Fuel Pressure

[jkh04200]

84 Mercruiser 228, 305 Chevy I/O
Thing started laying down above 3000. I'm gonna replace the fuel pump. Its 22 yrs old, everything else is in great shape.
I rigged a gauge on the fuel line, 5.5psi at idle. Took it to the water and when I get up around the 3-3500 mark needle starts jumping between 3 & 5psi, hard to tell exact because its moving so fast. I'm thinking pump can produce the pressure at low demands, like idle. But cant keep up with the volume at WOT. Sound Logical?????????????

[Moneypitt]

84 Mercruiser 228, 305 Chevy I/O
Thing started laying down above 3000. I'm gonna replace the fuel pump. Its 22 yrs old, everything else is in great shape.
I rigged a gauge on the fuel line, 5.5psi at idle. Took it to the water and when I get up around the 3-3500 mark needle starts jumping between 3 & 5psi, hard to tell exact because its moving so fast. I'm thinking pump can produce the pressure at low demands, like idle. But cant keep up with the volume at WOT. Sound Logical?????????????
After you turn the motor OFF, does the pump hold any pressure on the gauge? A good pump should maintain pressure for a few minutes. If it drops off right away, suspect it, and check for gas in your oil..........3-5 lbs isn't that bad for your application, as long as it doesn't go below 3. Also, check your fuel filter, marina gas is to be avoided if at all possible, as it is usually very dirty..........MP

[SmokinLowriderSS]

Had it happen to me after 25 years. Took sight plugs out of carb primaries and could watch fuel level drop as Rev's climbed. Had no press gauge at the time. Am now running about 5 to 5.5 idle, 9 WOT @5Krpm.

[Wicked Performance Boats]

Should have 6 to 7 pounds all the time, anything less is insufficent. Budlight

[jkh04200]

UPDATE:
Replaced the fuel pump and insulated the fuel lines. This weekne boat ran great, for a while. We went about 20 miles and then it started missing. Went ahead a continued to the bar/resteraunt. We were there about a 1 1/2 and I had the deck up the whole time it was tied up. Heraded back and I make it within 200 yards of the ramp almost 30 miles, and then it started missing again. I'm sure its and air flow and vapor lock issue. Thats just screwy, for it to run that good for that long then take a crap on ya.

[Bow Tie Omega]

What kind of ignition are you running? Just wanted to mention this. When I first purchased my boat, before the restoration it ran flawlessly for the first 20 minutes or so, then it would start missing, very noticable above 3000 rpm. I would park it, take off the fuel filter, change it out and fire it up again. Ran great for 20 minutes, then it would start to miss again and loose rpm. My brother had the same problem with his. To make a long story short, both of our boats had bone stock HEI distributors. The stock automotive coils heat up and causes the motor to miss and run like crap. We both went with high performance coils for the HEI, problem solved. Do you have a stock HEI?

[jkh04200]

Yes mine has the thunderbolt IV ignition. I'm planning on changing the coil. It's probably original, 22 yrs old. Wouldn't hurt to change it anyway. I'm also gonna change the t-stat back to original. original was 135 and I put in a 160. It has run great for two years. The only thing I have done is put sun deck on the back. I think its holding in too much heat and not getting enough air flow. So, I'm gonn do the changes I mentioned plus I have found some vents that I can put on the sides. They look like the original but are about 2" taller and 12" longer. That will definatley help with oir flow.

[SmokinLowriderSS]

What kind of ignition are you running? Just wanted to mention this. When I first purchased my boat, before the restoration it ran flawlessly for the first 20 minutes or so, then it would start missing, very noticable above 3000 rpm. I would park it, take off the fuel filter, change it out and fire it up again. Ran great for 20 minutes, then it would start to miss again and loose rpm. My brother had the same problem with his. To make a long story short, both of our boats had bone stock HEI distributors. The stock automotive coils heat up and causes the motor to miss and run like crap. We both went with high performance coils for the HEI, problem solved. Do you have a stock HEI?
Had coil failure the following year. :cry: Was 23 years old then tho, and on as points setup. $17 at a nerby auto parts placve, and we were hard on it again. Took a while to figure out the heat-crash pattern.

[jkh04200]

I was planning on buying an original replacement form the marine supply store I use. Do you suggest anything different? The stock one worked for years.
The one thing that confuses me about this whole thing is when it acts up it feels like its lean. the plugs are bone white. As I said before I'm gonna go back with the stock 135 stat and a stock coil. I've had the coil in the back of my mind since I changed it a few years ago while at the lake. Coil was not the probem back then. Auto-Zone 11.99 special. Not sure on the quality of that coil

[Bow Tie Omega]

Let me start by saying that I am no expert. That being said, my understanding is that stock auto coils are not made to withstand the high rpms that a boat motor is running for extended periods. Some people seem to not have any problems and some have problems like I listed above. In a jet boat you very seldomly run below 3000 rpm. I know you said you had an I/O. It is a cheap fix regardless, especially in the past 2-3 years with all the after market companies jumping on the bandwagon with "high voltage coils". All I can tell you is upgrading the coil resolved our problem with great results... Your problems sounded pretty similar. Anyways, I hope this info helps and good luck...Joe