Secondary Metering Plates

[EngineerDoug]

Hello all,

I've been slowly refining the tune on my six pack setup on a 493 stroker big block.

My question has to do with secondary metering plates. I came across a pair of secondary metering plates that accept screw-in jets, but they don't fit properly. They tend to interfere with the floats, as they are a bit thicker than the factory metering plates.

I've come across some comments that indicate the aftermarket plates are not to be used with the thin sheetmetal plate + gasket that lives between the holley metering plate and the carb body. I can see how this would yield enough clearance to allow the float to swing freely, but why would the thin plate = gasket no longer be needed? I would think you might be setting yourself up for some internal leaks across passages by omitting these?

What am I missing? Thanks.

 

[lewtot184]

The metal plate keeps the bowl gasket from squishing into the main channels in the metering plate. I bought some of those jet plates before and won't use them. They either interfere with floats or the main feed channels get reduced by gasket, and the jet plates I bought have narrower channels than factory. I really don't think there's a lot to be had by playing with the main metering orifices. But if I do I'm going to use short brass 6-32 set screws drilled for jet size and tap the metering plate orifices to fit them.

 

[493 Mike]

I'm running those billet jet plates on my 6 pack setup. They work fine, in fact I went up 10 jet sizes to get maximum power on Steve Morris' chassis dyno many years ago. Perhaps more than one plate design is out there?
Mike

 

[lewtot184]

The ones I have are quickfuel.

 

[lewtot184]

An interesting read would be the December 2002 Mopar muscle article where they dyno test those jet plates. Keep in mind that this is an engine dyno test, probably no mufflers, cammed up with alum heads. I've always felt, even prior to reading this article, that a .093" orifice was about the limit on a stock intake. Of course using an str or weiand cross ram would be another ball game.

 

[EngineerDoug]

Lewtot184,

Thanks - your explanation for the stiffener plate makes sense. I can see how the gasket could, especially over time, swell or shrink.

 

[lewtot184]

Lewtot184,

Thanks - your explanation for the stiffener plate makes sense. I can see how the gasket could, especially over time, swell or shrink.

If you had a stock plate and the jet plate side by side you would notice the narrowed fuel channels in the jet plate. Couple that with the removal of the steel plate that prevents the main gasket from migrating into the fuel channels and I think a problem could arise. I really believe the stock dual plane intake just has limits and adding gadgets isn't the solution, unless there's some work done to the intake.

 

[steve340]

The tin plate and gasket are there to provide support/sealing for the centre part of the metering jet plate were the carb body has a cast in recess for a secondary power valve.
If the carb is completely flat they will not need the support plate and gasket.
The main reason everything comes close is the short side hung float bowls.
The aftermarket plates like the Quickfuel etc are basically designed to work with the centre hung float bowl style like Holley fitted on the performance vacuum secondary carbs.
This info is right for 4 barrel I am not sure how Holley configured the 3 x 2 setup.

 

[lewtot184]

The tin plate and gasket are there to provide support/sealing for the centre part of the metering jet plate were the carb body has a cast in recess for a secondary power valve.
If the carb is completely flat they will not need the support plate and gasket.
The main reason everything comes close is the short side hung float bowls.
The aftermarket plates like the Quickfuel etc are basically designed to work with the centre hung float bowl style like Holley fitted on the performance vacuum secondary carbs.
This info is right for 4 barrel I am not sure how Holley configured the 3 x 2 setup.

I think the quickfuel plates would work better with side pivot floats than center pivots, or at least that seems to be in my case. Without the steel plate the gasket (blue gaskets) does push into the fuel channels reducing volume in my carbs. After spending the money and time I don't believe it was worth doing, but everybody has to make their own decision here.

 

[steve340]

I have used those replaceable jet metering plates on a fair number of carbs and had no problems fouling the floats - but I only use those blue non-stick gaskets and they are fairly thick and may just hold the float bowl out enough.
I have also had no problem with how the engines run with them installed and they allow the secondary mixture to be changed easily.
On a street car were you can do what you want I prefer to fit a secondary metering block.
He doesn't mention having jet extensions but those do foul the stock floats for sure.
I have not seen those support plates the OP is asking about on an aftermarket carb only on Holleys.

 

[EngineerDoug]

Thanks, guys I appreciate the inputs.

 

[Bird 426]

Holley makes a float with reliefs to clear the jets. I have Promax plates in my small block stroker 6-pack end carbs, base throttle plates too. No issues.
Stock metering plates were a bugger, that's why I switched.