# Yamaha F70 running weird at low rpms



## coconutgroves (Sep 23, 2013)

Shuddering where the vibration is in the boat sounds more like a misfire. I haven't had this issue myself on my Yahamas (2 strokes though), but those RPM ranges are when the most load and fuel consumption happen on the engine. That range is when getting on plane, so the fuel demand is higher since there is a lot of energy needed for the load. 

Question - did it do this before you did your recent maintenance? 

Does this happen all the time and in all conditions? Specifically, cold start / warm start / throughout fishing all day?

It not the impeller for sure - that only affects two things - how good the engine pisses, and if not getting enough water, the overheat alarm.


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## CAPTAlexMoran (May 2, 2020)

coconutgroves said:


> Shuddering where the vibration is in the boat sounds more like a misfire. I haven't had this issue myself on my Yahamas (2 strokes though), but those RPM ranges are when the most load and fuel consumption happen on the engine. That range is when getting on plane, so the fuel demand is higher since there is a lot of energy needed for the load.
> 
> Question - did it do this before you did your recent maintenance?
> 
> ...



Hmm interesting. It started just before memorial day weekend, so after the new plugs and just before the new lower unit oil. Yeah, it's pretty much all day. I can feel it as I'm jumping up on plane and then we're good to go. Is there a way I can figure out if/which plug might be misfiring?


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## coconutgroves (Sep 23, 2013)

CAPTAlexMoran said:


> Hmm interesting. It started just before memorial day weekend, so after the new plugs and just before the new lower unit oil. Yeah, it's pretty much all day. I can feel it as I'm jumping up on plane and then we're good to go. Is there a way I can figure out if/which plug might be misfiring?


May not be a plug - typically a misfire would happen at all times. But what is strange it happened after your service. I could be as simple as a fuel demand issue, so check your fuel bulb (if its old, replace it). But I'd take back to where it was serviced if it happened right afterwards.

Sometimes engines will do things cold (at first run) or after getting to heat. Heat and cold affect parts - that is the reason I asked. My 2 stroke yamaha would only do things when it was fully heated up, but also only while wide open throttle and on plane. Hard to diagnose, but turned out to be the high pressure fuel diaphragm. Another fuel issue (bogging down while getting on plane) was due to my fuel bulb getting too hot (in the sun) and also installed too far away from the engine and pointed down. Fuel bulbs have an arrow for a reason (flow direction) but also should be installed as close as possible to the engine and vertical if possible. Small little things can affect fuel flow and make it seem like the engine, when in fact it is a cheap plastic part on the outside not doing its job.

Someone on here with an F70 will be better suited than me, I am just giving observations I've learned from thinking it was an engine issue when in fact it was just fuel line related.


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## CAPTAlexMoran (May 2, 2020)

coconutgroves said:


> May not be a plug - typically a misfire would happen at all times. But what is strange it happened after your service. I could be as simple as a fuel demand issue, so check your fuel bulb (if its old, replace it). But I'd take back to where it was serviced if it happened right afterwards.
> 
> Sometimes engines will do things cold (at first run) or after getting to heat. Heat and cold affect parts - that is the reason I asked. My 2 stroke yamaha would only do things when it was fully heated up, but also only while wide open throttle and on plane. Hard to diagnose, but turned out to be the high pressure fuel diaphragm. Another fuel issue (bogging down while getting on plane) was due to my fuel bulb getting too hot (in the sun) and also installed too far away from the engine and pointed down. Fuel bulbs have an arrow for a reason (flow direction) but also should be installed as close as possible to the engine and vertical if possible. Small little things can affect fuel flow and make it seem like the engine, when in fact it is a cheap plastic part on the outside not doing its job.
> 
> Someone on here with an F70 will be better suited than me, I am just giving observations I've learned from thinking it was an engine issue when in fact it was just fuel line related.


Ok sweet. I'll take a look at all of that and see if there's an easy fix. I appreciate the input! Thanks for the help.


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## Ckirk57 (Sep 27, 2009)

My motor did that- ended up having to much engine oil. I changed the oil again and made sure it was on the lower end of the hash marks when I re filled it. Been good ever since.


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## coconutgroves (Sep 23, 2013)

Ckirk57 said:


> My motor did that- ended up having to much engine oil. I changed the oil again and made sure it was on the lower end of the hash marks when I re filled it. Been good ever since.


Wow - haven't heard this one. Any idea why that would cause the problem?


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## CAPTAlexMoran (May 2, 2020)

Ckirk57 said:


> My motor did that- ended up having to much engine oil. I changed the oil again and made sure it was on the lower end of the hash marks when I re filled it. Been good ever since.


Really?? I checked the levels and they're right on the money. That's a new one to cross my bow. Maybe I'll look into that.


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## Ckirk57 (Sep 27, 2009)

CAPTAlexMoran said:


> Really?? I checked the levels and they're right on the money. That's a new one to cross my bow. Maybe I'll look into that.


i changed spark plugs several times and ignition coils. Ended up being to much oil.


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## BrownDog (Jun 22, 2019)

2 ideas:
Run it on muffs out of water (not under load) and see if it is still happening. You can also try a spark tester (not expensive) to see if you are not getting spark in a certain range to any of the plugs.
Run it off an external fuel tank to eliminate most fuel issues.


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## CAPTAlexMoran (May 2, 2020)

Thanks for all the input guys! I'm gonna put some muffs on and drink a beer or three and see how she feels. Seems like it's a little smoother once it's warmed up. Worst case scenario I call my mechanic buddy and cook him some dinner while he helps me out.


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## Frank Ucci (Jan 20, 2019)

Did you check that your spark plug boots are pushed down securely? Also, I once had a brand-new NGK spark plug on one of my motors that was bad from the factory. It took 3 visits to the mechanic to sort it out.


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## Ckirk57 (Sep 27, 2009)

coconutgroves said:


> Wow - haven't heard this one. Any idea why that would cause the problem?


Nope, but I changed spark plugs 3 times and new ignition coils. Only thing that made it better was less oil.


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## CAPTAlexMoran (May 2, 2020)

Frank Ucci said:


> Did you check that your spark plug boots are pushed down securely? Also, I once had a brand-new NGK spark plug on one of my motors that was bad from the factory. It took 3 visits to the mechanic to sort it out.


Maybe one went bad? The thing was purring like a kitten after I changed them and then all of a sudden not so much. I'm running all over east jesus looking for tarpon tomorrow with my fishing partner so I'll see what the damn thing decides to do.


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## tailchaser16 (Sep 23, 2008)

Did you put NGK plugs back in?


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## CAPTAlexMoran (May 2, 2020)

tailchaser16 said:


> Did you put NGK plugs back in?


I sure did. I swapped out the fuel filters as well. The shop I took it to tested everything except the injectors (they're too small for whatever the machine you use is) and they said it's all running perfectly (fuel pressure is good, fuel pump is right in the pocket etc etc). We popped a different prop on there and it got up better so there's that. I'm going to run the piss out of the boat tomorrow with a bottle or two of "sea foam" to clean the injectors tomorrow and see what happens.


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