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Good evening, im new to this forum, I have a sbc 400 30 over, with 2.02/1.60 64 cc heads, Holley 750 dbl pump with the Edlebrock RPM air gap intake. The cam is a comp cams Xtreme Energy 292H Hydraulic Flat Tappet Lift .501"/.501" The car is running fairly well at idle, but im only getting a vacuum of 9.5,,, is this expected? i feel it is very low and contributing to the hesitation off idle to WOT. also it seems to not rev as high as it used to with a 282 cam and a 650 Holley vacuum secondary.. am I missing something ? I have disconnect all the accessories to ensure there is not a vacuum leak, there doesn't appear to be one because as soon I pull off one of the vacuum ports the idle goes up quick. also I am having trouble keeping the primaries closed in order for the idle circuit to keep the car running, i have to have them open slightly to idle at 950 or so.
 

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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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The vacuum reading sounds correct for that camshaft even with the extra cubes of the 400. It's not the cause of the hesitation you report having.

Most often an off idle hesitation can be attributed to a "lean stumble" or a vacuum leak somewhere. If you still have the previous 650 carburetor? It would be worth while to try it temporarily to rule out the idea of an off idle lean condition. This is with the assumption that it worked well with the 282 cam set up. Cracking the primaries open to maintain an idle can also lead to trouble by leaving the transition slots over exposed. We could go on for pages on the adjustment and tuning. I'm just tossing out a couple of ideas.

As far as the "revving" part. It would be good to know the rest of the combination there. The compression ratio, valve springs, torque converter and/or transmission type, gear ratio and what the vehicle is. It could be as simple as the 282 camshaft being a better match or you could be floating the valves with the larger cam.

This a jumping off point to get started. The members here that know a lot more than I do will be interested in that additional information, too.

Last, but certainly not least. Welcome aboard!
 

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Check valve springs for seat pressure. Make sure the accelerator pump is adjusted correctly. Should be ever so slight gap. Eric nova will give exact specs Im sure? I adjust mine so that is all but touching maybe .005". The accelerator pump cam has a couple different positions you can try or you can buy a more aggressive cam, they are colour coded. What colour is yours? 750 is a little on the small side but adequate. I hate that cam. Big hydraulic flat tappet cams can be a bear. Make sure your timing is set correctly and timing is advancing fast enough, You should have a full 34º advance by 3000 rpm with vacuum disconnected
 

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Discussion Starter · #4 ·
The vacuum reading sounds correct for that camshaft even with the extra cubes of the 400. It's not the cause of the hesitation you report having.

Most often an off idle hesitation can be attributed to a "lean stumble" or a vacuum leak somewhere. If you still have the previous 650 carburetor? It would be worth while to try it temporarily to rule out the idea of an off idle lean condition. This is with the assumption that it worked well with the 282 cam set up. Cracking the primaries open to maintain an idle can also lead to trouble by leaving the transition slots over exposed. We could go on for pages on the adjustment and tuning. I'm just tossing out a couple of ideas.

As far as the "revving" part. It would be good to know the rest of the combination there. The compression ratio, valve springs, torque converter and/or transmission type, gear ratio and what the vehicle is. It could be as simple as the 282 camshaft being a better match or you could be floating the valves with the larger cam.

This a jumping off point to get started. The members here that know a lot more than I do will be interested in that additional information, too.

Last, but certainly not least. Welcome aboard!
Thanks! i really appreciate the response, unfortunately i no longer have the 650, but i do know that it was very lean with that carb even after some jet work. the car is a 55 Chevy Belair with a fiberglass tilt nose, Turbo 400 trans, 2800 stall speed, with 3.73 rear. compression is 10.5 - 11to1. the springs are dual springs rated for .600 lift and the rockers are 1.6 roller.
 

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Discussion Starter · #5 ·
Check valve springs for seat pressure. Make sure the accelerator pump is adjusted correctly. Should be ever so slight gap. Eric nova will give exact specs Im sure? I adjust mine so that is all but touching maybe .005". The accelerator pump cam has a couple different positions you can try or you can buy a more aggressive cam, they are colour coded. What colour is yours? 750 is a little on the small side but adequate. I hate that cam. Big hydraulic flat tappet cams can be a bear. Make sure your timing is set correctly and timing is advancing fast enough, You should have a full 34º advance by 3000 rpm with vacuum disconnected
i double checked the valve lash and everything seemed accurate. the accelerator cam is the pink on that comes with it, so far I increased the squirter nozzles from .31/.28 to .32/35, HEI advance at 3000 is 36.
 

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It sounds like either the spark plugs are not or the valve springs are going soft.

I‘d put in fresh plugs gapped .030-.035 first as it’s a cheap check. If it still lays down it’s time to check spring pressure.

The carb falling lean up on the top end is usually accompanied with flame out the carb rather than just the engine laying down quietly.

For the cam your idle vacuum sounds correct.

Bogie
 

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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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Change the accel cam from pink to a blue cam and set it to #2.
Yes. Good call. I like the blue (favorite) and the green (if memory serves) for off idle lean conditions.

I'm not an expert, but I really never found a use for the pink cam.....on anything.
 

