1959 Lark 35 not reaching full RPM, sometimes???

[59 Sea Coaster]

I have this beautiful 59 Thompson Sea Coaster with an Evinrude Lark 35HP on it. It had been sitting in indoor climate controlled storage for a long time! Last registration sticker expired in 1967. I replaced the coils, points, condensers, rebuild fuel pump and carb, replaced hoses and plug wires and installed new impeller and wear plate. It had been running pretty good and would reach 23 MPH and what seemed like full RPM after a bit of a flutter in the mid range at a speed of 15MPH. Recently it stopped fluttering and only reaches that 15MPH mark but will on occasion flutter and get past that mark to 20ish MPH. Compression 135 in each cylinder, both plugs look decent maybe a bit on the rich side? I have not checked for spark on both cylinders at speed, primarily because I have not been able to get someone on the water with me. Hard to believe right!

If anyone has some experience with this condition or even some good ideas I am open to suggestions!

I have pulled the carb off and will go through it again, was thinking maybe my float is heavy or mis-adjusted not allowing enough fuel to keep up with demand? It does seem to like fuel!

Thanks,

Joe

 

[oldboat1]

You have tried adjusting the h.s. needle? Snug up the nut if it's vibrating loose. Nice rig!

 

[pro-crastinator]

How long have you owned the boat/motor combination?
What is your expectation of its top speed?
Length / weight of boat?

 

[oldboat1]

^^good points. thinking the SeaCoaster is 18'? Pretty beefy, anyway. Weight distribution is always relevant. Was it blocked on the trailer for all those years (nice straight hull, no hook?)

 

[59 Sea Coaster]

I have owned it for a year now but just got it in the water this late spring and have been out 5-6 times now. It is a 16' Sea Coaster it is the original engine/boat/trailer combination and I am now the second owner.

Expectations of the boat? Being it seems capable of 23 MPH with the current engine that is what I would expect it to do. The boat came with all the original equipment the 1st owner purchased, including a pair of water ski's that may or may not have been used? It would be nice to be able to pull a skier up on two skis with a dock start. The boat pulls hard enough to 15 MPH if I could get it to stop the flutter in the mid range and continue pulling to the 23MPH it should easily pull a skier.

I learned to ski behind a much heavier Lyman with a 35HP Johnson on it. Hoping to do the same with my grand kids!

I have quite a bit of experience with engines in general and a lot of two stoke tuning on small engines and have adjusted the low and high speed mixtures. Just cannot quite get the flutter out and now cannot even get to the flutter in the midrange. The carb is off waiting for the new float, it will go into the ultrasonic cleaner and get all packings replaced, new gaskets and i'll set the float a little higher.

 

[59 Sea Coaster]

Oh and the hull is pretty straight. I flipped it over this spring for a fresh coat of bottom paint, maybe a 1/16" deep depression where the rear roller sat just in from the transom.

Thanks! We're pretty proud of her! I'll try and get some pictures of her on the water.

 

[oldboat1]

sounds like you've been there, done that. What fuel ratio are you using? I used 24:1 in my '57s, but might be at 50:1 for '59 -- not sure offhand.

 

[59 Sea Coaster]

From what I could find 50:1 was recommended. Worked on a of small engines and snowmobiles over the years and tinkered with boats. This is my first attempt at something this old that has sat this long. I am starting to second guess myself and have concerns it might be the leaf plate behind the intake manifold. My guess is it may have deteriorated some in the past 50 years while sitting! I'll get the carb done, replace the fuel filter and see what happens.

Thanks

 

[lindy46]

Link & Synch?

 

[F_R]

Leaf plates are extremely reliable. Very little chance that is your problem.

 

[pro-crastinator]

Great looking boat. Am looking forward to hearing about it after the carb work is done.
Pics are highly appreciated. Best O luck - and welcome to Iboats.

 

[lindy46]

Run that motor with a 24:1 gasil ratio. They didn't start recommending 50:1 until 1964.

 

[HighTrim]

Start by ensuring you have good spark on both cylinders.

Spark should jump a 1/4" gap with a bright blue ZAP!! on an adjustable inline tester.

I would also try neon type testers, or use a timing light to ensure constant spark while running. Perhaps one cylinder not running.

If good spark throughout, Have you opened exhaust, and checked that it wasn't clogged up, restricting power?

 

[59 Sea Coaster]

Ok somewhat of an update. Went thru carb again and set float, nothing real surprising or evident during the carb teardown. Reassembled and headed to the lake! After some initial tinkering to get idle mixture set so it would stay running got underway and hit a wall at 16MPH, had that same flutter, like it wanted to give more but just couldn't pull it off! Exhaust seems to be clear, in all fairness it bubbles a lot of water at WOT under no load. That is my scientific diagnostic for the exhaust being clear. Without tear down I do not see a way to check it for obstructions.

One thing of note, while cruising along at full throttle and 16MPH (about 3/4 throttle speed) I had to make an abrupt hard turn to avoid a jet ski, during which time the prop cavitated and the engine went to full RPM? Brings me to think do I have a drive issue? Is there something binding or creating resistance during acceleration that is holding the engine back? When that frees up, only occasionally while cruising along it allows the engine to reach full potential? The drive was very clean when I had it apart over the winter, it had been sitting dry for a long time! But no water or rust seemed apparent in the gears.

The engine will always reach full RPM with no load just sitting in the water, not the most responsive but this is old carb technology I do not expect snappy throttle response!

