kcludtke

Hello all,

I have a 1966 Oldsmobile Delta 88 with a 425 Super Rocket. Originally, it had a single exhaust with a ultra high compression 2 bbl carburetor setup from factory with a 10.5 to 1 compression ratio (I believe this to be correct information as it stated on the original air cleaner that it had ultra high compression). I have now put in a true dual-exhaust set up. However, I have been having issues attempting to change my carburetor. Initially, I installed a 750 CFM Demon carburetor with vacuum secondaries. It ran horrendously with no torque whatsoever. I switched to a 750 CFM Edelbrock carburetor with mechanical secondaries and while it has torque on the high end, it still has no low-end torque (I was able to squeal the tire with the factory 2 bbl carburetor). Is this a vacuum issue or is there something else I'm missing? I've now ordered a rochester carburetor dated to be 1979 with an electric choke. Not sure if it's a 750 CFM or an 800 CFM carburetor but I've heard that the factory rochester carburetors work better on stock engines. The only modification that I've done to the original motor has been installing a new edelbrock intake manifold and an HEI ignition. This will be my third carburetor and I'm tired of playing games. Has anyone had any previous experiences installing a new 4 bbl onto an original 2 bbl 425 with success?

Thank you.

oldcutlass

Did you use the stock timing settings with the addition of the HEI?

Sugar Bear

Im guessing the "switch pitch" variable vane torque converter worked with the 2bbl and doesn't work with the linkage modification to accept the 4bbl.

Please post pics of the carb linkage from the driver's side aimed directly at the side of the carb with the air cleaner removed and one on a 45 degree angle from the front left corner of the car showing the carb linkage all the way back to the firewall.
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kcludtke

We put on a microswitch that engages on full throttle for the kickdown switch. It sends down 12 volts to the transmission because the firewall mounted switch that sends 12 volts down to the transmission is not functional. The switch pitch terminal has voltage as it should. However, the kickdown terminal has nothing as far as voltage goes. Basically, we bypassed the stock kickdownswitch configuration because the terminals were too worn and they had no voltage.

kcludtke

We had the timing settings tuned by a professional carburetor shop. We're not sure what the timing is set at.

cfair

Your thinking is totally sound.

A 4 barrel carb should run & perform better than a 2 barrel. Don’t get hung up on 425 vs. 455. The principles are 95% the same. That last 5% is displacement, which relates to jetting & exhaust.

The key thing is jetting & tuning the new carb on the engine. I have no problem Demon/Edelbrock/Cater or whatever. They can all be made to work well, but the time & effort is in the tuning. Whatever brand, it’s just an air/gas mixer - and mostly they use springs to work against vacuum and use vacuum (high or low) as a proxy for driver engine demand. Carbs need to set up for the engines they run on. If you win by just “bolting it on”, that’s just good luck. Occasionally this happens, but mostly unconfigured run pig rich, or stumble lean, or have no guts. Dig around here on classicolds and you’ll probably find some qjet specs, or maybe even Holley/Demon specs for jets & rods. Try those.

To _really_ get a handle on carb tuning, I recommend a wideband oxygen sensor in your exhaust to tell you whether your carb is tuned correctly at idle, part throttle and WOT. See Innovative Solutions or other wideband vendors so you can read AFR/Lamda in real time and see what changes your carb needs.

If you switch to an electric choke from the factory ‘66 hot air choke, you’ll need to wire in an ignition-switched line to the choke to be sure it starts heating up (loosening) once the car is switched on. Some people use a jumper wire from the windshield wiper motor. Joe P has a very good diagram that won’t fire the ignition until the oil is up to pressure. I recommend that, as a design principle.

Moving on to the HEI swap, (which I’ve done to my ‘66’s’), you’ll need to be sure you have 12v to the distributor. In 1966, HEI didn’t exist, so when upgrading, you have to do some wiring modifications. The factory power line from the key switch to the points distributor was a resistance wire which limits the voltage to something less than what the HEI needs. So, you need to open the engine wiring harness & replace the fabric covered/striped wire between the key switch and the distributor with a full voltage (non-resistance) wire that sends a full 12v to the HEI distributor. Somewhere here on classicoldsmobile.com, there are better instructions than I’ve given you here.

