13 replies to this topic

[ddelectrical]

Hi all. I have a 1997 TDM850 4TX 2 with 47,000 miles on it that has foxed me for a year now. When changing down a gear and reving the engine it cuts out on one cylinder. If I rev it up slowly it doesn't cut out and makes it to the red line. I have had so many issues with it. I found and replaced a faulty coil, (2 new ones fitted), a faulty new plug cap, (repaired), a faulty spark plug, (2 new ones fitted), dirty carbs, (cleaned and all jets clear), cracked inlet rubbers, (both replaced). It has a new air filter. Valves just been correctly shimmed.

 

It always had a bit of a mis-fire but is running better now I've replaced the duff ignition parts but still bogs down on one cylinder when grabbing a handful of throttle. The pilot mixture screws are both at two turns out as per the service manual. My carb balance gauges show both carbs are equal. The slides both start to move together when reving with air box off. The carb diaphragms are new. The air cut-off diaphragms are new. The cold start assembly is new.

 

Fuel level in both bowls is identical and within spec.

 

The pick up coil reads correct resistance over the temperature range from cold to hot. The bike ticks over OK now whereas previously I could not get it to idle evenly but that was all due to the ignition parts.

 

It has just been re-shimmed to correct valve clearances as it had low inlet clearance. All within spec now. Compression is good. It went up quite a bit after re-shim. All good there.

 

I have spent so many hours trying to get this bike running right over the last year and am running out of hair now!

 

I still think it is something to do with the carbs but I am now out of my depth as to what it might be. They are Mikuni BDST38 CV carbs. Could it simply be worn emulsion tubes and needles? There are some very slight wear marks on the needles but the tubes still look circular.

 

I am now wondering if it could be the ECU. Is it possible that the timing advance one one cylinder is out? This is the two coil version unlike the earlier wasted spark single coil version. Two output channels on the ECU so maybe a difference in outputs?

 

Any advice on what may be causing it very appreciated.

 

Thank you.

[fixitsan]

It's baffling why it would rev only on partial throttle, but then again, on partial throttle you maintain a high intake vacuum, where as with a wide open throttle the vacuum drops away rapidly, so there's at least one major difference.

So you could still be right about the carbs.

When you pin the throttle open  you expect maximum flow in the carbs, and if there's any debris in there it will likely get sucked into the jet then, more so than with a partial throttle, when fuel flow is much less ?

 

Maybe ?

 

My 850 carbs were also 'fun'. I had similar problems like other riders have here, you clean the carbs, the pump, the filter, and seemingly from nowhere a bit of something alien blocks open the float bowl needle !

Edited by fixitsan, 27 July 2024 - 10:24 am.

[Favs]

Greetings, I'm sure a carb guru will be along soon to help.
Done sooner than said

[ddelectrical]

Thanks fixitstan and Favs. I checked the two outputs from the ecu with an oscilloscope today and have nice clean pulses. Nice to actually see the 270 degree spark pattern.

 

The carbs came off again today and I found that one of the new cold start plungers had a damp patch of fuel around it. And the brass fuel tube on the side of the carb had some locktite around it that had cracked and come loose. So, maybe some clues. Sealed the tube and put heat shrink on cold start plunger shaft to get a better fit in the o ring / grommet. Maybe.... 

 

Just got to find enough sanity to put carbs back on again. And again........   

[dapleb]

Hello dare and welcome!

That's some shoppin list. I don't see mention of new ignition leads.

Since ewe have individual coil system, can ewe swap them over to see if problem swaps cyls?

Do you know which cyl cuts?

Do plugs look the same colour? Are they standard?

Plungers could be a clue.

Whereabouts are you to? Someone local may be able to offer a fresh pair of eyes!! I admire your persistence.

[fixitsan]

Do you know which cyl cuts?

 

I was going to ask the same question, but how do you know ?

 

I remember on my 850 when it dropped a cylinder, in order to work out which one it was I pulled the ignition lead off the plug while it was misfiring...on the basis that if it kept running I had identified the dropping cylinder. Easier to do in the garage than when riding it.

 

You could also put a hand over the carb throat, or, pull the low voltage 12V spade terminal from a coil.

 

If it happens from cold then the missing cylinder would have a cooler temperature exhaust downpipe

 

A fancy-pants Infra Red camera might help. Wish I had one of those !

[ddelectrical]

Hi. The HT leads were new with the coils. Both coils showing about 15k Ohms on secondary windings. Heat shrink on the cold start plunger shafts doesn't work. It goes tacky with petrochemicals. Jams up plungers. Removed sleeve and had a real close look at the plungers. Not the best design by a long way. Little plastic needle and flappy stainless thing. Very interesting to note that by 1999 the TDM was fitted with BDSR 38mm carbs which have metal cold start plungers and all internal fuel gallery! Like they always used to be. Unlike the BDST carbs fitted to this 1997 model with its plastic plungers and external brass fuel pipe. Also noted that the XTZ750 always had the BDST carbs and never updated. Wonder if owners of these had similar problems?

