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Hello Everyone,

I have a Holley 650 Double Pumper ( mechanical secondary} carb on my car for a long time, car runs great with instant throttle response. However, It runs too rich with unburnt fuel smell from exhaust especially at idle. My question is that if I were to switch the carb with a vacuum secondary one, will it make a difference as far as exhaust fumes are concerned ?

I am willing to give up some performance for a cleaner exhaust.

Any advice will be highly appreciated.


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...Have you had a recent 'Back-Fire' through the carb? Then check blown/Leaking Power Valve. Also check for a fouled spark plug...Intermittent or Miss-Firing. This will lead to Raw Fuel in the Exhaust. Last, the Idle 'Mixture' Screws are Open too far, And/Or, the Choke is Not Opening Fully.

Double Pumpers Always Run Rich out of the Box! They are tuned that way! In as, on 'Mechanical' Secondary Carbs, the Internal Passages are LARGER for Increased/Faster Fuel Flow, and Can NOT Be TRIMMED Down. The flow must be increased because the SECs Open almost Instantly. So we are driving around in Traffic with a Carb that is always running Rich! Changing the Jets will not make a difference, Nor any Adjustments...It's The Passageways.

In My View...they are for the Racetrack Only! A 650 CFM is NOT Too Big. And Yes! A Vac Sec of the same size Will Fix Your Problem, as far as Raw Fuel.

I took a 600 V.S. to the Desert Ran Clean NO Smells, Got Home, Put the 750 Vac Sec Back On, Now the Fumes are Back....just a Little.

The Plus is, with the Carb LESS Rich...the Throttle Response will be More Crisp!!

Be advised, there are Dozens of ways that cause UNburned Fuel...even in the Timing.

Good-Luck with It!


Last edited by marlinjack

The idle air mixture is adjustable as you probably know already.  We usually try to adjust the mixture to maximize vacuum at idle.  It turns out that you can adjust the screws to get maximum vacuum and set them on either the rich side or the lean side.  Maybe try setting them on the lean side.  I recently used this technique to help eliminate a dieseling problem on engine shutdown.

Are you using vacuum advance?

As Steve said, I would connect a vacuum gauge to it and assuming all else is ok (plugs, ignition, etc.), lean it out on all 4 corners of the carb to maximize the amount of vacuum you can get at idle.  I have a QuickFuel 600 mechanical secondary carb (new 2 years ago) and it is quite nice and no raw fuel smell beyond what any 53 year old muscle car smells like.

Without vacuum advance you might not be idling on the idle circuit because the butterflies are too far open.  If so, it will run VERY rich at idle.  With vacuum advance it will increase timing at idle and raise the the RPM's allowing you to close the butterflies (using the idle set screw) enough for the car to run on the idle circuit.  I cannot guarantee this is what's happening on your car but I had a similar problem recently and this helped.

There are two ports on the Holley carbs that can be used for vacuum advance.  One is the "timed" port and operates as you described above.  The other is the full vacuum port and is the one that may help in this case because it provides vacuum advance at idle.  The upper arrow is the timed port.  The lower arrow is full vacuum.


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First thing you can do is check your idle screw adjustments as recommended. Adjust them for highest vacuum level at needed RPM, then back out 1/8 turn to richen it up just a tad. (Recommended procedure in the books....!)

The stock FORD distributors generally run on "PORTED" vacuum, the upper connector on the primary metering block, as some have mentioned. There should be NO vacuum at this port at idle.

Some AFTERMARKET distributors may run a full "manifold" vacuum, the port on the carb base plate. Check your paperwork.......manufacturers recommendations.

Running a vacuum advance won't kill anything.....will help with off throttle emissions and low load running. Think cruising.......better efficiency.

The BEST thing that I found to reduce/eliminate the gas smell that used to envelope me as I pulled up to a stop light, was to install a Ford DuraSpark II ignition system!!   It allowed me to lean out my carb a tad more.  The "blue connector" control module also provides for a 10 degree spark retard upon cranking, so my 10.5:1 compression ratio didn't get in the way of easy cranking and good starts!!!     Read up on this system.....many articles.

(In a Boss 302 Frankenmotor, I ran 34-36 degrees total timing, with a 10 degree mechanical advance, which left my initial timing at the harmonic balancer at 24-26 BTDC.  Without the 10 degree retard at cranking, the engine was a bugger to convince to turn over. But with that 10 degree retard, it made the engine think that it had only 14-16 degrees of initial timing. MUCH better for starting!)

I have not yet run across any aftermarket ignitions that work as well as the DS II system in terms of having the 10 degree retard during cranking, or the ability to survive at 10 volts or lower in the event of a battery goin' bad!

Your mileage may vary!


The Pertronix 510 allows you to reduce the advance in the start-up phase from 1 to 10° configurable and would work (I have not tried) up to a voltage of 8.5V.

It has several configurable rev limiters (launch, burnout and fatal) and several operating modes, single or multi spark with two power levels plus one or two gadgets such as shift light control.

On the other hand, the installation manual is very poorly done, it is not clear and does not indicate important elements.

Last edited by rene4406

WOW!   Good job Pertronix!!!  About time someone stepped up.....

Also have not tried one......!

Would be interesting to know if that 510 Module would trigger with a DuraSpark II distributor!    

The last Pertronix distributor I saw on my son's 351W based 408, used a really tall distributor and cap affair....not sure if that would fit under the engine cover.