If it were my car, I’d solve the HEI thing first. If your distributor is kneecapped, you’ll get very little power out of the engine. If it knocks, consider Progression Ignitions HEI units. I fought part throttle and WOT ping for years until dialing-in timing on these. Simply put, these HEI’s get you out of springs, weights, and vacuum cans which don’t work as well with today’s gas as they did 50 years ago…

When you have a sec post your rods, jets and so on for your carbs. There’s a high likelihood that someone here has a similar set up and can offer your additional guidance.

Welcome aboard. I hope we can help you sort & enjoy your car!

Chris

cfair

If you have questions about the switch pitch/kick down switch go here:

https://classicoldsmobile.com/forums...vicing-149216/

Neat feature, but more or less unique to 65-67 Olds and mostly forgotten by mechanics these days.

Hope that helps,
Chris

Olds64

I agree that the OP should get the timing dialed in first. Make sure the shop you took it to used a timing light and disconnected the vacuum advance when setting base timing. There's no need to switch distributors though. The GM HEI units work great. I have one in my 71. The only thing you really need to watch for is poor quality 4 pin HEI modules.

BTW, aren't mechanical secondary carburetors better suited for cars with manual transmissions?

joe_padavano

Also, which intake did you use? As for the trans control switches, there are TWO separate circuits, one for the kickdown and one for the torque converter control solenoid. Are both circuits working? The torque converter (switch pitch) circuit is designed to go to high stall both at idle and at wide open throttle.

70W-32

What brand HEI did you install? The Chinese made units are known to have faulty modules.

joe_padavano

Yeah, but module failure is kind of all or nothing. Either it runs or it doesn't.

kcludtke

I installed a Pertronix HEI Distributor
from Summit Racing.

joe_padavano

Is that actually a GM-style coil-in-cap HEI, or are you saying "HEI" when you actually mean an electronic distributor? I just recently had a brand new Pertronix coil crap out on me on my way to OCA Nationals last month. Fortunately I had a brand new spare coil in the car, so I got back on the road pretty quickly. The failure started out as misfires, but finally ended up as "no fires".

kcludtke

It's a GM style coil in cap distributor

oldcutlass

I would double check what timing settings are being used and also make sure the throttle is opening all the way with the accelerator pedal pushed to the floor.

Fun71

I agree that you need to check the timing settings. The HEI distributors can have a significantly different mechanical advance curve than the original points distributors, and that can result in not enough total timing, which will reduce performance.

Olds64

Ummm actually, let me tell you about the many times faulty Chinesium HEI modules have failed on me while hot and are fine the next day.

joe_padavano

But again, the car didn't run when the module was hot. That's not the same as a car that appears to lack torque but still runs.

CANADIANOLDS

I have to say the troubleshooting advice here is horrendous.

everyone focusing on the dizzy when the op hasn’t even said at what point he changed it?

first thing to ask is at what point did the low power problem start? Was everything changed at once ? Did it run fine with the distributor, and bad after new crab installed?

first things first. Was all the work done at the same time? Was it ever running good with the distributor change? at what point in the changes did the poor performance show up?

CANADIANOLDS

no. GM style modules are known for retarding the timing with RPM increase. It’s called magnet pulse degradation.

70W-32

Its not a spark, no spark issue with the Chinese crap. Its erratic timing.
Ive seen it on 3 different Chinese HEIs.
Owners switched to a junkyard GM HEI and the problems went away

kcludtke

The original factory two barrel carburetor , intake manifold , and points distributor were all changed at the same time. Now installed is the Edelbrock 750 carb , Edelbrock intake manifold , and Pertronix HEI distributor. This is when the no low end torque issue started - after changing the mentioned parts out.

gs72

I would look at the timing curve first, even if it was set up by “A professional”. If the mechanical advance is coming in too late. It is going to fall on its face.

kcludtke

Do you mean just trying different timing settings with my distributor ? Thanks

oldcutlass

What are your current idle timing settings with the vacuum advance disconnected?

kcludtke

Not sure ? I would have to check , I live at an altitude of 4,500 feet - any ideas of what timing should be set at ? Thanks.

