Home page
SPONSOR AD

Support our Vendors!  |  Advertise Here

Topic: q-jet tuning

in Forum: C3 Fuel, Emission Control, and Exhaust Systems


Already a Member?
Login
Not yet a Member?
Register for Free!

q-jet tuning (1/31)
 10/17/17 10:19pm
vince vette
Former Member

Send Private Message


Vette(s):
1980 L82 White with doeskin interior


Joined: 1/5/2016
Posts: 72

Sent the Q-jet from my '80 L82 off for professional rebuild to Cliff's high performance. They set it up appropriately for the L82 engine compression and cam. Once back it started OK but of course there was some need for getting the electric choke, high idle, curb idle adjusted. I have that all pretty well tuned in now but there are two outstanding issues. 

First, it takes about 45 seconds to a minute before tapping the pedal gets it to drop off the high idle cam notch.

Second, once it is fully warmed up and idling at around 650 (and sounding like it's ready to die as L-82's always do) if I place my hand over the open choke plate it will rev up another 300 to 400 rpm. I assume this indicates that it is running lean and thus I need to open up the idle mixture screws. But I would like some confirmation first. Thoughts?


______________
As it once looked and hopefully will again
Our Sponsors help support C3VR
Re: q-jet tuning (2/31)
 10/17/17 10:55pm
73shark
Limited Member
Send Private Message

Overland Park, KS - USA

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.


Joined: 7/9/2003
Posts: 864

Assuming that you used the factory service manual method to set the idle speed and mixture screws, the engine shouldn't act like it wants to die. Mine never did.  Try shooting some propane in and see what the idle does.  FWIW, the idle is supposed to be as lean as you can get it.

______________

1973 L-82 4 spd

Re: q-jet tuning (3/31)
 10/17/17 11:28pm
vince vette
Former Member

Send Private Message


Vette(s):
1980 L82 White with doeskin interior


Joined: 1/5/2016
Posts: 72

I'll take a look at it and see what it can do. 

But the fact is even from the factory it sounded like it wanted to die at idle even when warm (yes I bought it new). It didn't die, but always sounded close even at around 650 RPM. When cold and in gear if it was stopped with the brake on I always had to keep one foot on the brake and the other on the gas to keep it from stalling. If you turned the curb idle up to run cold then it roared at idle when warm. The late 70's and early 80's Q-jets had the idle mixture screw access holes plugged so you couldn't adjust the idle mixture. They were intentionally very lean both in setting and the size of the idle flow ports to help meet emission standards. 

But I'll take look at the manual, ignoring all the set this at 19.21 degrees and that at 0.002" height etc. and just work the adjustments. 


______________
As it once looked and hopefully will again
Re: q-jet tuning (4/31)
 10/18/17 10:35pm
73shark
Limited Member
Send Private Message

Overland Park, KS - USA

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.


Joined: 7/9/2003
Posts: 864

Did you talk to Cliff's to see if they had any suggestions?

______________

1973 L-82 4 spd

Re: q-jet tuning (5/31)
 10/19/17 9:04am
vince vette
Former Member

Send Private Message


Vette(s):
1980 L82 White with doeskin interior


Joined: 1/5/2016
Posts: 72

Yes I did. After rebuild they test all the carbs they do. He did say that the idle mixtures would likely need to be adjusted specifically for the engine. I did make an adjustment and got to the point where warmed up I could lightly cover the choke plate with virtually no change in RPM. After that I though I had the Accel 300+ multispark ignition module fail (another thread) and took that off. Now when warmed up there is this issue with the idle mixture. I find it hard to believe that just going from 3 sparkplug firings back to the traditional 1 would cause this change. 

Anyway, once warmed up so that the choke and high idle factors are all taken out of play, that leaves only curb idle and idle mixture adjustments. So I think I'll just focus on those for now. Then I'll worry about the start up circuits in the amazing air-fuel mixture control mechanical/fluid/electrically driven analog computer also known as the Quadrajet. It really is an amazing device. 


______________
As it once looked and hopefully will again
Re: q-jet tuning (6/31)
 10/19/17 9:08am
vince vette
Former Member

Send Private Message


Vette(s):
1980 L82 White with doeskin interior


Joined: 1/5/2016
Posts: 72

PS - the trouble with making idle mixture adjustments is that when it's running it's virtually impossible to get to the screws with all the vacuum hoses and temp-vac switches sitting in front of carb. I'm working on making a tool that can thread through maze and then engage the oddball heads on the mixture screws. 

