1976 Chrysler 120 hp backfiring

bassboy416

Petty Officer 3rd Class
Joined
Jul 9, 2013
Messages
79
Heres the most brief recap I can give:
Bought a 1976 chysler 120hp outboard (2 stroke)
Previous owner said engine always ran great until the last time he took it out but that he replaced spark plugs and they were old and fixed problem.
I took boat out and it didn't like to shift into gear without dying unless I gave it some gas when I shifted. It ran great though other than that. Had plenty of power, got on plane no problem, towed me on a wakeboard, generally kicked ***.
Got home. Read plugs. Top two plugs were white, bottom two were brown, slightly black. Got worried that it was going to burn piston because plugs were electrode plugs and manual suggested surface gap. Replaced plugs with SG and ran in bucket at home. Engine sneezed like crazy with backfires all the time. Put the old plugs back in. Engine started and idled fine for up to 5 min but then backfired REALLY HARD out the exhaust and died. Did a compression test with engine cold and no starter. Used a makeshift pull cord and got 140-150psi out the top two and 125-130 out the bottom. keep in mind. Engine was not warmed up and starter was not used at all. Top two were the pistons I was worried about from running lean. Whats the next step?????? Carb Rebuild?
 

jerryjerry05

Supreme Mariner
Joined
May 7, 2008
Messages
17,923
Re: 1976 Chrysler 120 hp backfiring

Remove the plugs all 4
Then using a jumper at the solenoid do another comp test.
Post the results.
The carbs, check the air screw on the carbs.
Should be top front. It needs to be set at 1 turn out for inital setting.
The comp readings you gave.There's 25# difference between the top and bottom.
It should only be 5 maybe 10# at the max.
10# would make me crazy?
Pull the fuel pump apart and check the diaphram.
Could be ripped and causing some of the problems?
 

bassboy416

Petty Officer 3rd Class
Joined
Jul 9, 2013
Messages
79
Re: 1976 Chrysler 120 hp backfiring

Results of compression test 2:
cylinder 1= 140+ psi (spark plug looked like it was burning hot)
Cylinder 2= 130 psi (spark plug same as 1st cylinder)
cylinder 3 = 120 psi (spark plug perfect)
cylinder 4 = 120 psi (spark plug perfect)

Took apart carbs to clean. Checked fuel filter. Diaphragm was great but fuel line from pump bubble to engine was cracked on inside and will be replaced.
Engine ran for 1 trip out after buying boat but died when shifting into gear unless punching it and then backfired after idling perfectly when started after 1st trip.
 

jerryjerry05

Supreme Mariner
Joined
May 7, 2008
Messages
17,923
Re: 1976 Chrysler 120 hp backfiring

Set the carbs at 1 turn out.
Does it run after the backfire?
The comp is a bit low.
Try a decarb. Use Mercury's Powertune or OMC's Engine Tuner.
Seafoam isin't as good.
The floats set right?
Check the reeds behind the carbs?
 

bassboy416

Petty Officer 3rd Class
Joined
Jul 9, 2013
Messages
79
Re: 1976 Chrysler 120 hp backfiring

Thank you for your help.
I took the carbs off yesterday to give them a chem dip cleaning and new gaskets all the way around. Noticed the lower one had a little varnish coloring in the bowl. Upper carb looked fine. Haven't dug all the way into them yet. Looking for a carb kit. Hoping I can get this from franz marine. Noticed a few strange things. Line going from the primer bulb to the fuel filter was held onto filter port with a zip tie and inside of hose (outside seemed fine) was badly cracked without breaking the surface. Also, the ignition area had what I think was called a CDI unit. Its a small box with two wires going to the timing area. Its on the left side of the engine mounted near the top. What struck me as odd is that it has stickers on it with years like 99 and 00 01. It also has a bracket designed to be mounted with two bolts; however, on my engine it is only mounted in one bolt as it does not reach any other bolts to mount into. It also has marks on the one bracket with no bolt in it as if it was mounted at one point on another boat and might be some kind of used piece that was added later on. I'll try and post a picture of it hopefully you'll know what I'm talking about. The boat did run very well the last time I took it out. The only problems I noticed on the water were that it died when put into gear unless you punched it a bit or it was very warm and it always seemed to like to be choked after sitting a while. It had plenty of power and a nice smoothe power band. I'm going to keep on going until I get this thing fixed. It was way too much fun to just let it die. 0811131811a.jpg
 

bassboy416

Petty Officer 3rd Class
Joined
Jul 9, 2013
Messages
79
Re: 1976 Chrysler 120 hp backfiring

