Astern Limiter on Mercruiser

[Stormin Norm]

I posted the following question on the Marine Power website and received the attached reply, while the reply explains the idea it does not explain to me why I can only go astern at tick over and if there is anything I can do to remedy the cutting out.

Perhaps a users can give me a laymans answer rather than a technical one ?

Many Thanks

Norman


Question:

Having recently purchased my first boat (second-hand) It is fitted with a Mercruiser 3.0 Petrol with an Alpha One stern drive. The engine runs very well but on shifting the gear lever Astern it has a tendency to cut out. I have overcome this by increasing the tick over speed a bit but it now seems to tick over a bit fast. I have noticed a cam type switch on top of the engine which limits the power available astern, so it will obviously only go astern at tick over and no faster. I would be grateful if someone could tell me the reason for this and any other information that might help.

Answer:

On this type of installation (Alpha drive) there is a shift switch which make easier the shifting.In fact when the shift witch is activated,it reduces the timing of the engine which facilitates the shifting.Regards

 

[Guest]

Part guesswork, but whatever... On some cars there is a switch that cuts the ignition instantaneously to prevent the engine racing as you shift between gears. Sounds like this is related, except that you wouldn't be crashing from forward to reverse, you'd always be in neutral first. Hmmm. There is a cam thing on the throttle cable that limits you to about 1/3 throttle astern (not tickover only), so it sounds like you may need to adjust the cable mechanism.

 

[Chris_d]

If you go too fast astern, the back pressure on the exhaust will be to great and water may travel back up the exhaust and enter one of the cylinders, this is despite any none return flaps fitted.
This should be adjusted to more than tickover though, so check this. If the Alpha drive has a switch to retard the timing this may be sticking on for too long and stalling the engine, also check the timing is correct to start with, might be ok for most of the time except when you shift to reverse and the retard switch retards too far.

 

[Freebee]

Having had a few problems with my mercruisers I have become an armchair expert. The cam type device you have spotted is a microswitch. The purpose of the switch is to momentarily cut the ignition when you pop the outdrives into gear. The purpose is to allow the simple dog cluth to engage without excesive wear. You should be able to go full astern but In your case the switch is causing the engine to cut. The switch is doing its job but the engine cuts. speeding up the tickover is what I did and caused the engine to back fire and fill with water. so my advice is:
1.slow tickover down immediatly.
2.with the engine off work the shift a few times and see if it is just stiffness apparently this can make the problem go away.
3.Alteranaively you have to adjcut the gear shift cable and switch position. You can obtain details of how to do this visiting www.boatered.com engines forum.

Best of luck

 

[David2452]

Contrary to popular belief the shift interupt switch is not designed to help the shift into gear, but rather out of gear when going from forward to astern or vice versa.
It will either work or not, and will certainly not limit revs other than momentarily, exept that it may cause them to reduce to zero if it sticks open.

There is no "designed in" rev limiter when going astern, there is clearly a fault in the setup somewhere.

 

[Stormin Norm]

Thank you all for the helpful replies to my query. I have lots to be getting on with when I get a chance.

Cheers

Norman