Merc 260 Carburetor Idle/Adjustment issue

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mrdopes

Seaman
Joined
Oct 13, 2008
Messages
59
Okay, i have a merc 260 (5.7) with the stock Rochester 4 barrel in a 1985 Sea Ray Sundancer 260.

When I bought the boat last month, the previous owner said that he had rebuilt the carb about 12 months ago, drained the fuel tank, and has not used it since.

So, I finally got my boat in to the water. The idle seemed low and had trouble keeping the motor running, so I adjusted the idle.... I got it upto about 700+/- 50, but the engine rpm was a little sporadic, jumping up and down about 100 rpm. It would smooth out at about 900-1000 rpm.

I I adjusted the fuel/air ratio best I could by ear, and decided to bring it in and have the shop adjust the fuel/air, as I couldn't get the idle to smooth out.

The mechanic adjusted the fuel/air screws twice (i watched him so i could make sure I was doing the right thing). He couldn't get it to smooth out either. He said that these older carbs are commonly like this, and that I shouldn't rebuild it, just buy and new one with a warranty from him.... at a price of about 650 to 700 bucks for parts/labor. I kinda felt like he was just trying to push the sale.

Anyways, so, today I get it back on the water, and it is idling around 650-750. Running well. Then, after our 8th or 9th restart, it would not hold idle and I had to bring the idle screw in about 1.5 turns to start it. Then, backed the idle screw out to about 750 rpm. Went through that about 2 times today. I have to keep adjusting the idle screw in, raising the idle, then adjust it back out.....after the motor runs for a couple hours I have to do it again.

It almost feels like the idle screw is coming/vibrating loose/out and allowing the idle to drop too low, to the point it stalls. Anyways, I'm not too familiar with these systems....so i was hoping that someone might be able to give me some advice here.

I'm pretty mechanically inclined and am not afraid to tackle the job if someone can point me in the right direction.

Does this sound like a problem that can be fixed, or should I start saving for a new carb?
 
4

444

Senior Chief Petty Officer
Joined
Jul 16, 2010
Messages
704
Re: Merc 260 Carburetor Idle/Adjustment issue

Sounds like you have the rochester quadrajet on there. They are a decent carb in the hands of those who know how to work them in but they would be far from my first choice for a boat carb. 700 bucks for a carb and install, that guy just wants to bend you over. Right now summit has the edelbrock 600cfm remanufactured marine carbs on for 285 bucks. If you didn't run the marine specific one, you could get one for 200 bucks. You could get off even cheaper if you bought a used one and rebuilt it yourself if you know how. I don't usually recommend just plopping a carb on, but the stock chevy 350 is likely the engine that carb is jetted from the factory for. Dial in the choke or remove it if you want, then get the idle mixture/rpm set and you should be in business. 4 bolts and a piece of linkage... I think you can handle a carb change.

http://www.summitracing.com/parts/EDL-9909/

It is a square bore carb so if you're running a rochester quadrajet, you'd need to either put on an square bore/spread bore adapter plate or change to a square bore intake manifold.
 
A

Aloysius

Chief Petty Officer
Joined
Aug 21, 2010
Messages
484
Re: Merc 260 Carburetor Idle/Adjustment issue

Quadrajets are great carbs, but few people understand the subtleties and tricks to rebuilding them.

I rebuild carbs and distributors as a sideline, and my advice is 95% of carbs are rebuildable. The original equipment carb is properly jetted for your engine. Of course most mechanics will recommend a "new" carb..rebuilding is a dying art.

Find someone who truly understands quadrajets. Have it properly cleaned, repaired, resealed. and adjusted.
 
M

mrdopes

Seaman
Joined
Oct 13, 2008
Messages
59
Re: Merc 260 Carburetor Idle/Adjustment issue

Cool, thanks a lot guys. I'm going to order a carb kit and give it a go! Wish me luck!
 
A

Aloysius

Chief Petty Officer
Joined
Aug 21, 2010
Messages
484
Re: Merc 260 Carburetor Idle/Adjustment issue

Read the instructions CAREFULLY! Set the air valve spring to 3/4 turn. Be VERY careful with the power piston and the bowl gasket..takes 20 fingers and a thumb........
 
D

dan t.

Lieutenant Junior Grade
Joined
Feb 28, 2008
Messages
1,137
Re: Merc 260 Carburetor Idle/Adjustment issue

Order a float(preferibly brass) and a choke pull off at the same time, these are deterorating parts and should always be replaced during a rebuild,and yes stick with the q-jet, best marine carb built. Adaptor plates=power loss, they totaly screw up the air flow between the carb and manifold.Good luck with your rebuild
 
M

mrdopes

Seaman
Joined
Oct 13, 2008
Messages
59
Re: Merc 260 Carburetor Idle/Adjustment issue

Thanks for all the help and insight guys. I really appreciate it.

Is this carb kit here a decent one ya think?

http://cgi.ebay.com/ebaymotors/Roch...2028125QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear

Also, I was curious... does this 1985 model Merc 260 have any special procedure when adjusting the idle or timing? Someone mentioned to me that my timing sounded a little advanced, and that in order to check it with a timing light I would have to set the engine to timing mode by jumping a wire or something or another off the distributor?? Is that right? What would the procedure there be?

I'm waiting for my engine manual to come in the mail, so I was just wondering...
 
J

John_S

Rear Admiral
Joined
Jun 21, 2004
Messages
4,269
Re: Merc 260 Carburetor Idle/Adjustment issue

mrdopes said:
Also, I was curious... does this 1985 model Merc 260 have any special procedure when adjusting the idle or timing? Someone mentioned to me that my timing sounded a little advanced, and that in order to check it with a timing light I would have to set the engine to timing mode by jumping a wire or something or another off the distributor?? Is that right? What would the procedure there be?

I'm waiting for my engine manual to come in the mail, so I was just wondering...
Click to expand...

The person you are talking to, is refering to newer ignition systems. Yours should be Thunderbolt IV, which does not require a base mode jumper.

Idle mixture and idle speed are straight forward. Final idle speed is set in the water, in gear.
 
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