Posted By: Lawman 66 Chevy 250 dies at stop signs and red - Thu Jun 25 2020 12:10 AM
I have a 66 Chevy C10 with a 250 in line 6 engine with a 1 barrel rochchester carb. Recently it has started dying when approaching a red light or stop sign. She cranks easy, runs fine, accelerates fine, and will idle all day if you are not decelerating. It’s a 3speed manual transmission and pumping the gas to keep it running while stoping is not fun. When this started I noticed a small drip of fuel coming from the mechanical fuel pump, (it’s an airtex fuel pump). I tightened all the screws holding the thing together, stopped the little infrequent drip of fuel....still had the problem. So I replaced the fuel pump anyway.....still have the problem. I have set and rest the ignition timing, it was never off, 4 degrees before TDC. I have adjusted the mixture screw on the rochchester by small increments to no avail, it still dies when decelerating. So, I need some advice here....there must be something I am missing....I’ve owned the truck for 17 years and never had this problem before. Any help would be appreciated.
After it dies at stop light, does it start right up?

How can it idle fine and not idle fine when stopping? So when does it "idle fine" ?

It seems like debris is moving forward as forward momentum stops, then floats back on acceleration. Fuel filter, tank pick up or tank debris? If it runs good in all other modes like you say. Idle, acceleration, it doesn't seem like carb or pump?

Not flooding right?

Plenty of gas in tank right?

I know this sounds silly but can you go from 30MPH to a gradual roll and let truck stop without brakes or trans and see what is does. In a large parking lot or something.
Note to all: This is not me using another sign in. Lawman, scroll down a few posts (20-25) and you will find a similar thread with some good answers. “Idles all day but dies when stopping “.
I just read that. I was thinking what could change in the carb to cause this. Jerry had a good point about leaky/soaked float which would raise the level in carb BUT:
1. Poster doesn't say anything about flooding? that's why I asked
2. Martin, in that post you did find the float drop was off 1/2 inch and that fixed it. CAN YOU PLEASE TELL US WHICH WAY THE DROP WAS OFF 1/2 inch? too much drop or not enough? I'm guessing too much. That would be the flooding direction. Like a sinking float. Not enough would be the starving direction.

Here is the thread.

I have to stick with debris now that I committed. We'll see.
Too much drop and yes the float adjustment fixed it.
Ok, it always starts right up when it dies at a stop light or sign. It cranks easy either cold or after warmed up. It accelerates just fine. Doesn’t appear to have a flooding problem. Tank is new (5 years) all lines replaced in last year, fuel pump replaced after this issue started. Plenty of gas in tank. I have a glass in line filter installed on the intake side of the fuel pump, gas looks clean, but the level of gas visible in the filter does fluctuate, but the filter has been installed here for several years.

I’ll try the coasting stop tomorrow, but that is basically what I’m doing anticipating stops now, pressing in the clutch shifting to neutral and hoping the engine doesn’t die and staying ready to tap the gas if it seems like it will die. Some time it does even this way. If you just crank it in the driveway, it will idle just fine for as long as you want. If you decelerate (with coasting) and stop and it doesn’t die, it idles fine too.
Ok, I read the entire earlier thread offered. Seems this best fits my issue, so I’m thinking carb, it’s a rochcehsrer B, maybe the float level. I’ll wait a bit to see what other input I get before getting into the carb. I’ll have to order a rebuild kit anyway.

I can’t explain the fact it idles perfectly EXCEPT when slowing for a stop. Thought maybe the vacuum advance might be sticking, but it seems to work fine revving the engine in the driveway. Mixture richness may be an issue, but I have adjusted that screw in quarter turn increments with little to no effect on the engine dieing issue. APPRECIATE YOUR HELP AND ADVICE.
I’ve tried to post replies answering some of thes questions twice, but they don’t show up.

After it dies at a stop, it always cranks easily.

It idles fine when you crank it, cold or warm. Sitting in the driveway it will idle all day, after it dies at a light and you crank it it-idles fine until the next stop.

To try and keep it going I am shifting to neutral and coasting gently to a stop using brakes lightly and keeping a foot on the gas in case it stumbles and dies.....some times it doesn’t die, but more than half the time it dies. It’s an aggravating way to drive.

