Mercruiser 260 low vacuum

[bruceb58]

Re: Mercruiser 260 low vacuum

Well, I'm eliminating a mechanical timing problem as the source of my low vacuum. Found something on the 'net that explained my timing gear mark confusion. When the cam gear and crank gear marks are both at 12 o' clock, the #1 cylinder IS at TDC of it's compression stroke. When you're assembling the engine, and you line up the marks at cam 6, crank 12, the engine is actually at #6 TDC compression. So, I'm back to square one. Time to pull the intake, and reassemble it with new gaskets. If that don't clear it up, guess it's going to the shop.View attachment 109279

Like I said before, you have a 4 stroke engine. #1 will alway be across from 6 and that cam can be assembled 180 from what you show in your diagragm as long as the engine itself is at TDC.

 

[scf8169]

Re: Mercruiser 260 low vacuum

I am confused.....Why would an engine that has no vacuum advance have a vacuum port that is between the carb and the distributor? vacuum guage attached at the plug between the carb and distributor

Did I misunderstand? Are we talking about vacuum advance here?

Is the vacuum port clear and open? A clogged port will not give a good reading...

I'll sit back and read now.....

On a small block chevrolet engine, there is a 3/8" NPT plug in the manifold plenum that runs between #5 and #8 intake ports, right between the carb and distributor. To get the most accurate vacuum gauge reading, you replace that plug with a nipple to attach your gauge hose to.

 

[scf8169]

Re: Mercruiser 260 low vacuum

No, no vacuum advance, it's a Prestolite mechanical advance, originally points and condenser, now aftermarket electronic conversion.

 

[scf8169]

Re: Mercruiser 260 low vacuum

Well, after bashing my head against the hull for months now trying to figure out what is wrong with this thing, and resealing the intake twice, I have come to the conclusion that I have a bad harmonic balancer. When I run the timing up to what appears to be 16 degrees advanced at idle with a timing light, the engine smooths right out, and the vacuum comes up to 18-20". I have done some research on the 'net, and it seems that a lot of early 80's SBC's balancers have had a problem with the outer ring of the balancer "migrating", some actually starting to migrate off the front. You know balancers are actually comprised of an inner hub and an outer ring with the TDC groove, connected by a rubber membrane between them. Supposedly, sometimes that rubber becomes unbonded. Not very common, but it does happen. So, I ordered one from Summit that has an index stamping from 0-60 degrees. I'm going to put that one on and time it at idle, 8 deg. advanced, and at 3000 rpms, 32 deg. advanced. Hope this works, I'll let you all know next week when I get it.

 

[John_S]

Re: Mercruiser 260 low vacuum

Hopefully that will be it, but the test you did would increase vacuum and smooth out even a "good" running engine. Need to determine if the #1 piston is at TDC and timing mark aligned.

Milkshake on the dipstick says there was water in there.

Have you done a compression or the more revealing leakdown test?

 

[scf8169]

Re: Mercruiser 260 low vacuum

Yep, did a compression and leakdown test. 150-165 psi on all cylinders, no leakdown. The milkshake was from when I reset the intake the first time, gasket slid out of place. Got that corrected on the second reset. The mark lines up at zero when the piston is up and the valves are closed, but I haven't indexed it for the dwell. That's what, 20-30 degrees of crank turn while it's up, something like that? Gonna do that with the new balancer to make sure it's dead-on TDC.

 

[bruceb58]

Re: Mercruiser 260 low vacuum

but I haven't indexed it for the dwell.

Not sure what you mean by this statement.

 

[joewithaboat]

Re: Mercruiser 260 low vacuum

So sometime between it running good and running bad You moved the distributor to set the timming ?
Your problem could just be the spun balancer. I have had more than one of these over the years. I always mark mine now with a stripe of contrasting paint if using the stock rubber bonded 2 piece type. You need to check the key way location with the engine on TDC #1 then look to see if the timing mark is in the correct place.

Just thinking out loud here... but if that was re-powered... there are two different timing tab locations on small blocks, i forget when the change was but Ive seen folks mix balance'rs and front covers. You need to check this out.

 

[joewithaboat]

Re: Mercruiser 260 low vacuum

No, no vacuum advance, it's a Prestolite mechanical advance, originally points and condenser, now aftermarket electronic conversion.

