New 8hp Yamaha enduro runs poor

[belizesailor]

Fuel oil mix ratio can affect fouling of plugs. Given the symptom sounds like a fouled plug to me. Pull, clean, gap properly and see if that fixes it.

I would not risk running at leaner oil mixes. Enduros all over Central America run just fine at 50:1. Ive owned several from 15-60 hp and all ran on 50:1 just fine.

[hpeer]

Just to be clear

I replaced the plugs, they ran fine but then started missing a bit. But it didn’t solve the problem even when brand new.

I’m still in the “break in” period. 10 hours of 25:1 oil mix. I’m not deviating from that. I reread the manual, it does say 50:1 thereafter. My other 8hp Yamaha, not Enduro, was 100:1. I always ran that around 75:1. So anyway it appears the mix is different for different models of 8hp. Also makes it unlikely the mix is the problem.

It’s running on clean fresh fuel.

I took the carb off and cleaned it. But really there was nothing to clean. I had a carb problem with my Honda 2000 and I cleaned that carb and it works fine since. The discoloration from the varnish was noticeable on the Honda. NONE noted here.

Right now I’m in Mayaguana, Bahamas. Population 250ish, total in three “towns.” Looks like I’ll be here close to a week before getting a weather window. So it will be some time before I get to a mechanic to look at it. Annoying.

I think it feels a bit rough at the high end. Not 100% positive. I’m betting it’s not hitting it’s designed top end. The manual says mid throttle is 3,000rpm and 3/4 throttle is 4,000rpm. To my uncalibrated ear I’m not getting that. I’m guessing 3,500rpm tops.

Is it possible there is just that much difference in power between the two Yamaha motors? One Enduro and one not?

[hpeer]

Just to be clear

I replaced the plugs, they ran fine but then started missing a bit. But it didn’t solve the problem even when brand new.

I’m still in the “break in” period. 10 hours of 25:1 oil mix. I’m not deviating from that. I reread the manual, it does say 50:1 thereafter. My other 8hp Yamaha, not Enduro, was 100:1. I always ran that around 75:1. So anyway it appears the mix is different for different models of 8hp. Also makes it unlikely the mix is the problem.

It’s running on clean fresh fuel.

I took the carb off and cleaned it. But really there was nothing to clean. I had a carb problem with my Honda 2000 and I cleaned that carb and it works fine since. The discoloration from the varnish was noticeable on the Honda. NONE noted here.

Right now I’m in Mayaguana, Bahamas. Population 250ish, total in three “towns.” Looks like I’ll be here close to a week before getting a weather window. So it will be some time before I get to a mechanic to look at it. Annoying.

I think it feels a bit rough at the high end. Not 100% positive. I’m betting it’s not hitting it’s designed top end. The manual says mid throttle is 3,000rpm and 3/4 throttle is 4,000rpm. To my uncalibrated ear I’m not getting that. I’m guessing 3,500rpm tops.

Is it possible there is just that much difference in power between the two Yamaha motors? One Enduro and one not?

[Sailmonkey]

Have you checked to be sure the gap is correct on both coils? It’s possible that one cylinder is getting a weak spark.

[sandiegoskipper]

Sounds to me like it's running too rich on the high speed jet. If so both plug tips will be black and wet. The only way to correct an over-rich high speed condition is to go with a slightly smaller high speed jet. Keep doing what you're doing until you can adjust the mix to 50:1. If it is still burbling at high R's lean it one one jet size and try again. If one plug color is different from the other, then you likely have a plug wire or coil issue.

On other note - make sure you vary the throttle a lot during the break-in period rather than running at a constant RPM.

Skip

[hpeer]

Yes I’m doing that, varying the throttle a lot.

I wish I could get a parts or service manual for this thing. I can’t even find the correct users manual online. It’s an E8D. All the manuals I see, including the one I have are very generic, don’t always have the right prop pictured for instance. That’s how I got Cove nurses over the mid, online 8ho 2 strike manual says 100:1, but paper manual that came with it says 50:1.

[hpeer]

Just an update, not a lot to add.

The engine is now past the break-in period, I’m running it on 50-1 mix as prescribed.

It is very marginally better. I have cleaned up the plugs, they both look about the same. They are black but not terrible. No deposits or build up.

The engine still misses at low speed then the second cylinder cuts in. To my ear it still feels a bit rough. It starts easily.

Although slightly better it is still VERY weak compared to the old 8hp motor I took off.

I’m suspecting the power pack. Hard to imagine what else could be wrong.

[Mike Banks]

It sounds like one of two things--faulty ignition or some problem with the reed valves--which are located behind the carburettor. Something preventing a reed valve from opening (sticky varnish stuff) or closing (something stuck on the mating surfaces) will cause the problems you mention. I had a five horsepower single pot Yamaha--it was the reed valves sticking in my case. Cleaned them--completely new fuel and replaced the fuel lines just for good measure--and away it went good as new.

[hpeer]

After writing the above I was able to find a ignition coil assemble (power pack) on line at Amazon for <$50 shipping included. I won’t be able to receive it until I make Puerto Rico, but for that one price I’ll just do it. This looks to be the easiest thing to do, and that part is used on a whole bunch of different machines.

When I pulled the carb out I found no signs of varnish what-so-ever.

[sandiegoskipper]

First - go with new plugs. Second - make sure the plug caps are super tight to the plug cap. Third, look for anywhere the lug wires may be pinched against metal in their routing and correct. Fourth - test.

Skip

[a64pilot]

I have had some weird plug problems myself.
My Husqvarana ran like a scalded dog but once it got hot, if I ran it hard it would just suddenly shut down, not miss, but act as if the kill switch was pushed, it would usually come right back onto power in a second or two.
I first replaced the fuel pump as it was cheap, was going for the ECU next , but changed the plug on a lark.
It was the plug. Never seen that before or since, would start perfectly and run beautifully, until you ran it hard, then it would die. Sounds like a coil, but it was the plug, and the plug looked perfect

[sandiegoskipper]

The weakest link in the ignition system is between the plug cap and the plug. If the metal-to-metal contact there is at all loose, the vibration creates added resistance and the spark quality is greatly diminished - but also, the coil has to push harder and can overheat. The second weakest item is the plug itself, essentially a piece of glass glued to metal expected to function in an extreme environment. The porcelain around the electrode can crack (impossible to see) and occasionally, the spark will jump through this crack direct to ground (the cylinder head). High heat and vibration will exacerbate this phenomenon. What you are describing on the Husky sounds like an overheating ignition coil - likely cased by a loose plug cap. Suggest you start there.

Skip

[a64pilot]

I fixed the Husky, it was sold some time ago.
It was the plug. I posted it to try to illustrate that a plug can be problematic, and they are easy and cheap to replace. If it’s not the plugs, well then you have a spare set, which you need to have anyway

[hpeer]

Originally Posted by sandiegoskipper

First - go with new plugs. Second - make sure the plug caps are super tight to the plug cap. Third, look for anywhere the lug wires may be pinched against metal in their routing and correct. Fourth - test.

Skip

My frustration is I have done all this.

[a64pilot]

It can be reed valves, and it can be crankcase seals, or the ECU.
Coils usually starting acting up only after they get hot, usually.
It’s new, but you can’t get to a dealer? I guess maybe officially report the issue to get it on record and then get to a dealer when you can.

On edit, I believe it can be carburation also, most newer carbs don’t have a choke, they have fuel enrichment and I have seen that cause very rich low RPM operation and what we used to call four stroking, or missing every other stroke.
If it’s the enrichment circuit, then operation at higher throttle it will clear up as the circuit is only effective at low throttle operation.

This is pure speculation of course