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I have richened the idle on my car by inserting wire in the idle air bleeds with some improvement - but that was only after drilling holes in the primary plates and cracking open the secondaries slightly to get the primary plates closed down to show a square under the throttle plates and get the idle screws to be responsive.
You probably need to do something to get your outboard carbs to contribute a bit more air at idle so the center carb plates can be closed down some. And you may need to richen the accelerator pump circuit to with a plastic cam change and a bit larger squirter. I believe there are also different size accelerator pump housings to fit Holleys and you probably need the larger one.
It would probably shed some light on things if someone can out the specs on your cam.
I went from a stock 30 lb flywheel on my car to a middleweight 18 lb aluminum one and didn’t notice any change in driving around on the street. And I have over 100 fewer cubic inches and I bet a more radical cam for engine size than your car. I wouldn’t waste money on that at this point.
me neither Dan , i read for a street car the lobe separation is best 110 à 112 °. So that would be OK.
But i have no knowlegde how to understand duration etc...
114 LSA is probably better for a street car in the under 3000 RPM range through mufflers for drivability. Unfortunately, cams like that are hard to find in higher lift and durations as most cams like that are more for the "race" guy that wants max HP readings at higher RPM and is running open headers.
I have richened the idle on my car by inserting wire in the idle air bleeds with some improvement - but that was only after drilling holes in the primary plates and cracking open the secondaries slightly to get the primary plates closed down to show a square under the throttle plates and get the idle screws to be responsive.
You probably need to do something to get your outboard carbs to contribute a bit more air at idle so the center carb plates can be closed down some. And you may need to richen the accelerator pump circuit to with a plastic cam change and a bit larger squirter. I believe there are also different size accelerator pump housings to fit Holleys and you probably need the larger one.
It would probably shed some light on things if someone can out the specs on your cam.
I went from a stock 30 lb flywheel on my car to a middleweight 18 lb aluminum one and didn’t notice any change in driving around on the street. And I have over 100 fewer cubic inches and I bet a more radical cam for engine size than your car. I wouldn’t waste money on that at this point.
With tri-power's +900 cfm and 540ci, a 50cc center carb pump housing may be needed to enrich the tip-in a/f ratio, as the 30cc pump may be running out of fuel volume as squirter size and pump cam size increases (despite what Holley says is typical).
All 4150 and 4160 style Holley carburetors use the standard 30cc pump reservoir. This capacity rating is not the amount of fuel delivered per full stroke of the accelerator pump system but instead is the volume after 10 strokes. This is the total reservoir capacity. Depending upon the pump cam, the volume delivered after 10 strokes is usually less than 30cc. As an example, the white accelerator pump cam will deliver less than 20 cc of fuel after 10 strokes.
The nozzle or squirter size has no bearing on the volume delivered. A smaller nozzle like a 28 will merely create a longer duration shot of fuel versus a larger nozzle size.You may also have seen references to a larger, 50cc pump that can be added. For even aggressive, high-horsepower applications, it would be extremely rare for a tuner to find a situation where a street car was in need of the extra volume of the 50cc pump on a 4150 style Holley carb. These larger reservoirs are found on much larger, 4500-series Dominator carburetors.
One tuning point that will be stressed several times throughout this story is that the ideal accelerator pump tune is the least amount of fuel required to create optimal acceleration. When a hesitation occurs, the oft-used solution is to add more fuel with a larger nozzle or with more lift from a taller pump cam. As we mentioned, this shot of fuel is only intended to prevent a lean hesitation. It deserves mention that engine hesitation or bog under heavy throttle can also be caused by excess accelerator pump fuel.
