SBF diag help

Got a SBF with a new 600cfm edelbrock (electric choke) I used as a replacement after my Demon carb started acting up. So now the engine will idle smooth in park/neutral but stumbles and stalls once it's been put in gear.

I've got a holey blue pump with 6 psi at the regulator with the engine running (slowly goes up to 8psi with the key on/engine off)

Electronic distributor with vacuum advance, new plugs gaped at 40thou. (previously gapped at 35)

 

Sounds to me like large vac*** leak. Be sure of all our connections and gasket configurations. If it ran fine prior to the carb swap don't change anything else until you get back to running fine, other wise you may compound the problem. Good luck.

 

I checked for leaks but maybe just not well enough...Hmmmm.

Thank you.

 

It also could be too lean either in the idle circuit or main jets!! This **** they are selling as gasoline is causing all sorts of problems with carburated cars.Usually if it idles fine with no load but then stumbles or dies when in gear it is a lean condition. JMO,ROY.

 

Hey Comet, I've adjusted lean to rich without much change but I'm impatient at best with adjust a carb so end up ruling it out after an hour haha. I had that thing running rich enough to kill the flys and lean enough to purify water haha.

 

either your idle circuit is clogged. (ongoing edlebrock issue) or your engine is worm out.

On mine, I had both....That may have been your demon carb issue..... I would have rebuilt the demon myself...

What was the demons failure?

Keep in mind, don't change to many things. It really causes a mess, like Christmas lights.... all tangled up

 

The demon was way to much carb for the motor and started leaking out and needed constant adjusting, In reality I just never liked the carb and am just more familiar with the edelbrock.

As for the engine it was rebuilt a few years ago and has always been reliable, I drive my coupe often through the summer so for it to suddenly start acting up doesn't really seem like it's worn out.

I did just realize the vacuum line for the trans isn't connected so I'm going to do that and recheck the vacuum when I get home. Maybe that'll tell me something.

 

sounds like you found the problem to me.

 

How big was the Demon? I got a 525 Road Demon on the 302 (300hp, 9:1 comp) and it ripped *** right outta the box

 

If the ****** vacuum line not being connected fixes the whole problem, your engine is tired.... Normally it would just idle fast....

My experience with engine stall when put into gear has always been a warn out engine or at least 2 cylinders on a small V8 not makin power.... Do a compression check.

 

I hooked up the ****** line and no change.
Compression between 130-145 and it holds compression.
Ran a vacuum gauge and idled it down at the screw so it idle at 18 but it does have a slight stumble and drops to 17. If I tap the throttle it drops then comes right back up. When I put it in reverse or drive gears the needle starts dancin between 16 and 20.

Any thoughts with that info?


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I also canceled cylinders and it'll stumble then come back up when reconnected.


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It stumbled on every cylinder when you cancelled one by one? Or just on certain ones?


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It stumbled on each one.


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New info above for the day crew.


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I seem to remember 6 psi being pretty high for an Edelbrock carb. Also, have you gone thru the jetting process according to Edelbrocks recommendations? Between these two things, it may eliminate a lot of your issues.

Just checked, Max pressure was 6.5. I would humbly suggest a GOOD pressure regulator and get the pressure down, then do a jetting if it hasn't been done yet.

 

Thanks for the info, I'll have to look in to jetting.

I'm curios if a carb spacer could help the problem.

 

I just went through something similar with my sbf. I'd make sure all the vacuum ports are sealed or not leaking on the carb, double check your timing, adjust your idle speed and set the carbs idle enrichment screws back to the factory setting, or using the vac gauge. If the engine still idles ok, but stumbles in gear, I suspect you have a vacuum leak somewhere. Using some spray carb cleaner, I found mine to be the last intake runner on the right rear.

 

also, jetting has nothing to do with your idle issues. If you were experiencing a drivability issue, such as a lean miss under part throttle or cruising at a steady speed, then yes, look at the jetting. An edelbrock 600 with electric choke is a 1406, which is lean from the factory compared to a 1405 600.

 

Thanks for the infor VT.

 

tell us where your timing is.

 

Sadly I don't have a timing light at home, I did try to turn the distributor a little but there was no real improvement. It does have a vacuum advance distributor with electronic conversion.

 

I decided to put the timing light on each plug wire (I know that's not how it's done) and found the spark was erratic so I'm going to look deeper in to that.


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Did you check to see where the timing was set? The erratic spark could be from fouled plugs, a result of the malfunctioning demon carb.

 

How did the plugs look when you did the compression test (black and sooty, or white like out of the box).

Definitely verify you have good spark, borrow or rent a timing light.

Is your vacuum advance hooked to a timed vacuum (above the throttle blades) or a manifold (full time) port?

If you have it to full time vacuum and the mixture is lean enough, it may be that you loose enough timing on tip in to cause the stumble.

That would be exaggerated by a weak accelerator pump, or clogged accelerator discharge nozzles.

Have you checked to see if the pump squirts when you twist on the throttle? (Does not have to be running to check)

Otherwise, does it smell rich or lean at idle (ie, do your eyes burn while it's idling [rich] or just stink [lean] )?



FWIW your compression numbers seem low to me, do you have numbers from when it was running good to compare to?

 

Uhhhhhhhh.Are you sure that the carb base gasket is installed correctly?
Don`t ask how I know about this.I put the damn thing on upside down. Hehe.
Good luck.Have fun.Be safe.
Leo

 

Funny you mention that woodbutcher...I'm starting to think I may have put it in upside down...having looked at most everything else I'm gettin' curious!

Nex, I've tried running both vacuum and nonvacuum advance to see if I wasn't getting enough ignition while in gear, it runs the same regardless. As for the plugs they're black from running rich on the demon, swapped them out when I switched the carbs and the blacke right up. I can practically tune this thing at idle in park just based on the exhaust alone, I've had burning eyes and a migraine for a week haha. It's running just a touch lean now though...according to exhaust smell.

 

What kind of plugs you got in there?

 

They're just autolite 26's. I've had them gaped at 35 and now 49thou.

 

OP post #23 - "found the spark was erratic "

that would make me mighty su****ious, whether the erratic-ness was varying timing at idle speed, or occasional "missed" sparks. flash-flash-flash-pause-flash-flash

The hallmark of electronic ignition is dead steady spark