It doesn’t appear to flood. There is plenty of gas in the tank. The tank is new, 5 years ago, gas lines were replaced in the last year.

There is a glass fuel filter before the fuel pump intake, fuel looks clean although the level of fuel in the filter fluctuates between 30% to almost full routinely. Been like that for years.

Read through the suggested thread and it may be a carburetor issue, float level maybe, but as I said it doesn’t appear flooded, cranks right back up and no excessive gas smells.

Your reply is showing up now.

Something easy and free to try, take filter out of the system for a test drive or two.

I am just going to throw this out there, Have you checked for a vacuum leak? The intake manifold at the cylinder head, Base of carb at the manifold? I would be reluctant to say, since it is a rochestor B carb, there are two screws that hold the base of the carb(where the throttle shaft is) to the fuel bowl part of the carb where the float is, that could be loose causing a vacuum leak. What is the idle set at? Where is the timing set, you can advance the timing a little bit and see if it makes a difference. Check for vacuum leaks with the engine running and some spray carb cleaner or a unlit propane torch around the intake manifold and the carb base.

Good luck. Some people will say I am off base with my thinking, But I have learned over the years to always check the easy things first.
Check to see if the bolts which hold the carburetor on are tight. I knew a guy who had a similar problem when going around sharp turns. The bolts were so loose, they were about to fall out. Gravity applied enough down force most of the time. His started right up like nothing was wrong just like yours is doing.
Choke all the way open?
Alright, yesterday I found a quiet street and did coasting stops and the engine did not die on any of the thee drifting stops - 30mph to zero no gearbox, no brakes. That may indicate a float issue or some floating trash in the bowl. So to be sure I knew where everything stands, I reset the timing and mixture screw to original settings. I adjusted the mixture screw to where is was set before all this happened. She idles well, I don’t have a tachometer, but it sounds between 800 and 1000 a little high maybe but smooth. I set the timing at 5 degree BTDC a degree more advanced than before. I haven’t tried taking the filter out of the loop yet as suggested, but did add about 4 oz of sea foam to the quarter tank of gas just to see if maybe it’s bad gas, this all started after my last fill up. Then I went for another test drive. She died at the first stop sign...then didn’t die for the next eight driving normally. She did flutter a couple times, but didn’t die. So now I think I’ll drive it until the tank is near empty and get new gas from a different location and see where that leads before I tear into the Rochester. Oh, I did check for vacuum leaks and everything, carb bolts and hoses, were snug with no evidence of any leaks. I’ll update the forum later as I try and work this out, I feel closer now and I appreciate the help and suggestions provided so far.
You need to set the idle at 450 RPM before you set the mixture screw for highest RPM, then back the idle screw back down to 450 RPM and readjust the mixture screw to highest RPM. Repeat this until the idle stops climbing past 450 RPM when you tweak the mixture screw.
With the RPM at 800-1000 RPM, the throttle plate is open too far, uncovering the holes in the throat of the carburetor allowing extra fuel in. This will void any adjustments made with the mixture screw.
My 1963 Chevy book for Rochester B carb says 500 RPM, but no 250 in 63, they had 230 cu in with B carbs.
Wikileaks says 500 RPM and 6 deg BTDC timing for 1966.
Ok, so I will try to dial in the idle speed tomorrow, I bought a digital Tachometer today. Appreciate the inputs.

Ok, I’m back and nowhere near fixing the problem. With a digital tachometer I checked the idle speed after the engine warmed up. I had the idle set at 695rpm before I did anything today ( remember I said it was a little high, but turns out not between 800 and 1000).

Next I reset the timing to 6 degrees BTDC to establish a baseline. Then I dialed down the idle to 500 and worked between the mixture screw and idle to try and find the right balance. I finally got the idle stable at 550 rpm after a lot of back and fourth. But then when I test drove the truck around the neighborhood it died at every stop....wouldn’t even coast to a stop without dieing. It still starts right up after dieing, idles fine sitting still. As it is set now, it does not accelerate as well as before.

So I’m back where I started except the current set up doesn’t accelerate as well. Is there an easy way to check/ verify the float level in the Rochester B without having to do a complete rebuild?
Does it happen when you coast to a stop or use the brakes hard? Some wire could be rubbing or come loose if only on a hard stop. When it dies does the accelerator pump give a good shot immediately?

