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Author Topic: Question about Q Jet Tuning  (Read 3265 times)

Offline Aprules2

  • Posts: 169
  • Name: A Perna
Question about Q Jet Tuning
« on: May 05, 2014, 11:36:39 AM »
I have a 75 Coupe Deville, I just got a rebuilt carburetor from a company online. The numbers decode out to a 75 Cadillac so I'm pretty sure its the right carb. I installed it and changed my spark plugs. Once the car warmed up it seemed seemed real rich smelling at at idle so I turned the mixture screws in one at a time until they just bottomed out and brought them out 2 1/2 turns. The car ran very rough  and idled around 700 RPM with only 18 inches of vacuum. I started on the drivers side and brought the mixture screw out a 1/4 turn, then turned the passenger one a 1/4 turn, until I got a nice high vacuum its now at 24 inches at 750 rpm and idles fairly smooth. The screws now are out 4 1/4 turns which I always thought was a lot for a qjet. The problem is if I accelerate quickly let off and accelerate again I see the generator light come on, so the rpms must be dipping down a little too low at times. The best example is I live on a busy road so when I back out I back up quick stop then put it in drive and accelerate kinda quick. when stopping from reverse you can see the light flicker. I sprayed around with carb cleaner and found one vacuum leak which I fixed but that's it. The timing is set to spec with the vacuum advance disconnected. Ive heard of guys adjusting the difficult ones with all the vacuum lines plugged except the booster, should I try that?

Offline Aprules2

  • Posts: 169
  • Name: A Perna
Re: Question about Q Jet Tuning
« Reply #1 on: May 05, 2014, 01:38:46 PM »
Forgot to ad that the exhaust smells rich and is so strong that after I walk around the car and come inside I actually smell like I've been at the track racing all day. Oddly enough my highway mileage has increased but my city is somewhere around 3-5mpg...

Offline Dave Shepherd

  • Posts: 2425
Re: Question about Q Jet Tuning
« Reply #2 on: May 05, 2014, 04:58:57 PM »
What you are doing is only adjusting the idle mixture 4 turns out is very rich, but would not affect cruise mixture. I think there is an issue with the carb internally make sure the secondaries are closed completely also. How did it run before the overhaul?

Offline Aprules2

  • Posts: 169
  • Name: A Perna
Re: Question about Q Jet Tuning
« Reply #3 on: May 05, 2014, 06:00:29 PM »
When i bought my car it had a rebuilt 76 carb installed by the previous owner on it. It was rebuilt around 30 years ago. When i opened it, it was squeeky clean like a new carb. So I put a kit in it but never took it off the manifold.  My car always smelled rich and had a bog when you pushed the pedal from a stop. It had a slightly rough idle and I could never get it totally in tune. I removed it to get it rebuilt and found out someone had massively over torqued the mixture screw on the drivers side and actually cracked the casting. I called up a company off the internet and ordered a rebuilt carb.Now it runs smoother but its incredibly rich at idle but when set leaner it runs pretty rough. I'm wondering could a misadjusted piston be the cause? This model has the little cap on the passenger side front on top of the carb. Under it is a flat head screw you can adjust. When I received the carb the plug was missing. I got one from the local rebuilder and bolted the carb on.

Offline Dave Shepherd

  • Posts: 2425
Re: Question about Q Jet Tuning
« Reply #4 on: May 05, 2014, 07:15:03 PM »
Pretty sure you have an egr valve on this, no?  If so make sure it is completely seated, any by pass at idle will lean the mixture cause a rough idle and hesitation.

Online The Tassie Devil(le)

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  • CLC Number: 18992
  • Name: Bruce Reynolds
Re: Question about Q Jet Tuning
« Reply #5 on: May 05, 2014, 07:47:21 PM »
When I adjusted the mixture screws on my '72 Eldorado, according to the '72 Shop Manual, I ended up getting it right, and upon seeing just how far I wound them out, it turned out to be around 6 turns.

The Manual actually mentions the number "6 turns" in part of what I read.

One of the reasons I had to completely re-tune it was I had totally rebuilt the engine, and that included using parts of two carbies to make one.   The bast plate on the original one had worn throttle shafts, and was letting in air from the side, and the other carb was in a lot better condition, but missing parts at the top.

