[PMDRACER]

Looking for an idea of what to do from here, guys.

1979 Q-Jet #17059553, 40K rod, 76 jets. I've borrowed a buddy's FAST o2 sensor kit to get this thing dialed in. Cruising at 60-80 mph is around 14.0-14.3 and 30-50 mph is 13.8-14.1. Cruise and full throttle seem to be dialed in pretty good.

My problem is easy throttle from a stop or accelerating in traffic. Things quickly head toward 14.8 to 15.2 and will ping like crazy. Give it 1/4 to 1/2 throttle (staying out of the secondaries) and things will settle to 12.5 or so and no audible ping. I've tried tuning the APT out one full turn CCW (rich) and the numbers all changed about 0.2-0.3 across the board. Didn't do anything for the pinging.

I do have Cliff's book, but I'm getting a little googly-eyed from going though it so many times and don't have anything to verify the air bleeds, etc.

[Cliff R]

How much timing are you adding at light throttle from the vacuum advance. The pinging may not have anything to do with the A/F ratio. The A/F ratio at light throttle isn't supposed to be in the 12 to 1 range......Cliff

[guccieng]

since it's stabilizing at a good cruise afr, sounds like it's simply a squirter issue. you need more pump shot. opening the throttle will naturally introduce more air and it takes some time for the airflow to increase the fuel flow; that short time it takes will be lean. the pump shot is there to make up for that lean spot. you don't need to verify air bleeds or change the idle mixture.

[guccieng]

Originally Posted by guccieng

since it's stabilizing at a good cruise afr, sounds like it's simply a squirter issue. you need more pump shot. opening the throttle will naturally introduce more air and it takes some time for the airflow to increase the fuel flow; that short time it takes will be lean. the pump shot is there to make up for that lean spot. you don't need to verify air bleeds or change the idle mixture.

edit: cliff, you beat me to it! i was just about to add: see pg. 100 in cliff's book.

[PMDRACER]

Thanks for the replies. I understand about the accelerator pump squirt 'filling the hole' as the throttle blades open and what-not, but if I give the car light throttle and keep it there, it will rattle and ping forever. It doesn't go away at all. I have to give it much heavier throttle to get up to the desired speed, then back off to cruise.

I need to find my notes on my timing curve.

[guccieng]

15 should be normal for light cruise and not pinging. that lean light cruise requires more timing; using vacuum advance?

[PMDRACER]

Yes, vacuum advance is hooked up and working. I'm pulling 10.5" of vacuum at idle in gear. My vacuum advance can starts at 11" and ends at 20", giving 13-14* of advance. Currently I'm set to use ported vacuum. I've used manifold vacuum also, but throtle response seems to be better this way.

I'm currently set at 14* initial, 22* mechanical using #139 weights and #364 center plate installed numbers down and medium springs. I'll borrow a buddy's dial-back timing light to get the full mechanical curve. Mine died.

Thanks for the help.

[guccieng]

not anywhere close to enough vacuum advance. i have 13 at idle, cruise at 20, and use a can that starts at 5 and is all in at only 8 in/hg. lean likes lots of timing. you shouldn't see a change in ported or manifold because your can doesn't even start 'til after idle anyway. when you're driving and slowly lay on the throttle, you're losing vacuum and the can is pulling it out.

[PMDRACER]

Originally Posted by guccieng

not anywhere close to enough vacuum advance. i have 13 at idle, cruise at 20, and use a can that starts at 5 and is all in at only 8 in/hg. lean likes lots of timing. you shouldn't see a change in ported or manifold because your can doesn't even start 'til after idle anyway. when you're driving and slowly lay on the throttle, you're losing vacuum and the can is pulling it out.

OK, so what can are you running and I'll give that a shot.

I can add more total vacuum advance. I was told by someone here to limit the static+mech+vac to 50*, but changing my limiter won't fix at what point it comes in.

[Kenth]

#17059553 have .040" idle downchannels and are way too small for a performance engine, enlarge these to .055".
Idle tubes are at .035 somewhat small too, i suggest .038" for this unit.
Idle discharge port may be opened to .095" for better control of the idle mixture.

All this will cure the off-idle surge and you may use .073" main jets instead of the #76 wich are somewhat large for this application and may contribute to your ping?

Try a setting of timing at 10-12° initial, 30-32° total at 4000rpm and around 15° from vacuum advance.

[PMDRACER]

Originally Posted by Kenth

#17059553 have .040" idle downchannels and are way too small for a performance engine, enlarge these to .055".
Idle tubes are at .035 somewhat small too, i suggest .038" for this unit.
Idle discharge port may be opened to .095" for better control of the idle mixture.

All this will cure the off-idle surge and you may use .073" main jets instead of the #76 wich are somewhat large for this application and may contribute to your ping?

Try a setting of timing at 10-12° initial, 30-32° total at 4000rpm and around 15° from vacuum advance.

