Another Q Jet Mixture question
Thread Starter
Le Mans Master
Joined: Jul 1999
Posts: 9,206
Likes: 17
From: Sewell NJ
Another Q Jet Mixture question
I just read a post about adjusting the mixture on Q-Jets and I've always followed the procedure that was recommended. I turn both screws in until they bottom and then turn them out about 1 1/2 turns. Then with a tach and a vacuum guage hooked up I adjust for the best idle and when it's reached I turn them in about 1/4 turn to lean things out. My question is this. With everything set up like I just said if I uncap one of the vacuum ports the engine speeds up. Is this normal or do I have the engine set too rich. I always thought that when a vacuum cap was opened the engine should stumble because it goes lean. I usually pull the cap off the vacuum port that is on the right hand side of my Q-jet which is a full time vacuum port. My setup runs just fine the way it is but I was always curious about this. Can anyone shed some light on this.
Pro
Joined: Jul 2000
Posts: 654
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From: Now Frozen in Michigan
Re: Another Q Jet Mixture question (vettfixr)
Vettfixr-
I'm not gonna write a book on science for this (this could easily lead to that), just try to give you a little 'nut-shell' understanding.
Yes, when you're 'uncaping' an additional vac port, slight rise in RPM is normal. You're allowing a "metered" amt of air to enter, leaning the air/fuel some, but not enough to cause problems while idling. Under load, (while driving) it may or may not cause problems depending on what the overall air/fuel ratio (mixture) is.
For normal operating conditions, best economy is achieved from a ratio of 1 to 16 or 17. For good acceleration and max performance, a richer mix or about 1 to 12 - 14 is needed. But, for idling, a slightly richer mix is required than for operating conditions. Now when cold starting, an extremely rich mix is required. (hence the choke) I should add that the ratios stated (ex: 1:12) represent 1 part fuel and 12 parts air.
So as you can see, 'uncapping' your port leaned the idle mix out, raising RPM's, but to no adverse effects. Uncap all your ports and watch the engine stumble and maybe die. (too lean)
Hope this makes sense to you and gives a little clearer picture of 'what's happenin'. Good Luck -Rod :chevy
I'm not gonna write a book on science for this (this could easily lead to that), just try to give you a little 'nut-shell' understanding.
Yes, when you're 'uncaping' an additional vac port, slight rise in RPM is normal. You're allowing a "metered" amt of air to enter, leaning the air/fuel some, but not enough to cause problems while idling. Under load, (while driving) it may or may not cause problems depending on what the overall air/fuel ratio (mixture) is.
For normal operating conditions, best economy is achieved from a ratio of 1 to 16 or 17. For good acceleration and max performance, a richer mix or about 1 to 12 - 14 is needed. But, for idling, a slightly richer mix is required than for operating conditions. Now when cold starting, an extremely rich mix is required. (hence the choke) I should add that the ratios stated (ex: 1:12) represent 1 part fuel and 12 parts air.
So as you can see, 'uncapping' your port leaned the idle mix out, raising RPM's, but to no adverse effects. Uncap all your ports and watch the engine stumble and maybe die. (too lean)
Hope this makes sense to you and gives a little clearer picture of 'what's happenin'. Good Luck -Rod :chevy
Thread Starter
Le Mans Master
Joined: Jul 1999
Posts: 9,206
Likes: 17
From: Sewell NJ
Re: Another Q Jet Mixture question (ROD NOCK)
Hey Rod, thanks for the reply. I had about given up on this thread but I'm glad someone answered. What you said makes sense. I never really worried about it but I was always curious as to why it happened. Thanks again.
