K Motor Zenith 8983 Adjustment/Repair

[kmkemper]

Further to my previous post regarding my rich-running condition, I changed out the ignition coil and plugs, and fiddled around with the choke lever. The return spring on the choke was a little stretched out, and so I replaced it. After running in the driveway for about 20 minutes, at various RPMs (up to 2,300 at times), the new plugs still look more fouled than they should. Plugs installed now are Champion J8C. The exhaust smells a little rich to me, though there is no smoke. Not sure what should be normal for this boat, though. Boat = 1942 17' Special Runabout.

My boat has a Zenith 8983 carb on it. This, I determined from comparing photos of my carb to others on the Web, since the tag on mine is illegible. I am mechanically inclined, but I have no experience with this carb at all and how it functions, and I don't want to just start ripping into things. The boat is in restored/rebuilt condition, and did not have that many hours on the restoration before I bought it, but enough that things are generally sorted out on the boat.

Does anyone have a diagram for the 8983, or instructions/advice on how to tune the carb or check/reset the float? The more basic, the better. I have rebuilt carbs on British cars, but this one is a different animal and hard to see behind the engine, much less reach. Not afraid to get my hands dirty, or lose some skin, but fear that I can easily turn a small problem into a big one without guidance! The carb has the T-screw on the bottom, which I have not touched in all of this.

[Riviera Wish]

Try looking at "the carburetor doctor" web site. I recently downloaded a 13 page 'plain English" Zenith 28 series service manual
for $5. They have the Zenith carbs listed by group numbers as well as many other carb manufacturer's kits and literature.

[farupp]

I think the T-screw is the high speed mixture adjustment. The low speed mixture adjustment is done with a regular flat head screwdriver.

[jim g]

If your running suppression spark plug wires you will get sootier plug then you do with cooper wires. Running it on the trailer will not get you a good reading on the plugs. You need to run it down the lake at speed for 10 minutes or so then turn the switch will your at speed. Then pull a couple of plugs and see what they look like. If there slightly dirty you can turn the T handle screw in a little. This will lean out your high speed mixture. Be careful not to lean it to much. As the leaner you run it the hotter the cylinder temps get. Which can start melting stuff.

To set the high speed. You run the boat down the lake wide open and unscrew the T handle screw until the boat no longer gains speed. Then turn the screw in until speed starts to drop off. Then turn it back to the fastest spot. I tend to turn it about half a turn further out to make sure its not running to lean. A tad rich is better then a tad lean. Unless your racing. But you know what race motors do best. Blow up.

[kmkemper]

Jim - that's a good idea, to take a "plug cut" while on the water, I will do that. I don't believe that the wires are suppression wires, they look to me like reproductions of original, like most things on the boat. Attached is a photo of the wires, perhaps you can tell me what I have, I am new to this but learning. Which way does one turn the fast-idle T-screw to lean it out, clockwise or counter? I won't be able to mess with the carb while the boat is underway, I have a runabout with the engine under a side-hinged hatch, and the carb is located underneath the hinge. This is where a utility design would present a distinct advantage!

[farupp]

You turn the fast idle T screw clockwise to make the mixture leaner, and counterclockwise to make the mixture richer.

[jim g]

The spark plug ends on your wires probably work. But someone rig distributor ends to work on the plug end. I would recommend getting the correct spark plug ends. Farupp is correct. Clockwise to lean it out.

Once the mixture is set. double check the timing sometimes you have to advance or retard it a little.

The T handle screw is not fast idle. Its the high speed circuit. Your idle screw is the screw on the neck of the carb.

You will probably never get it right unless you adjust it while running full speed. Get someone to drive the boat. At idle open the hatch climb over the engine and sit on the rear deck with your feet in the engine compartment. Place your feet on the engine stringers. Tell the driver to go. As you bend over to adjust the mixture use your shoulder to hold the hatch open. Its best to do this on a flat calm day. If you worried about the hatch take it off and run the boat.

As a kid I would watch the old mechanics that worked on these go down the lake by themselves and do this. Some would tie the steering wheel off and the one mechanic that was about 6'3" would drive the boat with his foot will leaning over into the engine compartment to set the timing and adjust the carb.

Shutting down the engine while at speed to to check the plugs is how the race engine builders tuned the carbs back in the old days.

[kmkemper]

Thanks all - I suspected as much on the spark plug ends, and have had to mess with them to secure a good fit on the tips of the plugs. Wasn't sure if this was simply "the way things were" and retained as an attempt to maintain originality. I spoke today with the builder of the engine (John Matzen) who confirmed some of what you all have explained regarding the direction of the adjustment screws. The wires are copper, I confirmed that.

