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

Maintenance, repair, troubleshooting, etc.
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Lookin' Around
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Joined: Tue Jun 07, 2016 2:00 pm
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Motorcycles Owned Currently: 1993 Yamaha XJ600

Tue May 16, 2017 3:25 pm

Hi, thanks for the nice round up of the possible causes! The most scary to me is the less likely: I can't check the valve clearence as I don't have the right space and tools! The backfires happens only at certain RPMs and in a specific scenario (if I lightly open the throttle after a deceleration) and I think that if the outtake valves aren't closed the right way, the issue will show up all the time, no matter the RPMs...
I don't have any big news: I have the new starter jet but I didn't have the chance to replace it.
I also ordered an IR thermomter, hoping to get more stable and accurate measurements.
I checked the cold cylinder's spark plug: I'm not an expert of these readings, but to me it looks fine!
On the other hand, the spark plug of the fourth cylinder (which is the hottest of the 4) was very dirty with carbon buildup! I always thought that dirty spark plugs = temperature insufficient for the self-cleaning = cold cylinder = rich mixture, but this is totally the opposite I read from the thermocouple!
One question: are we sure that in this particular carb set, like in the more common one, backing out the pilot screw means more fuel hence richer mixture (and colder temperature)?
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One of the Regulars
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Tue May 16, 2017 4:57 pm

From what you're saying, your backfiring occurs at idle or at low revs, and that's why you're considering adjusting the pilot mixture screw. Is that correct?
A record of maintenance items carried out on my bike:

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Lookin' Around
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Tue May 16, 2017 7:40 pm

Hey just a quick question I noticed on your temp reading you listed your 3rd temp as 150*C was that from your 3rd cyl? The reason I ask is if so you have 2 cyl that are running cold that just happen to be companion cyl for the engine. If that is the case I would start to look at your ignition coil as it is the most likely culprit.

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Thu May 18, 2017 2:05 pm

Hi guys, thank you for your interest!
Crimson, you're right! But for what I see, turning in the screw doesn't make the temperature rise; this is why I'm doubting that these carb have a different principle.
North1987 yes, also the 3rd cylinder is a bit cold. Are you sure these 2 cylinders are coupled? I knew that the 2 coils fire respectively the couple 1&4 and 2&3, but I've never checked by inspecting visually...
Anyway, remember that the thermocouple that I'm using gives me really unstable readings!I hope that the IR thermometer will provide me with better values.

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Thu May 18, 2017 5:25 pm

Whoops I messed up you from another forum post it does say that the 1 and 4 are companion cyl I guess I have been working on the bi stuff for to long. I will confirm on my bike tomorrow, not going to brave the thunderstorm to check on my bike but as 99% of all automotive engines have a 1-2-4-3 firing order it would stand to reason you are correct. I would be curious to see the readings after you get an IR gun because it does seem like you have 2 dead cyl but as you said the method for measuring might not be 100% accurate.

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Motorcycles Owned Currently: 1993 Yamaha XJ600

Mon Jun 05, 2017 3:44 pm

Hi guys!
I did 700 miles in a week, so I can report my news!
I finally replaced the petcock's diaphragm, which was indeed in a horrible shape: despite the feeling of having done the right thing, I didn't notice any change.

I discovered that the PO screwed a bolt into the hole at the bottom of the air box (to which is supposed to be connected the one-way oil relief valve). I removed it, hoping that allowing more air will offset the rich condition: NO AVAIL! No changes either... what do you think is better: replugging the bolt or leaving it without? The valve allegedly doesn't allow air in, so plugging the bolt should be the right choice (assumed that no oil leaks into the airbox).

