25 Johnson missing

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Xterminator GT

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I have a 89 Johnson 25 jet. The motor has the 35 carb and intake and a set of Chris Carson reeds in it. I'm having some trouble wit the motor missing/backfiring through the carb. It will idle fine and run wide open but anywhere in between it backfires through the carb. Any ideas or suggestions on what to do will be greatly appreciated.
 
I'd never argue with Pappy.

That Sneeze happens when ignition takes place in the Crankcase instead of the Compression chamber.

Too lean is almost always the cause of it. The way to fix it sometimes involves changing the Reeds.
That is, if it's all stock and the parts are the one's the engineers designed for it.
 
The reeds that are in it are aftermarket Chris Carson marine reeds. They have only been in there for 5 months and have 15-20hrs at the most.
 
And I took it down to the river today and decarbed it with some sea foam. I checked the plugs before and after doing this and both times the top cylinder was slightly rich and the bottom is running really fat.
 
How do you tell "Rich from Lean"?
What's "FAT" mean?

Let it idle for a few minutes, then pull the plugs, see which is wet or dry (er) than the other plug. Put New plugs in for any test.

Sometimes a Timing light helps.

If Vacuum on one cylinder fail's (a miss etc), it stops (inhibits) the Vacuum (Flow) of the other cylinder. You might get a big bang outta that one!

Did you take some photos of your Aftermarket reed installation? That helps to make sure the Reeds seated etc.
Why did you put those after market reeds in? Were the originals failing?
 
When I pull a plug I look for a nice uniform brown color. When I pulled these plugs the top one was black with some gunk on it and the bottom one was very black and wet and gunky. And I installed the aftermarket reeds to help with performance. This motor is an outboard jet and you need all the help you can get whenever your running up a shoal in the river that is only 5 inches deep. The motor has always ran great before and we took it gigging last night and it started acting up.
 
Go here: https://www.atlanticjetsports.com/_techtalk/00000005.htm

See if you can find your spark plug colors.
Make sure your fuel Mix is correct and your using good quality Evinrude Oil.
 
Soooo.....because your aftermarket reeds have only been used for around 20 hours they must be good, huh.
What is one of the tell-tale signs of an aftermarket reed standing slightly open?
Anyway....gave you some other things to check as well.
BTW.....been there done that on aftermarket reeds.......several times.
 
I will check the reeds to see if one is open and I only use Evinrude/Johnson mixing oil, mixed at a ratio of 3oz a gallon. Thanks for all the replies
 
I checked the reeds and sure enough 3 of the petals on one of the reeds were lifted. I haven't got to take the boat to the river to run it but I'm sure this will solve the problem.
 
Is there any aftermarket reed that is good? When I installed these I noticed an improvement in throttle response. These little 25 jets need all the help they can get. It already has the 35 carb/intake mod, are there any other mods out there for these? Exhaust mods or shaving the head slightly?
 
Knowing I don't have all the experience in the world, but from the 50's on till last year, I've never seen the need to fool with those Reeds.

Back in the 70's I learned to buy good Gas. Water in the Gas from the El-cheapo stations had Water in the mix. The motor began to tap, then Knock, then loudly KNOCK!
Can't use the motor now.
I had to row nearly 30 miles, between tides. Three days of Sleeping on the Beach, worried about Bears. :oops:

All that was a result of Water in the Fuel, my 35 Hp Johnson suffered a bad Center main.
I replaced them all. :|
The guy I bought the parts from said I'd ought to change the Reeds too.
I wondered about that and took a look. The "Suck" test was OK, and nothing was awry looking.
Replaced the Bearings and that Motor was still running a few years later when I sold the boat.
I learn everything the Hard way. But it's sure made for an interesting life! :mrgreen:
 
I always buy 91 octane fuel from a gas station that gets their tanks filled everyday. The station owner told me that their 91 octane is ethanol free, whether there's any truth to that or not I don't know. And I always mix with Johnson/Evinrude oil. And I decarb the motor every couple months to try and keep it as healthy as I can.
 
Go back to production reeds.
You can shim the reed stops a little.
Cut the head if you want.
Go to 89 octane and stay there. Ditch the 91
You can port to your heart's content. Since you are running a jet the engine should respond pretty well.
New rod bolts of course if you go that far
 
The 91 octane burns substantially faster and is not needed.
The engine may actually produce more power on 89.
Yes, even if you cut the head resist the urge to go back to 91.
Have run engines with heads cut between .100 and .125 on 89 octane (you read right, that's right at 1/8")
 
Thanks a lot pappy. I'll make the switch to 89 octane. I'm taking the boat gigging tomorrow night. Hopefully putting the stock reeds back in cured the problem. And what is a good ballpark setting for the mixture screw on the front of the carb?
 
pappy how do you figure 91 burns faster then 89 since higher octane slows the burn rate of the flame. That's why higher octane is need to for high compression engines to keep from detonating. If lower octane burned slower racing motors would use 87 pump gas. I agree 91 is over kill on that motor but that's because all the gas isn't being burned by the time the port opens letting the exhaust out.
GT contact Chris about the reeds he'll try to make it right. Fiber reeds can be a pain but they do help performance to a point and if one breaks your not gonna be rebuilding your motor because it chewed on a piece of metal. maybe because it leaned out but you should be able to catch it before then As far as hot rodding your 25hp I could probably tinker with it. You've seen my boats run just shoot me a pm.
 

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