______________
As it once looked and hopefully will again
Re: q-jet tuning (7/31)
 10/19/17 12:58pm
Adams' AppleLifetime Member
Lifetime Member
Moderator
Send Private Message

Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas


Joined: 11/8/2003
Posts: 20126

 Most parts houses will have a tool for adjusting the idle mixture screws. Get ya one of these....


It's flexible, and has the "D" shape on one end, and a rectangular shape on the other. It will fit most all later model quadrapukes. Fairly cheap, too.

______________

Joel Adams
C3VR Lifetime Member #56    

My Link


(click for Texas-sized view!)
             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Re: q-jet tuning (8/31)
 10/20/17 11:59am
nosal1
Former Member

Send Private Message

Dunedin, FL - USA

Vette(s):
1980 Corvette Torch Red with Gold metallic 370HP/350 CU.IN.Turbo 350 w/shift kit.Originally CA.Car with 305 CU.IN.


Joined: 3/21/2006
Posts: 262

Vince Vette Good Morning,Quick question.What is the amount of vacuum do you have..It should be at 14 degrees mercury to a max of 18.You should have vacuum port behind carb to get max reading of vacuum.Let me know what you have now..

Sal C                                 nosal
Re: q-jet tuning (9/31)
 10/20/17 2:11pm
nosal1
Former Member

Send Private Message

Dunedin, FL - USA

Vette(s):
1980 Corvette Torch Red with Gold metallic 370HP/350 CU.IN.Turbo 350 w/shift kit.Originally CA.Car with 305 CU.IN.


Joined: 3/21/2006
Posts: 262

Vince Vette a correction on my email on vacuum,Should read

14 inches of vacuum not degrees..Sorry..........Sal C
Re: q-jet tuning (10/31)
 10/20/17 2:19pm
vince vette
Former Member

Send Private Message


Vette(s):
1980 L82 White with doeskin interior


Joined: 1/5/2016
Posts: 72

Holding my thumb on the port for 30 seconds turns it a light purple color - that should be 13 inches Smile. Sorry, I couldn't help it. I'll have to dig up a gage somewhere to find out. 

I should preface all these questions with one caveat. My objective on this engine, which is the original L82 engine in the car with 90,000 miles, is to simply get it running acceptably to drive it places to get it to an inspection station first (been 13 years) and then some other shops to swap our the transmission for a 700/R4 and get the rear gears changed over to 3.33 or 3.55. After that and when the bank account moves back up the engine will be replaced with 383 stroker and I'll then at a very slow pace to a rebuild on the L82 myself. So mainly with my current engine work I just want to make sure I don't damage it between now and swapping it out. 


______________
As it once looked and hopefully will again
Re: q-jet tuning (11/31)
 10/20/17 4:16pm
nosal1
Former Member

Send Private Message

Dunedin, FL - USA

Vette(s):
1980 Corvette Torch Red with Gold metallic 370HP/350 CU.IN.Turbo 350 w/shift kit.Originally CA.Car with 305 CU.IN.


Joined: 3/21/2006
Posts: 262

Vince Vette,after weeks of investigation of trans swapping I have found swapping T350/400 with a 200 R 4 is a better choice.The 200 R 4 is the same length as both 350/400 and will sit perfectly in place without cutting drive shaft and other changes..Using a 700R4 entails to much cutting including drive shaft..Good luck in whatever you do..

Sal C                       nosal
Re: q-jet tuning (12/31)
 10/21/17 5:23pm
vince vette
Former Member

Send Private Message


Vette(s):
1980 L82 White with doeskin interior


Joined: 1/5/2016
Posts: 72

I'm probably dealing with some other issue affecting my difficulty getting a good start and transition to idle. I got a vacuum gage and hooked it up to a manifold vacuum port. Started the car. It kicked on OK and kicked off the high idle nicely. Then went to a very low after about 30 or 40 seconds. By very low I mean it would quit if put into gear. Then after a long period like that, perhaps another 40 to 60 seconds it settled in at 800 RPM in park. During this entire time the vacuum gage was fluctuating rapidly over a range of 3 to 4 in Hg. I've posted two photos one of the 600 and the other of the 800 RPM readings. 

So, where do we start trouble shooting this?


______________
As it once looked and hopefully will again
Re: q-jet tuning (13/31)
 10/21/17 5:50pm
vince vette
Former Member

Send Private Message


Vette(s):
1980 L82 White with doeskin interior


Joined: 1/5/2016
Posts: 72

PS - I do hear a slight knock, but not sure if it's real. Does not sound like a lifter. I had a stuck lifter in the past and it was much more obvious. 

|UPDATED|10/21/2017 2:50:59 PM (AZT)|/UPDATED|


______________
As it once looked and hopefully will again
Re: q-jet tuning (14/31)
 10/21/17 5:47pm
73shark
Limited Member
Send Private Message

Overland Park, KS - USA

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.


