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

Posted: Sun Mar 26, 2017 12:02 pm
by Giuan
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 :
[SIMG][/SIMG]
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!