Good info though!! Lots of nice features!

Yes, the Pertronix 510 works with a Duraspark distributor. It works with distributor:

- Pertronix II or III

- magnetic pick up

- Ford duraspark

- breaker point or electronic


You can download the installation manual here:


I couldn't get my tachometer to work, I had to buy an adapter from Panteraelectronics.

ALL of the Holley "double-pumper" carbs were designed as "race carbs" in the sense that they are intended to be used with non-stock, aftermarket tube RACE headers. The term that Holley uses/used, is "header scavenging". Scavenging tends to lean out idle a/f levels.

What that means is that the idle A/F ratio is set by design to correct the fuel level at idle WITH HEADERS to be correct for the larger exhaust. This means a heavy, noticeable "gas smell" to the exhaust.

I ran a 4779, 750dp for years and just got used to the heavy exhaust smell. It is as bad as the idle smell of the Weber 48ida setup, which will smart your eyes.

You could bring it to a "carb tuner" and they could modify it to have a leaner idle but it isn't really that simple to do since it requires internal changes to the idle fuel circuit.

The simplest thing to do is to change to a carb like a 1850 or a 3310 and that will give you an idle designed for a cleaner street idle, i.e., a late 1960's "street idle" which is important to note, not able to be clean enough by today's emissions standards to be street legal.

If you are in a location like California, that still uses a tail pipe sniffer as part of the annual State emissions test, even those two carbs are going to be too heavy and you are going to need to use something like a '85 Mustang GT carb.

That one is NOT AN "1850" BASIC DESIGN. It is designed to idle with an idle of 14.6:1. That ratio is locked in and you cannot change it. It is around a 600cfm carb with annular boosters in the primaries and  vacuum secondaries or you can go to an EFI conversion like the Holley Sniper system which is designed to replace single Holley 4v carb applications and will give you a clean idle.

Other then that, you are in for quite a bit of work to get your 650dp idle levels down.

Last edited by panteradoug

I'm not aware of what CA is currently doing. I have my own issues with NYS DMV. I'm attempting to explain how to get the exhaust smell down to what many would consider a reasonable one, unlike what a typical Holley DP carb does at idle.

Admittedly I have to agree that in the case of lawyers actually being in effect on my side, as open minded as I may or not be, if I was sitting on a jury you would have a difficult time convincing me that a car built yesterday is a 1965 model. That's a real stretch. I suppose this is great for the Cobra replica guys? Someone footed a huge legal bill and lobbying effort in CA to get that one through? I wonder who paid that bill?

For me proving that the Pantera is an import, built with less then 5,000 units a year and as such is exempt from the 1973 US emission standards regardless of what Mrs.Detomaso's political influence was on her NJ Senators, is not something that I want to argue and document annually with the NYS DMV.

It may be easier to convince the chickens at feeding time not to fight over the kernels of corn? They both are at the same intellectual and commutative level yet empowered to deny everything.

"They" just don't like where my oxygen sensors are located OR EVEN that I have two on a 1973 model import vehicle that isn't supposed to have any at all? Some want to know if I am Detomaso and named the car after myself since they never heard of one and it looks like a kit car to them?

Last edited by panteradoug

I was able to get an AFR of 14.5 at idle with my Holley 750 cfm #9379 (4150 dual mechanical secondary pump) by drilling a 0.1" hole in each butterfly. The idle richness screws are open about 1/2 to 3/4 turn and are very sensitive.

In France, pollution control is compulsory for vehicles put into circulation from October 1, 1972. My #4406 was built in September 1972 and therefore necessarily put into circulation (date of first title) in the US later but I managed to get September written on the documents.....

I am in Ohio and cars over 25 years old are exempt from emission testing. I am just tired of smelling gas every time I drive the car. I am leaning towards replacing the DP with a VS carb something like Summit Racing’s 600 VS with annular boosters. I have read good reviews about it. My understanding is that a DP will always run rich compared to VS regardless of what adjustments are made. I have used the DP carb for the last 18 years and yes, car is very responsive  especially when driven hard and sounds great but maybe its time for a change.


Keep in mind that the DP carb is simply a fuel mixer.......and that it can be just need to find someone that understands the "mixer" and how to adjust the "recipe"!

I had a DP on my Boss 302 in my Mangusta.   It was very well tuned!  (Rest of the motor was Frankysteined by persons dorking with things after the initial setup!) It was cross jetted for proper performance of the fuel mixtures noted on the dyno.

I used the DuraSpark II ignition on the car and can only say from my experience that the fuel "cloud" was much reduced after a better ignition system was installed replacing the old points distributor.....which reminds me....what DID I do with that old distributor???    Probably worth something now!

The VS carb of any sort will be fine.   Again a properly tuned VS carb will work very well on the 351C!   But ya gotta know how to do it!

A How to Tune Holley Carbs by Dave Emanuel would be a great book for your nighttime reading habits!!!     The basic carb hasn't changed much since the original one barrel carbs if you get right down to it!   Only thing has changed is how many one barrels do you pack into a single piece!??

It's all about venturi flow, proper vacuum signals, and metering of fuel....probably a few other things, but the basic carb is just a mixer....

What IS amazing is that it took all of the computing power and bits and pieces of EFI to finally MATCH power levels that could be had from a carb based design from decades before.   The old timers did a good job!   (Note, not talking about drivability here......)


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