Very strange.....

[GTO Geoff]

I have been messing with TQs since the late 70s. I have struck a problem that I have never had before....with any carb.

Engine is a newly built 440 Mopar. 10.5:1 CR, Trick Flow 240 heads, sol roller cam 242/242 @ 050, 106 LSA. Headers. Mopar M1 single plane intake & 850 TQ, 6000 series production carb 4 speed. Elec fuel pump. Engine starts easily hot or cold, & runs at 170-180*. Since it is new, owner has not taken engine over 2500 rpm on short drives. Flawless performance, no flat spots.

Best idle quality/highest vacuum [ 13.5-14.0" ] was with 43-44* of timing at idle, provided by man connected vac adv.

Here is the problem: I cannot get the idle rpm lower than about 1100 rpm. That is with idle stop screw 1/2 turn from bottoming out the t/blades. I believe the engine would be able to idle slightly lower but cannot get it lower. It was actually worse, but this so far is as best as I can get it.

The IFRs were originally opened to 0.040". I have since closed those to about 0.035" by inserting thin wire in the IFR. Float level for these is normally 27-29/32" [ plastic floats ], but I have changed it to 1.0625", so the actual fuel level is lower. The PCV was gutted & is now a fixed orifice of 1/8", which is about the area of a stock PCV at idle. There were two bypass air holes in the base that are now blocked. Sec blades are fully closed. Only a smidgin of T slot is visible at idle. No air/vac leaks that can be seen or heard, yet cannot lower idle rpm. So where is the air coming from for the engine to idle on? Air bleeds? I do not want to remove the PCV function, & that small hole should not be the cause of this problem. Any ideas?

[rgalajda]

PCV 1/8" is .125" , I would think this is your idle air. Would it not be similar to having .125" idle bypass air. With 1 1/2 primaries they were 800-850 cfm depending on the source.

"Plastic floats" I assume these are Nitrophyl.

[GTO Geoff]

Rg.
Thanks for the reply.

Plastic = nitrophyl

1/8" PCV hole. Yes same as by pass air. PCV air gets delivered below the t/blades. So it is the only air that I am aware of that the carb is getting other than leakage air around the t/blades, t/shafts & idle air bleeds.

The 1/8" figure is the equivalent area of the pintle being closed at idle in a stock PCV. I was worried that low idle vac may causing PCV oscillation, so this is the fix, a fixed orifice. Have used it in many engines & have never had this problem of pri blades being almost closed but still getting high idle rpm with 440 cubes.

[tbarb]

If the carburetor has a hot air choke there is a calibrated vacuum leak in the choke housing.

I understand that you don't want to run without PCV but how does the engine idle (RPM) when it's plugged? (1/8" seems big, lots of air through there)

There is a procedure for testing the seal of the intake manifold that requires blocking all leaks in the crankcase and using a vacuum gauge to test if crankcase vacuum or pressure is present. I did this on my engine once and my memory tells me the crankcase held something like 1/2 to 1 1/2" vacuum at idle but this is from memory. Vacuum gauge on dipstick hole but I may need to refresh my memory on the test FWIW.

How does the engine idle with a ported vacuum advance set up, IMO, too much idle timing makes the engine very efficient at idle but does not allow for a good transfer slot exposure and powers off poorly. Maybe test some different combos (20* idle timing) and see how it idles.

[GTO Geoff]

If you read my entire post, you would see where I measured the area of of a stock PCV. It was 7/64", for this engine I increased it a tiny amount to 1/8".

I have done this MANY times on engines with big cams, never had a problem.

If the PCV is plugged, then the t/blades will need to be opened further at idle to supply the missing air....or....bypass air from somewhere else. Carbs that have a PCV provision are designed to work with the approx 3 cfm that the PCV supplies at idle.

GM used manifold vac adv up to about 1968, & then we got the useless ported VA because of emissions. My GTO left the room with 26* of idle timing: 6* init + 20* MVA added. Mild factory cam 198* @ 050. MVA gives better tip in response, smoother idle, better mileage & the engine runs cooler at idle.

Ported vac adv is useless, scroll down to post #6.

www.hotrodders.com/forum/vacuum-advance ... 47495.html

[tbarb]

I understand all the information related to ported vs. full manifold vacuum advance, thanks for the link to the post containing the article. I run full manifold vacuum advance on my chevy small blocks, the only difference between the two is idle timing after that they are both the same.

Could your engine have a small air leak at the intake manifold / head gasket area.

[GTO Geoff]

No air leaks. Combustible spray was sprayed around, engine idling, no change in engine note. Did find idle timing was erratic.
I have reduced the size of the IFR & re-built the carb. Intend on trying this weekend.
Thanks for your interest.