1998 Mercruiser 3.0l high idle

[Nick36]

Hi all, i rebuilt my Mercarb over the winter and and also had the distributer out of the engine. So this spring I reinstalled the carb and adjusted the timing and got the engine running. Problem is, I cannot get the idle down. I start the carb idle adjust screw at 2 turns in on the cam and the mixture screw 1 1/4 turns out. The engine runs fine at 1000+ rpm, but no amount of adjusting gets the idle down to the required 600-700 rpm. I'm going to check the fuel pressure, as the pump was changed, but other than that what issues in the carb could cause me not to get the idle down? I've verified float level and drop, the jets are new, etc. Really not sure what to try next.......

Thanks!

 

[dubs283]

Are you performing these adjustments with the throttle cable attached?

 

[Nick36]

I am, yes, after it's running at temp

 

[dubs283]

Idle speed and idle/air mix should be adjusted at normal operating temperature with the throttle cable disconnected.

Once you have achieved proper speed and idle/air mixture, adjust the cable to match the anchor and throttle arm stud with a bit of preload against the throttle arm stud

 

[Nick36]

Yup, I just can't get the rpm down to the ~650 number no matter how much I adjust the idle and mixture screws

 

[Rick Stephens]

Yup, I just can't get the rpm down to the ~650 number no matter how much I adjust the idle and mixture screws

Read dubs post again.

Disconnect the throttle cable and then tell us how your adjustments fix or don't fix the idle issue.

 

[dubs283]

Yup, I just can't get the rpm down to the ~650 number no matter how much I adjust the idle and mixture screws

You mentioned this in your original post.

You also mentioned you still have the throttle cable attached. You must remove the cable to perform the adjustments correctly, as I stated earlier

 

[Nick36]

Sorry, I had to re-read his post. The throttle cable WAS disconnected when I did the adjustments. Sorry about that confusion.

 

[dubs283]

The throttle cable WAS disconnected when I did the adjustments. Sorry about that confusion.

No worries

In that case first guess would be a vacuum leak, carb base gasket correct/sound?

 

[Nick36]

I believe it is correct and sound. Came with the rebuild kit.

Any tricks to checking for a vacuum leak?

One thing I tried was when it was getting close to stalling out, was give it a shot of carb cleaner down the throat, and if it revved up, it was likely too much air in the mixture, but when I did it, it killed the engine, so seems like too much fuel?

 

[Rick Stephens]

Spray carb cleaner around the base of the carburetor, see if the RPMs change.

May need to recheck things internally on the carb if that doesn't get it. Double check float level! What carb kit did you use? Sometimes the chinesium kits have funky needle valves.

 

[dubs283]

when it was getting close to stalling out, was give it a shot of carb cleaner down the throat, and if it revved up

This is the first you mention this.

Your original post makes it sound as if the engine runs at 1000 rpm or so at idle and you can't adjust lower. No mention of the engine stalling/choking out.

Try manually closing the choke when you're adjusting lower than 1000 rpm. If you can get it to idle that way, you are too lean. If the engine dies then most likely too rich

 

[Nick36]

This is the first you mention this.

Your original post makes it sound as if the engine runs at 1000 rpm or so at idle and you can't adjust lower. No mention of the engine stalling/choking out.

Try manually closing the choke when you're adjusting lower than 1000 rpm. If you can get it to idle that way, you are too lean. If the engine dies then most likely too rich

Oh, ok, maybe I wasn't specific enough, apologies, when I try to adjust it down from 1000rpm or so, it will choke and stall out before it gets to the recommended idle range of ~650rpm

 

[Nick36]

Spray carb cleaner around the base of the carburetor, see if the RPMs change.

May need to recheck things internally on the carb if that doesn't get it. Double check float level! What carb kit did you use? Sometimes the chinesium kits have funky needle valves.

I'll have to go back and check the kit P/N, but I bought it from a Merc dealer, so I assume it's good. I already have checked the float level and weighed it (9 grams), so seems all good. I will try what you said about spraying the carb cleaner around the base of the carb too! Thanks

 

[nola mike]

what issues in the carb could cause me not to get the idle down?

Thanks!

Vacuum leak...