3.7 Mercruiser, Crank no start

[wrenchfanatic]

Hey guys, I'm new to these forums, but I'm at my whits end to know what to do. I bought a 21' Thompson with a 3.7 in it. The engine was locked up when I got it, but I was able to bring it back. I had the piston walls cleaned up, the head pressure checked and the valves replaced. I have had the boat on the water several times and she runs like a charm. Now the issue I'm facing is suddenly she will only crank and crank and not even try to fire. Even with starter fluid. I had her to a marine tech and spent over $700 to get the boat back and have the same issue. Compression has been tested and is great, plugs are new, I have spark and I have fuel. Any ideas whatsoever? Any help would be appreciated!!

 

[Scott Danforth]

what is the compression?
did you file and re-gap the points and set dwell?

 

[wrenchfanatic]

what is the compression?
did you file and re-gap the points and set dwell?

Compression on all cylinders is around 150. Points are gapped correctly. I'm not familiar with dwell?

 

[tank1949]

What year is motor/boat/OD model? There is a tool you can buy and use but I do it the old fashion way. Pull plug. Attach plug wire to it and ground plug to block. Fire up motor see if you get spark. Of course have muffs on and a fire extinguisher near you.

 

[Scott Danforth]

if you have compression, and you have proper spark, at the proper time. all you need is fuel

 

[nola mike]

Compression on all cylinders is around 150. Points are gapped correctly. I'm not familiar with dwell?

Checking the dwell angle

When fitting new contact-breaker points in the distributor , the normal way of checking the gap between them is with a feeler gauge .

www.howacarworks.com

Tried to point to the adults only section, @achris , links #19 and 20 don't work.

 

[wrenchfanatic]

What year is motor/boat/OD model? There is a tool you can buy and use but I do it the old fashion way. Pull plug. Attach plug wire to it and ground plug to block. Fire up motor see if you get spark. Of course have muffs on and a fire extinguisher near you.

The boat is a 1987. I did verify that I have spark

 

[wrenchfanatic]

if you have compression, and you have proper spark, at the proper time. all you need is fuel

That's exactly what I thought. That is why I'm so stumped. It won't even fire with starter fluid. Only cranks. I bypassed the neutral safety switch in case that was not working, but no change...

 

[nola mike]

Spark, fuel, compression, only thing left is timing. Double check your timing and plug order, and make sure your #1 is firing on the compression stroke.

 

[wrenchfanatic]

Spark, fuel, compression, only thing left is timing. Double check your timing and plug order, and make sure your #1 is firing on the compression stroke.

It had been right and running, but I will verify that just to be sure

 

[achris]

Tried to point to the adults only section, @achris , links #19 and 20 don't work.

I'll sort it out tomorrow...

 

[tank1949]

Spark, fuel, compression, only thing left is timing. Double check your timing and plug order, and make sure your #1 is firing on the compression stroke.

and pray that timing chain has not jumped.

 

[Eric 3.7LX Alpha 1]

There is a timing chain tensioner on these engines, I'm not sure how loose it would get if that piece were to break. You could put a timing light on #1 and see where the light flashes on the balancer mark even if the engine is not running. Factory spec calls for 4° BTDC. See the attached TSB

 

[wrenchfanatic]

Ok so finally had success last night and she's running like a charm! I checked and here I did not have spark. I didn't know exactly what to check after that, but after an evening of messing around, we checked the points gap, and it was to wide! We re-gapped the points and she immediately fired up and runs great! Takes nothing to get her fired up now! Thanks for all the answers and input!

 

[wrenchfanatic]

and fyi... Probably most of you already know this, but I had replaced the condenser earlier, and I found out you should always replace the points whenever you do the condenser. For anyone else out there that's new to this!

 

[Eric 3.7LX Alpha 1]

Now you have to check dwell and then set your timing. By moving your point gap, your timing has changed.

 

[achris]

and fyi... Probably most of you already know this, but I had replaced the condenser earlier, and I found out you should always replace the points whenever you do the condenser. For anyone else out there that's new to this!

An 'ignition tune up' should be spark plugs, points, capacitor (only 19th century people call them condensers now), rotor and distributor cap... Then you set the plug gaps and points gap. Once the engine is running, check the points dwell (adjust if necessary), then set the timing, idle speed and idle fuel mixture..

Anything less is a half-fast job.

Chris...

 

[nola mike]

An 'ignition tune up' should be spark plugs, points, capacitor (only 19th century people call them condensers now)

Chris...

We're going to install condensers over here, and we're going to measure our point gaps in inches!

 

[poconojoe]

We're going to install condensers over here, and we're going to measure our point gaps in inches!

Good one Mike!
That's my story and I'm sticking to it!

Oh, and I'm going to park in my driveway and drive on the parkway!

 

[tank1949]

We're going to install condensers over here, and we're going to measure our point gaps in inches!

I wonder how difficult it would be to snatch out that REAL old stuff and replace with TB5?