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Discussion Starter · #1 ·
Hello everyone, I maintain an older 1982 yamaha 550. Im an auto mechanic Iv'e worked on anything with an engine for decades, I have expierence with engines, tuning rebuilding etc.. however I have an issue that just will not change in any way.

Background info: I'm just the mechanic that maintains this machine, its a family members and has always ran like a top...engine is OEM, new ngk plugs with propper gap, new valve cover gasket, clutch pack and oil(this year). She ran great up until last year when I noticed it had a slightly high idle that could not be adjusted out
(oem idle is recomended at 1200, prior to last year I had it running smoothly at 1100rpm, just sounded better and still ran great). I told him that the "pilot circuit" seems to be a little clogged because thats what it idles off of when throttle plate is closed.So he dropped it off last winter, and
I did a full carb rebuild, before I was done, he contacted me and aske if I would want to paint it, I said sure. So after tckling the carbs(already had them laid out on the bench) I pulled the engine off set it aside and stripped everything off the frame, painted it and re assembledharness coils engine etc..got the tank and side panels painted, set them aside. Installed carb rack and started to adjust the "pilot air mixture"and planned on moving to the vacume guage sync after,

however here is the issue:
Carbs have new OEM:
Needle/seats, main/pilot/enrichment jets,bowl gaskets, shaft seals,vacume diaphrams, fuel bowl link tube seals, bowl screws, upper vacume top hat bolts, proper OEM float heights set, air mixture screws set at recomended starting point at 2 3/4 turns out. brand new carb intake boots, brand new rubber vacume port caps. actual issue. will not start without using the enrichment circuit when warm, and will not stay running, and when forced to will never come down from around 2800 to 3800 rpm. adjusted the main idle center screw(adjusts all throttle plates at same time) to lower this, but its damn near closed as is.

Now Im used to these issues all the time, checked for any vacume leaks/Passed
Higher "floating" rpm tells me its either drawing fuel from somewhere it should not be, or vacum leak somewhere. Carb have been off multiple times an gone thru several time since to confirm jet sizes(correct), compressed air will blow thru all circuits individually, checked for any cracked carb bases etc... I have worked on anything from older prop plane engines to diesel(12v cummings), to car/truck engines, to scooters, harley's, gas RC, nitro RC, mowers,weed eaters etc... but for the life of me I can not get this thing to even stay running long enough to adjust any mixture or sync etc... I have exhausted all my contacts and have confirmed all thier thought/recomendations to pass, Boots not leaking, used a propane torch( gas only, not lit) around the boots/carbs while at this 3k rpm, no rpm increas so no vacume leaks there.

I have Never had anything I touched not be able to be fix or confirm as "bad/faulty" I almost feel embarrased... but at this point I am willing to be laughed at..lol:)
anyone have any thoughts?
 

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Have you checked the linkage and cables? Only thought that my brain is spitting out is off with the carbs, take them apart and start over.
 

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Can the timing be advanced too much on that engine?

Is there any possibility you have one or more carburetor slides inserted, 180 degrees rotated from where it should be?
... in a regular Mikuni type carburetor that can happen and will definitely cause the exact symptom you described, high idle and the idle adjuster screw will not function.
 

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Discussion Starter · #5 ·
@canadianguy theotle cable linkage has slight slack(connection to carb rack is slightly dangling) passed

@anybike Floats were tested in a clear bucket of water before first final assembly, and all tested to float aproximately 2mm within each other. Passed


@Trials The upper rubber diaphrams have a "bump out, that aligns the slides, and made sure the vent hole is aligned with the intake side per manual, The slide bores were polished by hand with mothers aluminum pollish(cleaned polish residue out and blew compressed air into all cavitys to ensure no polish dried in any holes.
 
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