I received the IR thermometer, which gave me the following readings: 100 150 150 150 C (210 300 300 300 F): so all cylinders are cold, the first one really cold.
During this week I noticed that the bike has a little indecision in acceleration, when giving a little gas; if I give a good twist on the gas, she takes off fine. This gets worse when the tank is full to the brim: I am now suspecting that the fuel pump is faulty and it allows too much fuel.
Just for give it a shot I ordered one of those Polaris fuel pump replica from china: If it solves the issue, I can think of buying a good one; if it doesn't, I wasted 10 bucks!
I also replaced the fuel filter, which was in a filthy situation (I could see a jelly blob hanging out the outlet :o ): this makes me suspect even more on the fuel pump! I forgot to tell that when I took this bike, the engine used to die every time I hit the reserve and it took a while to restart; eventually I discovered that the short tube in the petcock fell from its seat, so every time I switched to reserve, the engine pulled the fuel from the very bottom of the tank, where usually all the filth is collected...
In case the fuel pump is the culprit, do you guys know if it is possible to repair/restore the original Taiyogiken fuel pump? Do the membranes exist as spare parts?

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Tue Jun 06, 2017 5:36 am

In reference to the airbox drain. I would put a hose on it and feed it down along the frame and exit below the bike by the swingarm. That's how they are set up at least in the US. This way it gives a path to drain any crud that builds up in the airbox rather than to pool and maybe soil your air filter or get sucked into the engine.

I doubt the fuel pump is pumping too much fuel causing a rich condition. These vacuum operated pumps are pretty low pressure. Before replacing the pump thoroughly flush your gas tank and lines (maybe replace them with clear lines so you can see if the fuel is clean and flowing) & install a new fuel filter. You don't want to gunk up new parts.

You are running a little rich hence the low temp readings on the cylinders and the carbon on your sparkplugs. I would suspect your mixture is off. You can try leaning them out.

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Tue Jun 06, 2017 6:26 am

I had to replace my fuel pump and got this one Exactly like the one I had on.
just enter in ebay:
New 12v 60 LPH Fuel Pump Replace 3020 3010 3000 2510 2520 1000 For Kawasaki Mule

It works great have it on now for now 8000km and not a single problem. But the fuel pump stops pumping if float bowls are full so do not think that the fuel pump have a effect in running rich? :twocents:
I ride solo

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Motorcycles Owned Currently: 1993 Yamaha XJ600

Tue Jun 06, 2017 2:22 pm

Thank you for your answers!
Alec, I have a membrane operated pump! For fitting the one you suggested I should figure out a way to get the current and to adapt the holder...I'll keep it in mind, thanks!
I just went through a full tank riding all goody goody: I've got the same lousy mileage I get when I ride regadless (33 mpg :cry: )
The mixture seems to be unresponsive to the pilot screws week-end I'll try to play more with those, until I get the new pump!

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Tue Jun 06, 2017 5:41 pm

Hi, the general backdrop to your bike is it has had a lot of amateurs - that's all of us here, but with the use of a TARDIS if you get my drift! - so damage gets done. Here, I paste all my final notes on tuning up the carbs on my XJ600N Rosie Naked twin disk model year 2000 Seca II or Diversion bike. It's a UK/European bike spec. Yamaha sabotaged the carbs to lamely try meeting Euro emissions without retro-fitting fuel injection to these.

My carbs therefore are the wider bore ones I believe, the 28mm (BDS 28), and I have disassembled and cleaned and everything, the last thing I wanted to believe was Yamaha ballsed it up themselves - but that is exactly what happened. Yours, is not of that era, you just need to make it stock bearing in mind there are little undocumented differences in jets etc, but overall, if Yam reduced fuel jet bore, they also reduced air hole size, etc, to match. Different, therefore does not mean wrong.

Anyway, here is the story of tuning my carbs, I wrote these notes to keep myself straight, carbs are as complicated as fuel injection it sometimes feels - unless you have ever experienced issues with that in which case you will dream of carbs!

WRITE UP FOLLOWS includes carb jet details, I see I relied on some from XJrider!, back in 2015 :thumbsup: :

Having re-examined the carbs with the intake unit removed, and pics taken, it's clear Rosie's carbs have NOT got a cutaway to the leading edge of the throttle slides.

DETAILS of Rosie's jets:

Main jet - currently has DJ102 fitted, officially had MK100. Older bikes pre 1996 had MK102.5 and sometimes even the inboard cylinders had MK105. NB according to the chart below, MK100 is bigger than DJ102 by quite a way - seems odd. Others say MK102.5 is exactly same as DJ102. Given that it seems just slightly better running with the DJ102 than the DJ100, let's stick with that and just adjust the needle clip.

