Backfires, high consumption, cold cylinder...a mess

Maintenance, repair, troubleshooting, etc.
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Giuan
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Motorcycles Owned Currently: 1993 Yamaha XJ600

Sun Mar 26, 2017 12:02 pm

Hi y'all!
I've been reading a zillion of discussions about my problems:even though I believe they are all correlated, I can't find a root cause of them, hence I think the best thing is to tell the whole story, hoping someone could help me shed some light on the issue.
When I took this bike I could run 250km (155 mi) before hitting the reserve: considering that the tank has a capacity of 17 liters (4.5 gallons) and 3.5l (0.9 gal) of those are the reserve, I could do 18.5 km/l (43.5 mpg) which is in line with the mileages I can read here.
I had to drain the tank because the reserve tube fell off the petcock and was floating around; after having collected and put it back in place, I had the *brilliant* idea of turning the tap from PRI to ON while the unit was all dry: I heard a SNAP but I didn't care much.
Right after that, I had the feeling that the bike wasn't running very well, it was like sometimes the engine sputtered a bit.The following day I could smell gasoline; i found out that the fourth carburator was dripping, so I took off the tank again as it was clear that the peckock wasn't shutting off. Here,s what I found :

that tiny piece of rubber snapped off and made the petcock leak! In addition, it was clear that the 4th bowl's gasket wasn't sealing too.
I ordered a petcock repair kit and one of those expensive tour max carburator repair kit and I replaced that rubber seal in the petcock, the bowl's gasket, the needle valve and the o-ring of the floats' holder.
Now the petcock wasn't leaking anymore and the bowl wasn't overflowing. However, the engine sometimes showed a little indecision in the midrange when I didn't give a good throttle and I could ear some puffing sounds out of the mufflers. The thing got worse and those puffs became loud bangs, which could then be located as coming out of the left muffler: they usually happen when you open up the gas after a deceleration, between 3k and 4k rpm.
I did the math again and I calculated that now I hit reserve after just 200km (125 mi) which translates to a consumption of less than 15 km/l (35 mpg): a nightmare! At this point I gained a lot of confidence with this bike and I usually like to rev up at least to 5k before shifting up; I don't think these numbers are reasonable though.
It was clear that something was wrong with the carburation, although the bike always starts right away, even in the early morning, it idles perfectly and has both good acceleration and top speed. It seemed crazy, because when I restored the 4th carburator I dropped all the other bowls as well and did a good cleanup of all jets using compressed air.
After having read the nice How to: Tune Carbs with an I/R Thermometer guide, I decided to measure the temperature of each exhaust pipe. I don't have an IR thermometer, but I have a thermocouple-based thermometer equipped with a K-probe (TM 902C https://www.google.fr/search?q=tm+902+c ... 24&bih=657 ). I discovered that the first cylinder is really cold (barely 100 C, 480F) while the others were 300C 150C and 300C (570F 300F 570F).
So basically I had a really cold cylinder, a hot one, a lil cold one and another hot one.
My attention moved then on the first cylinder, which also fires through the left muffler.
In the beginning I was afraid it wasn't firing at all, but I noticed that if I disconnect its spark plug cap, the engine doesn't idle and dies, so it must be alive. All 4 spark plugs were replaced less than 1 year ago.
I took a look at the pilot screw and found it was set 2 turns out, so I tried setting it to 1.5 out (screw in= leaning out the mix=rising the temp, isn't it?) but no change ; I then tried 1 out and I felt no change until the following day, when I took the bike and discovered it didn't start in the cold of the early morning... I put it back to 2 out, ordered another carb repair kit and replaced the bowl's gasket, the needle valve and the o-ring of the floats' holder; I also pulled out the pilot jet and cleaned it to perfection, using nitro thinner and compressed air.
Nothing changed, still many backfires and low mileage. While taking long trips on highways I can reach 18 kml (43 mpg) but it's a lot anyway.
Cold cylinder and high consumption usually spell rich mixture, but I noticed two crazy aspects of the issue :
1) I usually spend my holidays in a different city than the one I live in: every time I go there, the backfires misteriously disappear! This other city has a milder temperature and it is at sea level. The city where I live is at 300m above, which frankly I don't think is enough for causing differences in the carburation. Anyhow, lower altitude means more air, then leaning out the mixture, so it would be reasonable that this makes things better for the 1st cylinder, but this should be repercussing on all carbs!
2) If I pull the choke a little bit I have no backfires; this makes no sense, because enriching a cold cylinder should be making the things worse, and not the other way around.