Joined: 7/9/2003
Posts: 864

Was your vacuum gauge hooked directly to manifold vacuum and not one of the carburetor modified ports? The vacuum at idle should be much higher than that and steady.

______________

1973 L-82 4 spd

Re: q-jet tuning (15/31)
 10/21/17 6:49pm
vince vette
Former Member

Send Private Message


Vette(s):
1980 L82 White with doeskin interior


Joined: 1/5/2016
Posts: 72

It was hooked to a three way manifold vacuum port. One hooked into a metal line (not the power brakes). I believe the one I was hooked into went to the cruise xducer - but that is laying in drawer so the other end of the line is plugged. 

So I swapped the gage over to a rear port on the base of the carb. This feeds into a couple different lines off a tee. I teed the gage line into that so I could keep whatever else they ran going as the vac advance ties into it. This source had some fluctuation also, but a lot less. The reading though was the about the same. I also ran the throttle up a couple times. I noticed when I opened it quickly the pressure dropped to 10" before rising. I ran up to between 2000 and 2500 RPM (by ear). I held about 20" but when I let it drop at first it rose up to 21 or 22".

Note, this engine is 37 years old with a little over 90,000 miles. It sat for the last 13 years being started or once or twice per year. 




|UPDATED|10/21/2017 3:49:12 PM (AZT)|/UPDATED|


______________
As it once looked and hopefully will again
Re: q-jet tuning (16/31)
 10/21/17 6:54pm
nosal1
Former Member

Send Private Message

Dunedin, FL - USA

Vette(s):
1980 Corvette Torch Red with Gold metallic 370HP/350 CU.IN.Turbo 350 w/shift kit.Originally CA.Car with 305 CU.IN.


Joined: 3/21/2006
Posts: 262

It looks like a valve adjustment is in order.Looks like possibly too loose of the rockers..As have been mentioned the VAC gauge should be at a minimum of  14 in.vac and steady,no flocculation and may stop the "clacking"noise.

Sal C                                 NOSAL

Re: q-jet tuning (17/31)
 10/21/17 7:19pm
vince vette
Former Member

Send Private Message


Vette(s):
1980 L82 White with doeskin interior


Joined: 1/5/2016
Posts: 72

Could well be. I replaced the valve seals back around 2000. Replaced the rockers with compcam roller tip rockers - didn't want to deal with higher covers roller rockers would have required as it was all I could accomplish to fit the covers under the air injector hoses (which have gotten lost somewhere since then). The rockers had a slight bump in ratio to 1.52 from 1.50. Their geometry pushed tips rocker tips off center so I did change out the pushrods to something a little longer longer I believe. 

As far as adjusting them I followed the shop manual which was rotate the engine until a given cylinder was top dead center and then you could adjust intake cyl X and exhaust cyl Y, etc. Per the manual the adjust was tighten until zero lash and then loosen a certain amount. This was obviously all done statically. One thing I do recall was the compcam stud nuts were so damned tight I thought I would snap the studs so I used the stock nuts instead. 

Any thoughts on all of that. 



______________
As it once looked and hopefully will again
Re: q-jet tuning (18/31)
 10/22/17 1:08am
73shark
Limited Member
Send Private Message

Overland Park, KS - USA

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.


Joined: 7/9/2003
Posts: 864

I've found the best way to adjust hydraulic lifters is the engine running method. Just loosen the lifter until the engine starts to run rough and then tighten 3/4 turn. Be sure to use the oil stoppers on the rockers to prevent a mess. This method can highlight a bad cylinder. If this doesn't help, then need to do a compression check and a leakdown test.

|UPDATED|10/21/2017 10:08:34 PM (AZT)|/UPDATED|


______________

1973 L-82 4 spd

Re: q-jet tuning (19/31)
 10/29/17 6:20pm
vince vette
Former Member

Send Private Message


Vette(s):
1980 L82 White with doeskin interior


Joined: 1/5/2016
Posts: 72

No luck with the valve readjustment. Set at 3/4 turn past zero lash. '80 shop manual says 1 turn which is what I adjusted them to last time. So I backed off a little as the manual makes no allowance for the different cam in the L82 and my recollection is a cam with more overlap ought to be backed off a little. I admit I didn't feel like spraying oil around so I didn't adjust them running. Anyway, a couple hours spent and no change. 

|UPDATED|10/29/2017 3:20:35 PM (AZT)|/UPDATED|


______________
As it once looked and hopefully will again
Re: q-jet tuning (20/31)
 10/30/17 10:39am
nosal1
Former Member

Send Private Message

Dunedin, FL - USA

Vette(s):
1980 Corvette Torch Red with Gold metallic 370HP/350 CU.IN.Turbo 350 w/shift kit.Originally CA.Car with 305 CU.IN.


