One Cyl. Running Lean?

[Rickrey1]

I have a 74 Alfa Spider with an Weber conversion. I installed an exhaust header last week. When I started and warmed up the engine to check for leaks, I noticed (after 10-15 minutes of run time) that the number 4 exhaust tube was cherry red--the others were not. The next day I checked the back carb filter (clean) and removed and blew out the main jet assembly and idle jet for cyl. 4. I drove the car and checked the header tube temps with a temperture gun. After normal driving the #4 tube is about 50 dgrees hotter. If the engine is held at 2000-3000 rpm (car stationary) the #4 tube is cherry red and about 100 degrees hotter than the other tubes. I looked at the stock cast iron manifiold taken off the car --#4 shows moderate white flakes --the other cyls. are black. Spark plugs---#4 is white on one side, but not showing what you would expect with a really lean condition. There is no real "ping" or detonation on acceleration. The engine idles smoothly at 800 rpm and seems to otherwise run fine. WHAT'S THE DEAL ?? Any suggestions? Thanks

[slyalfa]

my guess is that you have a break bootser leak. and the booster only feeds into #4.

I had this happen once and it was cracked where the tube goes into the booster. the plastic 90°

in my case the vac would suck it closed but when I was driving the vibe would split it and I would run lean.
I also notice that my breaks boost was somtime not as good but did not put 2+2 together at the time.

[Rickrey1]

Thanks Slyalfa, I will check the brake booster, hose and valve. By the way, is it possible to replace the plastic 90 degree fitting. I have a brake booster on a parts car, but the plastic piece is broken (clumsy engine pull). Thanks again, Rick

[slyalfa]

I got one at the local parts place. and it worked fine, but looks a bit different. and has no logo on it.
but it just pushes into rubber groment on the booster

[Andrew]

The check valve at the manifold can be failing too; they can come apart at the crimp joint and leak. Spray some starter fluid at the joins to see if you get an rpm rise; if so you have a leak.
Andrew