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TOPIC: 88 Johnson 150 high altitude jetting help

88 Johnson 150 high altitude jetting help 3 weeks 2 days ago #132530

Hey guys I could really use some help with this. I have been struggling with this Johnson outboard that came on a recently purchased 89 Larson runabout boat. I primarily operate this boat at attitudes ranging between 6 and 10,000 ft. I use high octane fuel only , rarely ethanol free ,as it is difficult to obtain in my area. I do get it once in a while though. I pre mix Walmart outboard oil at 16 ounces to 5 gallons. It usually starts ok, however once in a while really struggles with it. It also struggles at idle, which it doesn't like to do for too long before getting a bit of throttle. I must shift quickly to just past where 1st gear starts to avoid stalling. From there , if I try to accelerate much past this point it wants to and often does stall. The trick I've found to keeping it running is to push and hold the key in for a second or two just as it's about to bog out. This opens the choke solenoid which injects gas directly into the cylinder. This tells me it's running lean. Once it's past this crucial Rpm range (between 11 and 1300 Rpm) it's fine. Cruises at full throttle just fine, and when I back off, it's also fine at low speeds. It's just accelerating through this small range. I have another thread detailing all of these issues called " 89 Johnson" (before I figured out it was a 88). Another member on here "chinewalker" advised me to pull down and clean up the carbs. Last week I completely disassembled all three, removing every jet, and cleaning them with carb cleaner individually before reinstalling​. I was skeptical as they seemed pretty spotless upon disassembly. Nonetheless cleaned em well, put everything back together and took her out this past weekend. It ran pretty much exactly the same. I have been suspicious about the carbs needing different jets for high altitude. However I learned that high altitude jetting usually involves actually down sizing the jets, which would result in a ever leaner condition ??? Information about the jets is difficult to obtain. I have called several marine shops here in Colorado and none have had an answer for me. Any help would be greatly appreciated. Thanks

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88 Johnson 150 high altitude jetting help 3 weeks 4 hours ago #132573

No response yet.....either the post was too long for anyone to read, or nobody here can help with this particular problem. Is there any other sites I should attempt to get help ?

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88 Johnson 150 high altitude jetting help 2 weeks 2 days ago #132644

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88 Johnson 150 high altitude jetting help 1 week 2 days ago #132771

Thanks silver bay, I didnt find anything directly relevant to my situation in the thread you linked. I found out that johnson does not recommend changing the jets in these engines for high altitude. The problem has persisted. It is definitely fuel starved at the specific rpm range I mentioned. The primer bulb is difficult to get hard and doesnt stay so. High altitude jets are usually smaller, that would make the problem worse. The only way to keep it running is with the primer/choke solenoid, which injects fuel directly into the cylinder I am starting to think the problem may be somewhere other than the carbs. Would a bad fuel pump still work good at high rpm, but struggle at most other speeds ? Really hoping for insight here.

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88 Johnson 150 high altitude jetting help 1 week 1 day ago #132775

Both links were ideas on other places to ask, both have large populations of old hands (some who are on both sites)

If the fuel pump is keeping the carb bowl(s) full, I'd THINK the primer bulb should stay hard, since the pnly thing that keeps the bowls from overflowing is the float valve closing off the inlet (the same thing that makes the bulb hard to pump), so maybe it is a weak pump....
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88 Johnson 150 high altitude jetting help 1 week 1 day ago #132782

thanks silver, finding a replacement pump for my exact model number has been difficult. Unless there is a cross reference, it doesnt look available. What else could be causing this besides the fuel pump ? Some have mentioned a clogged fuel line, but again , I would think the problem would be apparent at all speeds. Since the nearest shop that can work on it is about a 2 hr drive and charges 96 bucks an hour shop rate , and is backlogged for 3 weeks.... it might be best to just throw some money at it . I am thinking of starting with the pump..if I can find one. The only other parts I can think to replace would be the primer bulb and hose....or the fuel pickup in the gas tank. Suggestions please ?

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88 Johnson 150 high altitude jetting help 1 week 1 day ago #132785

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I am no mechanic, but feel your pain. If fuel line with primer has some years on it, why not start easier and cheaper and replace first. It never hurts to have a good one before any hose does start failing. If that doesn't solve it, no great loss yet. Then maybe pull that tank pickup and check for anything including looking in tank. I can only do these simpler things, but at least that is 2 tries that don't cost too much. This is where I have to rely on professionals for further repairs as I have not learned further skills. Wish I could help more. Is your tank vented without restriction? Are any hoses pinched or kinked? You have probably done all of this and more... I am just reaching at this point. Maybe your tank pickup is near the front of the tank and the tank is lower on fuel so when you are planing off, the fuel runs to the back until you plane off. Ok - I am reaching at this point and am way out of my wheelhouse. Good luck. Hope you get better advice than mine!!
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88 Johnson 150 high altitude jetting help 1 week 1 day ago #132787

Thanks dr.go. Great advice sir ! those are all some possibilities I have not looked into. I am not sure about the tank ventilation. There is a vent below the filler cap, I noticed when I overfill the tank, gas leaks out here.

