20 hp johnson no start

milmo1 · Apr 8, 2009

I have a 1966 20 hp Johnson pull start motor that will not fire. Last year I had some difficulty, but pulled the carb off, got a rebuild kit, and got it running. I periodically start the motor in the garage during the off-season. A couple weeks ago, it fired, ran a few seconds, then I shut it down.

Now I pull this thing endlessly and the best I can get is a slight backfire when I use starting fluid. I have checked the fuel pump, it delivers fuel out of the hose (to the carb) on every pull. Compression is pretty good at appx 100 psi and 110 psi with me pulling on a cold engine. I have a thick blue spark from the new spark plugs. I feel it may be a spark timing issue, but have not pulled the flywheel yet. The upper plug had a large deposit on the electrode (lean mix?) after last season, but I believe that plug was of the wrong heat range.

As a side question, a fella that thought he was helping me last year jacked the lean/rich adjustment on the carb all over. It is disconnected from the knob on the outside of the motor, so it may be horribly out of whack. What do i do to get it back in range?

HybridMX6 · Apr 8, 2009

Re: 20 hp johnson no start

For the lean/rich, screw it all the way in (gently), and once it bottoms out, unscrew it 1.5 turns. That will be a basic setting, you will need to have it in the water and running to get it set perfectly from there.
As for the won't start, I'd set the idle lean/rich setting, then try again. If it's still not firing, I'd suspect carbs. Spark timing should not change by itself.

milmo1 · Apr 8, 2009

Re: 20 hp johnson no start

HybridMX6 said:
For the lean/rich, screw it all the way in (gently), and once it bottoms out, unscrew it 1.5 turns. That will be a basic setting, you will need to have it in the water and running to get it set perfectly from there.
As for the won't start, I'd set the idle lean/rich setting, then try again. If it's still not firing, I'd suspect carbs. Spark timing should not change by itself.

So, you're thinking it's not getting the mix?
Even when I spray either down it's throat I can't get more than a weak backfire. That's why I thought timing, although I agree, it shouldn't change itself.
Do you suggest I pull the carb and do a thorough cleaning?
Thanks for the ideas.

HybridMX6 · Apr 8, 2009

Re: 20 hp johnson no start

First off, don't spray ether in there, use a spray bottle with pre-mix fuel in it safe for gas if you can find one. I can't explain why, but have read numerous times on here not to do that.
I do suggest pulling the carbs off and giving them a good cleaning. Depending on how old the gas is, you may want to use some fresh gas as well with fresh oil mixed in, unless you use a fuel stabilizer in it as well. If you use fuel with ethanol, you also may want to consider replacing the fuel lines with newer alcohol resistant lines. The ethanol will break down the old lines and they will start to get lodged in your carb.
My thinking is you said you have good spark, the timing should not change by itself, so that leaves fuel.

milmo1 · Apr 8, 2009

Re: 20 hp johnson no start

Thanks!
I replaced the rubber fuel lines last year as they were quite bad. Here in the Detroit area, all gasoline has ethanol.
When I pulled the carb last year, we were amazed at how clean it was (as well as the reed valves). I know spraying either is bad as it can wash out the oil and cause engine damage on 2 strokes. It has worked well for me in the past as far as testing for presence of fuel in the cylinders. But, if you think the carb is the most likely cause, I'll give it a whirl.

HybridMX6 · Apr 8, 2009

Re: 20 hp johnson no start

I'd still say to check that idle mixture setting too before messing with it too much.
I am assuming you are choking it when you try to start it, does that not help at all either? It sounds like your low speed is gunked up or something.
How old is the gas you are using? You didn't say.

milmo1 · Apr 9, 2009

Re: 20 hp johnson no start

Yes, I am at full choke, although I have varied it out of desperation. All I can muster is a backfire.
The fuel is older, but stabilized. I have used some of it in my chainsaw just fine.
Incidentally, after pulling a number of times, fuel comes dripping back out of the carb. I don't remember that ever happening before.

EN2 · Apr 9, 2009

Re: 20 hp johnson no start

Check needle and seat also float. Make sure high speed nozzle gasket is in good condition and in place.

milmo1 · Apr 10, 2009

Re: 20 hp johnson no start

EN2 said:
Check needle and seat also float. Make sure high speed nozzle gasket is in good condition and in place.

