6 Cyl vacuum/carb issues

The 6 Cyl. motor in my 74 runs, but is not right at all. It has a new tune up and I rebuilt the carb. The vacuum lines, which there are many, are all rusted out, so I removed (and plugged) them. I am sure I don't have vacuum leaks. When I bought it, it ran, but was very rich. The previous owner had put a bolt in the PCV line. When I uncork that, it runs better, but I cannot get this thing to run normal. Does anyone have a procedure that might help me narrow down the main source of problem here? I have a new PCV valve for it and with all the other vacuum stuff blocked off, it still won't run right. The EGR vaccum diaphragm is rusted out, but the vacuum side of that is blocked as well. Is there likely to be a problem on the inside of the manifold with that?

 

The first thing I would do is a compression check. This will verify the condition of the engine internally. As long as all the cylinder are within 5-10% of each other, then I would start looking at other things. I would definitely hook up the PCV valve. You can kind of look at it like a controlled vacuum leak. Also it is possible that the EGR valve is allowing exhaust gases to leak by at idle, which will definitely cause a rough running condition. I'm sure others will chime in with other ideas. Good luck.

 

If it were mine, I'd first try to put everything back that had been removed. Some of the vacuum lines are needed for normal running, sp. the distributor vacuum advance. John's right about the EGR - if the valve is dirty, it will stick open and put exhaust gas into the manifold constantly. This will give you a rough idle. The EGR adds exhaust gas to the above-idle fuel mixture, diluting it and lowering combustion temperatures. If the EGR is inoperative, the mixture will be too lean off idle. Ideally you should make the EGR work properly or rejet the carburetor for a slightly richer mixture.

With the vacuum operated devices, disconnecting them leads to side effects that can be harder to understand than the stock systems. Typically you will have 2 CTOs (coolant temperature overide switches, often called ported vacuum switches). The first is for distributor vacuum advance (3 nipples), switching from manifold vacuum to ported carburetor vacuum as the engine heats up. The second is for the EGR (two nipples), disabling EGR until the engine is warm. I think these are both in the side of the engine block on the pre-80 258. You could also have a thermostatic air cleaner (TAC), and you should have a fuel vapor recovery system (charcoal canister). DOn't remove the fuel vapor recovery - your fuel tank won't vent properly without it!

I'd suggest you purchase the factory service manual and put everything back to stock condition. I know that the reprint manuals seem expensive, but they contain the best and most concentrated form of info for your vehicle. There's a '72 FSM online at c104.net. It's a slow link, though you can get just the chapters you need. The '72 FSM will be similar to a '74.

hth

 

BTW you don't need the steel lines - rubber works fine. Wears out faster though.