17057266 1977 W72 q-jet- changes necessary?

[Mr Hand]

I have a question about about the 17057266 emissions era q-jet:


I have a 1977 Pontiac Can Am and it has this original numbers matching carb still on it. It appears to have never been opened up before and this may be true since the car only has 40K miles on it.

I have never drove the car because it needed work but it did run ok and idled very well. The engine is currently out for a rebuild due to severe oil neglect and the damage it caused so I am unable to due carb testing.

Anyway, I have the carb apart and I am getting ready to install the basic rebuild kit I bought from Cliff when I noticed it has different primary jets and rods.

17057622 auto trans carb should have 71 jets and 42K rods.....mine has 72 jets and 45L rods. Why I dunno as this was a one owner car with no history of carb work let alone a jet/rod change. Float setting was still at the OEM 17/32"

For comparison a  1978 W72 carb auto trans 17058266 uses 72 jets but has 45K rods. Not sure what the difference is between 45K and 45L rods.

As it turns out the EGR system will be non-functioning when I get the new rebuilt engine installed due to a bad EGR backpressure transducer. Instead of trying to find this rare part to make the EGR work properly I will run without it.

Will the carb need to be re-jetted to be made a little richer since the EGR is disabled? If so will it be as simple as a jet/rod/power piston spring change? Or is it ok with whats in it now? I plan on raising the float up a bit to 13/32" if that will help with anything? I'll be using a new Carter mechanical pump.

Engine will be totally stock right down to the single exhaust with the original catalytic convertor.
Just a .040 overbore and a 066 cam which is almost identical in duration to the original W72 cam with the only difference being slightly more lift at .407 vs .364

Original HEI was rebuilt to stock specs with the exception of a vacuum advance unit limiting the advance to 10 degrees instead of the stock 25 degrees...... since the EGR will be non functional this change should help avoid pinging.
 
Thanks for any help.

[Cliff Ruggles]

45 L are slightly tapered at the upper section then taper all the way down to .026".  The "K" rods only taper .005" then "step" down to .026".

72 jets and fine for what you are doing, EGR or not.

It's a crap-shoot with currently available fuel pumps, some are high pressure like the OEM some are not.

If it's a high pressure OEM type pump the factory float setting will be fine.  You should be installed a new float and do NOT use a brass one.  This new fuel eats the solder out of them and they are too heavy and don't work right anyhow.

Any slight recalibration from not using the EGR can be done with the APT system.

Not sure why you would limit the VA to 10 degrees.  Those engines have really low compression and it may like, want and need a bit more timing from the vacuum advance for best efficiency.  I typically don't reduce timing from the vacuum advance for "stock" engines.  Completely different deal when you tighten up the squish, increase compression, improved cam timing, etc........Cliff

[Mr Hand]

The distributor was re-curved to be all in by 3000 RPM which is the main reason the 10 degree vacuum can was used.

Would it be ok to use the original 25 degree vacuum can with this fast timing curve?

Thanks for the advice on the carb.

[Cliff Ruggles]

Most of those HEI's had the timing all in about then anyhow so about all that would have been needed to "re-curve" it was to open up the "eyes" in the springs slightly or use one slightly lighter spring on one side.

I hope that whoever did the "re-curve" welded in a positive stop for the advance, they will add timing at high RPM otherwise as the weights are now able to "roll out" on the tips past the "flats" on the cam if the stock parts were retained.

If the "goofy" parts from one of those POS aftermarket kits were used it becomes a crap-shoot for timing control, they are absolutely pure junk and to be avoided.

As for adding timing with the vacuum advance those engines are really low compression and set up pretty lean for emissions.  It will like some fuel added from the carb as this new fuel lacks BTU's and needs about 5-7 percent more of it to act the same as the fuel they were originally tuned for.

They will also like a hot thermostat, and a lot of timing at cruise to be happy.  The factory set them up with pretty high initial timing specs as well.  Many were up around 18 degrees initial nearly as I can remember, around 20 degrees from the mechanical advance, and over 20 degrees from the vacuum advance.  That puts them up there pretty high at light part throttle cruising depending on the exact RPM's as the mechanical wouldn't be "all-in" till around 3000 or close to it.

So my advice would be to start out with at least 15 degrees in the VA and work from there.  I'd also use the APT system in the carb to add or take away fuel with the timing changes to find out what the engine likes the best.

My bet is that it will like a lot of timing from the advance and need 48-52 or so at light part throttle at freeway speeds in high gear..........Cliff

[Mr Hand]

Reviving this old thread:

Been a while but finally got the new engine in this spring and been slowly breaking it in.

300 easy miles on it.

'77 Can Am..............040 over 400..........stock 6X heads..........066 cam......2.41 highway gear...all emissions equipment hooked up and working including EGR, CAT converter, etc. Running at a constant 190-195 degrees which is thermostat temp.

Carb is a 17057266 stock rebuild with one of your kits, rebushed throttle shaft and all internals are stock......stock rods and jets...........72 jets and 45L rods. Only change was backing out the APT almost as far as it would go to eliminate a slight off idle bog, which it did for the most part.

Engine runs great, no bog, hesitation and great idle. Smooth acceleration and very throttle responsive. Pretty good power for what it is and how it's geared.

2 problems:
Runs extremely lean. After 300 miles plugs look brand new. Almost like they just went in yesterday. I was a little surprised by this because of how well it runs. Need to fatten up the carb so it'll run richer? What is the easiest way? Thought about playing around with jets/rods but is there another way? Will bigger idle tubes help with anything else besides idle quality?