Right now it happens no matter what, coast no brake, brake for a normal stop .... all the same. When you let off the accelerator the engine just does not return to idle. Once you stop and hit the starter she cranks and idles fine, no need to pump the accelerator.

You said you thought you may have gotten some bad gas, have you put some fresh gas in yet? Is the carb tight to the manifold, is the manifold tight to the head? I'm sure you could up the timing to 8-10 degrees without hurting anything. Does the truck have power brakes? If so check the line to the booster, sometimes the check valve goes bad it is usually plastic and time and heat are not it's friend. How about the vacum line to the pcv valve , the rubber hose get oily and will be loose fitting on the connection to the intake manifold or the valve itself, and then you have a vacum leak.
Just some thoughts.
Yes, I ran the old gas near empty and put new in. All the bolts are tight, manifold and carb mountings. It does have power front discs (installed about 3 years ago) that vacuum hose is installed with hose clamps on both ends. Hose to PCV valve is snug on both ends. I’ve had the timing as high as 10 degrees while working this issue. For years it was set at 8 degrees (I thought it was 4, misreading the timing marks as 1 degree instead of 2). I moved it to six degree because Bartamos suggested that was spec and I wanted to base line the engine to solve the idle issue.

Any chance a bad PCV could be the culprit?
Where ever you have a vacuum line, remove the vacuum line and put rubber caps on for testing purposes. This will eliminate the possibility of a vacuum leak from a faulty vacuum activated device.
Alright, I’m now convinced no adjustment I can make in timing, idle speed, or mixture will fix this problem. All the vacuums seem solid, all the carb screws, bolts and manifold bolts are tight. I think the gas supply is good and the pump is new. So, I’m going to take the Rochester off, clean and probably rebuild it. There must be something stopped up or flow restricted somewhere. Thanks for all your help everyone.

I don't expect you to do this. There are a few of us who could do this and have done this. BUT............if you could hook up a clean gas can on the passenger's floor and run the hose thru firewall to the fuel pump inlet, it would sure eliminate a lot of possibilities. It would point to pump or carb.
Thanks, but I don’t see what that would accomplish. I’ve already replaced the fuel pump early in this effort because I noticed it had a little drip leak. So that is already new and seems to be pumping, I can see the gas pulsing through the glass in line filter just in front of the pump inlet. The tank itself is about 5 years old, all gas lines are new within the last year including the sending unit and it’s filter sock. I treated the gas with sea foam and ran the gas in the tank nearly empty and replaced with fresh from a different station a few days ago. It doesn’t feel like a fuel supply problem.
As Carl says “It’s never the carburetor.......until it’s the carburetor “. It’s time to dig into that Rochester and see what’s happening.🛠
Could there be an electrical problem where the coil is grounding out when you stop?

A simple test would be to connect a test light to the + terminal on the coil and to Ground...make the test leads long enough so the light is visible from the drivers seat and take it for a ride and see if the light goes out when you stop...

Grabbing straws here...

Mike B smile
That was what I thought, but he didn't think so.

Sounds like a likely possibility. Should definitely be checked.
Ok, I’m back with the rest of the story. I took my carb off and broke it down, didn’t see any tell tale issues, but it needed cleaning overall. I Cleaned everything, blew out all the jets and went looking for a kit. That’s when I discovered my carb wasn’t a genuine Rochchester, it was an aftermarket clone and probably put on years ago while it was still my dad’s truck and I was off somewhere serving in the AF. So, knowing that, I looked into finding a remanufactured genuine Rochester B 1bbl just to be true to the rest of my build and get rid of the clone. So the reman Rochester came in yesterday and I installed it today. After a few adjustments, she purrs like a kitten drives and accelerates good AND doesn’t die at stop signs and red lights. So it was something in the carb. Now, I don’t know if they will accept my clone as a core exchange. If not I’ll go ahead and rebuild it and keep it around. But that’s an issue for tomorrow. Appreciate all the help and advice.

I guess Carl was right “ it’s never the carburetor....until it’s the carburetor”. The new carburetor seems to have done the trick. Thanks again!
Glad you fixed it.
Thanks for posting the follow-up. always is nice to see the problem both resolved and explained as we follow along. Lots of times guys are searching the forum because of the same symptoms. Glad you got it going again.
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