Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Idle and Perf Issues

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77cruiser:
Do you have a limiter so you don't have too much total?

RMA:
No.

Cliff Ruggles:
Third post I responded to on here this morning where folks have too much camshaft for the CID and compression ratio of the engine so it's liking, wanting and responding to a BUTTLOAD of timing at idle speed.

Running the initial timing clear off the scale plus a LOT of idle fuel and idle bypass air to make the engine happy tell you the cam is bleeding off too much cylinder pressure at idle and low RPM's. 

The fix is more timing and more idle fuel, although more of a "crutch" than a fix, that's what has to happen with these things.

Here is a basic "rule of thumb" to tell you if your cam selection is about right for the engine parameters.  Set the initial timing to 10-12 degrees. With the engine fully warmed up lower the idle speed to about 700-750 rpm's and pull a vacuum reading on it.  If it isn't makes at least 12-13" vacuum you need less overlap or more compression, or a little of both.

Many engine builds I end up custom tuning here can't even get down much below 900-1000rpm's with that test, so the work begins.  I end up going into both the carb and distributor to provide more initial timing, less total timing with a shorter mechanical advance curve, and have to modify the VA as well to get the total timing at light engine load where it needs to be.  The carb is going to require more idle fuel and idle bypass air as well. 

Although the end results will be 100,000 times better than when the vehicle showed up here it's still more of a "crutch" fix for a fundamental issue of having too much overlap and/or late intake closing for the static compression ratio of the engine. 

Not saying it in a bad way, that's just the laws of physics with these things and how we fix them.  Besides, who wants their new "high performance" engine build to idle like the 305 engine in a 1987 Chevy pick up truck?........FWIW.....

RMA:
Cliff, thank you for the advice.  Unfortunately my engine builder seems to lack your level of expertise at least with this engine.  My advice for anyone needing engine work, do your research.  Of 5 builders I met with this one seemed the most knowledgeable/experienced.  Thanks again.

Cliff Ruggles:
It's really not their fault.  I wasn't hitting "home-runs" with these engines right out of the gate. 

However it didn't take me too long to figure out that bigger camshafts REQUIRE more compression or you only degrade idle quality, hurt throttle response, kill off low end power and push power up higher in the RPM range. 

To offset the losses one needs to INCREASE the static compression ratio apprx one full point for every 10 degrees added duration.  This helps keep things happy at idle and greatly improves street manners, and much easier to tune without having to run the timing clear off the scale at idle speed and add a buttload of idle fuel to the mixture screws.

You also have to keep in mind that there is so much miss-information out there with these things one quickly gets confused trying to build a high performance street or street/strip engine. 

The biggest enemy of bigger cams in these engines is the idiot who came up with the 9.5 to 1 threshold for compression ratios for pump gas.  That's just STOOPID! 

Lowering compression then installing bigger camshafts is going backwards with these things and it NEVER works out well for the end user......FWIW......

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