I have a 66 L79 with a 3367 Holley Carb - just rebuilt the motor, etc, etc, but the carb I was told was rebuilt about 1500 miles ago before I got the car. Very pretty on the outside, but who knows on the inside! After about 300 miles, the carb is running very rich (I think!) as it will pop back through the carb when going around corners, downshifting and starting back up around 2-2500 rpm's, etc. If I "feather it", it is ok. Once I have the RPM's up in the 3-3500 range, it runs super! Only at lower RPM's. It seems to do this more when warm. Car starts up fine cold. After its warm and you shut it off, it is harder to start and seems to flood out - if I hold to floor will spit back through carb. No leaks externally. Just very strong gas smell when warm.

Ok, where do I start?? I have rebuilt numerous carbs over the years, but not a Holley! Anyone have any ideas? LARS?

Thanks, Ron

 

Sounds like it's time for a rebuild. Holley carbs, are by far the easiest ones to rebuild. They are extremely simple in operation. Go for it.

 

It could be a simple power valve popped. The early design power valves were suceptible to being "blown" when the engine has a serious backfire. This causes the vacuum diaphram of the power valve to rupture. This causes the power valve to stay open when it should be closed al idle and mid throttle, richening the mixture. The idle can be compensated by turning down the idle mixture screws. This results in an engine that idles, has very poor off-idle to mid throttle, but proper mid to wide-open throttle response. It also would be rich when both cold and warm, creating the good starting but problems when warm.
Check the power valve by removing the 4 bowel screws to get to the metering block. Remove the power valve. Use a vacuum pump with a 1" cup fitting. Grease the surface of the valve to hold vacuum. Pump up the vacuum and watch the valve close as the vacuum increases. There is a rating stamped in the edge of the valve. It should completely close at the rated vacuum. If not, replace it. When putting the bowl and metering block back on, be careful to position the accellerator pump lever below the push arm prior to re-attaching the bowl screws. As always, if any of the gaskets or screw seals are damaged, replace them. They and the power valve, can be purchased as part of a kit or separately (I always keep both gaskets and screw seals on hand for this along with an assortment of power valves left over from rebuilds). There is a wide variety of replacement and speciality power valves. Unless you have altered the internals of the engine, stay with the original power valve rating.
I made a quick look for your carb number to see what the original rating of the valve should be. I didn't find it so I'm not sure if it had only one power valve or two (one in each metering block). My suspicion is that it is similar to other 3310's with a 6.5" vacuum primary and no secondary.

 

Good luck with your rebuild.
Something to consider in the future tho...
Just my opinion.... Have used Holley, Performers and Q-Jets.....
Holley's are my last choice for street, but first for racing.
Performers are a great simple street carb.
My preference is the Q-Jet...Set up right, it is probably the next best thing to fuel injection. Probably will get a lot of flames for this, but the Q-Jet is the only carb I will have on my cars.
Nothing but taillights.....
Don

 

First let me say I do not have any real knowlege of the ins and out of Holleys. However, my experiance with carbs in general....backfires through the carb is a lean condition. Do you have a choke? That may help on the cold starts. I wonder if some dirt has has worked it's way into the low speed circuit. I would dial the choke in and see how it runs.

Tom

 

I couldn't agree more. Holleys work great at idle and W.O.T. Anywhere in-between, they suk. I've built several Q-Jets that perform as good as a Holley DP but have great throttle response and gas mileage. It's all in the set-up.

 

Those who know Q-jets love them, those of us who know Holleys love them. It's true of anything else.
As to the problems of mid throttle, they can be tuned just like a webber or Q-jet. You just have to know what you're doing. There are plenty of good Holley books out there that explain how. As I posted before, a popped power valve will do exactly what RONS65 has described. It's a simple test. Give it a try. Let us know the results.

 

[QUOTE=rgs]Those who know Q-jets love them, those of us who know Holleys love them. It's true of anything else.
As to the problems of mid throttle, they can be tuned just like a webber or Q-jet. You just have to know what you're doing. [QUOTE]

I disagree. I know both carbs. very well and stand by my beliefs. I've rebuilt & tuned many of both. Holleys tend to have larger primaries which kills throttle response and gas mileage. Anyone who truely knows carbs. will agree. They cannot be tuned to offset this fact. Q-Jets have the small primaries and large secondaries. They also have metering rods which allow fine tuning far beyond any Holley. Flame suit on.

 

It looks like on this subject we will not agree. It's unfortunate that you've had bad experiences with the Holleys. I on the other hand have always been able to make them work very well.

As to the size of the venturi issue you brought up, I agree that there are differences to the throttle response between the Holley and the Q-Jet. The small-primary-large-secondary will produce a higher velocity of air/fuel for a given CFM before the secondary opens. So for low power times, idle and low speed cruise, the Q-jet has a higher velocity and a more noticible response for a given movement of the throttle plate. Once the transition to the secondary opens, the velocity dramatically drops, reducing the the sensitivity of response. The larger primaries of the square 4 Holley will have a lower velocity initially, equating to less sensitivity off idle but will have a higher velocity for the same cfm where the secondaries open in the Q-Jet.

