Topic: q-jet tuning
in Forum: C3 Fuel, Emission Control, and Exhaust Systems
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Prior to more internal work, I plan to readjust the idle mixture again. The idle at the time was too high so there's a good chance it is off. After that, I'm not sure how much work I'll do on the engine. My only objective was to get it running well enough to drive it somewhere to get it pulled and replaced. Then I'll just crate it and let it sit until I'm retired and have the time to rebuild it at my leisure.
I am though curious about your recommendation. I'd like to understand the different approaches I've seen recommended for adjusting valves. The shop manuals use the approach of turning the engine by hand until the timing mark is at 0 deg. That then has either cylinder 1 or 6 at TDC. If it's 1, then adjust #1 intake and adjust, and 6 other valves, one each on cylinders 2, 3, 4, 5, 7, and 8 - a mix of intake and exhaust which I haven't memorized nor will I ever. Then rotate the engine another full revolution to bring 6 to TDC and adjust #6 int and exh and whatever 6 you didn't do on the first round.
The second approach I've seen is to go to #1 TDC and adjust the intake of 1 and the exhaust of some other cylinder. Then rotate 90 deg to the next cylinder is TDC (8 if memory serves) and adjust its intake and then the exhaust on some other. And then just keep repeating.
Your recommendation is kind of a mix of the two in that it adjusts 2 valves each time a piston is at TDC, but eh two are the intake and exhaust of the TDC cylinder.
Any guidance you could provide for the different approaches, or at least for doing each cylinder individually vs. setting 8 valves each time the timing mark is at 0 deg would be appreciated. Also, I'm curious about basing #1 at TDC on the distributor position rather then the timing mark.
Thanks,
As it once looked and hopefully will again
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Overland Park, KS - USA
Joined: 7/9/2003
Posts: 914
Vette(s): 1973 Orange Metallic Coupe (orig owner), L82, 4 spd (WR), PS, (A/C & PW (I installed from wrecked 73)), leather, AM/FM Stereo, ran with '65 FI unit earlier & will again some day.
2023 Accelerate Yellow HTC Stingray
I'm guessing that you don't want to do it with the engine running method.
1973 L-82 4 spd
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Overland Park, KS - USA
Joined: 7/9/2003
Posts: 914
Vette(s): 1973 Orange Metallic Coupe (orig owner), L82, 4 spd (WR), PS, (A/C & PW (I installed from wrecked 73)), leather, AM/FM Stereo, ran with '65 FI unit earlier & will again some day.
2023 Accelerate Yellow HTC Stingray
I'm guessing that you don't want to do it with the engine running method.
1973 L-82 4 spd
yeah, that's the case. Basically I'm just trying to get this running OK until I replace the engine, trans, and rear end gears. So I'm not killing myself on it.
As it once looked and hopefully will again
Good Morning Vince,Yes I been doing valves this way for at least 40 years and have done 1165 engines recorded including my '72 Chevy truck with Vette engine with now 130,000 mi. on the engine and have not a loose valve etc.,and it is less of mess and more accurate..Take Care. Sal C nosal1
Finally back to the game. I decided to readjust the idle mixture. I think previously the idle itself was too high when I did it and it may have been that the throttle was opened so much it wasn't terribly sensitive to the mixture adjustment. So after it warmed up I adjusted the curb idle to 800 RPM in park and then started on the mixture adjustments. When it was done I the vacuum was up to 15 to 16 inches, so a gain of about 2" vs. the first run.
The next question, now that this seems better, any thoughts on whether I should redo the adjustment with the tranny in drive?
As it once looked and hopefully will again
Good morning Vince, Great vacuum you have,however you must make all operating air fuel adjustments at the lowest RPM,i.e. about 600-700 rpm at operating temp with air cleaner off.then check again with air cleaner on carb.Be sure that the vac line to distributor vac advance is plugged off at carb... Hope this helps..
Sal C nosal1
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Overland Park, KS - USA
Joined: 7/9/2003
Posts: 914
Vette(s): 1973 Orange Metallic Coupe (orig owner), L82, 4 spd (WR), PS, (A/C & PW (I installed from wrecked 73)), leather, AM/FM Stereo, ran with '65 FI unit earlier & will again some day.
2023 Accelerate Yellow HTC Stingray
Won't the idle speed increase when you hook the vacuum advance back up?
1973 L-82 4 spd
Vince,If the idle is at 600-700 and idle vacuum is at the rate you stated earlier,you will have no vacuum on the metering outlet used for the vacuum advance (be careful there is 2 outlets on a all carb ( I forgot did you tell what carb you have. Let me know) SBC vac advance generally start opening at 7 - 9 inches of vacuum.You should have not vacuum at idle on metering port until you start to accelerate or increase from idle to 700 + rpm and full vacuum on total vac port.I hope this helps,let me know what carb and how you make out. Sal C nosal1
Thanks for the continuing advice. It's no problem having the air cleaner off during adjustment, in fact for me it's pretty much required. Between the size of the pan and the snorkels I doubt I could even get the screw adjuster tool onto the screws with the pan on.
Concerning having the vacuum advance off, if the car is idling 800 to 900 warmed up in park with the vac advance on and then the vac advance is pulled off, keeping it running may become problematic. I'll need to check. The carb is a Q-jet. There may be two possible sources for vac to the distributor. One is definite coming from the front driver's side of the carb through a TVS and then to the distributor. The other possible one is from the back of the carb to a TVS inside the air cleaner housing and which controls one of the air cleaner snorkel baffles and which branches off through a delay valve before joining into the first line. The question of what vacuum comes to the advance depends on whether these TVS's are open or closed when hot.
But, expect a follow up this weekend.
As it once looked and hopefully will again
in Forum: C3 Fuel, Emission Control, and Exhaust Systems
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