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#1 |
Registered User
Join Date: Jul 2011
Location: vacaville ca
Posts: 1,304
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Q-Jet adustment issues, jets?
Ok all, I got the vacuum hooked up to the truck, turned the jets all the way in the 1.5 turns out. Was at about 14 on the vacuum. Started turning out the jets and got to about 16 and kepts going. The right side jet came out, so the engine stumbled. Put it back in and went in until vacuum dropped which was about all the way closed then adjust out again. Decided to try left side, turned jet out until I was at 15 on vacuum. Now here is the kicker, I could not get the truck to stumble until the jets were just about closed and then again I could not get the truck to stumble until the jets came all the way out.... so my question is this, why can't I get the jets to come all the way out before I see and stumble in motor. There has to be something wrong? I replaced the filter and gasket, replaced all the vacuum lines... truck seems to runn pretty good. Timing set at 12-14. So what I ended up doing was giving it full throttle by hand and letting it off fast, motor would stumble then recover to idle, I figured it needed more fuel so I adjusted the jets out a 1/4 turn each until i could give full throttle and let off with out the motor stumbling and recovering to idle very quick and smooth.....
So why is it i can turn the jets all the way closed and get a stumble or engine about to die but I can take the jets all the way out with no rpm change untiol they fall out and motor dies..... shouldn't there be a point when the motor stumbles before the jets back all the way out? Sorry to ramble but I am frustrated
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04 CHEVY LLY LS DURAMAX 4WD. MODS TO FOLLOW, EFI LIVE, BUILT TRANS, LBZ MP, LIFT PUMP SOLD 1972 chevy c10 |
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#2 |
Registered User
![]() Join Date: Nov 2007
Location: Hermiston, OR
Posts: 947
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Re: Q-Jet adustment issues, jets?
All the way in closes it off completely, all the way out causes a big vac leak. This is good and means you don't have any vacuum leaks elsewhere!
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Running in place gets you nowhere fast. -me 1968 GTO 4-spd covnvertible 1970 K-20 4-speed, factory AC. |
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#3 |
Account Suspended
Join Date: Jun 2010
Location: Placerville Ca.
Posts: 982
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Re: Q-Jet adustment issues, jets?
It depends on the carb, Holley's do ...... Qjet's adjustment may all happen in the first couple turns, after that you may have already exceeded the taper on the needle ...any further out may do nothing. The main thing is that it does respond when turned all the way in, if it did'nt your idle circuit ain't working and your idling off the mains.
If your timing is at 12-14 degrees initial and your within 2 turns of closed on both sides w/smooth idle 14-16 inches of vac that would be good for a performance cam but low for a stock cam ...should be more like 18-20. (what cam are you running ?) Don't worry about the needles not responding after two turns out if your engine is running smooth with good idle and vacuum.....the Qjet you have may very well have a limit to how rich the idle circuit can be adjusted, I've had carbs which did the same thing |
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#4 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,616
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Re: Q-Jet adustment issues, jets?
The procedure.
Engine warmed up. Set both mixture screws at 1.5 turns out. While watching vac gauge turn left screw in 1/4 turn at a time until vac drops. Pause for a few seconds between each adjustment. Back off 1/4 turn. Go to right screw. Adjust right one the same. Go back to left one. Turn in a 1/4 turn. If vac drops again back off to last good setting. Adjust right one again. This should give you the best idle with your set up. Remember to pause between each adjustment. Gives motor time to settle. It's just a fine tune process. |
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#5 |
Registered User
Join Date: Jul 2011
Location: vacaville ca
Posts: 1,304
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Re: Q-Jet adustment issues, jets?
I just have your typical rv cam sttings slightly better than stock for low end tors I believe. I have the specs if you need me to post...
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04 CHEVY LLY LS DURAMAX 4WD. MODS TO FOLLOW, EFI LIVE, BUILT TRANS, LBZ MP, LIFT PUMP SOLD 1972 chevy c10 |
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#6 |
Account Suspended
Join Date: Jun 2010
Location: Placerville Ca.
Posts: 982
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Re: Q-Jet adustment issues, jets?
If you can pull em up I can give you a better idea of what range your idle vacuum should be ......also any other modifications to the engine you can think of.
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#7 |
Registered User
Join Date: Jul 2011
Location: vacaville ca
Posts: 1,304
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Re: Q-Jet adustment issues, jets?
Cam Style
Hydraulic roller tappet Basic Operating RPM Range 2,000-5,600 Intake Duration at 050 inch Lift 232 Exhaust Duration at 050 inch Lift 236 Duration at 050 inch Lift 232 int./236 exh. Advertised Intake Duration 288 Advertised Exhaust Duration 294 Advertised Duration 288 int./294 exh.
