dmruschell

I recently installed a reproduction Duntov solid lifter cam and a clone dual-quad setup on my 61 Corvette. It runs very well except for one thing that I've noticed that I hope some others can shed some light on.

Under 3,000 RPMs or so, if I floor the gas pedal and open the secondary carb up, the engine acceleration slows down. It's not falling flat on its face or causing the car to slow down, but the rate of acceleration slows down. If I back the pedal off to where the secondary carb is closed, the rate of acceleration increases.

Around 3,000-3,500 RPMs is where things change. At that RPM, if I open up the secondary carb, it makes no difference when compared to just having the primary carb open (as is expected, since the engine isn't spinning fast enough to use the additional airflow), and the engine accelerates nicely all the way to 6,000+ RPMs, eventually pulling all of the air and fuel it needs through these carbs.

Is this situation normal? If not, what can I try to do to fix the situation?

I (obviously) have them set up in a progressive linkage, as I've heard low RPM bogging can occur when set up in a 1:1 ratio and the pedal put to the floor. I've only used carbs that either had a vacuum scondary or had an air valve (like a Quadrajet, Edelbrock AVS, AFB, or WCFB) in the secondaries, so tuning a double pumper or two four barrels with a strictly mechanical operation (no air valve on the primaries of the secondary carburetor) is new to me. I've done a lot of reading on 2x4 WCFBs, but haven't found anything talking about this issue (though people have mentioned that their car doesn't have it). Thanks in advance for any help!

MikeM

It has been decades since I ran that combination but to me, reading your description, it sounds kinda' like what I remember as normal.

But then, you could have something wrong with one or more carburetors.

68hemi

I would contact fuelie dave as he builds a lot of clone 2x4 set ups as well as rebuilds original ones.

GTOguy

I am running original 270 HP carbs on my '61, which has a 383 stroker in it built many years ago. Mine does not run like you describe....when you open up the front carb, it flat takes off and pulls like a two stroke motorcycle coming 'on the pipe'. With the stock bias ply tires, this can be a bad thing if one is not prepared for it. You have to really hold on.....hopefully Frankie will chime in....he ran the same set-up on his old '61 for years. The tripower on my GTO feels the same way...give the outer carbs the boot and it takes off instantly. It sounds to me like you have a fuel supply problem....either at the pump or at the float levels.....so when you are dumping the carbs open, you are getting too much air.

dmruschell

I rebuilt the carbs myself and used the factory correct 270hp rods and jets, so hopefully I didn't mess anything up lol.

I guess I'm looking for other owners' experiences driving their 2x4 setups. I have no problem taking this engine up to 6,500 RPMs with the tach needle bouncing all over the place lol, but crisp throttle response is what makes a car feel fast

dmruschell

I'm also hoping Frankie will chime in lol.

Above 3,000 RPMs, the car takes off instantly. I'm not expecting it to slam me back as much as my 450hp 79 Vette with a 350 and QJet, but the 61 does instantly have power in the mid-high RPM ranges. And the sound when the secondary carb is opened is magical. It almost seems like the engine isn't sucking enough air to pull enough fuel (or get it properly atomized) when 4 barrels are forced open at lower RPMs.

GTOguy

You may just need to adjust your progressive linkage. Mine is set to kick in at about half to 2/3 throttle. Too much too soon= Bog City.

dmruschell

That had occurred to me as well. I set it up so that both carbs will reach full opening at full pedal travel, and everything else falls as it may.

But, I figured I'd ask around before I start messing with things. Even most local C1 owners don't work on their own carbs, so I'm kind of on my own unless I contact a local carb guru.

68hemi

You say that they are clone carbs. Are you aware that the weights are different for a 270 h.p. set up?

MikeM

The OEM 097 cam starts to come alive about 3-3200 rpm. Maybe this is what you are feeling?

dmruschell

Yes. I have a few WCFBs with different size weights and different length shafts on them. These visually look about right for the 270 application. I doubt the secondaries of the carbs are affecting anything below 4-5,000 RPMs anyways. A single WCFB can feed 230HP to 4,500RPMs, so the primaries of the two carbs should be able to do the same before the carb secondaries would start to kick in, I would think.

dmruschell

If I floor the pedal at about 2,000 RPMs, the car will accelerate. If I let off the pedal to where I can feel the second carb close, the car will start accelerating more quickly.