Pilot Jet/slow jet/idle jet - the name is in all respects misleading. They use VM28-486 pilot jet types (poss just for flat slides), or CR/YZ N224.103 similar ones and you get 15, 17.5 fitted as std. Many people up them to 20 (also on suzuki 600 bikes, they were too lean as well). Rosie's std one is meant to be 15 for the meant-to-be 135 air jet, but it'snot, she has 17.5 and a 140 airjet. The fuel out of this jet flows AT ALL TIMES and speeds. IE it's kind of underlying any other setting even the main jet, always there supplying fuel. So, keeping the main jet the same, and clip notch on needle the same, just changing the pilot jet for larger, richens the mixture all the way up the rev range.

Air Jet - 140 is fitted to Rosie - but it is meant to be 135, in keeping with a smaller pilot jet of 15, but Rosie has the older sizes of 17.5 pilot fuel jet, and 140 airjet fitted. Older pre 1996 UK and Canada models had 145 airjet and a very lean 15 pilot fuel jet, but had 102.5 and 105 (middle cylinders) main jets.

1 - Replace bowl and diaphragm cover screws with Allen screws. These std ones get chewed and are hard work, if Allen screws make it easier, use them.

2 - Jets of various sizes - the main jets come with the Dynojet set. The slow/idle jets though, may not - check out what spares are in the GPz750 - they should be bigger slightly? Are they mikuni though, the numbers may not match?

2a - XJRIDER and other websites, owners with bikes that run well, say the stock 17.5 slow or pilot jet is way too lean, that 20 or 22.5 is the way to go. NOTE that PILOT or SLOW or IDLE fuel jets, are FULLY OPEN THROUGHOUT THE REV RANGE ! So, despite their name, they actually have an effect of some sort across the range, BUT ESPECIALLY THE TRANSITION OF THE FIRST 25% of throttle. IT seems very clear that this is the thing that needs changing and has not been - the only thing I have NOT altered is this slow/pilot/idle fuel jet, and Rosie has 17.5 size. This is TWO sizes smaller than many people run these bikes on.

2b - AIR JET - my air jet is 140 size. This works in conjunction with the pilot fuel jet. The pre-1998 carbs in UK used 140 air jet, after that they used 135. So, Rosie is 'odd'.

2c - Notes in the manual - papers I slotted in - show some people swearing the only way to up the mixture is to increase the pilot/slow jet size.

2d - Suzuki Bandits had exactly the same problems - Suzuki leaned them down to 15 after y2k then riders and dealers had to put them back up to 20 to get rid of a terrible flat spot. Some manage in US with 17.5. It must be just on the edge, so the question really is, do I get 20 or 22.5? Bandits also use Mikuni carbs. What size are the Kawasaki ones - remembering they are actually for a 900 bike, as they kept the same carbs as that for the 750, but the jets could have been dropped in size.

3 - note the following from xjrider:
As I'm sure you've read, the Yamaha Seca II is jetted lean from the factory, at least, on the pilot circuit. Yamaha fit the Mikuni BDS26 carburetors with a #102.5 main jet and a #17.5 pilot jet. A common carburetor modification for the stock carbs is to replace the stock #17.5 pilot jet with a #20 pilot jet. This will improve cold-startup and will also improve partial throttle performance when warm.

Additionally, if you've modified both the intake and the exhaust on your Seca II, re-jetting the main jets is likely needed to restore proper performance. (Note: Main jet replacement is typically not needed on an XJ that is stock, or one that has only exhaust or one that has only intake modifications. Main jets are only replaced if you've altered the airflow through the engine by changing both the intake and exhaust).

NOTE - the carbs were upped to Mikuni BDS28mm from the 26mm ones (check) from 1998. So, if anything, the above is a minimum, perhaps even a size higher is needed for both?

This does not necessarily mean the problem is no longer an 'over-admission' of air at the critical point where the throttle is just cracked open - the 'second eighth' as I term it. When I set the jet needle etc to be really quite rich, and with the dynojet needle, it's the mpg that rockets down to the low 40's that's the issue - the bike actually runs perfectly fine.

