• Welcome to the Two Wheeled Texans community! Feel free to hang out and lurk as long as you like. However, we would like to encourage you to register so that you can join the community and use the numerous features on the site. After registering, don't forget to post up an introduction!

V-Twin Mikuni carbs bad throttle response

Joined
Sep 2, 2007
Messages
1,141
Reaction score
0
Location
Van Alstyne
First Name
Rob
Last Name
Windsor
Before you say "tear it apart and clean jets and needles", I need to point out that the carbs were freshly rebuilt by "WiredGeorge" down by San Antonio.

Hello, I'm an old member, coming back into the fold, "last visited 3 May 2011" up in the corner right now. Been ... *ahem* "busy" the past seven years. :zen:

Anyway, believe it or not, this is my (new-ish) wife's bike. I unloaded wife 1.0 in 2008, married wife 2.0 in 2012.

1995 Yamaha Virago 535, V-twin, pair of Mikuni carbs in downdraft configuration.

As I mentioned above, the carbs were pulled and sent off to SoTX for a rebuild. About a month ago I got them installed, got the system primed up and actually had a running motorcycle. It was a nice rumbly purr you'd expect from a V-Twin. Seemed to me that the bench-tuning was rather nice.

Noticed a leak in the fuel pump, coming out the body. :giveup:

Sourced a replacement, took a couple of weeks to get it in and installed. Went to fire it up and the front carb started venting all sorts of fuel. Coming out the intake horn (yeah, clamp not tight enough) and out of the emissions control goodies that are connected to the float bowl vent nipple.

Pulled the float bowl cover off and tickled the float and needle while running the fuel pump (small bursts) to flush out any gunk that was in there, all good.

Pulled spark plug and turned it over a few times to chug out any fuel that may have pooled in the cylinder.

Re-assembled necessary components and fired it up.

Horrible throttle response. Seems to me that one of the sliders is sticking and we'll go ahead and blame the front one since this seems to be the problematic carb.

What are the odds I got fuel on the dry (wrong) side of the carb, where the diaphram resides?

Any tips or should I just fire it up and try to "blow out the cobwebs"?
 
Last edited:
Before you say "tear it apart and clean jets and needles", I need to point out that the carbs were freshly rebuilt by "WiredGeorge" down by San Antonio.

Hello, I'm an old member, coming back into the fold, "last visited 3 May 2011" up in the corner right now. Been ... *ahem* "busy" the past seven years. :zen:

Anyway, believe it or not, this is my (new-ish) wife's bike. I unloaded wife 1.0 in 2008, married wife 2.0 in 2012.

1995 Yamaha Virago 535, V-twin, pair of Mikuni carbs in downdraft configuration.

As I mentioned above, the carbs were pulled and sent off to SoTX for a rebuild. About a month ago I got them installed, got the system primed up and actually had a running motorcycle. It was a nice rumbly purr you'd expect from a V-Twin. Seemed to me that the bench-tuning was rather nice.

Noticed a leak in the fuel pump, coming out the body. :giveup:

Sourced a replacement, took a couple of weeks to get it in and installed. Went to fire it up and the front carb started venting all sorts of fuel. Coming out the intake horn (yeah, clamp not tight enough) and out of the emissions control goodies that are connected to the float bowl vent nipple.

Pulled the float bowl cover off and tickled the float and needle while running the fuel pump (small bursts) to flush out any gunk that was in there, all good.

Pulled spark plug and turned it over a few times to chug out any fuel that may have pooled in the cylinder.

Re-assembled necessary components and fired it up.

Horrible throttle response. Seems to me that one of the sliders is sticking and we'll go ahead and blame the front one since this seems to be the problematic carb.

What are the odds I got fuel on the dry (wrong) side of the carb, where the diaphram resides?

Any tips or should I just fire it up and try to "blow out the cobwebs"?
So it sat 3~ 4 weeks or more non running with pump gas? If it was not treated, and sometimes even if it is, starting at the 3~4 week range is where I commonly see jets and passages plugged up partially to completely due to gas going bad.

The slide sticking?? possible though not common on those carbs, not super picky with cap tightening and binding the slide.
No, gas on the top side of the diaphragm is not likely, like, unless the bike was on its side or upside down not likely.
Though I have seen slides get gummed up when gas was in the carb bore and then dried up leaving behind the same nasty deposits as it does in the float bowl.
Sorry, but I'm gona say, carbs need to come out and be cleaned.
 
Last edited:
Looking at the calendar...

It was two weeks exactly. Had it purring the weekend before the Wanenmacher show in Tulsa, running like crap the weekend after.

If a carb gums up in just two weeks of sitting, doom and despair for the owner. :doh:
 
Wow, this may be hard to take/hear, buuuuuuut, as a long time mechanic the minute someone say "it cant be that" that's the first thing I check. Add to that someone else has "touched it" and with the symptoms you mention.....Its the carb(s).

I'm working on a Kawasaki VN125 eliminator right now that's been sitting for 5 years. I have a heated ultra sonic cleaner. The carb appears to look brand new after cooking. However, the unit will not idle properly. The idle circuit passaged are microscopically small and almost impossible to get clean. Buuuuut, it wont idle and it has all of the symptoms of running to lean at an idle. Point being, it may look clean, someone may have "rebuilt" them, but if it quacks like a duck..........
 
