Yamaha TRX850 Enthusiasts Forum

Please could someone give me some advice - my bike has been running rough so have had the carbs reset and balanced also had new rings fitted - have noticed that on tickover at about 1500 rpm when I put on the lights the bikes revs drop. She still runs rough under 3000 rpm which makes slow turns into junctions a dangerous occupation. Please help or she will have to go.

The revs dropping may be a red herring , but put a multi test meter across the battery terminals and maintain the revs at about 3k, you should see about 13V .

Before i get shot down this is from my faulty memory.

I remember having someting similiar with a 350 ypvs it turned out to be a corroded conection in the positive feed to the regulator.

Another thought , i know the trx suffers from Jet wear in the carbs.....

What do you lot think....worn jets?

How many Km's from a tank before reserve?

It isn't the jets as they have been re-newed, as for the distance travelled before I get on reserve I dont know as its been running so rough I hav'nt gone any distance yet. Some one has also suggested that there is an electrical piece of kit attached to the carbs, dont know what it is, any ideas?

The electrical thing is the TPS or Throttle Position Sensor which is bolted to the carb body... it sends (as the name suggests) information about the opening of the throttle butterflies to the ignition unit, and that in turn does a bit of juggling with the timing to set the ignition accordingly. You wouldn't need to disturb the TPS to do a simple re-set or re-synch of the carb and if it hasn't been moved and the bike was running before the carb job then I doubt the TPS is at fault. It's always worth checking the wires and that the connectors are clean -

It really sounds as though there's a bit of dirt in the carbs somewhere... if it's running ok over 3k then I'd suggest the pilot might be blocked. If I were in your position, I'd have the carbs off and give them a bloody good clean and a thorough blowing through with compressed air to knock out any small bits of junk.

While you're there I'd also check the choke valve assembly just in case there's a foreign body in there.

You said you'd had the rings renewed... piston rings? Are you sure the garage got the valve timing right? I don't know for sure whether it would actually run if the timing was one tooth out, but if it did it would be as rough as f*ck for sure! Dunno if it would have any impact on the tickover, but the additional load of the alternator dragging on an engine that's already compromised by valve timing that's off would certainly be felt!

As usual I'm probably talking a load of old bollocks, but hey look what it's done for Tony Blair!

G

Great thanks for that - yes had new piston rings fitted and the guy that fitted them has already checked the valves - so its going back tomorrow for him to have another look - another weekend without a bike - thanks for the advice anyway guys.

Greg wrote:The electrical thing is the TPS or Throttle Position Sensor which is bolted to the carb body... it sends (as the name suggests) information about the opening of the throttle butterflies to the ignition unit, and that in turn does a bit of juggling with the timing to set the ignition accordingly. You wouldn't need to disturb the TPS to do a simple re-set or re-synch of the carb and if it hasn't been moved and the bike was running before the carb job then I doubt the TPS is at fault. It's always worth checking the wires and that the connectors are clean -

It really sounds as though there's a bit of dirt in the carbs somewhere... if it's running ok over 3k then I'd suggest the pilot might be blocked. If I were in your position, I'd have the carbs off and give them a bloody good clean and a thorough blowing through with compressed air to knock out any small bits of junk.

While you're there I'd also check the choke valve assembly just in case there's a foreign body in there.

You said you'd had the rings renewed... piston rings? Are you sure the garage got the valve timing right? I don't know for sure whether it would actually run if the timing was one tooth out, but if it did it would be as rough as f*ck for sure! Dunno if it would have any impact on the tickover, but the additional load of the alternator dragging on an engine that's already compromised by valve timing that's off would certainly be felt!

As usual I'm probably talking a load of old bollocks, but hey look what it's done for Tony Blair!

G

Fair comments here I think Greg.

Also, make sure your garage has installed the second compression ring the right way up as it's tapered. Also, check to make sure that the correct rings are in the correct grooves, easy to get wrong as they're the same thickness and cross-section depth. The top ring should be the barrel shaped one.

Don't forget to check the ring end gaps as well, 0.30mm - 0.45mm for both rings, and 0.020mm - 0.055mm total side clearance.

TPS Adjustment.

1. Turn on the ignition.
2. Disconnect the TPS electrical connector.
3. Re-connect the TPS connector, (puts TPS and rev counter in test mode).
4. If the Rev counter shows 4000 rpm, TPS is set correctly.
5. If the rev counter shows any other value you'll need to loosen the TPS mounting screws and rotate the TPS assembly accordingly to achieve the 4000 rpm reading, then re-tighten the mounting screws.
6. To reset to normal rev counter mode, start the engine or turn off the main ignition switch.

Sorry about late reply - many thanks for the info

Steelback could be electrical. Faulty HT leads or coils sometimes give the same symptoms as fuel starvation and it seems strange alternator drag has a big effect. Lower voltage may be affecting it somehow. See if your electrics all check out.

Going into the garage on Monday morning - they are going to do a full electrical diagnosis, so fingers crossed might get to the bottom of it.

Fully sorted and running sweet - it had a hairline crack,that could only be seen with a magnifying glass, in the carb housing - found another on ebay. Sorted just in time for the BSB meeting at Oulton Park on Monday. Thanks for all your suggestions guys.