Thank you, Kenth, but now I'm just not following where you are headed. The idle of the car is just fine, and I'm not sure what you mean by off-idle surge. Are you saying to enlarge all the idle areas so to richen things up before and as the carb transitions to the primary circuit?

I'm also confused by your jetting comment. You say to drop from #76 to #73 jets. Won't this lean out my carb even further? My cruise numbers now with 76/40K are in the 14.3-15.0 range. When I was at 74/40K my numbers were 16.7 to 17.4 and the car was a dog. Wouldn't leaning out the mixture make the pinging worse?

[Cliff R]

What is the rest of the engine combo? Static compression ratio, camshaft used, etc?

Does it ping at heavy or full throttle?

Does it ping if you plug the hose to the vacuum advance?....Cliff

[PMDRACER]

Originally Posted by Cliff R

What is the rest of the engine combo? Static compression ratio, camshaft used, etc?

Does it ping at heavy or full throttle?

Does it ping if you plug the hose to the vacuum advance?....Cliff

Intake/Exhaust
@ .006 285/290
@ .050 235/240
Lobe Lift .338/.331
Gross Lift @ 1.5:1 .507/.496
Lobe Separation 106 Deg
Installed Intake Centerline 102 Deg.

Compression is 9.6 to 1 or thereabouts
462 cid/TH400/3.36 gears
6X-4 heads, 255cfm, 1.5 rockers
TRW's w/Butler lightened pins, Eagle rods
HEI, MSD 6AL
Performer RPM intake
Doug's headers, 3" exhaust

Ping at heavy or full throttle? No
Ping if I disconnect the vacuum advance? Haven't tried that yet.


Again, thanks for the help.

[guccieng]

i use the eschlin vc1810, found at your local napa (for points type dizzys). here's a link to a nova site that has the famous 'vacuum advance 101' article, and a long list of vac cans with their specs, hei and points types. i forgot which one i used for my hei, but i used the charts to find the one that works.

http://www.stevesnovasite.com/forums...ad.php?t=11689

[PMDRACER]

Thanks again. Looks like I'll be ordering the AR15. Nobody seems to have this one in stock.

VC1843 AR15 3-5 7.5 @ 9-11

FWIW, I just happened to come across a pdf version of the chart here - http://www.corvetteclub.org.uk/files...ance_setup.pdf

[kyle_blake]

if I'm not mistaken the crane adjustable vac advance can work to adjust down to the VC1843??

Also a different carb might be a test you have time for?

Interesting problem, I wonder what will fix it?

[guccieng]

when it comes to choosing a vac can, look at where your power circuit comes on, like your secondaries (power valve opening for holleys). when that additional fuel is added, the advance is no longer needed, so try to time the vacuum can with the power circuit. for example, my vac can is all in at 8, and starts at 5. i chose a 8.5 power valve, so when the vacuum drops to 8.5, the fuel increases the afr, and as the vacuum drops even more toward WOT, the additional fuel will remain the same and the vacuum advance timing will pull out, and will be all out at 5 in/hg. this 'nail the throttle' thing happens pretty darn fast, but that's what seems to work well for me. i'd rather be rich for a moment in the transition from lean/lots of timing to rich/not as much timing.

[PMDRACER]

Originally Posted by Cliff R

Does it ping if you plug the hose to the vacuum advance?....Cliff

Just took it for a drive. With the vacuum advance disconnected the pinging went away. FWIW, I turned up the MSD timing controller +6 degrees and the pinging came back.

[guccieng]

wow, too much timing! i now see the 106 lsa on a 9.6cr. lot's of dynamic compression! i said earlier 'lean likes lots of timing', but 'lots of dynamic compression don't' overrides that, huh?! it's going to be tricky to figure that combo out... you might need a can that quits just below your cruise vacuum (if you need a can at all; that dc with iron heads and pump gas can be ping city!). is the motor zero-decked? what gas are you running? have you driven the car with a vac gauge installed?

[PMDRACER]

Originally Posted by guccieng

wow, too much timing! i now see the 106 lsa on a 9.6cr. lot's of dynamic compression! i said earlier 'lean likes lots of timing', but 'lots of dynamic compression don't' overrides that, huh?! it's going to be tricky to figure that combo out... you might need a can that quits just below your cruise vacuum (if you need a can at all; that dc with iron heads and pump gas can be ping city!). is the motor zero-decked? what gas are you running? have you driven the car with a vac gauge installed?

The motor was not zero-decked, although I wish it was from what I've read. I'm running the "California Special" 91 pump gas, but for special trips, I have mixed 5 gals of VP110 and it runs really nice that way.

I haven't put a vacuum gauge on this motor. What should I look for?

I have a buddy ordering the vc1843 through his work tomorrow but its going to take at least a week for it to get here. The closest one in stock (according to NAPA) is Detroit.