Engine Bogs at Full Throttle

[Rebel96]

Hi All,

My in-laws have a 1987 20' Larson Delta Conic with a 305cu. 230 HP V8, and I believe a 4bbl carb. I cannot verify the carb or provide serial #'s because I wont be seeing the boat until this coming weekend. The boat went into stoarge last fall running perfect, aside from starter motor and alternator issues which the marina fixed while in storage. Boat came out of storage last week and while driving it home from the marina whenever the throttle was pushed past the 80% range up to full, the engine would get bogged down and wouldnt go anywhere. Plus from a dead stop and full throttle take off, the engine would get bogged down as well. But when the throttle is increase gradually (until you hit the 80% mark) the boat ran great. Could i be on the right track in assuming that the second set of barrels arent opening properly or at all? Could anyone offer a remedy or other way of diagnosing this problem further.

Thanks.

 

[Haut Medoc]

Re: Engine Bogs at Full Throttle

Welcome to iboats......
I would start by changing the fuel filters for starters.....
Is this Merc, OMC, VP?​

 

[bwthor]

Re: Engine Bogs at Full Throttle

It isn't just a problem with old fuel is it?

 

[Rebel96]

Re: Engine Bogs at Full Throttle

I doubt it, boat was pretty low on fuel when it was picked up, so we threw in $20 of fresh fuel before we set out from the marina to the cottage.

 

[Rebel96]

Re: Engine Bogs at Full Throttle

Engine is a Mercruiser, with Thunderbolt Ignition.

 

[Coors]

Re: Engine Bogs at Full Throttle

I doubt it, boat was pretty low on fuel when it was picked up, so we threw in $20 of fresh fuel before we set out from the marina to the cottage.

So the almost empty fuel tank got alot of condensation in it during the winter? Change filters and separators, and add seafoam, to begin with.

 

[Rebel96]

Re: Engine Bogs at Full Throttle

Alright thanks ill give all that a shot and report back.

 

[Rebel96]

Re: Engine Bogs at Full Throttle

Ok, back from the weekend and here is how it turned out.

I changed filters, seperators added some seafoam and 15 gallons of fresh fuel, burned through probably 5 gallons of gas to get it through the system.

Still getting the same problem, i removed the flame arrestor and monitored the carb while we were moving. Basically the top flap on the primary's is open 100% all the time no matter what level of throttle. The secondary's top flap opens about half way at full throttle and the engine bogs (i believe there is just too much air getting in there disrupting the mixture), if i manually close the top flap while at full throttle the boat takes off and performed perfectly. Or if i just keep my hand on the secondary's top flap and keep it closed from a dead stop to full throttle take off, works beauty...
Engine info which i confirmed.
305 Mercruiser Alpha One, with Thunderbolt Ignition, 230HP V8 4bbl rochester carb. Any help would be greatly appreciated thanks!

 

[180shabah]

Re: Engine Bogs at Full Throttle

(i believe there is just too much air getting in there disrupting the mixture)

There is no such thing as too much air, There is however, not enough fuel. It sounds like you may be due for a carb rebuild. This is no big deal as long as you are clean and organised. It is also pretty inexpensive if you do it yourself.

 

[180shabah]

Re: Engine Bogs at Full Throttle

Hopefully, while you had one hand on the carb(with flame arrestor off) you had the other hand wrapped firmly around a fire extenguisher

 

[Rebel96]

Re: Engine Bogs at Full Throttle

Alright carb rebuild it is, and yes the fire extinguisher was at hand!

Thanks for the info, how should i go about doing this? I am mechanically inclined to do the job, as long as there is some rough instructions. To be sure i get the correct kit where abouts would the serial numbers be located to know exactly and for sure which carb I have?

 

[mudweiser]

Re: Engine Bogs at Full Throttle

The Rochester Quadrajet has the Identification numbers stamped into the vertical boss on the driver's side secondary throttle shaft. The code of manufacture is also at this location. Rochester had some carburetors that were made by Carter and show the Carter designation.

I just went through the same issue. 89 Bayliner with a 305 LX. Rochester q-jet. Pulled the carb and rebuilt it. The kit was about $26.00. I still have the same problem as before. If you get yours going, let me know and i'll do the same.

 

[John_S]

Re: Engine Bogs at Full Throttle

I think they are right on for a carb rebuild. It sounds like there are at least three issues with it.

1) Choke is not closing - the plate over the primaries should close when you pump the throttle for cold start-up.

2) Hard Accell Bog - Probably Accelerator pump leaking or adj.

3) Wide open Bog - Secondaries mechanically open near the 80% throttle and if there is enough vacuum the air door (large plate on top) will start to open. On a 305, it should never fully open.

All of these are covered in a carb kit and standard rebuild calibrations in Merc manual. The special tool to set the air-door opening (gram scale) for spring wind-up may be the most difficult to obtain. Some people adj this by trial and error.

Make sure you use a new base gasket and torque the carb down per specs to prevent vacuum leaks.

 

[mudweiser]

Re: Engine Bogs at Full Throttle

Got mine figured out. I bought another rebuild kit, just so i had new gaskets again. I also replaced the float, but i don't think that was the problem. I found there is a plastic cam on the secondary's butterfly upper shaft. This cam was cracked, so it would never lift the metering rods. I superglued it for the time being and that took care of the problem. I know this is just a band-aide fix, but i will have to find the cam at a salvage yard or something. Anyone have a scrap Rochester q-jet laying around?