was initially concerned about my torque converter being the culprit behind the accel issue - that maybe its so bad that its causing the engine to work too hard to get out of idle, and the presence of that much metal only deepened my suspicion. But from what Ive learned, a TC thats in that bad of shape often will also cause stalling when pulling to a stop, and might lead to hard/missed shifts. I have no stalling or bad shifting, so idk

So, basically i dont know where to turn. I will put a timing light on it to make sure im not dealing with timing thats too retarded - like I said, when i get into the gas pedal, she straigtens out a bit. Im guessing that the timing advances when RPMs climb, so i could be a few degrees further BTC than i need to be.  But is the Trans issue related? I cant find a mechanic who is interested in helping me out. I found one on the googelz that was rated well for classics, but they basically looked her over, and sent me back home with a sorry we dont work on carbs themselves, and you should see a transmission shop for your leaking. They were concerned that there was internal issues with the clutch packs or valve body or something, and didnt want to get that deep. I dont want to trust a random transmission shop with my car, so I guess I could pull the trans and bring it in, but that still leaves me with the most common small block in existence that no one wants to touch...i guess im going to have to learn tuning SBCs myself, which is why im here now.

I realize diagnosing outright isnt really a valid expectation, but hopefully the hive can suggest some online resources to absorb knowledge from?

halp. And thanks (in advance) for (hopefully) the warm welcome back?

eastsideTim
eastsideTim UltimaDork
11/8/22 8:38 a.m.

Welcome back!  The Monte Carlo looks sweet!

I used to have a book on tuning Holley carbs, but I think I purged it a few years back, or I'd send it your way.  With it being a 670 CFM carb, does that mean it is a Street Avenger?  If so, those usually seem to have a good baseline setup out of the box, may not make as much power as they could with a tune, but should not have any problems.  All bets are off if someone fiddled with it, though.  This Holley article may help, but I am guessing you have already read at least that much information. 

I'd agree with checking timing first, but also maybe going over plug wires, distributor, and plugs, too, just in case. 

Afraid I can't offer any advice on the transmission, they are still kind of a magic box to me.  If it is leaking like that, I wonder if it was sitting for a long time, allowing seals to dry up.

NOHOME
NOHOME MegaDork
11/8/22 8:49 a.m.

Have the same carb on the Molvo. And fought pretty much the same issues that you describe.

Took me a year or two to figure out that the hesitation was an HEI distributor that contacted the tone ring inside the distributor when transitioning the throttle.   And the vacuum canister leaked down. What I thought was a bog was a misfire. Took a really good eye ( not mine) to spot the issue but it was obvious once seen.

 

Once past that, it was a case of tuning the carb as you are doing. I broke down and bought a broadband O2 reader and installed it permanently. As long as you don't have a header leak on the side it is installed, it is a godsend when it comes to not guessing what is going on with the carb. Get a passenger to note what happens during the trouble spots and fix what you see.

Uncle Tony on Youtube has the best carb tuning videos if you can stand to listen to his conversational cadence.

4cylndrfury
4cylndrfury MegaDork
11/8/22 8:56 a.m.

Yep, it is indeed a Street Avenger. I also read that article, and several others. Those are supposedly known for running well out of the box as you mentioned, and power comes from settings and jets etc. I cant say for sure if its been monkeyed with, but they do have the electric choke disconnected and set wide open. She struggles to stay running when cold, which is probably why the base idle is set a little high. it remains to be seen if there were any other adjustments made that I dont know about.

I do also plan to pull the plugs and see what they tell me. Distributor and wires look brand new, but who knows. A new battery did wake it up a bit, but didnt help the stumble unfortunately. The resto was just finished this summer, I dont think the guys who did it even had time to figure out what else they wanted to do or what else it might have needed. The owner sold it as he was getting to where he didnt trust himself behind a wheel that didnt have the nannies that todays cars have, so who knows how well they got to know her. 

Thanks for the comments though, she is a looker. I cant stop at the gas station without getting a thumbs up and hearing about a fond memory of an uncle or a buddy that had one once upon a time. Its fun to have a car with actual clear coat smiley

4cylndrfury
4cylndrfury MegaDork
11/8/22 8:59 a.m.
NOHOME said:

Have the same carb on the Molvo. And fought pretty much the same issues that you describe.

Took me a year or two to figure out that the hesitation was an HEI distributor that contacted the tone ring inside the distributor when transitioning the throttle.   And the vacuum canister leaked down. What I thought was a bog was a misfire. Took a really good eye ( not mine) to spot the issue but it was obvious once seen.

