Whos running a mechanical secondary carb on their BBB455 street machine

i have an 850 mech sec on my 455 in my 65 skylark with a 2.78 rear gear and a th400 with a tight converter. even though i don't get traction it handles it quite well. whn we dyno tested it everything was right there, just slightly rich. couldn't see it on the plugs though. i would use it.

 

I've been running a mechanical Holley with NO CHOKE for years. Never a problem cold starting and never an issue driving. When cold starting I have to keep my foot in it for a minute before it idles, but small price to pay.

 

Just wanted to give an update and possibly get some insight. After much tinkering with this thing I changed the plugs, re-jetted and adjusted the pumps and finally have it running like I believe a 455 should. The main culprit was the timing. I am a little confused though as with the 650 I had no issue with stabbing the throttle from off idle and it re-acting quickly. With the 850 I would get a nasty blow back through the carb with a flame. Turns out initial timing was at 26 degrees and overall timing at 2800rpms was at 47. I backed the timing down and this things is a tire turner with not much work.

My question is, does having a smaller carb on allow for more timing? The timing was so far advanced with the 650 I was amazed, yet it still ran good and didn't have the stumbling I was presented with when I dropped the 850 on.

 

I run an old Barry Grant Demon with the 1050 Venturi kit. Runs great (well, as good as an old Barry Grant can). Car is strictly street car, drive it all over. It took a while to get it tuned, squirter, jets, air bleeds, etc. I am going to swap it out for EFI, but as of now, I have run this carb. for 7 years on 2 motors.

---------- Post added at 09:57 PM ---------- Previous post was at 09:53 PM ----------

With a good backfire, you should check the power valve, it is probably shot now.

 

I found this to be the case. We were running a 650 CFM SpreadBore Holley with mechanical secondaries, a B&M Hole Converter, TH350, and stock size gears for '79 Cutlass. Did fine on the street and was 'okay' on the track. (1/8 ET 9.0 on factory rims and tires, no posi.) After we blew out the the third rear end, we swapped it for a 1972 Monte Carlo Diff, rebuilt the engine and bought a Holley HP 650, the SpreadBore was getting tired, and changed the converter to around 2200-2500 stall. (Gas mileage was paramount as this was my only car at the time)

I noticed that it would still go but, no more ability to really break the tires loose, which means that the gearing/converter combo was a little much for the set-up in regards to launching and racing but, we got 17 MPG or so...

Both carbs had mechanical secondaries and with the change in the gearing, their operating profiles differed greatly. I did notice that there was definitely an ever-so-slight delay in the motor when I'd slam the pedal against the floor, on both carbs.

I'm aware that a 650 cfm carb is inherently starving the ol' beast a bit, but the A/F ratio was always acceptable.

Both Holley carbs were fine on the street, got "okay" gas mileage, and supported the power-plant at the track. The Q-Jet that I ran for a bit, had vaccuum everything and I found it to be less responsive than the two Holley's. But, it's your choice on your car, and whatever you decide to do, it'll work out in one of two scenarios: One, you'll love your mechanical secondary having, double-pumping, Holley or equivalet, or two, you'll realize that it wasn't all it was cracked up to be and either deal with it, or buy something else.

Good luck,

V/R

Mark Richards

 

It sounds like you also have some distributor work to do, and Larry's power timing sticky is a great place to start with that. The pop through the carb could also be related to timing, as well as where you have your vacuum advance hooked up. Try tuning it with just the mechanical advance and once you have the timing curve and performance where you are happy then play with the vacuum advance to get a smoother cruise and gas mileage.

Part of your problem may be that your front butterflies are opened too much and you need to adjust the rear butterflies open more so the front ones can be closed back down a little bit. if the butterflies are opened to the point of exposing the fuel transition slot instead of just the idle circuit this is the case. Off-idle stumble is a Holley trademark that can be challenging to tune out, so be methodical and know it is still just a carburetor and that it CAN be done.

The closer you get to having it really right, the more of a balancing act it is to make it all work together seamlessly. If it's rich most everywhere, well it will run OK but will not be as crisp or get as good of mileage as you might like.

 

I appreciate the feedback but it seems as if some posts were skipped as I have already corrected the timing issue and the lark is running great. My question I did have regarding it is how was it running fine with the 650dp on there with the timing so far advanced and when the 850dp went on it gave me the issues. Does carb size allow for such drastic timing changing/ranges.

---------- Post added at 03:25 PM ---------- Previous post was at 03:20 PM ----------

I appreciate the feedback but it seems as if some posts were skipped as I have already corrected the timing issue and the lark is running great. My question I did have regarding it is how was it running fine with the 650dp on there with the timing so far advanced and when the 850dp went on it gave me the issues. Does carb size allow for such drastic timing changing/ranges.

