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Old 06-11-2016, 11:40 AM
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sbc 355 big mutha thumpr cam no idle barely any vacuum new rebuild

ok so i have a sbc350 over 350 out of a 72 stingray. just had it rebuilt with 190cc/64cc aluminum heads with maximum valve lift of 0.600 , big mutha thumpr cam, comp cams 1.6 magnum roller rockers, high energy pushrods, with a weiand dual pane action plus intaken and a 600 cfm freshly rebuilt carb with a 6.5 powervalve.
im no mechanic but know and understand enough to get by in certain areas but not enough to build an engine so i took it to a shop.after building the engine mechanic cant even get to car to idle, has very little to no vacuum and wont even stay on unless foot on pedal and even then it blows greyish smoke out exaust and reeks of ful. all mechanic can say is the cam is to big and rocker ratio to high but i have found a few sbc 350's and 355's with the same camshaft and roller rockers. anyone that can give any advice or help would be greatly appreciated

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Old 06-11-2016, 11:44 AM
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sbc 350 over .30
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Old 06-11-2016, 12:09 PM
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go thru this gallery.. just to get you to know the inside of your holley circuits..

Holley R1850-3 - Album on Imgur

you could have issues where your engine needs much more fuel thru the idle circuit and idle transition circuit ..

open your secondary throttle blades more.. so the primary throttle will be able to close down so you are only pulling fuel from the round idle feed ports.. not the idle transition slots..

you may end up needing to take the carb off to reset the secondary idle screw.. its up from the bottom..

is that intake for spread bore carbs.. you may want to check around the outside edges for vacuum leaks.

are you running a PCV valve..

is your power brake booster leaking.. you can disconnect the power brake booster hose and plug it to see if you can then get it to idle.
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Old 06-11-2016, 12:33 PM
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Quote:
Originally Posted by HiGhDRo View Post
sbc350 out of a 72 stingray. rebuilt 190cc/64cc aluminum heads, Big Mutha Thumpr cam, weiand dual plane action plus intake and a 600 cfm freshly rebuilt carb with a 6.5 powervalve. took it to a shop. after building the engine mechanic cant even get to car to idle, has very little to no vacuum and wont even stay on unless foot on pedal and even then it blows greyish smoke out exaust and reeks of fuel.
Basically, it's doing what Comp Cams warned you it would do! As Wayne says, you'll need to sort out carb. tuning and ignition timing and make sure no vacuum leaks... The carb. may not have been rebuilt for this type of engine... It's also a bit small for this type engine... A single plane intake may work better by avoiding fuel pooling problems in the lower runners of a low rise dual plane intake with this cam... Disconnect and plug hose from carb. to vacuum advance on distributor (leave it disconnected until engine runs right from tuning). Start with about 18 degrees of base idle ignition timing... 36 - 40 degrees of max timing after 2500 RPMs... #66 - #68 jets in primaries side of carburetor... A mechanic who only works on stock engines will prolly have no clue how to tune this raucous engine... Do not expect or tune for a smooth idle at low RPMs... this thing is going to really lope at idle... that's normal... Make sure a backfire hasn't blown out the diaphragm in the Power Valve...

Last edited by BuzzLOL; 06-11-2016 at 12:44 PM.
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Old 06-11-2016, 12:47 PM
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Originally Posted by waynep712 View Post
go thru this gallery.. just to get you to know the inside of your holley circuits..

Holley R1850-3 - Album on Imgur

you could have issues where your engine needs much more fuel thru the idle circuit and idle transition circuit ..

open your secondary throttle blades more.. so the primary throttle will be able to close down so you are only pulling fuel from the round idle feed ports.. not the idle transition slots..

you may end up needing to take the carb off to reset the secondary idle screw.. its up from the bottom..

is that intake for spread bore carbs.. you may want to check around the outside edges for vacuum leaks.

are you running a PCV valve..

is your power brake booster leaking.. you can disconnect the power brake booster hose and plug it to see if you can then get it to idle.
Yea i have a pcv valve. I will have to check power valve also as mechanic said he used a 6.5 and from my research an engine with as big a cam as mine needs a lower numbered power valve or power valve to be blocked off. I will try to check for vacuum leaks see what i find.
Also do have a hei distrubutor with 65k coil with vacuum advanced. Dont know if this has any effect on my issue

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Old 06-11-2016, 12:53 PM
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sbc 355 big mutha thumpr cam no idle barely any vacuum new rebuild

