converted an early 4.3 from 2 to 4 barrel carb. getting issue at WOT

pae0332

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Jun 8, 2017
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hey i got a question i want to through out to see if there is anything i need to look at. i converted a 1991 4.3, pre vortec,pre balance shaft, engine to a 4 barrel carb, it runs great until you get it opened up. it hesitates and stumbles.
is the timing different with a 4 barrel compared to 2?
im wondering if the fuel pump cant keep up but i doubt that is the issue.
or maybe the ignition system is breaking down.

anyone do this conversion and have this problem?
 

Lou C

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Nov 10, 2002
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Which carb did you install? Different issues for the 3 main types used, Merc/Weber/Edelbrock, Holley or Quadrajet. Is it a marine carb for sure?
 

Lou C

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Has it been gone thru, cleaned and put back together with new gaskets, new marine style accelerator pump?
A common problem on Quadrajets is the adjustment of the secondary air doors. The way the Quadrajet works for the secondary system, the secondary throttle plates are mechanically opened but the air doors above them are vacuum operated. The point at which they start to open is adjustable and if they open too soon you will get a big bog when getting into the throttle.
So look up how to adjust these, it is well-discussed on the internet...
 

Lou C

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The plus to them being set up right is the classic Quadrajet howl when they open, sounds just like classic GM musclecars most of which had Quadrajets, except the highest performance variants that had Holleys.
 

pae0332

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its a new build from a marine carburetor shop on the internet
gauranteed carburetors
 

bajaman123

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I'm thinking you may have TOO much carb for that little engine...it should be the same model as what is used on the 'LX' versions of this engine from Mercruiser. It may not be jetted quite right yet. Does it work okay (or better/different) if you GRADUALLY get on the throttle as opposed to dumping it, like for a hole-shot?
Being VERY careful, you could take the flame arrestor off and have an assistant throttle the engine whilst in neutral, while in a lake...don't try this on muffs!...and you should be able to see down into the carb, see how the secondaries are opening, see what the fuel spray looks like, etc.
 

pae0332

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this carb was available on this engine in the 80's & 90's so i do not think its too much carb for the engine. yes it runs great around the lake but at WOT is where there is an issue
 

ESGWheel

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@Lou C
Help me (others) understand what a 'marine style accelerator pump' is
thx
 

Lou C

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The Quadrajet was available with different size plungers for the accel pump and this could then very the amount of fuel the pump would dispense. Similar to how a marine carb is jetted more rich than an automotive carb…
 

pae0332

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not saying your wrong but i would hope that a company that speciallizes in building and selling marine carburetors would know which parts to install in said carburetor. however i have seen worse errors
 

bajaman123

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this carb was available on this engine in the 80's & 90's so i do not think its too much carb for the engine. yes it runs great around the lake but at WOT is where there is an issue
Good...that was the question, was it the "LX" carb.
Make a high-speed run and then shut it down, don't idle. Let it cool some and pull a plug or two and read them...see if you are running lean or rich.
 

Lou C

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While it may seem like it the Quadrajet is not too much carb for a 4.3 especially in marine use, a properly set up QJet is almost like a variable venturi carb.
Post your concern up at Cliff's High Performance Quadrajets, they have a forum there and he does answer questions.
My OMC 4.3 came from the factory with the Quadrajet so it was set up right. This is a problem with rebuilt or remanufactured Quadrajets and keep in mind there are no new ones, and have not been for about 30 years. The ones you buy can be good, or a combination of various carbs, making it hard to get it dialed in.
 
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