Joined: 3/21/2006
Posts: 262

Vince, Sorry to hear your adjustment did solve the problem.May I make a suggestion?? with cap on distributor,mark the base (where cap meets dist.) with a very bright marker below each spark plug post.Remove #1 plug and have someone "bump" starter until you are at top dead center #1(air should push thumb off spark plug hole).Remove cap and note position of rotor..It should be pointed at mark on base of distributor.Now you are ready to adjust valves at TPC #1..Do your adjustments as you did before.Get to zero lash then 3/4 turn.Rotate crank to next in firing order(#8),and continue on forward. Good Luck... Sal C 

Re: q-jet tuning (21/31)
 10/30/17 12:10pm
vince vette
Former Member

Send Private Message


Vette(s):
1980 L82 White with doeskin interior


Joined: 1/5/2016
Posts: 72

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
Re: q-jet tuning (22/31)
 10/30/17 10:05pm
73shark
Limited Member
Send Private Message

Overland Park, KS - USA

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.


Joined: 7/9/2003
Posts: 864

I'm guessing that you don't want to do it with the engine running method.

______________

1973 L-82 4 spd

Re: q-jet tuning (23/31)
 10/30/17 10:05pm
73shark
Limited Member
Send Private Message

Overland Park, KS - USA

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.


Joined: 7/9/2003
Posts: 864

I'm guessing that you don't want to do it with the engine running method.

______________

1973 L-82 4 spd

Re: q-jet tuning (24/31)
 10/30/17 11:05pm
vince vette
Former Member

Send Private Message


Vette(s):
1980 L82 White with doeskin interior


Joined: 1/5/2016
Posts: 72

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
Re: q-jet tuning (25/31)
 10/31/17 6:23am
nosal1
Former Member

Send Private Message

Dunedin, FL - USA

Vette(s):
1980 Corvette Torch Red with Gold metallic 370HP/350 CU.IN.Turbo 350 w/shift kit.Originally CA.Car with 305 CU.IN.


Joined: 3/21/2006
Posts: 262

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 

Re: q-jet tuning (26/31)
 11/8/17 6:55pm
vince vette
Former Member

Send Private Message


Vette(s):
1980 L82 White with doeskin interior


Joined: 1/5/2016
Posts: 72

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
Re: q-jet tuning (27/31)
 11/9/17 7:10am
nosal1
Former Member

Send Private Message

Dunedin, FL - USA

Vette(s):
1980 Corvette Torch Red with Gold metallic 370HP/350 CU.IN.Turbo 350 w/shift kit.Originally CA.Car with 305 CU.IN.


Joined: 3/21/2006
Posts: 262

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
Re: q-jet tuning (28/31)
 11/9/17 10:35am
73shark
Limited Member
Send Private Message

Overland Park, KS - USA

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.


Joined: 7/9/2003
Posts: 864

Won't the idle speed increase when you hook the vacuum advance back up?

______________

1973 L-82 4 spd

Re: q-jet tuning (29/31)
 11/9/17 11:39am
nosal1
Former Member

Send Private Message

Dunedin, FL - USA

Vette(s):
1980 Corvette Torch Red with Gold metallic 370HP/350 CU.IN.Turbo 350 w/shift kit.Originally CA.Car with 305 CU.IN.


Joined: 3/21/2006
Posts: 262

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  
Re: q-jet tuning (30/31)
 11/9/17 1:39pm
vince vette
Former Member

Send Private Message


Vette(s):
1980 L82 White with doeskin interior


Joined: 1/5/2016
Posts: 72

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
Re: q-jet tuning (31/31)
 11/10/17 7:48am
nosal1
Former Member

Send Private Message

Dunedin, FL - USA

Vette(s):
1980 Corvette Torch Red with Gold metallic 370HP/350 CU.IN.Turbo 350 w/shift kit.Originally CA.Car with 305 CU.IN.


Joined: 3/21/2006
Posts: 262

Good Morning Vince,The metering vacuum port is as you say on the driver side of the carb and a little passed center on the carb and the full vacuum ports are in the base plate of the carb.You should be able to reduce idle to 700 rpm and then adjust air/fuel primaries.The vacuum gauge of total vac( 14-16 inches)may fall a little and if so advance distributor to get back to the vacuum you had and again reduce idle to where it my die or at least 700 or less.As you advance the timing so does the curb idle.Let me know how you do,good luck.          Sal C             nosal1 

Our Sponsors help support C3VR