70hp Vapor Lock Telltale Reroute

Puncheon50

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I have the Vapor lock issue with my 1981 J70ELCIM . The telltale needs rerouted. I can't seem to find service bulletins for this issue for my particular model year. The best I can find is this thread on iboats....

https://forums.iboats.com/forum/eng...n-cooling-system-modification-additional-work

will The OMC Service Bulletin 2276 and OMC Water Pump Kit 432955 work for models prior to 1986? I can't seem to find anything else
 

racerone

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That modification is NOT NEEDED on 1981 model 70 HP motor !!-----And how did you determine that you have vapor lock.-----Or what is your motor doing / not doing ?----What trouble shooting have you done if any ??
 

Puncheon50

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I have the issue where I can run my motor all day but if I shut it down it will not restart because the fuel is vaporizing before it can fire. I have to wait for the engine to cool down a little before it will fire. Water pump and peller new and Stream is strong. Everything I have researched on the Internet points to a reroute of the telltale. It is a common issue with the 70 hp through the 70s and 80's. I have talked face to face with more than a few mechanics and they sound very familiar with the issue. I have had recommendations to simple take out the thermostat, route the telltale over the top of the engine, I had a guy tell me put a tstat from a 150 hp in. From everything I can tell the Correct fix per the official service bulletin is a reroute of the tee and telltale line. Plenty of threads on iboats that have the same issue that are 70's and early 80's models like mine . Only problem is they don't list details. That link I provided is the best I can find
 

oldboat1

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What is the operating temp at the top of the head? Too hot to touch? When was the impeller last replaced?
 

Puncheon50

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No, nothing seems too hot to touch. You can hold your hand on the jacket after shutdown. Impeller was replaced in June of 2019. I never checked operating temperature at the top of the block. I can run it for about 20-30 minutes, shut it down and it will usually restart . After running for 30 minutes it isn't going to restart. Runs like a champ all damn day at any rpm but shut it down and you have to wait 30-45 minutes for it to fire up. I found that tilting the engine with the cover off speeds up the time needed for cool down. The boat is winterize and in storage now but I wanted to put in a new tstat and clean out the water chamber thing to see if that might do it. That reroute doesn't seem that difficult and if I need to order gaskets etc, I might as well do it
 

racerone

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Find the real problem !----Moving the tell tale is not the solution !!
 

flyingscott

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I have the issue where I can run my motor all day but if I shut it down it will not restart because the fuel is vaporizing before it can fire. I have to wait for the engine to cool down a little before it will fire. Water pump and peller new and Stream is strong. Everything I have researched on the Internet points to a reroute of the telltale. It is a common issue with the 70 hp through the 70s and 80's. I have talked face to face with more than a few mechanics and they sound very familiar with the issue. I have had recommendations to simple take out the thermostat, route the telltale over the top of the engine, I had a guy tell me put a tstat from a 150 hp in. From everything I can tell the Correct fix per the official service bulletin is a reroute of the tee and telltale line. Plenty of threads on iboats that have the same issue that are 70's and early 80's models like mine . Only problem is they don't list details. That link I provided is the best I can find

That mod is for the 86 and later 56 cid motor NOT the 85 and earlier 49 cid motor. And it is not to cure a fuel issue That mod will have no effect on your fuel issue. How is the fuel vaporizing before it can fire? Once it is in the crankcase it cannot just disappear. Have you checked for spark? Have you checked compression?
 

Puncheon50

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Yes spark and compression are good. I fully understand that service bulletin is not for my model, but it is my understanding that one exists for my model because this is a common issue in the 70hp through the 70's. The reason I say that is because there are quite a few threads on iboats and other forums with the same issue. I have run into a couple people on the lake with 70hp that have had the same problem. I have talked to mechanics that have said they remember doing the reroute a lot back in the day. From what I have read is there is an air pocket at the top of the jacket that causes to run hot. There are quite a few people that say they have taken out the tstat and it solves the issue. I don't think that is a good idea. I really don't believe the feul mix is the problem because I am pretty sure I am dead on 50:1. I also use NO ethanol gas. I did start 1oz/gal seafoam but I had this problem before I started adding it.
 

Puncheon50

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Also the overheat alarm works but never goes off. I know it works though
 

Puncheon50

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Also had someone say it might be the power pack overheating but I replaced the power pack in June. I also put in a regulated rectifier and tach
 

racerone

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Some trouble shooting on your motor needs to be done.----I have never encountered vapor lock on one of these motors !-----Vapor lock is NOT a common issue on these motors !!
 
Last edited:

oldboat1

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Yes spark and compression are good. From what I have read is there is an air pocket at the top of the jacket that causes to run hot.

But your motor is not running hot, right? An infrared gun might be a good purchase -- test at top of head. Operating temp. should run around 130-140F, but never at 160F or over.

Is there any indication of ignition parts overheating? Black goo around the bottom of the stator, for example?
 

Puncheon50

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Well maybe I am confused as to what Vapor lock actually is but I had somebody tell me that is what it was
 

Puncheon50

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I guess I will change the tstat and clean the jacket. I am pretty sure I have the air pocket issue so water isn't flowing right somewhere. Tstat is old and has sat unused for we'll over 20 years so hopefully that will do it.
 

racerone

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And how can you be " pretty sure " that there is an air pocket.????----Why not tell what you think " vapor lock " is and how it affects this motor.
 

Puncheon50

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I say there is an upper air pocket because the behavior of the motor is exactly like the threads I see all across the Internet. On further examination, those threads confirm what you are saying about earlier models. A majority of those threads are for 70hp after 1986. Don't know why Johnson kept pumping out engines with the same defect but it looks like it was fairly common after 1986. Anyway my point is reading through all of those threads, they have the symptoms and behavior of the engine is exactly the same. Same story in every one, runs like a top but behaves like a flooded engine and wont restart until 30-40 minutes of cool down. I have been through the electronics on this thing. Coils are not overheating. Powerpack is new. I rebuilt the carbs, pretty sure I am at 50:1 non ethanol gas. So as beautifully as this thing runs, it behaves the same as the later engines that require that service bulletin. If you are saying it's rare for my model to have the same issue I buy that but I feel like it's a jacket water flow issue. Who knows what's in there. This boat sat in a barn and had been unused since 1993. Who knows what kinda bugs might have crawled up in there and called it home. Maybe the thermostat isn't opening all the way. It is pretty old
 

racerone

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Well I give up.-----I bought 21 of these motors like yours in one deal.-------None had this modification made when I bought them.---And I did not modify a single one.------So you say it behaves like a " flooded engine ", how do you relate that to vapor lock and heat ????
 
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