- It is worst when the engine is cold

- If the throttle is cracked way, way open, the problem almost disappears even at low rpm.

- The worst throttle action to invoke the issue is gently opening as if taking away nice and easy/normally.

- This means the usual jetting culprit would be the

- Divvies are famous for being dreadfully lumpy at cold, especially I might suggest when trying to pull gently away, that's most often how you ride when engine's not fully warm, so this second 1/8 throttle issue is very common indeed, it's just not usually as bad as Rosie.

So best to focus on all the things that could sort the low end, that tiny sliver of just off idle, first just opening the throttle, what happens or does not happen riding that way, as opposed to cracking it wide open, when the issue seems to almost magically disappear even at the same low rpm the issue was cutting the engine out with, using gentle throttle opening.

Dynojet say they don't usually alter the idle or start circuit so this should be as before. Therefore, the issue is that crucial transition from idle to 'whatever' is just above idle.

NOTE maybe this is part of the problem - here's an extract from a Harley write-up:
'In standard trim it was only the early California models that had any problems, with an off-idle hesitation caused by a lean slow-speed jet (to beat emission requirements). The problem was easily cured however by changing the slow jet to a #45.'

So, what size of slow jet iis in Rosie's carbs, and would one slightly larger be worth fitting, rather than mess with other stuff eg jet needle, which is dealing with speed ranges and throttle openings that mainly are working fine.

JET SIZE EQUIVALENCES - KEihin/Dynojet/Mikuni ... _Chart.pdf

Width------Keihin # -- DynoJet # -- Mikuni #
0,0350---- 92,5--------- 92----------- 86,3
0,0360---- 95----------- 94----------- 88,1

0,0370---- 97,5--------- 96----------- 90,0
0,0380---- 100---------- 98----------- 91,9
0,0390---- 102,5------- 100---------- 93,8
0,0400---- 105--------- 102---------- 95,6
0,0410---- 107,5------- 104---------- 97,5
0,0420---- 110--------- 106---------- 99,4

0,0430---- 112,5------- 108--------- 101,3
0,0440---- 115--------- 110--------- 103,1
0,0450---- 117,5------- 112--------- 105,0
0,0460---- 120--------- 114--------- 106,9
0,0470---- 122,5------- 116--------- 108,8
0,0480---- 125--------- 118--------- 110,6
0,0490---- 127,5------- 120--------- 112,5
0,0500---- 130--------- 122--------- 114,4

Here are all the possible ways the mixture could be turned lean at the point the throttle is cracked open. In the old days this problem was solved by an accelerator pump - this rather unscientifically squirted a thin jet of fuel into the air intake as the throttle was yanked hard open! Cars and bikes both employed this, but obviously it's a wasteful use of fuel. Anyway, here are the possibilities as to why the mixture gets suddenly lean at this critical point, yet the engine is fine at idel and once the throttle is cracked right open, it's able to act properly, EVEN IF THE RPM IS QUITE LOW AT THE TIME:

1 - Throttle slide flies up too fast, too far. This is unlikely, but as the butterfly gets opened just a fraction, the slides' 'average' position will be open a little more, and it is theoretically possible the mixture is too rich at that point, by the slides being too high. NOTE this is the same as saying the jet needle is TOO LOW (ie too lean a setting) for the air volume admitted at that point.

1a - Watch the airbox clamps around the lip of the carb mouth is airtight - some appeared willing to rotate under screwdriver pressure when slackening them off, even though they were tightened up full ie the clamp faces were touching so it couldn't tighten any more. Perhaps some packing under the clamp a thin strip of rubber encircling the carb rubber connector?

2 - Idle or slow jet - as done by Harley owners in California because under pressure of emissions, the slow jet selected was way too small, in their case they upped it to 45.
Current bike year 2000 Yamaha XJ600N also called Seca II or Diversion 600. On an older bike air cooling is a blessing, no sludge and complexity of badly-maintained liquid cooling. Other bikes were: BMW K100 roadster; Honda VF750S Sabre with anti-dive.

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