Not enough rpm

Author Message   Robert Carlson Member Username: floater1 Post Number: 72 Registered: 10-2008 Posted on Tuesday, September 22, 2009 - 08:11 pm:    Finally have my rig seaworthy and running after the engine swap. Its a '78 225 I/O. I kept everything thats OMC intact that originally came stock on this engine. On the water I'm only able to pull 3200 max rpm at WOT. I did previously put in a new fuel pump and filters. I have adjusted and re adjusted the throttle/carb linkage to what it should be. Where could my extra rpm be hidden? I am running the original coil (yeah I know I should have upgraded with the swap) could that be the issue? I should also add that before dropping the engine in the boat it checked out with 150 psi in each cylinder. I have yet to check the spark plugs and see how they look. Any chance a fouled plug or bad spark plug wire could be holding the rpm back here? Any suggestions are much appreciated.   Hy Stat Senior Member Username: hystat Post Number: 1497 Registered: 05-2007 Posted on Tuesday, September 22, 2009 - 10:57 pm:    failing coil and other possibilities you list usually show more at idle, not high rpm. What is this engine? Another 305? Does it have a marine cam? Some smog-cam california 305's made as little as 150HP with a 4BBL carb. aside from that, triple check the firing order, timing, timing advance and look for vacuum leaks.   chiefalen Advanced Member Username: chiefalen Post Number: 266 Registered: 03-2008 Posted on Tuesday, September 22, 2009 - 11:15 pm:    yea could also be the valves set to tight, try a vacuum test heres the link let us know whats what after the vacuum test. http://www.secondchancegarage.com/public/186.cfm   Robert Carlson Member Username: floater1 Post Number: 73 Registered: 10-2008 Posted on Wednesday, September 23, 2009 - 07:31 am:    Yes its another marine 305 ('84 Mercruiser 228). I will do the above and report back when completed.   Ricardo E. Advanced Member Username: ricardomarine_vp Post Number: 622 Registered: 03-2009 Posted on Saturday, September 26, 2009 - 10:50 am:    Robert, Your hull's bottom must be clean.... drive/prop must be clean..... prop must be in tune..... boat cannot be stern heavy..... drive ratio must be correct for the engine type! Prop must be correct for your WOT RPM spec! This cannot be determined until the engine is in near perfect tune. It's sort of the chicken and the egg thing! All too often, TA (total advance) is not discussed. If you are timing for BASE timing only, I'd recomend that you at least check to see what your TA is doing @ 3,200 RPM. See your OEM specs for your engine. Do NOT use automotive specs for this. Too little TA, and the sacrifice will be power. Too much, and you risk detonation..... a very destructive force in a gasoline marine engine. You may find some info here that will be helpful. http://www.marineengine.com/discus/messages/12487/293828.shtml   Robert Carlson Member Username: floater1 Post Number: 75 Registered: 10-2008 Posted on Sunday, October 04, 2009 - 10:06 pm:    Update here: Rechecked timing (Ricardo thanks) and yes its set to factory specs. The hull is clean and I'm running the same pitch prop as what originally came on this boat. Checked for Vacuum leaks and found none. Firing order checked all plug wires correct. Pulled and checked all plugs, they are a nice brownish color. Now here is a question for you all. When I'm throttling up to WOT I'm not getting that woooosh of the secondaries opening. It feels and sounds like its just running on two barrels. Further checking indicates that my brand new Sierra replacement 4 bbl carb doesn't want to open up the secondaries at WOT. With the engine running in neutral, I held the secondaries wide open by hand and my buddy revved up to 5500 rpm no trouble. The carb gasket is correct so the butterfly is not getting held up on that. Its my understanding that automotive carbs the secondaries are vacuum operated. Is this the same case for marine 4 bbl carbs? Looking at the linkage on the Sierra carb its clear that the way the linkage is set up on it, doesn't allow for the secondaries to open up more than 1/8th of what it should. I have previously adjusted my throttle cable and rechecked it again recently to make sure its where it should be set. A buddy recently rebuilt my old Holley 4 bbl (which came on the engine when I got it) so I'm going to swap that back on for now and see what happens. Only a week or so left before the weather drops below freezing. Could the issue be the secondaries not opening?

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