Dropping RPM over 3K – almost like starving for fuel – opinions?
 

Rebuilt engine (was HP500C). No fuel or rpm issues before pulling engine in October. Upgraded everything but the block and crank over the winter. Bore .030 over, rods, pistons, AFR heads, prosystems venom II 1000cfm carb. On the dyno we used my full exhaust, the engine and exhaust cooled by the seawater pump. Engine ran well to 6K with full dyno load and made 600hp.

Engine installed in boat and run one day a few weeks ago for ~1 hour under 3K rpm checking for leaks, strand noises etc. The second time out this past weekend I can maintain any RPM less than 3000 without problem. When I go over 3K it almost like it’s running out of gas. At 3200 it takes a while for the rpm fluctuation to develop, and much quicker to happen at 4k rpm – leading me to suspect a fuel supply issue.

The significant differences between the dyno and boat – Dyno used the shops 12V system, fuel supplied by their electric pump direct to my carb, and the load. Boat – 12V system appears okay, 13.8V on the gauge with no fluctuation. Load- I think not an issue, if my drive was bad it would be bad at lower than 3k as well as above and there were no changes to the drive other than fresh oil from last year (maybe this is a bad assumption). The fuel supply again is leading the list of differences

I have the Airtex 60932 mechanical pump; it was bolted to the seawater pump-housing while on the dyno. Is there potential to ruin or damage the pump running it w/o fuel for an hour or two? Can the pump supply the necessary amount of fuel for the new engine configuration? Did I get a faulty out of the box pump? I still have the OEM Mercruiser pump to swap back in as a test – the Airtex was brand new for this build. I swapped to the new pump because the Merc. pump has some wear on the arm from lack of lube in the seawater pump housing.

My plan is to plumb in a fuel pressure gauge and test this weekend to see if I am dropping pressure above 3K – I have checked the supply from the tank to the fuel/water separator, there does not appear to be any obstructions, clogging or kinks that would reduce flow.

For the fuel pressure gauge, is the plug in the unused fuel inlet on the port side an okay place to hook into or do I need to be plumbed into the supply line?

Is there anything else I should check?

Thanks for any help and insight.

Ryan

http://i531.photobucket.com/albums/d...psluoc3uio.jpg
http://i531.photobucket.com/albums/d...pskxrjtpkf.jpg

An Airtex 60932 is only a 40GPH @ free flow pump. That is a problem within itself. That supported the original 500HP?

Rookie,
I never used the airtex pump on the HP500, the stock pump was on it and during the R&R of the seawater pump housing I found wear in the pump arm so I replaced the whole fuel pump. I found an old thread on OSO indicating the airtex was an equivalent to the OEM pump - I think this is the thread

http://www.offshoreonly.com/forums/g...ion-hp500.html

If it's just this easy and the airtex will not support the fuel needs of the engine, that's good news - simply get a different pump and I can finally enjoy what's left of the season :)

BTW - Thanks for the photo description of the seawater pump rebuild you posted last year.

Ryan

Even at free flow rates 600hp would need 50gph, and many internet mechanics argue that you need to at least 3-5x free flow for real world demand. My stock mercruiser pump (65gph) peters out at 4500 on my 502/500hp engine. I'm ordering the Holley 130gph where the bottom bolts directly on to the top stock mercruiser pump. I'd think that would be the minimum you need for 600hp.

http://www.jegs.com/i/Holley/510/712-454-13/10002/-1

Optimal GPH = (Max. HP x BSFC) / 6

http://www.onallcylinders.com/2013/0...ions-answered/