Post the sec. rods & hanger letters from the '72 455 Buick Q-J & that will give an idea of the jetting.

Where would I find these letters?

Sad to say, my carb rebuild skills are limited to tearing down, cleaning, replacing the float and gaskets. I never mess with changing parts and adjusting linkages. So short of squirting carb cleaner through the jets, I have never really looked at them.

 

On top in back between the barrels is 1 screw in middle between sec. holds the hanger (1 letter) & sec. rods (2 letters). Undo screw & lift out. Very easy.

 

That was easy.

So the hanger either has a W with a line under it or an M with a line over it. The rods both say CT on them, vertically and horizontally.

Also, I just did the Matt Gruber test. Wow, what a difference a little choke makes. Smoothes the engine right out. If I wire the choke half closed, I think the car would run great.

Make me wonder what previous owners were thinking?

Thanks.

 

Disonnect your vacuum advance and set your timing for 30-32 degrees at 3000RPM and see how it runs. This 56 degrees advance is crazy stuff, I would have an expensive piece of scrap metal between my fenders if I did that

 

My vacuum advance adds 20 degrees, so you are suggesting total advance of 50-52 degrees total? Not much of a change, but easy enough to try. Wouldn't the engine start pinging if the timing was too advanced?

I did try a prior recommendation of running without the vacuum advance connected (line plugged), to see if it made a difference (so only 36 degrees total advance). It did not help, and actually made the car run worse.

So far, closing the choke down really made a difference. No way for me to drive like that, but I think the running lean line of logic is probable.

 

No plug the vacuum advance and set your total timing for 30-32. If it did'nt make any difference I would be surprised but at least you rule out too much advance. I would give it another try. 50 degree advance is for Ralph Naders smog mobiles, not anything to do with performance motors. JMHO

Anyway the lean surge you are feeling is probably just that, the big motor doesn't like the A/F ratio at that RPM. Invest in a wideband O2 sensor so you can "see" what the A/f is at that RPM, my guess is way too high. Now you can adjust the carb for correct A/f at idle, cruise and WOT.

Probably the one best investment I ever made, everyone should have one. 24 mpg on the highway at 80mph with previous 550HP 406

 

The way the hangers are stamped makes some appear to have a line over & under. Roe's book lists hangers B-V so it should be M. Hold hanger up w/ letter at bottom.

I would expect the carb to be rich so CT does not sound right for 72 Buick 455 according to Roe's book which lists tip as .0774 (very lean). I was expecting about .0337-.0440.
Check CT again.

 

You need a good carb, for a beginner stick with Holley. That 4360 is too small and was designed for a small block economy engine. A Qjet is much more complicated too deal with.

For a beginner u are much better off with a Holley. And a 4175 should be fine as it has vacuum secondaries and a secondary metering plate - uses an electric choke too. U will need to correct and adjust the throttle linkage for the Holley conversion but parts are readily availible. The 4165 Holleys are mechanical secondaries - some are 650 cfm and some are 800 cfm but 800 cfm are no longer in production. U want vacuum secondaries - go for the Holley 4175.

Summit Racing sells the 4175 for $380 but my recommendation would be an rebuilt spreadbore from the Carb Shop out here in california: http://www.customcarbs.com/index.html
A few years ago i got quotes for their rebuilt carbs much better than new prices and they calibrate them for your car and test run them in too. NO CORE CHARGES! They rebuilt and calibrated Qjets too for a great price.

Now as far a 6" vacuum at idle and 20" vacuum at cruise those numbers are reversed. Highest vacuum is at idle (your 'bout 15" as expected) and low vacuum readings with the throttle plates open (no where near 20"). That's why u don't have vacuum advance at higher rpm because the vac can drops out its advance. U may well have 56* total but the distributor will never see it. By the time u get to 3000 rpm the vac adv drops out.
Now i have seen what 66* of actual advance does to brand new 383" - similar to what MotorGuy said it had a big hole in #8 cylinder wall (broken piston too). Nice shiny new pistons told me the motor was brand new stroker - what a big waste of $$$$ (and time). I would reduce mechanical advance to like 32* or less - that would be 8*-10* initial plus 18-22* mechanical from the springs and weights.

BTW do you have AC in that car? Yes i see its a convertible but i can feel a tiny surge in my '94 Z28 when the AC cycles on and off.

Good luck and hope this helps,
cardo0

 

Just checked again, definately CT. If the rods are at the top of the lever, M is the letter at the bottom.

I could not score a qjet at the swap meet today, so if I do not rebuild this Buick carb, I am thinking about buying one off ebay. Would a 7045212 1975 Chev Truck qjet bolt up?

