What Throtle bracket to use 2bl to 4bl upgrade

[kpiazzisi]

I just upgraded my 4.3 LX Gen + alpha (0F821555) to a 4Bl setup. I ordered the Spread Bore Intake Manifold (8 bolts VOrtec 1996 +) and a Rochester 4bl Carburetor. The old 2bl throttle bracket will not work. From what I can tell I need part 805359T. The 2bl bracket is considerably longer and has room for the Circuit Breaker and Starter Slave Solenoid to mount on the bracket. The new bracket is considerably shorter and I am concerned that I won't be able to mount it on the bracket. I checked the Merc Manual and the 4.3 LX (2bl) and 4.3 LXH (4bl) use the same wiring harness, so that leads me to believe it would? I do not yet have the bracket (in transit). If anyone knows for sure where these parts mount on a 4bl setup, please let me know

 

[SteveH_1919]

Re: What Throtle bracket to use 2bl to 4bl upgrade

The new engine I replaced came with a bracket for the throttle.
I did have to fab a bracket for the slave solenoid and breaker. I used a piece of flat alum stock and drilled holes.

Until I figured out what the included bracket was for, I was planning on fabing a bracket for the throttle.

Just my .02
Steve

 

[kpiazzisi]

Re: What Throtle bracket to use 2bl to 4bl upgrade

Steve,

Where did you bolt the fabricated bracket to? I did some more research and it looks like the circuit breaker and the solinoid both attach to Mercruiser bracket 805368T. That same part number is show on both diagrams for the 4.3LXH Gen + (4bl) and the 4.3LX Gen + (2bl). Now I just have to figure out what 805368T bolts to. Like I said, on the 2bl application it bolted to the throtle bracket, but I don't think that will be possible with the new shorter 4bl bracket.

Thanks

 

[John_S]

Re: What Throtle bracket to use 2bl to 4bl upgrade

On my 5.7L the plate (w/CB,etc) that was on the 2brl bracket fit the 4brl bracket.

BTW, once you have that Q-jet tuned for a 4.3 gen+, post it here. I am sure others would want that info for a good starting point. Most use a 1409 with the 4.3 jet/rod kit for starting point.

 

[SteveH_1919]

Re: What Throtle bracket to use 2bl to 4bl upgrade

I removed one of the carb bolts. Purchased another slightly longer bolt and used the the longer bolt as the mounting point for the fab'd bracket.

Good luck,
Steve

 

[kpiazzisi]

Re: What Throtle bracket to use 2bl to 4bl upgrade

ACE has the bolts they are 5/16 x 18 (threads per inch). They have them in Stainless Steel and Grade 8. I am going to try out the bracket I bought (UPS suppose to deliver today) and if that doesn't work, I will fab one. I am not sure what imported_john is asking in reguards to the tunning? I am hoping I won't have to do any tuning. I am assuming he is talking about changing the metering jets or main jets? I did some research on here and apparently ther are different Rochester part numbers for the different CFM ratings. The different CFM ratings and coresponding part numbers we dependent on the different engine sizes. I believe the Rochesters went on V8 sizes 260, 305, 350, and finally 454. I asked Carlos at I5 Carburetor what the CFM was for my particular carb and he told me it was only 450 CFM. He also said that physically all the Rochester's were the same size and the internals (assuming metering jets) determined the CFM. This sounds strange to me, since I always assumed the physical size of the barrels would determine the amount of air that would flow through the carb. I made Carlos at I5 aware of my application, so I am assuming he set it up for me with the correct jets/tuning etc. I had it running in the driveway with the ears on it, and it worked perfectly. I plan on taking it out tomorrow and seeing how she does on the water. I suppose I could pull a spark plug at after running different RPM ranges to see if it is running too Rich or too lean? Then I could check with Carlos to see what jets are inside my particular carb. So far I am very impressed with this Carb. It looks very factory and everything with the exception of the bracket fits perfectly.

 

[John_S]

Re: What Throtle bracket to use 2bl to 4bl upgrade

As far as I know, the Q-jets used by Mercruiser on 4.3L. 5.0L, 5.7L, and up were all 750 cfm. The air valve opens in accordance with vacuum, so on a 4.3L, they are barely open at wot. If you are buying an elec choke version from I5, it is a newer style auto carb converted to marine, and their tuning is different than what merc had. It is highly unlikely that it will be a bolt on and go. One it is a Q-jet, and two it is coming from I5.

 

[kpiazzisi]

Re: What Throtle bracket to use 2bl to 4bl upgrade

I believe this carb originally came off of a non vortec V6. It's original part number was MCM 205/4.3LX or part 3304-9354A. I am assuming 1996 Vortec v6 with the vortec heads is capable of flowing more air and fuel then the older V6 the Rochester came on. I guess if anything I should expect this to be on the lean side? I will pull a plug and let you know what I find.

KP

 

[kpiazzisi]

Re: What Throtle bracket to use 2bl to 4bl upgrade

Took the boat out, and I am not impressed. First off I have a flat spot going from cruise to WOT. The engine will hesitate until I go to full throttle. Secondly I just don't feel much of a horse power increase. I was hoping to go from 190 HP with the 2 bbl to about 220 HP with the 4BBL setup. I really doubt that is happening now. I think I would feel the extra 30 HP. I guess now I have to educate myself on the Rochester Q-jet and figure out if I can tune it and hopefully gain some more HP and get rid of the flat spot. If anyone has any literature on these carburetor, please provide me with a link.

Thanks
Kevin

 

[haulnazz15]

Re: What Throtle bracket to use 2bl to 4bl upgrade

You do realize that 30HP on a boat probably won't hardly be felt like it would on a car, right? It takes a lot of horsepower just to increase top speed by 2-3HP.

 

[Howard Sterndrive]

Re: What Throtle bracket to use 2bl to 4bl upgrade

I have a flat spot going from cruise to WOT. The engine will hesitate until I go to full throttle.

I have had the same issue on every marine quadrajet. a few issues and checks I recommend:
1. which hole of the accel pump arm is your linkage in? needs to be in the one that no car uses...so the geometry gives it a full stroke on accel
2. Ethanol fuels dry out the accelerator pump cup quickly. Apparently the blue cup is the ethanol resistant one - I just going to make it an annual maintenance item to put a new one.
3. probably need to tighten up the secondary air valve. You'll be leaning out wickedly if the big barrels open up and the fuel isn't there yet...good for kissing the windshield when you punch the throttle

You might also hear a lean backfire on holeshot if the accel pump is weak

I was hoping to go from 190 HP with the 2 bbl to about 220 HP with the 4BBL setup.

the 4BBL is only a 15HP gain on a V6
Only the MPI's make an honest 220

 

[Howard Sterndrive]

Re: What Throtle bracket to use 2bl to 4bl upgrade

the interwebs tell me Edelbrock #1982 is the ethanol resistant pump with the blue pump cup - I just ordered two.
It also apparently gives a bigger accel fuel shot over stock pump.
we'll see...

 

[christoner2002]

Re: What Throtle bracket to use 2bl to 4bl upgrade

Ah, yes, Carlos at I5 carbs. If you ask him about the flat spot, he will tell you to bend up the Secondary metering rod bracket. When you do that, check to see if the secondary metering rods are the same from side to side. Mine had 2 different ones, and neither of them were right. Like Imported John said, if it is electric choke, it is not a marine carb. Carlos himself told me that, after I questioned mine. If it is electric choke, is there vacuum ports on the front that aren't blocked? He told me mine were blocked(epoxied). And they wern't. Also, I am told the marine carbs, the rods that hold the throttle blades are green in color. And, I called them, and bought it direct, so it was cheaper then buying it off Ebay. Oh how I wish I would have bought it off eBay, so I could leave negative feedback.

If you need a book on Qjets, let me know. I bought one. Still have it.

My I5 carb is good as a boat anchor. Mine had the same flat spot or bog when the secondaries opened. And it was way lean in the top rpm range. I messed around for awhile. Till I burnt 4 valves in my engine do to running lean, and had to tear the cylinder heads off, and have them rebuilt. I then bought a 1409 edelbrock. No more bog, and no more lean spot. I know the qjet can be a good carb, but i will never buy a thing from I5 again. Hope they did better with your carb then mine.

 

[imported_TheMan]

Re: What Throtle bracket to use 2bl to 4bl upgrade

the 4BBL is only a 15HP gain on a V6
Only the MPI's make an honest 220

Thinks may be different on newer engines I suppose, but the output listings in the past were 30 HP for a 2 barrel to 4 barrel complete swap.

 

[John_S]

Re: What Throtle bracket to use 2bl to 4bl upgrade

Info: http://www.florida4x4.com/tech/quadrajet/

I found this helpful: http://www.florida4x4.com/tech/quadrajet/qtune.pdf

Walk through the whole process. Trying just to attack the flat spot, may just spin wheels.

Your first step, before tuning, is to identify exactly what carb I5 sent you.

 

[kpiazzisi]

Re: What Throtle bracket to use 2bl to 4bl upgrade

I spoke the Carlos yesterday and he also recommended bending the secondary hangers to give more fuel at the higher RPM. It hardly sounds scientific. I have looked at some publications where they are talking about using calipers and calculating metering area using equations, and this guy is telling me to bend something in the carb. Wow! I also told him that the engine runs better with the mixture screws about 4-5 turns out (pretty much unseated). I implied that I think it is running too lean and started to hint around about possibly re jetting the carb. In my opinion be became uncomfortable at this point and kept repeating that I the mixture screws should be about 2 1/2 turns out. He asked where I would get the jets, maybe insinuating that they are hard to come by? Maybe even for him, and this is why the others have experienced mismatched parts? I had problems with the 2bbl I purchased from him and now this 4 bbl isn't right either. I wish I would have purchased the Eddlebrock. Does anyone know if the jets for the Rochester are easily available and if so from whom?

Thanks
Kevin

 

[Howard Sterndrive]

Re: What Throtle bracket to use 2bl to 4bl upgrade

runs better with the mixture screws about 4-5 turns out (pretty much unseated).

I'd try the propane enrichment test or other method to look for a vacuum leak around the base gasket and manifold.

Looking through my records, I see I have an I5 carb. I wasn't sure where I'd gotten it. My accelerator pump needed changing day 1. Gas leaking around it and washed the black paint off. I might just get a whole kit and go through it.

2 turns out on the idle screws runs fine on mine though.

 

[John_S]

Re: What Throtle bracket to use 2bl to 4bl upgrade

Does anyone know if the jets for the Rochester are easily available and if so from whom?

I answered your PM. Some jets and rods are available for the Q-jet from edelbrock. They had a q-jet clone carb. You need to get a good book on the q-jet to help decipher which version I5 sold you and what the specific rods/jets/springs that fit it.

 

[kpiazzisi]

Re: What Throtle bracket to use 2bl to 4bl upgrade

Was it Eddlebrock or Carter who made the clone. Mine actually says it is a Q-jet made by Carter. It's part of the casting. Is Carter and Eddlebrock somehow connected?

 

[John_S]

Re: What Throtle bracket to use 2bl to 4bl upgrade

As far as I know Carter is a seperate company that made most of the original Q-jets. I don't know if it is still around, if edelbrock purchased them or rights to mfg etc. Edelbrock only made automotive versions and I don't see them listed anymore on their website. They do offer q-jet parts:
http://www.edelbrock.com/automotive_new/mc/carbs_acc/access_q-jet.shtml