Other prob is it pings like crazy at speed once I am into the secondaries. I kept the timing specs stock just to establish a baseline and go from there......16 initial, 24 mechanical, 25 vac advance. It was re-curved to be all in by 3000 RPM.

I backed initial down to 14 but have not test driven it yet to see if it did anything, but based on how bad it's pinging I dunno if 2 degrees will make much difference. I have other vacuum cans to try if need be........a 10, a 15 and I think a 20. But is vacuum advance much of a player at heavy acceleration? There is no positive stop welded in on the advance as you mentioned in your reply earlier BUT, the guy who did the recuvre, Bob Davis, initially had only a 10 degree can installed on his re-curve job, I am the one who swapped it out with a 25 degree can. Did he put the 10 degree can on there for a reason to go with his weight/spring combo? Did I mess it up by putting on the 25 degree?

advice where to go with this?

[Mr Hand]

one correction: mechanical advance was set up at 20 degrees not 24.

[Cliff Ruggles]

Without a positive stop welded in place and lighter springs to bring all the timing in early the HEI can and will add additional timing at high RPM's. 

With your application I'm not sure you'd get up that high, but ALL HEI's need a positive stop in them.

For your "low" compression build it still needs a lot of timing from the VA, so a 10 degree can may not be the best choice for it.

It is NOT lean if it works well.  The plugs should be light tan to nearly white.  If it's lean on the primary side the engine will surge, ping, lackluster power, poor throttle response etc. 

We NEVER tune carbs in this business by reading spark plugs.

For sure you need more fuel at WOT.  The factory DB rods have huge tips.  I'd get a set of our DA rods and if you want more control with the primary side fuel so you don't have to raise the APT clear up out of the bore get my custom full taper 45 rods at the same time........Cliff

[Mr Hand]

thanks cliff I will look into ordering them.........I recall a post of your at PY showing how to add a stop. Do you mean adding it to the mechanical advance or the vacuum? , I will try to find it unless you have a link or something?

[Cliff Ruggles]

We add a positive stop to the mechanical advance in the HEI. 

The factory designed the HEI to stop advancing when the "flats" on the weights reached the "flats" on the center cam.  It does OK with the stock spring but IF you install lighter springs the weights can and will "roll-out" on the tips and continue to add timing at high RPM's.

Installing a positive stop is NOT an option, they all do it and I'm sure plenty of engines have had the rod bearings pounded out of them from detonation at WOT because folks trust those cheap POS spring weight kits or the folks who install lighter springs with their stock parts.....FWIW.....

[Cliff Ruggles]

I also modify the VA cans if/as needed to add exactly how much timing I'm wanting.  I use VA cans with the spring tension I'm looking for (start/stop points), then simply MIG weld the slot and "adjust" the amount of travel with a round chainsaw file.  Pretty easy stuff there, and no need to have more than one VA, I can put it where it needs to be for the application........

[Kenth]

Other prob is it pings like crazy at speed once I am into the secondaries. I kept the timing specs stock just to establish a baseline and go from there......16 initial, 24 mechanical, 25 vac advance. It was re-curved to be all in by 3000 RPM.

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

[old cars]

Good or bad positive stop?

[Cliff Ruggles]

I've seen them done in similar fashion and it is effective.

It really doesn't provide an easy method to control how much timing is added. 

Here I MIG weld up the "slot" then use a round file to precisely control the stop point, which also controls how much timing it adds at the same time.

[Cliff Ruggles]

"Engine will be totally stock right down to the single exhaust with the original catalytic convertor.
Just a .040 overbore and a 066 cam which is almost identical in duration to the original W72 cam with the only difference being slightly more lift at .407 vs .364"

I'm curious as to which pistons you are going to use?  All of the "builder" pistons being sold have 8 valve reliefs and end up .030-.050" in the holes at TDC.  NOT a good place to be as this will reduce the compression ratio nearly a full point and even more if you use a thick head gasket.

Pontiac 6X heads have flat combustion chamber floors and are more sensitive to quench area than most others.  They thrive on tight quench and will run cooler, less timing required, and less octane sensitive (detonation) at the same time.  They will also make a LOT more power.

What else is being done to it to deviate from "stock"?  Hopefully it's not getting a "double roller" timing set or a high pressure oil pump.  We've seen nothing but issues with currently available double roller timing sets including being inaccurate and the chains stretching out very quickly.  I wouldn't use roller rockers on it either, and if you venture that direction avoid anything but Crane Gold Race, Harland Sharp or Crower Enduro.  Everything else we've seen used have failed miserably and/or put metal into the assembly.  The WORST of the bunch are Comp Cams roller tip rocker, absolute JUNK.....IMHO......Cliff

[Mr Hand]

"I'm curious as to which pistons you are going to use?  All of the "builder" pistons being sold have 8 valve reliefs and end up .030-.050" in the holes at TDC.  NOT a good place to be as this will reduce the compression ratio nearly a full point and even more if you use a thick head gasket."

"What else is being done to it to deviate from "stock"?  Hopefully it's not getting a "double roller" timing set or a high pressure oil pump.  We've seen nothing but issues with currently available double roller timing sets including being inaccurate and the chains stretching out very quickly.  I wouldn't use roller rockers on it either, and if you venture that direction avoid anything but Crane Gold Race, Harland Sharp or Crower Enduro.  Everything else we've seen used have failed miserably and/or put metal into the assembly.  The WORST of the bunch are Comp Cams roller tip rocker, absolute JUNK.....IMHO......Cliff"

Pistons are TRW speed pro forged.
Timing set is all NOS parts. Cam gear, crank and original style wide link chain.
Stock Melling 60psi pump.
No roller rockers, no trick aftermarket parts anywhere.