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miss at full throttle

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Lowflyer51357

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2000-01 j30esia 30 hp johnson - jet drive on an older tracker 2 flat bottom. have replaced fuel pump, fuel filter, and rebuilt carb. motor idles nice, but open it up, and there is a very noticeable miss, that to me, seems as if it is starving for fuel. it just will not get up to an even, nice steady rpm. back off the throttle a little, and it seems even out, and to be running perfectly, no surging, just a nice steady purr. push the throttle down again, and hardly no increase in speed, just an uneven rpm, spit and sputter.

ordered new spark plugs for it today, but don't think thats the total problem. hose and primer bulb are not very old, and seems to be no change if you squeeze bulb when it is missing. thought for sure fuel pump was the culprit. any ideas?

IMG_2786.MOV
 55 second video, with about last 8-10 seconds of it running with slightly reduced throttle. you can hear the change in evenness more than anything.
 
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spark on both cylinders. went through the synchronization linkage adjustments, and timing. no change. motor idles perfect, and is good as rpm goes up. get close to full throttle, and surging starts, back off a little bit, and it evens out.
 
Typically you would run with a timing light.----Will help determine if it is a spark problem.----May help in deciding if it is top or bottom cylinder acting up.
 
So determine if its a spark interruption, then check all your wires under the stator. Movement here may cause wire abrasion or deterioration and act up at only certain throttle positions. This motor has a bunch of moving wires there.
 
update on what i've found out. was able to get a independent mechanic to take a look at it. he checked out fuel flow and pressure, then checked out the synco and linkage. timing is good, spark is steady. then with it running in gear, in a test tank, discovered that if you covered the two 1/2" holes that are in the front of the exhaust housing with your fingers, the surging stopped. totally baffled about the reason, but speculated that the inner exhaust tube might be missing. or possibly has been shortened. i don't remember if it was there, when i changed the water pump, but with the amount of work it would take to remove it, i doubt that it is missing.

my question then becomes, what would happen if i plugged the holes in a more permanent way? something that i just left in place. it definitely changed the way it was running.
 
Honestly never saw this demonstrated before, can't advise you, but it should be tested at top speed again......could be lack of back pressure causing the lean condition, but you can try cleaning the main jet better, it may be slightly closed up. This would create the symptoms you described.......lean. If it is running lean you will score the cylinder......likely the upper. Take note of spark plug color.
 
i had just rebuilt the carb, and the high speed jet, is mostly an open tube. looked good to me?

this guy said he it was changing back pressure. he thought the engine overall sound, was louder than it should be., but quieted down some, with the holes plugged. if the lack of backpressure is causing a lean condition, would not plugging the holes, create more back pressure and not remedy the condition? and be something helpful? he also mentioned replacing the gasket between powerhead, lower cowling, and the exhaust tube, as being a possible help. what would i look for in testing at speed again? thanks for your replies.
 
The high speed jet ---" is mostly an open tube "-------Well I do not believe you were looking at the high speed jet.----That jet is about 5/8" long and screwed into the bottom of the float bowl.----It is not part of the carburetor body !-----Not sure if I would use this " guy " for my trouble shooting work.----I do not believe the inner exhaust tube can be missing !
 
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Which "guy" you referencing, Racer? I think I know which one. I probably wouldn't trust him just with what has been said either. Lowflyer, you apparently missed the main jet in your carburetor overhaul. Take a look at the schematic picture of a carburetor on this site and you will see where the main jet is located. You have to get the right screwdriver to fit in there and remove the jet, next you have to make sure even though the jet looks to be open it is probably a reduced size from deposits of old fuel. I use a welding tip cleaner and throttle body cleaning spray to clean it out to its correct size. This has become an all-too-common problem now with jets engineered just on the edge of lean, as well as poor fuels which have a very short "shelf life".
 
part of this problem, may be in that it is a jet conversion. if i remember correctly, the inner exhaust tube doesn't extend down inside the housing, to be anywhere near the water level. if it is basically just dumping inside the housing, why would back pressure be changed, from a standard prop drive. forcing the exhaust into the water could increase back pressure, but when the exhaust can exit through the small holes,(above water level when underway), pressure is relieved somewhat. i have a fsm, for this engine, and in the last notes pertaining to carb mixture adjustment, it says, if engine does not respond properly to these adjustments, check the following - #4 is sufficient exhaust back pressure. but i haven't seen anywhere how to change or adjust back pressure.
 
pulled the carb back off, and checked out the high speed jet, (it actually is a short brass tube, threaded on outside, choked down in center, then expanding again. 1/16 drill bit passed through without binding in anyway. also used torch tip cleaners to make sure nothing was inside. 67d stamped on it.) pulled powerhead loose. nothing exciting discovered. exhaust tube housing dirty, but didn't seem to be leaking. (didn't look to have been shortened) have to order gaskets and o'ring before reassembly.

question remains on what to do about low back pressure.
 
What size jets are in there? Might be the wrong ones. Maybe jet drives require a size or too larger because of less back pressure. This is where we are leading, just as you suspected.
 
Find out if you switchbox has a rev limiter. Jets are great for letting a motor overrev especially if the impeller is in less than steller shape. If your switchbox has a rev limiter put a tach on it and find out your RPMs. You.also want to make sure the throttle butterfly is not rotating to far. I dont think you have a back pressure issue. Because if you did you probably would have burned the motor down from running lean.
 
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i seldom run at full throttle, but would like to have the ability, if need would arise. if it were not a back pressure issue, why would the engine smooth out, when plugging the relief holes on the exhaust housing? impeller and liner are in great shape. butterfly opens properly.
 
thanks for everyone that replied, trying to sort this out. i think i have it finally figured out. i pulled the powerhead and exhaust housing in an attempt to find any issues. nothing really jumped out as indicating a concern. replaced gaskets, o rings, and cowling seal. i also pulled lower unit as it seemed it would be troubling to align water tube from the top. rebuilt water pump, and as i'm getting ready to put lower unit into place, i'm looking through factory service manual still looking for a solution. in a picture of the water pump servicing section of the jet drive service, i see a part referred to as impeller housing spacer. i realize that's something i don't have. i bought this motor in pieces, supposedly complete, but it's a piece i've never seen before. always had merc's before this johnson, and they are a little different.

anyway, this looks like it seals the tube that the driveshaft runs through from water pump to powerhead. the holes through the midsection, that i could plug up, to make the engine smooth out, are located inside this passageway. engine should be slightly quieter, and have increased back pressure with this rubber section in place. i have hopes that all issues will finally be worked out, after i get this part delivered in a few days. thanks again.
 
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