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A friend of mine has a boat with a big block chevy that runs up to 3000rpm then bogs down.It recently overheated due to failed impeller.Due to the overheat it blew a head gasket and got water in oil. He took heads and intake off, had heads checked,repaired.he is getting fuel to carb(4bbl).The bogging down happens just as the 4bbl open.He has adjusted carb per factory info.It seems to me the ignition is not advancing which is where i am going brain dead.How does the electronic distributor advance? I’m pretty sure it doesn’t have the centrifugal weights like they used to have and there is no computer like in a car and i probably know the answer but like i said I’m going brain dead.The boat is about a 2000(yr). Any help with cause of bog and how dist. advances will be appreciated.Thanks
ps.I had to leave before we could put a timing light on it and he was pretty fed up so i am hoping to get a solution so he can have some boating on his last week of vacation.Thanks to all for the help.My friend decided not to waste he vacation time working on it so he is going to take boat home to work on it. He has another boat to use while on vacation anyway.
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Did the motor bog before the over heat? can you see fuel spraying
down the carb. Usually an electronic distributor would control when
spark is fired. The fuel is controlled from the carb.It sounds like a fuel problem to me too. At 3,000 revs full advance has occurred, typically, and there is more than ample siginal to pull fuel from the secondaries regardless of cam choice. I’m thinking this thing has a Holley carb with a bad power valve. I’d also bet it’s running rich and stumbles a fraction of a second after the throttle is advanced to wide open. If this has a magneto or some other type of point set triggered ignition, look there first. That’s my best guess…
P.S. Is this a Mrec Cruiser?
It is a Merc Cruiser and I told him before I left to check the power valve not sure if it’s a holley. It doesn’t have a mag just a standard electronic distributor which he put in new rotor plugs and wires
It did not bog down prior and it accelerates and runs fine up to the 3000rpmand he said he can see the fuel spray but i do not know if he ment he can see it from the mains AND secondaries.with it bogging down as the secondaries open i am betting on the power valve or a pressure problem like the other guy said in his answer.
Where Merc Cruiser is concerned I can only guess as to the timing/advance curve they use, however, any BB Chevy will respond well with 35 to 40 degrees total advance @3,000 revs.
Your friend should look at the secondary side of his carb. I’d bet it’s a Holley, and I’d bet the vacuum dash pot controlling the secondaries is blown along with the power valve. Holley carburetors are known to suffer this kind of damage from a back fire.
I’d bet when your friend advances the throttle to wide open, that’s exactly whats he is getting -WOT (in short the secondaries slam open creating a lean bog). They should open gradually as engine demand dictates. A mechanical double pumper carb compensates for the lean bog with the second accelerator pump, but not so for the vacuum secondary carb without metering rods like Holley.I will let him know.I forwarded all the info from you and the others to him early this am but have not heard back as yet. will let you know what ends up as the problem when i hear from him.Thanks
[quote=”a88racefan” post=106396]It is a Merc Cruiser and I told him before I left to check the power valve not sure if it’s a holley. It doesn’t have a mag just a standard electronic distributor which he put in new rotor plugs and wires[/quote]
I did a quick google on Merc Cruiser, and it was all over the map where carburetors are concerned. Seems Merc used every carb known to man.
That said, since it’s a boat there is a very good chance the fuel is contaminated (and I have doubts about 10% methanol too).
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