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Old 07-02-2009, 09:16 AM
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Carb Recommendation please

I have a new 355, 10-1, edlebrock aluminum RPM heads with 1.6 roller rockers, Comp Cam- Dur @ .050 243/257, Lift .531/.517 (I believe that's based off 1.5 rockers ?).. The intake is a Performer #1201 and I installed a new Summit 750 VS carb on it but I'm having all sort of tuning issues and believe its all related to the carb so rather than throw a bunch of money and more aggravation into this carb, which I think is a POS, I'm searching for a new carb and looking for recommendations.. The motor was dynoed just over 450hp with the summit carb but it was never tuned for idle quality or tuned with a load on it because right now it idles extremely rich, gets an off idle stumble and runs lean at both cruise and WOT.. The throttle response off idle is terrible (cant even spin my tires on my smooth garage floor with a 3000 stall) and in order to change squirters you have to remove the top plate of the carb (not very tunable friendly).. The idle is also very inconsistent.. It will idle nicely at 1000rpm when all warmed up but after a mild cruise it rises to about 1300 and I'm getting real tired of screwing with this thing..

Anyways, what I'm looking for is-- A carb that is already modified, especially for a large cam.. Ya know, one that already has an idle circuit designed and easily tunable for large cams to minimize fouling at idle (without having to drill holes in the throttle plates).. It would also be nice to get something that only requires some mild tuning out-of-the-box such as jets and squirters without having to tear the top half of the carb apart to get to them.. This time I want a DP too.. I totally modified my last 750DP holley myself but it was allot of work and I would prefer to have most of this already done this time.. Can something like this be had for under $500? And if so, what would you recommend?

Thanks!!

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Old 07-02-2009, 09:35 AM
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Sorry to hear that the Summit carb never worked out for you.

I'd use a Holley 4777 650 DP that rocks, in my opinion. About $340 from Summit. The 4778 700 DP is about $384 and is as big as I would think of going.

They also sell a Holley "Street HP" series 650 DP for $502.

Last edited by cobalt327; 07-02-2009 at 09:42 AM.
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Old 07-02-2009, 10:17 AM
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Quote:
Originally Posted by cobalt327
Sorry to hear that the Summit carb never worked out for you.

I'd use a Holley 4777 650 DP that rocks, in my opinion. About $340 from Summit. The 4778 700 DP is about $384 and is as big as I would think of going.

They also sell a Holley "Street HP" series 650 DP for $502.
Do you know if these carbs have idle circuits already designed for low vacuum, large dur cams? How about 30cc acc pumps with #31 squirters? The description at summit or jegs dont give these kind of details.. I see Demon carbs have idle circuit provisions for large cams but through my searches feedback seems marginal.
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Old 07-02-2009, 01:19 PM
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With that much duration you would need to go with one of our Mighty Demons as it is intended for cam duration ranges of .240 - .260 @ .050. What happens typically with the larger duration cams is that you cannot get the car to idle without really opening the butterlfies up or drilling holes in them. The Mighty Demon has an idle-eze feature which lets you open a valve in the base plate to let air by it as opposed to drilling the holes. When the butterflies are opened too far you get into the transfer slots and start pulling that fuel which makes the mixture screws non responsive.

As far as size we would recommend the 750 with annular boosters.
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Old 07-02-2009, 01:43 PM
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Originally Posted by Tech @ BG
With that much duration you would need to go with one of our Mighty Demons as it is intended for cam duration ranges of .240 - .260 @ .050. What happens typically with the larger duration cams is that you cannot get the car to idle without really opening the butterlfies up or drilling holes in them. The Mighty Demon has an idle-eze feature which lets you open a valve in the base plate to let air by it as opposed to drilling the holes. When the butterflies are opened too far you get into the transfer slots and start pulling that fuel which makes the mixture screws non responsive.

As far as size we would recommend the 750 with annular boosters.
Funny you popped in, I was just reading about your Mighty series and noticed they were designed for cam dur above 240.. I only see the 850 with annual boosters on your site? Do you have a part#?

Do you think the pump shot is enough right out of the box to give me maximum launches?