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04 CHEVY LLY LS DURAMAX 4WD. MODS TO FOLLOW, EFI LIVE, BUILT TRANS, LBZ MP, LIFT PUMP SOLD 1972 chevy c10 |
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#8 |
Wide n'Low
![]() Join Date: Jan 2007
Location: La Center, WA
Posts: 1,511
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Re: Q-Jet adustment issues, jets?
Just make sure when you are adjusting your idle mixture screws, that you are in the idle circuit. By that I mean you should be at or around 750 rpm. Anything much above that and may not get valid readings. Also, keep an eye on the idle after each change to make sure you are not getting up above the idle circuit. If you need to, turn down the idle a bit before you start. I agree that many of the Q-Jets seem to run out of adjustment beyond 3 turns or less. Remember, you want the highest Steadiest vacuum reading, not just the highest reading possible. Here is a good visual description with some troubleshooting explanations.
http://www.classictruckshop.com/club...ts/vac/uum.htm Plus, I have a great Q-Jet tuning guide from years ago, http://www.2heeldrive.com/Chevrolet/...ning_Paper.doc
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Trippin Hazard.. 67 swb, Gold Member.. 68 Suburban Air Force Retired |
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#9 |
Account Suspended
Join Date: Jun 2010
Location: Placerville Ca.
Posts: 982
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Re: Q-Jet adustment issues, jets?
That's a good sized cam I wouldn't exactly call a 288 cam "rv" ....if your pulling 14-15 in hg of vacuum at idle I'd be very satisfied with that....your tune on the carb is probably as your going to get with that much cam duration. You should even be able to run power brakes at that vacuum. Leave the carb right where it is for now and drive it around in different situations and take note of how it reacts. Here's a few easy tests:
--off idle acceleration....quick throttle snap from idle (this tests accelerator pump) --accelerating from a cruising speed quickly to WOT (this will test for secondary opening rate and main jet circuit) --cruising steady speed, low or no load (checks main jet circuit for rich or lean condition) |
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#10 | |
Registered User
Join Date: Jul 2011
Location: vacaville ca
Posts: 1,304
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Re: Q-Jet adustment issues, jets?
I was told that its a basic rv cam... I didnt build the motor but broke it in, I bought the truck right after the motor was built....
Quote:
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04 CHEVY LLY LS DURAMAX 4WD. MODS TO FOLLOW, EFI LIVE, BUILT TRANS, LBZ MP, LIFT PUMP SOLD 1972 chevy c10 |
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#11 |
Registered User
Join Date: Jul 2011
Location: vacaville ca
Posts: 1,304
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Re: Q-Jet adustment issues, jets?
twisted, thanks some very great info you provided, I read the q-jet set up link and it has tremendous wealth of information, thank you....thank all of you for the responses...
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04 CHEVY LLY LS DURAMAX 4WD. MODS TO FOLLOW, EFI LIVE, BUILT TRANS, LBZ MP, LIFT PUMP SOLD 1972 chevy c10 |
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#12 |
Registered User
Join Date: Oct 2005
Location: Overland Park, Ks.
Posts: 5,216
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Re: Q-Jet adustment issues, jets?
Q-jets are designed for low emissions. The passages in the idle system are designed so they can only go so rich. That is a pretty high performance cam. You wont get any more vacuum then that. Sounds like you have it pretty down pat. You might get away with a little more timing. Usually that system will work best with the vacuum advance hooked to full vacuum. You may need an adjustable vacuum chamber or one designed for lower vacuum.
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#13 |
Who's got steelies?
![]() Join Date: Feb 2006
Location: Morrison, Oklahoma
Posts: 1,026
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Re: Q-Jet adustment issues, jets?
On an engine with a non-stock cam, in order for the engine to get enough air at idle to keep running, you may have to have the primary throttle blades cracked open enough that the idle screws seem to have little or no effect apart from what you describe. Factory performance Q-jets had extra internal idle air bleeds so the engine would get enough air at idle. You may want to consult a Q-jet book for additional info, as there are alternatives/mods to solve this problem.
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#14 |
Registered User
Join Date: Jul 2011
Location: vacaville ca
Posts: 1,304
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Re: Q-Jet adustment issues, jets?
thanks all, you have guided me in the righr direction
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04 CHEVY LLY LS DURAMAX 4WD. MODS TO FOLLOW, EFI LIVE, BUILT TRANS, LBZ MP, LIFT PUMP SOLD 1972 chevy c10 |
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#15 |
Account Suspended
Join Date: Jun 2010
Location: Placerville Ca.
Posts: 982
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Re: Q-Jet adustment issues, jets?
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