Idle and Perf Issues

[RMA]

Model: 7040251

1970 Cutlass SX 455 TH400.  Rear is probably a 2.56 or 2.78.  Complete stock rebuild last summer (200 miles) except 0.03 overbore and comp cam hyd. flat #42-114-3 (so called factory muscle).  LSA 113, Intake/exhaust dur. 308/308, Int/Exh Duration at .050”: 233/233, Intake/Exh. Lift: 0.296/0.296.

After rebuild Poor performance, idled in park ok, idle dropped in gear & continued to stall.

My carb guy purchased Cliff’s book and parts.

Changes made - Parts from Ruggles:

Idle Tube .039

Idle Down Channel drill .059

Upper and Lower Air Bleed .070

Idle Bypass Air drilled .110

Mixture screw through hole drilled .090

Main Air bleeds plug & drill .040

Primary jets .070

Primary rods .026, .048, .061 (was .026, .038, .062)

Secondary rods .045, .084, .132 (was .053, .102, .134)

Secondary air valve ¼ turn from zero

Power valve unstuck and new “red” spring

High flow needle & seat, new cam on secondary air valve

After above changes very slight improvement.  It will idle in gear if idle set to 850-900 rpm in park.  Initial timing = 12 BTDC (tried 8-14, 12 seems most stable).  Running rich, black soot in pipes, burns your eyes.  Manifold vacuum at idle erratic 10-14 inHg (advance plugged).  Checked for vacuum leaks multiple times w/ carb cleaner, none found.  A/F mixture screws very little to no effect.  Secondaries will not come on (due to lack of vacuum?).  Also as rich as I’m convinced it is, it still seems to gain a few rpm if I hold hand over the carb.  PS – it ran smooth as silk/great acceleration (broke tires loose 1-2 shift) prior to the engine rebuild/carb work!  Any advice is much appreciated.  Thank You.

 

[Cliff Ruggles]

What is the static compression ratio of the engine?

[RMA]

Engine builder estimate is 9 to 9.5:1, running 93 octane.   Thanks

[77cruiser]

Is the cam degreed?

[RMA]

Yes.

[77cruiser]

Just for yucks try about 20 initial.

[RMA]

I did try up to about 16 degrees, (eyeball it Pointer stops at 14).  Idle got worse.  I could try again with a dial back light.

[Cliff Ruggles]

Your main problem is the cam selection.  Not enough compression for that much duration even in the big 455 engine.

It's going to want a LOT of idle fuel, bypass air and initial timing to be happy. 

The exact same engine build at 10.5 to 1 compression would be making a LOT more vacuum and pretty happy with that cam, but even then it wouldn't be making a lot of vacuum at idle speed and still require some help from the carbs idle system to be ideal......

[RMA]

Thank you for the feedback.  Any suggestions  on initial timing?   Also, any ideas why the secondaries will not kick in?  I have full travel on the throttle linkage.  Thanks

[RMA]

Bumped the initial timing to 20 she seems happier.  Mixture screws out about 6 turns in park Idle is 750 and drive around 600 which is an improvement.  Check on vacuum tomorrow.  Hoping for better weather and take a drive.   I appreciate all the feedback!  Thank you.

[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......