Pontiac 400 - Idle bypass and Secondary rod hanger questions

[Cliff Ruggles]

A Pontiac 400 with 9.5 to 1 compression and that cam doesn't need a lot of idle bypass air.

If the throttle angle is too great, you will see two things.  Nozzle drip at idle and no control with the mixture screws. 

If you have no nozzle drip, and no control with the mixture screws, it may be getting enough fuel thru the transition slots even with the mixture screws seated.

What is the vacuum at idle, and idle quality?

With that cam it should be relatively "smooth" with a deep/heavy sound and little if any "lope".....Cliff

[DieselPower]

Cliff,

At 850 rpm, I am seeing a very steady 14" vacuum with no nozzle drip.  However, in gear, it is on the edge of stalling and any input into the power steering and the engine dies.

The mixture screws do almost nothing.  It seems to run slightly better when they are full closed.  Anything more than 1/2 turn out and there is no difference.

Thanks

[DieselPower]

I was just thinking, does the carb gets some "bypass air" from the PCV system?  Is it possible that some carbs have different sized PCV ports, thus the difference in required IAB diameter in similar applications?  For example, if someone else has an engine with the exact same specs as mine, but he has a factory carb and I have an Edelbrock 1910, is is possible that the 1910 has a smaller PCV port and thus less airflow for a given IAB diameter?

EDIT:  I just remembered that Pontiac engines have the PCV port in the intake manifold.  I wonder what the flow area of the elbow fitting is compared to the PCV port on my Edelbrock 1910?

Maybe the PCV valve itself is the limiting factor to airflow?

Either way, I still cannot get this thing to idle like it should.  I have the IAB at .130" now with little change.  I was able to close the throttle about 1/2 a turn.  Mixture screws still do basically nothing, but engine runs best with the screws all the way in.

[Cliff Ruggles]

Sounds like too much idle fuel for the application.

I'd also check the timing, and make sure it is rock solid in and out of gear......Cliff

[DieselPower]

Cliff,

Timing is steady as long as vacuum stays above 12" or so.  The vacuum advance can I installed is fully actuated above that point. 

The idle recipe I used is per emails from you and recipe 3 from your book.  If the idle circuit is too rich, how does that keep the throttle blades from being closed more so less transition slot is exposed?  Not arguing, just trying to understand.

Thanks!

[Cliff Ruggles]

None of the symptoms you are seeing are typical for that camshaft if a 400 build at 9.5 to 1 compression.

I have tuned several of them, and they idled fine in and out of gear with less idle rpm, less idle fuel, and less idle bypass air.  I also didn't run nearly the timing either, about 12 initial and used ported vacuum to the advance.

So why is your engine wanting all that bypass air with the timing at 24 degrees?  It should idle fine in and out of gear with less timing.

I have ran into several engine combinations that hated a lot of timing at idle speed, so you may want to try less timing by using a ported source to the advance, and start the idle tuning process all over.

With the HUGE idle fuel delivery from the Edelbrock 1910, you should be fine without all that timing, and very little idle bypass air, at least from what I've seen here tuning similar set-ups with the same carburetor.....Cliff

[77cruiser]

What does the exhaust smell like?

[429bbf]

someone correct me if my understanding is wrong. the only way the idle screws get fuel if from the size of the  idle tubes and and the size of the dcr.another small amount comes from the transition slots .if those 2 are to large you have to ether give it more bypass air or open the throttle plates . now you are not truly running off the idle circuit. my experience has been if the idle is to rich i have to smaller the idle tubes and the dcr so i can get control of the fuel with the idle screws and keep the throttle plates nearly closed so I'm not pulling a bunch of fuel from the transfer slots.this works for me. if my understanding is wrong please correct my way of thinking.fwiw

[Cliff Ruggles]

The transfer slots are not in the exact same location in every carburetor, so some will provide more idle fuel to the engine with the mixture screws fully seated.  This fact is not usually a big player, but once in a while one will run into a carb that's extremely rich at idle with no control of the mixture screws because it's getting plenty of fuel from the transfer slots.

