Help! Mercruiser 228HP - Not Charging & Backfire at hi speed

[phiml]

Hi all, I have a 1979 228HP mercruiser, chevy v8 305 CDI s/n 5064363 with Quad barrel rochester caburator. Recently, it keep draining my batteries down only at idle or slow trolling speed 600-800 rpm. I have replaced the alternator with new one and now it doesn't charging at any speed at all. Second issues is the boat will die out after about two hours of slow trolling. I restart the boat and ran it to 2800 rpm and it start to backfire. Check the Gas and it is clean (no water). thanks for all your input.

 

[Bondo]

Re: Help! Mercruiser 228HP - Not Charging & Backfire at hi speed

Ayuh,.....

I'd say you either got a Bad Alt. or wired it Wrong,.....
And,....
Your other Issue sounds like a Fuel Delivery problem.....

 

[Maclin]

Re: Help! Mercruiser 228HP - Not Charging & Backfire at hi speed

If you are running the batteries down then I think the second problem could be due to the low battery situation as well. Ignitions need full voltage to be able to maintain the current draw from the coil as rpm demands increase. I mean, could be what Bond-o wrote also, or a combination.

 

[phiml]

Re: Help! Mercruiser 228HP - Not Charging & Backfire at hi speed

thank you Bondo and Maclin for your replies. Here is what I have measured
The Battery measure 12.7V. I check the purple wire (excitation wire from the alternator to coil + terminal) voltage is @ 4.0V with ignition key "ON". if remove the wire - (from the coil to the distributor) and remeasure the voltage at purple wire is 12.68V.

Check the Alternator output
Start the engine and jump +12V direct to the purple wire only. Output at the alternator terminal is +12.4V. It seem to be bad alternator. I will get another alternator and see if this will resolved the charging system issue.

I am using Hi-oct gas (91). The Back fire issue may due to running to lean - your though on this.

 

[Maclin]

Re: Help! Mercruiser 228HP - Not Charging & Backfire at hi speed

The next few comments assume you have a typical points ignition setup with ballast resistance in the primary coil circuit and all is wired correctly.

Voltage at the + terminal on coil should be around 9 volts with ignition in ON position. It should read a full 12 volts while the ignition is in START or cranking position. 4 volts sounds too low to me.

The jump in voltage at the positive terminal when the wire from the negative terminal is removed is confusing, but could indicate a low battery charge condition as well. The wire from the negative terminal to the distributor opens and closes the coil primary circuit depending on if the points are closed or open. When the points are closed the current draw is higher and this could be drawing the voltage down, then when the wire is removed so is the load.

The 12 volts reading when the negative coil terminal wire is removed is too high though, makes me wonder how you are checking the voltage. The negative lead on the meter should be attached to a good ground. It will not need to change where it is connected when checking voltage at any engine connection. The positive lead when touched to the + terminal should read 9 volts or so with key in on positon and 12 volts while cranking.