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When it comes to off idle stumbles there are several factors that come into play. Nice ride by the way and sound good. I myself would ask what is your timing curve like? Timing will play a big important role in the carb tune. I would not go out and get a vacuum secondary just yet. I myself have never found a use for the pink pump cam regardless on what carb I have used on what engine build from mild to wild. It gives a smaller but longer stroke of a shot compared to an orange or blue pump cam.

What you want to make sure is your primary transfer slot is not showing no less then about .020 and no more then .040 and you want to keep it square. When I know my timing is right and I have my carb properly adjusted I always write notes down on where my throttle is set at on the primary and secondary side. I always write down how many turns it takes to get my transfer slot to about .020 and then put down how many turns it can go to .040. I then make sure it stays within that range so I know my transfer slot is not under exposed or over exposed. As far as the pump cam setting the number 2 spot will give a slight delay and that is for a lot of times engines that have a higher rpm idle above 1000 rpm and you have to use that in order to get as much pump cam profile as you can. If you have to go past the .040 square transfer slot range then you would want to open up your secondary side more to get the correct idle setting. You want to make sure your pump shot will start the instant you hit touch the throttle but you don't want the adjustment screw to tight on the lever arm either as if the engine vibrates enough it can cause the shooter to very faintly dribble just a hair.

Also when setting your idle mixture screws on a four corner idle carb you only want to adjust each one about an 1/8 of turn at a time as they are more sensitive vs a 2 corner idle carb. You also want to make sure your idle circuit is calibrated correct and that involves the metering block idle feed restrictor size and also the idle air bleeds but I will leave that info for another time if needed. If an idle circuit is not calibrated rich enough for a big cam it will cause you having to open up the transfer slot to much and make the idle mixture screws not adjust well or not at all.

It will only have issues on the idle setting and off idle and in low speed cruising conditions until the main circuit starts to flow fuel through the boosters. With a big cam like that and a low vacuum reading and with a weak signal going to the carb it will always require a richer calibration compared to something milder do to the overlap of the cam and exhaust reversion and other factors.

You also need to have the secondary side open up enough as well and that is a trial and see how it does area just like the mixture screws. Also how are your mixture screws adjusting and how far out do you have to go on all four? As far as how I set my pump cam accelerator arm I will loosen mine up to where it just starts to have some play in it and be loose on the arm. I then will tighten it up just enough to where there is a tiny bit of drag by moving the lever front to back and it will have a slight drag on the accelerator arm screw bottom. I after that will open it up to wide open throttle and push up on the accelerator arm and check to see that it is not bottomed out and still have some travel left.

I don't bother with the .015 feeler gauge trick as it can at times still throw things off and not be right. On a double pumper I always start with the front pump shot first and tune it and drive around in different conditions and se how it reacts and get tuned first before going to the secondary side of things but I make sure I don't give it more then about a slight hair less then half throttle because after about 40 percent of throttle opening the secondary side will begin to open and it can cross into the primary side function and you can get a false signal on what your carb is doing.

I have heard that some folks will unhook the secondary side until they get the primary side hesitation free first before they go on to the secondary. I leave mine hooked up and never have had a problem tuning them it just takes some trial and error and see what it wants. Once I get my idle circuit all adjusted well and good then I proceed to work on the secondary pump shot and its a little bit tricky as sometimes you might have to tweak the primary side a hair to go along with the secondary side as they transition from one to the other during a big wide open throttle romp. Here is a chart below on the pump cams. On the pump cam the smaller the cc the quicker the pump shot will be be have less throttle duration but on the more cc on the pump cam like the pink one for example it will have a lesser pump shot but have more of a shot across throttle duration opening. Orange has worked for me in most applications to where a quick shot is needed especially on engines with steep gears and quick acceleration and is not a heavy vehicle. Its a test and try sort of thing as all builds will differ.
Rectangle Font Slope Parallel Pattern
 

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Good evening, im new to this forum, I have a sbc 400 30 over, with 2.02/1.60 64 cc heads, Holley 750 dbl pump with the Edlebrock RPM air gap intake. The cam is a comp cams Xtreme Energy 292H Hydraulic Flat Tappet Lift .501"/.501" The car is running fairly well at idle, but im only getting a vacuum of 9.5,,, is this expected? i feel it is very low and contributing to the hesitation off idle to WOT. also it seems to not rev as high as it used to with a 282 cam and a 650 Holley vacuum secondary.. am I missing something ? I have disconnect all the accessories to ensure there is not a vacuum leak, there doesn't appear to be one because as soon I pull off one of the vacuum ports the idle goes up quick. also I am having trouble keeping the primaries closed in order for the idle circuit to keep the car running, i have to have them open slightly to idle at 950 or so.
Having primary jets that are too small can and will cause flat spots, and decrease engine power, and will decrease top engine rpm. Your primary jets are always a factor at any and all rpm. Sounds like your new camshaft needs more fuel than the previous cam. In another time and place, I used to play around with jetting and metering rod sizes. Amazing what a difference a few thousands of an inch will make. Latest foray into jet sizes was in my 21 horsepower Briggs Intec in my riding mower which was over fueling. I made my own jet and started with a #57 drill bit and ended up with a #53. Small changes = huge change in performance.
 
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