 

I know super old thread, but that 4011, do you recall what jets you had in her?
I just picked up one for my 455 and tore down to find 60s in front and rear, and knew it to be far too lean,
Wondered if you knew what you ran as I want to pull this stock 800 cfm q-jet off for now and attempt to get some mileage with this 455.
Has Comp 268h, B4B, 2" primary headers, 3.90s with TH400 and stock stall,,
I am not out for racing, will stay under 5k rpms, cruising 3k tops, so looking for jet recommendations for the 455 on a 4011 carb...
thanks if you recall...

 

I think that through the run of this thread not a single person mentioned that the Q-jet is a mechanical secondary carb. p

 

Perhaps that was a given, they meant another perhaps?

I posted here due to asking this on another page and I could not get an answer to just jet sizes, but all sorts of comments about changing my gear ratio, trany, cam, tire size, and in use this or that.. I gave them what I had and what I wanted to do and answers were all over the place, none were helpful, most were argumentative, people with deep pockets chiming in, of which I do not, the shallowest of most on her I am certain, working with broken back limits my ability to just change out the cam for another, or other extra work, some wanted me to put on dyno and use O2 readings from emissions, I almost bought collectors to do that in another vehicle fine tuning before, found leaky base gasket that time..

I just know prior to intake-cam upgrade, plugs looked good, fronts were tad lean, EGR and AIR systems were bypassed but leaked back into system, and that rear mounted stock carb on intake could lean fronts and wanted to center carb... and if I was getting 4 MPG in town I would have been impressed, I know I have leaky vent tube, cracked open, need to drop tank to repair and fix sender issue, but never noticed any fuel leaking or smelled any fuel, locking gas cap.. Might just see if the improved induction will change what I had, but in case I ever used this carb, is the 60/64 that came with it stock, info I could find off web, adequate to be starting point for any size engine? 650 CFM is all the charts say I need cruising up to 5500 RPMs which I do not plan on exceeding...

Just after jet recommendations knowing 60/60 could not have been correct for a spreadbore, the rear would have to have more.
I know having a rear power valve complicates jet sizing, and that these carbs were not popular, so finding info is a challenge..

I will start a new thread if no replies, but there are only a few with mention of a 4011 in it anywhere on a Buick...

 

I am running an old holley 750DP and have no trouble with it at all.Off idle I can floor and it never bogs or hesitates.Wonder I Ishould go for a more cfm carb or not. Remember I am at 6500 ft. should you increase cfm or leave it alone for increase in altitude. mt headers,TA413 cam,port matches heads,all the good stuff. 12 bolt chevy with 3:73 gears,10 inch convereter ect. Thanks Ken.

 

Every engine is different, and for this reason I like to TRY things , its sometimes expensive but its the ONLY way to know what will truly work, dig around on eBay and buy a used 850 or 950 and try it, if it doesn't work no sweat, Holley DP carbs are probably the easiest hot rod part on earth to resale

 

I took the GS out for a run on freeway out to the lake this weekend, temps all over the place on ride out there 190-220.
When I got to lake I added a bottom air dam to insure air was pulling from radiator, there was no real shroud, just a small arched top mini-shroud that reached fan.
After adding this damn, my temps were even more aggressive, I've never had a capillary gauge act up but I saw temps as high as instantaneously going from a steady 190 to wrapping back to 0 on gauge, a seemingly 300F reading, SunPro gauge, I tapped throttle, no pinging, no smoke, no loss of power and temps shot down to 210 in seconds. I did not pull over since she wasn't displaying any issues of overheating, was assuming gauge was acting up, get into town, lower speeds, freeway was 70, and temps back to the 190 she had been for months, get home 5 minutes after this 300F reading and no boil over, no gurgles, anything to make me think it had overheated. I take peek at radiator, 4 core, cannot see through it, the fins have louvers and some debris is trapped, tried a hose and air, minute amount comes out, still no clear view through cores...
My question is, I ran this carb with 63 front and 67 rears, 3 stages richer both front and rear from stock, could it be possible I am running lean, I was cruising between 3k and 4k on highway vs the 1500-2000 I was cruising in town when it ran cool, even without lower or a full shroud, would think city would be tell me if fan and shroud were issue and most of airflow on freeway would be from actual air resistance not the fan, correct?
Were these older 4 core louvered finned radiators an issue? I know I added a few ponies from what it had, headers-cam-B4B, and it cannot dissipate the heat any longer? I ran same radiators in the past with 400HP GTOs without concern and in fact this fan is off one of the GTOs...
So running lean, not enough cooling ability, or just a bad gauge? I just never had a capillary gauge that was bad before and it is not laying upon anything hot, it is strapped to my vacuum hose for my gauge.
I could try the Q-Jet again on highway real quick, takes 20 mins to swap, that is the 800+ cfm version stock off the 455 I rebuilt, just need a different spring for a slightly lower vacuum from the cam selection if I can find one.. The Holley had 3-4# more vacuum than the Q-Jet, ran way smoother, started up with 1 pump no matter how long it sat, and was really hoping I could use it...
Sorry for the highjack of old thread again, but since relies were coming and related to OPs 6011 650 Spreadbore question thought keep it going...