Trojan Boat Forum

Not taking away from all the good suggestions but don't overlook the simplest of things...

Just because your engines throttle up to a high rpm in neutral does not mean there is NOT a fuel issue. There is a difference in fuel pressure and fuel volume.

With this being said, it is very possible that you have a fuel issue. Here are some things to check but you never said which manufacturer you have as engine. Mercruiser, Crusader, etc. Also, whether you have two tanks or four tanks.

Be sure that whichever tank you draw from, that the manifold crossover valve is closed.

Crusader engines for example, use a fuel water seperator, a fuel filter between pump and carb and finally, a filter at carb inlet. These 3 must be spotless.

Pick up tubes, can surely rust and develop leaks which will draw air. Reving in neutral can surely produce high rpms because fuel is still in float bowl, but on demand and under a load, you'll need the volume to keep up...

Check fuel tanks to be sure they are vented. Mud dobbers, spiders, etc can make the vents their home and block the tank vents. We all know what happens when a fuel tank goes into a vacuum mode. Try loosening caps to vent.

Also, if modified, be sure all flexible fuel lines are correct in application and are not collapsing under load demand.

Engine room also needs to be ventilated, it needs to be able to draw so much cubic feet of air, into engines..

I have had just one of my filters get enoguh dirt in it to not get past 2500 rpm. It does not take much...

Don't spool the engines up that much with no load (neutral). I wouldn't go any further than 2500 rpm out of gear.

Ok, first let me report that we had some success. 16 new sparkplugs and nothing else. Port motor ran up to 4600 RPM, starboard motor up to 4200 RPM equated 25 mph. New wires, cap and rotor this afternoon. Hopefully will get equal performance from each motor. Huge improvement already though.
My mechanic and I were discussing what reaction I would receive from this board...... Admittedly it sounds silly not to have replaced plugs first but - the previous owner said he just had - and they looked fine. Who knows.
I will post updates and thanks again for all the info

Even though it didn't run up to RPM before it was running smooth?

Looks like you found the major problem with the engine. It just sometimes is a simple thing to overlook the wires and plugs but if someone had told me they just changed them, I would have tended to not look at that as a problem first.

maybe your tachs could still be off by 400 rpms unless the bridge tachs agree with the lower helm tachs. So trying to get equal performance might not happen if th tachs are off. You might try to see if the two tachs agree with a known good tach.

Yes, the engines were running smooth before the new plugs.
I checked the lower helm tachs against the timing light and they were off by 100 RPM or so. Funny thing is the flybridge tachs seem to match almost exatly. It is tough to tell a 100 rpm difference though.
Without buying a floscan I am wondering what fuel burn numbers to expect. At 3500 RPM I was averaging 18 mph.
Perhaps I should start a new subject? Advice welcomed.......

18 mph seems pretty slow with 350's at 3500rpms
what were you measuring your speed with?
i get 18 with my chrysler 318's at around 2800 rpms. my props are 16x15.
i installed flow sensors that are nmea2000 that work with my lowrance hds system but heres a chart a member landvf38 posted in 2008. it is for a f36 but i have always thought it is a good reference. with the 350's you have it would be in the ball park

http://www.trojanboats.net/wforum/viewt ... highlight=

You should be getting around .8 mpg burn rate or at 18 mph about 22.5 gallons fuel burn per hour

glad you are now getting revs but something is still wrong. 18 mph at 3500 rpm is still 50% slip - Way too high.

jav wrote:glad you are now getting revs but something is still wrong. 18 mph at 3500 rpm is still 50% slip - Way too high.

For a point of reference, with my Chrysler 360's (260hp each) and 16x16 cupped props I get about 24 knots (on gps) at 3500 rpm. I don't tend to run like that much as the 4 barrels open up just before that... Cruise is best on my engines between 2800 and 3200.