Technical Why is my SBC running on?

The problem I had with run-on was that the idle adjustment screw held the throttle blades open and allowed the engine enough fuel and air to run. Once I closed this off, no more run-on.

You state that the engine will not idle in gear if the "park" idle is less than 1000 rpms. Perhaps you could adjust the idle speed in gear with the timing and back the idle screw all the way down. The AC solenoid method would work too.

Either of these would be easier than a converter swap.

 

I've seen this also..

 

really? If you want to see a neat demonstration of how stall speed affects idle speed, try a car with a switch pitch (variable pitch) torque converter, as used in some GM cars in the mid 60s. I've installed SP converters in several of my rides over the past 30+ years....it's pretty obvious when you're in D, stopped with the brake on, engine idling, that the idle speed will change by a couple hundred RPM when you flip the switch....just because of the change in stall speed. Try it if you don't believe me.

As for lean best idle, that's fine with a stock cam, but usually a cam that's big enough to cause a rough idle, requires a richer mixture to keep from coughing when you open the throttle.

 

First problem: SBC. Second: Edelbrock.

Seriously though, I'm curious as to what your dwell is set at on the points (we're talking points, right?) and what kind of vacuum you're pulling at idle. I like numbers, and what's got me here is the fact that you're "dieseling" with an idle set at 1000. This seems like a separate problem from the RPM drop from your converter, which tells me it is most likely a fuel delivery or timing issue.

To that effect, start with timing it. Get it where you want it there first, fuel comes later.

Just my .02.

Mike

 

I generally get run-on with idle set to 1000, unless i shut the engine off in gear. I blame it on modern fuel, which is way too volatile for use with carburetors.

 

I don't think the octane is causing your run on, I have a 327 with a edlebrock carb (not my choice thats how it came) camel hump heads and a th350, I don't have any run on issues running 87 octane. 93 octane might just be masking other issues. I don't recall off hand what the idle is set at but I'll check it later and get back to you.

 

Doesn't that cam call for a 22 or 2400 stall?
Too much throttle position to keep it running in gear makes for a high idle. The converter is too tight. With more slip you can lower your idle speed and it will still idle when in gear.

 

Octane is only one part of the equation. I noticed that my 55 (when it had the blower) would run on with 87 octane, but not with 91. Run on is also called dieseling, which means compression ignition...which is what higher octane is used to prevent--engine knocking is the result of compression ignition, before the plug fires. Makes sense that increasing octane would reduce the tendency to run on.

 

Well lets start with the obvious, turning the idle trim screws out 2 turns then setting the idle is not how you trim a carb. If you don't have a feel for it hook a vacuum gauge up to it then play back and forth with the idle speed screw and the trim screws until you are pulling max vacuum at idle.

Your timing is way slow even for an thumper cam. Your stock out of the book static timing is going be up in the 12 degree range unless you are looking at '70s smogger specs. You stopped running a wheezer motor when you changed the cam and hopefully you are not running a smog era HEI behind all that.

There are several reasons one with detonate, too much compression, carboned up combustion chambers, too much quench, too much lead (timed to fast), idling too fast, overheating. I don't know the condition of your engine or the rest of the build so I would probably start with the way you set your carb and work my way out from there.

 

That intial timing spec seemed a little low to me also..should be 12+ base. But if it was fine before and now it's not unless there was a mechanical reason for the change (broken distributor weight spring, balancer slip) why the sudden change?. I had a GM 1 wire alternator do the very same thing to me once, installed a one-way diode in line, fixed the problem.

 

Well there is one that I missed and should not have. I don't know why that sometimes causes a problem and other times it doesn't.

 

Hello Jay,

Having just sorted through similar issues with the SBC in my C10, I think I can offer you some advice. You'll have to try it out and let me know if it's good advice

First, that 1800 RPM stall converter sounds too tight for a Thumpr cam. A 400 RPM drop is not normal. I suspect that a higher stall speed converter would help out a lot with the large RPM drop that you're seeing between park and drive. My guess is that a "tight" 2800/3000 RPM 10" converter would be perfect, but I'm just some guy on the internet so you should talk to a converter company instead. I recently purchased a converter from Freakshow Converters in Texas. The price was great and the guy who runs the place asked a lot more questions than the "big guys" and seemed to care a lot about getting me the right converter. Search the Yellow Bullet forum for more information about the company if you're interested.

Second, you are not running enough initial timing. I'm pretty sure this is why you're getting run-on. A cam with that much overlap is going to want A LOT of initial timing to "clean up" the idle. You probably have some build up in your combustion chambers and that's what keeps the engine running after shutdown. My guess is that your engine wants something like 18-24° initial, 34-36° with mechanical, and another 10-15° of MANIFOLD vacuum advance on top of that. Try it and see if it works for you. The vacuum advance will be a little tricky, as your cam isn't making enough vacuum to work with most factory HEI vacuum canisters. You can buy an aftermarket adjustable one (a headache IMHO) or look up "Lars Grismund HEI tuning paper". He wrote a great paper on tuning HEI distributors. At the end of his paper is a list with all the factory HEI canisters and straightforward instructions on how to pick the one that matches your engine. You're going to need a vacuum gauge for this one. For the mechanical timing, you are probably going to need to modify your distributor to limit the advance. An easy, cheap, and crude way to do this is to add a screw to the advance plate to limit its travel. You grind the head of the screw to dial in the amount of timing. Crude but it works! If you have interest in this I can dig up a picture.

