Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Another Idle question
richie49:
I have a problem with the idle.I got parts for my carb from cliffs recommendations. Now I havent drove the car to see how the new jets and needles respond but (not the best cam for my needs ) the cam has a 108 lobe sep. and 296 duration so the idle is rough. The engine is a 1976 caddy 500 that has been rebuilt other wise stock .Iam trying to see how much I can do with this cam before I go thru the problems of replacing cam.The cam says 950 idle spec but when I get it to 950 in gear it is 1250 out of gear,any suggestions on what to look for that would make a 300 rpm change.Tryed manifold vacumn for advance and it didnt like it so went back to ported. ignitial timing is 19 degrees advanced.
Richard richie49
77cruiser:
How is the idle circuit set up? What didn't it like about manifold vac.? What size engine & comp. ratio?
Cliff Ruggles:
Take a look at the timing to make sure it is ROCK SOLID in and out of gear.
Look into the carb at idle speed and check for nozzle drip. If you have nozzle drip you will need more bypass air and/or more idle fuel to the mixture screws.
No big secret here that I am NOT fond of tight LSA camshafts with a lot of seat timing for street engines.
I get more complaints from folks who go that direction than any other. The Thumper and Thump your Mutha stuff tops the list, followed by the Comp XE cams. Hardly a day goes by someone doesn't call here bellyaching about rough idle, doesn't want to stay running when placed in gear, or the wife woln't ride in the car now that the new engine is in place because it stinks her hair up!
On a more serious note, I get involved in a LOT of big CID "stroker" engine builds, and FAST Class set-ups. Over the years the wider we've gone with LSA the quicker the cars are at the track and milder and easier to manage on the street.
I attached a dyno sheet below showing how this works. The first cam was purchased after the engine builder called Comp for a recommendation. The XR276HR is a very popular hydraulic roller cam from their catalog.
The ONLY change I recommended to our customer after everyone at the dyno shop right down to the janitor was blaming the Q-jet for the piss-poor power production of the fresh 455 engine build and some pinging when they tried to crank up the timing to make more power.
I told them to leave the carb alone and change the cam, and provided the specs. The new cam idled better, improved throttle response, broader power curve and made more power as well.......Cliff
richie49:
Thanks Cliff
I'll check for that and repost the results ,probably be another week before I can do that.
Richard richie49
richie49:
Cliff
I havnt forgot about the post but the car is killing me. I have been having starter problems. Cant tune anything till I get the starters behaving.I burned up 3 oem style and waiting on a gear reduction style .So I soon as I can get it to start and I'll reply with what I have to do with the tuneupand idle.
Thanks
Richard richie49
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