Blew off a vacuum plug, what could cause that?

Recently took my carbs to church, and I'm in the process of tuning them up again.

When I got it back on the road after doing the bench sync, and float level adjustment just to get an idea of the plug color, I heard a POP and the engine started running roughly.

After an inspection, I noticed one of the vacuum caps on my #2 manifold had blown off. What would cause this? I had no clamp on it, but I feel like without a backfire, there shouldn't be anything but vacuum in this spot. The valves have been adjusted during this whole process, too.

What gives?

 

the vacuum pressure in the intake manifolds pulses. if the caps are not a tight fit, the one that popped off may have just worked it's way off the vacuum nipple. the "pop" you heard was probably the cap coming off the nipple. the resultant "rough running" was a caused by the new vacuum leak.

CN

 

Yeah sorry, I meant to imply that the pop was the plug coming off.

So it's "normal"...

I guess I'll be buying some clamps.

I'm also assuming that the mixture is a tad lean, anyweay. The plugs came out pretty light (ha, one was super white with all that extra air!), so maybe it'll happen less once I add a quarter turn out or so.

 

You need clamps, but it's probably out of sync'.

 

Never had clamps, never blew a cap off. Get some new caps

 

With clamps.

 

Okay, I found the likely culprit of my misfire.

A mixture screw O-ring was not properly seated, so it busted in two and wasn't letting the #3 cylinder perform correctly.

Has anyone come up with a trick to get the assembly seated correctly, or is it best to drop it, the washer and the spring in and hope they lay flat?

 

put some grease on the spring, then see if the o-ring holds the washer and itself on, the grease should hold the spring

 

I was going to say earlier that it could have been caused by a lean misfire. I would use silicone grease to hold the spring/washer/o-ring combo on the screw and insert them as an assembly.

CN

 

Okay, got everything seated correctly thanks to silicone grease, as suggested. I started with all of my screws 3 turns out, and have done a couple 4k rpm chops to get an idea of what's going on.

For the first round, the 1, 2, and 4 have been adjusted to 3.25, 3.5, and 3.75 turn out respectively. The #3 cylinder was a darkish brown color but not overly dark. Like a glazed donut. This chop was done doing 55-60 down the highway, and revved to 4K as I pulled into my driveway.

Second round was done more about town, 4K into my driveway, and the #3 plug is now dry sooty black.

Any suggestions? Either the first run only took it lean enough to turn it glazed donut and the town run took it lean enough to have the carbon start building up, or it has become rich again on that cylinder for some reason.

Anything helps...

 

recheck your float levels.

CN

 

As in, running VACUUM SYNC? The first step, before tweaking mixtures?

I'm seeing all sorts of discussion about mixture tweaking; did you do your RUNNING VACUUM SYNC FIRST?

Were the floats accurately wet-set to spec?

Re-check the floats; do your running vacuum sync with all screws set at 3 turns out (or 3.5 if you want) THEN do your mixture tuning. Sounds like you may have the cart before the horse.

 

Floats all set to spec, bench sync, running vacuum sync.

All done.

Adjusted mixture a bit, and re-synced.

Possible sticking needle? I paid attention to the hanging position of the floats and they where all equal, and all had very minimal play.

That being said...when checking one of my levels(one which later happened to be in spec), the bowl overflowed. A SLIGHT tap with the handle of a screwdriver, and it stopped.

I'm using metal float needles and gave them and the seats a once over with 1500 grit. Maybe it wasn't enough.

 

Or too much. 1500 isn't very fine; personally I don't use sandpaper in carbs.

You might go back and POLISH the parts you took the 1500-grit to. A sticking float needle is a very good possibility.

 

During the last phases of fine-tuning the Pilot Mixture Screws should be adjusted in degrees, ... NOT quarter and half-turns.

Visualize a Stop Watch.
A 1-minute sweep equals 60-seconds.
A quarter-turn would equal 15-seconds.
Fine-tuning is a matter of TWEAKING the Pilot Air Screw, ... just a few seconds.
Somewhere between 1 and 8 seconds after getting a good Idle and Sync is all that is needed.
In some cases you hardly move the Screw at all. A (my-nute) adjustment!

 

Gotcha, I think I'll just need to wait for a color tune. It was my understanding that the utra-fine threads could be turned in larger increments. The coarse-threaded makes more sense that they be turned only very slightly. Thanks for all the help, you all are legendary.

 

Okay, so I think I'm pretty close, but wanted to see what you all thought of the plug color.

#1


#2


#3


#4

 

Also, does anyone know what a good running XJ exhaust temp reading should be coming out of the head? I'm going to borrow an infrared thermometer from work and try to see how they look....

 

Still lean & a bit odd, you should be aiming for the brown paper bag look, on the porcelain as well.