It is surprising that some people will always revert to the "wind in, then back out 1 1/2 turns" without actually thinking.   That was, and still is as far as I know, the base setting for Holley Carbies, and therefore MUST be right for everything else.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

Offline Scot Minesinger

  • Posts: 6004
Re: Question about Q Jet Tuning
« Reply #6 on: May 05, 2014, 10:09:57 PM »
The 76 carb has an electric choke so that is a popular way to go, and you can only use 76 carb parts to fix it (different year carbs work on 472/500, but the carb must use same year parts).  Is there a dash-pot on this?  There is on 1970 which kept rpm from dropping too quick on high throttle to low throttle.  The EGR and adjustments area good place to start-agreeing with other posts.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

Offline Julien Abrahams

  • Posts: 473
  • Name: J. Abrahams
Re: Question about Q Jet Tuning
« Reply #7 on: May 06, 2014, 02:46:10 AM »
I have  '67, so I have no experience with the EGR valve. But, I have also found that working with a tachometer and tuning each idle mixture screw at a time, adjusting so that you get the highest idle gives very good results. Do one screw at a time. If the idle is too high after tuning, then adjust the idle screw. The 1 and 1/2 turn is the basic setting when the idle mixture screws have been removed. Good luck.
1967 Cadillac Sedan De Ville HT
1969 Austin Healey Sprite
1979 Opel Kadett

Offline Aprules2

  • Posts: 169
  • Name: A Perna
Re: Question about Q Jet Tuning
« Reply #8 on: May 06, 2014, 09:57:06 AM »
Its funny back 12 years ago I used to go to an older guy who used to rebuild my carbs for me he told 2-2 1/2 turn is your base line and then you adjust each screw equally from there. But then my friends fathers who were old hot rodders told me to do it the way J Abrams is saying unscrew each screw one at a time to get highest idle. What is the way Cadillac recommended?

Scot 75 actually also has an electric choke, from what I understand there is an extra power piston in 75 model year carbs that helps regulate the mixture. They did away with it in 76 so the 76 carb actually makes more power, but the 75 will yield more MPG. From having both carbs on my previous 75 I can tell you the 75 yields at least 4 mpg more and the hp difference on the butt dyno isn't very much.

How do I check the EGR valve? I remember my carburetor guy just lifting the diaphragm and watching the idle drop. Is that the best way or is there a better one?

Offline Scot Minesinger

  • Posts: 6004
Re: Question about Q Jet Tuning
« Reply #9 on: May 06, 2014, 12:23:04 PM »
EGR valve first check to be sure it holds vacuum and it is not ruptured.  The other indicator is that the car looses power only after it is well warmed up (at least 25 minutes) very noticeable uphill at 50 miles an hour, symptomatic of a vacuum leak, but not.  Sorry about electric choke, that was told to me and I must have misconstrued.  If in doubt and it is original, consider replacing it with a quality new one, not used.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

Offline Aprules2

  • Posts: 169
  • Name: A Perna
Re: Question about Q Jet Tuning
« Reply #10 on: May 06, 2014, 11:23:25 PM »
Is there a way to see if the EGR is all carboned up and its not seating without removing it from the manifold? The last couple I did, were a huge pain to remove. I had to heat the bolts until they glowed then break them loose a little at a time.

Re: Question about Q Jet Tuning
« Reply #11 on: May 07, 2014, 12:07:57 AM »
An inspection and cleaning of the EGR should be a regular maintenance task.  It doesn't take much to clog up the passage and keep the valve from closing.   The answer is no, you just have to remove the two bolts.  When you have the EGR out you can of course see if it is actually functional by applying vacuum to the port on the actuator and watching ther valve open and close.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

Offline Aprules2

  • Posts: 169
  • Name: A Perna
Re: Question about Q Jet Tuning
« Reply #12 on: May 09, 2014, 08:25:44 PM »
I worked on the car a little more today trying to diagnose it. I checked everything a second time, and then started messing with things. I plugged the hose going from the carb to the PCV valve and I noticed i was able to set the carb much easier. I had it set with each screw between 2 1/4 and 2 1/2 turns and it idled pretty smooth with 22 inches of vac. Then I put everything back together and it idled like crap the vacuum was bouncing up and down, between 21 and 22. So I tried tuning it again and it just wants me to set it very rich. it starts to idle better when I hit 2 3/4 turns and idles best at 4. I know you need to run PCV I just dont know what to do about it..... I think Ill pick a new one up tomorrow to see what happens.

