Adjusting Rebuilt Carburator - Mercruiser 260 / V8

[ideffects]

Hello,

I've been browsing this forum for a couple of years getting the answers to my questions and concerns from this great resource and I have a question that I can't seem to find the answer to. Late last season I got a rebuilt carburetor after my old one started acting funny (I can't remember what was wrong with the old one now) and the boat ran good once installed.

However, there was one side effect. My fuel consumption increased and considering the cost of fuel and of course it's never good to run rich (or lean). I am hoping to adjust it so the boat runs perfect without extra fuel cost but I don't know how to accomplish this. I know there are screws to adjust the idle and air/fuel mixture but how don't know how to tell if the boat is where it needs to be. I've looked on google and this forum but I couldn't find a detailed guide. If anyone can suggest a site or give me some more info, that would be great.

The engine is a Mercruiser 5.7L 260/V8 and the year is 1987. If I remember right, the carb is a Rochester 4bbl with an electric choke.

Thanks for the help!

 

[Maclin]

Re: Adjusting Rebuilt Carburator - Mercruiser 260 / V8

I would start by looking at the spark plugs, their condition will give an indication of the fuel mix they are seeing. Then I would preerve my labor efforts and put all new spark plugs back in, make sure they are the correct ones for the engine, heat range makes a difference.

I would check (or re-check if you have had a look already) the condition of all tuning components. Also check dwell and timing. Dwell has a big effect on how well an engine can ignite the fuel and if dwell is not within specs the effect is the same as too rich of a mixture. I did not see the year of your engine, but if it has points then I would put new ones in and check dwell and adjust timing if necessary. If timing is off towards the retarded side the effect at idle is the same as too rich of a mixture.

If the engine is indeed in a good state of tune then with enough patience and time you can check the fuel mixture at idle conditions, cruising conditions and wide open throttle conditions by buying a few extra new spark plugs and using them as data recording devices. If all the spark plugs from your engine look relatively the same then pick one cylinder as the test one. Put in all new spark plugs (remember to have some spares) then start it up and let it warm to temperature and then do a close to wide open throttle run for 30 seconds or so. Then pull out the plug in the "test" cylinder. Put in a new plug then let it idle for a few minutes, then pull that one. Put in another new one then go for a moderate cruise for 10-15 minutes then pull the test cylinder plug again (and install a new one). Reading these plugs should show what is going on.

Other changes could have affected this as well, like anything done while the other carb was on to get it to run with whatever was wrong with it.

It would be good to know your WOT rpm's and speed. Also, can you give any numbers that can show the fuel usage now as well as from before? It is entirely possible that the fuel usage is normal for a 5.7 in that boat and you are just using it more because it runs so much better now

 

[Maclin]

Re: Adjusting Rebuilt Carburator - Mercruiser 260 / V8

Sorry, I now see the year of your rig is 1987 so it more than likely has electronic ignition but post back the type of ignition as well if you can. Also report current WOT rpm and speed and prop pitch and what kind of boat. All of those are factors.

 

[ideffects]

Re: Adjusting Rebuilt Carburator - Mercruiser 260 / V8

Thanks for the help.

The ignition is a Thunderbolt 4. WOT RPM is about 4500RPM which is right where it is suppose to be. I don't remember what the prop pitch is - I can check tomorrow. The plugs were black with a lot of carbon buildup on them. Before the new carb, the plugs had a brown tint to them - just like how they were suppose to be. The boat is a 21' openbow.


I'm in the process of doing a lot of work to the boat to get it ready for Lake Powell this weekend so I was hoping for a quick fix to see where the carb should be...

 

[Uraijit]

Re: Adjusting Rebuilt Carburator - Mercruiser 260 / V8

Assuming your carb is already jetted properly, the best way to set the mixture is to use a vacuum gauge on the manifold.

First things first. You want to make sure your timing is set properly. If your timing isn't right, none of the rest of this will matter. If you don't have a timing light, you can "rent" one from Autozone for free. Just pay a deposit, and you'll get it back when you return it.

Once your timing is all good, the next thing is to get your idle speed set up properly. Start the engine (while cold) and set the idle cold idle to the proper speed.

Drive the boat for a bit to get it nice and warm. Once it's warm, set the warm idle.

With the engine still warm, hook up a vacuum gauge to the manifold (not the carb). If you have a healthy engine, you should have a good steady reading on the gauge.

Start with the mixture screw turned all the way in, and then back out about 1.5 turns. Start the engine back up, and turn the screw slowly until you get the highest vacuum reading you can, on the gauge. If you find that there's a "range" or multiple "spots" of high vacuum, you want to set it to the "leanest" spot with the highest vacuum.

It's not a bad idea to double-check your idle settings again, once you're all finished.

The poor-man's method of setting it up, is to turn the screw in until the engine starts to stumble, and note the position. Then turn it back out until it stumbles, and note the position. Figure out the mid-point between those two positions, and set the screw in the middle of those two positions (usually a little on the leaner side of things).

