inline 250 Vacuum/Emissions System

[bob77]

Ok. At idle, my truck seems to run just a slight bit rough. It's a 30 year old engine, it happens. But, I am barely getting 11 mpg out of that little 250 and I don't get it. I'm looking for a place to start. I have traced all vacuum lines and checked for leaks. So far, I don't see any issues. Am I correct in my understanding that I really only need minimal vacuum lines in order to run? I have made sure that all vacuum lines are as originally run. I NEVER need to pass any emissions where I live, so could someone please let me know what my essentials would be. I am assuming that I could disconnect the lines for the EGR valve and such. Would operating on a minimal Vacuum line system possibly make a significant difference? I really believe that I should be getting at least 15 - 16 mpg out of the truck.
Also, my muffler has blown out. There is a hole next to the outlet pipe that is bigger than the pipe. As soon as I get the time and money to replace it, obviously I am going to. Could this change in back pressure cause any kind of significant fuel mileage issues? I believe that part of the EGR system operates somewhat with backpressure (I think). I am going to have the cylinders pressure tested, but I don't really believe I have an issue there. I changed out the 4.10 rear end to a 3.07 and I thought that would make a significant difference.... I was wrong. I changed out the plugs, wires, cap, and rotor yesterday. None of the plugs were wet or oil covered, but they did have quite a bit of black (carbon) buildup on them.

If anyone has any ideas, let me know. The fella I bought the truck from said he rebuilt the carb, but i had to make a few adjustments to the idle and choke. So, I'm sure that the problem could be in the carb itself. I think I am getting some unburnt fuel coming out of the muffler ( the giant hole in the muffler), but I am hoping to try some other ideas before rebuilding the Monojet. I have never rebuilt a carb before and I am a little hesitant.

Thanks all

[VileZambonie]

Check for vacuum leaks at the intake manifold to cylinder head.

[bob77]

It has the integrated intake/head.

[Lt.Del]

I wouldn't start removing vacuum lines just because it will still run without them.  Just check for leaks---idle your engine then take some WD40 and start spraying around all gaskets and vacuum lines.  If there is a leak, the rpms will increase instantly.

You can do diagnostics on your engine like compression tests that will help you get a start of where to look.  A hole in the muffler won't do anything to your mpg if is after your O2 sensor, if you have one. (being 30 yrs old, i don't think you have an O2 sensor).

Driving habbits will decreae your mpg drastically.  Keep your right foot light--no rabbit starts, etc..

Make sure your parking break isn't too tight
Adjust air/fuel mixture correctly on carb
after about 5 minutes of running, your carb butterfly should be completely open


 

[bob77]

I do seem to be getting a fuel smell from the exhaust. I thought it was the vent line on the fuel tank filler ( because it was leaking a bit until I replaced it yesterday), but I am starting to think maybe the truck is just running too rich. I can't seem to find any vacuum leak issues ( sprayed all around with carb cleaner), so I think I will try to adjust the mixture.

[bob77]

Ok. So I turned the Idle mixture screw all the way in, then turned it out about 1 and a half turns. (It had been out about 4-6 turns. Then, I started the truck and turned the idle speed screw in until the truck would hold and acceptable idle.
With the idle mixture screw out so far and the idle speed screw out so far as well, I am assuming that the idle mixture was turned up way too high in order to compensate for the idle speed screw being set too far out to hold an idle. This, (I believe) would have been causing the carburetor to dump much more fuel into the system just to make the truck run. I could be completely wrong on this, as I am still trying to learn a little about carburetors. It makes sense to me, but if I am terribly mistaken, please let me know. Obviously the proof will be at the gas pump, but I am just curiuos as to whether or not I have the principle correct.
Thanks again for everyones help. It is greatly appreciated.

[VileZambonie]

Have you checked your ignition timing and what happens if you hold your hand cupped over the carb?

[bob77]

Holding my hand over the carb doesn't seem to do much. It decreases the engine idle just slightly. I doublechecked the timing and it is properly set.

[bob77]

Ok. Bit of an update. I guess I wasn't getting a ver good seal just putting my hand over the carb. I held a wrag tight down over it and it stalled. A fella where I work tells me that he can tune a chevy 6 by ear... Ok, so I was curious. He fiddled with the distributor for a few second and did find a bit of a sweet spot and the turned the idle mix screw quite a ways out. It does sound a bit better, but I still seem to smell gas when it's running. He tells me that the Idle mixture screw is 3 to 4 full turns out now. That seems wrong to me. It was quite a ways out before and it still seemed to be sucking gas. I suppose I will drive it for a few days and see what happens. If it is no better, I am stumped. I guess I either rebuild or look for a new carb.

One little note. On the top/front left side of the carb there is a Vacuum connection ( I think it's a vacuum connection). It's quite a bit bigger then the other connections and it doesn't seem to appear on my vacuum connection schematic. It comes out of the top, drivers side of the carb and sort of points toward the drivers side front marker light. I'd say it's maybe 3/8 to 7/16 diameter. Right now I just have it plugged. Any ideas?

[bob77]

If this picture came through, could someone tell me exactly where the Red Vacuum connection in the upper left of the carburetor is supposed to go. I can't find that fitting on my vacuum schematic. Currently, that big fitting is just capped

[Captkaos]

Have you looked at this:
http://econtent.autozone.com:24991/znetrgs/repair_guide_content/en_us/images//0900c152/80/07/a3/41//large/0900c1528007a341.gif
http://econtent.autozone.com:24991/znetrgs/repair_guide_content/en_us/images//0900c152/80/07/a3/4f//large/0900c1528007a34f.gif

That red hose isn't used on the Chevy for 1977.

[bob77]

Well, I am going to replace the fuel pump as I am told it could be leaking a bit back into the oil giving me that gas smell around the engine that I can't seem to locate. I suppose the more research that I do I am learning that 10-11 mpg out of a 250 isn't that far off from what some other folks are getting. I am going to relpace that fuel pump, check the compression, and probably try and rebuild the carb. If the compression checks out ok and the fuel pump and carb rebuild don't help, I will explore other possibilities. Is it true that if I put a 2-barrel on there instead of the monojet and stayed out of the throttle, that I would see better mpg?
Would anyone recommend a particular 2-barrel carb that would bolt right up to my 250, integrated intake/head without too much trouble?

[ccz145a]

Well, I am going to replace the fuel pump as I am told it could be leaking a bit back into the oil giving me that gas smell

Only if your oil smells like gas.

[bob77]

Yeah. I'm not smelling gas at the dipstick. Getting a good deal on a fuel pump and a carb rebuild kit from a neighbor who works at an anuto parts store, so I am going to replace the fuel pump anyway.
I think I am going to tackle the carb rebuild tonight or tomorrow. I've never done one before, but as I understand, the monojets are pretty easy. We'll see...

[Captkaos]

BTW, if you want to tune the carb yourself, get a vacuum gauge hook it to the carb and adjust the mixture for the highest vacuum reading.