I also see a very similar problem with some engine combinations where folks use manifold vacuum to the advance and put too much timing in at idle speed.  Might be a player here, not sure.  From what I'm reading the engine isn't very happy with the tune it's getting.  Could be simply too much timing at idle combined with a lot of idle fuel available at the mixture screw holes and transfer slots.

It's also no big secret but I am NOT a big fan of Comp XE cam lobes, especially in Pontiac engines.  I've tuned a number of them, and not in one single instance have I liked the end result(s).  Those cams drop the valves too hard on the seats, and shove them up too fast.  LSA is too tight, and intake valve closes too early.  This makes for a "quirky" idle quality right to start with. 

I actually built a 400 Pontiac WAY back when those cams first came out, with very well prepared #16 heads.  I thought at that time I'd buy into the "new" technology and find some additional power.  I couldn't get that cam out of that engine fast enough, very disappointed in it at every level......Cliff

[73ss]

I agree on the comp XE cams.   Had one in my 454 chevy. noisey valvetrain, stinky exhaust, poor vacuum...  Swapped it out for a summit 1302 cam. Not for the price but for the specs. Did nothing else to the car. Now it has a very quiet valvetrain, Idles like a champ in & out of gear.  Picked up 2 inches of vacuum. (The power brakes really improved) & picked up 10lb ft of tourqe on the dyno. Horsepower stayed the same. Came in 200 rpm sooner. Couldn't be happier.

[DieselPower]

OK,

Got back to working on this car.  Haven't touched it since my last post.  Installed a new AC Delco PCV valve and did nothing else.  Engine started right up (40* or so in garage) and went to a fast idle speed of 2000 rpm.  I had previously had this set to 1400 rpm, so I knew something had changed.  Readjusted fast idle and let it warm up.  Was able to dial the throttle back quite a bit.  Now the mixture screws do something with the best setting about 1.5 turns out.  It still stalls if I have idle speed in park set to 800 rpm.  Vacuum is about 14" still.  Set timing to ported vacuum and changed base timing all the way from 12* to 36* just to see the effect.  Vacuum keeps on increasing all the way to 36*.  Previously, I had about 28* with manifold vacuum. 

My questions now:

1)  Is it realistic to have this thing idle in park at 800 rpm and not have it stall when in gear and turning the steering wheel?

2)  Should I hook back up to manifold vacuum to get timing at about 28* with base timing of 12* (I have a vacuum cannister that is fully actuated at 12" of vacuum.

Thanks a ton guys!

[Cliff Ruggles]

My book describes timing engines using manifold and ported vacuum to the advance, and how that deal really works.

In any and all cases you have to give the engine the timing it wants/needs at idle to make things happy, same as fuel delivery.

Using MVA brings in some problems in some applications as the advance can may not fully employ, or stay employed when a load is placed on the engine.

Light distributor springs can also make idle tuning next to impossible.  Make sure that the timing is rock steady at idle speed, and not moving around some, especially when a slight load is placed on the engine......Cliff

[DieselPower]

Thanks again guys for all of your help.  I think I am done tuning for now.  Maybe I will go with a slightly smaller cam sometime in the future, possibly a Summit 2801.

For now, engine idle in neutral at about 950 rpm and in gear about 650-700 rpm (stock converter, listed as 1700-1900 rpm stall).  I ended up putting a "B28" vacuum canister on the distributor as a "B20" would not fully actuate in gear.

I now have total timing at 36* by 3000 rpm with base timing + manifold vacuum timing at 28*.  This engine definitely likes as much timing as it can get at idle and since the vacuum can fully actuates at 8" of vacuum, it is always actuated at idle.

Part throttle, cruise, and WOT are all awesome.  This car has only 2.56 gears and 29" tall rear tires but is still able to easily spin the tires easily even with limited slip diff.  I am probably going to put shorter tires on it in the future or maybe even a lower ratio to get a little more out of the car, but for now, it is a great highway cruiser.

Thanks again guys!

[Cliff Ruggles]

Good news!  Good results are good results no matter where you end up with these things. 

I've never owned an engine that ran worth two squirts of duck poop with 28 degrees initial timing in it, but if that's what it takes to make your engine happy it's a good thing in my book......Cliff