ok first off i must say im just a begginer at car stuff but have help from a really good car guy.. ok the problem is that my qjet is running to lean at WOT but perfectly at cruise ok heres the run down..
secondaires blocked off.. original jets where 76 ran 14.5:1 at cruise and 14.5:1 at WOT, made 79 jets ran 14.5:1 at cruise and 14:1 at WOT
then 82 jets 14.5:1 at cruise and 14:1 at WOT,.. we would of thought going to 82 jets it would of been atlest to the correct 13:1 at WOT, everythign is running perfect just at WOT its to lean the APT is set right everything is.. anyone have an ideas whats going on here.. its a 78 q-jet on a 305 enigine.. :confused:

 

Dont quite know what yur ratios mean but....as far as jetting is concerned..its a day to day basis...air temp that day can make a big difference....not likely to get it perfect always...find a happy medium and stick with it...this is why fuel injection has taken over...it measures and changes all this constantly to provide optimum performance via computor...carbs could never do that...not without lots of human time and effort

heck i dont know..yur #'s may indicate a perfectly tuned Qjet.


[Modified by Jvette73, 8:08 PM 8/21/2002]

 

How are you testing? Q-jet secondaries will not open unless you are under load (as in on a dyno or on the street).

Are you sure the secondaries are opening, as in the spring is to tight.

Just some thoughts.

Terry

 

Maybe I'm not reading it right but, if you have the sec blocked off you will run leaner no matter what because there is no sec to put extra fuel in. :confused:

I'm running 75 jets and 46 rods (primary) on my mild 350. Get Lars' tech paper on QJets from www.corvettefaq.com. Makes life easy.


:cheers:

 

ok what were doing is blocking off the secondaires and just runing the primaries and trying to get them, and only them selfs to run 13-13.5:1 air/fuel ratio at WOT but the problems is stated as above cant seem to get it to run any richer when useing really big jets......

 

When you say blocking off the sec, are you blocking the top of the carb or blocking the actal sec fuel flow? If you are blocking sec fuel - are the secondary butterflies and the sec air door opening? If they are, aren't they helping dump more air into it without the benefit of the sec rods allowing more fuel?

Why do you want to get it richer with just the primaries? Once the sec rods let more fuel inyou'll be running too rich at WOT.

Just trying to get across what you want to achieve. With my combo I run about 12.8 @ WOT.

:cheers:

 

we are blocking off the 2ndaries at the top of the carb on the air horn..

 

my advice is to tune it as you actually drive it. with all 4 bbl's working i assume


[Modified by Matt Gruber, 10:46 AM 8/22/2002]

 

A couple of thoughts for you:
First, if you're blocking off the secondary airvalve, the secondary throttles are still opening, and this will draw some air leakage through the closed secondary airvalve without the addition of any fuel. Block off the secondary throttle shaft by engaging the choke lockout lever before doing the testing. This will produce more accurate readings.

Secondly, I have found that at WOT on the primary side only, the engine can actually pull enough manifold vacuum to prevent the power piston from popping all the way up to the full rich position. You might want to install a good, stiff spring under the power piston during this testing to assure that it's really popped all the way up to full rich.

Third - the '78 carbs have a very low float level, inducing a lean condition - around .450". Try raising the float up to .375" - .400". This will richen you up across the powerband.

Finally, make sure that your primary metering rods are not stamped with an "M" designator after the size number. The "M" rods are truck rods, and have .036" diameter power tips on them. The passenger car rods (Vette rods) have .026" power tips. This makes a HUGE difference in WOT metering area.

A '78 really should not need to be jetted richer than 77 on the main jets. You might want to keep this as your upper limit size and manipulate the secondary jetting from this point to get the WOT mixture you need after verifying the above parameters.

 

.....well it's about time Lars! ;) :p:

I'm still trying to figure out what is the benefit of getting the primaries 13.5 @ WOT. Wouldn't that all be out the window once the sec come in?

 

ok from what ive gathered is that the primaires mostly run on the front cylinders and the secondaries on the back.. you tune the primaires as best as you can.( getting them to run 13/13.5:1 at WOT) and then set your secondaires becasues there much easyer to tune and actaully not that much more power comes out of them at WOT well least on the small v8's ie 305,307 wich its running on now.. personally i dont really no much about qjets my friend does im just riding off of memory.

 

OK, I'm no guru so there is my disclaimer.

The primaries run 70/80% of all the 8 cyl. The sec come in gradually and the air door should open gradually as well. Too fast and it will bog. The sec compliment the primaries in the last 20%, so yes the primary circuit is the one to get right first. (How am I doing so far Lars?)


I really recommend printing the Q-Jet Tuning Paper doc from www.corvettefaq.com or I could e-mail it to you. It is easy to read and understand - even for a dummy like me. I pulled mine apart and rebuilt it myself and I have stayed away from carb's for many years as I just didn't want to know. I just asked a lot of questions. I did a dyno run a while back and I have it set up almost perfect.

At least you are asking questions and are giving it a go! :cool:

:cheers:

 

71 -
The primaries do not feed the forward cylinders with the secondaries feeding the back - primary and secondary sides feed all cylinders equally through the stock dual plane intake manifold.

You are correct in tuning the primary side first to its best condition, and then supplementing the primaries through secondary tuning. On the smaller engines, the primaries really do the majority of the airflow and metering work, so getting the primary mixture set up right at both cruise and WOT has a big impact on performance and drivability. But even the smaller engines will dip into the secondary side at high-rpm WOT operation, so having the secondary airvalve and the secondary metering set up right certainly produces a significant kick in the butt.

ChrisO -
The nice thing about getting the WOT mixture close to correct on the primary side rather than letting the sec side compensate for a lean mixture is that you avoid the erratic secondary transition that occurs by having a lean WOT condition when you first mash the pedal to the metal (primaries go full open, but secondary airvalve is still closed), and then the secondaries suddenly come into play and give you a massive change in mixture. I've seen guys jet their carbs like this, and they think they're going faster because they get a sudden surge when the secondaries open. Fact is, if you can get the primaries jetted right so they're producing peak power, and eliminate the rich surge upon secondary opening, you're actually going faster even though you can barely detect the secondary transition. A properly jetted carb, primary & secondary side, will produce completely smooth power through the entire power, rpm, and throttle range, with a barely-detectable transition into the secondaries.

 

71-307
good advice from lars.
.
where is the sensor?

 

71-307

My best advice? Listen to Lars.

You're new on the forum (great to have you on board!) -- the one thing you will learn is that NOBODY on the forum knows more about Carbs, esp. Q-jets, than Lars.

Any other advice you get is either a variation on what he is saying, or just wrong. :D

Good luck! :cheers: