Logo
FULL SITE MENU

2bbl to 4 bbl conversion 4.3 problem

J

James Andrew Kenny

New member
Hi there I recently had a problem with my inlet manifold so I have updated to a new manifold and a new 4 bbl Rochester carb .....boat runs eat up to the point where the second stage opens at approx 3800rpm then the secon stage open and the engine seems to bog down ...if I back of and wait 10 seconds it picks back up....if I leave the throttle ...it stalls it sounds like over fueling in the second stage but I don't know how to adjust it .....any ideas
 
Not sure of the cure but if it was me I would talk to someone knowledgeable at a dealer and seeing if you could go to smaller jets for the secondaries.
 
The secondary's are probably not set up correctly with the opening tension/rpms (openning to soon or under to litlle load) and it may not be jetted properly.

Underneath the secondary top butterfly on the 2,4,6 side of the engine there is a very small set screw that is used to set the spring tension on the butterfly. Now this works in conjunction with the VACUUM and is set with a grams wieght scale.....YOU SHOULD HAVE A MANUAL!!!

If you are unfamiliar with any of this it would be best to get someone who understands how these carbs work!! Not for the amature!! Going from a 2 bbl to a 4 bbl is fine but if the carb was not set up for that specific motor then this is typical of a issue because of this...

could also be a vacuum leak,
 
Welcome aboard !

Year of engine ? Got a manual pump?

See there is a reason for my questions.

What pressure the fuel pump giving you.

Before you start fiddling with the carb reply !
 
I agree with what Paul is suggesting.
This may need attention and adjustment.

The Q-jet secondary throttle plates open progressively with the primary throttle plates... IOW, these are mechanically linked to the primary throttle plate operation.
The large air door over the secondary plates prevents venturi action until it begins to open at the correct manifold pressure (vacuum).
The air door is linked to metering rods that control secondary fuel metering.
All of this must work in sync.

See a good Q-jet manual for this, or go on line and look for an article.
Most will be for automotive, but should help you through this.


.
 
Master mechanic said:
Welcome aboard !

Year of engine ? Got a manual pump?

See there is a reason for my questions.

What pressure the fuel pump giving you.

Before you start fiddling with the carb reply !
Click to expand...




Because he had a carb before it would be expected that the fuel pump is correct. The V6 has been using a electric pump since the early '90's maybe longer... and it is preset for that application.

Not sure that is the issue,

It sounds like He simply is over carbed,

The question would be what did the carb come off of???
Was it a V6, 4,3? A V8 small block or possible even a big block? Maybe it is a automotive??

I think that is the first question that needs to answered......in my opinion......
 
James Andrew Kenny said:
a new 4 bbl Rochester carb
Click to expand...
what part number carb?
from what supplier?

you can probably get the "windshield kiss" out of it by adjusting the secondary spring as suggested, but if you have the carb numbers - good to see what it has for primary and secondary rods/jets

I bought one for my V6 from i5 on ebay... it was a beatiful build - brass float, very clean.... but it was jetted for a 7.4L... no spring adjustments were going to fix that
 
Is this carb your talking about new or used?Did you rebuild it?Or is it a rebuilt carb?
Many other questions need to be answered before a answer can be given.
Sounds like the jets need adjusted or was touched before.Or the carb needs cleaned or rebuilt.
 
jets.jpg


mine shipped with .072 primaries and .036 rods....that's a huge opening for fuel....in Rochester data book, I could only find that combo on a 1969 or 70 GTO Judge with a big block... and I forget which secondary rods were in mine, but they were way huge secondaries - so I was super rich at cruise and super lean at WOT- I found some very very close to DH's in another Qjet I had around here so I swapped them in - I thought I might get some more top end speed, but I didn't - still 47mph or so
the weird thing... my engine is quieter now at cruise...I can't figure out how the carb was making the engine that much noisier, but it definitely was.

If you need jets and rods- OMC (BRP) actually sell a jetting kit for about $40 which I discovered after I ordered two .066 jets from some carb place in Washington state - it cost me $40 just to get the two jets here

I now have a big plastic tool box with about 6 Qjets in it - all the jets and rod specs are now labeled in sharpie right on the secondary plate - why doesn't every carb rebuilder do that?.... e.g. 66/36/DH

funny i5 claims they run every carb on a flow bench....maybe, but then they mix them all up in inventory?
 
Last edited:
Back
Top