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C40 TLRX Issue

B

BAS

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C40TRLX Yamaha
125/124/125 comp
Carbs rebuilt, new trigger coil, charge coil, rectifier, CDI, fuel pump and filters, lines, clamps
Spark jump @ 10mm
Sparkplugs look as they should, no tell tales. Gapped at .10mm per spec
Timing at 7deg AFTC Idle, 27deg BTDC WOT. Tested for cylinder drop by running thru the range while shooting timing light looking for drop. all good
running ethanol free only high test

Motor will start and run thru 3500 rpm but tops out WOT@ 3800 rpm, spec is 4500-5000. Sluggish hole shot but not bogged down just not what it should be.
Rebuilt ignition system after stock CDI gave out, prior to that motor ran well hitting 4600 RPM, no changes to prop or damage to prop (Used test prop in same size and results the same) I should note I have a known good CDI from a test motor and results are the same
When running at 3500-3800 rpm it sounds and feels somewhat rough but not that bad, had I not known the engine I might think thats normal. Tested tach by running shop tach with same results.
I do not believe this is a fuel issue... during rebuild I ran a fiber camera to all ports we could get into and verified clean ports and I set floats to spec, set idle screws at 1.25 turns and tuned with inductive tach. Engine is also emitting a wet soot from exhaust.
My Question: All DVA readings check out as per CDI, however, when testing CDI unit (Black wires with W/Y/O stripes) I put the positive lead in my test harness and negative to the coil ground as per Yamaha Factory Manual, when I go to touch the ground a light but faintly audible spark jumps from the lead to the ground screw. Im assuming this is indicative of a loose ground, live wire making contact somewhere on the harness the coils and starter attach to or a bad coil. That said all coils in continuity as per CDI. Before I tear this apart (Again) looking for the culprit, is it normal to have a small electrical discharge when testing DVA on CDI? Only on CDI, does not happen when testing other ignition comps. (I did fire up at night looking for dancing spark, clean. also ran a screwdriver close to block and coils hoping to see a spark jump but did not.)
One other thing I noticed, when at idle I put a timing light on each cylinder, when testing #3 (Yamaha marked TDC for each cyl on the flywheel) I notice a very quick and intermittent fire to the #2 cyl, it is very fast, almost unnoticeable, if It wasnt dark and I had my readers on I may have missed it) Thanks!
 
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Just tested some grounds while the starter ground wire to the rest of the block and grounds for the coils tested for continuity I am perplexed by one thing. I tested the battery with a multimeter + to - and got 12.68v. When I tested positive post of the battery to the ground wire that is attached to the starter it read 5.97v. Shouldn't this read the same or very near the same as the battery test?
 
Was the starer under load when testing? If so you've discovered a major volt drop. If not tehn i reckon the neg post of starter is bad or going bad bc if it were truly grounded you'd read 12.68 or lil less.

Were you getting 4600 with both test prop and standard prop, or just test prop?
 
First the starter issue. Dumb dumb mistake! I was reading off the acc battery which is not attached to the motor... its hot in Louisiana and I think its affecting my judgement! Lol. When using the correct battery everything is good as far as the starter/block readings go. That said, yes both props tested prior to the engine’s current issue at WOT of 4600 RPM. Prior to this issue nothing in the lower unit or mechanicals have changed. Just the ignition and fuel system changes and verifications as list in op
 
I've always proceeded with troubleshooting by the advice given to me by a yam master mechanic...when encountering an engine issue, 90% of the time it's fuel system related. This method has always worked for me. I'd start from fuel tank amd work way back to carbs, checking for restrictions pre-fuel pump and adequate pressure post-pump. Be sure jets are clear, as well as time amd sync after reinstalling. Check that throttle arm, linkage, lever, etc, are dialed in to spec. Refer to service manual.
 
Im in the same camp that fuel is often the issue. I did recheck all the fuel systems short of pulling the carbs again and checking them and the reed valves. That said, I ran last night in the dark and after about 5 minutes of running the #2 ignition coil is making a very small spark right in the crux where the plug wire meets the coil. It is a very faint spark but its there. I tried resettigg the wire and it didnt help. Hopefully this is the issue and I can get on with my life. Will report when new coil is installed
 
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