Here is a walk around video I just did.. I wish it was as fast as it sounds.

http://www.youtube.com/watch?v=DYN30llch90


Thanks
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Old 07-02-2009, 01:54 PM
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Quote:
Originally Posted by My68Vette
Funny you popped in, I was just reading about your Mighty series and noticed they were designed for cam dur above 240.. I only see the 850 with annual boosters on your site? Do you have a part#?

Do you think the pump shot is enough right out of the box to give me maximum launches?

Here is a walk around video I just did.. I wish it was as fast as it sounds.

http://www.youtube.com/watch?v=DYN30llch90


Thanks
The site hasn't been updated recently as we have been putting a new one together so you won't find them on there but the part number is 5402020GC .

In regards to the pump shot they are at a good starting point but may require tuning based on the engine. You can change out the squirters and/or pump cams to change the pump shot if needed. The Mighty's also have replaceable air bleeds on top so calibration changes can be made if needed.
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Old 07-02-2009, 02:15 PM
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Quote:
Originally Posted by Tech @ BG
The site hasn't been updated recently as we have been putting a new one together so you won't find them on there but the part number is 5402020GC .

In regards to the pump shot they are at a good starting point but may require tuning based on the engine. You can change out the squirters and/or pump cams to change the pump shot if needed. The Mighty's also have replaceable air bleeds on top so calibration changes can be made if needed.
Ok, I just bought one.. At least I know where to find tech support if I need it
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Old 07-02-2009, 02:29 PM
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Quote:
Originally Posted by My68Vette
Ok, I just bought one.. At least I know where to find tech support if I need it
We are usually on here often as you can see by our post count but sometimes it may be a few days in between if we are real busy. If you need any help feel free to pm us or email us and we will see what we can do.
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Old 07-02-2009, 02:32 PM
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Sounds like you don't have enough ignition timing at idle. A cam like that needs at least 25 degrees at idle. 35 to 36 total.

more timing increase the idle speed which allows the throttle blades to be close more which will ensure the carb is in the idle circuit only, so no need to drill throttle blade holes

I have a similarly built 350 and I use a Holley 3310 750 vacuum secondary carb. runs great.
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Old 07-02-2009, 02:55 PM
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Quote:
Originally Posted by 454C10
Sounds like you don't have enough ignition timing at idle. A cam like that needs at least 25 degrees at idle. 35 to 36 total.

more timing increase the idle speed which allows the throttle blades to be close more which will ensure the carb is in the idle circuit only, so no need to drill throttle blade holes

I have a similarly built 350 and I use a Holley 3310 750 vacuum secondary carb. runs great.
Actually I had the initial set at 24 when I drove it yesterday and just backed it down to 18 about an hour ago.. I plan on taking it for another spin later tonight.. Right now its set at 18/36 and all in by 3000..

After setting it back down to 18 intial, I noticed it didnt seem to run as hot..
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Old 07-02-2009, 04:46 PM
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Try the timing "locked out" 36deg fixed at idle. Remove the carb, flip it over and equallize the throttle bladeopening idle stop /idle fuel transfer slot exposure at idle .020-.030" on pri and sec barrels.. Swap in a power valve that will stay closed at idle. 3.5-4.5" rated. Then reinstall.
Yes the motor needs this much timing at idle. it will idle much cleaner, without the rich exhaust and poor throttle responce.
The high valve overlap of your cam requires this kind of base inital idle timing at idle.

Save your money and just dial in the carb you got.

The edelbrock #2101 performer manifold plenum tends to get way to hot if the exhaust heat riser passages are open at the manifold gasket. Block off both sides.

From your vid it is clear the PCV is not hooked up.

Make sure the PCV is hooked up to the base of the carb and functional. The other opposing valve cover must be
vented. The PCV is your friend and gives the extra air flow required at idle to allow correct throttle blade/transfer slot idle position. Without the PCV the throttle ends up too far open on the transfer slot at idle.
There is not good reason to not run the PCV.

These mods will have to be done reguardless of the carb you use.
Your carb is not a POS, but it does require abit of special setup when running a high overlap cam.

18idle/36 timing is not enough at idle. If you just advance the timing to 24 deg the max advance will be way to far advanced. (42deg total)
The motor needs a full 36deg at idle. Lock out the mechanial advance curve out and set the timing at 36deg BTDC. Its easy to do.
Drilling the throttle butterflies is not nessessary at all. Do not drill the throttles.
These recomendations are not opinions, They are based on 35 years of experience in correctly setting up and running high performance vehicles just like yours (and a lot lot more radical).

