Q-Jet adustment issues, jets?

[dukcaln]

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

[webfoot]

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!

[RUSHNBOBO]

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

[geezer#99]

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.

[dukcaln]

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...

[RUSHNBOBO]

Originally Posted by dukcaln

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...

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.

[dukcaln]

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.

[Twisted78SS]

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

[RUSHNBOBO]

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)

[dukcaln]

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...

[dukcaln]

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....

Originally Posted by RUSHNBOBO

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)

[Wrenchbender Ret]

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.

[oldgold70c10]

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.

[dukcaln]

thanks all, you have guided me in the righr direction

[RUSHNBOBO]

Originally Posted by dukcaln

thanks all, you have guided me in the righr direction

the bills in the mail