OMC Exhaust Plugged?

[gottaboatnow]

So, I managed to complete my swap from 3.8 to 4.3 on my Stringer 800. Everything works perfectly! Well, okay, there is one issue that has come up. The exhaust isn't coming out of the prop hub but is coming out of the intermediate housing where it meets the drive unit. The engine won't start unless you raise the outdrive, then she'll start right up. Lower the drive, and she'll stall right away. I can't seem to find any topics on plugged exhausts for OMC's, so I'm guessing this isn't common? Everything inside should be completely clean, as I was pretty detailed when putting the pieces back together. I cannot figure out what might have stuffed up the exhaust, but when it was running, you could go for about 15-20 minutes before she just suddenly quit. Then, you were dead in the water. I only figured out the raising issue when I was near some rocks and had to pick up the drive to avoid banging into them. Any thoughts from anyone out there? Thanks!

 

[Boomyal]

Did you have the drive apart when you swapped motors? Also, it is normal for the exhaust to exit via the relief port in the intermediate housing. Do you use muffs or do you have the flush fitting? When you apply water, does any of it come out the prop hub? I just cannot see how the exhaust port thru the leg would be plugged up.

 

[Howard Sterndrive]

I think your boat's just playing tricks on you. Unless you have it sitting on some kinda weird trailer or wood cribbing under the intermediate housing. You can 100% plug the exhaust in the drive and lower it without causing the engine any trouble. There's 20 or so big holes in the anode plate at the bottom of the intermediate the exhaust can go out (about 10 square inches of wide open). It doesn't have to go through the drive with the drive down.

 

[Howard Sterndrive]

what distributor are you using in the 4.3?

 

[gottaboatnow]

Okay, we're not talking about muffs or flush tubes, this is on the open water. She'll run for 15-20 minutes at full throttle and just flat-out quit. Done. Won't restart for about an hour. The last time that happened, a storm was coming, so I trolled over to a cove to wait it out. Had to pick up the drive because there were rocks and the skeg was scraping. While waiting for the storm to pass, I tried to start it, fired right up! Lowered the drive, engine quit. Did this about 4 or 5 times. Not sure if the two are related, high speed stall and exhaust appearing to be plugged, but the exhaust didn't used to squeeze out between the intermediate and the lower unit. I don't have any exhaust (that I can see) coming out the prop, at least at idle in the water. As for the Distributor, I did replace with aftermarket HEI after reading the research that Chevy did that proves the HEI system is safe in heavy fuel fume testing, so there is an aftermarket unit in there. I can turn the idle waaaay down and keep her running, so I can ignore the loss of stumble during shifting. What has occurred to me (thank you, Howard) is that the ignition module could be causing the problem with stalling. Hmmmm....

 

[gottaboatnow]

Boomyal, yes I tore the whole thing apart, checked, inspected, cleaned, which is why I'm wondering what the cause could be. If she'll run without exhausting through the hub, then I'm lost. I know the only way something could plug it up is if it crawled in there and got stuck, which isn't impossible with our desert critters. The last time I ran it on the muffs, I did get water out of the hub, but that was before all of this happened. Thanks!

 

[gottaboatnow]

What I will say, is this, that motor was a great idea. Plenty of power, hole shot, and for me, the conversion was pretty easy. The intake bolted right on, made 1/2" spacers (laser cut, cost about $75.00) for the exhaust manifolds because the valve covers are more square, hardest part was finding a new flywheel (BTW, early Corvette is cheap, lightweight and beautiful!), had to hand-cut thicker gasket material to make up for the spacers which re-angled everything. Oh, and Silicone Exhaust Tubing, great stuff. Used on Semi-Trucks for the stack exhausts, flexible, rated to 600 degrees, and won't rot like rubber or melt if there's a problem. 4" fits with a little silicone on the intermediate side as they're a little bit smaller, but it all came together nicely and works really well. If you want to know about the HEI data, I'll be happy to explain that, too. The HEI unit, because of its high voltage, creates ozone in a pretty well-sealed environment. Ozone is not flammable and will not allow for combustion under the cap. It's part of the design, which is why the cap is so big, to allow enough ozone to build up and insulate against crossfire from the carbon dust that builds up with time. There is research out there if you want to look for it.

 

[Howard Sterndrive]

but the exhaust didn't used to squeeze out between the intermediate and the lower unit.

yes it did.

maybe someone has blocked those holes in the anode grate at the rear of your intermediate. If the exhaust needed to go out the prop your boat would be very difficult to start - the back pressure is too great with the prop shaft 24" or so under water. That's why that grate is there.
18 holes so your engine will start.

 

[gottaboatnow]

I will have to look at that, because it is extremely hard to start with the drive in the water. I was assuming it was a carburetor issue originally, but after rebuilding and tuning it, I can't believe that's the problem now. I will check the anode grate for blockage. So, if the drive is raised up, I should see this in the back end hole of the intermediate housing, correct?

 

[Boomyal]

I will have to look at that, because it is extremely hard to start with the drive in the water. I was assuming it was a carburetor issue originally, but after rebuilding and tuning it, I can't believe that's the problem now. I will check the anode grate for blockage. So, if the drive is raised up, I should see this in the back end hole of the intermediate housing, correct?

Looking back into the intermediate exhaust port, this is what you will see. The opening at the bottom of the port is more than enough to let exhaust escape without having to force it down two more feet to get it out the prop hub. The anode block is bolted on below that and I doubt there is any blockage in it.

 

[gottaboatnow]

Thanks for posting the pic and I did manage to figure it out. I have two problems, one is electrical, the other is fuel related. I'll get into both issues in the coming days. The boat will start with the drive in any position at all when POWER is applied to the distributor (faulty relay or failed wire to the coil side of that relay). A jumper worked to get the boat back to the dock. As for the fuel system, she starts great, idling well for the most part, and will go into gear and drive as long as you don't ask for too much throttle. Since I've never had the tank out, I'm sure there's probably some gunk in there, plugging up the pickup. I don't think it's ever been out of the boat since 1984. The hoses are so old I couldn't get them off a year ago. I should have done this back then, I suppose. I was smart enough to have the carb open then, cleaned and inspected, and all seemed well once I got the garbage out of it. I'll open it up again once I see the condition of the fuel tank. Thank you both for your help and ideas!