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Bf150 surging rpms

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Parker23

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I have twin '04 bf 150's. The stbd engine doesn't max out rpms like the port (5600). In addition, the engine will begin to surge rpms at 4400 and higher and this typically doesn't begin until after 30 minutes or more of cruising speed. We have vacuum tested fuel lines, replaced all filters , vst clean, cleaned injectors and replaced high pressure fuel pump and still have this issue. I think we have ruled out fuel as the problem. My mechanic hooked up computer while at wot and no codes. Everything appears normal but it is not. Any suggestions on where to spend my $ next? These motors have been great up until now. I can do basic maintenance but am not mechanically inclined.
 
You may not have eliminated fuel being the issue yet. Did your mechanic monitor fuel pressure as the motor was surging? Was it constant?

How old are your spark plugs. Maybe one is breaking down and not firing correctly.

Did the computer indicate whether the vtec solinoid was activated. The VTEC should kick in somewhere around 4400 rpm's.

Did he try a cylinder drop test as it was failing. Although not normally done at that speed, it might isolate the problem to one cylinder.

Pull off your vtec solinoid and clean the screen. Could be partially plugged. See #8 and #9 http://www.boats.net/parts/search/Honda/Outboard Engine/2004/BF150A4 LA/CYLINDER HEAD/parts.html

Just some thoughts.....but mostly questions.

Mike
 
Thanks mike. Fuel pressure was normal when tested at wot but engine not warm enough to begin surging. Will try to replicate surging by getting sufficiently warmed up then test fuel pressure and hook up computer. The spark plugs have been replaced. The vtec kicking in at 4400 is interesting. Will report back after today's test with mechanic
 
Honda mechanic came to boat today. I ran the boat for about 45 minutes prior to his arrival to get them warm. The surging began as I got it up to 4,000 -4,200 rpms and then it wasn't as pronounced the more I ran the boat. The mechanic arrived and put on a new fuel regulator, hooked up Dr. H to the engine and his laptop and off we went. The engine surging was not evident at all above 3,800 but there was very minor surging at 3,500. The VTEC came on/off right around 4,300 per laptop so solenoid appears fine. fuel pressure constant the whole time. Ran up to WOT and there was a 500 rpm difference which is noticeable. pulled port throttle back to match Stbd which was 5,000 to 5,100. Ran it for a bit like this and experienced no surging at all. Brought it back to lift and he inspected/adjusted throttle linkage which was fine The engine sounded smooth just a loss of rpm at the top end. We looked at prop and zero dings. The rpm difference starts about 3,500-3,800. Honda mechanic has a call into regional/national Honda tech on ideas for what to try next.
 
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