Is there something else I could look at or a recommended "test" that I could perform?

Thank you for your help so far!

PS: When the engine does come to full RPM and 23 MPH under a load I can back off the throttle some, no less then 3/4. And can accelerate back to full throttle and speed. If I drop back any further with the throttle it stall at that 15MPH range. Am I over propped? Not a lot of selection available for these old dogs!

 

[lindy46]

If it bogs at 3/4 throttle, you may want to check the carb synchronization to the magneto. This is from memory, but look at the cam on the mag plate. There is a roller linked to the carb which rides on that cam. As you advance the throttle, the roller riding on the cam causes the butterfly in the carb to open. There is a scribe mark on the cam. I believe on your motor, there is a "pointer" just to the left of the roller. When that pointer hits the scribe mark on the cam, the carb butterfly should just start to open. There is an eccentric screw and clamp on the throttle shaft which adjusts this. Loosen the clamp and adjust the eccentric to make the butterfly just start to open and then tighten down the clamp. Now with the engine in "forward" (not running of course), advance the mag plate all the way until it is up against the stop. Now look for a long link rod running from the cam roller to the rear on the port side of the engine. There is a brass collar on the rod with a set screw toward the end of the link rod. Adjust the brass collar so that it is up against the "clevis" on the vertical throttle arm. There should be no play or just a hairs' play between the clevis and the brass collar. Once adjusted, further advancement of the throttle causes the cam roller to move away from the cam and causes the tension spring at the mag plate to allow the vertical throttle arm to move farther which now advances the carb butterfly to open fully via the link rod. The link rod takes over at about 3/4 throttle. Hope this isn't too confusing.

 

[oldboat1]

Lindy! Impressive memory!!

Confess I'm not sure what "fluttering" is. I wonder if it's hesitation in hull speed, maybe tendency to fall back off plane -- something you feel in the seat of the pants. If so, there may be a trim issue involved -- thinking you could experiment trimming another notch in, maybe negative trim as it sits on the trailer. You sure shouldn't have to hold WOT to keep it on plane. 3/4 throttle, as I think you mentioned, strikes me as a comfortable cruising speed if you can hold plane with that. I guess with a few more ponies or a cupped prop or bigger bite, another few more mph could be found. But I guess I'm not sure it's that kind of rig.

On revving through turns, ventilation I think, the Thompsons I remember might have been prone to that -- we liked sliding around corners. (Actually, had an aluminum Lonestar that was even more fun at it with a Big Twin.)

On the other hand, if it's alternating rich/lean that sounds like air/fuel mix or oil ratio -- and the fix can be counterintuitive. You might need a richer ratio, for example, or a leaner mix at the h.s. needle -- 24:1 ratio, and tweak the needle a bit leaner (clockwise) at WOT. But treat it right (Primary color enamels for the tear drop fenders, and shine up the baby moons...)

 

[racerone]

Hook up a timing light to # 1 cylinder.-----Lay timing light on a seat.----Run boat and observe behavior of the flash.----Repeat the test with # 2 cylinder.---Checked clearance on compression relief valves ?

 

[59 Sea Coaster]

LIndy that is impressive! I will check these adjustments and confirm, not confusing at all. Oldboat1 The trailer will get a full restoration with as close to original Tee Nee colors as possible and re- chromed moons! We plan on keeping this one and the kids can have it after us. I swear this has the original air in the tires! To precious to let slip away.

Fluttering - Have you ever heard an engine bounce on a rev limiter? That is what this almost feels like but not as extreme. Lets say it pulls hard to 15MPH and is sitting at 3k RPM (just for reference, RPM's unknown) and it sits there bouncing between 3000 and 3200. Feels like it is starving for fuel, not quite enough there to achieve or maintain full RPM. Make sense?

I had considered timing being an issue but had not researched the settings as of yet. Lots of linkage on there! Also wondered if the compression release linkage could be a factor? Assuming this boat ran fine back in 1966 when it was last used, one would think everything would be in order? What would wear or otherwise fail while sitting for 50 years? Fuel pump has been rebuilt, carb rebuilt (twice), coils, points, condensers replaced, new plugs, drive resealed. Leaf plate? I know reed valves can fail or dry out from sitting.

This really is a labor of love and I will get this figured out. I have a great running older Merc 80HP with controls and ignition that could be thrown on here. But this is a Jubilee edition Lark that was original to the boat and it looks awesome! It would be shameful to remove it!

 

[pro-crastinator]

Has anyone thought about the fuel tank, and the possibility that fuel delivery could be an issue?
Is there an old filter screen that has varnished up somewhere? I note that the float was replaced - and that fuel damage to the float is seldom a single "problem".

 

[59 Sea Coaster]

Replaced the fuel tank and all lines. Which is a shame the original tank looks beautiful but has some rust on the inside that I have not been able to resolve ret. Might try a sloshing compound someday when I get really bored!

I'm going to try some old school troubleshooting with carb spray near the upper seal to check for a leak there, doubtful but what the heck! I also have an old vacuum gauge I will try and install someplace to monitor and check for leakage. I think I may have an old school timing light still lying about! Thanks racerone!

Just found a new leaf plate and gaskets cheap, ordering that and another fuel pump diaphragm. I need to make sure that was reassembled correctly just so I can sleep at night!

Now I need to order a day of free time so I can do all this. It would seem free time has been backordered indefinitely!