Use carburetor cleaner to spray around the perimeter of the intake manifold and listen for the idle to go up. Also get a vacuum gauge and do some test on the engine. The instruction booklet will tell you how to do all the test. Weiand recommends the Fel Pro 1256 intake manifold gasket set. Also use Permatex 80697 Copper Spray-A-Gasket Hi-Temp Adhesive Sealant on the head surface and intake manifold. Use Permatex Ultra Black Maximum Oil Resistance RTV Silicone Gasket Maker in the four corners where the intake manifold and heads meet and a 1/4" bead along the china walls. Use a Fel Pro Carburetor Base Plate Gasket 1900. Make sure your idle transfer slot is adjusted correctly on the primary and secondary throttle blades. http://www.crankshaftcoalition.com/w..._transfer_slot Scroll to top for index.

Last edited by cdminter59; 06-11-2016 at 01:06 PM.
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Old 06-11-2016, 01:06 PM
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Quote:
Originally Posted by HiGhDRo View Post
Yea i have a pcv valve. I will have to check power valve also as mechanic said he used a 6.5 and from my research an engine with as big a cam as mine needs a lower numbered power valve or power valve to be blocked off. I will try to check for vacuum leaks see what i find.
Also do have a hei distrubutor with 65k coil with vacuum advanced. Dont know if this has any effect on my issue
Make sure PCV valve seals up good, isn't leaking air back through it... use Power Valve about 1/2 size of idle vacuum...idle vacuum prolly in 6" - 10" range somewhere's... distributor should be fine, but keep vacuum advance disconnected and hose from carb. plugged until done tuning...
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Old 06-11-2016, 01:18 PM
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Im pretty sure carb wasnt rebuilt correct cause before carb was rebuilt due to fuel leak motor sounded alot better and didnt give off smoke out the exaust.
Mechanic says cam just to big for engine but i know of 350's and 355's with the same cam. After all i spent on parts and labor wont be able to get a bigger carb for a bit.

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Old 06-11-2016, 02:08 PM
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Originally Posted by BuzzLOL View Post
Make sure PCV valve seals up good, isn't leaking air back through it... use Power Valve about 1/2 size of idle vacuum...idle vacuum prolly in 6" - 10" range somewhere's... distributor should be fine, but keep vacuum advance disconnected and hose from carb. plugged until done tuning...
Plugged up vacuum advance hose and still cant get it to idle or stay on. When mechanic tested vacuum trying to tune the car after he rebuilt the carb he says vacuum was below 5". So pissed off i spent 3k in parts and paid a little over 2k and car still not running.

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Old 06-11-2016, 03:02 PM
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Please get that guy to explain how a cam that is "too big" wont idle? You are looking for big power,obviously! How did you decide on a mini carb and dual plane? Beg/borrow/steal a Holley 300-25 intake and a proper set up Holley "HP" series or better in 850 cfm version.
Im hoping you have a stud girdle,,,That is a big cam,not a huge cam
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Old 06-11-2016, 03:10 PM
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If this is the intake you have 8004: Weiand Action Plus Aluminum Intake Manifold 8004 Chevy Chevrolet | eBay You need this under the Carb https://www.summitracing.com/parts/edl-2732
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Old 06-11-2016, 03:52 PM
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I have the 8120 .

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Old 06-11-2016, 04:01 PM
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Originally Posted by vinniekq2 View Post
Please get that guy to explain how a cam that is "too big" wont idle? You are looking for big power,obviously! How did you decide on a mini carb and dual plane? Beg/borrow/steal a Holley 300-25 intake and a proper set up Holley "HP" series or better in 850 cfm version.
Im hoping you have a stud girdle,,,That is a big cam,not a huge cam
The carb and intake where originally on the engine before the rebuild. Since they never gave me an issue i decided to keep em.
Yea i want power. Lol who doesnt.
Yea about the mech saying cam to big and wont idle i stayed lookin at him all weird. Said only way to get it to idle it change to smaller cam and change to stock rockers cause they are 1.6 ratio roller rockers. Valve springs have max lift of .600 and he said i have intake lift of .5344 and exaust lift of .5184 cause of rocker ratio and that im lookin at about 9 more hours of labor.

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Old 06-11-2016, 04:13 PM
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sbc 350 over .30
Do you mean over .030?
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Old 06-11-2016, 04:34 PM
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Do you mean over .030?
Yes over .030 lol

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