 

For a beginner u are much better off with a Holley. And a 4175 should be fine as it has vacuum secondaries and a secondary metering plate - uses an electric choke too. U will need to correct and adjust the throttle linkage for the Holley conversion but parts are readily availible. The 4165 Holleys are mechanical secondaries - some are 650 cfm and some are 800 cfm but 800 cfm are no longer in production. U want vacuum secondaries - go for the Holley 4175.

I could not find either of the Holley's for less than $200 at the swap meet. The price would not bother me if the venue was not so temporary. I will check the carb shop, thanks.

Currently the car has a manifold heat "activated" choke, so I would prefer to stay away from electric.

Now as far a 6" vacuum at idle and 20" vacuum at cruise those numbers are reversed. Highest vacuum is at idle (your 'bout 15" as expected) and low vacuum readings with the throttle plates open (no where near 20"). That's why u don't have vacuum advance at higher rpm because the vac can drops out its advance. U may well have 56* total but the distributor will never see it. By the time u get to 3000 rpm the vac adv drops out.

6" was from the EGR port at idle. If I connect to the manifold port for the brake booster, I get the 15" at idle. Besides the momentary drop, however, the vacuum does not drop at higher rpms.

The way I came up with total advance is to run the engine up to 3000rpm, with the vacuum can attached, then adjust the dial on my timing light until the balancer timing mark is at the 0 degree intent. The dial on the timing light shows 56 degress. That should be 56 total advance, correct?

Now i have seen what 66* of actual advance does to brand new 383" - similar to what MotorGuy said it had a big hole in #8 cylinder wall (broken piston too). Nice shiny new pistons told me the motor was brand new stroker - what a big waste of $$$$ (and time). I would reduce mechanical advance to like 32* or less - that would be 8*-10* initial plus 18-22* mechanical from the springs and weights.

By nature, I am kind of a paranoid guy, so tomorrow morning I will dial the total advance down to 50. That will end up being about 14 degrees of advance, w/o vacuum, at idle, which is still more than the emissions sticker suggests.

The 56 advance really smoothed out the engine, but I do not think pulling 6 degrees off will make much of a difference. It appears that the biggest problem is the carb. Once I get that replaced I can start the whole process over.

BTW do you have AC in that car?

The car does have AC, but it is not cycling on (or working for that matter).

Thanks for all the input.

 

I backed the advance down to 14 degrees at idle, which left me with 52 total advance at 3000rpm. Actually all the advance is pretty much in by 2000rpm.

It did not seem to make a difference on how the car ran, but I feel better that I will not grenade the engine.

Thanks for all the tips.

 

Something is not right here as you should not have 52* or 15" vacuum at 3000 rpm. I suspect you may have a bad timing light but i can't account for the vacuum or rpm numbers. Try a different tachometer like an auto diagnostic (if you can rent or borrow one from the auto parts stores) and definitely a different timing light. FYI here is good article on chevy distributor timing curves: http://www.chevyhiperformance.com/ho...438/index.html

To me 14* of static timing (initial) at idle with the vac adv line disconnected is too much - especially with a 20* vac can. Look in your shop manual (Chiltons will do) and u should see 8* to 10*.

But u really need a good carb to work with. Trying to learn carburetors by rebuilding a Qjet is the hard way. In that case its better to make just one adjustment at at time to the carburetor u have now and watch the results of every change you make (yes write it down).

I see you are trying to figure this out.
Good luck,
cardo0

 

Thank you cardo0. Eventually I will have this figured out.

I think my Sunpro multimeter is probably dead on as a tach. The reported rpms are very close to those on the car's mechanical tach. My guess is the timing light is probably right too, but I can see if O'Reilly's has a loaner.

As for timing, keep in mind I installed an aftermarket HEI distributor, and I set the initial advance to 14, based on keeping the total timing at 52 degrees of advance. The GM Service Manual and emissions sticker say 10 degrees of advance at idle, with the vacuum disconnected, but the car runs worse at that level. Other posts led me to the higher advance.

I also have the vacuum can attached to a ported vacuum line, so it really adds almost no advance at idle. Flip open the throttle plates, however, and that quickly changes.

Why do you think 52 degrees is too much? A lot of posts recommend total advance in that neighborhood, and the total timing in the How to Set Timing sticky is 52. As long as I do not get any detonation it should be fine, correct? Your post about trashing an engine really gets my attention.

Because I never loose vacuum, my vacuum advance never drops out, as you mentioned should happen in your previous post. Could this be because of the undersized carb?

I agree I really need to get the carb swap done. Other than learning a whole lot, I will have to start over at that point anyway.

