Brand new 5.7 runs poorly

13crazyhorse · Jun 20, 2013

I have an 86 Four Winns boat that I just installed a brand new 5.7 Mercruiser motor in over the winter. It starts and runs at idle perfectly. It won't rev up very well and when under load runs very poor. I can go around 1500 rpms with no problem but slightly above that it "coughs" every couple of minutes. The more gas I give it the worse it gets. I can get it up to around 3000 rpms and it runs very poorly with no power and sputters alot. Top speed at this rpm is about 8 mph. I can hear the 4 bbl open but the boat will not accelerate. The motor came with new heads, I had to put on my old intake and carb. I put on a brand new Delco ignition system, wires, plugs and new risers. The exhaust manifolds were good. Last week I removed the carb and totally rebuilt it including soaking, etc. and also changed the fuel/water filter and the boat runs basically the same. The timing was set and rechecked with a digital timing light. Any suggestions would be greatly appreciated. I think it's probably something simple like maybe a carb adj but I don't see any on the unit. Thanks ahead of time for suggestions.

Bondo · Jun 20, 2013

Re: Brand new 5.7 runs poorly

Ayuh,.... What killed the old motor,..??

How old is the fuel,..??

It's goin' Lean, everything from the tank's vent, to the carb is suspect,....

13crazyhorse · Jun 20, 2013

Re: Brand new 5.7 runs poorly

Have no idea what killed the old motor it's not talking. Bought it like that. The fuel was all drained and replaced with new. I didn't check the vent but didn't suspect that since there was no pressure when I removed the carb or fuel filter. I think I blew that out before but not 100% sure could give that a look. Like I said the carb was redone this week without much change. Fuel pump also was put on new.

Bondo · Jun 20, 2013

Re: Brand new 5.7 runs poorly

13crazyhorse said:
Have no idea what killed the old motor it's not talking. Bought it like that. The fuel was all drained and replaced with new. I didn't check the vent but didn't suspect that since there was no pressure when I removed the carb or fuel filter. I think I blew that out before but not 100% sure could give that a look. Like I said the carb was redone this week without much change. Fuel pump also was put on new.

Ayuh,... Which carb,..??

Carb rebuilds are 'bout a 50/50 shot, in a well equipped shop,... mine included,...

How bad was it,..??

13crazyhorse · Jun 20, 2013

Re: Brand new 5.7 runs poorly

Thanks Bond-o for the input. Rochester 4Bbl didn't seem to be in real bad shape. I don't think there's any kind of adjustment I can make on this model that I see? This backfire or cough through the carb in indicative of a lean condition? I guess the lack of power or bogging might indicate that.

Don S · Jun 20, 2013

Re: Brand new 5.7 runs poorly

Do a vacuum test on the engine. The secondaries are vacuum operated. No vacuum, no secondaries.

Should be a pipe plug on the back of the intake you can install a fitting in for a vacuum test.

View attachment Vacuum testing.pdf

If the carb rebuild was not done properly, it's not going to work either. Rebuilds takes more than just making it look nice.

13crazyhorse · Jun 21, 2013

Re: Brand new 5.7 runs poorly

Thanks Don it's a brand new motor but I'll give the vacuum a look this weekend. This is not the first carb I've rebuilt I've been doing this stuff all my life but I'll give that another going over if all else fails.

Volphin · Jun 21, 2013

Re: Brand new 5.7 runs poorly

Sounds like your spark plug wires are out of order or you are one tooth off on your distributor.

TheHunter990 · Jun 21, 2013

Re: Brand new 5.7 runs poorly

Is the fuel pump ok? Try T in a fuel pressure gauge some place after the fuel pump. You would want to see at least 5 psi at any throttle position.

TheHunter990 · Jun 21, 2013

Re: Brand new 5.7 runs poorly

When checking the timing did you rev engine up till 3000 rpm? What was your timing there? 30-32 degrees?