Online The Tassie Devil(le)

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  • CLC Number: 18992
  • Name: Bruce Reynolds
Re: Question about Q Jet Tuning
« Reply #13 on: May 09, 2014, 09:12:57 PM »
The only vacuum supply you need to plug is the one to the Distributor.

But, seeing as there could be a problem with the PCV Valve, next time I have to tune my car, I will block off that, and see what happens.

Thanks for the idea.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

Offline Julien Abrahams

  • Posts: 473
  • Name: J. Abrahams
Re: Question about Q Jet Tuning
« Reply #14 on: May 10, 2014, 07:17:16 AM »
Is it possible that your PCV is stuck in a position (open or closed)? My manual states that if the PCV valve is not functioning properly, this can lead to rough idle conditions you describe. As you say that the carb is much easier to tune when the PCV hose is disconnected, I would see if changing the PCV-valve makes a difference (and I think its a cheap part, not sure though).
1967 Cadillac Sedan De Ville HT
1969 Austin Healey Sprite
1979 Opel Kadett

Offline Aprules2

  • Posts: 169
  • Name: A Perna
Re: Question about Q Jet Tuning
« Reply #15 on: May 10, 2014, 10:42:43 AM »
Tassie Devil(le): I only plugged it because the car was running like it had a vacuum leak and I couldnt find it. So started to test all of the diaphragms with my Mity Vac and they all held vacuum.  Then I thought maybe one of the new lines had a tiny hole or tear in it or something so one at a time I started plugging vacuum lines and seeing if I found a difference in idle quality and vacuum. Eventually I got to the PCV and my vacuum got much steadier.

When you set the carb are you supposed to disconnect the Vacuum Advance an plug it? Ive always left it Connected..

Julien Abrams: I am going to order one from the store today. I havent bought one in a few years but I figure it cant be more than $10.
« Last Edit: May 10, 2014, 08:04:04 PM by Aprules2 »

Online The Tassie Devil(le)

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  • Name: Bruce Reynolds
Re: Question about Q Jet Tuning
« Reply #16 on: May 10, 2014, 08:41:54 PM »
.......When you set the carb are you supposed to disconnect the Vacuum Advance an plug it? Ive always left it Connected..
Yes, the Workshop Manual for the carburettored engine says, and I quote:

"NOTE:   Before idle mixture screw adjustment is made, distributor vacuum advance hose must be disconnected at the distributor and plugged to eliminate the possibility of advancing the timing and increasing engine rpm.

1. Disconnect parking brake vacuum hose at the vacuum release cylinder and plug.   Disconnect air levelling compressor hose at air cleaner and plug hose (but yours will have the Electric ALC?),  Remove air cleaner, but leave vacuum hoses connected."

It goes on to mention more, but my finger is getting tired from typing.

Do you have the Workshop Manual?

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

Offline Aprules2

  • Posts: 169
  • Name: A Perna
Re: Question about Q Jet Tuning
« Reply #17 on: May 11, 2014, 10:20:41 AM »
Thanks Bruce. No I don't anymore. I had both a GM and a Haynes manual but during one of the storms our power went out, the sump pump didn't come on, and flooded the basement. I lost a lot of gm factory manuals, Haynes, Fisher and Chilton manuals I had collected over the years. I only have the 75 Shop supplement left I need a 74 book for the majority of the info.

Oh an my 75 does still have the vacuum level control. But right now the compressor is out and the vacuum line is capped. Im in the process of having diaphragms made.
« Last Edit: May 11, 2014, 10:23:47 AM by Aprules2 »

Offline Hankk17

  • Posts: 255
  • Name: Hank Kuczynski
Re: Question about Q Jet Tuning
« Reply #18 on: May 11, 2014, 11:43:36 AM »
Aprules - shoot me a private message with your e-mail - I've got electronic versions of the '76 manual which should at least help a little.  I'll send you a link to my dropbox so you can download them.

-Hank
Wherever you go... There you are

Offline Viator Trudeau

  • Posts: 332
Re: Question about Q Jet Tuning
« Reply #19 on: May 11, 2014, 11:09:09 PM »
Hello again::   one of the post in this group talks about the secondaries leaking air, could be causing bad idle??.  how tightly closed are they to be?  They are spring loaded closed.  Can they reground for a air tight closure?  Do rebuilders look into this?  I see where loose shaft bores leak and give problems.

V. Trudeau  #8697



V.  Trudeau  #8697

 

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