I have a 1971 convertible with correct casting but wrong numbers anyways I have to live with 81/2 to compression and make the best of it.i have installed Holley street dominator manifold,600cfm Holley electric choke,roller rockers and 3.55 rear.with Huron SS. Long tube header ,side exhaust.car has turbo 400 trams wit shift kit.from at dead start with engine fully warm up she stumbles then get going and runs strong.any idea what's wrong? Plugs look to be a " brown paper bag". I am stump.

 

is this on very hard accelleration or pretty much anytime?? if it's only on hard accelleration, it could be fuel sloshing out of the front vent into the throttle blades.

 

Possible incorrect jetting or accelerator pump. Is this a new carb or did it just start acting up?

 

I purchase car with this carb. The Holley looks as if new car has 1000 miles in last 14 years.stored a very long time . She stumbles only when you put the pedal to floor from dead stop as a drag strip take off.normal driving perfect I checked plugs (brand new) and they are light brown .i recently remove OEM. Exhaust manifold and installed long tube headers with side pipes,roller rocker arms,push rods, 3.55 rear.

 

could it be a vacume leak ? there is a lot of vacume hose on a C3

 

I took the hose off from the Holley carb to the distributor and put vacuum gauge on.reads perfectly steady and quite high at 24 inches.i set advance at 12 degrees inertial at 700 rpm vacuum advance blocked off.float level checked ok.now what?

 

can you post a picture of the carb or the model of it.

 

You've indicated that you had a shift kit in your Turbo 400. Did you also install a stall converter in there as well? Because most of the automatic drag strip cars I've seen, don't start from the line at an idle (correct me if I'm wrong, please!).

From experience, most of them are 'taching it up' before they launch hard, so the engine already has some RPM's going. Same with a four speed car. You're gonna be giving it a good deal of gas before you even dump the clutch so it doesn't die or stall out on you at the light, I mean line.

If you don't have a high RPM stall converter in your trans, you may not be having any trouble at all. If you do... then that's a different story. And anybody else, please feel free to add, because I'm just surmising here.

 

here's an old drag racers trick, it's easy to try, reversible and cheap-- the three things racers like to hear---

anyway, take short piece of fuel line that will fit snuggly over the vertical vent "stacks" on the bowls, jump from one stack to the other making a nice smooth arch in the hose, then take a sharp knife and cut a small round hole in the top of the hose right in the apex of the arch. this will keep the fuel from sloshing out of the front bowl vent into the throttle blades which basically causes the motor to flood out just for a few seconds then it picks up and runs good.

 

Sure can't hurt trying that and that was on my mind too, I just didn't know what caused it. I'd still advocate a stall converter too, if he's going to be racing it regularly.

 

from my short experiance in racing, too low of a stahl doesn't make the car stumble as much as it does just make it a pig off the line, I'm reading his post as the engine is coughing or running rough during take off, stahl doesn't usually cause that. IMHO

 

Is it a 600 double pumper? I had this problem and this is the cure I found.

The problem is that when the secondaries kick in, the A/F goes dead rich and this creates the bog you feel. Eventually it recovers. This dyno sheet will show exactly what I mean. Note the drop in power accompanied by a 10:1 A/F. Eventually, this fuel atomizes and is used and the A/F returns along with the power.


A partial cure was to use the smallest squirters offered by Holley (21 vs 25 stock). This actually took care of most of the problem, but as shown in the following, not entirely. CarCraft Magazine also had this problem with their 600 dp and epoxied the squirters and redrilled them to 22 then realized Holley offers 21s. They said it fixed their bog too. Note that the A/F ratio in this plot is approximately 1.2 lean based on my prior readings and a bunch of other guys with late modeles with on-board A/F meters who also had their cars dynoed at the time/event.


The next step was to change the Holley pink pump cam on the secondaries from the more "mild" setting of "2" to the more "wild" setting of "1" that is supposed to provide a bigger pump shot. But don't believe anything you read about the "1" vs "2" settings because on a 600 dp secondaries at least, I found that all the literature is bass-ackwards based on my testing, and the "2" shot is much larger than the "1" shot.

This seems to have alleviated the bog, but I've not dynoed the car since.

 

jig a Low that is brilliant i have never heard of that tip.right now it is real cold out like single digits i will have to wait until monday the weatherman says to venture out and do burnout (thats not happening) in front of house.i can get all information off carb and post , incidentally i would like to further describe "stumble" actually more like back fire thru carb then she run solid to 100mph.

corvette brothers you guys are terrific like my combat veterans helping each other out.

 

Todd, great stuff and it just reminded me of a recent experience with my Holley 650. Captain Bob is running a low compression 8 1/2 to 1 engine and that's not far from my L48 dished piston engine. With your dyno chart, I'd bet you're running a heck of a lot more compression and engine than Bob and me.

My brother totally disconnected the rear pumper on my Holley, because my engine and the horsepower it has, doesn't even need that much fuel! It was running way rich and you could smell the gas all the time (does this sound familiar Bob, if not, then ignore this) and after he was done, the car ran much better and didn't smell nearly as rich! It still revved hard all the way to the redline.

All he did was remove the adjuster on the back pumper.

 

carb is holley list 1850-3 on the bottom of that says 0548. electric choke, primary and secondary float adjustment and float level screws on both sides primary and secondary.