Vacuum gauges are cheap though, so you might as well just do it the right way.

Whatever method you use, after you get it set up the way you think it ought to be, go drive it around a bit, and do a plug reading. Make minor adjustments accordingly.

Good luck!

 

[John_S]

Re: Adjusting Rebuilt Carburator - Mercruiser 260 / V8

Assuming your carb is already jetted properly,/QUOTE]

Allot of good advice and should always tune-up, before assuming it is something. Given the statement of the elec choke and it being an '87, I suspect this is a "generic" marine carb, which the jetting may or may not be correct. If the idle mixture screws are at the default setting, and not finally tuned, I doubt that is causing it to run that rich. Look for dribbling in the carb, which might mean bad adj of float or sticking needle.

 

[ideffects]

Re: Adjusting Rebuilt Carburator - Mercruiser 260 / V8

Thanks Guys!

Everything is good but the carb. My first car was fuel injected so I never learned about carbs.

I knew there was a way to adjust it with a vacuum gauge but I didn't know exactly how so I will do that. I adjusted the idle last year but I didn't want to touch the a/f mixture until I knew how. I'd definitely take an engine that runs rich over an engine that runs lean. I'm going to try to take the boat out this evening for a shake down and I will pull a couple of plugs to see how I did.

It just difficult to test by pulling the plugs since the closest launching ramp is about an hour away and I don't like to pull plugs on a hot engine at the lake.

The carb is a Rochester marine carb - the same as I removed. It's possible it wasn't rebuilt properly.

 

[Uraijit]

Re: Adjusting Rebuilt Carburator - Mercruiser 260 / V8

To clarify, you don't want to run lean. You just want to be on the "lean side" of the optimal range. Big difference!

 

[John_S]

Re: Adjusting Rebuilt Carburator - Mercruiser 260 / V8

Thanks Guys!
The carb is a Rochester marine carb - the same as I removed. It's possible it wasn't rebuilt properly.

Merc had different jetting for different models of the same size engine, all with the same size (cfm) Q-jet. So a rochester marine carb is not standard.

And yes, like anything, it could have been done wrong.

Idle mixture should have been adjusted on install, or at least set to the default starting position. The one I bought last year had the idle mixture screws turned all the way in, and then had to find a special tool.

To use the vacuum gauge, you will probably need to add fittings on the intake. It is the best way to adj it, but might not meet your timeline.

Given your looming trip, I would record where they are now, by counting the number of turns to lightly seat the needles. Record both sides, such that you could get back to where it was. Now turn them out to the default setting. Mine was 2-3 turns, so went to 2.5 to start from. Adj in until stumble, adjust out to stumble, set at mid-point. Re-adjust idle speed (if neccessary), repeat. This should be the procedure in your manual, but in a much easier to understand way.

For idle mixture richness, you don't have to splash boat. Running on muffs at idle speed is enough. Most idle richness is caused by other things, choke not fully open, leaking float needle.

 

[sdbraun]

Re: Adjusting Rebuilt Carburator - Mercruiser 260 / V8

I have a similar issue with my 1987 Merc 260 5.7L w/Thunderbolt ignition. I have been experiencing rough idling. I can smell it is running rich and it has great power so I figure the choke is not opening up all the way or the carburetor mixture has richened up/the screws have slipped out over the past few years. The choke is going to be easy to see but I don't know how to view the two carb screws - is one air and one fuel or are they both fuel? (I grew up with a mid-70's Ford and friends with GM vehicles and I have been handicapped ever since). The screws I have do not use a screwdriver but some special tool - I'll have to use just the right size hex driver or a long-nose pliers to adjust them.
Greatly appreciated,
Scott

 

[6meter]

Re: Adjusting Rebuilt Carburator - Mercruiser 260 / V8

First of all idle mixture screws ONLY adjust the air/fuel mixture AT IDLE. Anything above idle they do nothing. Plugs shouldn't be black. They are black because of a carb issue, generally nothing else. A choke staying on, leaky needle seat, sticky float or way oversized jets make a motor run rich.

 

[sdbraun]

Re: Adjusting Rebuilt Carburator - Mercruiser 260 / V8

It is certainly the IDLE that I am having issues with now. The idle has slowed to 500 RPMs and has become less stable. I replaced the cap and rotor and plugs and took a compression test while I was at it and everything looks good. My temp looks good, oil looks good... I have 900 hours on the engine. I have owned this boat for three years now and it has been very reliable. I can certainly plan to undertake a project at the end of the season, but at the moment I need to do an idle test with my jetting. Are both these jets for fuel?
Thank you very much for your time.
Scott

 

[Maclin]

Re: Adjusting Rebuilt Carburator - Mercruiser 260 / V8

What brand and kind of Carb, 2bbl, 4bbl, etc.