MIDNIGHT BLUE

i have a 1970 454 LS6 in my vette that just doesn't seem right when it's idealing. it will run smooth then a stumble or bobble in the ideal. i read that it cuold be a vacuum leak some wear. i disconnected all the vacuum wipers and lights no change. i would like to do a leak down test, i'm told that will tell if i have a leak in the engine some were. the question is how do you do a leak down test and what is needed to do it, and could there be any thing else i should look for .the engine is stock with 2000 miles on and it is a 70 LS6.

jim

MikeC

A vacuum leak will usually raise idle speed, not cause a stumble.

I would look at distributor timing. I'd set it to 10-12 degrees with the vacuum advance plugged and see if that gets it running better.

The LS6 is a engine with a fairly ratty(pun intended) cam in it, its gonna have a bit of a lope at idle.

After you straighten your timing out, then start looking into the carb.... if you are not experienced in this, plan on a learning curve or get someone who knows their stuff to do it.

A leak down test will not help you with the problem you are having. A leakdown test will tell you the condtion of how "tight" the engine is on the inside(valve sealing to heads, piston ring sealing against cylinder wall.

PRNDL

How are you going about disconnecting the vacuum operated accessories? When I was trying to determine if I had a vacuum leak I first disconnected the hose on the vacuum port on the intake manifold behind the carb, and put a rubber cap on it. This effectively disconnected the entire wiper/ headlight vacuum canister system. Next you have to rule out any leaks around the carb. I think the generally recommended technique is to spray some carb cleaner around the base of the carb - if the idle speed changes then you have a vacuum leak.

Although I have never tried it, I also like the idea of blowing cigarette (or cigar) smoke into a vacuum hose to locate a leak in the system.... but I wonder if the smoke will just pour out of the top of the carb?

I have never done a leakdown test. Would it be easier to run a simple compression check first? (not sure) MJ


[Modified by MNJack, 3:47 PM 1/21/2003]

Jvette73

A major vacuum leak will cause the idle to increase. I more minor vacuum leak could cause the stumble you describe. If you suspect a vacuum leak you can check for it with an unlit propane torch. Turn the gas on and move it around the areas of suspect leaks. If you find one, the idle should increase as the propane is pulled in and combusted in the cylinders. Works great for narrowing down intake and carb base gasket leaks as well as split or cracked hoses. of coarse if yur too close to the top of the carb, it may pull in through the throttle bores and mislead you. If so, pull it away for a second or two and zero in on suspected area again. Ive used this technique before and it has worked well for me.