Why is it so hard to overcome compression on pull start?

[erikpn]

It's just a 35hp force from the 80's. But it is incredibly difficult to pull start. I can pull it easily with the spark plugs removed. With the spark plugs in, I can still turn it over slowly even with just 1 hand.

Disconnecting the plug wires didn't make any difference.

Am I setting it up wrong? I put a rope through the notch on the top of the flywheel, wrapped it around a few times and try to pull. I almost jerked the boat and trailer around. The compression is 145, 145 in both cylinders. What could it be?

Backstory:

Outboard stiffened up pretty bad over winter. Worked flywheel loose by hand until it turned over easily with the plugs out.

Electric starter had been getting progressively weaker over the past year and after the winter it wouldn't turn the flywheel around fast enough to get it to start, it barely spins it around until it gets to a high compression point in the rotation and gets stopped.

 

[jerryjerry05]

Re: Why is it so hard to overcome compression on pull start?

Lube the cylinders(WD-40) and rebuild the starter.The battery any good? Load test it.J

 

[Frank Acampora]

Re: Why is it so hard to overcome compression on pull start?

Because the flywheel rope pulley is there only as an emergency starter. You have almost no mechanical advantage with it like you do with the reduction gear pull starter.

Let's run some numbers: Stroke is 2.8, lets round to 3 inches. Flywheel diameter is about 9 inches so the rope is about 4 inches from center of crank. That's only a 1.25 reduction ratio. 145 PSI on a 2 square inch piston (guesstimate) is about 290 pounds. SO with a 1.25 reduction, you need to exert over 200 pounds pull to crank the engine. Yeah, if you are outside the boat and it is small, you will move it and the trailer around. These numbers are not by any means accurate but they serve to show why it takes so much effort to rope start the engine.

You need to have a sturdy rope with a handle. You need both hands on the handle and brace a foot on the splashwell. Then, give a good yank, making sure no one is behind you to be whipped by the loose rope.

Like jerry said, rebuild the starter or at least disassemble it and clean the brushes and commutator. Check the cable connections and check the battery.

 

[SwampThing]

Re: Why is it so hard to overcome compression on pull start?

Also it seems to make a big difference if you rotate the flywheel to #1 TDC before attempting to pull start it.

 

[erikpn]

Re: Why is it so hard to overcome compression on pull start?

just thought anybody might want an update.

1. I sprayed some lube inside and let it sit, then just continued to jerk on it with a rope until it became easier to pull start.

2. tried starting again, didn't start with my current electric starter.

3. Thought current battery might be worn down, tried jump starting with a known good battery. still no go.

4. then bought a used electric starter off of ebay, which didn't work. opened it up and saw corrosion on the brushes, replaced it with brushes from my old starter. Still didn't start. Then tried jumped off the car battery again. It then started up just now.

so overall...
-cylinders were still a bit stiff,
-2 bad starters,
-and a drained battery from sitting all winter

4 problems which were each individually able to keeping it from starting. Old motors often have multiple problems that contribute to the overall performance issues.

 

[foodfisher]

Re: Why is it so hard to overcome compression on pull start?

Cool, a learning that can be shared if we make it past 6!@#$%^&*()_+ Done deal.