396/375 hp/motor issues
 

hey guys,i have a 396/375 nova ss...i run a class that runs bias tires through stock manifold.recently had the motor rebuilt from a well known guy a year ago,it was dyno tuned ,i recently placed it in the car...been messing with it for a week after i got it running.it has a pulsating miss at idle and clears up somewhat at 2000 rpm but never goes away fully..has intial timing of 28 degrees,total of 38...it has a flat tappet bullet cam ,to factory specs...run through the valve lash ,ok..turned over the motor with starter switch and watched to see if all the valve springs seem to be moving up and down all the way,looking for lobe wipeout,seems ok..one thing it has basically has no vacuum until 2000 rpm...and then it has 15 on the vacuum gauge..also it goes hand in hand,when i put my hand over the carb the idl picks up showing a lean condition.wil be looking for vacuum leaks today...but starting to lean towards cam lobe issues ,by the way it runs,,,any help appreciated

Re: 396/375 hp/motor issues
 

Try fattening the idle by backing the idle mixture screws out 1/2 turn.

Re: 396/375 hp/motor issues
 

If you suspect that it is wiping out a lobe, drop the oil and look at it closely. Running the engine further at this point would be a mistake until you can confirm whether or not a wiped out lobe is the culprit. No reason to circulate oil through the rods and mains that may have junk in it and do more damage. If your lash was correct when you checked it (no loose rockers), then you can start looking for a lean condition. Have you looked at the plugs?

Re: 396/375 hp/motor issues
 

If it were wiping a lobe you would have found a rocker arm with an extreme amount of lash.

Re: 396/375 hp/motor issues
 

checked the valve lash again,only a few about 2 thousands loose...no vacuum leaks that i could find,will try jetting the carb up to cure a lean condition and changing the power valve.ran a compression test,and all is well...

Re: 396/375 hp/motor issues
 

Why "jet up the carb" over a lean idle???? Turn the damn screws. That's why God put them there. LOL

Re: 396/375 hp/motor issues
 

Ed,I turned the idle screws out .it didnt change the lean condition or change the results as i had a vacuum gauge hooked up while i turned the screws out.

Re: 396/375 hp/motor issues
 

Changing jets will have very little effect on idle fueling. Sounds like you need some help. Where are you located?

Re: 396/375 hp/motor issues
 

Actually losing cam lobes does not necessarily affect the lash as the base circle is not really affected. The fact that there is no vacuum at low engine speed indicates possibly having a cam ground off spec, maybe lobe center off, if the vacuum gauge fluctuates wildly that shows a vacuum leak. At least a good look inside the oil filter and the engine should tell you if there is mechanical trouble.

Re: 396/375 hp/motor issues
 

Most of the cams goig flat I've seen do change the lash because the lifter always gets shorter. Put the vac gauge back in the tool box and use it to tune a car with a stock cam. You might need a smaller idle air bleed or check it for Vac leaks if it is lean at idle. If you have a PCV hooked to it like it was a stock car get rid of it or get one that is designed for low vac. Get an O2 sensor on it with some Data logging if you can't figure it out.

Re: 396/375 hp/motor issues
 

There ya go.

Re: 396/375 hp/motor issues
 

Make sure your headers are not leaking. We picked up a miss with an engine fresh off of the dyno and it turned out to be a leaky header gasket.

Re: 396/375 hp/motor issues
 

Not to oversimplify things but do you have a plugged idle air bleed?

Re: 396/375 hp/motor issues
 

What Billy said,did you hear engine on dyno ?Has carb had fuel sitting in it since dyno?
Had BBC that was working on one time,idling sounded like carb or valve lash too tight,idled sound like was on 6cyl. all that was good,with idling tried to get feeler gauge in between rocker/valve would'nt go,shut off lash would check right, long story short tore down found lobes just strarting to go.

If have temp gun may shoot ex and find if one cyl. colder than others,no vac @ idle I would guess you are getting comp. back in intake,ethier valves being held open some reason ,bad lobes? bent valves?

Mike Taylor 3601

Re: 396/375 hp/motor issues
 

Plugged air bleeds make them richer, not leaner.

Re: 396/375 hp/motor issues
 

Ummmm, if the air bleed is completely plugged the idle circuit won't flow any fuel.

Re: 396/375 hp/motor issues
 

The air bleed in the top of a Holley? Stick a tooth pick in one and see. Very rich, the air bleed won't bleed off any of the signal. Bigger bleeds=leaner.

Re: 396/375 hp/motor issues
 

Always go back to the basics!

