# Diagnostic fun - Yamaha 30hp 2 stroke



## lemaymiami (Feb 9, 2007)

Did you re-build the carbs - and have you tried (before anything else....) hooking up a known good fuel tank and line to see if you've got fuel troubles (the boat end of things..). Most pros I know will do that as a first step to make sure you have a "motor problem" and not a "boat problem"... Hope this helps and must admit - don't think I'd ever want to attempt to re-build a carburetor without a pro at my elbow to take me through it step by step...

"Aren't boats fun?"


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## bryson (Jun 22, 2015)

Yes, first test run was a brand new tank and short pre-made fuel line. Then the tank was installed with a longer (also new) fuel line and a filter/water separator. No noticeable difference between the two setups.

The carbs were "rebuilt" in that all available passageways were cleaned and a rebuild kit was installed, which really only replaces gaskets and float needles. Jets and nozzles were not replaced. Pilot screws adjusted per the manual.


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## Smackdaddy53 (Dec 31, 2012)

You need to pull the carbs and do a proper rebuild. That entails pulling the jets and cleaning them. Jusf a little varnish or buildup will cause the engine to run like crap. That’s the heart. 
Get a gallon of ChemDip and soak all metal parts in it for 30-45 minutes then spray them with carb cleaner and reinstall all seals and gaskets. Carb kits are cheap so use Yamaha, no aftermarket junk to save $10...ask how I know!


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## LowHydrogen (Dec 31, 2015)

Seen this before, had one of these motors for a LOOOONG time. I won't bet the farm on it but I'd bet my next paycheck.

If it's not needle and seats flooding the carb (I'm guessing it's not if you just checked all that)....

CYL 1 & 2 are still firing but way rich that's why they're black, CYL 3 is already fouled, that's why it's wet (not burning the fuel anymore).

You air idle mix (pilot) screw is likely too far out (OR) they were over tightened in the past and when they are out to spec they are still dumping too much fuel due to flared seat and dished needle (they're brass).

Run them all the way in then back them out in 1/2 turn increments until it'll run. Run it for a couple min then pull and read the plugs. Continue until you get a combination of proper running and correct color on the plugs. Make note of how many turns this is because your motor will be unique to this setting and any time you take it in for a carb clean they're going to screw it up because they're going to set it to factory setting.


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## LowHydrogen (Dec 31, 2015)

Forgot to mention you should start that ^^ process with fresh plugs. I recommend NGK B7HS

Good luck let us know what you figure out.

Edit to add: if I'm right you're going to have to back the idle way down once the idle mix screw is adjusted correctly. It was idling low like you described because you were drowning the plugs but not giving them any air (throttle blades closed w/ too much fuel). 

Once the screw is right it'll be idling leaner and trying to rev up, if so you'll know you're on the right track.


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## bryson (Jun 22, 2015)

@Smackdaddy53 We didn't pull the jets out initially since we didn't want to risk stripping any threads or breaking anything off, but they were cleaned while we had the carbs apart, and it seemed like everything was alright. The Yamaha "carb kit" was surprisingly light -- basically just new floats and needles. New jets are $30 per carb. New pilot screws are $20 per carb. I don't want to throw $150 worth of parts at it unless I know it's the problem, so hopefully I can reuse most of what I have after I disassemble it a bit more (or hopefully I find something that's clearly the problem). The fact that she's running rich rather than lean was making me look away from the fuel pathways, though there may be something in one of the vacuum ports or something.

@LowHydrogen That's a really good point -- I just set them to factory and never messed with them. I didn't even think to check if there's a port for a manometer to balance them all out, but checking the plugs should get me close.

Thanks guys, I feel like there have to be at least a few things I've overlooked, since my brain has felt like mashed potatoes for the last month. On a side note -- I didn't pull the reeds and look at the crank case. Do the intake ports stay separate all the way from the carbs to each cylinder, or do they mix together in the crankcase? Just trying to determine if a lean/rich condition in one cylinder is absolutely caused by the corresponding carb, or will extra fuel/air from one carb make it (even if only a little bit) to the other cylinders? I was thinking that all seem to be running rich, and if the intake opens up into a single "chamber" in the crankcase, cyl 3 could be the worst simply because it's at the bottom.


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## devrep (Feb 22, 2009)

Smackdaddy has it, full disassembly of carbs, everything into chemdip, blow compressed air thru all passages and jets. you can hold the jets up to light and see if they are clear. btw there is the good old style toxic smelling chemdip and the new eco friendly calif compliant chemdip. guess which one works?


