Author Topic: Carb Adjust  (Read 2531 times)

Offline Johnbravado

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Carb Adjust
« on: August 24, 2017, 10:39:05 AM »
So i have done a full tuneup
Replaced spark plugs, cables, fuel filter, fuel pump, belts, vacuum hoses and adjusted timing.

I think i am failing to understand the carb adjust concept.

So i got 2 tools: INNOVA 3568 Digital Timing Light and OTC 5613 Vacuum/Pressure Gauge Kit

My carb is a Carter AFB 1707 which is i believe and equivalent to the edelbrock 1406 600-650cfm 4bbl carb. i bought it rebuilt from some old dude who worked on carbs for 40 years or so. He had like 10 he was selling

So i look online and basically i have 2 IMS screws. a right(driver side) and left(Passenger side). i turn the left, which i assume is for the rear WOT ports, clockwise or off complete. Then i hook up the pressure gauge to the manifold vacuum and adjust the right IMS until i get maximum vacuum while i am revving the engine a little, I take it to like 1500 rpm or whatever. i don't think it matter just so long as i am doing something.

I then do the same for the left IMS screw except now i take the engine to WOT and make the engine scream until i get max vacuum.

I dont think i do it right because i get backfire and bad power outputs. so i look online and find the Edlebrock carb manual and it has me adjust the carb using timing at idle, which i am trying t set idle at 650RPM as per spec for the crate engine. Same basic concept as vacuum except looking for max RPM and adjusting if idle RPM gets too high and then turning the screws back 20RPM for perfect lean mixture.

This too caused backfire and poor performance. I think it is down to the carb adjustment. Anyone have any suggestions on how to do this properly.

Also the crate engine i have is internally balanced so the distributor does not have the vacuum advance hooked up ever so i dont have to worry about making sure distributor is disconnected in order to properly adjust carb

Offline Stewart G Griffin

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Re: Carb Adjust
« Reply #1 on: September 04, 2017, 10:23:11 AM »
Engine being internal balanced or external balanced has nothing to do with distributor vacuum advance or any other type of distributor advance.

Offline Dr_Snooz

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Re: Carb Adjust
« Reply #2 on: September 04, 2017, 09:52:34 PM »
Where did you learn to tune carbs like this?
1989 Chevy Suburban V-2500, 5.7L, TH400

1990 Chevy C-3500 Ext. Cab, 7.4L, 3L80

2009 Chevy Silverado 1500 WT 4WD, 4.8L, 4L60

Offline Johnbravado

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Re: Carb Adjust
« Reply #3 on: September 14, 2017, 02:20:42 PM »
max vacuum from guy at work who builds race cars. the other adjustment from edlebrock manual

Offline Henry

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Re: Carb Adjust
« Reply #4 on: September 17, 2017, 11:53:41 AM »
Hi John:
Have you had any luck getting your Edelbrock carb adjusted? If not, why dont you compare your adjustment method with the standard Chevy method for the original Q-jet carb? Here is the method:
1. Get engine warm, make sure choke is fully open, air cleaner installed, air conditioning off, and put transmission in park or neutral. Make sure truck is on level surface and put on parking brake and block wheels for safety.
2. Set timing: pull vacuum hose from distributor and plug hose. If you have a fuel tank vapor cannister, disconnect the line from the tank. Then set timing to whatever it is for your  model/year truck. Example: 8 degrees BTC. When this is done, leave the vacuum hose to distributor plugged and vapor cannister fuel tank line disconnected.
3. With engine off, turn in idle mixture screws full in CW until they stop and then turn out by 2 turns each. These are the two screws on front of carb at bottom of carb base.
4. Idle speed:Turn engine on and set idle speed to the idle speed specified for your model/year truck. Example: 700 RPM. On Q-jets, this idle speed screw is on the right side of the carb when facing the engine.
5. Idle mixture: Re-set idle speed to the MAX idle speed specified for your truck for lean drop idle mixture. Example: 875/700 RPM, use 875. Turn each of the idle mixture screws 1/8 of a turn sequentially while you are doing this step. Turn out (CCW) both screws (one at a time equally) by 1/8 of a turn until you achieve max engine rpm possible (or max vacuum possible) with just the mixture screws. Once this is done, reset the idle back to the 875 rpm with the idle speed screw if necessary. Then, sequentially turn in (CW) each of the mixture screws by 1/8 of a turn until the lower specified idle speed is achieved (700 rpm). If you have modest roughness in the idle at this point, you can back the mixture screws out a little bit each and equally until the roughness stops, then reset idle to specified 700 rpm with the idle speed screw.
6. Adjustment is complete, reconnect distributor vac line and fuel tank vapor line to vapor cannister.

*This procedure assumes the carb does not have a idle stop solenoid on the idle speed screw. It is a little different if you have one but I am assuming you dont. I am also assuming you have set your cold start choke beforehand per your Edelbrock instructions.

Good luck,
Henry