General Category > Quadrajet Carb Talk and Tips
Idle tuning
Cliff Ruggles:
You got poor advice from the experienced engine builder and from Comp Cams.
The intake closing point is a critical event in an engine, and advancing camshafts closes the intake valve much earlier so everything above the piston starts to get compressed sooner, plus you change scavenging by the exhaust moving things ahead some.
Been hearing all sorts of remarks over the years about "advancing the cam to make more low end power" and to account for timing chain stretch, among other things. Those comments are mostly regurgitated information from very poorly informed folks and from doing this for many years I know that they have very limited experience with actually moving camshafts around and looking at the results....FWIW.
The cam you used was already on a tight LSA with an advanced intake lobe position so moving it further ahead obviously was not a good move since you have high cranking pressure and the engine doesn't like much initial timing. It shows 292 degrees duration @ .006" tappet lift and 232 @ .050", so it's PLENTY of cam for what you are doing. It shouldn't be making a lot of cranking pressure or finicky to a lot of initial timing either being on a tight LSA and sporting a lot of seat timing.
Not a big secret here or anywhere else that I'm not a big fan of tight LSA camshafts in "street" engines. They are done that way for the "bling" factor more than anything else. Folks love to hear the miss-fire in the exhaust or "lope", but at the same time they narrow the power curve, pull power down in the rpm range, increase octane requirements, and put a lot of unburned fuel out the exhaust (stinky).
Cam position also effects idle quality to some degree, as well as efficiency, power output, and vehicle performance.
Difficult cranking when hot, "kicking back", and breaking starters with as little at 8-9 degrees initial timing is a HUGE indicator the camshaft is too far advanced, or your actual compression ratio is much higher than predicted, or a little of both.
From what I've seen tuning engine with similar characteristics, it will also be finicky with the timing curve, total timing, and how much timing you can add by the VA at cruise as well.
For idle tuning you simply have to get enough fuel to the mixture screws to provide full control from rich to lean, and enough bypass air to keep it on the idle system (no nozzle drip).
Many late model carburetors, especially truck units will also have a LOT of idle transfer slot in the baseplate. This can provide a good amount of fuel to the idle system even if the mixture screws are seated, so it can make idle tuning a little more challenging.......Cliff
55 Tony:
It runs pretty darn well, maybe someday I'll have an excuse to take it apart and back off the 2 degree advance and see if there is a noticeable difference. For now it's staying the way it is.
I would like to get a pair of "normal" idle mixture screws, the ones it has are rectangle shaped heads. Do you know what size they are and do you sell what I need? Also what version fix do you recommend for the leaky bowl? I just got a set of numbered drill bits today so I may work on the idle circuit soon.
Oh, and no I'm not one to choose a cam for how it sounds. But I will also admit I don't know a lot about choosing a cam. I saw they listed some good for "street machines" but this one said for "hot street machines". And as it is it's very streetable. No problems at all cruising through town in stop and go traffic or slow on back country roads. My torque converter is only about 500rpm higher stall than a stock factory one. No problem going from idle to WOT, well except for keeping rubber on the tires.
Cliff Ruggles:
Any seat of the pants assessments of engine power and vehicle performance are next to useless, especially with big block engines with decent compression.
A well thought out low compression 454 will pull the end out a ditch, and completely IMPOSSIBLE to hold it with any sort of street tire, even with a stock converter and stock gearing. The last 454 I helped a customer out with here cranked out just over 500hp/600tq with a smaller custom ground HR cam that you are using, less compression, factory "flat" intake, factory Q-jet, and original points distributor.
Matters not in the big scheme of things, what matters is that you don't knock the rod bearings out of it with too much timing. If it's finicky to initial timing and high cranking pressure, it will not do well with much total timing either, so be conservative in that area when tuning.
We sell EVERYTHING for the Quadrajet carburetor, and many of our parts are custom made for us and far superior to anything else out there. I push parts to keep the doors open, no big secret there, but even more important is to have folks get good results and dead solid reliable in long term service.
This is why our accl pump have custom wound springs on them, and a custom made seal that's at least 1000 times better than the "soft" blue seals showing up on accl pumps these days, and no one else out there has a lifetime Warranty on them......Cliff
55 Tony:
:-[ :-[
If it seems impossible maybe it is ... I found out my timing light is shot. My timing for this ordeal has been right around 17-18* not 7 or 8. Still got to dig into that book again, haven't yet found what I'm looking for.
Cliff Ruggles:
17-18 initial is too much for that engine combination, imho. It can and most probably will "buck" the starter on hot restarts especially when it's well heat soaked.
I don't like to see about 10-12 degrees initial timing, with 10-12 degrees in the mechanical curve (20-24 at the crank) and another 10-15 degrees from the vacuum unit.
As you improve fuel delivery from the carburetors idle system it will idle better with less initial timing........Cliff
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