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## JC Designs (Apr 5, 2020)

All great advise so far, and I will confirm @Smackdaddy53 and @LowHydrogen on their assessments! I will also add that I wouldn’t rule out the fuel pump diaphram, “I don’t remember which cylinder drives tge fuel pump” but you could have torn it during reassembly. In your situation, I would start with a proper rebuild of the carbs, factory sync n link, and factory air idle settings “I’ll post them for you later straight from my pocket guide”. And if problem persists, buy a new fuel pump after verifying proper spark.


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## bryson (Jun 22, 2015)

@Smackdaddy53 @LowHydrogen @JC Designs @devrep

Okay -- finally got some time to get back on this thing. Rundown and recap, plus my thoughts:

Played with the pilot screws, it affected the idle some but didn't seem to lean it out much. The needles look good -- much better than I expected, actually. Noticed the air bleed (and bowl vent) circuit seemed like it might be a little clogged. Pulled the carbs back apart, Chem-dip soak, the whole nine. They came out beautiful, all passages nice and clean and free-flowing. Set everything to factory settings and put new plugs in and... pretty much the same deal, a little smoother out of the gate. Adjusted the linkages for the oil pump thinking it might just be running oil-rich. No change.

At this point, I'm ruling out the carbs. I've also put 2 brand new fuel pumps in thinking there might have been a leak allowing fuel to get sucked into the crank case, no change.

So, I feel confident that the following are not the culprit: air, fuel, oil, and compression. That leaves ignition, and I guess possibly reed valves. Visual inspection of the reeds are okay, and the engine starts relatively easily, it just runs badly. I will remove them to check that they meet specs, but I don't have a lot of faith in that. Also, I feel like a bad reed valve would cause issues primarily with one cylinder (assuming each cylinder is fed strictly by the associated carb).

I am getting spark, but I haven't verified the "strength" of the spark yet. My tester just has a light, but the manual specifies that the spark needs to be able to jump a 9mm gap. I'll probably pick up that style of tester tomorrow. One thing that I noticed was that the *resistance across the charge coil was 508 ohms*. I don't know what my particular engine should be, but the service manual for my buddy's similar Yamaha 60hp specifies 160-240 ohms (or something around there). I'm not too familiar with small engine ignition systems, so I'm not quite sure how to interpret this info. Could this potentially cause a weak spark? *Does anyone know what the resistance across the coil should be? *The pulser coils and the lighting coil seemed to read very closely to what the 60hp manual suggests, so that makes me think my charge coil should as well.

I also highly doubt that it would be the ignition coils or the pulser coils, since the problem seems to be across all 3 cylinders. I don't know much about the CDI box, but I feel like when they go bad they typically just stop working, not necessarily give out a poor signal. I have some "ballpark" numbers from my buddy's 60hp manual for CDI resistance values, but I don't know how helpful that will be for me.

Would love to hear opinions on this. I may have a local shop test some of the ignition components for me, rather than buying the equipment I would need to test "peak voltage" like the manual recommends. The slow decrease in performance initially seemed like a physical clog that was accumulating crap and creating more of a blockage, but I feel like all mechanical passageways have been addressed. It could also be electrical, even though it doesn't seem to be heat-dependent like electrical issues can be sometimes. It's also not intermittent, and it's not an on/off thing, it's just running poorly in general. The plugs do come out looking wet, and the residue left in the "idle bucket" is outrageous. My guess is that raw fuel is making it through the cylinders and basically cleaning all the soot off the exhaust manifold. It looks like I spray-painted the grass around the bottom of the bucket after letting the motor idle for 5-10 min.

Sorry for the novel, just wanted to get my thoughts laid out.


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## JC Designs (Apr 5, 2020)

I’ll text you the page from my factory pocket guide later. Or, drag it down here and I’ll diagnose no charge and we’ll go catch some fish!🤙🏻


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## bryson (Jun 22, 2015)

JC Designs said:


> I’ll text you the page from my factory pocket guide later. Or, drag it down here and I’ll diagnose no charge and we’ll go catch some fish!🤙🏻


Sounds good -- I'll send you my number, can't remember if you have it or not. I'm really surprised my FSM doesn't include anything other than peak voltage output.


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## JC Designs (Apr 5, 2020)

Texted ya and think I answered your question without seeing this lol!🤙🏻


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