Armed with the basics of how the circuit works, we can now approach the tuning aspects. Among the two most commonly used accelerator pump components are the accelerator pump squirter or nozzle and the pump cam. But before we get into those pieces, it’s always important to make sure the current system is working properly. The most common cause of an annoying off-idle hesitation on Holley carbs is when the primary bowl has been removed and re-installed. It is very easy to replace the fuel bowl in a slightly different position that can create additional clearance between the accelerator pump cam linkage and the pump. This will delay the pump shot into the engine and can cause a hesitation.
made a order at Jegs , with the pump jets 25,31,40. And the hollow bolt.
Also a cam set , and Accel plugs ( heat range 6 and 4 ) to keep spare IF my Bosch would fail.
And a HEI advance kit with weights and springs..... also a tune up kit for my "old" HEI distributor , to have a good spare.
Local i bought all parts to make a exhaust H... in 2 1/2" .
so we can soon play "hobby"
PS according Jegs , up to 37 one can use the massive bolt , from 40 the hollow one. But i assume it doesn't harm using that hollow one on the smaller ones. As they mentioned in the vid , we want the flow of the jets , not restricted by any other .
Last edited by belgiumbarry; 04-20-2023 at 09:48 AM.
I see a similar 500HP LS3 , 10.7:1 CR , LSA 112° , has a duration 219 int / 228 exh. Mine "can" be 236/244 so 17° more int / 16° more exh.
valve lift .525/.525 .... mine .610/.632 so 15 to 20 % more lift
Yes, it’s a pretty aggressive cam, but with all those cubic inches, they will take the edge off it. It’s not a real aggressive or race cam IMO for that engine. The LS3 is quite a bit smaller than your Merlin so the same cam in it as is in your car, would be a little more radical yet. Assuming it’s typical of good roller cams, the ramps are more aggressive ( faster rate of rise and closing) than the old flat tappet cams. So you have to look at the valve opening and closing curve (lift vs duration) and study the area under the curve, which is indicative of how much flow can be cycled through the head. The roller cam will have a wider curve for a given lobe lift which equates to more area under the curve which is indicative of ability to flow more intake or exhaust, dependent on the head, valves, ports, intake, etc. So it’s capable of making more power over a broader rpm range, compared to a flat tappet cam of equal duration and lift. To make close to the same power, the flat tappet cam will need even more duration and lift. About all I know about cams.
Yes, idle vacuum is often a good measure of how aggressive your cam is. My old Cam Dynamics cam pulls 8 Hg on a good day. I bet you will be somewhere more in the 12 Hg +/- range if you are fairly close on the tune now
i was thinking , a 2300 Holley comes standard with a "21" pump jet. That is for engines 200...250 CID ? as it is 350 CFM.
So i need at least the double with 540 CID. (The outer carbs don't have a pump )
The double section of dia "21" is a dia "30". Don't know if it likes more or less , so with 25,31 and 40 i must can find a solution for ideal pump effect with transition.
From the Porsche with Webers i learned that we had to be carefull not to pump to much . It is raw fuel , not as good atomized as the main circuit does. With the Alusil there is danger of washing the cilinders.
Don't know if that counts too for steel cilinders.
I wonder what pump jet # the original 427 tripower used ?
for the vacuum cutout valves i installed 2 small electric valves , one to vacuum the system , one to break the vacuum.
I mounted the hoses "visual" the nicest way.... and the system worked, so no suspision.
Just tested it and it leaks.... seems i must use one specific nozzle of the two for vac "in" , which i connected reversed on one of the valves.
that will be the nozzle on which vac helps closing it .....as long as the 12V doesn't open it .
correct Dan , the exhaust cut out valves ( at the end of the sidepipes ) are vacuum operated. ( best against water/heat /dirt etc ).
Vacuum comes in via 2 electric valves , dry mounted high in the engine bay , one to vacuum the system , one to let air in and break that vacuum. ( Open and close )
I needed two , as otherwise as long as the engine runs , i could not close the valves once opened. ( or perhaps at WOT when there is no vacuum )
Parts are ordered to make a H connection just after the headers. I have perfect room for that , going under the pan. Job for next week
04-19-2023, 09:19 AM
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