Yamaha TRX850 Enthusiasts Forum

Ok, so yes this is another carb tuning thread but this thing has me a little stumped.

First off (stock carbs) the condition of the carbs is they were stripped and cleaned about 1,500km ago and the needle jets and jet needles were replaced new with stock replacements.

Plugs are NGK iridium and have been in the bike about 1,500km

Main jet is stock. Float height is 5mm below the line on the bowl (both sides) and I am on the middle clip position (plus the washer). Fuel screw is 2 turns out both sides. Carbs are most definitely balanced

Throttle chop tuning and checking the plug tells me the main is right and riding at above 1/4 throttle and killing it also gives me a great reading on the needle. No flats spots or surging between 1/4 and full throttle. Running great plug checks at the side of the road after I have been riding around normally, so on the needle gives me textbook plug readings.

So my issue is on the pilot. Now, all starts well but after about 20mins I get an uneven tickover and surging up to about (not even) 1/4 throttle. It is only evident at tickover and very small throttle openings.

Now the weird thing is that the symptoms say lean running, but a problem that gets worse at the motor heats up say rich running.

Normally I would say, "no worries, just go an extra 1/2 turn out on the pilot but, and this is very strange, bearing in mind both carbs are set the same.

After letting it tickover and riding about very slowly at less than 1/4 throttle I get a rich reading from the plug on cyl 1 and a normal reading from the plug on cyl 2.

So a solution may be to leave the carb on cyl 2 alone as my readings are good and go an extra 1/2 turn out on cyl 1. But I have never encountered this difference before and have never run different pilots screw settings on each cyl so I am interested to hear what people think.

I have attached a picture I my plugs

Yes weird alright!
Can I ask, did you completely strip the cars? Those nasty black plastic choke arrangements have a habit of shaving the needle end `notchy`, I wonder if a very small peice of choke needle end is fidgeting around in there?
Go to page 21 of the parts pdf and it`s listed as `starter set` (weird Japanese) item number 26, 3EN-1410A-00.
Edit:

Here`s another idea, use two zip ties to close them off or pull the small carb heating hoses off and plug the ends then go try it too, it could be that extra heat in one of the carb bodies creating a lean mixture. While you`re under way / at speed their is an air cooling effect remember.

I thought you might have nailed it on the choke, and i was just on my way out tothe garage when i remember one important fact i left out of my first post.

I was getting the rich reading on cyl2 and the normal reading on cyl 1. Last night i took the pilot jets out and replaced the o-rings. I popped them back in and tonight i am rich on cyl1. So more than likely the pilot i had in cyl2 is now in cyl 1 and vice versa

Ok, well that would seem to indicate a badly made or less likely, worn pilot jet would it not?
Those choke needle arrangements are really nasty little blighters, in the carbs my TRX had on when I bought it they were seriously notched and all those little nylon / plastic shavings have to go somewhere!
I binned stock carbs in favour of my FCR`s, nuts and bolts carbs, everthing manually operated, you can`t go wrong.

they are on the wish list,

I will have another look at the pilot screws and let you know how I go.

...a little bit embarrassing, but when I had removed the pilot scews last night and found that I was missing a washer and an o-ring from one of the pilots. I didnt dwell on it and put in a new washer and new o-rings both sides

then tonight I got the weird plug reading.

So anyway I went back and took out the pilot screw on the side reading rich and found it looked good so i got my torch and had a good look in where the pilot screw sits. Well as I am sure you can guess I saw what looked like an extra washer. Sure enough there was a washer and an o ring in there which I missed last night so today I was running with 2 washers and 2 o-rings in the cyl 1 carb.

So that explains my problem.

I will see how it runs tomorrow.

I am in the process of tuning these carbs mostly by throttle chopping and I am very close to getting everything spot on.

Throttle chopping confirmed my main jets were good and after finding my needle jets and jet needles were worn I found a good setting there, again using a throttle chop.

Float hight is at 5mm below the line and I am getting great power delivery with no flat spots or surges and good plug reading on everything (well except idle today) so hopefully the fuelling will be sorted now because I am heading down to the Snowies next weekend.

I got my sag set up at the weekend so she is handling like a dream and new pistons and seals have fixed a stinking rear brake.

The info on here has been great to use while rebuilding and now setting up the bike. The worn needles for example would have been a very though one to suss out without the info on here, so thanks to all for giving a very rich source of info from the search function.

Just loving this TRX and now i am in the final stages of getting everything just right I am really discovering a bike I can see myself keeping in the garage as other bikes come and go.

Hahahahaha, you`re far from the first to have done that. I had two O rings in one pilot jet drilling of a pair of carbs on an old Honda twin I owned years ago, easily done.
The good thing is you`ve VERY likely found the problem.
Do check on those choke needles though, nasty little blighters that they are.