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bog at low revs, 2K to 3K

Started by carlharri123, June 20, 2020, 06:42:59 AM

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carlharri123

Hi,

My vision bogs quite badly at the lower rev range roughly between 2-3K revs, if I open the throttle quickly it also bogs, I also have the stumble around 4.5-5.5K revs and low power in top gear, it barely pulls at all in 5th gear, I'm guessing I have a number of fueling and jetting issues?, and also one for luck when I start up with choke on it revs highly up to 4Krevs and its very touchy trying to regulate the rpm on choke either races or dies, does anybody have any suggestion on what I can do to address each problem?

My bike is a European model 81-82 year, it has an air box flapper that is actuated by air flow it does not have the pump and rod actuator.

I do know for sure that my charging circuit is working correctly, I have synced the carbs also.

thanks

Carl

Walt_M.

There could be more than one problem, first being an air leak. Then a leaking YICS and the ever popular dirty carbs. First thing I would do is cap off the YICS and see if it helps.
Whale oil beef hooked!

fret not

Cracked(leaking) carb boots, and worn butterfly shaft seals are well known issues, but EVERYTHING needs to function properly or you get disappointing results.  The bike is 38 years of age, materials succumb to time and the elements.  We will await your announcement of success and the issue(s) that it turned out to be.  Hopefully it turns out to be a simple fix.
Retired, on the downhill slide. . . . . . . . still feels like going uphill!

jefferson

Is this something new that has cropped up or the way it runs all the time? One of the tricks back when was to glue nickels to the flap to make it harder for it to rise with the airflow. As mentioned, start with the easy stuff and progress from there. Disconnect the yics and plug the ports. My bet is a very thorough carb cleaning is in order.

Rikugun

I read some of your previous posts and didn't see if you had checked the valve clearances. If not, I'd check compression before and after doing so to confirm the top end is healthy. No amount of carb work will cure leaky valves. I think you mentioned the bike had sat in a container for 10 years so maybe there is a valve that's hanging and/or stuck rings?  Also, you noted the tank was rotted. Even with a fuel filter, fine rust particles will get through so clogged carbs are of course a concern.

A simple, low cost test to determine where any vacuum leaks exist is helpful. Get a can of carb cleaner spray and direct the stream around the various intake bits at various RPM's  to note changes in RPMs.  Eliminate the YICS and plug of the connection points first then spray around the intake boots, vacuum lines, throttle shaft ends, etc.
It is far better to grasp the universe as it really is then to persist in delusion, however satisfying or reassuring.  Carl Sagan

carlharri123

Thanks for the helpful replies, I have now fitted the mod kit for the rod actuated flapper, this also involved change the a rear carb jet and the front main jet, this has helped significantly with the stumble it now pulls nicely through the revs in most gears, although it is still weak in top gear above 70 mph, referring to a previous reply, I'm sure its not a piston ring issue as both top ends where dismantled, inspected and new gaskets fitted, however I have not yet done any compression testing and have not checked valve clearance, I still have an issue where I can set the carbs up nicely and then go for a 20 minute ride and it audibly appears to start misfiring on one cylinder, I think its the rear and then the idle goes bad and the bike will cut out, I can start it and re tune the carbs and it will run okay for another short run then it goes bad again, does anyone know where I can get hold of a valve lifter tool from?

jefferson

One thing that will limit top end is the proper vent line placement. Your situation sounds a little severe for this, but it is worth checking. The vent line or lines from the carbs has to be run down all the way to the crossmember under the battery and inside the hole in the top. If there aren't any vent lines there or they aren't routed to that hollow section crossmember the jetting can go rich and you will lose power and speed. This is due to positive pressure in the float bowls that push more fuel through the jets.

Walt_M.

Hmmm, a cylinder is cutting out. Couldn't be the, wait for it, rev limiter, could it?
Whale oil beef hooked!

injuhneer

#8
YICS related air leaks (any part: port, host, chamber) seem to exaggerate the stumble (fueling drop-off). I recently got to ride my Vision for more than a few minutes. I had capped the YICS ports until I decide what to do with them.

While riding one of the caps popped off and two things happened. The low throttle behavior became just the same as what was described and the engine would die at idle without a bit of choke or throttle help.

I suppose other similar vacuum leaks would cause the same problem. In my case it was YICS ports uncapped because the cap had popped off and because all the intake parts are new on my engine I am certain there were no other vacuum leaks.

- Mike O
1982 Yamaha XZ550RJ