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#1 |
Registered User
Join Date: Nov 2012
Location: Mexico, NY
Posts: 559
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Re: Carburetor Question (To Jet or to Go Up In CFM)
So here's the thing... You'll never figure it out unless you troubleshoot. You always start with the easiest/most obvious things. Checking timing is a must IMO. Its also not the only thing to check but its the easiest so get that out of the way.
Your original questions were 1) do I need to rejet my Edelbrock? 2) should I move to a 650 CFM? 3) if theres anywhere else I should check for problems please let me know The answers were; 1) possibly need a rejet although Edelbrock does not recommend the 600 cfm carb for an engine with cam duration over 220 @ 0.050" lift and you are running a cam with 243* duration. That in itself could make it difficult to resolve your initial problem. It was also pointed out that with a 1200 rpm idle you are now on the power circuit of the carb and the idle screws are no longer at play. That alone means you cent even begin to dial that cab in properly without getting initial timing and idle speed set properly. 2) Edelbrock actually recommends their 800 cmf carb given the large cam duration you have. 3) As we all keep saying, proper timing is the first step. Get the initial timing at about 10-12 degrees at 650-750 rpm then adjust the idle mix screws. If your ignition timing is too low then it will have a hard time idling and you will have to compensate by turn your idle speed screw (not mix screws) in more which opens the main butterfly to get your rpm up. This combined with the retard timing will also make it want to keep firing when you shut it off. So, start with timing then move to the carb. Good luck.
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'84 K10, SM465, NP208, 3.42 Gears, 33x12.5-15 GM 4 bolt 880 roller block, 062 Vortec Heads Comp Cams XR258HR-10, MSD Street Fighter HEI Edelbrock: Performer Intake 2116, 1405 Carb Long tube headers, 2.5" duals to Flow Master Super 40's Vortec Engine Build |
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#2 |
Registered User
Join Date: Oct 2013
Location: Maine
Posts: 2,357
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Re: Carburetor Question (To Jet or to Go Up In CFM)
I think that cam is made to idle rough. I'd say it will be trial and error with the timing to get it to run as good as it ever will. I'm sure the low compression 1986 engine isn't helping. You may get it to sound bad ass though. Keep tinkering.
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76 Chevy K20 76 GMC K15 77 Chevy C10 77 Chevy K10 |
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#3 |
Registered Truck Offender
Join Date: May 2008
Location: hells training ground (aka Ariz)
Posts: 3,118
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Re: Carburetor Question (To Jet or to Go Up In CFM)
Agree. The "thumper' cam will never idle right
BUT, a 243°@ 0.050" is far from HUGE in a 454. The Edelbrock RPM small blockcam is only 7° less than that, intended for a 350" motor, and will idle at 750-800RPM all day long. Another 100 cubic inches would gobble up 15+° and never know its there. The fact that you have issues with TWO carbs,, you should listen to the fella's here and check elsewhere. Timing,, absolutely if you don't KNOW where the timing is you need to find out/ My first suspect in a backfireing, poorly running motor is the cam has lost an exhaust lobe or 3. Get the timing set properly and if you still have issues, that neither carb will cure,, time to cut a oil filter open to see how much iron debris is in it,, , and then pull a rocker cover and see how many lobes are wiped.
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Still playin with trucks, even at my age! When you're dead, it's only a problem for the people around you, because you don't know you're dead. .....It's kinda the same when your STUPID. I just did my taxes and reviewed my SS statement. Thanks to the current administration it looks like I will only have to work till noon on the day of my funeral. |
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#4 |
Senior Member
![]() Join Date: Mar 2014
Location: Toano, VA
Posts: 874
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Re: Carburetor Question (To Jet or to Go Up In CFM)
As I am repeatedly saying: I cannot get it to run long enough or at constant enough RPM to get a timing reading. I put a 5 psi fuel regulator on it and it fires up easier now. I'm going to try turning the distributor a little each time firing up until I find a spot that is okay.
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"Sometimes I do serve as a good bad example. Glad to be of service." My Build Thread (1967 C10, lowered, EFI, daily truck): http://67-72chevytrucks.com/vboard/s...d.php?t=830809 |
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#5 |
Senior Member
Join Date: Apr 2000
Location: Delta,Pa
Posts: 14,950
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Re: Carburetor Question (To Jet or to Go Up In CFM)
Put the engine at 3k rpm and see where the timing is. You will need an advance timing light but you need one anyway if you want to set the total timing properly.
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Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member. |
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#6 |
Senior Member
![]() Join Date: Mar 2014
Location: Toano, VA
Posts: 874
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Re: Carburetor Question (To Jet or to Go Up In CFM)
Put a new 750 double pumper, a 4776 not a 4779-9 750, but a regular street 750. Fired up. We're just adjusting it down now.
Posted via Mobile Device
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"Sometimes I do serve as a good bad example. Glad to be of service." My Build Thread (1967 C10, lowered, EFI, daily truck): http://67-72chevytrucks.com/vboard/s...d.php?t=830809 |
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#7 |
Senior Member
![]() Join Date: Mar 2014
Location: Toano, VA
Posts: 874
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Re: Carburetor Question (To Jet or to Go Up In CFM)
Tuned that 750 DP I purchased, drilled the butterflies and threw a tiny powervalve in it, corrected the MSD box wiring, put a new distributor and spark plugs in it and its driving around again. Another issue I had was a bad alternator. New one fixed it.
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"Sometimes I do serve as a good bad example. Glad to be of service." My Build Thread (1967 C10, lowered, EFI, daily truck): http://67-72chevytrucks.com/vboard/s...d.php?t=830809 |
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#8 | |
60-66 Nut
![]() ![]() Join Date: Mar 2007
Location: Coeur d'Alene, Idaho
Posts: 23,247
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Re: Carburetor Question (To Jet or to Go Up In CFM)
A 4776 is a 600 cfm double pumper, not a 750.
Quote:
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carburetor, edelbrock, holley, jets, tuning |
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