 

Once past that, it was a case of tuning the carb as you are doing. I broke down and bought a broadband O2 reader and installed it permanently. As long as you don't have a header leak on the side it is installed, it is a godsend when it comes to not guessing what is going on with the carb. Get a passenger to note what happens during the trouble spots and fix what you see.

Uncle Tony on Youtube has the best carb tuning videos if you can stand to listen to his conversational cadence.

thanks for this. Its the first specific advice Ive been given. 

Were you able to adjust the distributor, or was a new one worth the cost?

ShawnG
ShawnG MegaDork
11/8/22 10:49 a.m.

This isn't the answer you want but get rid of that thing and put a Quadrajet on it.

I've tuned lots of Holleys and they can make you pull your hair out. 

If you want to keep it, start with the basics. Tear it down, clean it well and put a full rebuild kit in it. You have no idea how much the last guy messed it up for you.

Verify the jets, power valve and accel pump cam are correct for that list number carburetor. 

Verify that the idle circuit is clean. It sounds like it's not working and the carb is running on the the transition circuit.

Make sure the fuel filter is clean too.

Carry a fire extinguisher because Holley carburetor.

 

4cylndrfury
4cylndrfury MegaDork
11/8/22 10:55 a.m.
ShawnG said:

This isn't the answer you want but get rid of that thing and put a Quadrajet on it.

Tempting - i also am potentially looking to ditch the slushbox for a muncie 4sp, but SWMBO isnt interested in having to choose from so many pedals. Theres also the easy button for iginition:

  1. Write check
  2. install efi
  3. TBD
  4. Rule planet

ShawnG said:

Carry a fire extinguisher because Holley carburetor.

LOL - i have one in the glove box 

NOHOME
NOHOME MegaDork
11/8/22 12:20 p.m.

My HEI distributor was new. From China. Of course who suspects new stuff? I do now.

One thing to be aware of is that Holley quality is in the dumspter as of late, or at least that is what I kept hearing when learning about mine.  You could just have a duff unit.

 

What kind of vacuum is your engine making at idle? Is it a steady signal?

 

The high idle might be a clue. A carb tune needs to be built from the idle up. The engine should run with the throttle plates pretty much closed and the fuel/air for idle delivered via the idle circuits. The Avenger has 4 of them.

If the car will not idle  with idle jet adjustments with the throttle open 1/2 to 3/4 of a turn, then stop and find out why. DO NOT open the throttle more to smooth it out and then try to do more tuning. The reason is that once the throttle starts to open it gets into what is called the transition slots. You are then feeding off the main jets, not the idle mix screws.

Transitions events in a Holley are mind boggling complicated: You have the accelerator pump that needs to be set for clearance. You also have a "shot size" that you can select ( stay with stock) . Then there is the spring that controls the vacuum secondary opening that is dependent on I am not sure what? ( car weight I am told). Then you have the squirter that delivers raw fuel down the carb throat and the sixe of the orifice that delivers the shot, along with a cam that controls the rate of the shot volume. Confused yet?

 

The throttle body EFI is tempting and known to work for some people, but having gone down the Fitech rabbit hole, I would do an LS swap with a stock tune before I ever went aftermarket EFI. Aftermarket EFI is for people who want it to be their new hobby.

4cylndrfury
4cylndrfury MegaDork
11/8/22 4:59 p.m.

In reply to NOHOME :

OMFG thats a lot, but Im here for it. Thanks for this insight! I will put a vac gage on it and see where Im at there. I appreciate the guidance about starting at idle and going from there. Im not opposed to looking at a different carb that may be more friendly to newbs considering Im not in a position with a hot cam or forced induction etc that makes the carb the weak link, necessitating an option with a zillion variables to tune. Do you have any resources that you can share to help educate a home wrench who's new to last century tech?

I havent hardly put a wrench to this car because its running well enough to get me where I need to go so long as I drive it the way it wants to be driven. mechanically its super strong. The only real issue that I know of outside of the trans and tune is the heater core leaks and is bypassed, so nothing too difficult. I had been taking it to cars and coffee and a few local shows this fall, so i didnt want to twist a few screws and end up with a car shaped lump of metal that doesnt move on its own while the weather was nice. Now that the temps are dropping, Im moving into garage hermit mode, hence this thread.

This winter I also want to add boxed rear trailing arms and a rear sway bar, a bit of love for the interior, and really dig into detailing the engine bay. Build thread forthcoming...

Shadeux
Shadeux Dork
11/8/22 5:19 p.m.

In reply to ShawnG :

Truth. My rebuilt Q-jet ran like EFI for 15 years on a 350.

ShawnG
ShawnG MegaDork
11/8/22 5:27 p.m.

