Rough Idle

Rough Idle
One plug will not fire at 900 RPM

If you're sure it's not a vacuum leak it's probably trash plugging up 1) the idle jet or jet carrier holes, 2) idle air jet (brass bushing with a hole in top of the carb body), or 3) an idle jet passage.

Also check the sparkplug wire and connector for that cylinder, sparkplug (they can short), and the distributor cap electrode.

Also, make sure that cylinder's idle mixture screw taper or seat isn't messed up.

Johnny,

George is correct.

You have to isolate certain items so you know what is functioning and what isn't.

Replace the plug that isn't firing properly and then test the engine with the new plug. If it still doesn't fire, replace the spark plug lead with another plug wire (just swap the plug lead). Start the engine and without getting shocked, pull the lead from the distributor
going to the problem cylinder/plug. You will have spark jumping from the cap to the lead. If you have spark, it probably isn't the plug or plug lead. If the problem still is present, persue to the Carburator.

A quick way to check for a vaccume leak is to spray carb cleaner on the intake/head flange and carb/intake flange. If the engine evens out, you have isolated the vaccume leak.

Good Luck,

Jack Blake

Jack, your vacuum leak test will work for most leaks but it won't work for one where the leak is across ports (gasket bad in the middle between ports.

You can make a simple tool that bolts to one carb flange bolt-pair at a time and has a compressed air hose fitting in the center. Remove carb, install the tool onto a port runner (use the carb base gasket), put a cylinder at TDC on the compression stroke, connect a pressure line and hold about 15 or 20 psi. With a piece of rubber tubing listen around the outer base of the manifold for a hiss, then down the other port runner, then down the oil filler tube (intake valve leaking?).

Then switch to the next port, put that cylinder at TDC on compression, etc.

Miss found. It was the gasket between ports.
You can

Thank you! Johnny for having the problem. Thank you! George for solving the problem. Saved me untold frustration. Mine has K-duds and started spitting and sputting and belching fire out the tail pipe (back firing). WOT Ok. Miss is in right bank,pulled to carb and saw no cause of the problem. Pulled the intake and WALLA a hole in the gasket right there between the ports. Off to the parts house I go.
Thanks again guys

AHHHH, two [sick] birds one stone, who said forums were a waste?

Glad you both found your problem. Since the firing order is such that each side fires two cylinders consecutively (4 then 3; 2 then 1) and the leading cylinder (4 or 2) intake valve closes after bottom dead center (ABDC) while the following cylinder (3 or 1) is just opening before top dead center (BTDC) a gasket leak between the dual-ports can kind of mess things up. Not to mention the fact that a cylinder on the intake cycle is trying to draw through both carb throats (although somewhat weakly through one, but enough to screw things up).3:25:51 PM)
(Message Edited 6/26/2003 5:15:35 PM)