Last edited by F-BIRD'88; 07-02-2009 at 05:17 PM.
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Old 07-02-2009, 04:58 PM
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The carb he has is a Summit-branded 4010 vacuum secondary Holley. Not the hot set-up, compared to the excellent carb he just got, IMO.

I do believe the Summit carb can be made to work- actually he's able to drive the car now, albeit w/lousy manners.

The BG will allow the carb to easily be tuned properly. The cam is big, but not so big that it should be necessary to lock it at 36 degrees, IMO.

In any event, hopefully the OP will keep us all informed of his progress.
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Old 07-02-2009, 05:11 PM
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Holley 3310 750 vacuum secondary carb. Cole
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Old 07-02-2009, 05:22 PM
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Quote:
Originally Posted by cobalt327
The carb he has is a Summit-branded 4010 vacuum secondary Holley. Not the hot set-up, compared to the excellent carb he just got, IMO.

I do believe the Summit carb can be made to work- actually he's able to drive the car now, albeit w/lousy manners.

The BG will allow the carb to easily be tuned properly. The cam is big, but not so big that it should be necessary to lock it at 36 degrees, IMO.
I fully understand what he's saying because I had to do that on my last 355 build, which was a little more stout but your right, I shouldnt have to do it on this motor because the cam was actually designed specifically for the street (Comp Cam #12-602).. I personally think this Summit carb just needs more pump shot but that doesnt solve the rich idle mixture either.. I already have the curb idle set 1/2 turn from bottom so even if I can bring the idle up by drilling holes in the throttle plates or opening the secondaires slightly, there isnt much adjustment left in the curb idle to go down. If I look down the carb from the top, it doesnt look look like the primaries are open much at all at idle. I dont think I could fit a .030 feeler gauge in there..

I just tried moving the cam position on the squirters from #2 to #1 and it didnt change anything..

If the new carb cleans up the rich idle but still has a stumble off Idle, I think more pump shot is all I will need..
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Old 07-02-2009, 05:32 PM
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Quote:
Originally Posted by My68Vette
I fully understand what he's saying because I had to do that on my last 355 build, which was a little more stout but your right, I shouldnt have to do it on this motor because the cam was actually designed specifically for the street (Comp Cam #12-602).. I personally think this Summit carb just needs more pump shot but that doesnt solve the rich idle mixture either.. I already have the curb idle set 1/2 turn from bottom so even if I can bring the idle up by drilling holes in the throttle plates or opening the secondaires slightly, there isnt much adjustment left in the curb idle to go down. If I look down the carb from the top, it doesnt look look like the primaries are open much at all at idle. I dont think I could fit a .030 feeler gauge in there..

I just tried moving the cam position on the squirters from #2 to #1 and it didnt change anything..

If the new carb cleans up the rich idle but still has a stumble off Idle, I think more pump shot is all I will need..
When you are finished experimentingt withe the new carb and your incorrect timing etc, and are tired of fouled plugs , overheating and acrappy throttle responce, set it up exactly as I outlined and you can leave the hood shut.
The thumpr cam you have is a "street cam" but needs a full 36deg at idle.
Take the carb off the intake flip it over and reset the throttle opening on both the pri and sec barrels so that .020-.030" of trnsfer slot edge is exposed and reconnect the PCV as designed . It will now have the required idle airflow.
If you find it hard to hot start with loked 36deg idle timing, install a simple ignition power interupt switch on ther HEI 12v power feed wire. to allow crank with the spark disabled. Your GM starter will outlive you.
The gear drive is gay on a other wise nice looking vette. big Thumbs down.

The off idle stumble is caused by the basic incorrect pri and sec throttle blade position at idle, created by the lack of PCV airflow at idle and lack of base idle timing.
Pop the distributor cap and rotor off and lock out the timing. 2 minutes job with two common plastic electrical cable ties around the advance mechanism, once the weights and spring are removed.
The spark plugs will clean up once you have set it up correctly and cleaned er out a few times with a few good power giv'ers.

Last edited by F-BIRD'88; 07-02-2009 at 05:47 PM.
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