My cruise goes 3 or 4 mph below then 3 or 4 mph past my set cruise speed. Up and down then floors it back up past the set speed then back down. Occasionally it will hold the proper set speed. The vacuum line to the cruise is new. I have now completely replaced all components including the speedometer and speedometer cable to my cruise system. Has anyone got any ideas? At idle my truck runs a little rough. It is new GM complete engine and carburetor from Summit, with 286 miles on odometer now. It sounds like it is a vacuum leak to some of the mechanics that have listened to it for me. I replaced all rubber vacuum lines and still cant find a leak anywhere.Any suggestions will help me greatly. Thanks BigNick

Does your cruise control system have the vacuum diaphram or is is all electronic?

It has the vacuum type and its brand new. Looks like the one in your picture.

I have a skip in the engine and I think its vacuum related. I have stopped off all the vacuum ports and still the skip is there. Is it possible that may be the cruise problem or part of it. The engine is new. The mechanic may not have done a good job on the manifold and carb installation. I dont know how to check for a manifold vacuum leak. Do mechanic shops have a tool or machine to check for vacuum leaks that are not hose related?

You can use one of those little hand held propane torches to check for vacuum leaks.

DO NOT LIGHT THE PROPANE TORCH AFIRE

Just open the valve on the propane torch, allowing the gas to hiss out....move the tip of the torch around the manifold and vacuum hoses....If rpm'* change, you've found your leak.

A spark plug with a really tight gap could cause a random misfire associated with that particular plug.

Also, how are the brakes....could be a ruptured brake booster causing the vacuum leak. You could pull the vacuum hose off the brake booster and cover the hose with your hand and see if the miss goes away (obviously, have the engine running ).

My brakes have been working kinda strange. Like they needed bleeding. I bled them and they tightened up but seem spongy or softer than usual. I will give that idea a shot. I never thought about that vacuum supply. In my situation it comes off a separate port on back of carb.

Is your carb a Q-Jet.....that vacuum supply coming from the carb would be unmetered. Do you have an inline check valve near the booster hose?

My carb is a summit M2008, 600cfm. it came with the new engine. There is a check valve inline on the vacuum line. The pointed in is toward the vacuum source. Someone mentioned that there may be a leak in the brake stop line. I will check that when I test brake booster line.