OldNBold51 · Jun 21, 2013

Re: Brand new 5.7 runs poorly

timing locked down in "base mode"?

13crazyhorse · Jun 21, 2013

Re: Brand new 5.7 runs poorly

I set the timing as per recommended by Delco. There's a jumper wire to use for the base timing and then I set the timing at the recommended 8 degrees. I will also recheck that this weekend. I did not check it at 4800 rpms as they recommend - where it should have a total advance of I think 27 since I can't get it to rev that high.

Don S · Jun 21, 2013

Re: Brand new 5.7 runs poorly

I set the timing as per recommended by Delco.

Did you change ignition systems? If OEM, you don't have a Delco ignition, you have Merc Thunderbolt IV.

Assuming you have the stock Thunderbolt IV, you don't have to go to 4800 rpm to check advance, and there is no base timing mode. ignition system. You should see full advance by 2500 rpm.

13crazyhorse · Jun 21, 2013

Re: Brand new 5.7 runs poorly

Don I stated in my original post that I had installed a new Delco Voyager EST ingition system. After spending some time thinking about this I tend to lean toward maybe I did something wrong in setting the timing. I will also double check that this weekend.

Don S · Jun 21, 2013

Re: Brand new 5.7 runs poorly

13crazyhorse said:
Don I stated in my original post that I had installed a new Delco Voyager EST ingition system. After spending some time thinking about this I tend to lean toward maybe I did something wrong in setting the timing. I will also double check that this weekend.

I just went back and read it and didn't see it, then read it again and did see it.:facepalm: Sorry

vetting · Jun 21, 2013

Re: Brand new 5.7 runs poorly

If you put all new gas in it, it could have stirred up the sediment in the tank. Replacing your fuel filter wont even help it. Take the fitting off you your tank and check the screen - look for chunks of crud/rust on the screen.

Had the same thing when I first bought my boat. Would start to hickup at higher RPMs and the fall flat on its face when you gave it more gas. Cleaned the screen off on the pickup and from there it was fine.

13crazyhorse · Jun 23, 2013

Re: Brand new 5.7 runs poorly

Well this boat is 150 mi away but I had a little time to tinker with it over the weekend. I pulled the fuel line off and blew into it and it is very free so there's no obstrucion there. I did the same with the vent and that's also clear. I put the shunt on the dist and rechecked the base timing. It was at the correct 8 degrees BTDC. I actually retarded it alittle since I don't think I can do any harm going in that direction (?). The boat ran quite a bit better like this. I set the base at 4 degrees. It now will run at about 10 mph at 2200 rpms. Still when I go above that it bogs down. I can hear the 4 bbl open up but the boat does not respond it actually runs worse. I did not get a chance to check the vacuum. I'm going to add some dry gas next time since it did "cough" a couple of times but not nearly as bad as before. It also revs up much better when it's not under any load. Thanks for your replys so far.

13crazyhorse · Jun 24, 2013

Re: Brand new 5.7 runs poorly

Any thoughts on the timing issue or why the motor would rev better when not under load? thanks.

Bondo · Jun 24, 2013

Re: Brand new 5.7 runs poorly

13crazyhorse said:
Any thoughts on the timing issue or why the motor would rev better when not under load? thanks.

Ayuh,.... Because it don't take any gas to rev to the moon, Without a Load,....

I still think yer lookin' at a Carb, or fuel system issue,...

NHGuy · Jun 24, 2013

Re: Brand new 5.7 runs poorly

Don't bother putting vacuum to the intake. The vacuum that opens the secondary air flaps occurs when the secondary throttle flaps in the base of the carb open. You can see them open by looking down the bores of the carb and having someone operate the throttle lever. The air flaps, which are on top, are spring loaded and as they open they cause metering rods to lift out of their bores allowing more fuel to the secondaries.
Since turning back the timing helped I'd go back through the ignition installation.
Seems to me someone on the forum just solved a Delco EST problem like this recently. Time to search...

Brand new 5.7 runs poorly

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