So I'm trying to get my truck to run a bit better, it has quite a bit of hesitation off idle since I installed headers. I spent a few hours messing around today and here is what I found:

Timing with vacuum disconnected from dist:
14 BTDC @625RPM 5" vacuum
25 BTDC @2100 17"vacuum
38 BTDC overall @3200 RPM 18"vacuum

Timing with vacuum connected to dist:
18 BTDC @625
28 BTDC @2100
42 BTDC overall @3300

It seemed to idle better the higher I went (27 initial!?), what do you guys think of this dist. curve??

Also how should my carb be tuned for 3500 ft above sea level.
It is a 600 CFM Holley with vac secondaries
I know basically nothing about carbs

My motor is a 390, 9.5:1 compression, stock 4v intake, headers, dual exhaust with cross pipe
Ignition is stock coil, stock dist with pertronix, 8mm accell wires, autolite plugs

Thanks for the help!

 

You set the timing with the vacuum line to the distributor disconnected and plugged. A good place to set it is at 10 degrees BTDC. After you're done setting it, plug the vacuum line back in...

To get it to idle right, there should be two small screws on the primary metering block. It's right behind the float bowl. These are the idle mixture screws. To set them, turn them in until they stop. Dont tighten them, just go until they stop turning. At this point the engine should be running craptacular. Turn it back out until the engine is running smooth. It's something that has to be set by ear. Then go over to the other side and do the same thing.

Is it an out of the box carburetor? If so, the jets and power valve may need changing. Does yours have the secondary metering plate, or block? Look at the secondary float bowl. If there's a block in front of it like the one in front of the primary float bowl, you have a secondary metering block. If there's just a thin plate there, you have a secondary metering plate. If you have a secondary metering block you can change secondary jets. If you have the plate, you cant.

I'd suggest #68 primary jets, and #72 secondary jets. That's what always worked good for me.

Any more questions just ask.

 

When you adjust the idle screws in the metering block as described in the previous post, make sure the rpm is below 1000. If you get much above that you will be into the intermediate circuit and the idle screw adjustment will mean nothing. If you still have the hesitation after youi get the timing figured out you may need to increase the initial shot of fuel the engine needs just off idle. That can be done with larger squirter tube, (the proper name for it escapes me at the moment), or a different pump cam, a little piece of plastic on the linkage that actuates the pump diaphram.

 

Accelerator pump discharge tubes.

Yeah, they come in different sizes: 31 up to I believe 50. It's something you can play with. If you go to big, you'll get popping out the top of the carb (too much fuel).

Get the Accelerator pump cam kit as well, that way you can play with the different size cams for the accelerator pump.

I agree with rusty70F100:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>I'd suggest #68 primary jets, and #72 secondary jets. That's what always >worked good for me.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Right now I'm using the largest accelerator pump cam, size 37 accelerator pump discharge pump nozzel and size 69 main jets and whatever came in the secondaries.

I battled a totally annoying hesitation over the last few months and it's finally gone.

Good luck.

Sam

 

I have been told that the way to set initial timing is with a vacuum gauge. The engine will make the best vacuum at the optimum initial timing at idle. I have also been told that the idle mixture can be set the same way. I am going to give this theory a try in the next couple of weeks to see if it works. My plan is to bring the engine up to operating temp, then set the initial timing first since my carb is fairly close as is. After I have the timing close, I'll readjust the idle mixture while keeping the engine at my target of 850rpm. Then I'll start over, kind of a round robin approach until its nailed. Once I have that baseline I'll start on pump, pump cam, squirters, power valve, etc...

I'll post the results just as a FYI.

-Scouder

 

Rusty70-there is no secondary metering block, just a plate I guess

I checked the float level when I started and it seemed to be a bit high (running out of sight hole) so I lowered it a bit. I also pulled a couple plugs and they are pretty black except the end of the electrodes are a nice tan. It seemed to me like it was rich before, wouldn't idle that well and took a few seconds to clean out when you revved it in neutral.

I do run it with the vacuum advance connected, I was just trying to get an idea of how much advance is mechanical and how much is vacuum. It seems like mine is mostly mechanical advance. Is this any good or should I try adjusting it? My dad and I were messing with it before and there is an allen screw to adjust the vacuum in the distributor. The allen key just fits in the vacuum inlet.

So to recap- I lowered the float level and set the idle mix screws to 1 1/4 turns out. It seems better off idle but there is a bit of a pop at HIGH RPM(sorry no tach). The timing is still at 14, It seemed like it wanted more but I was scared to go any higher Going to do some towing today so I guess I'll see how things are now

 

The initial timing should not change with the vacuum plugged in. That is if you are using the ported vacuum off the carb. If it changes when plugged in, either the carb is bad or you have the idle stop screw cranked up way to far. You shouldn't use manifold vacuum ever. What you really ought to do is pull the points plate and see which mechanical advance curve you engine is using. I will bet it is at least an 18 degree curve(it is marked). There will be another curve on the other side of the mechanical advance weight system, maybe 13 degrees. If it is less than the one you are on, use it. The shorter the curve the better. The higher the initial setting the better and a short curve will give you that. Also 38 degree on regular gas is too much advance. 32 degrees is better. I use to run 42 total in first and second gear and 38 total in high(with a C6) in a 428 SCJ with 10.6 heads and it was very quick.

 

Bear, how do you change the total timing depending on what gear you're in?

 

Yeah, I'm curious about that too. I know how to do it based on rpm, but not gear selection.

-Scouder

 

You have to run a dual point dist. You set the set the dwell of the first set of points for 42 degrees of advance. You run a set of wires to a switch mounted on either the shifter(my preference) or steering wheel. That switch puts the second set of point in our out of series with the first set of points. When you turn the second set of points on the dwell changes and drops the timing back. I knew one guy who had a switch mounted on the shift linkage to do it automaticly for him. I found it nice some times for a little extra mph to flip it back to 42 degrees just before the timing lights. But mph isn't what drag racing is about, but a tie gets decided by the fastest mph under the AHRA rules back in the dark ages.

 

Ingenious. I would have never thought of that.

-Scouder

 

I'm not that smart. Got the idea from a how to section in a Hot Rod magazine way back in the late 60's. It made about .15 of a second increase in the ET. MPH of course didn't change as only more horsepower would help there. But again quick is what drag racing is all about, not fast.