Could this be the cause of raw fuel draining back out of the carb?

milmo1 · Apr 20, 2009

Re: 20 hp johnson no start

Carb is now completely cleaned up (although not much was found to wrong) and reinstalled on motor. Still only manages a backfire through the exhaust. I'm still thinking it's spark timing, but I am unsure how to check and verify...

lindy46 · Apr 20, 2009

Re: 20 hp johnson no start

Is the float floating? If gas is coming out the carb throat, the needle isn't closing - either a bad float or float adjusted too high. Or may be an internal problem with the carb (crack, air leak). I just had a 5.5hp carb which I completely rebuilt and had the same problem as you. I just couldn't get the motor to fire. I decided to buy another carb, re-built that one and installed it on the motor and she fired right up.

milmo1 · Apr 20, 2009

Re: 20 hp johnson no start

Well, the float, the needle, the seat, are all new. Yet, the result is the same as before - backfires out the exhaust. Now, just like before, I can disconnect the fuel supply and run out the remaining fuel. Then I clean and dry the plugs, reinstall, shoot a little either in the throat - backfires out the exhaust.

lindy46 · Apr 20, 2009

Re: 20 hp johnson no start

You can easily check synchronization to see if that's the problem. Take the cover off, make sure the throttle grip is in STOP position. Look at the throttle cam - there is a mark on the cam. Slowly advance the throttle grip until the cam roller aligns with the mark on the cam. In this position, the throttle valve (butterfly) should be closed. If it is already opening, you need to loosen the cam mounting screws and adjust the cam so the throttle valve is closed and there is no play in the linkage. As you further advance the throttle grip, the throttle valve (butterfly) should start to open. It should be fully open when the throttle grip is in the FAST position.

milmo1 · Apr 22, 2009

Re: 20 hp johnson no start

OK- the saga continues...
I checked the throttle cam, and the cam follower (which is fairly chewed up) doesn't get anywhere near the throttle cam when the cam is advanced to it's timing mark. Should the throttle cam be adjusted, and/or the cam follower?
I see there are 2 small bolts under the throttle cam, are these merely loosend to adjust the cam, or must they be removed? I do not see slots to "slide" the cam - its pretty hard to see under the cam. Any advice is greatly appreciated.

lindy46 · Apr 22, 2009

Re: 20 hp johnson no start

Yes, just loosen the two bolts and adjust the cam until the roller (follower) just touches it at the mark. Then re-tighten. The throttle should just start to open at this point.

milmo1 · Apr 22, 2009

Re: 20 hp johnson no start

Will do. I'll report back after I reassemble and test. Thanks again!

rusirius · Apr 22, 2009

Re: 20 hp johnson no start

I wouldn't ordinarily think this, but if the sync was that far off then who knows... Have you checked the coils/spark plugs to make sure the RIGHT ignition coil is feeding the right cylinder? In other words, if they go switched around you could be firing the wrong plug at the right time...

ditchen · Apr 22, 2009

Re: 20 hp johnson no start

most of the plug wires I have seen (and on my 66 20hp) has a metal band on one of them. That one has been on the top clylinder

milmo1 · Apr 23, 2009

Re: 20 hp johnson no start

Update:
Yesterday I checked the throttle cam timing. I found a loose spring on the cam follower, installed it, and that took up some slack. I adjusted the timing mark to coincide with the cam follower, so that timing is on.
Incidentally, the night before I found a tune up set of condensors and points that I had purchased a while before. I installed and gapped these, too. Following the direction of an old auto tech, I manually checked the point opening/piston position. It "seems" to be opening at or just a hair after TDC. That would explain the timing issue if that is correct (its just a manual observation at the judged idle speed position). However, my Clymers manual states that this magneto point ignition does not have a spark time adjustment. Where do I go from here?
BTW, I have checked the spark plug wire configuration probably 5-6 times. After every few days I convince myself there is something very simple I have missed. Unfortunately, each time I find that I cannot physically cross these two wires as the shorter one will only reach the one cylinder. And yes, the metal banded wire goes to the top cylinder.

lindy46 · Apr 23, 2009

Re: 20 hp johnson no start

Is the flywheel key OK? Flywheel torqued properly?

20 hp johnson no start

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