Idle problem pontiac HO 7237262

[Kilraine hoGTO]

 400 ho engine.. .30 over 670 heads HO manifolds 21/2 " exhaust..xe 274 comp can 230/236 duration .. 481 lift at .5 ,roller rockers700 r4 3:90 rear.. Car performs well idles rough and loses idle (stalls). Petronix2 ignition , 14" vacuum idle 2200 stall torque converter ...all newwites , plugs(ac44) .timing 12 deg initial .. Total36 deg.. Opened up idle plugs on top of carb just a little , which slightly improved ... All other components stock ideas?

[omaha]

Probably need more fuel and idle air. Like maybe open up the idle tubes and  maybe add some bypass air.The cam is a bit "aggressive" especially in a 400. Think you may need to pull the idle tubes out and enlarge them maybe to .036. I think the idle speed will end up at around 900 (maybe 950). for the idle air I think I would start at two-1/8 inch holes and maybe go up from there if needed. I think the timing should be ok. If anything may need a couple more degrees on the initial but the total mechanical at 36 sounds right. The early casting is nice for a restored car but has its limitations. It can be made to work but I would put it on the shelf because of its originality and find a carb from the '70's to use on the street. those 1967 Q-jets are getting harder to find these days. Good luck!

[Ethan1]

400 ho engine.. .30 over 670 heads HO manifolds 21/2 " exhaust..xe 274 comp can 230/236 duration .. 481 lift at .5 ,roller rockers700 r4 3:90 rear.. Car performs well idles rough and loses idle (stalls). Petronix2 ignition , 14" vacuum idle 2200 stall torque converter ...all newwites , plugs(ac44) .timing 12 deg initial .. Total36 deg.. Opened up idle plugs on top of carb just a little , which slightly improved ... All other components stock ideas?

 Welcome to CHP!

[Kilraine hoGTO]

Thank you for reply .. I was told to open up tubes , but no one knew how much .. I dont understand what I should be doung to which part of card tol get 1/8 " .... You are correct in that I should look for a later model carb .. But car is otherwise numbers matching ... I should pull cam and switch to comp xe272. ..i had that cam in a ho 2+2 and it ran extreenly well as well as having good idle manners..

[omaha]

the 1/8 inch holes are drilled into the base plate. there is a place to drill them in the casting (usually) sometime they are drilled already. the idea is to let some air get passed the primary throttle blades that is not goin through the venturi. this fills the needs of the engine at idle without the need to open the throttle blades too much (causing nozzle drip.)  usually when a larger than stock cam is used the bypass air is needed. how big the bypass air is depended on the size of the cam or size of the engine or both.  If you  look at the bottom of the base plate you will see 2 grooves that extend from the edge of the throttle bore. at the end of the groove is where the holes need to be drilled. one groove is straight and one is dog-legged. these holes will connect with passages in the main body and will eventually connect with a passage in the top casting that lets filtered air in through holes/openings the air horn casting located near the choke. It acts as a metered vacuum leak though the air is filtered cause the openings are within the inside of the air cleaner.

[Ethan1]

 

[Ethan1]

 The photo I posted above, I believe this is what Omaha is referring to. The Idle Air Bypass. They are located just above the white circles. One is straight and the other looks like a dog-leg.

[Cliff Ruggles]

The 1967 Pontiac carburetors didn't have idle bypass air, and it will most likely need some for that cam in a 400 application.  It can be added to the baseplate, and main body/airhorn modded as needed, or you can try drilling two very small holes in the throttle plates directly in line with the mixture screw holes, about 1/4" back on the throttle plates.  This will help, but it is not as accurate as using idle bypass air (controlled vacuum leak).

The carb will also need larger idle tubes, and idle down channel restrictions opened up some for that application.  Do you have one of our books?....Cliff

[Kilraine hoGTO]

Throttle plates already have small holes in them , looks like factory ..., I dint want to "booger" up the casting , so if possible would like to send to you for mods ,, thank you all for great responses .. They all make sense...

[Cliff Ruggles]

Probably a good idea for that one.  The early carbs don't use idle bypass air, and that carb needs some for the XE274 cam in a 400 application.  Needs a LOT more idle fuel as well.

If it were mine I'd put a different cam in it, and it would NOT be a Comp XE grind......Cliff