Yes, I'm aware that I'm in the "denial" phase of grief. ;)
 
Something I do on the '86 Wing (four carbs) to keep the carbs clean with the Ethoncrap, is to add 1 oz. of cheap ATF per gallon of gas. Appears to do 2 things, help lube the rubber and cleans the carbs with use. 3 or 4 years ago I'd had them rebuilt twice inside of a year, the last fellow that rebuilt them suggested this trick. Haven't had a problem yet, knock on wood :)
 
No, gas on the top side of the diaphragm is not likely, like, unless the bike was on its side or upside down not likely.

This is a Virago, carbs are in downdraft arrangement. One big difference in these is the jets are contained in a "jet body" that is removable once you gain access to the float and remove it. The V-Max uses much the same, just four of them instead of two.

Regarding the diaphragm, the "carbs" you see in the starboard-side photos are the diaphragm cover plates:
https://en.wikipedia.org/wiki/Yamaha_XV535#/media/File:2014_Yamaha_XV_535_Virago.JPG

All that said, you were correct -- no fuel in the diaphragm pieces.

I got the carbs out (again!) and checked a few things. One of the choke plungers was missing the spring (grr.). Aside from that, everything looked fine. I'll remove the pilot jet and clean it up and try again (with stabilized fuel).

Looks like for future jet work, I don't need to completely remove the carbs from the bike.
 
Something I do on the '86 Wing (four carbs) to keep the carbs clean with the Ethoncrap, is to add 1 oz. of cheap ATF per gallon of gas. Appears to do 2 things, help lube the rubber and cleans the carbs with use. 3 or 4 years ago I'd had them rebuilt twice inside of a year, the last fellow that rebuilt them suggested this trick. Haven't had a problem yet, knock on wood :)

I'll probably end up doing that. :giveup:
 
carb tbl. you say it's the front carb giving trouble? have you done the "pull the plug wire while running" make sure the front cyl is ok not ok? swap out the ign coils and retest? plug not fouled. plug wire broken, spade clips loose on coils, if running good before the fuel pump repair, what happened?
carbs are so simple, and dang near tbl free I would eliminate other things before tearing into carb. Oh yeah, do you need to sync the carbs? good luck
 
I've squirted fuel into each cylinder (independently) and gotten it to fire a couple of pumps, so I'm pretty confident it isn't the ignition system.

I'm guessing I got some crap in the fuel downstream of the filter, might have even been buried in the fuel pump (leftover from the manufacturing process?).

That would explain why I suddenly had problems with the float bowl needle, immediately after replacing pump.

The way this bike is set up (from factory): tank -> fuel filter -> pump -> rear carb -> front carb
 
Last edited:
Yeah, I figured out what happened.

Or, more like, what happened slapped me in the face until I finally saw it.

Pulled the jet bodies out and let them have a soak in mineral spirits until I could deal with them. Yesterday I pulled them out and extracted the jets and hit the jets with compressed air. Suddenly my hand is all wet from mineral spirits (expected) and gritty (unexpected). Same happened when I ran the air hose over the jet bodies.

That was my ah-ha moment.

I went back to the carbs and took a closer look in the float bowl pockets. Sure enough, there was *SAND* piled up into the corners.

Some jackwagon in the supply chain packed the fuel pump full of sand. :brainsnap :angryfire

I'm going to bag up what's piled up on the workbench and have a calm cool chat with the manager in charge of parts. I don't expect any compensation, I just want to convey what happened and bring home the point that this is a good way of losing customers. If their guys didn't do it, then their supplier did it. Either way they get the credit for this misdeed.
 
Check the float bowls for debris from the tank. Also drain the gas in the carbs into a clear glass and look to see if there is any water in the bowl. Make sure you put a gas filter into the line coming from the gas tank. Carbs will not run on water, debris will clog passages inside the carb body. If the carbs came as a original equipment item, then search about and find out what jets the carbs came equipped with, and what setting the needle was set at. Usually you will find that the middle position on the needle will be a good place to start. Check to see if any of the rubber carb mounts are leaking, a air leak will really foul up how a engine will run. Check to see what the manufactuer suggest as the proper amount of turns out for idle and air are and start there for tuning those two settings. Good luck
 
Already a fuel filter in place.

Cleaned out the float bowls on Saturday (before post #11), that's where I found the sand.

Tank was emptied, cleaned, and fresh fuel put in at the beginning of this project. Did it again over the weekend.
 
Just hope anything I wrote helps in some way. I tuned and repaired Mikuni's as a racer and mechanic for years. one more suggestion, if you ever power wash a motorcycle carb always drain the water from the carbs float bowl before you try to start the motor in order to save you some hair pulling later. If provided the brass plug on the bottom of the float bowl will let out the water and other junk that you might find.
:rider:
 
These are downdrafts with jet body. As soon as you gain access to the jet body, you've drained the float bowl.

Ever touched the Mikuni carbs on the V-Max? Same setup as those.
 
Back
Top