Today I had nothing to do so I checked the other pilot scew settings: from carb 1 to 4 they were [2 2 1.5 2] turns out.
I played a little bit with them but it seems that only the second cylinder is responsive, so I lowered its temp to 250C (480F) by setting its pilot screw to 2.5 out.
As for the others, any change makes no difference! Specially for the first carb's pilot screw, even if I screw it in all the way, its temperature remains very low. The fourth cylinder on the other hand, is a bit hotter than the ideal temperature (it's more that 300C or 580F) and the combination of these two results in a backfiring muffler.
I don't know how much I can trust the temperatures measured as described, but this is all the info I collected so far.
I hope that some of you had the patience to read to this point and would kindly give me a hint on what's to check next!

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Giuan
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Motorcycles Owned Currently: 1993 Yamaha XJ600

Mon Mar 27, 2017 3:52 pm

Ok I got some news:
today I replaced the last two items of the repair kit, which are the jet holder's gasket

which didn't look in a bad shape, and the pilot screw o-ring, which was indeed flat and hardened.
One weird thing I noticed is that compared to this pic

I am missing the wahser that goes between the spring and the o-ring!
I don't know if it is strictly necessary: worst case scenario is that if you screw it all the way in, you are missing that tiny width...but you usually have to back out!
Anyways, guess what? NO DIFFERENCES, still backfires while revving at 4k and cold cylinder...I think is even colder now, as I could touch its exhaust for a short! :o
Now I'm thinking of ordering the whole service kit A from http://litetek.co/Carb_Kit_Yamaha_Seca_II_NonUS.html as it comes with those washers and other interesting stuff, but I realize that I'm falling for the trick of cherry picking!
Anybody has a tip for me, before trying this one too?

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radare
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Fri Mar 31, 2017 9:55 am

I don't think your problem is carburation, at least, not fully. The fact that #1 is cold makes me think it is rich (which you do, too, since you screwed the pilot jet inward). What I suspect is happening is that your petcock is still leaking. Specifically, leaking past the vacuum diaphragm, through the vacuum tube and into the #1 cylinder.

Here's a test. But don't leave it this way. Cap the vacuum port on the #1 cylinder. Plug the corresponding vacuum hose on the petcock with an M6 bolt. Put the petcock on prime and take the bike for a spin. When you get back, carefully remove the bolt from the vacuum hose and see if fuel leaks out. Also, when riding, see if there is any change.

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Giuan
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Sun Apr 09, 2017 12:16 pm

Hi radare!
Thank you a bunch for your opinion!
Indeed I believe that the 1st carb is running very rich and the extra fuel that doesn't get burnt in the combustion chamber makes it to the exhaust: when it hits the hot muffler, it burns istantaneously making noise.
I did what you advised:

Image

while running I didn't feel any difference, the left exhaust was backfiring as usually, loud; however, after a 5 minutes ride I actually found some drops of fuel in the tube! Very few fuel to cause the cylinder to run erratically, I'd say, but it certainly contributes to the high consumptions I'm experiencing. Way to go for the intuition! :thumbsup:
When I bought the tap repair kit there was also replacement for the diaphragms



the point is that, since a plastic element has to be sandwiched between those two diaphragms, you have to take out one of them from that metal part you see me holding, and I was afraid of ripping it... I sticked to the "never try to fix a working system" mentality back then, but I guess the time has come!

Going back to the carbs, I have to clarify I have a 1993 european model, hence with oil heated BDS28. After reading viewtopic.php?f=10&p=85403 i was convinced that maybe my main jet and o-ring are bad; since I remembered to have seen a jet in a bad shape, I bought a 102 main jet (102.5 wasn't available) and a box of metric nitrile o-ring.
When I dropped that bowl again, I discovered that the jet in a bad shape was the starter jet :o

Image

this jet doesn't have an o-ring so I don't see the point of unscrewing it; I don't think that worn notch affects the carburation and besides this jet is not supposed to be working in the range I'm having troubles...anyways, I think the only way to remove that jet, would be using an extractor screw and replace it with a new 47,5.
As for the main jet, as you can see it's not marked so it's hard to guess what size is it; I bought a 102 jet because I read several topics here that say it's the right one, but I realized too late that they were all about the US model! This carb set is supposed to have 105 on 1st and 4th isn't it? Would you suggest to try the 102 I bought in the 1st carb anyway? This way, 1st and 4th carbs will be jetted differently..
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radare
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Mon Apr 10, 2017 9:07 am

Your observation is correct: The US carbs are BDS26 and the Euro carbs are BDS28. They are different animals and use different jetting.

