Running premix - what jets?

[daveg]

I am running 30:1 premix on my 31K, should I up the size of the main jets or would moving the needle up a notch be sufficient. If I understand correctly, running premix will weaken the mixture as less fuel is going through the carbs due to the addition of oil - At the moment the plugs look good (coffee coloured), however that is after a normal run and not a high speed plug chop.

Should I try to up the main jet?

[Dirtbag]

I would upjet one size.....if the plugs are a good colour at normal running- then one size up should do the trick nicely for higher RPM.

[daveg]

I just discussed with my local Yam dealer on this, on he reckons that I could try a bigger jet size, but he believes my problem (in fact I had a four point seize on one piston). This is due apparently to the piston heating up quicker than the cylinder, which could be due to a number of things - wrong plug temperature range, wrong main jets, and/or bad warmup. He thinkgs that my problem is due to the fact I am running Wiseco pistons at +1mm (which is 2 sizes over the maximum that Yam recommend, the dealer reckong that the cylinder sleeve is too thin and maybe not able to conduct the heat away rapidly enough, and therefore overheating ..... not sure I belive this as Wiseco would not have a market in selling oversize pistons, but I am really stuck on this one!

[Dirtbag]

Sounds like he's spinning you a yarn mate.......if anything the bike will run cooler than a standard bore engine......because the liners are thinner - they will make the heat transfer process quicker..............in other words your pistons are closer to the coolant . As for the plugs thing.....what are you running in there ? , should be NGK BR9ES or equivalent with other makers.


If its an overheating problem , I'd recommend overhauling your cooling system..............junk the thermostat , rad-flush all the coolant ducts overnight , as too the radiator..............you'd be amazed the amount of shit that comes out.

As for the +1mm thing mate.....thats crap ! , when you see how thick those liners are....trust me 1mm is f**k all

[daveg]

I am running BR9ES, the cooling system has been emptied and refilled loads over the last few weeks, but I must check the thermostat you are right! One thing I forgot to mention is that I am running just water in the radiator as we are not allowed any anti freeze (it's slippy on the track!), but I don't think the cooling properties of water are any less than an antifreeze mix - what do you think?

[daveg]

Just checked the thermostat and it is fully open at 86 degrees, measured with the Jam thermometer - so that's not the issue. I will just have to try 250 main jets, and do some high rev. plug chops, after running in - anything else you could suggest?

[Dirtbag]

There could be several things left..............

1 . Your radiator is shafted........furred-up or the cooling fins bent ,
Get some good quality Radiator flush...STP is the best I've found ,
take the rad off , bung up the hose outlets with corks or summat,
mix a full bottle of rad flush in a bucket of hot water (enough to fill the rad) , pour it in , replace the rad cap and leave it stood up for a full day. After this empty the gunge out and flush radiator through with a garden hose.

2. Water pump is not working properly or the channels are scaled up.
The best way of ridding this from your motor is to fit all the cooling system as normal , and if you can lay your hands on some.....Steam cleaner de-scaler (Acetic acid)....really good shit !......leave it in there for a couple of hours , flush it all through thoroughly.

3 . Throw the thermostat away........theyre shite anyway...they slow down the flow and will contribute to overheating.

4. If you're not allowed to use conventional coolant ....get yourself a bottle of water-wetter....it looks like water anyway...so no-one's the wiser , and it works alot more efficiently than water on its own.......if memory serves me right - its 200% more efective than water on its own.



If this lot doesnt sort it out mate I'll eat my own head ;D ;D

[billbott]

daveg said:

I just discussed with my local Yam dealer on this, on he reckons that I could try a bigger jet size, but he believes my problem (in fact I had a four point seize on one piston). This is due apparently to the piston heating up quicker than the cylinder, which could be due to a number of things - wrong plug temperature range, wrong main jets, and/or bad warmup. He thinkgs that my problem is due to the fact I am running Wiseco pistons at +1mm (which is 2 sizes over the maximum that Yam recommend, the dealer reckong that the cylinder sleeve is too thin and maybe not able to conduct the heat away rapidly enough, and therefore overheating ..... not sure I belive this as Wiseco would not have a market in selling oversize pistons, but I am really stuck on this one!

Funnily enough, i spoke to my local yam dealer when i was struggling with my bike. All he wanted to do was sell me loads of new parts - he had absolutely no idea. He claimed to know all about every model despite the fact that my lc was older than he was.
Maybe try running premix at 25-1

[daveg]

Thanks for the advice about the cooling system, I will try flushing through and see what happens. I will also increase the ratio of oil to petrol.

Thanks,
Dave

[daveg]

I upped the jets to 250's, and increased the petrol oil ratio to 25:1, however the bike would not pull any revs over 6K. Went back to the standard 240's and it would rev nicely. I still don't know what made is seize the last time, but this weekends run in and racing were great. The bike was slightly catchy at low revs, but up where it counted it was great!! No heating problems all weekend, the bike stayed firmly in the middle of the gauge all weekend. And no clutch slip thanks to the new TZ springs - thx Norbo!

[Dirtbag]

Nice to hear you got it sorted