Carb Adjustment And Compression Problem

[tinytanic1]

I have a 1977 Mercruiser 120hp with a 2bbl rochester carb. At the base of the carb. are two adj. screws. I know one is the idle adj., and I assume the other is idle mixture. First question....... is the forward screw idle, and aft screw mixture, or vise versa. Second question is what should initial settings be? My Merc. manual says idle screw should be 1 1/4 turns out, but doesn't show or suggest setting for the second screw.
Also I couldn't get engine started so I reset timing and points to specs, and was getting strong spark. However compression check showed #2 and #3 piston only getting 50lbs, while #1 and #4 were 145lbs. I took valve cover off and found most of the push rods were bent. Would installing new pushrods be good enough to get compression back or do I also need to dig deeper into lifters???????
Engine only has 20.3hrs since complete rebuild, so I know rings and cam are good.

 

[Reel Poor]

Re: Carb Adjustment And Compression Problem

If you have bent push rods you have other problems. Either the pistons hit the valves or the valves are stuck in the valve guides.

 

[Don S]

Re: Carb Adjustment And Compression Problem

Engine only has 20.3hrs since complete rebuild, so I know rings and cam are good.

Don't bet on it if water was getting into the cylinders from a defective manifold or riser, causing the valves to rust open.

From the sounds of your post, I would suggest you get the proper OEM manual for your engine and study it.

 

[tinytanic1]

Re: Carb Adjustment And Compression Problem

Don, what the best way to check for a manifold or riser problem. Does anyone have advice on setting the two screws (idle and mixture) at the base of the carb. and which one is which?

 

[Don S]

Re: Carb Adjustment And Compression Problem

There are 2 idle mixture screws on a rochester carb.
But you can't adjust a carb on an engine with bent pushrods.

You have some serious problems, and like I said before, you need an OEM service manual........... Big time.

 

[tinytanic1]

Re: Carb Adjustment And Compression Problem

I have an OEM manual. I've studied it almost as well as my Bible. Problem is it doesn't show the two adj screws at bottom of carb, only one. This boat didn't get used Last summer ('06) and I haven't pulled it out till last month. I had trouble starting it so like an idiot, I took a 'shortcut' and tried to reset the timing by playing with the distributor. I think, at least I hope, my rods got bent when I did this by advancing or retarding the spark too much. I got new rods yesterday. I'd like to put them in, without tearing anymore apart, set the timing, CORRECTLY, and set the screws on carb. Then see what happens.

 

[Bondo]

Re: Carb Adjustment And Compression Problem

Pushrods Don't bend from changing the Ignition Timing,.....

You've got Other Issues........

 

[Don S]

Re: Carb Adjustment And Compression Problem

What manual number do you have? It should be book #2 (1974-1977). The carbs with a single screw are Mercarbs (Didn't even come out till the early 80's). The Rochester carbs have 2 idle mixture screws (like yours). And the OEM manual does show them.
When I say OEM manual, I do not mean Clymers or Seloc. They are aftermarket.

 

[tinytanic1]

Re: Carb Adjustment And Compression Problem

I have Merc. Svc. Manual #13 for GM 4 CYl. It coveres the 3.0L, which mine is a 2.5L, but I figured they were fairly similar except for the cu. in. and some wiring. Do you know where I can find a #2 manual?
As for the motor, today I install new pushrods and adjusted them. Now the two outside cyl. (#1 and #4) are getting about 165, which is about 20 more than they were, but the center two chambers are still about 65. This weekend I'll probably pull the head off. What should I look for when I do? Is it a coincidence it's just the center 2 chambers and the outside two are fine? What are the possible and most likely culprits???? I've never messed with the exhaust system other than remove and reinstall during engine rebuild.
I realize I'll probably need a head job done but do you think my lifters and cam are still good. How can you tell?
I thank you in advance for any help!!!!!!!!

 

[Silvertip]

Re: Carb Adjustment And Compression Problem

You have a two-barrel carburetor. Hence the two screws in the base are idle mixture screws (one for each barrel). Your service manual tells you how to adjust them. As for the push rods -- be certain you understand how to adjust the lifters or you will bend the push rods again. Your service manual tells you how to do this. As for timing -- what made you think you could "fiddle" with the distributor and stand any hope of getting timing correct. Your service manual tells you how to do this. That was not the reason for the bent push rods. Pull the plugs on those cylinders and I suspect you will find they have evidence of water intrusion as Don indicated.

 

[Don S]

Re: Carb Adjustment And Compression Problem

I have Merc. Svc. Manual #13 for GM 4 CYl. It coveres the 3.0L, which mine is a 2.5L, but I figured they were fairly similar except for the cu. in. and some wiring.

That manual is the wrong one, it's for the 90-97 engines, you have a 77. While the block may be similar, the fuel systems and ignition systems are different.
If things were still the same as 77, they wouldn't have changed the manuals.

Ebay or your local Mercruiser dealer are a good place to start for the manual or try here
http://www.stemtostern.com/catalog/displayproductdetails.asp?DepartmentInventoryRefID=365943

 

[jamyers]

Re: Carb Adjustment And Compression Problem

About the only thing that'll bend a pushrod is a valve being held open (either stuck in its guide or hitting the piston) or severe over-revving (which will cause a valve to be held open).

Two side-by-side cylinders with the same low psi usually indicates a blown head gasket allowing pressure to bleed over to the other cylinder. Shoot some oil into the cylinders and see if the psi #'s jump significantly (like 40 psi or better). If so, it's likely worn rings - if not, then it's the head gasket.

I'd guess that the above comments are dead on: You've got water getting into the engine somewhere, and it caused the valves to stick and the head gasket to blow.

Looks like you'll have to pull the head for the gasket, might as well have the valves / guides checked when it's off.

But you're going to have to check out the riser, etc for where the water is coming from anyway (it *could* be from the blown head gasket, but not likely).

At any rate, get the right manual for your engine - it'll be the best money you'll ever spend on your boat.