Idle Adjustment

B

BillyT0020

New Member
Apr 11, 2008
263
Catawba Island
Boat Info
1980 260 DA
Engines
Merc 260
Guys,
Is the idle adjustment a huge process, or is there an Idle Adjustment Screw on the Carb base? I think I remember reading something about having to take off the throttle cable while the boat is throttled up in neutral and messing with the air/mixture screws. If that's the case I will probably have a mechanic do it but am hoping to save the bucks if its just a screw somewhere.

Basically, I had a bunch of junk in my gas tank and had to have it pumped out, and last weekend I installed a digital tach and changed the fuel filter on the carb body which was full of junk as well. After making those changes, the new tach says its idleling at 1000 RPM's after it warms up and I don't want to ruin the gears by slamming it into gear at 1000! I am assuming 750 is where it should be.

Thanks,
 
Last edited:
Billy,

Yes, the idle adjustment screw is on the right side at the base of the carb (to the port side of your boat).

The idle rpms you are targeting are at idle, in forward gear (as if you are idling out of your marina). Idle in neutral will be slightly higher.

To do this, you'll need to make sure that your spring line(s) are very secure, and your boat will not crash into anything. put your throttle in gear, then adjust you rpms where they need to be according to your manual, return to neutral. Also, you'll want to make sure that your engine is warmed up to operating temp. for this procedure.
 
ghuskin is right re idle screw. I had a 1978 26 weekender 260hp and I believe your idle should be more around 650 rpms, but if you have a manual go by that, if not I would be between 600-650 rpm at idle, that will be lots enough so your boat won't stall and easier when you shift.
 
I didn't want to comment on his ilde rpms, as I do not have the 260 hp (350) engine. My idle speed is 650 rpms also.
 
Awesome,
Thanks a lot guys, I don't have a book so I will go with the suggested 650. So when I put the boat in gear and adjust the screw to 650, thats the only adjustment I make correct? Then when it goes back to neutral it will kick up a bit? Or do I adjust it both in gear and in neutral?
Thanks,
Billy
 
that should be it. Again, make sure your engine is warmed up to operating temp. You need to be in forward gear to set the idle. if it seems that you idle in neutral is too high, you might want to turn it down a bit before you put it in gear.
 
Adjusting the mixture while your at it might be a good idea. You could adjust the mixture first, then put the boat in gear as described above and adjust the idle.
As long as your down there you may want to check the timing too. I don't have the same boat as you do, but most of the small block Chevy motors (that's basically what your motor is) of that vintage should be set somewhere beteen 8 and 12 degrees BTDC.
It's amazing what a difference you can make in a few minutes with a timing light, wrench, and a flat screwdriver.
 
My manual says 650-700 in-gear idle speed for a 260.
 
Personally, I would get a manual before I started turning any carb screws. I think its easy for us to suggest changing the mixture, adjust the timing, etc but if billy hasn't done it before and without specific direction he can ruin a weekend or two waiting to get the boat in with a mechanic.
 
Thanks a lot for all of you help guys. I do agree that I am a little hesitant to start turning mixture screws and timing etc. but the idle is something that really needs to be done before I take the boat out again. If its just a matter of putting the boat in gear and turning the screw until I hit 650 ish in RPM's I'm okay with that. I will probably have the mechanics put a new carb on it next season anyway so I may leave the other adjustments for them to do when they do that.

I can't really screw anything up too bad by doing the idle right?
 
BillyT0020 said:
I can't really screw anything up too bad by doing the idle right?
Click to expand...

Well, maybe...

Why is it high? If it's been off or rebuilt then idling down is part of the process. In my experiences, if the idle is high then it's because the PO or PM set it high to cover up another problem. So idling down to 675 may expose other gremlins (ie carb problems, etc). All you can do is try and see.
 
KRB64 said:
Well, maybe...

Why is it high? If it's been off or rebuilt then idling down is part of the process. In my experiences, if the idle is high then it's because the PO or PM set it high to cover up another problem. So idling down to 675 may expose other gremlins (ie carb problems, etc). All you can do is try and see.
Click to expand...


Im guessing the reason it was so high is because there was so much junk in the tank, which was eventually pumped. The PO must have set it high to get some good gas in the tank. Also when I replaced the filter in the carb, there was all kinds of crap in there too, had to have been restricting the flow, and when I cleaned all that out it probably kicked the idle up as a result. But then again who knows. I think its an original carb so it could very well be shot. Ill find out soon enough!

Thanks all!
 
Billy

I have had several mid '80's boats with 260 Mercs. If you had that much crap in the tank you should have replace all fuel filters. Try and find an engine analyzer that has the tach with multiple ranges. The tach on the boat is not always accurate. You could borrow one from a friend but look on ebay. I bought a really nice Craftsman Engine Analyzer for $25.

Connect the analyzer and set the tach to the most sensitive range. Get the idle set to about 900 rpms in neutral. Watching the tach, turn one mixture screw clockwise until the rpms start to drop off. Then turn the screw counter-clockwise until the rpms come up and continue turning until the rpms drop again. Now turn the screw back clockwise about 1/2 to 1 turn or until the rpms reach the maximum level. Repeat for the other mixture screw. Then adjust idle with the idle screw to 600-700 rpms in neutral. As previously suggested, tie 'er tight.

That is how I have always done this task but the analyzer is very helpful. I know trained mechanics that can make these adjustments by ear.

http://cgi.ebay.com/ebaymotors/Craf...ryZ43989QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
I just looked up Engine Analyzers on ebay and this is like the one I have.
Have fun
 
NorCal - what all else will that analyzer do? And how "technical"s it to use?
 
It will do about 30 tests and this thing is good for the older motors like ours.

It will do:
  • Point Resistance
  • Battery-Starter Test volts/amps
  • Batery/Charging system
  • Alternator Output
  • Dwell TestRPM/Carb adjustmentsVoltage Loss Test on Batery/starter
  • Ohms test
  • and others
As I said, I paid $20 for mine on ebay. It came in the original box with the manual and looked like it had never been used. I use it mostly for the tachs and carb adjustments.

Shawn
 
Great advice! I have a MAC analyzer that I haven't used since I found a DVOM that does it all including temperature with a thermocouple for $80. It's a Craftsman too. Digital is easier to read.

DG
 
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