In reply to Shadeux :

They're great. 

200cfm on the primaries and the secondaries are only as big as you need them.

Ford even used a Q-jet on the Mustang for one year because the Holley wouldn't flow 900cfm.

4cylndrfury
4cylndrfury MegaDork
11/8/22 5:33 p.m.
ShawnG said:

This isn't the answer you want but get rid of that thing and put a Quadrajet on it.

any reason why this carb isnt the right answer?

NOHOME
NOHOME MegaDork
11/8/22 6:12 p.m.

In reply to 4cylndrfury :

Only because you already have a holley bolted on and it wont need a lot of $$$ to get right.

Like I said, Uncle Tony on Youtube does a lot of good videos on tuning the Holley and covers a lot of basic theory that applies to all carbs. Below is just one of them.

 

ShawnG
ShawnG MegaDork
11/8/22 6:54 p.m.

Seriously, before you do anything else, take if off, take it all the way apart, give it a good clean, verify everything is correct and put it together with all new gaskets, seals, accel pump and all that.

Until you've baselined the carb, there's no point in trying to tune it because you're chasing your tail.

Same thing with an engine that won't run properly. Have you done a tune-up and returned everything to factory specs? You can fiddle with it after you've established a known point to start from.

Don't use that Q-jet, it has no choke. You'll hate it.

 

03Panther
03Panther PowerDork
11/8/22 7:09 p.m.

A holly has a lot of different circuits that can be tuned in to dead nuts of a specific need. That makes them an excellent carb for drag racing. Those circuits, with the right explanations, are not really that complicated. But it is a tedious process, even with understanding. 
I personally recommend against a holly UNLESS drag racing. For a street car, there less tedious options. 

03Panther
03Panther PowerDork
11/8/22 7:16 p.m.

Since you bought a street car, we need some additional info before a thought out recommendation:

What intake does it currently have on it?

A Quadrjet is definitely one of the best street car carbs, and one of the least understood. If you decide to go with a used unknown quadrajet, you will definitely want a guy in your corner that knows em. 
If you only have a square bore intake mounting flange, and eldelbrok carb is a nice out the box friendly street carb, as well as a bit more user friendly to tune close enough. There are folks that can't stand them, but they are ok for a street car 
 

bobzilla
bobzilla MegaDork
11/8/22 7:47 p.m.

Carbs are the devil. Black magic, witchcraft and voodoo all wrapped up in an impenetrable box that makes no sense. 
 

sorry I am of zero help. I always had a friend come help that understood this black art. I never figured it out. 

03Panther
03Panther PowerDork
11/8/22 8:25 p.m.
bobzilla said:

Carbs are the devil. Black magic, witchcraft and voodoo all wrapped up in an impenetrable box that makes no sense. 
 

sorry I am of zero help. I always had a friend come help that understood this black art. I never figured it out. 

Nope... much easier for us simpleminded backwoods hill billies!

I can't even get Torque to communicate with my dongle consistently...

That said while I understand them ok, I'm still only mediocre at tuning one in on a car. Two of my best go to guys are dead now, so I'm about on my own!

bearmtnmartin (Forum Supporter)
bearmtnmartin (Forum Supporter) UltraDork
11/9/22 2:03 a.m.

Float level is pretty important on a holley and if it is too high or low it causes lots of drivability issues.

4cylndrfury
4cylndrfury MegaDork
11/9/22 3:38 p.m.
bearmtnmartin (Forum Supporter) said:

Float level is pretty important on a holley and if it is too high or low it causes lots of drivability issues.

this is something that has been on my mind a bit - you could convince me that the issue is in part that when at idle, the bowl is filled too high, and coming off idle, it takes a moment to run thru the excess. When it bogs down, if you blip the throttle, it wakes back up, like youve cleared the excess and get back to a proper flow.

I will take the hives advice and give it a proper teardown > clean > rebuild with new bits for good measure. 

@NoHome, I will be taking a close look at Uncle Tony's channel - many thanks for that suggestion!

4cylndrfury
4cylndrfury MegaDork
2/7/23 4:49 p.m.

Im slowly learning - between the UTG videos (thanks guys for this advice) and a few books, im getting there as far as carb knowledge. 

I started by baselining my carb by resetting the throttle idle position and the accelerator pump settings. This had some impact . Im starting to think that its the float level - this carb has little windows instead of a plug in the float bowls, and as far as I can tell, the fuel is all the way above the top of the window. According to Uncle Tony, this is waaaay too high. This could coincide with the bogging off idle - too much go juice dumping as you roll into the accelerator, and it doesnt clear out till you move some air thru the unit.