The idea of running the boat at top speed with me in the engine compartment with a screwdriver gives me the shiver-shakes. I would have to put my wife at the helm, creating trust issues that may require more effort to overcome than the proper air to fuel ratio

[jim g]

If it makes you feel better I've had my son driving customers boats since he was 5 with me in the back adjusting the engine. All against my wife's wishes and the law.

You don't need the screwdriver just your hand. If you don't won't to do it from the back you can do it from the front. By leaning over the seat back into the engine compartment. It just a little more difficult.

[kmkemper]

Update - this weekend, we brought the boat back up to Donner Lake in the mountains for the weekend. Before the trip. the boat seemed to run well enough in the driveway after playing with the choke and the spark plug wires, but once we got it up here, it was still running rough under load. Adjusting the carb while under throttle had little to no effect, so I figured the problem was the ignition.

Took the distributor out and brought it over to Sierra Boat, where the Petronix was switched out for points, Sierra Boat having found that the hot wire to the Petronix was rubbing and had a potential short-out. After re-installing the distributor and setting the timing, the boat still ran rough. Out of ideas, I ran to NAPA and purchased a compression gauge. Turned out that Cylinders 2 and 3 had no compression.

Went to Ace Hardware for a torque wrench, Sierra Boat for a new head gasket, and changed the head gasket out in the parking lot of the resort where we are staying. Boat is now running well, and I am stoked because we are here for three more days of fun on the lake. I figure that the ignition was the reason for the fouled plugs, and masked the separate problem with the head gasket; now both are resolved, thus far it appears! Thanks for all the advice on tuning the carb, which I have put to use since messing with the thing.

[martinfeletto]

Great job Kevin - nice to have Sierra Boat in the neighborhood. Enjoy - great weather, I just left Tahoe. I went through this earlier this summer with a little A or B, can't remember, for Chuck Kelley. I owe you a beer for not bringing up this possibility while you were searching! Keep a close eye on the temp gauge - something caused the head gasket to go. Probably got loose so also continue to torque and re-check almost every time you are going out for quite a while then off and on during the season.

Marty

[jim g]

Good job in finding the problem. I would recommend over the winter pulling the head back off and have a machine shop give a light milling to the head to make sure it is flat.

Get a rethreading die and tap and clean the threads on every head nut and stud. Then using 320 or 400 grit wet/dry sandpaper sand both sides of the washers and one side of the nut that touches the washer. Then clean the block well.

Then get from napa a can of Permatex copper spray coat and some moly lube. Moly lube is C.V. Joint grease. ARP has some in a squeeze tube. Coat one side of the washer with the moly lube and the side of the nut you sanded make sure a little bit goes in the hole. Take your head gasket { you can reuse the one you just put on if you can get it of without damaging it } and hang it up through on of the bolt holes and spray it with the copper coat. Do it evenly one side then the other and repeat until the can is empty. Wait 15 or so minutes and install on the engine. Put the head on then install the washers. Install the head.

But some lube on the threads of the studs. Install all your nuts. You should have 2 bolts. Make sure they are in the front and rear corner holes on the manifold side. If there not and there is a stud there I recommend removing the studs. But depending on how long the studs have been there they could break. With studs in those 2 corner holes you could crack the corner of the block if the stud turns while your torquing the nuts.

Torque to 60 ft-pds. { pretty sure on this but I would have to double check } Start at 30 and start in the middle and work your way fore and aft. So do the middle row the the next row forward then the row aft of the middle row and repeat. Then do it again at 40 the 50 then 60. Put the rest of the engine back together. Then go to the lake. Run it around the lake for about 5 minutes at speed. Quickly come to idle and shut off engine and retorque to 60. Run it again for 10 minutes and retorque. Run it again for 15 to 20 minutes and retorque. At this point the nuts should not move or move just a hair. I usually check it one more time after a long run.

A flat head engine needs the head retorqued once a year. Using the copper coat will help your head gasket last a longer time.

When a flat head gasket burns through between two cylinders its from the head running loose on the block.

[kmkemper]

Thanks Jim and Marty - glad to have the engine together and the boat running. Yes, per the factory manual, the torque spec on the head is 60 lbs, something Sierra Boat looked up for me while I was there. I re-torqued the head again after running it, which it needed on most of the nuts. I appreciate the detailed advice on how to install the head the right way. I couldn't find any specific direction in the manuals on the torquing sequence, but having done this on cars before, I started in the middle, working across and to the outside. All good.

Marty - I'll enjoy that beer, and the next one, when we get together next, hopefully soon and on me!

Breakfast this AM, at sunrise, served on the water