17057266 1977 W72 q-jet- changes necessary?

[Mr Hand]

All in at 3000 rpm´s is NOT stock, 4400 rpm´s is.
I will NEVER understand the urge some folks has to get "all in" earlier than factory settings. The factory ignition curve is there for a reason. And the reason is engine design/parameters.
You do NOT help the engine by igniting a too lean mixture by ignite it earlier, the real cure is to give the engine a proper, fully combustible, mixture at factory ignition settings.
How many times do you read "i have all in at 2500-3000 rpm´s and now my engine pings", big surprise!

FWIW

You're right. The spec from the 1977 MVMA spec form is:

START:                 0 degrees @ 1000 rpm
INTERMEDIATE:   9 degrees @  2000 rpm
MEDIUM:            17 degrees @  4600

This is for a base L78 400 engine which is basically what mine is other than the 066 cam. The '77 350 engine dist spec is a little lower, 16.5 degrees  @ 3640 rpm.

Here is the instructions that came with my distributor after the rebuild/recurve:

Set initial timing @ 12
Centrifugal allows  10 dist degrees 20 @ the crank
Curve starts @ 1250 crank rpm all in by 3000 crank rpm
Vac advance allows 10 degrees at crank....starts a 7hg max 12hg.


Only change I did was swap the 10 degree advance can for a 20........and I originally had base timing at 16 but backed it down to 14 to see if it will help with the higher RPM pinging but haven't test drove it yet.

[Cliff Ruggles]

Unless they changed the pin height the TRW forgings will be WAY below the deck at TDC, I will not use them here.

A couple of years ago we were hired to assemble a late model 400 engine where the owner had picked all his own parts.  He supplies the TRW/Speed Pro forged pistons hung on stock cast rods.  I mocked it up and they were around .032-.035" in the holes at TDC. 

That is NOT a good place to be with a Pontiac engine build especially using stock iron heads on it......FWIW.

I'd add here that the TRW's are also extremely heavy and have thick heavy pins in them.  Good news is that they are very strong parts and the new alloys with the skirt inserts allow them to be fit much tighter than previous designs.

We've been using ICON's or Ross/SRP forginings for the last 20 years or so instead, two valve reliefs, end up near close to the top of the block at TDC, lighter, and modern ring packs.  Their alloys are also superior and you can run them nearly as tight as a cast piston for less "rock" in the bores on cold starts and accellerated ring wear till they heat up some.  They are also full-floating if you chose to upgrade to forged rods instead of the stock cast iron parts.......Cliff

[Mr Hand]

Been playing around and made some progress:

Before I did anything to the carb I knocked the timing back from 16 to 14 and it eliminated any heavy throttle pinging. I don't hear it at all at any throttle position light or heavy. I'd still like to get the HEI recurved to be all in at 4K rpm instead of 3K but that's for another day.

As to this 17057266  carb, I made an error in my notes as to what jets/rods it has on primary side. I opened it up to check some things and as it turns out I did put the correct  71/42K set up in it  not 72/45L as I previously stated.

Float was way low for whatever reason so I set it 13/32nds. I also replaced a weak PP spring with a new one I got from you Cliff along with the DA secondary rods. Not a tire burner for sure but the mid range power seems better. It's not as flat as it was during heavy acceleration while still on the primary circuit just before getting into the secondaries. Better, but I feel there should be a little more. I'm sure putting the float to where it should be and a stronger PP spring helped. The engine has a solid 20.5" vacuum at 650 rpm idle.

It roars nicely once into the secondaries.

No real problems anywhere except in the idle circuit it seems.

A slight surge that only happens at light throttle under very little load, around 1000-1400 rpm........if I give it gas a little quicker it goes right past the problem and moves out well. Also seems more pronounced as the engine is warming up, once at full operating temp I can still feel it slightly but not as much. I fear once the weather gets colder it'll be more obvious....right now the idle is good, pretty smooth......main metering and secondary circuits seem to be dialed in ok.........no hesitations or cruise surging. It's a problem under very light load still using idle fuel apparently.

Playing with the APT didn't do anything. Was at 4 turns out, tried another turn, then one more but didn't change anything in regards to the light throttle surge so it's back at 4 . Idle screws are 4 out and the idle is smooth.

I thought of going up one jet size to 72 but it that worthwhile? I never changed the idle tubes in this carb...if the idle tubes are too small to begin with possibly causing this light throttle surge, will going to a one size bigger jet be pointless since the tubes are already too small? Or will it help eliminate the surging?

[Cliff Ruggles]

I remove the idle tubes on all carbs built here.  It will help to open them up .001-.002" if you have issues at very light throttle/light load.  Those carbs were pretty lean from the factory and that was on older fuel that had more BTU's than this new stuff.

Recently I worked on a carb for the basic set-up you have.  It was for a really low mileage car and all original.  The owner had the car since new and said that it was always somewhat lacking at very light load.  During the rebuild I opened up the idle tubes .002" and DCR's slightly.

He called back after installing the carb and said that it had never ran that good since he'd owned it.......

[Mr Hand]

Does jet size effect the idle circuit at all?

[Cliff Ruggles]

Not really.  It may have a minor effect on things if you have a HUGE rod stuffed too deep into the jets and blocking off flow to the idle tubes. 

I see that on quite a few carbs that come in here as they will have bent PP hanger arms and often one or both of them has the rods pretty deep in the jets.  A bit more common to see hanger arms bent up that down however.  If I take in 100 carbs and least 90 of them will have PP hanger arms horribly bent and the metering rods not correctly located in the jets. 

I did three carbs yesterday, a 1968 Chevy, and two Big Block Marine units.  The 1968 Chevy had one hanger arm bent up at least .150".  Both Marine carbs had one hanger arm bent up about .040-.050" and on the side where a previous "builder" most likely used a screwdriver to pry the piston out of the bore.  It amazes me so many carbs come in here with fundamental issues causes specifically by someone with good intentions but seriously lacking in attention to detail.

I also see folks using early style (long) primary rods in later carburetors which doesn't help anything when it come to tuning......