Just wont idle....argggggg!
 

Need some thoughts on things to check off and try to narrow the cause for this......

I have a Chevy 350 40 over......Brodix Tk1 heads....Holley 4150 750CFM. Engine drops off quickly and dies when I adjust the idle to about 1800 rpm. Engine throttle is crisp and revs really quick on the peddle. No smoke....but just wont idle.

Recently installed new plugs, wires (MSD 8.5mm), new dist cap & rotor.

I'm thinking must be a vacum issue......so going to get a tester tomorrow and pull the cap off of the back of the throttle plate and check.

Good place to start?.....what type of reading should I get (or should be) on the intake vacum?

Other things to check....or possible causes (i.e. timing?)

Thanks!!

Have you made any adjustment to the air mixture screws? If these are turned in to far then your car will not idle no matter what you do with the idle screws. If this is not the issue, then yes, look for a vacuum leak. Take a can of carb cleaner and spray around the intake with the car running. You will find a vacuum leak if there is one generally.

check for vacuum leaks like shawn said...make sure that your vacuum lines are hooked up properly...that carb has a hook up in the base plate for your vacuum advance....also has a hookup at the back of the carb for a pcv vavle/brake booster hose....then set your timing to about 10-12* btdc on the balancer with a timing light....then adjust the carb idle mixture screws for the final time and set your idle rpm about 800-1000rpms

Sounds like a vacuum problem, but if you haven't adjusted the air screws, screw them in to the bottom and back them out 1 1/4 turns, good place to start, if it still doesn't idle you most probably have a vacuum leak. It's hard to check vacuum at 1800 RPM and almost impossible to get a correct reading.

JMO

Zip.

If you have a vacuum modulator on the tranny, check to make sure the hose didn't fall off of it. You won't find this vacuum leak under the hood.

Thanks for the ideas....I will use carb cleaner spray and see what happens.

Another observation....I removed the air breather to see the top of the carb....and when I turn off the motor (or dies) there is a lot of fuel vapor (like a fog) hanging over the bowl of the carb.

Not sure that adds anything to the equation....but that does not seem to be right to me.

I got another tip that I'm currently investigating. Prior to my car the engine was in a Camaro bracket car.....using a crank trigger ignition system. The current setup in my Nova is using the 6AL. My distributor is the MSD ProBillet p/n 85551....which has the ability to "lock out" the timing or manual adjustment using springs and bushings. Now I'm certainly no expert on ignitions but I'm wondering if it's still locked out from when using the crank trigger.....or if not how the current manual springs and bushings might be working?? We have changed the timing once from 28 to 34 degrees advanced...using a light on the balancer and moving the distributor....if that matters.

Thanks again.....still digging.

You can tell if its locked out, while you have your timing light out check to see if the timing changes while you raise the RPMs if it doesn't its locked out. Make sure you set your total timing at 2500 RPM or more.

might have a blown power valve in carb , has it ever popped back through carb?? if thats the problem , you can install an anti backfire kit to keep from blowing power valve...just a suggestion,,,Paul :D

Just pulled the dist cap and rotor....definitely locked out....weights & springs removed.

if this is an all out drag car , I would leave the dist. locked ,,if a street car I would want the advance working..I dought the dist being locked is causing your idle problems...Paul

I agree with ya...starting to think not related to distributor. But this possible blown power valve has my attention.

I've only had the car for a couple of weeks, and when I picked it up....it idled fine....been to the track once and dont know of any major backfire....but if you read one of my initial posts....I was experiencing alot of hesitation and engine miss under load (just off the line) and the engine would not get above 5000 rpm. Just in park....I can rev to 6000+ no problem. Thats why I changed out the plugs/wires/cap & rotor....just to get those components out of the equation.

We checked the timing at the track which was at 28 and moved that to 34....and although I dont think advancing the timing is at fault....thats the first time I noticed a problem with idle....really had to keep on the throttle a bit just to get it back on the trailer.

So if it were a blown power valve....how best to test this since I cant get it to idle below 1800?

Still sounds like a vacuum leak, maybe base plate. I'd try another carb. Also, if it has a stock dampner it may have slipped inner ring to outer ring making the degree markings irrelevant, if thats the case your actual timing could be anywhere.

Well I sprayed carb cleaner all around the intake and carb base plate and did not stall the RPM any....so I'm assuming no leak in those areas.

The damper is an ATI Super Damper. Dont have access to another carb to test....might check around.

A good friend of mine who is a mechanic is stopping by tonight....see what he thinks. I think I will have him check the timing and we will set up the vacuum gage and try to get a reading as we adjust below 1800 RPM.

Still could be the power valve in the carb....so trying to figure out how to test that without just taking it out and inspecting.

If you have a blown powervalve you would know it. It will be running extremely fat.

Update:

My buddy stopped by....and we adjusted timing a bit...he worked on the air/fuel mixture on the carb. Idleing much better although still seems to be running a bit rich....meaning when you "crack" the throttle she bogs just a split second.....like the engine is loaded with fuel.

I still suspect the power valve in the carb....it maybe leaking some and adding more fuel in during idle.....which you have to open the air up to get to run...which obviously keeps you at a higher idle or she dies because of the excess fuel.......my thinking.

Next steps:

Going to purchase a temp gun to check the exhaust temps on each cylinder just to see if we have anything funny going on. Those guns have become really resonable in price and a quick way to check individule cylinder issues.

My buddy is going to make some calls today and see if we can get our hands on another carb just to throw on to verify we have a carb issue......it may be time to do a rebuild kit.

Track is open tonight...so may take it up to test under load.

Still digging.....but making progress.

Carb
 

It would take MAYBE?1 Minute to pull the Bowls off the Carb and Check the Power Valve???
DID I MISS SOMETHING?Is the Carb Mounted somewhere,where you cant get to the BOWLS???
Later
G 8)

Re: Carb
 

Yea....looking at the drawings....does not look to be a hard thing to get to at all....just not sure I would know if its bad once I got there....unless its very obvious....just no experience with it.

p/v
 

Pull the Bowl off,turn the Plate on end that the Power valve is Down.Put some type of Thin Liquid(Carb Cleaner,Brake Kleen)in the Threaded hole that the Valve Screws into,if it runs Threw,the Diapham is Bad!
Or you can use a Vac.Pump.They actually make a P/Valve tester.But,you can usually see if they are Blow!
Hope this HELPS?
Later
G 8)

Re: p/v
 

Ok....I pulled the primary bowl and metering block off and guess what??.....the dang power valve has a plug in it......

So help me understand the reasoning behind that move??......I guess for racing applications....which this is a bracket engine.

Atleast I know now that its not a blown power valve.....but not sure what this means about the idle issue....

Thoughts?

see my reply in your other thread asking about the point of plugging the pv.

Re: p/v
 

If someone has plugged the power valves, did they jet up afterward. When you plug your powervalves you usually have to jet up 6 to 10 sizes.
Maybe something to check.

I use a spray bottle of water ( $1 walmart ) spray each header pipe with the water, if the water vaporizes on contact, it's firing, if not then you got a weak or misfiring cylinder = just a thought> Cp

....the other thing I was thinking...if this has an electric fuel pump....chk the pressure at the carb...should be 7-7.5 lbs....if it is higher adjust the regulator down...