Slight burble and light cruise

[Daniel828]

I have a 1971 Oldsmobile 455. It was rebuilt this winter, the cam is 228 @ .050 .504 lift   Performer intake and headers into 2.5 exhaust with H pipe.  The carb is a 7040240 Buick 455 carb with 69 jets and 42B rods and DA secondary rods. The engine idles perfectly and has great response  up and thru WOT. The problem is, at light steady cruise there is slight burble thru the exhaust. I'm also using the original points dist with pertronix unit, and I got it back from bubbas hot rod ignition.  He found some advance problems and replaced a defective vac advance can. But the burble is still present. I have an HP kit and a APT kit installed. Any suggestions would be greatly appreciated
Thank you
Dan

[Daniel828]

I've tried a few different things. I changed the jets from 69 to 71 and still has a slight burble in the exhaust note. I put the 69 jets back in, it just seems to idles cleaner. I also screwed the APT screw in and it seems to be very far in. I don't have a base line on this carb. I got it off of eBay and rebuilt it. Is it ok to use a Buick carb on an olds? I know it seems like a silly question.
I also went thru the valve train again. It's an hydraulic flat tappet with adjustable roller tip rockers. I'm not using the olds rockers with aluminum bridges. And the last thing I tried was adjusting the Aconcagua air door.  First tighter, then looser but it didn't do anything.  And I quicken up the choke vac pot. That was originally over 2 seconds, now it's about 1.5 seconds .
But the burble is still there
I'll try taking a vid and see if it will pick up the audio
Thanks
Dan

[77cruiser]

How much timing do you have at that RPM?

[Daniel828]

I started at 10 at idle, about 650 rpm and bumped it up to 12. Now it's at 15 initial and don't know where it is at total. When it was at 10 initial it was all in at 2600 34-36 degrees.  I sent the distributor to Jim at bubbas hot rod ignition. He recurred it. I'm thinking it may be the pertronix unit

[Daniel828]

And I've tried ported and manifold vacuum. Doesn't make a difference. And I even tried with the vac advance disconnected.  Still burbles in the exhaust note.  I had the exhaust completely welded front to back except the header collector, that's bolted on. 

[Daniel828]

One thing I noticed.   The more the APT is screwed in (this is a baseplate style apt) the higher in the rpm range the burble happens

[Hillbillyenginering]

Not sure if this helps you or not.
I have a 69 w34 toro. Has comp cams 280h. Similar to your can. Fought a carb issue for years.changed carbs, etc. Rebuilt a carb using cliffs book and kit.
Finally the coil completely gave out. Not before installing a msd distributor and wires.
Gurgling went away when I installed a new msd blaster 2 coil.
I have equipment to test kV. It was showing good at idle.

[Cliff Ruggles]

If the problem is only occurring on the primary side of the carb at light to moderate throttle nothing done to the secondaries will effect the issue.

I'd continue to raise the APT even if you hit the maximum adjustment.  If things continue to improve but you don't cure it, go to a 70 jet and do it all over again.....Cliff

[Daniel828]

Ok ill give a try with 70s. I'll have to order a set.  I only have 71 n 73s.  Something I noticed today while driving around.  If I'm accelerating moderately it's smooth, until I'll reach the speed I want to stay at. Once I maintain speed it'll start burbling even if I slowly increase throttle.  It starts around 2500 and up. If I get hard into it and into the secondaries it'll clean right up

[Daniel828]

So I've been racking my brain about this issue. The carb adjustments I've been making on the car weren't making much of difference. So switched my attention to ignition. I removed the vac line to the vac advance.  The surging/ burbling went away. What does this indicate? Too much advance or not enough? Or the car just works best without vac advance ?  My head is spinning, I've been so engrossed trying to figure this out the last couple days. I know this is a carb forum but any suggestions would be welcome. Thanks
Dan

[77cruiser]

Too much vac. advance. How much is it giving, about 20 I'm guessing?
Can you add a limiter similar to this?

[Daniel828]

Hey Jim, that plate is slick. Great idea.  I have an accel adjustable vac advance on there. So I'll trying adjusting it. The motor doesn't seem to run any worse with the vac advance disconnected so I'm not really sure if I should run it.  Cliff sent me email stating I should run hooked up with it providing only 10*. I'm not sure how much it's adding currently. I only checked the timing with it unhooked   Thanks for your help
Dan

[77cruiser]

I think I'm running 12 now, depends on what it wants.

[Cliff Ruggles]

Some VA units can be adjusted thru the opening.  On some it only adjusts the spring tension, others the travel of the arm or how much advance is added.

Some send a "goofy" piece you can put under the back screw to control the travel distance of the arm.

I much prefer to adjust them by MIG welding up the slot and using a round file to establish travel distance for the arm.

.125" of movement is apprx 10 degrees timing at the crank......Cliff