MikeM

I started to say above that when you open the front carb primary, it is open manually so you in effect have four barrels at least partially open whereas with a single four barrel you have two barrels open and the other two may be open, depending on air flow.

You can always set your linkage so it's not progressive, disconnect the primary carb and run off the secondary and see if the engine performs well.

The last time I ran these carbs was 1963 and they were on a 327. Before that, on a 265. Four different sets in total. They responded to throttle opening better on the 327 than they did on the 265.

If you do have a carb problem, I'd be looking at the metering rod piston, spring and rod adjustment.

I'll bow out now that you're getting professional help.

Frankie the Fink

What you are describing is not remotely normal...

I've had original 1956 "one year only" dual quad WCFBs on my '61; followed by a set of clones sold to forum member Bill Pilon with a Chrysler rear carb and an unknown front carb, followed by a set of 'real deal' 270hp dual quads.

NONE, exhibited those symptoms...all pulled like banshees from a stop, no hesitation, no bog, no flat spots and despite a popular myth a totally smooth transition when the second carb kicked in. No kick in the *** transition. Ran the car around Sebring with the clones, having to back off behind some whuzzie C6s several times in the twisties getting down to 1,500 RPM then 'nailing it' to redline on the straight-aways, chirping tires at each gear change.

Frankie the Fink

If you have a true Duntov 097 solid lifter cam (check the specs they are unique) and correct solid lifter/springs those are some things to check. Next check your advance weight for sticking (you may need to remap your advance curve); original cars had dual point NO vacuum advance distributors. Check your fuel pump and make sure the pin for the actuator arm is not "walking out". That issue can cause really zany symptoms.

As to the carbs, its critical that the accelerator plunger seals well in the bores and that the check valve works properly. Flaring of the leather/plastic diaphragm slightly on the pump is a good idea for a tight seal. Leather needs to be soaked in oil slightly before installation.

The rear flappers, metering rods, vacuumeter spring and counter-weights (and their arms) ALL have to be set up for the vacuum characteristics of the Duntov cam...

And, of course, you should have the progressive linkage hooked up so the rear carbs starts to engage at half throttle. All things you can check in the ST-12 and in the WCFB service manual:

http://carbkitsource.com/carbs/tech/...fb-service.pdf

After you've eliminated the non-carb possibilities then you can come back on here or talk to Ron or Tim at Daytona Parts for more advice...

It can't hurt to check fuel pressure but if the car pulls strong after 3,000 RPM I'd tend to rule that out...

MikeM

The OP is talking about part throttle symptoms.

I remember one fella' here was having problems with his WCFB's and he took the front carb off and bolted a piece of plywood over the manifold to cover the hole.

Frankie the Fink

This is what I read Mike:

"..Under 3,000 RPMs or so, if I floor the gas pedal and open the secondary carb up, the engine acceleration slows down."

vettepoor

I'm running dual wcfb clones on my 350 in my 57, and I have zero problems with bogging, even from a standing start. It will set you back and keep pulling well past my comfort level. When I first got them, the jets were WAY oversized, and I did experience some "bog" when I jumped on it. After re-jetting to factory specs, it really smoothed out. I too have progressive linkage setup. These carbs can certainly be finicky, but when properly set up they are great!

Frankie the Fink

I put 20,000 miles on my clone carbs and Bill Pilon has put on an add'l 30,000 IIRC...the carbs have yet to be opened up or screwed with in all that time.

MikeM

Yeah, I got tangled up in his rpm description.

But depending on posters experience, the description of a problem is a little fuzzy to reality.

Still, if he has a carb problem, I'll stick with the metering rod check and also, make sure he has the right counter balance weights on his carbs.

And I'd still run it on the front carburetor to verify it's operation.