carb question

[John]

This weekend I experienced a proplem like I have never had before, therefore I have no idea what may have caused my problem. We were in the Ozark Mts in northern Arkansas, with an elevation of around 5-6000 ft. Before we had reached Harrison, where we spent the night, I had noticed that my bike was starting to run kinda sluggishly, and by the time we got there I was dragging a dead hole, but only at high RPM. It would idle smoothly and run well until the transition to the main jet circuit, then it ran rich. In the morning, I just cleaned the fouled plug, #4, and swapped it with #3 and it was fine...for a while. When it got bad again, I was thinking of some way to lean that carb out when it hit me...pull the vacuum cap off #4 carb boot. That worked. The bike ran much better, but not perfect. On the ride home, as we got to lower elevations, it started running lean with the vacuum cap off, so I put it back on. That helped. By the time we crossed back into Texas, it was like nothing had happened. So...why would just one carb go rich at altitude? The carbs have been on my bike for four years now. They are from a 78. Because of the pipes and air filter, I am running 140 mains and stock pilots. I just synched the carbs two weeks ago, and got them spot on. The plugs were brand new, I just put them in Thursday. Anybody got any ideas?

[geniusloerts]

-must be that thick air of Arkansas in the Ozarks(such high mountains and such)LOL J.K.

that seems very odd. did you check the plug gaps? but if #2 did it ven after you swapped the plugs i bet it wasent that. I have heard of blowing out sparks at higher RPM's, infact i have done it, but not on the XS (it was in a modified turbo car)


maybe the jet is loose or something like that.

[COOL TOM]

Carbs

The bike is use to the FLAT PLAINS of Texas, so it don't like to run on the hillside, tilts bike to side floods #4 carb. LOL Maybe junk in needle valve? Other than that I have no other idea what would cause one carb to run rich then run normal.

[egsols]

Yamaha specs for 80SG says 125 mains in 2 and 3 and 120 in 1 and 4. If you are running the 78 carbs with 140's wouldn't that be roughly 6-8 sizes above stock? (Unless you're running older heads)

I'm thinking with the exhaust/filter combo you would probably only need two to three sizes up like 125's and 130's. JAT

[DiverRay]

John,
Check the plug on the pilot tower and the main jet. I DID have a jet come loose on E.Liberties bike when I first set the carbs up for him. Ran just like you said.

[LoHo]

This is based on outboard motor experience, but might apply here:

That one carb is slightly richer than the others, but not so much that you notice. As the entire system was starved of oxygen, all the carbs moved toward richer, and the richest carb became rich enough to foul out. As you returned to normal altitude, they all returned to normal as well, all within an acceptable mixture range. The chance that all four carbs are set exactly the same is small, given the tools we use. You probably have a air/fuel mixture of, for example, 14.5-14.7-15.1-14.6, and all run fine under normal circumstances. But low-oxygen/altitude-richness would affect #3 the first and the most. IIRC, the #2 and #3 carbs are sometimes jetted more richly to balance the greater heat of being interior cylinders.

I had an outboard that ran great at sea level, but the top two cylinders (which shared a carb) would foul out when run at an altitude of 7200'. This carb was set slightly richer as the top two cylinders were at the end of the cooling water flow and therefor ran hotter. The richness was to counteract the extra heat during regular operation, but made them too rich at altitude.

[John]

Originally posted by DiverRay
John,
Check the plug on the pilot tower and the main jet. I DID have a jet come loose on E.Liberties bike when I first set the carbs up for him. Ran just like you said.

That's a good thought Ray. We just got back from a trip to Maine, and I haven't touched the bike since the weekend. I'm going to do a compression test on it also. The vacuum was great 14 inches mercury on all four cylinders.
egsols, I realize the Special jetting was as you stated for 80 carbs, but remember, I have 78 carbs. They require the larger main jets because the pilot circuit gets it's supply through the main jet, so the mains need to be larger to accomodate the extra fuel that must pass through them. The 80 carbs pilot circuit gets it's supply directly from the float bowl, so smaller main jets are used, and they should be richer on the center carbs, but the 78 wasn't set up that way, so I kept all the main jet sizes the same.
I gapped the plugs to .035 when I installed them, so I ruled that out.

[trbig]

????? No Harley gremlin bell on it?? You were riding in areas that lots of those little critters must hang out due to the high volume of V Twins?

The little bas%$^d fell off before you got home... the air was too hot and dry there.


Tod

[prometheus578]

"You've got a bad altitude!"

John, I used to foul plugs on my rides to Denver.
LoHo makes sense concerning one carb a little richer than the others, but not so that you'd normally notice, etc.

[egsols]

Originally posted by John

They require the larger main jets because the pilot circuit gets it's supply through the main jet, so the mains need to be larger to accomodate the extra fuel that must pass through them. The 80 carbs pilot circuit gets it's supply directly from the float bowl, so smaller main jets are used

Wasn't quite sure how the fuel flowed through the two models, only was aware of the smaller jetting. I assumed that the larger valves on the newer models made up for the smaller mains.

Good to know.

As an aside, would a set of 80 carbs run enough fuel to a '79 motor or would the carbs require large mains to avoid a lean condition (engine config being otherwise stock)?

[boyat68]

egsols:

The 80 carbs are drilled, a port from the pilot tower to the main tower can be seen if you look down in them. A rubber plug is inserted in the bottom of the pilot tower to force fuel from the main tower into the pilot tower for the pilot jets. I think that Ken Talbot has a pictorial of this some where in the archives. I believe this was done to maintain a controlled fuel supply on the pilots through the mains to be used in the pilots. I am led to believe that the 80s used a non clip needle jet to ensure that fuel for the mains were not available until the machine reached 5000. At this vacuum pressure the needles lifted allowing the mains to cut in and reducing the available fuel for the pilots, which in effect cut out as the greater siphoning power of the upper chamber diverted all of the fuel to the main channel. Apparently the 78s and 79s did not have this crossed drilled and stoppered pilots, but allowed fuel through both towers, but the setting of the needle clip governed when the greater siphoning power of the mains cut in reducing the the flow to the pilots. The crossed drilled towers also caused the need to allow more fuel in the bowls, i. e. the different float heights. The 80s use 23.xx which is more fuel vs 25.xx in the 78s and 79s.

Of course this is just one man's opinion only backed up by anecdotal evidence.

[John]

What I found

A very embarassing oversight. Before departing on our trip, I changed all of the fluids and even synched the carbs, but I forgot to check the air filter When I pulled the lower section of the airbox off I noticed how nasty, stopped up the filter looked. I was able to roll some of the crap off with my fingers. eewe! I washed the filter, and pulled the carbs anyway. I found nothing out of sorts. No missing plug on the pilot tower, all jets and passageways were clean, I checked them all with can of carb spray. The floats are some plastic ones I had installed and they were set at 25.5mm, but I reset them to 23mm. I cleaned the carbs real well, and put them back together, and reinstalled them. It fired right up and ran well. I hooked up my vacuum gauge, and resynched the carbs. I made sure not to disturb the carb adjustments, but they did need to be done again. I took it out for a short blast, and it ran great. I feel pretty stupid now, knowing it was just the airfilter causing the problem.

[egsols]

A lot of times we get too wrapped up in troubleshooting the difficult and overlook the obvious. Happens to all of us.

Ever forgotten to check kill switch position when starter doesn't turn?