1977 35HP johnson backfire's then wont start

[agentjss]

Howdee all, Im new here and my motor wont start. where do i begin?
firstly the motor is an 1977 35HP johnson with electric start.
The motor cranks over ok with plenty of spark but when i crank it for about 10/15 seconds or so it backfires through the prop and wont start. things ive tried so far.
0: purchased clymer shop manual
1: replaced spark plugs with correct gap.
2: cleaned/rebuilt the carby 3 times checking the float level is correct and cleaning the jets and stuff with a few strands of wire. turned it upside down check float again then checked it again, all seems great.
3:replaced all fuel lines and clamps, fuel pressure seems ok
4:replaced 2 vacumm lines (i think it's called the oil return line)
5:replaced top ignition coil coz i thought it wasnt creating enough spark, plus the original looked dodgy.
6:checked fuel pump, looks like new
7:cleaned all earths and wires so terminals are shiny.
8:replaced water pump houseing, impellor, bla bla bla

there's proberly something else but cant think of it right now.

now i have got it started a few times in a tub of water (with great difficulties) but would not idle, i had to open up the throttle over half to get it to stay running.

i dont know what else to do.

can anybody please help?

i wont let this beat me!!!

cheers

 

[Benny1963]

Re: 1977 35HP johnson backfire's then wont start

may have sheered fly wheel key,
also run compression test ,

 

[bonz_d]

Re: 1977 35HP johnson backfire's then wont start

Would also check the plastic pin on the bottom of the throttle/timing shaft linkage. I have purchased 3 different rudes this year, two 18s and a 78 35hp.
One the pin was missing, one had both sides sheard off and the last one had one side broken off. All also had the bushing the gear sets in worn out. The two 18s would barely run until I replaced those pins.

 

[tashasdaddy]

Re: 1977 35HP johnson backfire's then wont start

99% sheared woodruff key between flywheel and crankshaft, this throws timing completely out of sinc. you need to pull flywheel, and replace the key with a original replacement. not a hardware store key.

PULLING FLYWHEEL

you need a harmonic balancer puller, you need 3 - #8 harden bolts, that fit the 3 holes in your flywheel. standard hardware store bolts will strip out, causing more problems. loosen the flywheel nut, leaving it on so it protects the thread. thread the bolts in 7/16 of an inch. then tighten the puller, adjust so it has a straight pull. now tighten the puller, rap the center bolt with a hammer, then tighten some more. you may have to let it sit over nite, and repeat in the morning. this is a compression fitting, it will eventually POP!!. do not loose the woodruff key that goes in the groove of the flywheel and the crank shaft. then installing the flywheel, it has to be totally clean, no oil, woodruff key in place. then torque the nut to the specifications for your particular motor. if you do not do this, the woodruff key with shear, and you will be repeating these steps.

 

[agentjss]

Re: 1977 35HP johnson backfire's then wont start

Great info, i'll try it this weekend, i did read about that in the manual but i thought i would ask you guys anyway, other wise i would be randomly replacing stuff until it worked, which as we all know can be uneconomical.
thanks again and i'll let you know.
cheers

 

[tashasdaddy]

Re: 1977 35HP johnson backfire's then wont start

always ask first. that's why we are here.

 

[agentjss]

Re: 1977 35HP johnson backfire's then wont start

well i have change the flywheel key and still the same problem occurs. it's like it's on the edge of starting but then "bang" out the exhaust.

 

[EN2]

Re: 1977 35HP johnson backfire's then wont start

Make sure your spark plug wires are not reversed. Should be a tag for top cyl.

 

[EN2]

Re: 1977 35HP johnson backfire's then wont start

Make sure your spark plug wires are not reversed.

 

[agentjss]

Re: 1977 35HP johnson backfire's then wont start

just check the manual and they are correct. it's weird coz the backfire isnt consistant, some times it's louder than others and sometimes it takes longer than others, but either way it's there.

 

[ba_50]

Re: 1977 35HP johnson backfire's then wont start

My 1979 Johnson 35hp manual says it can backfire from a leaking intake manifold gasket.

 

[Harker]

Re: 1977 35HP johnson backfire's then wont start

If all it is doing is backfiring...and not starting at all, I would say the leads to the coils are reversed, making the plugs fire 180 degrees off. The leads go from the pack to the coil. I did this early this year when putting new coils on.Also, you have to have the washers on the coil bolts exactly how it shows in the manual.

 

[agentjss]

Re: 1977 35HP johnson backfire's then wont start

i have checked to see if all the leads and connectors and all are in the right place and it's all how it's supposed to be according to the manual. maybe i didnt put the flywheel key in correctly. it cant be the power pack coz im getting spark at both leads.
if it was 180 degees out then it should happen all the time.
also another problem there is fuel comming out of the carby throat like it's trying to put to much fuel in or something, maybe the float is not doin it's job.

 

[Harker]

Re: 1977 35HP johnson backfire's then wont start

If it was mine, just for giggles.....I'd switch the leads to the coils and give it a few pulls on the rope. Just reread your original post, you had it running a few times....make sure you have the fiber washers and wires in the correct sequence under the coils.

 

[mikesea]

Re: 1977 35HP johnson backfire's then wont start

i agree for giggles,but all you need to do is rev the plug wires,give it a shot,flywheel key goes only one way,but you do need to torque it down to prevent shearing

 

[Benny1963]

Re: 1977 35HP johnson backfire's then wont start

sounds like fireing order is incorrect ign prob for sure

 

[Harker]

Re: 1977 35HP johnson backfire's then wont start

i agree for giggles,but all you need to do is rev the plug wires,give it a shot,flywheel key goes only one way,but you do need to torque it down to prevent shearing

If they will reach. Mine won't on my 90's 30/35 but the coil leads are just like plug boots so are easier for me than the plug boots. Unless this motor has a major air leak somewhere, I would say it's firing in the wrong order or way out of time.

 

[fuzzeywiggler]

Re: 1977 35HP johnson backfire's then wont start

also run compression test ,

Have you run one yet?

Can save alot of hassel.

Fuzzy

 

[twostroke87]

Re: 1977 35HP johnson backfire's then wont start

After a few pulls try pulling the spark plugs and see if they are soaked with fuel, if they are then the carb is flooding the engine, the float could be stuck down and the needle is just letting the fuel flow out freely.

Have you checked your fuel qaulity, although you have rebuilt the carbs, they may have just been fouled up again by bad fuel.

Pull the line off before it goes into the engine and pump some of it into a clear, clean plastic bottle. Make sure that you have cleaned it free of water if thats what its held before. Your fuel sample should be green or blue depending on your choice of 2 stroke oil. You should be able to see clearly through the sample, if it looks foggy it is probably contaminated by water, if the sample is extremely foggy let it sit and see if it settles, if there is water in the bottom you can assume that water is making it into your carb.

 

[agentjss]

Re: 1977 35HP johnson backfire's then wont start

ok i have reversed the ignition leads and the same thing happened only with a louder bang.
the fuel is brand spanking new coz it's a brand new fuel tank, (but still could be dodgy) still have to check that. i will proberly rebuild the carby (again) just to be sure.
bottom spark plug didnt have fuel on it and it looks like it has been firing, top spark plug had fuel on it and still looks rather shiney.
so i guess the top coil is not getting power & i reckon that the power pack is at fault.