Merc 260 low engine vacuum

[tfret]

5.7L Mercruiser, only pulling a vacuum of 10 at idle (750rpm), should be closer to 20. The timing is right on 8*btdc as it should be at idle. I have leak tested with propane as well as carb cleaner and can not find any leaks. The compression is very good, around 160 on all cylinders. The manual suggests possible late valve timing could be the cause. Is this to say the timing chain is stretched? The engine has just under 900 hours on it.

 

[mkast]

Re: Merc 260 low engine vacuum

Engine leak down test will verify condition of the cylinders.

 

[Apollo75]

Re: Merc 260 low engine vacuum

Your compression is very good

Late timing is a possibility ---- timing chain after 900-hrs will be stretched --- how bad I do not know --- depends on quality.

I really cannot give you any more than this link

http://www.gregsengine.com/vacuum.htm

Good Luck

OFM

 

[riverrunt1971]

Re: Merc 260 low engine vacuum

Where r u checking vacuum at, is this tied in to something else or right off the intake

 

[tfret]

Re: Merc 260 low engine vacuum

The only place I have to attach the vacuum gauge is on the choke pull off of the carb. There are no other hoses or barbs. There is a plug in the manifold, which would perhaps be a more ideal location, but it has a 7/16" square top on it and I can't get it out.

 

[PowerAddict]

Re: Merc 260 low engine vacuum

hmmm, If memory serves me, late valve timing results in low compression, but with a bouncing needle on the gauge. You sure you don't have an intake leak, i guess you said you tested, but it seem fishy to me.

 

[John_S]

Re: Merc 260 low engine vacuum

That does seem low. Reading steady? Have you verified the gauge didn't get wacked? ie it reads properly on another engine? Using choke pull-off maybe the problem if it is not connected direct to intake. I have replaced the plug in the intake to hook-up a vacuum gauge as needed. I had low vacuum after receiving a new rebuilt carb. The idle mixture screws were nearly closed.

PS: I think 16-20 is considered good for SBC.

 

[tfret]

Re: Merc 260 low engine vacuum

the gauge reads 10" but bounces + or - 1. I suppose the gauge could be bad. I'll have to confirm on another motor as suggested. I was planning to pull the distributor cap, then manually turn the crank shaft while watching the timing marks on the crank pulley VS the rotation of the distributor. If I go in one direction, then the other, I should be able to measure how many degrees of play are in the timing chain and sproket.

 

[Maclin]

Re: Merc 260 low engine vacuum

Just wondering why you checked the vacuum, what problem you might be chasing down.

It does depend on where you measure it. At the intake plug it should read like Imported_John said, 16 or higher.

 

[tfret]

Re: Merc 260 low engine vacuum

I discovered the low vacuum after replacing my Q-jet 4bbl carb with a new one. I had to back the idle screws out almost 4 turns to get top rpm at idle. Most instructions say this will be achieved at around 2.5 turns out. The old carb screws were also backed out about 4 turns. So I started looking for reasons why. Then I found the vacuum low.

 

[jtybt]

Re: Merc 260 low engine vacuum

Is your engine running badly...cuz if it's OK, the reason for your low vacuum is where you're taking the reading from.

The best place is the plug/port on the intake manifold between the carb and distributor. Next best is at the base of the carb.

This is the best site for vacuum readings I've found. There's 2 pages.

http://www.users.bigpond.com/ergoff/vac1.htm

 

[lion2008]

my 85hp mercury has trouble to start

my 85hp mercury has trouble to start

it is 1973 motor,it run very good,but only got trouble when i start in the morning, imean when the engine is cold.Even i already do tune-up and service but it still happen.any one know about this please help me. thankyou

 

[180shabah]

Re: my 85hp mercury has trouble to start

Re: my 85hp mercury has trouble to start

Lion start a new thread in the proper(Outboard) forum.

 

[180shabah]

Re: Merc 260 low engine vacuum

tfret - off topic, but relevant. Your idle is too high at 750 RPM.