Idles great, dies when I drop into gear

[soultech]

Hey there. Just got my carb back from a rebuild here at CHP and mounted it. Looking for a little guidance to get it tuned now. Sorry for my ignorance, I've never tried tuning a carb before.

Model Q-Jet 71085213
86 Chevy K20 Silverado, 350 engine

Problem: The truck idles great, but when I drop it into gear the engine stumbles and frequently dies. I had a ton of other problems before the rebuild, but this one seems different.  If I goose the gas when the stumbling starts sometimes I can keep it running.  Other times it just dies. 

Also, sometimes when I leave it idling after a good 10 min warm up and short drive, it will idle for 5 or 10 mins (or just fine for some amount of time anyway) and then it starts slowing down and dies.

The only thing I've done since I mounted the carb is I had to lower the idle speed as it was really high after initial startup.  Other than that I've not tweaked any other screws or settings.

I did a quick google and came up with a couple of potential causes:

• Vacuum leak on one of the larger 3/8" hoses might be enough to kill the engine when I drop it into gear.
• Idle mixture screws might need adjustment

Examined all the vacuum tubes, they definitely not new but there's no glaringly cracked or warped hoses, they fit snugly on their fittings.

I traced the vacuum lines using the air cleaner routing diagram and it looks like everything's hooked back up correctly. Also I took pics before I removed the carb and labeled the hoses so I could know that I restored them to their proper locations when I remounted the carb.

Just wondered if there's any tips / guidance web links or faq's on how to tune the carb once I get it back from the rebuild and install it?

Thanks for any help.

Regards, Scott

[77cruiser]

Besides checking for vacuum leaks, make sure your timing is good. Most carb issues are timing related.

[soultech]

Could it be a blown bladder in the brake booster? Would that affect the vacuum dynamics and cause disruption to the tuning?

I noticed my brake light's stuck on now (the old truck's been sitting since late Feb) and I've lost some fluid in the master cyl, but the wheel cylinders are not leaking anywhere... thinking maybe the fluid's going into a broken bladder in the booster perhaps. Pedal's squishy too now.

[Cliff Ruggles]

Most likely some timing is dropping back when a load is placed on the engine.  Verify with a timing light that the timing is ROCK SOLID in and out of gear, I'll bet it's retarding a few degrees based on the symptoms provided.

Also set the idle mixture screws for best idle and the leanest settings, they may need to be backed out slightly to help things out when a load is placed on the engine......Cliff

[blazer74]

Too light springs or a broken spring in the distributer fly weights can cause what Cliff is talking about.

This would affect the mechanical (centrifugal) advance coming in too early with the higher idle iin park then Dropping Out when idle drops when  placed in drive.

[von]

It can also be caused by the particular vacuum advance can being used with full manifold vacuum. I had a '70 L78 Nova with TH400 that had this problem when the vac advance was connected directly to full manifold vacuum. To maintain correct idle speed in drive, it would idle way too high in park/neutral. Adjust idle speed down to correct in park/neutral and it would die when put in gear. Disconnect the vacuum advance or connect to port vacuum and the problem went away. The "activaton point" of the vac can was evidently where it would provide advance with the manifold vacuum the engine had when not under any load, but when put in gear it was enough of a load (with the L78 cam) to reduce manifold vacuum enough to take away enough advance to kill the engine.