Rough Idle, 1967, Mercruiser, 160 HP, Straight 6

[John3]

It's a newly rebuilt engine (Have about 11 hrs on it), the mechanic used my Block re-bored with new rings, bearings etc., (Rebuilt) Head, Valves, Valve Seats, New Plugs, Plug Wires, Coil. When he finished it idled rough, the mechanic said the Carb needed work & he wanted to send it out to be rebuilt. I got tired of spending money so I rebuilt it myself using the proper Mercruiser kit.
It's the original Carb, I disassembled it & soaked it in Carb Cleaner for a couple of hrs. Cleaned all the parts per instructions. Cleaned all the small holes by passing a single strand of copper wire thru them. the wire was smaller than the hole dia, didn't want to enlarge the holes.
The engine starts ok. After warm up it Idles at 600 rpm sounds a bit rough with some vibration, as though a cylinder is not doing it's fair share, vacuum fluctuates between 10 & 11 inches. At higher rpm's the Vac is steady, tops at 17.5" @ 2100 rpm.
It seems to me that with everything new it should idle smoother. Am I trying to pick the fly turds out of the pepper? If it is the carb I'll have to send it out, otherwise the mechanic should fix it under warranty.

 

[Don S]

Re: Rough Idle, 1967, Mercruiser, 160 HP, Straight 6

If it is the carb I'll have to send it out, otherwise the mechanic should fix it under warranty.

I thought you rebuilt it ???
Did you adjust the idle mixture? Is the timing right? Plug wires on right and in good shape? Any arching around the coil tower? Might want to do a compression test as well. Could even be a tight valve or two.

 

[Bondo]

Re: Rough Idle, 1967, Mercruiser, 160 HP, Straight 6

It's a newly rebuilt engine (Have about 11 hrs on it), the mechanic used my Block re-bored with new rings, bearings etc., (Rebuilt) Head, Valves, Valve Seats, New Plugs, Plug Wires, Coil. When he finished it idled rough, the mechanic said the Carb needed work & he wanted to send it out to be rebuilt. I got tired of spending money so I rebuilt it myself using the proper Mercruiser kit.
It's the original Carb, I disassembled it & soaked it in Carb Cleaner for a couple of hrs. Cleaned all the parts per instructions. Cleaned all the small holes by passing a single strand of copper wire thru them. the wire was smaller than the hole dia, didn't want to enlarge the holes.
The engine starts ok. After warm up it Idles at 600 rpm sounds a bit rough with some vibration, as though a cylinder is not doing it's fair share, vacuum fluctuates between 10 & 11 inches. At higher rpm's the Vac is steady, tops at 17.5" @ 2100 rpm.
It seems to me that with everything new it should idle smoother. Am I trying to pick the fly turds out of the pepper? If it is the carb I'll have to send it out, otherwise the mechanic should fix it under warranty.

Ayuh,.... Even when done Right, with a dip-tank, 'n compressed air, ya got maybe a 50/ 50 shot at rebuildin' such an old carb,....

It sounds like yer still fightin' a Bad Carb...

 

[Silvertip]

Re: Rough Idle, 1967, Mercruiser, 160 HP, Straight 6

With vacuum that low, that indicates there is a vaccum leak somewhere. Very likely the intake manifold or improperly adjusted lifters. I've seen intake manifold leaks many times before. Since the roughness was present immediately after rebuild, my guess is going to be as follows in the order listed:

1) Intake manifold installation
2) lifter adjustment done improperly
3) distributor/ignition/timing issue
4) Carburetor issue (cleaning, choke, assembly)

Lastly did this engine every run smooth? What's the history before you owned it and since you owned it? What prompted the rebuild.
Who installed the manifolds after the rebuild (you or the builder). Who adjusted the lifters (you or the builder). Who set the timing (you or the builder). How old is the fuel? Fuel filters been replaced? Compression on all cylinders? Spark on all cylinders? Good wires and plugs. What do the plugs look like after 11 hours? Details, details, details are necessary. Diagnose -- don't shotgun and hope you get lucky.

 

[John3]

Re: Rough Idle, 1967, Mercruiser, 160 HP, Straight 6

Lastly did this engine every run smooth? What's the history before you owned it and since you owned it? What prompted the rebuild.
Who installed the manifolds after the rebuild (you or the builder). Who adjusted the lifters (you or the builder). Who set the timing (you or the builder). How old is the fuel? Fuel filters been replaced? Compression on all cylinders? Spark on all cylinders? Good wires and plugs. What do the plugs look like after 11 hours? Details, details, details are necessary. Diagnose -- don't shotgun and hope you get lucky.

Thanks for your replies: Yes it did run smoothly. It's a 1967 Mercruiser, I bought it in 1970. I had it re-built because it was old and using oil. The Coil wires are new, no arching, I adjusted the Idle to 600 rpm both jets are set as well as I can make them.
The re-builder did all the re-building and all the adjustments, except for the carb. He removed the engine tore it down had it re-bored exchanged the head for a re-built one. Re-assembled it and did all the adjustments.
The fuel is fairly new. I drained the tank last April before we went to Lake Powell for a week (Before the re-build, it ran smooth but used a lot of oil). I installed a large Water Separator filter, next to the stainless steel gas tank, before the trip. The fuel in the glass filter bowel attached to the Carb is clear & clean.
I took Don's advise and just did a Compression test: #1 140, #2 150, #3 145, #4 150, #5 150 & #6 142. I then installed new plugs. The roughness is still there. The old plugs (11 hrs on them) all looked the same, black.
It starts very rough when it's cold, takes a couple of minutes to settle down at about 1500 rpm, then it will idle steady at 600 rpm, with the Vac fluctuating between 10 & 11, at 1000 rpm the vac increases and fluctuates about 1/4".
The mechanic apparently put in a 150? Thermostat, once it warms up it is steady at 150.
You said the Vac is low, can you tell me what it should be & at what rpm's?

PS I have an Air Compressor so I blew everything out with 80 to 100 psi Shop air after I Soaked & rinsed the carb parts.

 

[Bt Doctur]

Re: Rough Idle, 1967, Mercruiser, 160 HP, Straight 6

Might have a bad dist bushing resulting in a cyl not firing or a worn lobe. I`m assuming it still has points so check each lobe at its high point
for the same gap.