UPDATE:
Boat ran for the first time in about a month yesterday without any backfiring and with a smoothe idle. I bought an aftermarket starter for $80 from DB Electrical. COMPLETE WASTE OF MONEY. If you are going to buy aftermarket parts, be cautious because I bought several parts and double checked that they fit my model 1209hd. NONE of them fit as specified and this starter was a complete POS. Worked once until the engine caught then siezed up and wont rise on the shaft anymore to contact the flywheel. Also, the carb kits I ordered were junk and I made better gaskets myself. Started boat with a pull cord and it fired up after one compression stroke. I mean fired right up. I noticed that in nuetral it only revs about half throttle and I was concerned, but it seems normal as all it does in nuetral when you rev it is advance timing. When I pulled the throttle on the carbs it revved just fine with absolutely killer response. After turning the motor off, I checked by putting boat in gear and the throttle did engage all the way. My idle is a bit low, and I have tried adjusting the screw by the spot where the timing clips hook on, but I haven't gotten it right quite yet. It idles great at about 800 rpm, its just at about 500-600 when in dead center on the control. So I need a new starter, and a little adjusting on the carbs (my fuel bowl is leaking from a funky gasket). But the point of the matter is that it is idling great, I ran it in water for about 15 minutes and rpms at about 800 and it was nice and even with not one backfire. I started it a bunch of times with a pull cord and it fired up like a dream. If you're reading this, don't lose hope. I was about ready to burn it and move on, and it is amazing how its running now. Any advice on adjusting idle and ordering gaskets or making gasket for bowl is much appreciated! Thanks to all who helped me through this process. WIll be going out next weekend for a lake test.
 

Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
Re: 1976 Chrysler 120 hp backfiring

A couple more photos showing both sides of the engine block would be appreciated. With 4 coils mounted on the head, I believe that is a Magna-Power II ignition system. The component mounted on the exhaust cover MIGHT be an aftermarket rectifier/ voltage regulator. If all other components are mounted under the flywheel, and there is no distributor (not with the 4 coils mounted on the head) then for sure it is Mag II ignition.
 

bassboy416

Petty Officer 3rd Class
Joined
Jul 9, 2013
Messages
79
Re: 1976 Chrysler 120 hp backfiring

So, I took the boat out twice last weekend. Its running better than ever. Went wakeboarding behind it and everything. Great idle, throttle response, and power. Wow, what an awesome feeling! Dont give up hope.
 

Nordin

Commander
Joined
Jun 12, 2010
Messages
2,424
Re: 1976 Chrysler 120 hp backfiring

That?s for sure MAGII ignition.
 

bassboy416

Petty Officer 3rd Class
Joined
Jul 9, 2013
Messages
79
Just looked and yes its a MAGII ignition. Bought a new boat with merc 1150 and the chrysler 120 I was posting about here is a far superior engine
It is more reliable, more responsive, and less complicated to work on. Even with mismatched compression readings, when tuned properly, this motor did everything I asked of it and did it with reliability and consistency. Very surprised. Very impressed. Great Engine! Been sitting now for a year and change and it still fires up easily. Any help with prop shaft seal and lower unit seal replacement is appreciated. That's my only issue this year
 

MickLovin

Master Chief Petty Officer
Joined
Feb 18, 2013
Messages
822
No Title

That is a rectifier at the back of your motor, the LU seal below the water pump is wuite easy to change out, the prop seal on the other hand isn't so easy. I had a 100hp with Mag1 ignition, so I am a bit surprised yours has mag2 with individual coils are you sure it is a 76 model, my 100hp was a 79 model. Is your LU a one piece or two piece?
 

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