Force 120, backfiring on start

[chris lell]

Hey fellow boaters, I have a problem that seems odd. When I try to start my 120 force it spins then backfires(see video).

I have checked the compression(good),the spark(good(new plugs)),replaced flywheel key(new),brand new starter and battery with 850cca, reset the timing to 28 BTDS,and the carb needles at set to one turn from light seat. I have looked all over the forums and cant find anything similar so maby you guys can help me. She ran but a few times since i had her but she was sluggy and come to find out it was the key shifted. so once i replaced that and then re-timed it. Any help you can offer is much appreciated.

Here is a video of me trying to start it. The kinda air puff is it backfiring so loud that my camera cant pick it up. It was loud enough for the neighbors to ask if we were shooting guns .

force 120 wont start - YouTube

 

[procraftguy]

Re: Force 120, backfiring on start

Check and make sure plugs are in right order i had that happen thats what it does

 

[chris lell]

Re: Force 120, backfiring on start

How would i go about checking it? I did compare it to its diagram and everything is how its suppose to be.

 

[woody66912]

Re: Force 120, backfiring on start

Pull the flywheel and try to turn the center magnet with your hand, force had a big problem with them letting lose. If it is spun is probable that you need a used or new one, sometimes you can see some type of glue line to line it back up and re-glue. You can use J-B weld.

 

[jerryjerry05]

Re: Force 120, backfiring on start

("Pull the flywheel and try to turn the center magnet with your hand, force had a big problem with them letting lose")
What lets loose?The timing magnet in the flywheel? If it came loose then all kinds of damage would happen?
Old flywheel.If the magnet was loose it wouldn't start at all.It would just stick on the flywheel?
Newer style flywheel/ It has 6 magnets.
Take the wheel off again and see if the key has slipped?
The plug wiring is a hard thing to wire wrong.They run in a line from top to bottom.
The OLD distributor style, it was possible to cross leads but not the newer ones.
You have spark?
I know that you did a comp test what were the readings?
Backfiring could be a bad fuel pump diaphragm, bad reeds, timing off.
Second post is a link to a few procedures to do while looking.

 

[pnwboat]

Re: Force 120, backfiring on start

One question, when you set the timing, was the throttle set all the way open to maximum forward speed?

 

[chris lell]

Re: Force 120, backfiring on start

Thanks for the help guys. Been working day shift so hard to test and check stuff.

woody66912: The flywheel magnet looks to be in place.

pnwboat: Yea it was set at WOT on plug 1.

jerryjerry05: I do have spark on all cylinders and it looks stong(and feels stong(and painful lol)).
The flywheel has the one single magnet and looks to be in place.
I cant remember the exact numbers but it was all between 143 and 145.
On my next day off im going to try and pull all the fuel lines and clean the carbs. Replace all the lines and
clean the fuel pump.

 

[foodfisher]

Re: Force 120, backfiring on start

Shearing keys, are you using enough ft/lbs on flywheel reinstallation? One backfire can damage/tear the fuel pump diaphram.

 

[Jiggz]

Re: Force 120, backfiring on start

There's a good possibility you have damaged reeds already if it backfires to the carbs. Do an inspection when you decided to work on the carbs. And then re-verify your timing by first setting #1 TDC and verify marking on flywheel. From there you can just follow the sticky post video on how to do it properly. Goodluck.

 

[chris lell]

Re: Force 120, backfiring on start

Sorry haven't got back to you guys the last day or so but i been on the lake : D.

Well I took the fuel pump apart and found what seemed like a candy wrapper stuck in one of the diaphragm valves not a big piece but probably enough to hinder fuel flow. Then I pulled the header and cleaner all the cylinders with gas and checked for any scaring and it looked brand new so i rubbed a little 2 cycle Pennzoil on the walls and sealed it back up. Then I got to the flywheel, I no longer got the nut off and seen there was a problem. The key steel had broke AGAIN and turned in to thousands of fragments and was turned at least a quarter from where it was suppose to be. Pulled the flywheel and replaced the steel, vacuumed the little bits out and re installed. Got out the timing light and it was at like 40 deg BTDS at WOT so I set it to 28. Primed the fuel bulb and decided to try it. TA DAH start right up the revved up and died lol. So I worked with the carbs and got it to idle smooth thanks to the sticky you guys suggested. Loaded up and took her down to the lake.

Oh a little bit off topic, what should i be running in a 88 bass tracker 1600tf with a 12 gallon fuel tank? My speedo isn't working so i estimated 35-40, but when I had a merc 70 on the back it would do 33-35. And what is the best rpm range? at WOT and 35-40 she was at 4900-5100 rpm. I didn't keep it there long due to i didn't know what redline on it was.

Again thanks for all the help and If it wasn't for your suggestions i may have been still working on it.

 

[jerryjerry05]

Re: Force 120, backfiring on start

The wot is about 5200-5500
Your speed should be a lot more than a 70hp.
If it's setup right and the right prop you should be close to 45-50
At 5100 your not revving all the way.
The prop? Say it's a 17p then you need a 15p to bring up the rpm's.
You put too much motor on that boat?What's it rated for?
If it had a 70? I'd guess it's not rated for much more than that?
Some cops will give you a bunch of crap for over powering?
Plus it's dangerous.

 

[chris lell]

Re: Force 120, backfiring on start

Went and started the boat this morning and she fired right up on the muffs. So I went to advance auto and picked up a can of sea foam. Mixed it with a few gallons of gas + oil and let that run thru. It seemed to make it idle a little more smooth.

Thanks for the reply Jerry and for the concern for safety, I know some people would say throw the biggest motor you can find and see what it will do. I still need to fine tune the carbs cause I think its a little lean, but after that I might be able to get a better rpm and speed measurement. I think the prop it 19p due to its a quicksilver laser 2 and that was the smallest it came in. Is there a way to tell, I looked it over for markings and seen nothing. Anyway the transom said its rated for a 115 and the 70 that came off of it was 265lb so I looked up the weight of the force 120 and its # is 275-300 and I know guys running the old merc 115 tower of power on the 1600tf without issue and it weights in at 300-350 and there hitting 50+. not to mention the fact that they came with a 17 gallon tank from the factory and mine only has a 12. So I looked around for the weight of the 1600tf and found its at 830lb + 300 for the motor + 75 for a full tank and 100 for battery + the few odd and ends, and of course my fatness at 275. That brings us to a total of 1580lb. And as for the cops, were i live (WV) cops dont care what you do. as long as your license is current and your not drunk.

 

[foodfisher]

Re: Force 120, backfiring on start

Safety! It's not so much what you do to yourself, (yes it is ). It's what you do to those around you.