720 running on one cylinder

[miniclubman]

Hello,
Took my boat out today and ran fine for ten mins. Then LH engine died and would not start.
Checked fuel, spark and compression (all be it without a comp tester).
Pulled the aft plug out and cranked the engine and it fired.
Swapped cylinders and tried again with nil result. After that the engine would start, but only running on the fwd cylinder.
Got home and pulled the engine apart. Ring gap is within tolerance. No obvious signs of a problem with pistons or crank.

I am going to buy a compression gauge tomorrow and test the compression.

I have been reading the forum and am thinking perhaps the rotor valve might be playing up. Can they operate fine and then suddenly fail or is it a gradual decline?
Any suggestion would be greatly appreciated.

Thanks

 

[Dr Honda]

You lost a jug.

There is no way to really check the compression without a gauge. If you lost one ring, and that jug is down to 60 psi... it would still force you finger off the spark plug hole. The only time the "finger Gauge" works is if you have a full meltdown.


BUT... you could have had a carb get totally blocked too. I had that happen last year, were a screw vibrated out, and held the diaphragm closed.

Good luck, and let us know what you find.

FYI... I have a 720 for sale in the classifieds.

 

[Minnetonka4me]

Did you clip the plug wires back? You need a compression gauge. It could have saved you from pulling that motor apart for no reason.

 

[seadooya]

Did you clip the plug wires back? You need a compression gauge. It could have saved you from pulling that motor apart for no reason.

Yeah really, did you try a new sparkplug. Need to start with the obviuos before ripping things apart, bud.

 

[Minnetonka4me]

Yep pulling apart the motor is the absolute last thing on the list.

 

[miniclubman]

Thanks for your advice thus far.

I swapped the plugs from the other engine and they ran fine in the working engine but not in the dud one. Access to new plugs was a little difficult as I live in rural Australia. I will order some today and see how I go.

I have checked the compression and found that all cylinders are producing about 125psi on good and dud.

When swapped leads and tried that also with no change to the working cylinder.

I will pull apart the carb and see if there is any crud in the jets etc and also trim back the plug leads.

Cheers!

 

[RFoster130]

aYeah i agree, dont tear it apart yet. That and Always keep at least 2 spare plugs. Fixes a lot of issues.

 

[miniclubman]

So I've replaced the plugs and pulled apart the carb with only a little bit of muck inside.
I've reinstalled and the engine starts but only with partial choke.
Once started it only runs for a few seconds and then stops.

 

[seadooya]

you got the fuel selector to "res" or "on"?

With removing the carbs, you dumped all the fuel from in the line/s. It'll take some finessing to "purge" the air out, along with, whats in the fuel strainer. Also make sure your battery is good and full.

 

[miniclubman]

I pulled the flame arrestor off and cranked the engine and noticed fuel and vapour coming back out the carb.

On closer inspection I saw that it was only coming out of one side of the manifold.
So I removed the manifold and plate for the rotory valve to find that as I was cranking the engine over the rotory valve was not turning.
The shaft is locked in a fixed position but has a small amount of movement of about 20 degrees.

Now to figure out weather to fix the engine or replace motor???

Suggestions? Anyone Bueller? Bueller? Anyone?

 

[seadooya]

remove the rv assy, and install new one. The brass gear on the assy is stripped. You do not need to split the case to remove the assy.

 

[miniclubman]

I am so glad I joined this forum.

Thanks for your help.

I pulled out the Rotary Valve and as you predicted the brass gear was stripped.

I've ordered a new one and a seal and should be back in business by the weekend.

 

[Dr Honda]

Glad Seadooya got it figured out for you. :cheers:

 

[seadooya]

Miniclub, dont go the route of installing "just" the brass gear. It doesnt take much, to the tune of .003 outta whack, for the new brass gear, installed on the existing rv assy shaft, to be stripped again. For what it worth, pay the extra couple buks, and get the entire assy, bud...:cheers:

 

[miniclubman]

Ok! Will do.

Just as extra info. It appears that the RV has dug into the edge of the intake port and that is what has cause the sudden stop.

I cant quite figure out why it would have though.

The engine was starting really easy, so I tend to think that there was minimal movement in the Valve..

Before reinstall I will measure up me thinks!

 

[seadooya]

Alright bud, check in the rv/intake manifold, and doublecheck to see if theres BOTH brass fittings. If so, do you remember removing the 4-allen bolts, securing the flame arrestor to top of carb/s? These 2-things can contribute to the damage you got on the rv and its surface.

The surface of the rv area, can be scored and not cause harm. Was does effect its performance/start-up, is if theres a groove going from port to port holes....:cheers:

 

[phough5757]

Been there!

I lost an RV brass gear. I pulled the Motor, removed the RV cover and rotor.
I made a puller to get the shaft out, heres the hard part, flushed the chamber
out, and with a dental pick, scratched all the brass off the crank shaft gear.
I failed to do this before and wiped out another brass gear. Take Your time
and get it all out.
Good Luck and Cheers.