New Motor same power issue

[jststrd1]

Replace my 5,7 vortec with a rebuilt vortec. Block was rusted. When I get up to cruising speed, around 4000 rpms, it starts to pop through the carb. I have Rochester 4 barrel. The carb was replaced last year on old motor and i have a replaced one on this one also. Still pops through carb. I carried over the distributor which is an HEi, The module was replaced with a oem module last year, The cap and rotor are 10 hours old and the wires are 2 years old. I also used the old electric fuel pump. Timing was adjusted by tech that replaced the motor and he rechecked it. I was out 2 weeks ago and was ccruising and the boat just bogged down and could not get above 2000 rpms. I turned around to limp home and ran at 1500 rpms. I slowly accelerated got up to speed and after about 30 seconds bogged down again. did this several times. It feels like I am running out of fuel and whn I give a chance it catches up. I am leaning toward a fuel supply issue. Like fuel pump starting to fail, an anti siphon valve or fuel pickup. in tank. Any suggestions?

 

[Scott Danforth]

Mercruiser doesn't use HEI. They use their own Thunderbolt ignition

The Rochester carb went away after 1989. Electric fuel pumps were not needed until the vortec motor in 1996. So what vintage Frankenstein motor do you have?

Did you test the boat side of the fuel system? Popping is running lean

Test fuel pressure while running at WOT under load.

Could be anti-siphon valve or filter in fuel pickup tube

 

[Bondo]

Ayuh,..... Shoulda ran the old motor off a remote tank fulla fresh clean gasoline to test it,.....

 

[tank1949]

Replace my 5,7 vortec with a rebuilt vortec. Block was rusted. When I get up to cruising speed, around 4000 rpms, it starts to pop through the carb. I have Rochester 4 barrel. The carb was replaced last year on old motor and i have a replaced one on this one also. Still pops through carb. I carried over the distributor which is an HEi, The module was replaced with a oem module last year, The cap and rotor are 10 hours old and the wires are 2 years old. I also used the old electric fuel pump. Timing was adjusted by tech that replaced the motor and he rechecked it. I was out 2 weeks ago and was ccruising and the boat just bogged down and could not get above 2000 rpms. I turned around to limp home and ran at 1500 rpms. I slowly accelerated got up to speed and after about 30 seconds bogged down again. did this several times. It feels like I am running out of fuel and whn I give a chance it catches up. I am leaning toward a fuel supply issue. Like fuel pump starting to fail, an anti siphon valve or fuel pickup. in tank. Any suggestions?

The popping sounds like timing issues, but issues at slower speeds sounds like what Scott says. Also, check screen in gas tank pickup tube, which may be blocked and starving motor for fuel. I had quad's on a 95 that also had electric fuel pumps because the boss on the blocks didn't exist. Stay away from automotive replacement electric fuel pumps. I tried the cheap route, but it became a nightmare. I replaced the crap with Sierra's. Nightmares went away.

 

[kenny nunez]

HEI distributors require more amperage than the original Mercruiser ignition system. When the HEI is on a GM auto it is fed with a #10 gauge wire. Try jumping from the + side of the battery to the + side of the HEI with a #10 gauge wire and take the boat out to see if that helps. If it does then add a continuous duty power relay triggered by the original feed from the ignition switch and with the #10 wire from the + post on the starter solenoid to the input and out put to the coil on the relay.

 

[stresspoint]

how was the lash set on the new motor ?.

 

[jimmbo]

New Block, same ol Ignition, same ol Carb. Perhaps you are getting Crossfire, make sure the Plug Wires aren't all nicely tied together, check the Cap for Carbon Tracks inside. Electricity want's to complete the Circuit, and doesn't care what path it needs/uses

 

[QBhoy]

You’re either having timing issues or fuel issues. Be almost sure of it. Perhaps even both.

 

[jststrd1]

Mercruiser doesn't use HEI. They use their own Thunderbolt ignition

The Rochester carb went away after 1989. Electric fuel pumps were not needed until the vortec motor in 1996. So what vintage Frankenstein motor do you have?

Did you test the boat side of the fuel system? Popping is running lean

Test fuel pressure while running at WOT under load.

Could be anti-siphon valve or filter in fuel pickup tube

Scott it is rebuilt vortec purchased from a reputable company. It has a carb. Those are things I will be checking this Friday when I go down.

 

[jststrd1]

Ayuh,..... Shoulda ran the old motor off a remote tank fulla fresh clean gasoline to

HEI distributors require more amperage than the original Mercruiser ignition system. When the HEI is on a GM auto it is fed with a #10 gauge wire. Try jumping from the + side of the battery to the + side of the HEI with a #10 gauge wire and take the boat out to see if that helps. If it does then add a continuous duty power relay triggered by the original feed from the ignition switch and with the #10 wire from the + post on the starter solenoid to the input and out put to the coil

 

[jststrd1]

Putting that on the list to check.

 

[jststrd1]

how was the lash set on the new motor ?.

I do not know as it I purchased a long block complete.

 

[jststrd1]

You’re either having timing issues or fuel issues. Be almost sure of it. Perhaps even both.

I am leaning towards fuel. I purchased a new fuel pump and am going step by step to test things in fuel syatem

 

[jststrd1]

Ayuh,..... Shoulda ran the old motor off a remote tank fulla fresh clean gasoline to test it,.....

Old block full of rust did not feel comfortable with it. Thought I would do it right..

 

[stresspoint]

I do not know as it I purchased a long block complete.

ah ha , long block should have had lash set by builder , just to rule it out , you could ask the builder to recheck or do it yourself to be sure its in spec.

 

[Scott Danforth]

I am leaning towards fuel. I purchased a new fuel pump and am going step by step to test things in fuel syatem

do your troubleshooting first vs firing the parts cannon.

test the fuel pressure before spending money. little or no fuel pressure could be a plugged pickup tube filter or a stuck anti-siphon valve.