I have a 66 Dynamic 88 with stock 425 with remanned 2 barrel Rochester Carb. Haven’t done a whole lot to the engine…valve springs and seals, timing chain. Overall engine is a good runner. Doesn’t smoke at all. Main change this year was I put an electric pump on it as the mechanical quit and just haven’t replaced it. Car starts right up and idles fine and when I hammer down on the road it takes off great. But it has a slight stumble at cruising speeds. If I feather pedal it clears up. At a stop light it doesn’t bog or anything but the stumble is there. Any ideas? Idle screws are 2 full turns out. Choke seems ok. Plugs and wires aren’t that old. Timing is set to 7.5-8 degrees but I’m not an expert in that area. Any suggestions would be great. Thanks

 

I'd probably look at your distributor and insure the advance weights move freely as a first quick step. Next, confirm the advance moves the breaker plate via vacuum at the proper rpm. Your engine might like more advance but thats easy to check. A dial back timing light might be useful in this case to ensure the total timing at the proper rpm.

It might be helpful to tell the forum what kind of distributor you are running. Points, HEI or some other electronic box?

My comments assume this is an ignition issue and not carb related, since you describe it running pretty good otherwise.

 

I am running a points distributor. New points, distributor cap and rotor button last year.

 

Try tuning with a vacuum gauge.
Hook gauge to an intake, not a carb, vacuum source. Then try to achieve the highest possible vacuum reading by adjusting the air-fuel screws and timing at 650-750RPM. Verify the dwell is set to 30°
Report back and we'll go from here.

 

Ok so would the brake booster vacuum line running to the intake work?

 

Yes the booster line would work as would any direct to the intake source.

 

Is the remanned 2 barrel original to the car or is it a replacement? If after checking all the basics doesn't bring joy I'd check for completely clean passages in the venturi cluster, free non-sticking power valve in the carb and lastly increasing the jet size 2 numbers larger. It may be a lean stumble, what color are the plug porcelains and the tailpipe after a long highway run?

 

When the engines idling just look for a nipple that has vacuum on the intake or on the throttle plate. The booster line/port is rather large.

 

If the car stumbles in a certain RPM, you may be lean in that range. Something to try might be smaller rods or bigger jets. I’d do it in that order. The idea is to enrich the air/fuel mixture just a bit to eliminate the stumble.

Rods are a finer adjustment, jets change the whole range of air/fuel ratios across the entire engine RPM band.

Before you start randomly swapping in different parts, make notes on what’s in the carb now & what rods/jets represents a 5% change, 10% change, -5% change, etc. Those notes will also be your “trail of bread crumbs” back to whatever you started with in case your experiments go wrong…

With a written history you can trace out the changes you make and correlate them to the way the car drives.

If you’re sure the sparks/distributor are in order, usually a “bog” is too rich and “stumble” is too lean.

If you’re interested in upgrading, I can recommend a latter day (75 & up) quadrajet of the 800 cfm variety on a factory Olds or Edelbrock Performer. If you dig around you can probably find a factory cast iron manifold for the 425/455 for $100-200 bucks and carb for about $500 or so. Less if you can rebuild it. 4 barrels really wake up an Olds big block.

Having said all that, I sincerely hope you get your 2 barrel working right. If the car is fun and reliable, let it be & enjoy it!

Chris

 

Ok so we pulled all the plugs and they look good overall, Id say on the lean side if anything. We timed it at curb idle to around 12* up a little from where I had it. Then I put a vacuum gauge on it and got around 20-23 inches. Dwell is 30 and carb screws turned 2 turns out currently. Idles almost perfect and the stumble at cruising speed, while still there, is much less. No issues under acceleration. Beyond that I am past my expertise lol. I'm thinking I have a slight lean condition causing it.

 

If a lean condition is suspected it can burn a valve if left as is. I'd suggest opening the carb seeing what number jets are in it and then increasing (e.g., if it has # 72 try #74) the jet size by two and retesting.