Also I suppose you could be having trouble with the electronic conversion, my experience says they work or don't work but i guess anything is possible.

 

[John_S]

Re: Mercruiser 260 low vacuum

Not sure what you mean by this statement.

+1

With number 1 cylinder at TDC, the harmonic balancer mark should be at the zero degree mark on your tab. If it is not aligned than balancer has slipped. If tab was bent, that would be very obvious. In your case, you will be able to compare old balancer to new. Align the keyways and if the tdc mark are aligned, it wasn't slipped.

I am assuming, based on having an old point distributer with a single mark on balancer and timing advance tag on a metal cover. In the late 90's they went to a composite cover with single mark, and advance on the balancer.

 

[scf8169]

Re: Mercruiser 260 low vacuum

I may not be using the correct terminology, but the time between when the piston hits it's highest point in the cylinder on the compression stroke, and when it starts back down on the power stroke. There is a "dwell" time that the piston doesn't move, but the crank is turning and the connecting rod is shifting. TDC is actually when the connecting rod is in a parallel line to the centerline of the cylinder. An older guy I know was explaining it to me, he had to index balancers on race car engines because the factory marks are not always exact. The width of the groove itself was enough to throw it off by a couple of degrees, which can be critical when you're running an engine at high RPM's.

 

[bruceb58]

Re: Mercruiser 260 low vacuum

I see what you are saying. Actually, there is zero "dwell" time when the pistin hits its highest point. It may seem like it is not moving but it is...just a very tiny amount.

 

[John_S]

Re: Mercruiser 260 low vacuum

And if you measured any "dwell" with a dial indicator, probably a worn lower end, bearing, wrist pin, etc.

A 1 to 2 degree difference in initial timing is not causing that magnitude of low vacuum.

If the old and new dampers compare, keep looking. If they don't, it still may only be part of the problem. Depends how far off it was.

 

[joewithaboat]

Re: Mercruiser 260 low vacuum

Come on guys measuring dwell of the piston.... if there were anything to measure the engine would rip itself apart. The only way you can change the dwell is to change the length of the rod.

See this link for timing tab different locations zero is almost strait up on newer stuff, late eighties i think. the older is around two o'clock.

http://www.mre-books.com/chevy/engines/parts_interchange/timingchains.html

adjustable tab to really zero in true zero with a dial indicator and account for balancer inaccuracies.
http://www.cfrperformance.com/CHEVY_SMALL_BLOCK_TIMING_TAB_W_ADJUSTABLE_POINTER_p/hz-9179.htm

 

[bruceb58]

Re: Mercruiser 260 low vacuum

Come on guys measuring dwell of the piston.... if there were anything to measure the engine would rip itself apart.]

I think we all agree on that except for the old guy that gave the OP this advice.

 

[scf8169]

Re: Mercruiser 260 low vacuum

I don't know, maybe I'm not explaining it right. I do know that this older friend of mine has built a buttload of race car engines, for himself and a bunch of other people around here, so he knows infinitely more about engines than I do. And I also know that I've checked everything that has been suggested on here, and a bunch of things that haven't, sometimes three and four times over, that has made no difference. All I have left to do is try the new balancer. I'm trying to avoid pulling this engine and taking it to the machine shop, 1) because I'm relatively sure it only has about 180-200 hours on it, and there is no good explanation for it to start running like crap out of the blue, and 2) Christmas is here and money doesn't grow on trees. I'll keep you all posted on this "mystery illness".

 

[CharlieB]

Re: Mercruiser 260 low vacuum

Pull the top of the distributor apart and check the mechanical advance, if it is hanging up and not advancing it will cause a low vacuum.

 

[bruceb58]

Re: Mercruiser 260 low vacuum

One thing I would do is put the points back in to make sure its not the electronic ignition conversion that is causing your problems. Those things do fail.

 

[scf8169]

Re: Mercruiser 260 low vacuum

Pull the top of the distributor apart and check the mechanical advance, if it is hanging up and not advancing it will cause a low vacuum.

Already did that, a couple of times, and sprayed the springs and weights with silicon lubricant.

 

[scf8169]

Re: Mercruiser 260 low vacuum

One thing I would do is put the points back in to make sure its not the electronic ignition conversion that is causing your problems. Those things do fail.

Replaced it with a brand new Mallory e-spark module, with a new coil.