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10/17/17 9:19pm - Original Message: 'q-jet tuning '
vince vette
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1980 L82 White with doeskin interior

Joined: 1/5/2016
Posts: 60
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?


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10/17/17 9:55pm - Reply: 'Re: q-jet tuning '
73shark
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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: 394
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.

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1973 L-82 4 spd

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10/17/17 10:28pm - Reply: 'Re: q-jet tuning '
vince vette
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1980 L82 White with doeskin interior

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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. 


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10/18/17 9:35pm - Reply: 'Re: q-jet tuning '
73shark
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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: 394
Did you talk to Cliff's to see if they had any suggestions?

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1973 L-82 4 spd

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10/19/17 8:04am - Reply: 'Re: q-jet tuning '
vince vette
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1980 L82 White with doeskin interior

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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. 


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10/19/17 8:08am - Reply: 'Re: q-jet tuning '
vince vette
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1980 L82 White with doeskin interior

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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. 

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10/19/17 11:58am - Reply: 'Re: q-jet tuning '
Adams' Apple Lifetime Member
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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

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 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.

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10/20/17 10:59am - Reply: 'Re: q-jet tuning '
nosal1 Gold Member
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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: 237

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


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10/20/17 1:11pm - Reply: 'Re: q-jet tuning '
nosal1 Gold Member
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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: 237
Vince Vette a correction on my email on vacuum,Should read

14 inches of vacuum not degrees..Sorry..........Sal C


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10/20/17 1:19pm - Reply: 'Re: q-jet tuning '
vince vette
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Vette(s):
1980 L82 White with doeskin interior

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Posts: 60
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. 


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10/20/17 3:16pm - Reply: 'Re: q-jet tuning '
nosal1 Gold Member
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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: 237

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


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10/21/17 4:23pm - Reply: 'Re: q-jet tuning '
vince vette
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Vette(s):
1980 L82 White with doeskin interior

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Posts: 60
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?


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10/21/17 4:50pm - Reply: 'Re: q-jet tuning '
vince vette
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1980 L82 White with doeskin interior

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Posts: 60
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|


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10/21/17 4:47pm - Reply: 'Re: q-jet tuning '
73shark
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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: 394
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.

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1973 L-82 4 spd

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10/21/17 5:49pm - Reply: 'Re: q-jet tuning '
vince vette
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1980 L82 White with doeskin interior

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Posts: 60
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|


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10/21/17 5:54pm - Reply: 'Re: q-jet tuning '
nosal1 Gold Member
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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: 237

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



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10/21/17 6:19pm - Reply: 'Re: q-jet tuning '
vince vette
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Vette(s):
1980 L82 White with doeskin interior

Joined: 1/5/2016
Posts: 60
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. 



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10/22/17 12:08am - Reply: 'Re: q-jet tuning '
73shark
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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: 394
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|


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1973 L-82 4 spd

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10/29/17 5:20pm - Reply: 'Re: q-jet tuning '
vince vette
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Vette(s):
1980 L82 White with doeskin interior

Joined: 1/5/2016
Posts: 60
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|


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10/30/17 9:39am - Reply: 'Re: q-jet tuning '
nosal1 Gold Member
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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: 237

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 



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