Q-jet jetting 4.3 vs. 5.0

[ramster]

I bought a Q-jet from a 4.3 LX Merc, and put it on my 5.0 Volvo GL, it runs pretty good, I gained around 200-300 rpm and a couple mph over my 2 barrel Holley, I think it is a bit lean both primary, and secondaries, does anyone know what the jet specs are for a 5.0? I was hoping someone knows offhand without doing a bunch of searching. My current primary jets are .099, and I have HO secondary rods.

 

[jtybt]

Re: Q-jet jetting 4.3 vs. 5.0

Are you getting the 'lean' condition from reading your plugs? What RPM do you generally run at? Check your plugs after a typical run. Don't idle around or you'll change the color of the plugs.

I'd have to haul out my Q-jet books and notes but I remember having HO secondary rods. You can adjust mixture in the secondaries by the hangars. Take a look at the hangar specs and change to whatever you think you need. You seem to have good instincts. The higher the hangar holds the rods, the richer it will be.

On the primary jets, check at what throttle opening/RPM the secondaries start opening the lower secondary plates(by watching the linkage). You can do it with the engine off and open the upper 'trap doors' and watch when the secondary plates op in relation to your primaries. Put a mark next to you throttle as a reference point.

Now run around just before that throttle opening and stop to read your plugs. That will be good enuff to tell you if your jets are about right.

Dark brown to slightly black is the safest color. light brown or white and you're too hot...but a lot will depend on how and what load you put on your engine(s). Read up on the Q-jet .

 

[ramster]

Re: Q-jet jetting 4.3 vs. 5.0

I Haven't looked at the plugs yet, I have a good holeshot, and cruise, but at higher rpm right after the secondaries start to open, there is a flat spot where the rpm's seem to just hang for a moment and it feels like the engine is being held back. I think factory jetting specs are on the lean side on most boats by default to meet EPA standards. My secondaries only open by vacum, there is no linkage. With motor off you can open the primaries all the way and secondaries will remain closed, there is a vacum pull-off for the secondaries. You can really hear when they open, the motor just roars, I love that sound! I do run WOT alot, so I don't want it on the lean side, rather be a little fat especially on topend.

 

[dr_bowtie]

Re: Q-jet jetting 4.3 vs. 5.0

if the jet size is 99 that is good...

what are the stamped size on the primary rods...

factory Q-jet specs are on the richer side not lean...from everyone I have worked on and I still have over a hundred in the shop for cores...

the flat spot you notice may be the air door on the secondaries either too tight or too loose...

if it still has the pull-off rod on the secondary air-door then I'll bet it is too tight...

the vacuum pull-off holds it closed and then on full throttle vacuum drops to zero for a quick burst then starts adding back...

the air door must overcome the pull-off to open and they are usually not enough engine vacuum to do so...this will result in a short lag..

I have always pulled the rod off and manually set the air-door spring for each individual engine but, this will require some minor adjustment for every tune and as the engine wears or weather change...Q-jets are temperamental carbs...

 

[jtybt]

Re: Q-jet jetting 4.3 vs. 5.0

I have found the 'trap doors' are usually too loose when they lag when you get into them. Next time you're out, have someone else drive while you close the trap doors slightly(with your finger) when you feel the bog.


Yeah, that causes a lean condition if the upper doors open too much/too soon.

 

[ramster]

Re: Q-jet jetting 4.3 vs. 5.0

The secondary rods are stamped 40, I put a 1 1/2" spacer under the carb as an expirement, not sure of the differance. Still using the vacum pull off rod, and I did have the carb apart and gave it a good cleaning, has been working pretty good, but alway's looking for a little better.