 

The reason why an engine tolerates more than 30*-32* advance at part throttle is due to the lower cylinder filling at part throttle. But at WOT when the cylinder fills up that mixture is much denser and the burn rate becomes much faster/more rapid. If the piston is still moving up the bore at peak cylinder pressure u lose.
Maybe that's why you are getting that surging - too much timing for that part throttle rpm or maybe too lean also. Too lean is kinda like a bog when u accelerate where too rich is like a stumble as you open the throttle. Could be your tach and timing reading are good and its the mixture. Just my 2 cents, 52* of timing at 3000 rpm would scare me. That vac can should have dropped some adv by then or may not be operating at all. Or could be a vac can the brings in a lot of adv at very low vac (<15").
Here's how i choose a vac can. I measure engine vac at idle and then at 'bout 3000 rpm (vac line plugged). Then i take my "Mighty Vac" brake bleeder and connect it to the dist vac can. Now i can pump down the vac can and see where the vac adv starts to move the dist plate ('bout 5" for my car) and then read on the Might Vac gauge where full adv is (for you should be 15" at idle). Now i know what i need and at what rpm it should work but i still don't know how much adv i have. Again read that article to adj the vac can - takes <5 min to read.

Hope this helps ya,
cardo0

 

I read the article, but the idea of swapping weights/springs or welding the can slot is not very appealing. Seems like I should be able to get this right with how it was designed, or at least close to right?

Now i know what i need and at what rpm it should work but i still don't know how much adv i have.

So at what rpm do you want all (mechanical and vacuum) of your advance in? I know there are dozens of variables, but I am just looking for a ballpark number. The article talks about all the mechanical advance being in at 2500rpm, but vacuum would still be high since it is less than half of WOT. My vacuum is not going down at 3000rpm, and I am not sure at what point it does. I do know the vacuum can adds almost zero advance at 7", and is all in at 15".

It makes me nervous reving the crap out of my engine in the driveway, but I guess I need to get vacuum readings at rpm increments (I'll try 500rpm increments) up until WOT. That will probably be 6500rpm, which should be significantly scary, but that is the only way I will know what vacuum can specs I need (If my engine holds vacuum until 4000rpm, then I need change the vacuum can to 10 degrees max).

I'll wait until I swap the carb to run the test, as I do not want to risk one of those coffee can pistons shooting through the block at me.

To your point on advance being the issue, the car accelerates really well, during which time the vacuum advance would be almost zero. That would make me want to back down the total advance, but when I drop it to the mid-40s, it idles terribly.

Thank you for all the help. Very educational.

 

STOP!!!
Dont run that thing up to 6500 in tha driveway. If you do you better have a suit of armor on.
Just run a vaccume line out of the hood and put your hand held vacume gague in cab with you or your copilot. A copilot is the better idea, he or she can watch the gague and take notes while you drive.
At WOT weather at 2000 or 3000 or 6000 there is very little or no vacume, ergo, no vacume adavance.

Read the sticky note at the top of this forum and go by that. If it was that far off base it would have been taken down long ago. I read this sticky several times before I grasped it all. Yes my wife says I'm a little slow.

 

Ghunt, good call on using a copilot. I feel much better running the revs up on the road.

I'll report what I find.

 

you dont need to rev the crap out of it as it was put
3K rpms is all you need to go.
to get it set right you will probably need to change the springs.
the timing article buy Lars tells you which kit to buy and what springs to use.
with the vacuum line disconnected your mechanical advance should stop advancing around the 2800-3000 mark according to the article. he then says to let the idle timing fall where it will. then reconnect the vacuum to a manifold vacuum source. again straight from the timing article. it goes on to give you a timing number to not shoot for as an upper limit with the vacuum line connected (52 i believe)
if its more or less you have to adjust the vacuum can.

all of the manuals and stickers are for factory settings which are set for emissions reasons.
again just quoting the performance timing article. i followed the article to the T and got a little surging on cruise as well. i went up 2 sizes on the mains and it went away.

 

My total timing is set at 52 degrees, and I did it just like the timing sticky. I did have to back the initial advance down 4 degrees, because I have 20 degrees of vacuum advance, but otherwise I am dead on to the sticky.

Besides being under-carbed (which is most likely causing my surge) my newly realized concern is my vacuum appears to stay high even at 3000rpm, so my vacuum advance is not dropping. So even at 3000rpm I still have 52 degrees of total advance.

That might not be too much of a problem at 3000rpm, but I can understand how it would be at higher rpm. So I just need to make sure my vacuum (or at least vacuum advance) drops off at some point, hopefully before WOT.

Getting on it, for me, is about 3500rpm, and cruising is much closer to 2500rpm. Most likely I will not have an engine melt-down with my current setup, but I would like it to be right.

 

baxsom nailed it for you. don't rev an engine unloaded over 4,000 maybe 4,500 rpm - easiest method to ruin the valve train, maybe the bigger parts too.
I read you are sorting things out now and just need to experiment now to get it right. By Next week u will be the tuning expert here on the forum - and Big Blocks too.

Good effort,
cardo0