Mileage Quest 2011!

[Montana71-F100]

I can't get it to idle less than about 800 rpm. I backed the idle screw all the way out and it won't go any lower. Any ideas?

I was having trouble getting mine to idle at a lower RPM. It needed more tension on the spring. On mine, there are two holes in the plate that holds the spring so I moved it to the one further from the carb and it works fine now.

[woods]

Tell you what else can happen. If the timing has been set with ported vacuum its whole life (also see low cylinder pressure at idle). When you advance the timing, the cylinder pressure comes up, has more downward force on the piston, thus idles higher. If it has spent it's life at the low idle, the carb can actually wear a small groove in the base plate where the throttle plates have been returning to for years. It sometimes makes a "sticky spot" in the throttle. You can test if this is indeed the case. Back the idle screw out so its not touching, then grab the throttle arm with your hand while it's running and give it a little twist closed. The extra force from your hand will usually push it past the little ridge if it's there.

[fomocoguy]

I know that I can twist it real hard and bring it down by hand, but it doesn't feel right. It's a brand new distributor with new vacuum lines so it should be ok in the leak dept. Once again it's raining here, so I'll have to mess with it some more in a few days when the weather clears. I'm so tired of working in the rain...

[woods]

I know that I can twist it real hard and bring it down by hand, but it doesn't feel right. It's a brand new distributor with new vacuum lines so it should be ok in the leak dept. Once again it's raining here, so I'll have to mess with it some more in a few days when the weather clears. I'm so tired of working in the rain...

If you twist it and it slows down, does it come back up when you let the pressure off of it? I think we are getting somewhere here.

[Ranchero50]

I'd check for binding on the choke side and unhook all the linkage and see how it idles. You may end up pulling the carb abd checking the throttle plate fit in the bores. Wouldn't be the first time they were locked down on a bind...

Jamie

[woods]

Yeah...if it comes right back up to 800 when he takes his hand off of it, I would say the fast idle screw is preventing it from closing any further. If it stays lower after he takes his hand off, it's probably the throttle bores.

[AzSaintsFan]

Thanks for the topic I learned a great bit. I had always set my timing to the factory spec. 69 f100 w/302 getting roughly 10 MPG I will definetly try playing with the timing to see what changes I get. I did change from point to eletronic and have a considerable more high end pickup. It use to be at about 80 MPH the pickup would deminish and I'd top out at about 90ish with the electronic ign I hit 90 and it is still pulling hard and gaining speed but 90 was plenty fast enough and I backed off to my norm speed of 60 so I know I have much more power. The true test will be next month when I get the horse feed and see what it does pulling 4000 lbs of pellets.

[woods]

Following up a little with the timing talk....

I feel that many folks underestimate the importance of proper timing in a vehicle. I mean, most know that it's important that it be set correctly for the engine to run, they set it at whatever the initial timing recommended is at idle and call it good. Well...it will run that way and will normally run good enough for you to think there is nothing wrong with it, but doing that can leave a lot of power and drivability on the table.

Total timing and at what rpm it comes in at are very important in making your engine run like it should. A lot of people are shocked at how well something runs and how much power it has when the distributor is set up properly. The easy thing is to take out the distributor and bring it to a guy like me with a machine to set up your advance curve...but, you won't really learn anything that way and you can do it yourself with a little trial and error.

Lets start with total timing. Forget about the vacuum advance talked about earlier, it has no bearing on total timing when your in the throttle hard. Here is the problem with a lot of distributors...when you set it to the total advance you want, many times the initial timing is too low. So it's a bit soggy down low, but screams to life once you built up some rpm. This is because the distributor has too much mechanical advance built into it. In example...lets say your distributor has a total mechanical advance of 36 degrees in it and you want to run 36 degrees total timing. Well, that means you will have to set your initial timing (at idle) at 0. This leaves a lot to be desired down low but runs great up high. You can put more initial timing in which will improve your bottom end power, but moves your total advance up way too high.

Well, there is an answer for this. Many of our distributors have a plate under the breaker plate. You may not have this, but many do.

dist15.jpg

You will notice in the picture that one side of this plate says 10 and the other says 15. This is what limits the amount of mechanical advance in your distributor. Since the distributor turns half the speed of the crankshaft, you double those numbers, so 10 will be 20 degrees of total advance and 15 will be 30. Most of the time, it's set on the higher one. By the way, there are a lot of different plates, some are 13 and 18 some are 12 and 20...you get the idea. If you turn it around so you're on the lower number, you can run more initial advance (for good power down low) and not go past the full amount of total timing you want. On this one take for instance, you would flip it over to the 10 side, this will give you 20 of degrees available in the distributor. Lets say you want to hit 36 total advance now. You would set your initial at 16, total would be 36. Good power down low and up high.

To flip this around, there is a small clip in the top (you will see it), you can get it off with needle nose or a small flat screwdriver. Now, since you turned it around, your rotor will be pointing 180 degrees off. The best way to deal with this is just to lift your distributor up a little, turn it back where it was and set it back in place. Or, you could be like a buddy of mine and just switch the plug wires around (because his distributor is stuck and he does not want to mess with it).

Anyway, doing this makes a huge difference in the way the car or truck will run and drive.

Now...they also make a spring kit so you can adjust what rpm your mechanical advance starts. I cannot give you an answer as to what rpm your truck should be at full advance, because it will be different from engine to engine, vehicle to vehicle. Have it come in as early as you can and not ping is the only suggestion I can give you. Up to you to find out where that is.

Once you have your distributor all tuned to your engine/truck and driving habits, you will be VERY glad you did. The difference is tremendous.

[390bump]

Just wanted to thank everyone on this thread! This has motivated me to start recording my gas milage and to start tweeking to see what i can improve (milage and performance).

[67 428 F250]

Hey Joe,
#1) If that is a four barrel carb, check to make sure the primary and secondaries are not opened a bit. If you close all the plates, the engine will stop, it doesn't mater where the timing is. It takes fuel and air to run.
#2) Put your hand over the top of the carb when it is running, and If it stops, then the plates are not closing all the way.
#3) What carb is it?
#4) How old is it?
Will

[woods]

He has a two barrel.
Many plates on carbs do not close completely, as in air tight enough enough to kill it with the idle screw backed all the way out. Mix this with the engine running a bit faster from advanced timing and you can get what he has. It's probably hitting on something, but I have seen this many times.

[cdeal28078]

I read over the last posts pretty quick so I probably missed it but when I hooked up to full manifold vacuum years ago I had trouble with good smooth idle and a high idle. I had an 1850 Holley 600 on there then and my trouble was the secondary stop screw adjustment. It was too far out causing the secondaries to be open just enough to raise the idle. Thing is they were set just like Holley recommends them. I had to back them off enough to close down the secondaries to get a half decent idle.
Reading this made me remember why I went back to ported vacuum.
clint

[fomocoguy]

I assumed with the choke all the way in that the fast idle wouldn't interfere, but I'll check it out tomorrow if I can. I'd check it now but mama's at work and the baby doesn't care for carburetors...

[woods]

Yeah, and having the ol'lady find you with the baby bungie corded to the inner fender and smelling gas fumes seems to create a slight rift.

[fomocoguy]

I did figure out now that she's old enough I can pull the truck in the back yard and she'll run around and play while I do basic work on it. That'll come in handy compared to last summer...