What this all means, the two different styles of carburetors have different velocity profiles. Now compare this to the Holley spreadbore vs the square 4 versions, same issue. If you want the velocity profile of a Q-Jet, you can obtain it with a Holley spreadbore.

 

Thanks for the reply's! I will check the power valve this weekend and see what I find. I know Q-Jets pretty well.....and agree that they are great for the street - this 66 is a very nice number matching frame off, so at this point will try to work with the Holley to keep everything fairly correct. With that said, if I have to change out, I will. I will keep you posted.


Ron

 

Have to agree with Sky65, popping through the carb indicates a lean condition. Bad power vlv diaphrams ,push lots of gas through the intake flooding the exhaust and creating loud pops out the mufflers along with flames.
Since you said this engine was recently rebuilt, I'de suspect the carb stood for quite a while. This may have had the gas starting to congeal and turning to shellac ,guming up your fuel circuits.
I'de take the primary fuel bowl off and take a look inside for debris. These old cars also have problems with rust from the fuel tank messing the carb up also. So if you see red powder thats what it is. With the bowl and metering block in hand use some spray gumout to blow all the passages out removing the jets and pwr vlv. Use the gumout to spray the passages on the carb body and the air bleeds at the top of the venturies.
If your not that adventurous at least take the primary bowl sight plug off and chk for fuel just dribling out. If its not adjust the float level by loosening the large pan screw on the bowl 1/4 turn while turning the nut c/w to raise the fuel level. LOL
Bob

 

Hey Ron
Did you ever sort this out?

 

Tried to get to it last weekend, but failed - I went to a car show and the car drives great until you are 'easying along' and then either stop/start up or go around a corner, downshift and roll the gas on - if I stay on it and keep the RPM's up, it runs super. Idles fine, but is hard to start when warm. Someone said 'plugs fouled', but I do run the freeway and with 3.70 gears it running 3200-3500 RPM's and does not miss a beat...oh well, I hope to get to it this coming weekend......I will post what I find when I finally figure this one out. I understand your "lean" condition comment, but the gas smell is strong and just feels like too much fuel. But, I need to follow-up on my plans to double check all the timing, spark etc before I take the carb apart - no external leaks thank goodness!
Ron

PS: With the 80-90 degree sunny days its hard to work on the car - just want to put the top down, jump in, and put the hammer down!

 

Here's what I have found on this problem:
Modern fuels have a much higher vapor pressure (lower boiling point) than the old fuels that were in use when these cars were built. The Holley and BG carbs have a fairly small accelerator pump well (fuel reservoir) located under the float bowl(s) as the closest point to the intake manifold. What happens is that after engine shutdown, radiant heat from the manifold hits the accel pump well. This causes the fuel in the well to partially vaporize (it doesn't have to boil - it just needs to create some vapor pressure). When this happens, the accel pump well is pressurized, and the checkvalve in the accel pump will allow fuel to be squirted out of the accel pump discharge nozzles, just as if you were sitting in the car pumping the gas pedal. Once the fuel squirts out, pressure is relieved, and the accel pump checkball allows the accel pump to "suck" in another load of fuel, which is then heated and pumped right back out the discharge nozzles. This will flood the engine out and make it smell like gas after shutdown.

The simple and effective solution is to install a reflective heat shield between the carb and the manifold. Holley makes such a shield under part number 108-70. The shield, in the as-supplied sondition, is too big, so it has to be trimmed down to fit most of our small blocks. The key is to leave adequate shield material so that there is a shield between the accel pump well(s) and the intake.

Note that the problem has nothing to do with heat getting up through the carb body and the float bowls, so installing an insulating gasket under the carb will not solve the problem: you have to reflect heat away from the accel pump wells.

If you'd like a copy of an article where I discuss the fabrication and installation of this shield (along with photos), drop me an e-mail request and I'll send you an MSWord document on the subject:
V8FastCars@msn.com

 

New to Forum- old to this Holley - 3810 on 300 Hp; the percolating described in lars post is all too familiar along with the power valve problems. Have installed extra manifold gasket (one above and one below stainless heat shield that was originally installed on manifold) and it helps some, but more improvement would be great. It also seems that the bowl/metering block (primary) are constantly leaking and require torquing every couple months to just keep it running.
Would appreciate a copy the accelerator pump shield info if it would not be imposing/hijacking this post.
Any other thoughts, ideas, and experience would be greatly appreciated.
Thanks, Al

 

Rons65,

Let me throw my $0.02 in for grins. I have had plenty of experience with Holleys over the years and what you are describing sounds like a float problem. What type do you have? Nitrile or metal? The nitrile type is a black dense foam that over time and exposure to fuel additives can absorb fuel and become heavy. As such, the float doesn't level the fuel in the bowl, the pintel in the fuel inlet allows fuel to enter, and the engine runs rich. If you have a metal float, you could have a pinhole leak allowing the float to get heavy creating the same condition. That would explain why your engine runs fine at high RPM. My experience with blown power valves has shown that the engine runs extremely rich, and will barely idle, if at all. Also, you might want to check the gasket between the power valve and the metering block. I have seen power valves in Holleys that weren't quite tight enough allowing fuel to enter behind the power valve. Good luck.