I still need to reset and baseline the air/fuel settings. I feel like im getting there however. Wanted to pass my thanks along to the hive!

On a separate note: Im slowly cataloging my adventures .  Just added drilled/slotted rotors to this beast, and Ive been checking off little nitpicky stuff like interior trim and the like. Is there interest in a build thread on these forums? This is not a homegrown racer or a resurrected fan favorite niche car - is Classic Motorsports the better place for this?

Dusterbd13-michael
Dusterbd13-michael MegaDork
2/7/23 5:09 p.m.

Build thread!!!!

ShawnG
ShawnG MegaDork
2/7/23 5:55 p.m.

In reply to 4cylndrfury :

If your fuel level is that high, you've probably got some fuel sloshing out of the bowl vents into the carb throat on acceleration.

Not sure if those are the externally adjustable floats on there but a good start is having the fuel level with the bottom of the hole. I just take a plug out and bounce the car to see if a the fuel sloshes a little. That's always been good enough to get the carb dialed in properly.

 

jimgood
jimgood Reader
2/20/23 11:30 a.m.

Certainly germane to my car. Thanks for all the references. I too need to learn carburetor-ese.

Cyclone03
Cyclone03 New Reader
3/17/23 11:31 p.m.

I know this is near dead...

Im a Holley guy,so I will offer my $.02.

First take it off and flip it over ,locate the transition slots on the primary side.

Make sure the choke fast idle is not holding the blades open,then back the idle screw out until it's not making contact. Now slowly turn the screw until it just touches and moves the blades.Note this position. Then while looking at the transition slot open the blade until the transition slot looks like a square. Stop.Note how many turns from screw just moving blades til transition slot square. Make a note of it. From transition slot square you DO NOT want to open the blade more than 1/4 turn,or close it more than 1/4. While it's flipped over note the transition slot of the secondary as well.

Next set both idle mixture screws out 1 1/2 turns from lightly seated.

Install carb.

Start engine ,of course check for leaks,warm it up.

Check float level ,you have windows so set floats just above visible in glass but no more than 1/2 the window. Remember the screw locks the adjustment the nut moves the float. Set both ends the same (for now).

Now here is where I would make sure the timing is between 10 and 16 deg advance with the vacuum disconnected at idle,you may want to rev it to about 4000rpm (or less if the timing stops advancing) SBC? I like about 38total,no vacuum.Believe it or not todays fuels seem to want more base timing than the sticker on the valve cover says. If your total is more than 40deg your a candidate for a custom curved distributor.

Hook vacuum advance up.

now it should be running without fiddling,not perfect but running without working the throttle, hopefully a little high.

Now you need some help.This is where automatic trans tuners miss it.

Have your TRUSTED helper set the parking brake,hold the brake and shift to reverse or drive. THEY are to do NOTHING unless told to! HOLD the brake!

Now you are ready to adjust the mixture screws.

Turn each screw IN 1/2 turn,What happened? If RPM went up turn each screw in another 1/2 turn. If rpm continues to go up repeat,if engine start to die back screw you turned back to previous setting. Once you have the RPM dropping with 1/2 turns adjust again with 1/4 turns. 
You do not want to turn one screw until engine stumbles then turn out till it smothers then repeat on over side because the left and right sides will not be balanced. Going back and forth first in 1/2 turns then 1/4 will keep the side balanced.

If on your first adjustments turning screws in lowers the rpm,turn them each OUT 1/4 turn if rpm goes up continue 1/4 turn at a time. IF while turning screws out rpm does not increase STOP and go back in to previous point.

your helper is still on the brakes.

with engine idling quickly with your finger push down on the accelerator pump arm and releas(just a quick stab) NOT THE THROTTLE the engine should near die but recover. If it does not recover or dies turn each mixture screw in 1/8 turn.

Now what is the idle speed? 
About 800 in gear. Too high turn Idle (throttle) screw out 1/4 turn. Not low enough? REPEAT all of the above.

Below 800 in gear but acceptable leave it. Or turn throttle screw IN 1/4 the repeat above.

your looking for a balance between the open throttle blades and mixture screws giving you the rpm you want in gear.

From square idle transfer slot you can turn the screw 1/2 turn each way.

If you need more than 1/2 turn you will have to remove the carb,flip it over and find the secondary throttle blade stop screw on the bottom of carb and turn it in 1/2 turn. Reset primary to sqaure transfer slot and repeat all of the above.

Once you have it idling in gear between 600 and 800 rpm you will find the rpm in park at about 1100-1200. Doesn't matter it's all about in gear.

Go for a drive report back.

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