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Rochester 4bbl tuning

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chris_f

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"I put a Rochester 4bbl on my

"I put a Rochester 4bbl on my 1986 170 Four Winns 4.3. omc. It is a factory rebuilt marine 4bbl that was listed for a 4.3 mercrusier. Four Winns offered it as an option in 1986.It does do what I want- it gives me more power at hole shot with a full boat and pulling a skier. It allows me to do the same at a 5000' altitude lake we visit (which I couldn't do before).Problem is- engine runs great at WOT 3800-4000, it runs great at lower end of rpm. It starts to stumble at 2900 to 3400. It acts like it is getting too much fuel. I have looked into the carb while this is happening (with my wife driving boat) and the secondaries are just starting to open at this point. I have adjusted the air valve spring on the secondaries from tight to loose and at sea level it doesn't seam to make a difference. (At 5000' the air valve has to be set loose to run correctly maybe thinner air?) I was wondering if changing the metering rods in the secondaries would help smooth out engine at that rpm? Any suggestions welcome, Thanks"
 
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"<center><table border=1><tr><td>http://www.marineengine.com/discus/icons/mime_html.gif""" align=left alt=""text/html"">Secondary adjustment
[url=""][b]repairInfoPages.htm[/b][/url] (80.4 k)</td></tr></table></center>

It might be worth starting from scratch and synchronize the bottom secondary throttle plates "Secondary Open Adjustment" with the secondary top air valve "Air Valve Spring Adjustment". Please see the attachment with regards to the adjustments. There are a few other adjustments mentioned in the attachment with regards to secondary lockout and secondary closing that may not be applicable to your marine application carb. If the bottom plates are partially open however the carb cannot flow the proper cfm due to the position of the top air valve you may get the same symptom. The carb is definetely flowing enough fuel and air at the low end and high end it is just the transition point of the secondary opening up.
info on carb adjustments found on [url=""]www.autozone.com[/url]"
 
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