350 MerCruiser SBC smoking and erratic vac

6

69GTOby

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Re: 350 MerCruiser SBC smoking and erratic vac

That's interesting about the timing. Certainly something to ponder. Lots of differences between cars and boats.

What are welch plugs? As far as I know, this carb has never been off the engine since it was put on at the factory in 1988, which is probably why it is problematic. I am thinking more and more that the carb is responsible for the erratic idle and random plug fouling from dumping raw fuel and the boat rocking. I am considering picking up a Holley marine carb. I have more experience with Holley, and none with the Q Jet. Good or bad idea?
 
NHGuy

NHGuy

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Re: 350 MerCruiser SBC smoking and erratic vac

What about doing a carb kit on the OE carb? The one that was designed for the motor... Or if you don't do carbs yourself - be smart and get it rebuilt by a carb expert. Why reinvent a tried & proven system unless you are looking for a specific change.
 
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69GTOby

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Re: 350 MerCruiser SBC smoking and erratic vac

Well, the reason I brought up ditching the Q Jet, is because Holley carbs are arguably easier to tune, less sensitive, simplified, and more mechanic friendly. That's all up for debate, but I personally have experience with Holley carbs and very little with the Q Jet. So, it would be nice, for me, to have something more familiar atop the engine. But, a kit for Q Jet would be much cheaper, and gaining experience with the Q Jet would not be a bad thing. Maybe that's the route to go. Plus the Q jet will probably yield better fuel mileage, which is always nice. :) Hopefully I can rebuild it right on the first try.
 
series60

series60

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Re: 350 MerCruiser SBC smoking and erratic vac

69GTOby said:
What are welch plugs?
Click to expand...
welch plugs.jpg​
These welch plugs are prone to leaking. Many folks have used epoxy/JB weld to seal these plugs from leaking. Common practice for DIY rebuilders.
 
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69GTOby

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Re: 350 MerCruiser SBC smoking and erratic vac

Finally got the boat to the house. I quickly discovered the hose in the transom area that bring water from the impeller up thru the hull was disconnected on the outdrive side. Not sure how it happened. I thought I had installed it too tight... Well, it is on there now. We ran the boat in the driveway for about 10 minutes and it ran great and right at 140*. So, the cooling issue seems to be fixed.

Now, I removed the exhaust elbows and discovered some stuff... Sure enough, the y pipe flappers were the source of the clicking noise. I knew I felt it in there! I am not sure how these should look, but I know something is not right about them - you tell me. The couplers certainly need replaced. They were super brittle and misshapen from the overheating incident. Check out the pics:


P1050258.jpgP1050259.jpgP1050260.jpgP1050261.jpgP1050262.jpg

We really want to route the exhaust out the back of the boat. The pros seem to outweigh the cons. We would retain the y pipe and have it capped when running thru the hull. When we would want it thru the y pipe we would simply reroute the exhaust and move the couplers to the inside end of the tips. The problem is, if we go thru the hull above the steering rack, we would need to use small 2 1/2" tips, which are hard to find (maybe someone can help with that), otherwise, it has to endure a couple turns and go below the rack, outside of the y pipe.

All input appreciated! Thanks!
 
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69GTOby

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Re: 350 MerCruiser SBC smoking and erratic vac

Sorry to revive the old thread "mercruiser 260 low vac" Don and others. It's just that I wasn't getting much feedback here in my own thread and thought it would be helpful. Again, sorry for that. Anyway, I have made more progress. Here is where the project stands:

So, the temp issue is resolved. Now, I am dealing with the idle issue. It's smoky at idle - will make your eyes water - and has an erratic vacuum at 12"Hg +/- 2"Hg. I turned the mixture screws all the way in and it kept running the same. It's an untouched original Q Jet. I pulled it off and put on my 750 Holley from the GTO which made 20"Hg on the stock cammed 350 Pontiac, just for diagnostic purposes. It ran the same... Still smoky, eye watering and low erratic vacuum.

I'm thinking the plugs are still so foul from the Q Jet dumping fuel that it won't run right even with a good Holley carb. I'll clean the plugs tomorrow and see if it clears up.

Any ideas, input, suggestions - much appreciated!
 
NHGuy

NHGuy

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Re: 350 MerCruiser SBC smoking and erratic vac

Go to mityvac.com, they have a ton of diag methods in their book. If you don't get what you need PM me or post back. I car pool with a master tech who does this stuff in his sleep. I can ask him or try to get him to read your thread.
 
Thalasso

Thalasso

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Re: 350 MerCruiser SBC smoking and erratic vac

NHGuy said:
Go to mityvac.com, they have a ton of diag methods in their book. If you don't get what you need PM me or post back. I car pool with a master tech who does this stuff in his sleep. I can ask him or try to get him to read your thread.
Click to expand...

You are a good man.No sarcasim meant.
 
NHGuy

NHGuy

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Re: 350 MerCruiser SBC smoking and erratic vac

There is much more detail available, but I'm a bad typist. Only reason I took that class was to sit near the pretty girls.
My little book says with motor at idle speed, vacuum pointer should hold steady between 16 & 22 lbs vacuum. With motor at idle speed, dropping back of pointer indicates sticky valves. With motor at idle speed, floating motion right and left indicates carburetor too rich or lean. With motor at idle speed, low vacuum reading indicates late timing or intake manifold leak.
No mention of the smelly smoky stuff behind the boat.
My transom is gray and it's a good thing, it gets pretty rotten from exhaust soot...maybe you are trying to get a clean smell from a plain no emissions motor?
The new style exhaust flappers are different and won't jingle. You must have gotten things pretty hot to cook those rubber exh tubes, hopefully your test information is from after the cooling repair.
I have had my eye on these pipes, I might do it if I can fabricate a diverter that will stand up to the heat in there.
http://www.ebay.com/itm/MARINE-MODU...&otn=1&po=LVI&ps=63&clkid=4480748321746095719

On the other hand, if it's underwater at rest it won't be loud til I take off.
 
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69GTOby

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Sep 20, 2011
Messages
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Re: 350 MerCruiser SBC smoking and erratic vac

Thank you so much for all the help! I replaced the plugs and it still ran the same - bummer. I wonder if I pull them back out if they will be black again, or if the carb was the cause of the fouling. I know that old Q Jet that came off certainly needs attention. All the manifold bolts were tight and I could not find a leak with brakleen, but it could be leaking internally in the valley. In fact, maybe it is sucking up oil from the valley due to bad intake gaskets... I found a reman Holley 600 Marine carb on an ebay store for $270 with a lifetime warranty. I might go that route and ditch the Q Jet, as well as reseal the intake.

As for the thru hull... I have been thinking about it, doing research on it, and looking up parts for it for years. I decided what to do. I found a couple places that have 3" tips for about 80 bucks each. They will fit right above the steering rack and below where the hull angle changes. It'll be tight, but they should fit. I'll leave the y pipe in and cap it off at the top. To make it thru prop again, all I have to do is reinstall the elbows and put the caps on the thru hull fittings. Inexpensive and effective. BTW, I have been running the boat in the driveway on ear muffs with no exhaust, just straight off the risers, and it sounds beautiful! Way too loud for all the time tho.

You can also make the tips wide, on the outside of the steering rack, or below the steering rack which is below the waterline.
 
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