Need help with engine running rough

[vtwinsport]

I would do as Donut suggested, plug off the line to the vaccum booster for brakes and see what happens. If no change, then just keep going on all your vaccum lines until you see improvement. If theres none. That would narrow it to the intake or carb.
Not sure about a bolt in for that year but edelbrock seems easiest. they bolted in on mine- 76. Qjets fit on edelbrocks with a simple base adapter that edelbrock sells

[chicklin]

Did a little more troubleshooting. Replaced the EGR and I think that helped a little. I was able to dial back the idle mixture to 3 turns out and bump the idle screw up a little. However, I still have a vacuum leak at the intake that needs fixed.

The other problem I'm having is (apparently) a miss at high speeds. I thought it was a clogged exhaust, but getting all that fixed didn't seem to help. I've got a "spitting" noise in the exhaust that sounds like a miss, but I can't hear it in the engine (although, I can't diagnose it at idle until I get the vacuum leak fixed).

Checked compression, all cylinders around 140psi. Slapped the timing light on all wires while running and they are all firing. All the plugs look good and are the same color. Already done new plugs, wires, cap, rotor, coil, fuel pump, carb, EGR, set timing. Checked the heat riser for free movement.

So, a couple questions. Can the vacuum leak be bad enough to cause a miss at higher RPMs as well as at idle? A vacuum leak miss usually goes away upon acceleration.

What else could cause a miss at higher RPMs or what else could cause that spitting noise in the exhaust (kind of an intermittent "pooof" noise)? The truck still feels a little low on power. Replacing the cat and muffler seemed to help a little, but it's still sluggish. The motor sounds and feels smooth on acceleration, though. Weird.

Ooops, two other things. I've got some valve clatter or lifter noise in the passenger side valve cover at idle. It diminishes on acceleration and disappears completely when it's good and hot. Not sure if it's related.

Also, I noticed the water temp goes up to about 215 when it first warms up and then, about 10-15 seconds later it settles back down to 195. I guess it's not a big problem, it just seems odd. I'm thinking the thermostat is just opening late, but maybe the truck is heating up faster than it can open and it recovers eventually. Any chance this could be related to the vacuum leak and running lean?

Sorry, probably chasing ghosts here, but this is driving me nuts. Guess I need to get the intake sealed up first.

[donut]

I believe my old Edelbrock performer was a bolt in replacement for the intake. It's spreadbore, so no adapter for the Q-jet. http://www.summitracing.com/parts/EDL-2101/

Your temps could be related to running lean, though if your running a 195* thermostat, it looks about right.

Miss- IMO, bad ground, lean,? could be more, but you need to find/fix the leak first.

Exhaust pop, could be above or possibly small exhaust leak.

Valve clatter, If the valves are out they "may" cause problems.

IMO, if you have to pull the intake to fix the vacuum leak, check the valves then.

Disclaimer: I am not a mechanic, but chasing down similar problems on mine gives me a little insight. (or enough knowledge to be dangerous. lol)

[chicklin]

I think I've pretty much got this figured out. Turns out the choke was not opening fully b/c it was adjusted incorrectly. I've seen it posted two different ways on this same site: adjust so it snaps completely closed when dead cold and adjust so it's 1/8" open when it's dead cold. Apparently, the latter is correct. This also fixed the "clogged exhaust" symptoms I was having. It wasn't a problem of air getting out, it was a problem of air getting in.

I've still got a tiny little miss in the exhaust at idle, but the "spitting" noise on acceleration is gone and performance is much better. I still need to go back and re-check the timing, idle and carb mixture now that everything seems to be set up correctly. Also, I think I probably need to pull and clean the plugs since I've been running it with poor combustion for a week or two.

I guess I will probably never get it perfect, given that it is carbureted and I know nothing of the internals and history of the motor, but I'm kind of anal about these things, so I want to get it tuned as well as possible.

[donut]

Great! Glad it was something simple (relatively).
I didn't think of choke. Maybe it's being in New England and using my truck to plow the driveway, that I NEED the choke and usually rule that out first thing.

[chicklin]

Originally Posted by donut

Great! Glad it was something simple (relatively).
I didn't think of choke. Maybe it's being in New England and using my truck to plow the driveway, that I NEED the choke and usually rule that out first thing.

Me too. The choke was kind of the last straw and masked some of the other problems, but the list was something like:

- Needed full tuneup (plugs, wires, cap, rotor, coil, PCV, etc.)
- Choke on old carb was fried (just replaced the whole carb)
- Major vacuum leak in the intake manifold
- Small vacuum leak in the EGR
- Catalytic converter was deteriorating inside
- Timing way off
- Idle mix and throttle need reset after fixing vacuum leaks
- Choke set incorrectly

On the plus side, everything has now been replaced or repaired so I should hopefully have a reliable motor for some time.