Yam 70hp 2 stroke Bogging/Cutting out mid RPM

[mitch184]

Buddy just bought an Arima with a '95 70hp 2 stroke. On the first run, it was bogging real bad and cutting out around 3000 rpm. If you really feather the throttle and stick with it, it'll eventually plane out and go WOT of around 5200. So I don't believe the prop is to big.

We did all new fuel pump, fuel filter assembly, water/fuel separator and mount (the fuel line diameter is slightly larger ID now), plugs, thermostat and adjusted the pilot screws on carbs back to nominal.

Went out yesterday, and the motor just falls flat on its face around 3000-3400. Then, got frustrated and just pinned it WOT for about 15 seconds and it finally got after it, planed out and we were moving 5000 rpm. However, after running, and idle it would sneeze, snort and cough and eventually die. Thinking it was too lean, adjusted the pilot screws out to richen it up and then it wouldn't ever get on plane. Adjust the screws leaner, and after WOT for 10 seconds it would go. Used that method for a couple hours, then trying to run home, it wouldn't even fire up. Tried for 2 hours while we came in on the kicker and it would never even fire up. Fuel bulb is solid, smells of fuel, but nothing.

I'm thinking possible its the CDI now? Any thoughts? It did have 1/2 tank of old gas in it before we topped it off. When we were able to get it on plane, it would ONLY run from 4600-5000. Anything less and it would just die.

Thanks

[99yam40]

need to start from scratch.
check compression, you may have killed the motor running it when you knew it had problems.
topping off a 1/2 tank of old bad gas means you have a full tank of bad gas.

if compression is OK them check for spark on all plugs with a spark tester
or use a induction timing light and see if it lights up and if timing is in spec

if no spark then test inputs and out puts of the CDI with the proper test equipment
make sure the kill switch clip is in place

[robert graham]

Have the carbs ever been removed, disassembled, cleaned and replaced?.....could be time

[mitch184]

Have the carbs ever been removed, disassembled, cleaned and replaced?.....could be time

I don't think so. That was also on the list of "next to do" items.

I guess getting an accurate compression test is probably the most important next. If that's bad, everything is doesn't really matter

[TownsendsFJR1300]

Have the carbs ever been removed, disassembled, cleaned and replaced?.....could be time

^^^ It is / was time. You have crap floating around your carbs, varnish inside.

Keep running it as such, you'll lean out the engine(already did) and very possibly cook that engine...

BTW, your getting 5200 @WOT does not indicate a bad (low compression) cylinder. If a cylinder wasn't "hitting", you wouldn't see that high an RPM.

[99yam40]

^^^ It is / was time. You have crap floating around your carbs, varnish inside.

Keep running it as such, you'll lean out the engine(already did) and very possibly cook that engine...

BTW, your getting 5200 @WOT does not indicate a bad (low compression) cylinder. If a cylinder wasn't "hitting", you wouldn't see that high an RPM.

seeing how it will not even start anymore, it is hard to say what all problems it has at this time

[mitch184]

Thanks for the input. Got a few things to check out and hopefully it's an easy fix. No more running it though.

[TownsendsFJR1300]

seeing how it will not even start anymore, it is hard to say what all problems it has at this time

We know good fuel was added to bad fuel, that needs to go. What ever was in the tank got mixed up and pumped into the engine (instead of just sitting on the bottom of the tank).

Simply draining the carbs will probably show some debris in there.

Spritzing a little bit of pre-mix into the carbs while cranking and seeing if it lights off momentarily will confirm fuel issues.

[zenoahphobic]

Also an indication that screwing out the adjusters to enrich does worse and conversely screwing them in brought the engine back to improvement does not indicate the carb, tank, lines etc are permanently blocked.
That is at the beginning anyway.
I fear you may have caused some damage, as implied by others, by continuing to run this engine.

[mitch184]

Thanks for all the help. We drained the fuel, did a quick clean on the carbs then did a shock treatment of sea foam. We also checked spark, compression and timing, which were all good. There was also a slight gas leak at one fitting, so we fixed that too.

Engines runs much much better now. There is a minor hesitation/bog around 3500 still, but everywhere else is smooth. I think the carbs just need a full rebuild at this point. Set the pilot screws back to nominal and it idles great.

[TownsendsFJR1300]

Thanks for all the help. We drained the fuel, did a quick clean on the carbs then did a shock treatment of sea foam. We also checked spark, compression and timing, which were all good. There was also a slight gas leak at one fitting, so we fixed that too.

Engines runs much much better now. There is a minor hesitation/bog around 3500 still, but everywhere else is smooth. I think the carbs just need a full rebuild at this point. Set the pilot screws back to nominal and it idles great.

Glad you made a bunch of progress, but a complete re-build is probably not necessary. Simply pulling the carbs apart and CLEANING ALL the jets should solve your issue.

Seafoam works ok, but if you don't want to pull the carbs apart, Yamaha's RingFree works MUCH better than Seafoam.

A SHOCK DOSE is 2 oz / 1 gallon. Six gallons is enough to use..