Quadrajet Setup for 350 w/cam+manfld change and

[rbezdon]

Does anyone know what primary jets, primary rods, secondary rods and secondary hanger were standard for a 1988 OMC 5.7L 260HP with the 4MV Quadrajet?? I have a OMC Cobro 5.0L which I converted to a 5.7L (350 4 bolt mains truckblock) a few years ago with a slightly increased lift cam and an Edelbrock Performer manifold. I seem to start to run "flat" WOT arond 4400 or 4500 RPM. I have shanged the secondary rods from CH to DA (both on a J hanger) and had a slight improvement. I am not sure what the primary setup is as I have to open the carb (planned for next week end). I have also reset the airvalve spring (tighter as the secondaries seemed to open too soon) which also had a slight improvement. It seems like I am running tooo lean and starving when getting near the WOT RPM but I am not 100% certain. I guess it could too rich as well. Does any one know the stock setup for the 350 and have any recommendations for which changes I should consider for the improved breating from the higher lift cam and the better manifold??

 

[timbrittain]

Re: Quadrajet Setup for 350 w/cam+manfld change and

as far as the carb goes try closeing the choke slightly to see if its rich or lean if it improves its lean

 

[John_S]

Re: Quadrajet Setup for 350 w/cam+manfld change and

Merc specs for the 4MV Quads they used:

5.7L: 0.068 Pri-jet, 0.041 Pri-rod, CL 2nd rod

350 Mag: 0.069 Pri-jet, 0.036 Pri-rod, CH 2nd rod

I suspect OMC's were similiar, and someone probably will post that info.


The CH is a rich rod, and you went to a DA, which is significantly richer. I wouldn't expect that with only a small cam change (what cam?). Something else might be impacting your tuning. What vaccum are you pulling at idle? Very steady?

 

[bigdan1]

Re: Quadrajet Setup for 350 w/cam+manfld change and

run your engine at 2500 RPM for about 10 minutes .( in water ,driving)

stop it , remove 3-4 spark plugs : look at the color of the porcelain

is it crystal white ? way too lean enlarge jets by .002 to start .
is it tan , great mixture ......

start by this ! ......

the secondary enriching method is just like a plaster
the main thing is to have he primaries on par ....

comme back with results

 

[rbezdon]

Re: Quadrajet Setup for 350 w/cam+manfld change and

Tim, closing chike on qudrajet not as effective as seems as the choke only relates to the primaries. John, thanks for the info. The carb I have is actually off of a Merc, the old OMC was damaged, and that is where I got the CH rods. They were hanging on a J hanger which is bent. I haave a second carb I bought off of EBay which had the DA rods on an L hanger which is what I am running now and seemed to have a slight improvement (shown as a 100 to 200 RPM increase at WOT. I will check the primery setup next week end (the boat is an hour away). I did Dan suggested and the plugs are a light color some where in the white to tan range but but clear. They were not black which is why I suspect the issue is lean, not rich how ever, the inmanifild choke does run rich during warmup so plugs get a bit black during that. Choke is fully off warm. I have also notice some times backing off ever so slightly, and I mean slightly, for WOT can actually increase the RPM. Obviously, with the way the secodary throttle plates open on the qudrajet, this closes the plates again leaning toward lean. The only issue is the rods I am using are already on the rich rich. Maybe the primary needs to be opened a bit. After all, a 350 running 4500RPM will hardly activate the secondaries.

 

[John_S]

Re: Quadrajet Setup for 350 w/cam+manfld change and

If the merc carb came from a 305 or 350, the primary side jetting should be close enough for wot testing that you wouldn't have to be going so rich on 2ndary. You are most likely trying to compensate for another issue in carb or engine. Low vacuum or a vacuum leak can/will cause this. A vaccum gauge is your friend. A performance cam can lower intake vacuum, as well. It would at least help understand why the tuning would be so far from stock.

Being you are doing a tear-down, clean and inspect everything closely. With the absence of any other OMC specs, would set it up to Merc 5.7L specs. If you want to do the math and calculations for the jets/rods, the performer intake plenum is somewhere between the low-profile 4brl intake and the high-rise of the 350 mag.

 

[rbezdon]

Re: Quadrajet Setup for 350 w/cam+manfld change and

John, Couldnt agree more and have ordered the CL rods and new hangers several days ago. BTW, just for the record I checked timing, dwell and the exhaust is clear. Plugs wires rotor and cap are fairly new. As far as vaccuum, I checked it after the cam change and it hadnt changed from the stock. The cam is a CompCam 12-206-2. Lift went from .39 to .44 I think and dwell and timing nearly the same as stock Merc. Engine idle is smooth enough to put a wine glass on Flame arrestor, no lumpy idle. I boat in a place called the Chain of Lakes (near Chicago) and you spend more time idling in no wake channels between lakes than crusing across the lakes. A lumpy idle would be a nusiance.

 

[John_S]

Re: Quadrajet Setup for 350 w/cam+manfld change and

Keep in mind, you are talking to a fustrated Q-jet tuner. I would get mine where it seemed to run well, but then a few weeks latter, not as well. The hangers and how much bending to accelerate or retard the "thick" part of the rod entering the jet, may be your issue. Keep us posted on your progress.

BTW: I gave up on the Q-jet and joined the Edelbrock 1409 crowd, after extensive searches could not locate my q-jet tuning parts. I will say, it has been a much better experence.

 

[rbezdon]

Re: Quadrajet Setup for 350 w/cam+manfld change and

I also am a frustrated QJet user. It is not only having to make the adjustments but the cost of the extra rods, jets, springs etc is almost in the range of another new carb!! Wish there was some where to "rent" the entire range to make the adjustments and find the desired combo before having to purchase. One note, I have found several acticles on ways to tunre the QJat but none seem to clarify the effect of the Sec hanger size. Seems the higher the letter, the deeper the Secjets are inserted and the leaner the sec will be. Any one know the effects of the secondary hanger rating??

 

[jy118lfd]

Re: Quadrajet Setup for 350 w/cam+manfld change and

You all can benefit from this book


http://www.cliffshighperformance.com/buy_book_2.html


Also any and I mean any parts or questions you have or need for a q-jet cliff is the man.

http://www.cliffshighperformance.com/index.html


I called him when I did my carb for my 350 and it is now as close to Fuel injection as you can get. Ended up talking to him for an hour or more each time I needed help.

 

[timbrittain]

Re: Quadrajet Setup for 350 w/cam+manfld change and

well i hate qjets get a holley lol good luck on it