 

There is residue around the area where the plungers clip into the body. Something is leaking for sure. There's a few carbs of similar age on ebay showing similar residue.

 

I'm going to seal the carbs and blow some smoke through them to see if anything leaks. Hard to tell by vacuum cleaner test so pressure test instead.

 

Next up is plugs out again. Check colour. Watch sparks. Down pipes gave same temperature readings from cold start with infrared thermometer on last run up. Oh for a thermal camera though!

 

I haven't separated the carbs from each other yet but I am wondering if one of the link pipes has an air leak too.

 

I tried the hand over the carb trick but before finding and replacing the duff spark plug. Hand over left carb - engine dies. Hand over right carb - engine just about continues to run but laboring. Hence why I think it is sucking air into the right carb somehow. Manifold rubbers / o rings all fine. But then again it could have just been down to the faulty plug. Didn't check it again before taking carbs off this time. Opps.

 

Good tip about pulling spades from coils. Safer than pulling plug caps. Thanks.

 

To the bat cave..............

[Bjørge]

Hi all. I have a 1997 TDM850 4TX 2 with 47,000 miles on it that has foxed me for a year now. When changing down a gear and reving the engine it cuts out on one cylinder. If I rev it up slowly it doesn't cut out and makes it to the red line

Does it run on both cyls when revved slowly (to high rpms) and then being given full throttle?

Edited by Bjørge, 30 July 2024 - 10:06 am.

[dablik]

Hm, no guru here and you seem to know what your doing so i'm just throwing this out there, throttle butterfly shaft seals ? if leaking could cause to run lean then misfire !  i've seperated carbs before, rebuilt with a viton kit from LiteTek.. good luck it's bloody frustrating innit.

[fixitsan]

Hm, no guru here and you seem to know what your doing so i'm just throwing this out there, throttle butterfly shaft seals ? if leaking could cause to run lean then misfire !  i've seperated carbs before, rebuilt with a viton kit from LiteTek.. good luck it's bloody frustrating innit.

 

If that is the problem, I've got an new seal kit on the shelf and no 850 to put it in...

[ddelectrical]

Hi. A little further with the detective work. I managed to seal all the ports on the carbs and put some low pressure air into them via the float bowl atmospheric pressure breather. Then sprayed soapy water over the outsides and there are no air leaks to the outside, (Thank you dablik). I did find, however, when blowing into the cold start fuel feed tube port that goes from the jet block in the float bowl up to the cold start plunger via the brass tube there is some air bypass on one carb. Putting a very small amount of pressure on the end of the cold start plunger stopped this. The plungers are new but there is still a very slight bleed through. There is a spare hole on the white plastic arm that slides the cold start bar across so I am thinking I will attach a light duty spring to this to pull the plungers fully shut and see what happens.

 

Reply to Bjorge: Yes, it will rev all the way if opening throttle slowly but will cut out on one cylinder when opening throttle quickly. It initially picks up from idle revs nicely but then looses it at higher RPM. 

 

And now I have cracked a carb top. Oh bugger. My bad.

 

Reply to Fixitstan: I am interested in the seal kit if available. I am located in Devon. Can we PM on here? Quite new to the forum. Cheers.

[ddelectrical]

Hello dare and welcome!

That's some shoppin list. I don't see mention of new ignition leads.

Since ewe have individual coil system, can ewe swap them over to see if problem swaps cyls?

Do you know which cyl cuts?

Do plugs look the same colour? Are they standard?

Plungers could be a clue.

Whereabouts are you to? Someone local may be able to offer a fresh pair of eyes!! I admire your persistence.

I have yet to repeat the spark test. In my haste I put the plugs back in after finding that one was bad and didn't look to check the spark quality of the new ones. Idiot. I was so sure I had finally found the last fault. Ho ho. New coils / leads / caps / plugs all fitted. All resistance checks good. I will run the spark test again some time next week for sure. Previous test showed bad sparks swapped sides when swapping plugs over so bad plug for sure. As it has had a decent run since fitting new plugs the colour check will be next. Just need more time! Thank you for your input.

[Bjørge]

I had a somewhat similar problem when my tank cap seal started leaking, resulting in water in petrol and, over time, crud (rust ?). 

The bike would typically run smoothly at tickover, but having problems when opening up throttle - somewhat randomly as water droplets/crud got sucked into jets. 

 

The bike felt almost as if it was constantly running out of fuel. In fact, the first time it happened I was on my way to fill it up, and of course thought I was running out when it stopped running properly a few hundred meters from the gas station. Only that adding a can didn't help much...

[fixitsan]

 

Reply to Fixitstan: I am interested in the seal kit if available. I am located in Devon. Can we PM on here? Quite new to the forum. Cheers.

I've sent you a PM