SBC stumble off idle

Got a 68 327 with rochester 4 bbl carb (new) recently the car (engine) stumbles off the line thru to about 2200 rpm. after that its gone (and so is the car) performance is not an issue it goes like fire to gasoline. But this "miss" or fuel related anomolie..is pissing me off.
Is it possible to have my timing to far advanced? or too close to being to far advanced? or is my carb acting "lean"? in the acceleration mode? It is hard to determine if it is Ignition related or fuel.

 

Check your plugs.
You may be running lean.
r

 

raven ..was thinking that too..do i turn the adjustment screws clockwise to richen or is it counterclockwise?

 

What kind of ignition are you running? Points? HEI? How old are the plugs and wires? Lean should give you more of a stumble, where ignition will be more if a miss.

 

well the ignition is a new HEI with new 8.8 accel wires and shorty spark plugs fron accel all was working in beautiful harmony until recently..the only think ..one of my friends was helping me get it closer in tune i know we advanced the timing a bit and also he did something to the carb adjustments.."lean" or "rich" im not sure. i wasnt watching him. I may pull a plug tonight and see what that tells me.

 

Clockwise lean, counterclockwise rich. Remember, those are only idle circuits.

 

yep only idle ..but it is possible to have them too lean and have a stumble near or just off idle..yes?

 

If you're looking at it after dark, try to load the engine by accelerating in drive while holding the brake. If you can make it miss, watch under the hood for spark jumping. This really does work, pretty exciting when a spark leakage is visible.

 

ahhh good tech idea..will try that too. shouldnt be the issue , but you never know

 

I've got a similar problem but mine is only the moment when gas is appllied from a stand still. If I let it roll a few feet and smoothly apply the gas it's just fine. The plugs and points look fine and the tailpipe emissions have been scanned and everything there looks good as well. I know your frustration well.

 

So JoeG No end to this problem in site? dam but my engine didnt do this at first with lower timing and richer idle settings im thinking of trying the carb adjustment first, than the ignition second..if it doesnt go away than i will start pulling out the rest of my grey hair

 

I normally use a vacuum gauge to set the idle mixtures. Plug it in to a manifold vacuum port and adjust the idle mixture screws to give you the highest vacuum reading.

 

i got one of those somewhere
i should give it a try.
each screw is for a seperate side of the carb..so i adjust them evenly? or do i go as high a vac. on one and get it higher with the other one?

 

Kind of sounds like you and JoeG are fighting a carb related problem. JoeG's sounds like the "squirters" aren't quite big enough or are way to big. Your problem I don't think is the squirters because your worked ok initally. If your buddy doing the timing got it to far advanced and it back fired through the carb it could have popped the accelerator pump. I've had both issues in the past with different cars and Yes, they are frustrating until you figure out what the problem is. Check the idle mixture and if that's ok you may be looking at replacing the accl pump.
Good luck with it
Ron

 

Ditto on the Accel pump!

 

I do it one side at a time, get the highest reading there and then move to the other side. Then I usually go back to the first side again just to make sure

 

Ok i looked at the adjustment for idle mixture screws and noticed this the left (passenger) side screw was out about 3 or 4 turns . the right one was out only 2 to 2.5 turns..I was under the impression that these needed to be about equal and at about 4 to 5 turns out..is that correct? maybe my engine is loading up (rich) when at Idle and sitting at a light. And when i go to accellerate off the line the engine is needing to clear itself of all the rich fuel mixture..?? possibilities? should i go and get both the mixture screws out to about 4 and see if that makes any significant change?

 

BTTT please

 

Manafold stock or aftermarket? RPM powerband range?????

 

Is the 327 stock? When you say 'new' Rochester, are you talking about the Edelbrock repop's? I have one of those and it has not had a problem (other than having to spray the linkage on both sides of the carb with carb cleanter every couple of months). If you are lost and it says Edelbrock on a tag over the fuel inlet, call them. 1-800-416-8628

I would do as you suggested, check the mix, then the timing. But if it is still giving you fits, call your buddy and ask him what he did.

 

Manifold is a Perfomer Edelbrock aluminum dual plane. the engine is pretty much a stock 300 HP vette engine. Has a NOS rochester Believe is a 4mv mod. carb. with the electric choke set up.
let me understand something..if i open the idle mixture screws does it become leaner or richer? is the idle mixture an air screw or fuel screw?

 

Idle screw should be fuel, counterclockwise is more fuel (richer) clockwise is less (leaner).

The only carbs I have come across that have air screws for idle (on cars) are some Holleys off a 70's smog POS.

 

Been reading this post and it seems to me a lot of details are missing. In the first place, if the motor wasn't acting up before, then you did something to it, then it is what you did to it. If it had a problem before and you cleared that up but another complaint emerged, then that is important to know in order to diagnose.

Idle mixture screws; make them the same, start wherever you like, but adjust them the same, not just highest reading ( I actually don't use a vacumn gauge except as a diagnostic tool) but smoothes and fastest, then blip the throttle and adjust to rid the motor of stumble, it should be crisp, but the timing is also important, and you have what? A stock HEI? If so, get a recurve kit, not the cheap kit either, but the one with the adjustable vacumn advance, then take the time to follow the instructions entirely. Chevies in hot rods hate stock HEI's, but they can be made to perform very well.

 

well not if you read everything. I really dont know what exactly he did but i did say i found one screw to be 2.5 turns out and the other to be at 3 or 4 turns out and that we bumped the timing up what more do you need? those were the only two things that he changed. i am in the process of trying to get it back to where it was 1 step at a time rather than changing two things and not knowing what one was wrong

 

hey and its a possibility that what he did isnt the reason for this..that is possible

 

try val hedworth on the carb issues..... brandon

 

Val is he a vendor here? how do i find him?

 

Just the facts, man, nothin' but the facts.

 

normally the idle air screws are only 1 1/2 turns out.turn them in all the way then come out 1 1/2 turns,and try it.

-danny

 

ok HRP will try that too..I have gotten alot of different opinions on the setting of these screws but im up for any suggestions. I do feel this is an Idle load up issue. Due to the fact i dont have any problems with anything until the car sits at idle for a while at a stop light or in the driveway type deal. it drives fine doesnt blow any smoke black or other wise. it may just be too rich at idle. just wasnt sure or absolutly positive about these idle mixture screws on this carb. Im mostly into Mopars and i decided to build the chevy i had first due to cost. Mopars are not cheap builds from my experiance so that ones next year.