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MCM 170 Mercruiser 1986 backfires

T

talbert966

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"I have a 1986 MCM 170 and I j

"I have a 1986 MCM 170 and I just rebuilt the carb, put new points, condenser, plugs and wires. It is back firing and stalling. Once I get it going it runs good. It does this more when I shift it into gear than it does just using the throttle and not in gear. Any ideas would be great!"
 
I guess the problem was the po

I guess the problem was the points gap was off. I didn't tighten the screw enough and it slipped. I reset the gap to .018 and adjusted the timing to about 5 degrees BTDC. The backfiring stopped when hooked to the hose. I am taking it to the lake today to test it. Hopefully everything will work fine now.
The only problem I have now is the first the alternator isn't charging. I put a conversion kit on last summer but for some reason the alternator stopped charging. I called Breezeworks and they said I can send it back and they will send me a new one. I just want to wait until after the 4th to send it back. I keep 2 batteries fully charged so I am safe on the lake.
The second problem I have is blow by so I plan a full rebuild but for now I put a oil cap with a breather on it and a vent on the valve cover instead of the tube from the flame arrestor. Hopefully this will reduce the amount of pressure and not blow oil out to the flame arrestor like it was doing. This is just the allow us to enjoy the lake for the summer and I will rebuild in the fall.
 
The backfiring was only occurr

The backfiring was only occurring when accelerating from stop but once it would go then it was fine. I did rebuild the carb so I thought it was an adjustment there but it was the point gap. We will see today.
 
"[url=""]http://www.youtube.co

"http://www.youtube.com/watch?v=LQ8UndWhU3g
Here is a link to the video of the boat backfiring when you hit the accelerator. If you ease into it it goes fine but when you go straight to wide open throttle it bogs down and backfires or stalls. The points are set to .022. I am not sure what to do next, I guess I will try putting in new plugs, maybe one cracked or went bad? Any help would be appreciated"
 
"6 deg. BTDC timing

worn cr

"6 deg. BTDC timing

worn crossed plug wires or crack in the dist. cap., too lean accellerator pump setting."
 
"I put new plugs again but the

"I put new plugs again but the didn't Fix the problem. The plug wires are not crossed I double checked again. The only carb adjustment is the needle screw at the base. I have adjusted it in and out to multiple locations, it seems better out then in. I checked the distributor cap and it is good. Anyone have any other ideas?"
 
"It was doing the same thing b

"It was doing the same thing before I rebuilt the carb but if someone could look at the attached picture and tell me if I have the choke rod and the accelerator pump rod installed correctly.
287444.jpg
carb photo
 
I compared it to the schematic

I compared it to the schematic and the rods seem to be correct. Any other suggestions before I order the pertronix electronic ignition and flamethrower coil?
 
i think you solved your own pr

i think you solved your own problem when you said pertronics and flamethrower coil also like guy advised it wouldnt hurt to put a new acc pump in
 
"How do you adjust the acc

"How do you adjust the accel pump?"

It's hard for my eyes to see but it looks like there is one more hole in the accell. pump lever. Install the pump rod in it and see if it stops back firing. In the upper hole it will allow the accell. pump to draw about 1cc of fuel or 33% more each time the pump lever is actuated.
 
I put a new accelerator pump i

I put a new accelerator pump in when I rebuilt the carb last week.
Does everyone think the pertronix is the only thing left?
 
""The only carb adjustment

""The only carb adjustment is the needle screw at the base. I have adjusted it in and out to multiple locations, it seems better out then in."

That last part of the statement says the engine wants more fuel (richer) in the mixture. A thicker base gasket will help w/the mixture atomization and allow a finer mixture screw adjustment. I'll bet there is little engine stumble moving the screw in/out. If that's true; get the thicker gasket or laminate 3 together w/gasket glue."
 
The 125 psi comp. reading is s

The 125 psi comp. reading is suspect; it could be the root cause of the backfire from a slightly burned exhaust valve.
 
"When I rebuild, if the cylind

"When I rebuild, if the cylinder walls are good can I just hone the cylinders and use .010 oversize rings?"
 
You'll have to measure the

You'll have to measure the cylinders to be sure. There is a big difference between honing a cylinder and de-glazing a cylinder.
Bert
 
I installed the Pertronix II a

I installed the Pertronix II and the Flamethrower II everything seems fine running on the hose. The real test will be on the lake in the next day or so.

The next thing I will do is pull the head and have a valve job done. A local machine shop said it would be $60.00 plus any parts if anything is bad.

I hope to wait until winter to pull the engine and rebuild.
 
Does anyone know if I can buy

Does anyone know if I can buy just the needed gaskets for the head when pulled? Will gaskets from a Ford 460 fit?
 
I thought the head was a 428 f

I thought the head was a 428 ford head and not a 460???

Anyone know specifically? or are they the same?
 
"Not the same at all. The 460

"Not the same at all. The 460 is the modified block, from the 351-400-429-460 series of motors.
The 428 is from the 332-352-390-406-410-428 series of motors. The 351 modified was the truck version. In cars it was called the cleveland.
Ford have been famous for coming out with a motor and then stretching it out to maximum cu inch displacement."
 
"Does the accelerator pump del

"Does the accelerator pump deliver a healthy shot of fuel when the throttle is opened??? Even though the pump is new, there are some check valves inside the carburetor which can malfunction. With the engine off, open the throttle and check for proper operation of the pump assembly."
 
I put a new accelerator pump i

I put a new accelerator pump in when I rebuilt the carb. When I turn the pump lever it does squirt gas into the carb. I replaced everything in the carb when I rebuilt it.
 
"Number one: Don't use a c

"Number one: Don't use a car head gasket on a marine motor, it won't last long. From everything I've read and heard the head is a 460 Ford. Never heard the 428 before.

Two: Check your distributor advance weights. They may not be stuck but they may be sticky and slow to advance the timing. You may be able to see it if you put a timing light on it and blip the throttle if the advance has been slowed enough. I think this is one thing that gets over looked a lot since they are out of sight-out of mind. And if you run in salt water, corrosion gets every where.

Running on the hose and running under load are two enitely different things. Report back when you can do a sea trial."
 
OK I did some research and YES

OK I did some research and YES the Merc 4 cyclinder head is a 460 Ford head

OMC uses the same head gasket as they used Ford blocks for many years........
 
We took the boat out yesterday

We took the boat out yesterday tubing and it ran much much better after the pertronix install. It isn't 100% of what I would like but much better. I think if I tweak the carb adjustment a little more it will be the best I can do until I rebuild this winter.
 
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