396/375 hp/motor issues

christopher teed · 04-07-2013, 09:05 AM

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

Ed Wright · 04-07-2013, 10:03 AM

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

Steve Williams · 04-07-2013, 10:12 AM

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?

Ed Wright · 04-07-2013, 02:05 PM

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

christopher teed · 04-07-2013, 08:00 PM

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...

Ed Wright · 04-07-2013, 08:18 PM

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

christopher teed · 04-07-2013, 10:18 PM

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.

Ed Wright · 04-07-2013, 10:31 PM

Quote:

Originally Posted by christopher teed

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.

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

c612pro · 04-07-2013, 10:46 PM

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.

Adger Smith · 04-08-2013, 12:11 AM

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.

04-07-2013, 09:05 AM	#1
christopher teed Junior Member Join Date: Feb 2013 Posts: 11 Likes: 0 Liked 0 Times in 0 Posts	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

04-07-2013, 10:03 AM	#2
Ed Wright Veteran Member Join Date: Mar 2005 Location: Sand Springs, OK Posts: 8,132 Likes: 896 Liked 389 Times in 168 Posts	Re: 396/375 hp/motor issues Try fattening the idle by backing the idle mixture screws out 1/2 turn. __________________ Ed Wright 4156 SS/JA

04-07-2013, 10:12 AM	#3
Steve Williams Member Join Date: Aug 2007 Location: Detroit, Michigan Posts: 352 Likes: 0 Liked 1 Time in 1 Post	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?

04-07-2013, 02:05 PM	#4
Ed Wright Veteran Member Join Date: Mar 2005 Location: Sand Springs, OK Posts: 8,132 Likes: 896 Liked 389 Times in 168 Posts	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. __________________ Ed Wright 4156 SS/JA

04-07-2013, 08:00 PM	#5
christopher teed Junior Member Join Date: Feb 2013 Posts: 11 Likes: 0 Liked 0 Times in 0 Posts	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...

04-07-2013, 08:18 PM	#6
Ed Wright Veteran Member Join Date: Mar 2005 Location: Sand Springs, OK Posts: 8,132 Likes: 896 Liked 389 Times in 168 Posts	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 __________________ Ed Wright 4156 SS/JA

04-07-2013, 10:18 PM	#7
christopher teed Junior Member Join Date: Feb 2013 Posts: 11 Likes: 0 Liked 0 Times in 0 Posts	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.

04-07-2013, 10:46 PM	#9
c612pro Junior Member Join Date: Jan 2012 Location: surrey bc canada Posts: 2 Likes: 0 Liked 0 Times in 0 Posts	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.

04-08-2013, 12:11 AM	#10
Adger Smith VIP Member Join Date: Dec 2001 Location: Texarkana Ark/TX Posts: 2,365 Likes: 462 Liked 694 Times in 270 Posts	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. __________________ Adger Smith (Former SS)