Re: 396/375 hp/motor issues
 

thanks for the help so far...removed the carb and checked all the idle circuit for blockage etc.and checked the carb in general ..noticed that the diaphragm front pump... appeared to be sticking.changed it appears to run better...two things i really notice is the first is with the timing light ; at idle or throttle up the timing mark is steady ,then all of the sudden it jumps back about 20 degrees and then moves back to the stationary mark on timing tab this happens about every five seconds,,,the other is the fact that the bullet flat tappet cams is not much above the stock 396/375 cam...it does have a alittle tighter duration and a tight valve lash...but should have a good vacuum right at idle.....have drained the oil and removed the filter and will cut into the filter and see what is up.i live in orlando florida and the motor was dyno tuned on Bo Laws dyno by a guy named Dave Crunes ,Dave used to run stock eliminator years ago

Re: 396/375 hp/motor issues
 

well. drained the oil and cut away the oil filter to be on the safe side...everything looked good..drove the car down the road at 3000 rpm and up it runs fine...starting to think it may be a distributor issues..stock gm distributor with a advance curve kit and a pertronix ignitor ignition in distributor...have the same setup in my 396/350 camaro with no issues...will keep plugging ,but at low speeds and idle it runs and sounds like sh...will put my AFR innovative motorsports gauge on it ,and work on getting the air fuel ratio correct

Re: 396/375 hp/motor issues
 

Last time I had a car with that Petronix in it it ran pretty much as you are describing. It was in the car when I bought it. I went to auto zone, paid about $55 for a rebuilt single-point distributor and the AMX instantly ran perfect. Also picked up about 800 RPM.
Pull one plug wire at a time, ground it and see if you have spark out of all 8.

Re: 396/375 hp/motor issues
 

i will give it a try Jeff.thanks...with the timing light hooked up ,timing mark it is steady for about 5 or 10 seconds them its instantly jumps about 20 degrees.starting to make altittle more sense.

Re: 396/375 hp/motor issues
 

Me too-went through 2 of those junkers then bought a ford dura spark dist to power an msd box-cured months of frustration! now I just say no to pertronix

Re: 396/375 hp/motor issues
 

Chris - try swapping distributors and see if the problem goes away

Re: 396/375 hp/motor issues
 

changing distributors this weekend will let you guys know what happen...

Re: 396/375 hp/motor issues
 

I have used the Pertronics conversion for years on my boat. It has worked perfect. I doubt that is your problem. Google the wiring for your unit online. I cant rember but I think you have to eliminate the ballast resistor. If the ballast resistor is still in the system that could be causing some issue. Also some ignitions used a resistor wire. Run new wiring to the coil and pertronics unit.

Re: 396/375 hp/motor issues
 

The way you say your timing is jumping around,it sounds like ignition control/box problem.
I have used alot of pertronix on restoration projects have'nt had any problems,I use ignitor 3 has protection circuit and built in adjustable rev limiter,pertronix tech line has always been helpful when I have called,oddball installations, I would probaly give them a call.
I keep a hei w/wires so I can test and elimnate whole ign. system@ one time then have hei setup for msd so can determine box or dist.
Don't take for granted that any part can't be or go bad.
Mike Taylor 3601

Re: 396/375 hp/motor issues
 

Definitely check out the ignition as noted above. As a last resort, the carb to intake, or intake to head gaskets could be leaking vacuum.

Re: 396/375 hp/motor issues
 

hope to change out distributor tommorrow...changed the advance springs in the current distributor from light to medium to heavy.....light is just simply to light and a great deal of difficulty cranking.heavy is ok ,but slow gettin to the desired timing mark from the dyno tuning ,tried the medium spring ,but when timing mark was set at the 40 degrees under a load,the intial timing at idle was just too low and the car ran crappy...hopefully the distributor change wiil do the trick..the 396 nova has 11;5;1 compression .via purestock drag rules... and a msd 6a.

Re: 396/375 hp/motor issues
 

try eliminate msd box had one bad on customers car,had trouble starting and timing did weird stuff,was trying to get running could'nt twist dist to get running enough to even get light on it.
Mike taylo

Re: 396/375 hp/motor issues
 

well I bumped motor over to TDC to remove the distributor before i went to work..noticed when removed the cap that the rotor is actually pointed between number 1 and 8 on distributor cap .swore i placed it dead on when installed..possible ,could this have been the problem?

Re: 396/375 hp/motor issues
 

well,changed distributor .it is a stock gm distributor with a curve kit and a pertronix ignitor ll.it was in another 396 car and runs just fine...it basically ran the same.installed new cap and plug wires.still the same ....will try eliminating the msd 6a box and see if that is the issues..getting out my AFR and see if the car is that far out of tune,u wouldnt think so ,it the car was dynoed tuned..stumped on this one

Re: 396/375 hp/motor issues
 

Only way rotor would be pointing directly at #1 is if timing was on 0 degrees,you can set timing close as example you run 36 degrees line pointer up w/36 drop dist in line up w/# 1 on rotor &cap if dist. has advance twist rotor where it advanced and twist dist to line up w rotor,in case you don't know put mark on dist below cap where it's center of #1 electrode on cap.

I say you are going to find problem w/ ign, box
Mike Taylor 3601