Rough Idle

[Glyn_Turner]

I have an all new 383 chevy small block with a fairly mild roller cam, and an Edelbrock 600 cfm carb, which has only done 300 miles, but it has a much rougher idle than I would have expected. Once the engine is above idle it appears to be quite smooth. The problem is accentuated as the rubber engine mounts have little clearance between the two sides of the metal casing, so when idling the metal outers of the engine mounts knock against each other, which makes it sound worse.

I have checked the timing which is 16 deg BTC at idle with the vac advance disconnected. I have set the initial idle mixture screws at 2 turns out each, but turning them out any further has no real effect on the idle speed or smoothness.

Does anyone have any other ideas?

Best regards

Glyn

[wilf]

What idle speed are you expecting?

[Glyn_Turner]

Wilf

About 800 RPM, but even at 1000 rpm it is still rough and knocking and does not smooth out until about 1500 rpm.

Glyn

[craigh]

you can also experiment with the vacuum ports on the carb,my engine likes the full vacuum port and quite a bit of advance at idle to get a fairly smooth tickover ,around 20 degrees,some will advise not to use the full vacuum port but personally my engine prefers it

[mikey]

Give March 717 a mail. Sounds like it could be idle mixture at tickover. Does the exhaust smell sweet and kick you in the eyes and throat.

Unfortunately I only know how to set the Fuel mixture on the Holley type carb to bring it into the range of the mixture screws (ie re-sizing IFR's) not the Edelbrock. By the way is a 600cfm big enough for a stroker.

Other than that have you checked for a vacuum leak etc. Also what is your vacuum at tickover.

Additionaly try going up to 18 or 20 degrees of initial advance to see if it improves it. If it sorts it out you will need to restrict the overall advance to 34 or 36 degrees with bushings in the dizzy.

Let us know how you get on
Mike

[wilf]

Edlebrock carb book says to set the idle mixture for best vacuum. Start both idle mixture screws at 2 turns out, turn one screw in, if the vac rises, turn the other in and repeat. Adjust idle speed as you go.

Screw in = lean, screw out = rich.

[snakebite]

Found that a 650 Speed Demon borderline for my "383", went back to my 750 Street Avenger, much smoother right up through rev range. If you have a mild cam, bring your static timing back to 12 degrees and as Wilf states, tune carb with for best vacuum connected to the mainfold with the distributor vacuum disconnected.

[Glyn_Turner]

Thanks for the feedback, did some checking over the weekend, and improved slightly.

However I did notice that when I removed the pipe from the carb to the PCV valve, the tickover increased noticably and was smoother. Blocking the pipe from the carb had the same effect as putting it back on the PCV valve. Is this correct or is the valve not working correctly?

Best regards

Glyn

[wilf]

Glyn - that effect is to be expected - what you are doing is opening up another source of air - a little like cracking the throttle open a touch. The PC Valve controls this air bleed.

It might also point to your tickover mixture being rich - what does the exhaust smell like when it is ticking over? If you can easily smell petrol in the exhaust, it is too rich.

[callum]

If your cam has any valve overlap you may find the vacuum advance is aggravating the situation by giving you too much advance too soon and creating a stumble as the gas is just not flowing fast enough. I ended up blocking the vacuum lines and running a max of 12 degrees initial advance with all further advance and retard on the centrifugal bob weights. It transformed the engine. Vacuum cans are not so clever with hot cams...... You would not regret a trip to the rolling road at Thruxton - forget the Imprezas they love big V8s.