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88 Johnson 150 high altitude jetting help 6 days 6 hours ago #132837

Could I get the model number of your engine? They built a GT version & just a 150 in those years. Each has its own size carb. You are correct to assume that if the engine will run at WOT (wide open throttle) then the fuel pump is putting out adequate pressure for idle speeds in most cases. You are also correct that at altitude jets (high speed jets) are usually reduced in size to lean the engine since there is less air for the engine to breathe. The confusion comes from the fact that the low speed (idle jets) in this motor are air bleed jets. That means in order for them to make the engine run richer (more fuel) the jets have to be smaller in number (smaller hole). Restricting the air makes more fuel enter the carb throat. Now this brings up another interesting point? Air leaks become a very important consideration. Your carburetors have a top cover on them with a gasket under it. Did you put new carb kits in when you cleaned the carbs? It is important that these gaskets have a good imprint to show that they are sealing in all areas. The tiniest leak will cause the motor to run lean. Does the engine lean spit when it dies? Do you trim the engine up level until you get ready to take off. This can help get you past that point. You will notice the motor will run smoother trimmed up (top of motor cover) level with the water while idling around in no wake zones. You may have some other issues, but let's start there for now.

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88 Johnson 150 high altitude jetting help 5 days 22 hours ago #132843

I posted a link to this thread on the AOMCI forum in a members-only area (which may have brought t2 over) and another trusted veteran (who also posts on iboats, and worked professionally on OMC engines) wrote (his emphasis in bold):

Well since I live at about one foot above sea level, I'm no expert when it comes to 10,000 feet. But my first thoughts are check the synchronization between the carburetors and spark advance (link & sync)---by the book. If the carb butterflies do not close completely and start to open at the correct time, nothing else is going to work. Related to this is the timer base sticking and moving late, a notorious problem, caused by old sticky grease.
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88 Johnson 150 high altitude jetting help 5 days 53 minutes ago #132859

Hello t2, and welcome to fiberglassics ! Thank you for joining in, I need all the help I can get here. Additionally thank you for all of the great information you just provided with one post, so many great questions. First off the model # J150TLXCA.

As far as the carb kits go, no I did NOT replace those gaskets nor any other parts in the carbs(no kits used). However the gaskets looked like new. The problem didnt change at all after I cleaned them. A member here "chinewalker" advised me to just pull em apart, clean up everything, especially the jets, and put it all back together. They were all very clean upon disassembly.

The engine usually (I dont think always) makes a spitting noise when it dies. I have tried starting with the engine trimmed up and down, It doesnt make much of a difference with this problem, it IS more difficult to get her through that rpm range while trimmed down, but does the same thing with it trimmed up, just gets through it a smidge better.
Thanks again for the helpfull insight

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88 Johnson 150 high altitude jetting help 5 days 49 minutes ago #132860

silver-bay wrote: I posted a link to this thread on the AOMCI forum in a members-only area (which may have brought t2 over) and another trusted veteran (who also posts on iboats, and worked professionally on OMC engines) wrote (his emphasis in bold):

Well since I live at about one foot above sea level, I'm no expert when it comes to 10,000 feet. But my first thoughts are check the synchronization between the carburetors and spark advance (link & sync)---by the book. If the carb butterflies do not close completely and start to open at the correct time, nothing else is going to work. Related to this is the timer base sticking and moving late, a notorious problem, caused by old sticky grease.


Thanks again silver bay ! Really appreciated. I have looked over that section of the manual (link and sync) it looks like a PIA ! I hope the issue isnt there. The timer base thing sure would suck too !!

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88 Johnson 150 high altitude jetting help 4 days 22 hours ago #132863

I got your message (email). For the good of all we will go over the details here as to how to proceed. If you don't have it off already remove the front cover from the carbs. The black plastic cover. I know there are a lot of screws & the bottom three are tough to get too, but you can leave it off till we get this figured out. This takes a little time, but is not hard to do. Get you some round wooden tooth picks. Any restaurant usually has them. Look up into the mouth of each carburetor. You will find two low speed air bleed jets in the top roof of each carb body. These are little 8 X 32 brass jets. According to the parts books they should be .030 in size for your model engine. You may be able to see the number on the face of the jet? If you have a small mirror it helps. Take a tooth pick & stick it into one of the jets. It will stay in the hole if you push it in. Start the motor as normal and warm it up. Remember to keep the engine level. We are going to be putting the tooth picks into the various jets randomly. It's OK to put in more than one tooth pick at a time. You may want to put a pair in one carb? Give the engine a minute to respond. If you start to notice a difference then trim down and try to accelerate ahead. Caution! Do not take off enough for water to come over the back of the boat & go into the motor! Just goose it a little to see if it is better. Idle around to see if it idles better. By blocking the jets off with the tooth picks you are making the low speed circuit of the carb go full rich. The motor should load up some, but stop sneezing when you find out which cylinder (cylinders) are lean? I have different jets you can change to here that I can send you. We are not going to leave it full rich. If it takes a totally blocked jet to make it run correctly you have an air leak somewhere else. We will go looking for it. Like I said this can take a little time to accomplish, but better you do it than a mechanic @ $95/hr! Some won't even do it!
Let me know when you have an idea where you stand. We will proceed with the next step.

Dan in TN

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