Best way to find carburetor vacuum leak

I was suspecting a vacuum leak based on simple troubleshooting charts, and based on the fact that the insulator did not seem to be making a good seal anymore.

Another thing is that choking the carb further using a piece of cardboard allowed the engine to run instead of die, pointing to an overly lean mixture. If its not a vacuum leak, what could it be?

There is no smoke out the tailpipe when I start it. The engine races to about 2500 rpm, runs for a couple seconds, and dies very quickly, and throttle does not help. It doesn't take that much cranking for it to catch, only a few seconds.

I installed a different carburetor because the car would have trouble running when cold and failed emissions (NOx was over the limit). These two symptoms, along with a diagnosis from a auto clinic, made me believe there was some vacuum leak happening. Once i installed this carb, the car displayed the symptoms I listed above.

Currently I put the original carburetor back in with a new insulator plate and the car runs but still isn't quite right. I had encountered a couple of broken hoses when i was removing the carb for the first time and was hoping that would account for the problems I was having. Unfortunately, now the car idles too high when hot (but at least it will idle) and adjusting the throttle does not help.

i've been work>

Sounds like a plugged exhaust system. It would give similar symptoms to a lean mix because the air cannot get through the engine. Have a helper hold a hand over the tailpipe when you fire it. 2500 revs should push a good stream out the pipe. Does it have a heat valve in the manifold? Ran a compression test?

Ok That does sound like a lean condition. I assume this has a choke of some sort (besides cardboard) It sounds like you have not been able to run it long enough for it to run without the choke so if that is inoperative it could be the cause. Also if that was a carb from a junkyard where it sat and the gas was allowed to evaporate in the carb it could be pretty gummed up inside which might be the cause of the lean condition.

Your original problem could hve been timing and/or bad EGR now it sounds like you have messed up vacuum hoses and possibly messed up carburetor and possibly fouled spark plugs to also contend with. A vacuum leak could have been the original problem, but its doubtful that was the problem with the replacement carb. At this point you need to get everything back to where it was. If the the emissions were good all except the NOX that was most likely a plugged EGR. The problem running when cold is likely a completely separate issue.

-jim

BINGO! You have an exhaust system clog. Disconnect the manifold from the tailpipe and try again. When you reconnect it again, use stainless bolts and nuts and lots of copper gunk so it will be easier next time.

--scott

The engine was rebuilt recently so I haven't checked compression, however it ran smooth on the highway when I had to initially drive it home. I'm not sure how much air should be coming out the tailpipe but its somewhat less than my 1985 Corolla.

Thanks for the help. I will test this out today.

-J