mercruiser 260 low vacuum

[chucky112233]

Good morning everyone need some direction on this one..
1980 Mercruiser 260 hp engine low vacuum at idle(10-12)..
Vacuum gauge does fluctaute slightly (no quick flicker) between 9-13 at idle,steady throughout range off idle up..
Tune-up done,timing spec set a 8 btdc(was 17 btdc)
compression all around 130 psi..
No external vacuum leaks found(used smoke machine).
Replace carb due to 4 bb linkages being damaged,slightly better but still low
Engine runs o.k at idle,max rpm though is 3000
motor has approx 140 hrs on it,new in 2008
I did have a water in fuel issue in spring,plugged up old carb and water seperator,drained fuel tank got rid of all water(i think lol).
carb idle adjustment doe change idle, changes vacuum very slightly
Am i missing something?
Would be nice if i could get it to idle better,,last year vacuum was approx 18 at idle,i unfortunatly dont know when it changed...

 

[tfret]

Re: mercruiser 260 low vacuum

I have the same problem with a 1984 Merc 260. About the same readings as yours. I tried leak testing with WD40, carb cleaner, and with Propane and couldn't find anything. Then while winterizing last year by accident I discovered a loose intake manifold bolt. This Spring I pulled the carb off and retorqued all manifold bolts. It definately idled and ran better, although I never vacuum checked it again (I will when I winterize it this Fall). My point being, your intake manifold bolts could be loose and creating a leak that you can't find.

 

[scf8169]

Re: mercruiser 260 low vacuum

Chucky, how are you coming along with this? I started a thread of the same title as yours, I'm quite a ways into it, and I still haven't found anything to explain my low vacuum, which has the same symptoms as yours. Find anything yet?

 

[69GTOby]

Re: mercruiser 260 low vacuum

I have the same problem. Was any progress made here?

 

[dan t.]

Re: mercruiser 260 low vacuum

Exhaust back pressure can cause low vacuum, did you check the Y pipe for a broken off exhaust flapper?

 

[achris]

Re: mercruiser 260 low vacuum

If you don't find anything external, try a cylinder leakdown test. That tests more things more thoroughly than a compression test...

 

[Don S]

Re: mercruiser 260 low vacuum

69GTOby, welcome to iboats, you really need to start a new thread of your own. Per the announcement at the top of this forum.

http://forums.iboats.com/announcement.php?f=21