260b Quadrajet Bog

SHOTGUN RUSS

Seaman
Joined
Nov 30, 2016
Messages
53
i have a late 80’s 260b with a qjet on it and am having trouble with a bog under heavy load. If I have 4 people in the boat and go to wot the engine will just bog, if I go to 3/4 throttle it will put fine up on plane. If I am pulling a tube or put on my higher pitch prop I can only go 2/3 throttle before the bog. Took the qjet apart and cleaned it and installed new gaskets ( have full rebuild kit ). Secondary air plates seem good and have good spring tension to keep closed( known problem). Secondary Air plate vacuum diaphragm does not leak or bind. Noticed the qjet does not have the four tubes in the air horn, only two( missing the well- feed tubes and holes in air horn) but from a little searching it seems that is correct? To me all seems to look good with the carb but I have not watched it operate under load. Thoughts? Advise? Anything is appreciated. Thanks
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
Messages
47,307
What is your fuel pressure and compression numbers
 

kenny nunez

Captain
Joined
Jun 20, 2017
Messages
3,056
Follow Scott’s advice first, if every thing checks out then do this. Get a 6 gallon outboard tank with 91-93 ethanol free gas and connect it to the fuel filter with the primer bulb on the hose and take it for a cruise. Watch the primer bulb if it collapses at W.O. T. then you have a delivery problem from the tank. If the bulb does not collapse then have someone start squeezing it to see if the engine start to pick up rpm. This will help isolate the problem. The anti siphoning valve, collapsed fuel hose or the tank suction tube which may have a small screen filter inside may be clogged up.
 

SHOTGUN RUSS

Seaman
Joined
Nov 30, 2016
Messages
53
Can’t drive the boat yet, still ice on the lakes here. So I have not done any running checks yet. Compression is acceptable, can’t remember exact #s but when I purchased this boat I had done one and only have a dozen hours since. Forgot to mention everything runs very well in all other aspects except this problem.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
First things first, Have you had this boat very long? If so how long has it been doing this? If a While, did it appear and gradually get worse?

It sounds like a lean out. When you say "if I go to 3/4 throttle it will put fine up on plane. If I am pulling a tube or put on my higher pitch prop I can only go 2/3 throttle before the bog".
I will assume you mean 3/4 or 2/3 of the Throttle Lever Travel? Does the bog occur at a specific RPM Or does it vary based on Load and Prop Pitch

Is the Power Piston for the Primaries moving freely?

Do you have the correct Float, and is it set correctly?

Is the choke plate fully open? If not, the secondary throttle shaft will be locked and will not rotate. The Primary Throttle shaft can still be opened fully.

The Q-jet has many variants, so some circuits are on some carbs and not on others. Example, some Q-jets, but not all, have holes/orifices just above the Secondaries Air Valve, These will discharge a bit of fuel just as the Air Valve begins to open, acting like an accelerator pump/enrichener.
Do you have access to the Manufacturer's service manual for your Engine? It would probably show the Carb's various Circuits specific to the OEM carb. If yours is not the Original Carb, well all bets are off, as there are, as I said many variants
 
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SHOTGUN RUSS

Seaman
Joined
Nov 30, 2016
Messages
53
I believe the carb is original. The pt# did show as a Volvo packaged qjet. This qjet does not have the secondary butterfly lockout. Yes this definitely is a load based issue. I have put the carb together and will wait to get on the water and start from scratch checking all parameters. Primary power piston is free moving. Had the boat for two years now and has had this problem since I have had it. Seems to be fuel based, the engine will fall on its face every time you get to the certain load thresh hold. You can pull the throttle back to a lower position and engine always works good. But I will grind through checking everything.
 
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