HELP! Merc 170hp/470 Backfires and quits on plane.

I am a fairly new member here, joined about a month ago to research problems with the boat we just bought. It is a Four Winns Horizon 190 with the 170hp motor. Initially it bogged on acceleration and pinged on cruise plus the shift cables were all out of adjustment, those problems were addressed and fixed (thanks to this forum).
My son borrowed the boat to take the girls tubing and it worked great for over an hour then developed the latest problem. It would run great at idle and cruise off plane but when you got up on plane and went above 22mph it would pop and want to stall if you caught it and brought it back below 22 it would continue to run.
I did the research and replaced the coil, points and condensor, cleaned the carb again, Changed the water seperator (no water present) and two full tanks run through (Last one non ethonal), added marine stabil.
We went on a vacation and just ran it at or below 22mph as the problem wasn't fixed. The problem has progressed 2 days later into not being able to get off plane for more than 3 or 400yds before it pops (backfires) and will only run at idle and up to off plane speeds.
Did the research and replaced the Carburator, added the Pertronix ignition with the coil. Plugs look like new.

I'm at my wits end. Somewhere I read about the tach being in the loop and the tach does need replaced. There was something about a jumper wire from the interupter switch to the coil so yesterday brought along a jumper and ran it from the interupter to the neg coil wire. Didn't help but I also can't find the thread I read that on.

Hope someone knows what it might be. Checked all connections on the ignition switch also and fuse connections.


URRGGG!!

Re: HELP! Merc 140hp/470 Backfires and quits on plane.

Re: HELP! Merc 140hp/470 Backfires and quits on plane.

I need help here.....is it a 140 or a 470?....they are not the same engine....no year or other details about the motor here....does it have a carburetor and conventional mechanical fuel pump? How is the fuel pressure? How is the compression? Many here will be able to help but more detail is necessary.....also research is good but testing and making a diagnosis from the results of the test is much less expensive....just a thought.....

Re: HELP! Merc 140hp/470 Backfires and quits on plane.

Re: HELP! Merc 140hp/470 Backfires and quits on plane.

I believe it is a 470 block and 140hp the serial number on the valve cover is B436251 and the carb is Merc 1389-8489 on tag.
Thanks

Re: HELP! Merc 140hp/470 Backfires and quits on plane.

Re: HELP! Merc 140hp/470 Backfires and quits on plane.

There are 2 kinds of backfires: Lean Backfire and Rich Backfire. Does the backfire sound like a gunshot? If so, then it is a lean backfire and it happens when the engine gets too much air or not enough fuel: Vacuum leaks, starving for fuel, or an accelerator pump failure.

Double check the fuel system from end to end, start in the tank and remove the pickup tube and make sure it is not plugged.

Check the anti siphon valve.

Check your accelerator pump to make sure it is pumping a very healthy stream of fuel into the carb. With the engine off, make sure the choke is all the way open and then look into the carb while moving the throttle to wide open and watch for the fuel to squirt into the carb. If it dribbles or drips there could be a problem with the accelerator pump. You said you cleaned the carb again. Did you put a carb kit in?

Where does the backfire happen in relation to movement of the throttle? Does it back fire if you gradually run the throttle up and then jump on it to get on plane? Does it just backfire and stumble after it reaches Wide Open?

Re: HELP! Merc 170hp/470 Backfires and quits on plane.

I didn't mention that I checked the fuel supply from the tank on and no problems. The pickup is an aluminum tube with a v in the end and no filterand it was wide open when I looked through it. Where is the anti siphon valve? I replaced the carb filter and added a hi-flo inline filter also between the tank and carb. As stated in the initial post it runs great and comes up on plane then 3 or 400yds into the run I get a backfire and it comes off plane. It will stall if I don't yank back the throttle. The back fire seems gentle. I rebuilt the original carb and replaced original Merc accel pump. Seems a steady stream. Also, ordered a reman flow tested carb doubting my work and same exact result.
Thanks again for the help.

Re: HELP! Merc 170hp/470 Backfires and quits on plane.

Check the fuel tank vent, it may be plugged, which would not allow fuel to flow from the tank. The anti syphon valve is at the tank, the fuel line slides over the ASV barb in most cases. Looks like this: http://www.iboats.com/Moeller-Alumi...1297666--session_id.722634483--view_id.392106

Re: HELP! Merc 170hp/470 Backfires and quits on plane.

Next time it backfires and starts to poop out, shut it down and pull a spark plug or 2 and look at the condition. see if they are soaked with gas or if they are bone dry. A spark plug check should reveal if the engine is starving for fuel, or perhaps floods itself and then backfires.

Re: HELP! Merc 170hp/470 Backfires and quits on plane.

The anti syphon looked perfect, took it out and had it in hand worked normally. I did check the vent line and it is clear. I should've thought to check the plugs after it pops, good idea. Also, it is a mechanical fuel pump. They either work or don't correct?

Re: HELP! Merc 170hp/470 Backfires and quits on plane.

Did you check the timing, both initial and advance?

Re: HELP! Merc 170hp/470 Backfires and quits on plane.

The mechanical pump could have a leaky diaphragm. Check for fuel in the clear tube. How is the condition of your engine oil?

Re: HELP! Merc 170hp/470 Backfires and quits on plane.

I did check the timing and advance and all is correct. Engine oil changed month ago and is still like new. I didnt see a clear line on the fuel pump.

Re: HELP! Merc 170hp/470 Backfires and quits on plane.

there should be a clear line that runs from the pump up to the back of the carb.

Re: HELP! Merc 170hp/470 Backfires and quits on plane.

Pull the cap and make sure the pertronix unit is installed correctly and everything is tight and cannot move around.

Re: HELP! Merc 170hp/470 Backfires and quits on plane.

1977 SILVERLINE said:

Pull the cap and make sure the pertronix unit is installed correctly and everything is tight and cannot move around.

Click to expand...

Did it all according to instructions and besides the problem was there before pertronix.

Re: HELP! Merc 170hp/470 Backfires and quits on plane.

verbious said:

there should be a clear line that runs from the pump up to the back of the carb.

Click to expand...

Mine is black, but will check asap, 12hr day today and now it is dark. Thanks again.

Re: HELP! Merc 170hp/470 Backfires and quits on plane.

Just did a 350 with same problem, the rubber gasket on the back side of the intake blew out in one small spot, use brake clean and sprayed around until it reved up, took off intake replaced gaskets, good thing, it was time!! Used permatex on the intake ends, fired it up, and no probs

Re: HELP! Merc 170hp/470 Backfires and quits on plane.

slyoell said:

Just did a 350 with same problem, the rubber gasket on the back side of the intake blew out in one small spot, use brake clean and sprayed around until it reved up, took off intake replaced gaskets, good thing, it was time!! Used permatex on the intake ends, fired it up, and no probs

Click to expand...

so the intake was sucking air? was the brake clean used as an imitation for fuel ?

Re: HELP! Merc 170hp/470 Backfires and quits on plane.

Blacksting said:

so the intake was sucking air? was the brake clean used as an imitation for fuel ?

Click to expand...

it replaces the air and will speed up the motor, verifying the leak, i do this on all motors that i test, dirt bikes snowmobiles ect...