Vintage Chief

"Generally" you can/should advance the timing 1°/1000 feet of elevation. IMO, often more importantly, adjust your A/F ratio to the highest idle vacuum achievable.

kcludtke

Thanks for the information.

Fun71

The total advance (initial + mechanical) should be around 30-36 degrees. The initial will be whatever it will be once the total is set. After setting the total, measure the initial at idle speed for reference on future tuning activities.

oldcutlass

It would be nice to know what the initial is prior to resetting it.

CANADIANOLDS

you are making this very difficult to troubleshoot

go one step at a time when you post…

step one, you changed the carb and intake, plus the dizzy at the same time..did it run strong at that point?

kcludtke

No it never ran strong since changing parts.

matt69olds

I have never heard of this with factory Delco modules, but I can tell you I have experienced several cheap parts store module that ran “ok” but would run MUCH better with a factory or good quality modules. They always started, always ran, just felt lazy.

If it was a one time thing, I might agree that it was a defective module. But several different cars leads me to suspect some modules are just better.

I have said for years I’d much rather have a good used Delco module than a brand new parts store part.

matt69olds

I’d start with the basics. Verify the balancer TDC mark is accurate. Verify there are no vacuum leaks, make sure the carb opens fully when the throttle is floored. Don’t check it from under the hood, have helper physically floor the pedal, while you make sure the carb actually opens all the way. Make sure the switch pitch converter works as it did before. I assume you followed the setup instructions for the new carb? Make sure the timing and advance curve is somewhat correct.

Assuming everything is installed and adjusted correctly, I wouldn’t be the least bit surprised if you will need to do some custom tuning to the distributor and carb.

DR_DAN

I bet you set the timing to 1966 specs correct? If so it’s about 12-14* retarded. HEI distributors have to have 18-22* initial timing. Then check your total timing WITHOUT the vacuum advance hooked up. You need to know what rpm it comes in and how much. If it’s over 36* total most likely some welding of the slots is needed. Same thing with the vacuum advance or buy an adjustable. 12-* of vacuum advance, manifold vacuum only.
this is where you need to start

73aussie455

Is your new Edelbrock manifold a single plane Torker or Victor, or a dual plane RPM Performer or AirGap?
A single plane may affect your low end torque a considerable amount.
Joe also asked, not sure if you mentioned which one you have.

olds 307 and 403

Don't just shove on a 79 800 cfm Qjet, it won't fix anything. They ran lean at idle with the stock 8 to 1 or less 350 and 403. It would need idle passage modifications to run right on your high compression 425. Was the first carb a Street Demon? A wide band should be used to tune either carb, it could be way offand a dial back timing light should be used to check your total timing, let the base fall where it may. That and a proper 12 volt key on source to your HEI distributor. Why is this so complicated?

CANADIANOLDS

its a common problem with genuine A/C modules too. You don’t usually notice because the spark curve on a stock of slightly modified app isn’t bad enough to notice sometimes…they are erratic

when used with a good ignition system that can record/playback the timing curve, that’s when you see it. I’ve seen it

tell you what..google GM hei module spark retard with rpm.

matt69olds

Interesting. Never heard of that. The article I read on cheap aftermarket modules refer to dwell. The “good” Delco module increase dwell at low rpm to help with coil saturation. The aftermarket modules have a fixed dwell.

I did read up on the subject, the A/C Delco 990 keeps coming up in discussion as the pinnacle of modules.

Like most things GM, they tend to rush the release of new technology, let the customer be involved in the final step of development, perfect the design, then discontinue it for something new and repeat. .

kcludtke

I ordered a Rochester Quadrajet through Carb Exchange and had them jet it for 4500 feet of altitude , I installed it and right out of the box it fired up and the car runs with so much more power !! It is a completely different car , I am very impressed with the Rochester Carburetor. Previously had a Holley Demon and an Edelbrock - both 750 cfm carbs , had them both tuned by a shop but neither one of those worked well at all. I wonder if it's the shop not knowing what their doing or not ? It's supposedly a reputable shop and I spent a lot of money having them tune the Demon and Edelbrock carbs with very poor results. I am inclined to believe that the Rochester Carburetor with it's small diameter primaries must create more vacuum which is significantly helping with my low end torque. The car is literally a different Beast now !! I am very happy with the power it has now !!