Switch from Holley to Q-jet with adapter
 

I have been pretty pleased with the "problem child" as of late....I have my original Quadrajet still lying around and want to bolt it up to get a comparison of the two between my Holley I am running now and my old Q-Jet......NOW I know ALOT of people say the adapters suck and don't really cause a good a/f distribution but hell thats not my point:D I can't stand the fact that this perfect Q-Jet is lying around getting dust..I would like to know how it would do on my new motor...You know a little test and tune. If it works great if It runs like **** well I won't feel bad at all selling the thing......What all is needed to switch? A thottle bracket and manifold adapter (qjet-to square bore)? Im running a SBC with L31Vortec Heads and a RPM Air-Gap manifold....What does the Q-Jet flow CFM wise? It was on my original motor a 260hp mercruiser..Any body done this swap before and have some part recomendations?

Thanks
BH

All marine Q-Jets flow 750 cfm.
I'm not sure how a Q-Jet will work with the adapter. Have you seen what those adapters look like?
Trying to squeeze the air flow from the large secondaries down to the smaller square bore holes will hurt performance. I would think it's close to a NASCAR restrictor plate.
If you hade a Q-Jet manifold then bolted a Holley to it would be a better test.

Re: Switch from Holley to Q-jet with adapter
 

Given some of your past posts and problems...if it ain't broke , DON'T FIX IT:p :D

I'm running Q-Jets on top of OLD torkers, with the Mr Gasket open adapters. Mine is a single plane mainfold so I don't think it hurt anything. I had to trim the crap out of the adapters, because 'as cast' they don't line up with either of the two applications. I just layed the corresponding gasket on each side, and worked with my trusty die grinder till I didn't see aluminum inside the gasket surface.
I also had to make a spacer to raise my throttle cable bracket.
The gas milage with the Q-Jets is awesome! Yea, like anybody on this site cares about that!
DS

I say, go for it! It would be fun and the performance difference may be relatively minor. The idle and mid range performance may be better. Use the cheap Mr.Gasket adapter. You may get a performance gain from having additional plenum volume from the open adapter.
Let us know how it turns out.
Sincerely
Dennis Moore

I forogt about the open type spacer. The one I had in mind was a four hole type but the secondary side was beveled to match the Q-Jet secondary size. That's why I compared it to a restrictor plate.
My mistake.

Bad Habit. I have everything you need to make the switch if you want to borrow them to try let me know. Im in Huron, OH but travel all over.

Thanks Formula31....What all do you have and what is needed? Have you had any experence with this type of switch? I would like to switch them out maybe in a week or so....I'll wait to hear from you and thanks again

BH

I tried to run Quads on my engines and they didnt work at all. They were stock right off of 7.4's that ran great. On my engines, they wouldnt start and when they did, wouldnt run under 2000 rpm. Not knowing much about Quads, I just gave up. I also use single planes with open plenum's. I have the open adaptor, screws, fuel line connection (for 3/8" hose) and the stock merc throttle clamp (post for the barrel). I suspect the open plenum manifold and my advanced cam may be causing the quad problems. My holley 780's work great, start and idle like fuel injected.

Dear Bad,

I have a vortec intake for the quad-jet. It is cast iron and I would love to get rid of it. It is a stock height and was designed for the flat trackers. i bought it new from salee gm. It has just a few hours on it.

Let me know if it might work for you.

thanks

I love Q-jets, so my vote is give it try. Better yet, yank off that aluminum intake and take a die grinder to it to mach the Q-jet base plate gasket.:D

What is your current motor? If still a SBC, run 68/41 primaries and CH for 350, CK for 383. If a BBC, let us know what flavor.

Motor is a 358SBC Roller motor. RPM Air Gap intake with L31Vortec heads. Stainless Marine 4" thru hull exhaust. Im pushing about 375HP @ 5300rpms....

BH

I still have my first intake I bought for the "problem child" it is the Edelbrock Performer...It will accept the Q-Jet but the intake side is warped ever so slightly....I hear this can be fixed though...It's just a paper weight now:D Currently Im running a RPM Air Gap Vortec manifold...What is the part # for the intake you have?

BH

Here's my .02 for your set up. BTW, I'm no expert, I just tinker with marine Q-jets for grins.

If yor Q-jet is original for a 350 marine, it should have something similar to .068 pri jets, .041 pri rods and CL secodary rod. The CL will be too small on the top end.

Leave the primary side alone for now:
.068 Pri jets
.041 Pri rods

But get a set of CK secondary rods (may also be too lean, if so step up to DA)

Probably need the 0.147 hi-flow needle and seat. but don't put more than 7psi of fuel agaisnt it. Probably need to discard stock filter and go inline.

I'm sure your cam has duration greater than 216 at .050. You may find that you don't have enough adjustment with the idle screws. If so, you'll need to open up the idle mix screw ports to the max diameter of 0.093" (jewelers drill). That'll give you good idle mix adjustment.

You can get all the parts at Carbs Unlimited (carbs.net).

Good luck.