Rough idle, low vacuum

[Cruiser804]

Previously I had an issue with truck hesitating on take off. At that point I had a manifold vacuum reading of 17 ish. I replaced all the vacuum line and that seemed to resolve the issue.

Its a 3/79 2F with cali smog that appears to be complete.

So I drove it for a few days and it ran fine. Now, though, it idles rough and only has a vacuum reading of 5. I assume this is my issue but I don't know where to start. I took the reading at the manifold were it connects to the carb via 12 rubber hose (near the top of the bowl). When I pulled the hose the idle kick up but still ran rough.

Fuel in the bowl looks good. I can see fuel running through the filter. I have pulled one plug wire at a time and it doesn't seem to have an effect. Will run fine under load or without. Pulling the choke out improves and increases idle as I think it should.

Hot or cold engine does not make a difference. Also, sprayed carb cleaner around the carb and manifold did not detect and vacuum leaks or at least no increase in idle.

Whats my next move? Thanks

 

[saryon7]

Not an expert by any means, but I might try a few different things. Since no one spark plug seems to be adding to the problem when disabled, it might be something that effects all of the cylinders. I would check the rotor and cap. Look for any arcing inside the rotor. The other system that is in common with all the cylinders is the fuel system. You might try misting carb cleaner over the top of the carb with the air cleaner off. If it smoothes out for a second or two, it is probably a fuel problem. There is an idle port on the carb too that meters the fuel at idle, it might be clogged or in need of an adjustment via the adjustment screw. Again, I am not an expert, but hope it helps.

 

[saryon7]

I was just thinking that the low and steady vacuum leak could be an intake manifold gasket or carb mounting flange gasket too. Both of these symptoms will disapear at higher rpm because the relative amount of vacuum is far less. Also, retarded ignition timing can cause low steady vacuum readings.

 

[Cruiser804]

I was just thinking that the low and steady vacuum leak could be an intake manifold gasket or carb mounting flange gasket too. Both of these symptoms will disapear at higher rpm because the relative amount of vacuum is far less. Also, retarded ignition timing can cause low steady vacuum readings.

Would spraying carb cleaneraround the base of the carb increase the idle if there was a leak?

I pretty sure the intake is not leaking but I suppose it could be.

I know I have an exhaust leak at the manifold and exhaust pipe (not heat riser).

 

[65swb45]

How does the exhaust smell? Rich or lean? How do the plugs look? Smell?

Do you see gas running out of the emulsifier [nozzle] in the carb at idle?

These would be good things to know.

Best

Mark A.

 

[Cruiser804]

The exhaust smells a little rich. Not too bad. But worse than it did. I only pulled two plug, no fuel smell and didn't look too bad.. I will pull a couple more and take a couple pictures.

I assume you mean in the nozzle in the bowl. I will look post the findings.

 

[Cruiser804]

Ok, here a couple pictures of the plugs. Also, looking down the carb while it was idling I did not seen any fuel coming out of the jet(s). When I give it gas I do see it come out of the accelerator pump and then at higher RPM out of the jet.

So while I had the air cleaner housing off I decided I spray some carb cleaner in there. Emptied an entire can into the carb and now it appears to idle fine. I still don't see any gas coming into the carb while idling but I assume it as to be.

 

[65swb45]

So what is your vacuum now?

 

[Cruiser804]

Of course I didn't check. I will do that tonight.

 

[Cruiser804]

So what is your vacuum now?

At high idle with choke 1/4 out 19psi, at low idle with no choke 18psi. Both are solid.

 

[KB45]

Are you measuring vacuum in psi? Usually it is measured in inches mercury IIRC, if your running 19in/hg then your doing just fine for vacuum.

 

[Cruiser804]

Are you measuring vacuum in psi? Usually it is measured in inches mercury IIRC, if your running 19in/hg then your doing just fine for vacuum.

Hell I don't know. I have a pretty Actron vacuum gauge, reads vaccum and fuel pressure. You are proably right about the units.

Truck wasn't driven very much for a few years. When i got in back on the road in the spring it was basically out of gas. I filled the tank, changed the fuel filter added fresh gas and clearner. I bet I had some gumming or crude post filter that found its way to the carb. Gas looks clean no discoloration or sediment. I will proably change the filter again soon.

Thanks!

 

[65swb45]

At high idle with choke 1/4 out 19psi, at low idle with no choke 18psi. Both are solid.

Those are good numbers. So what symptom are left?

 

[Cruiser804]

Those are good numbers. So what symptom are left?

We're good! Thanks for you help. Back to running fine.