Link: http://www.digitalcorvettes.com/forums/showthread.php?t=75830

Third, I think you may find that too much of your transfer slot is exposed. You can pop off your carburetor and measure how much is exposed. Lots of pictures and info on the web. You want .020-.040". Any more than that and you're going to be pulling too much fuel at idle. That's another thing that will contribute to carbon buildup, which in turn contributes to run on. Getting the right converter and timing will allow you to turn down your idle speed, which will close up the transfer slot.

Fourth, copy and paste this into Google: "F-BIRD'88 thumpr site:hotrodders.com". The guy has a lot to say about getting the Thumpr cam to run as hard as it sounds. His posts will give you advice about gear ratio, converter size, ignition setup, etc. Smart guy. Read what he says.

 

This is really good info here! Thanks for the input. How would one actually open the throttle plates more if that was the case?
Thanks
Jay

 

The car that this motor is in is a show box. It is a friends car that I am tunning. Ok 350 with stock bottom end. Probably dish piston 305 HO heads (smaller combustion chambers) No idea of the compression. Doesn't have an electric fan.
Jay

 

Here is the link Jim

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=107&sb=2
What do you think?
Jay

 

Regular style fan. No electric.

 

I agree fixing it is the priority.
Jay

 

The best timing I've had with a SBC was with a Comp Xtreme energy 268H cam with:
18 degrees initial, 36 total with mechanical and 14 degrees manifold vacuum. Ran great, strong mid range, ran 190 temp in 90 degree weather all day long. Also ran a 2800-3000 rpm converter and 3.73 rear. This was an awesome setup.

 

SO you are saying a half turn each on the front two fuel air mixture screws and retard it again? I guess I can try it.
Jay

 

Electric fuel pump!

 

I like the suggestion made above about a "tight" 2800-3000 stall 10" converter. Kinda depends what rear gears you have, and what you will do with the car....if you have mild rear gears, and will be putting a lot of highway miles on it, then you don't want so much stall as the converter would be slipping a lot in normal driving. If you have at least 3.50 gears and mostly putt around town, then 3000 would be good. Like was mentioned, talking to the converter company is a good idea.

The timing suggestions are good...anything you can do to make it idle faster with less throttle opening is good.

 

Some solid info here burger! Stupid question but is there an adjustment one can make to get the right amount of adjustments on those transfer slots??? I saw a couple pics but no info on how to adjust to get that little square that they talk about?

Jay

 

The slots are exposed more or less as you move the throttle blade. What he's saying is that you want to have the idle speed adjustment screw set to about where it is designed to be. If there are things like a lumpy cam and too far retarded ignition timing making it so you have to adjust the idle speed screw too far, then the slots will be over exposed, and the carb won't work right. So...the slots are just a symptom of another problem.

 

Ya I love the xtreme energy series cams. I run one of these in my 55 chevy. (Avatar pic)
Jay

 

Jim did you see the post I made about the cam specs?
Jay

 

yes, see my post above about the converter. That was in response to your post with the cam specs.

 

Sorry brother I didn't see that lololol I will find out the gear. The car is primarily a runner on the highway and in the city a little. He puts a lot of miles on the car every year and drives the wheels off it.
Jay

 

Good advice. I did try this already. Jay

 

Am I missing something,or is there anything wrong with shutting off the engine while in "gear" or drive? This only seems a temporary stopgap measure until you are able to figure out the underlying problem,most of the suggestions appear to all be legitimate answers to your problem. Flatheadjohn47 Texas.

 

Hello again Jay,

The transfer slot gets bigger and smaller when you adjust the carb's idle screw. If you don't have a "square" slot (which would be .020-.040 tall measured with feeler gauges) that means something is wrong.

In your case I think it's the timing and converter. Read the timing article that I gave you a link to. It's all in there. As far as the converter goes, you'll need to match that to the cam. You've already picked a cam that's going to be hard on gas. Don't try to crutch a big cam with a small converter. Either buy the right converter and go fast or buy the wrong converter and have a poor running engine with bad mileage. All the parts have to match. You get the best performance AND mileage that way.

It seems you may have some confusion as to how to set the idle mixture screws on your carb. Here's how I do it. First, you need to get the timing set to where you want it. In your case I would start at 18*. After you get this baseline set and keep playing with it, you may find that your engine wants even more initial. Next connect your vacuum advance. You want this connected to a manifold source. Turn the engine off, and turn both idle screws all the way in. Turn each one out exactly 2-1/2 turns. Connect a vacuum gauge up to manifold vacuum or use a tach (maybe on your timing gun). Start the engine up and get it warm. Measure the vacuum and/or rpm. Turn both screws in a 1/4 turn. Check vacuum/rpm again. If the numbers went up, go in another 1/4 turn. If they went down, back the screws out. You want to keep doing this until you get the highest readings, then back out a 1/4 turn. That's it. Done.

Hello