Yours should be #105 on cylinders #1 and #4 and #102.5 on cylinders #2 and #3. I would stick with stock jets if possible.

While you have them out, though, I would remove the jets and clean them. That one that is buggered up has been removed by a PO. You should check and see what damage it has and what it looks like on the other side. I'd recommend removing it, carefully, with either a tight fitting screwdriver or a pair of pliers. If totally buggered, you should consider replacing it.

What do your carb's diaphragms look like? Leaking diaphragms can cause problems too.

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Giuan
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Wed Apr 12, 2017 1:55 pm

:) This bike has 23 years, who knows what kind and how many hands were laid on her... :???:
Ok, I'll try to replace the main jet and the starter jet with the proper ones; I won't make any attempt until I don't have the new ones, because the chances of breaking the buggered up one are high!
I have never taken a look at the diaphragms so far, I avoided removing the air box and stress the upper manifolds (which look great though) just out of curiosity. I'll have a look if nothing works: to my experience on twin engines a ripped diaphragm causes the engine not to rev up to the red zone, which is definitly not my case!
Thanks again, I hope to come back with good news.
Cheers!

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Giuan
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Tue Apr 25, 2017 1:39 pm

Hello again! I'm still looking for the replacement for that jet, but I'm not sure of what type it is.
If it is identical to the main jet it should be a N100.604, but it looks like that it doesn't exist with a jet size 47,5 :?:
There isn't much information about these BDS28 carbs around..for what jet model should I look for? I need to buy it in advance, because I don't think that jet is going to be reusable, once removed...
The only news I got is that I check the float height of that carb and is perfectly set!

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radare
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Tue Apr 25, 2017 5:51 pm

I started down the path of verifying your jets. From the manual, that 47.5 is your starter jet.

Image


Can you measure your existing starter jet with a pair of calipers? I suspect it may be a N102.221 but thats only if the dimensions match up.
This jet IS available in a 47.5:

http://www.jetsrus.com/a_jets_by_carbur ... _round.htm


Image
OVERALL LENGTH = 9mm
HEAD DIAMETER = 6mm
THREAD DIAMETER = 4.9mm
THREAD PITCH = 5.0 x 0.7


Otherwise, here are the dimensions for the N100.604, which does not come readily available in sizes that small.

Image

OVERALL LENGTH = 9mm
HEAD DIAMETER = 8mm
THREAD DIAMETER = 4.9mm
THREAD PITCH = 5.0 x 0.7

I suppose its also possible that both of your jets are actually the N102 jets (with the smaller head).

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Giuan
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Sun Apr 30, 2017 4:08 am

Hi! for 5 bucks I ordered one of those jets with a 6mm head so that, next time I open the carb again, if it matches I can replace it right away.
I doubt it will solve the backfiring issue (this jet should work only with the choke on) but I'd like to have a new one anyways.

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radare
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Sun Apr 30, 2017 9:17 am

Giuan wrote:
Sun Apr 30, 2017 4:08 am
Hi! for 5 bucks I ordered one of those jets with a 6mm head so that, next time I open the carb again, if it matches I can replace it right away.
I doubt it will solve the backfiring issue (this jet should work only with the choke on) but I'd like to have a new one anyways.
Your backfiring and high-consumption indicate an excessively rich condition. Fuel being passed through the engine, unburned and igniting in the exhaust. But you know that. Potential causes for rich-condition include, listed in order of likelihood IMHO:

Fuel related:
1. Leaking/failed seals on the float valve OR float heights set incorrectly
2. Pilot jets set too far out (they would have to be really out-of-whack for this symptom, though)
3. Leaking petcock vacuum diaphragm or fuel pump diaphragm leaking fuel into cylinders #1 or #2
4. Stuck slides or damaged diaphragms

Ignition related:
1. Spark plug gap improperly set or resistor cap worn resulting in insufficient spark on one or more cylinders
2. Intermittent coil failure

Mechanical:
1. Valve clearances too tight or valve seat wear resulting in incomplete sealing of the valves
2. Ignition timing out of whack

Unfortunately, this covers the gamut of maintenance items. A through going-through of the fuel, ignition and valvetrain might be required to ultimately remedy the situation.

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