Unstable idle

[nennedk]

Hi. I have a 2002 Mercruiser 5.0L Gen+ with a Mercarb 2 barrel carburetor. Last year just before the season ended the carb got a new kit installed together with a ultrasound clean. That was because the rev jumped to 1500 rpm when put in neutral for 5-10 seconds and thereafter settled with around 650 rpm. i didn?t have a chance to test the rep./clean properly last year, but now i have just put her in the water again. Now the behavior is different. When put in neutral the rpm drops to around 650 and then jump slowly up and down between 650 and 1000 repeatedly. Any suggestion why that is?

 

[alldodge]

Hi. I have a 2002 Mercruiser 5.0L Gen+ with a Mercarb 2 barrel carburetor. Last year just before the season ended the carb got a new kit installed together with a ultrasound clean. That was because the rev jumped to 1500 rpm when put in neutral for 5-10 seconds and thereafter settled with around 650 rpm. i didn’t have a chance to test the rep./clean properly last year, but now i have just put her in the water again. Now the behavior is different. When put in neutral the rpm drops to around 650 and then jump slowly up and down between 650 and 1000 repeatedly. Any suggestion why that is?]

Changing font size just makes it smaller and hard to read

You need to adjust your throttle cable so that when the throttle is in neutral, the cable has a very slight pressure keeping the throttle plate closed on the carb. To do this have the motor OFF and shifter in neutral. There is a barrel type connector either held in place by a nut and washer, or a cotter pin. Remove the barrel and rotate the barrel so that when its reinstalled there is a slight bit of pressure holding the throttle plate on the carb closed.

 

[nennedk]

Sorry about the font. It was copy/paste from another program. When I tried to post it, it timed out, or at least I thought so, so I didn't see the result online.

Could you please point out which numbers I should tamper with at the picture. Thanks.

 

[nennedk]

I have uploadet a little video of the behavior
https://www.icloud.com/sharedalbum/da-dk/#B0vG4TcsmJIWoSA

 

[Scott Danforth]

AllDodge is talking about the throttle cable. its not shown on your carb breakdown

when you cleaned the carb, did you chase every passage with a fine wire or a piece of fishing line? cyclical idle is usually crud in the idle circuit and improper adjustment

 

[alldodge]

This is a pic of a throttle body and not a carb, but the adjustment is the same. Also looked at the link and it doesn't play, may need to use youtube

 

[nennedk]

Oh now I get it So you think the problem is that the throttle plate isn't steady. I will try to set a little pressure to it.
I can't understand why you can't play the video. I have just tried from both a Mac and Windows machine (not my own) without any issues?

scott. It wasn't my self that cleaned the carb, but it was ultra sound cleaned so it should be as good as new. But of course never say never. I will try the little pressure first.

 

[alldodge]

I click on the vid, the pic shows a pic of the helm gauges, then it stops and says

Der opstod en fejl under afspilningen af denne video. Prov igen senere.

 

[nennedk]

Strange, the error means something like: There was an error playing the video. Please try again later.
I will try Youtube

 

[Scott Danforth]

It wasn't my self that cleaned the carb, but it was ultra sound cleaned so it should be as good as new.

just because someone vibrated the dirty carb doesnt mean that the passages are clean. I have had carbs that I have had to take pin drills to and drill the dirt out of the passages. Any good carburetor repair shop will flush out each circuit and make sure there is no obstruction in any of them.

 

[nennedk]

Maybe this link is better: https://youtu.be/WsQZV3dt59o

 

[alldodge]

Was able to watch it and it goes up and down in rpm very slowly. Not positive but a loose cable could be the issue. Boats don't have a return spring to keep it closed so it uses the cable to do the work.

Don't think this has any issue with your current problem, but your voltmeter might be in error, but if your alternator is actually putting out over 14V at idle there may be an issue. Don't know how long the boat was running prior, but 13.5V would be more normal

 

[nennedk]

Good point, I haven't payed attention to the voltmeter readings. I will test the alternator next time i'm in the boat. Just in case. I believe the engine had been running for about 20 min. and maybe also just been rev'ed up to max just before the video was shot.
About the rpm, could some electronics like the thunderbolt V ignition be a suspect or would the behavior only point to the carb?

 

[alldodge]

Don't see the TB-V being the issue, I'm still thinking carb

 

[nennedk]

Ok thanks. I will return with updates when I have tried the different things. Thanks again.

 

[nennedk]

I'd checked the alternator this weekend, and my Garmin chart plotter told me that it put out 14,4v at idle and also at 4000 rpm's. When I turned on the deck lights, navigation light, bilge pump and blower it dropped to 14,2v at idle.
Does it sound right?

 

[alldodge]

Disregard what I said previous

 

[nennedk]

After some longer runs at sea and a little adjustment at the idle and air screw (practically nothing), the problem has stopped. It is steady now at 600 rpm. Maybe i turn it up at notch so i hit 650 rpm.
I noticed that when i had the problem, there was at lot of small liquid drops at top of the throttle plate, maybe condensed water, and they are complete gone now.

 

[Scott06]

Might be a vacuum leak . Check the carb base gasket and nuts are snug. You can spray starting fluid at this gasket if rpms climb there's a leak

 

[nennedk]

Might be a vacuum leak . Check the carb base gasket and nuts are snug. You can spray starting fluid at this gasket if rpms climb there's a leak

Thanks, but there aren't any problems anymore. The idle is stable