[Birddog1970]

Here is the short and sweat issue: Fresh complete rebuild on 396 - street pef build, Cam - Valve list .513; cam lift .302 duration (SAE) 230; 100 miles on motor; motor smokes blue at idle when warmed up and gets worse when throttle and let off; checked PCV system which is fine; 16 degs timing at idle 39 degs at 3K RPM with vacuum advance line plugged; intake bolts tight

Reading through lots of posts and hearing anything from rings need to still seat to intake gasket or valve guide/seal issue. Many have said to put more miles on it to help seat the rings which I understand, but the vacuum test below has me concerned its something else.

I did a vacuum test using the small vacuum port on the manifold I utilize for my cowl induction. The needle goes VERY rapidly from 5 - 10 in (almost hard to read its so fast). I rev motor up and it still goes very rapidly but from 10 - 15.

Thoughts? I looked at http://www.secondchancegarage.com/public/186.cfm but dont see my problem.

Appreciate your help, I am trying to trouble shoot this logically before pulling the entire motor apart.

Thanks!

Dave

 

[Tom Mobley]

reading like that usually indicates a problem with a valve not seating or similar. Who adjusted the valves? do you know what method was used? could be a valve adjusted too tight or a cam lobe going flat. New engines usually don't have issues with the valves not seating but it's not out of the question.

The smoking and vacuum issues may not be related.

 

[Steven's 72]

Hey Dave, Hope this helps, I had a 454 built for me and it was smoking, It did not help that my Odometer is broken. I drove the car for one season, and it smoked at start up and when deceleration. The beginning second season same thing, This time I drove it as much as I could and sure enough It stopped smoking. Just needed to get seated. Good luck.

 

[Birddog1970]

The Engine builder did the valves and my mechanic checked them when he did the initial fire up, not sure what method he used.

I was hoping this reading would help me pin-point the smoke issue as well. I can hardly stay in my garage with the door open for more than 20 min's without stinking. Just seems like its burning alot of oil A few of my spark plugs are wet which appears to be oil. Having another issue on top of the smoke issue is not what I was hoping to hear on a $4K fresh rebuild.

Just to be clear on my post .. my vacuum needle will go very quickly back and forth between 5 - 10 at idle and when rev'd up very rapidly back and forth between 10 - 15 in

 

[Tom Mobley]

it's got a valve not seating properly to generate that needle movement. probably an intake valve. Do you have a leakdown tester or know somebody that does?

 

[Birddog1970]

UPDATE: Thought my vacuum gauge was crap so I bought a good one. Readings different but still not correct

Hooked up the gauge to my port on my manifold I use for my cowl induction.

At Idle Im at a steady 9 (almost 10) inches
Throttle it will quickly drop to about 6 inches
then quickly up to 15 inches (put to about 2500 rpm and held 15 inches)
when I release the throttle it will quickly spike to 13 inches (or so) then right back to 9 inches at idle.
*I'm in Denver at 5000 Ft btw if that matters*

So reading one of the website it states:
"A low steady reading of about 10 inches indicates Late Valve Timing or a possible slight intake leak" -- The site does not say if the quick drop at throttle is normal and if the quick spike letting off is normal.

Thoughts?

 

[Tom Mobley]

>>> "quick drop at throttle is normal and if the quick spike letting off is normal." Both are normal and expected.

9-10 is low for a stock engine. normal for a hot rod engine with a big cam. Also, retarded timing causes low vac. What is your initial timing set at?

 

[Birddog1970]

My 396 isn't stock but street performance. Not sure if you would consider my cam a "big cam" (Duration (SAE) 230; .513 valve lift; int centerline 106 exh centerline 114 and 110 spread.

My initial timing (vacuum line plugged) is 15 degrees with 39 degrees full advance (3500 rpm)

I read that I drop 1 inch of vacuum per 1000 ft .. so if that's true my 9 inch of vacuum at idle would be 14 inches at sea level. Is it normal holding the throttle constant about 2000 rpms to have it hold steady at another 5 inches of vacuum? I was holding at 15"

I really trying to determine where my smoking issue is from and thought the vacuum test would give me an indication of rings, valve guides, intake leak etc.

I did put a propane torch around my intake and carb but the motor didn't rev up ... guess I could still have oil passing through my gasket underneath.

So if my readings are normal for my motor then what should I try as the next test to determine where I'm burning oil at?

Thanks for the response.

Dave

 

[SWHEATON]

The vacuum readings with the new gauge are pretty normal so your ok there.

r;yAn aftermarket perf cam with 230 deg dur @.05 in a fairlyy small 396 cid bbc needs at least 18 even 20 deg for best throttle responce/best idle vacuum.

Also,39 deg total could be a bit high so when running 18-20 deg base timing you need to reduce mech adv in dist to give to approx 18 deg at maybe 2600ish rpm with vac adv unhooked plugged when setting/checking timing.

So thats 18-20 deg base + 18 deg mech in dist = 36-38 deg total not in vac adv.

Limit vac adv to approx 10-12 deg max using a crane cams limiter plate or new vac adv cam that only has 10-12 deg timing when fully activated .

With vac adv cruise timing with be approx 48-50 deg depending on base timing setting & vac adv setup,but dont go over 50 deg timing at low load part throttle cruise to avoid detonation on todays crappy low octane enthenole'd pump fuel.

Your current 9-10" idle vac should come up a bit when going from 15 to 18-20 deg base timing.

Run 93 octane fuel with this etup if no ping then try 91 to see if it will tollerate slightly lower octane fuel.

Thats a good place to start for as street perf timing curve in your 396 with a fair amount of duration for a street 396.

On the smoking,before doing anything drastic try loading the motor hard @ WOT thru the gears while also letting off gas in a lower gear a few times too at fairly high rpm to also let the motor act as a brake slowing the car down to load the motor/rings durring accell & decell.

Also put some miles on it too and if you get approx 1k miles or so on it and it still smokes and uses a lot of oil then its time to dig into it at that point.

Sometimes doing this can help better seat rings as long as there are not other issues like oil getting past intake gaskets or even issues with your newly rblt heads like oil getting past the valve seals & guides or issues with pcv sucking oil/etc.

Scott

 

[Steven's 72]

I was also fouling my plugs at first, Changed plugs and kept driving it. All is well now, just need to put some miles on it.

 

[Birddog1970]

Limit vac adv to approx 10-12 deg max using a crane cams limiter plate or new vac adv cam that only has 10-12 deg timing when fully activated.

I am currently running a Carquest 57-7535 vacuum advance which is marked at a B26 (VC1808 B26 5-7 8 @ 11-13), if reading correctly that would be 11 - 13 deg advance

So I should go with an adjustable like :http://www.summitracing.com/parts/MOR-72315/?rtype=1 or the limiter plate http://www.summitracing.com/parts/CRN-99619-1/

Hoping for the snow to melt and put some good miles on the car to see if the smoke issue gets cleared up.

Thank for the great info.

Dave

 

[cv67]

Had a very similar issue to yours with less miles.

Upon also getting a bouncing vacuum reading it was suggested (as above) to check the valvetrain.

Bent pushrods in my case. Maybe recheck your adjustment?

Will say the intake gasket was wicking oil up from the backside of the gasket, dry on top. Sure enough most the tops of the valves had a nice coating of oil on them.

Hope that helps.