 

I had a double pumper on my '82 T/A LT-1 drag car, chased the stumbling/coughing issue around the block more than once, I could not figure it out until one of my racing buddies walked by and asked why I was under the hood--when I told him he said wait a second, went to his trailer and came back with 6" of hose, took about 30 seconds and all that issue was gone, when I sold the car it was still on the carb.

I'm not saying for sure this is your issue, but as I said, it's cheap, fast and reversible

 

F22 you may be on to something essentially the engine is a 350" 270hp with street dominator intake, 1.6 rollar rockers and long tube header side exhaust,L88 long hood with large air cleaner , positively no fuel leak yet you do have gas smell

 

Bob, my brother is a 30 year experienced master mechanic at a Ford Dealership and he cut his teeth at dozens of shops, including Tune-Up Masters and has rebuilt hundreds of carbs on a variety of vehicles, so to say I trust him is an understatement. He's also owned some pretty bad-*** rides too!

He told me that my engine doesn't need the double pumper feature. Try this out. With the air cleaner off, engine off and the carb exposed, pump the throttle linkage just a little bit, not even 1/4 of the way and watch the secondaries. I instantly saw fuel squirting into the secondaries and they weren't even barely open yet!

The carb was squirting fuel into the secondaries all the time! The linkage is on the rear of the carb and the squirter can be found under the float. It's a squat little metal square and you'll see a linkage with an adjustment. If you have this situation, then go ahead and remove the linkage and you won't have that extra, unnecessary fuel squirting into the secondaries.

Right away, the richness went away, the gas smell inside the car went away and it sure ran a lot cleaner and got better mileage after that. A double pumper is only necessary for a high horsepower engine that demands that amount of fuel.

 

Check out this article
http://www.badasscars.com/index.cfm/...d=79/prd79.htm

"First of all, lets take a 600 double pumper, like Holley's #4776. It's a great little carb but as the throttle is pulled back, the primaries begin to open and a shot of fuel gets squirted down the throat from the primary accelerator pump. When the primaries get about halfway open, the secondaries begin to open (mechanically) along with another shot of fuel through the squirters, from the second accelerator pump, hence the name "double pumper". This is all fine and dandy unless you've got an automatic transmission, a heavy car or high rear-end gears and you stab the throttle off the line. The engine will probably choke, cough and spit before it sluggishly starts to go"

Another article:
http://www.mustangandfords.com/techa...s/viewall.html

3. Vacuum Secondaries vs. The Double-Pumper

Assuming you've got spot-on tuning, a carb with vacuum secondaries is easier to live with on a street car or one that sees only limited duty on the dragstrip. That's because the secondaries kick in automatically for more consistent performance; and they usually deliver better fuel economy. Vacuum secondaries are also tailored for the typical street-engine combination, such as a dual-plane intake and a "smaller" camshaft, as well as an automatic transmission-all of which typically provide better vacuum signals.

A Double Pumper carb with mechanical secondaries is controlled strictly by your right foot and uses an accelerator pump to shove more fuel down its throats when you quickly go to WOT. It uses more gas this way, but is more accommodating to big cams, single-plane intakes, and other vacuum- limiting factors. These qualities generally make the Double Pumper better suited to high-rpm combinations used mostly at the track.

Actually, the above Mustang article has some great tips on bogs and hesitation too!

 

No, just pump friendly 9.5:1.

My combination:
331 CI, pump-friendly 9.5:1 CR
K&N 14"x4" air filter, Corvette drop base and lid
Holley 600 dp, choke horn milled and blended, Primary - 69, Secondary – 74, Squirters - 21
1/2" Aluminum open spacer port matched to manifold, exterior polished
Edelbrock Performer RPM intake manifold, port matched, exterior polished with all extraneous castings and lettering removed
Homemade lifter valley splash shield to keep hot oil off manifold bottom
Manifold heat riser crossover blocked
Camel hump 1.94/1.50 heads hogged out to 2.02/1.60, pocket ported, port matched, pump-friendly hardened seats, 3-angle valve job
Cylinder bores clearanced to unshroud the valves
Comp Cams 1.52:1 roller-tip rockers
Crane Cam Vintage Muscle 327/350 hp cam, 222 degrees @ 0.05, 0.447" lift (with 1.50 rockers)
Doug Thorley headers, dechromed and ceramic-coated
2.5" mandral-bent exhaust (including tips), 2" cross-over just before rear axle
DynoMax stainless Ultra Flow mufflers
Millerspeed 1-1/2” Gilmer under drive belt drive system
Mallory Hyfire IV CD ignition box triggered off Accel points
Mallory high voltage chrome coil
Mallory spiral-wound coil wire
Mallory solid copper plug wires, ends soldered to wires
Champion plugs
37 degrees total ignition advance
Carter high volume fuel pump
Melling high volume oil pump
Open breathers
Polished aluminum high flow water pump
160-degree thermostat
Flex fan with polished aluminum spacer
Polished aluminum one-wire 100 amp alternator
Header Power Bracket
Keisler TKO600 5-Speed
Hurst shifter
3.70:1 positraction with Big Block yokes
225/60/15 Firestone Firehawk SZ50s on 7”-wide Western 30-spoke Turbine Wheels

Does even better in colder temps as shown in these older runs. There's that bog again.



Slightly modified 600 dp.