70 hp ignition woes

I recently bought a 1973 chrysler boat with a 1972 (707HE) 70 hp chrysler motor attached to it. The p/o said it ran last year - sort of! Compression is 130 lbs on all 3 cylinders. Carbs are clean. It has good spark, starts quick and idles smooth. I took it out to the lake yesterday and once it is put under load it stumbles, bogs, sighs and dies! I took the cowl off in the middle of the lake attempting to determine the cause of problem and concluded its an ignition problem with the timing advance, or some type of safety switch - somewhere. Here's why: I got my wife to sit behind the wheel and put the motor in gear, but not throttle it up. I sat on the back and grabbed the throttle linkage and opened it up - wide open. It ran fantastic and according to the almost 40 year old speedo was actually doing 35 mph!! I was able to repeat this time and time again, but as soon as I tried wot with the control, it would stumble, bog, spit and die. I am not too familiar with outboard electrics, but it seems as though there is something preventing the motor from opening up under wot with the control column - perhaps a faulty rev limiter or safety switch or??? Has anyone had a similar situation, or perhaps some advice on where to start looking?
Thanks -- Al

Re: 70 hp ignition woes

First: Previous owners ALWAYS say the engine ran last year or when put away--The check is in the mail, I'll respect you in the morning, the computer fouled-up, etc.

130 is decent compression so the engine basically should be in good shape.

If you can move the linkage a lot at the engine then either the control cable is broken or not attached. At the engine, on the tower, there is a clamp that holds the cable jacket. Check to be sure that the screw has not fallen out. Further back at the end of the cable there will be a stainless quick connect and a mounting ball on the engine. Check that the cable and connect are still attached to the ball.

Finally, check to see that the plastic fittings on the distributor have not broken and the distributor is advancing with throttle.

Re: 70 hp ignition woes

Frank
Thanks for the reply. And yes previous owners generally tend to exaggerate the condition of anything - usually in their favor!! Anyhow, the linkage is correct and all attached. The cables work as they should and the spark advance link is together at the distributor. By the way, I have a type IV ignition with points and a bunch of other stuff... When I mention grabbing the linkage, I am grabbing the butterfly linkage that joins the 3 carbs and by-passing the control linkage that engages the carbs with a cam. When I open the throttle manually I can get full power under load. As soon as I use the control and advance it forward it sputters, bogs and dies. It seems to me that the timing advance gets way ahead of the throttle. The distributor rotates almost fully way before wot is achieved. Is this normal? I had it going again last night and it runs awesome in the shop!! My problem is related to the control, distributor, advance stuff and wot under load. I have the seloc on-line manual, but find it kinda useless for this problem.

Re: 70 hp ignition woes

Timing generally advances to full at about 3/4 of the control handle travel. This is correct and on some controls is marked as cruise. That is where you will get the best fuel mileage.

If you look at the throttle advance cam you will see on it a "finger." This finger is what opens the butterflies fully at full throttle and will not do so before 3/4 throttle.

The difference between 3/4 and full throttle is generally a lot more noise and only 3-5 MPH more (varies depending upon hull and load).

Now to the nitty-gritty! Timing and throttle opening are linked and must be synchronized.

First remove the ball end link to the carb cam from the timing tower. Move the cam back out of the way. Now loosen the screws on the aluminum bar connecting all three carbs and allow all carbs to snap closed. Tighten the screws. Now, all three carbs will open the same amount--actually, because of slop in the link, the top two will open slightly less than the bottom and you can correct this by hand, loosening the screws and removing the slop--but it is not really necessary.

Move the cam to the point where the scribed line is pointing directly at the black roller on the lower carb. The black roller is on an eccentric screw. loosen the nut and adjust the screw so the roller just touches the cam at the line. This is the starting point and just sets the closed position-- it will not be the running position.

Now reattach the ball link. open the throttles to full position with the control box until the black button on the tower arm hits the block. Remove the ball link from the timing tower and adjust the length --keeping both ends equal-- so that all three butterflies open fully and substantially equally. They don't need to be perfectly horizontal, just substantially so.

At full throttle, with the plugs removed and grounded and with the ignition on, turn the flywheel by hand and adjust timing to 32 degrees before top dead center. (Watch for the spark on the top, #1 Cylinder) Adjust with the link on the distributor. The 36 degree mark on the flywheel should be 4 lines left of the long TDC line on the block pointer.

Carbs are now synced with timing and should open fully. When put back in neutral, the line on the cam should be a little below the black roller and the carbs should be cracked open.

Now start the engine and set idle using the idle stop screw at the bottom of the tower. Idle should be about 700-750 RPM in forward gear, in the water. Note that adjusting mixture on the carbs will affect idle speed. Do not adjust mixture unless necessary.

Assume that nothing on the engine is correct and everything is worn. Check the mixture screws on the front of the carbs. They should be at least 3/4 turns out from lightly seated. Less than 3/4 is too lean and the engine will melt piston(s) at or near full throttle. Replace the water pump impeller. Replace the fuel pump diaphragm. (fuel pump is on the side of the engine above the cables and will have either a glass bowl or a black plastic inlet fitting.) Change the lower unit oil. Install a cneap plastic fuel filter BETWEEN the fuel pump and the carbs.

Re: 70 hp ignition woes

Frank
I followed your set-up step by step. It appears when I set the static timing originally, I didn't do it at wot. #1 was sparking waaaaay too soon. I adjusted the carb link and tower to distributor link as per your instructions. I'm hoping for good results in the lake tomorrow. I did notice that the timing tower didn"t return to the idle set screw block stop. I am not sure if it is because it wasn't running or ?? I greased the distributor and it moves freely. I'm hoping a running motor will correct the minor problem. Now, I have replaced the impeller but not the gasket(s). I also replaced the lower gear oil. The t-stat was junk so I took it out and have not replaced it - mainly because I can't find a listing with the correct part # and rubber o-ring seal. Is there a lower unit gasket kit available for these old chrysler motors? Most marina's in my area cannot be bothered helping with the "old stuff". If tomorrow proves successful, and I think I have a viable motor I will take the time to replace all gaskets prone to failure - mostly from lack of use. Thanks for your help so far. I'm sure there will be many more questions to follow. Hopefully not though.