Hey there decided i wanted a bigger carb a while ago nothing daft 30-32mm then i realised i could just open the standard carb up 2mm with no difficulties :) so...... i did. took a spare carb top to work made a holding jig then slapped it in the wire eroder at work and sparked her out to 30mm with no probs . looks good to me dont think you could really go any wider as its getting close to a few passageways and the needle seat sticks out abit more then before :P iv not tried her on my steed yet as i thought i would ask some experts on the matter :) my question is what main jet would you guys go with its got the standard 240 in at the moment what do you reckon was going to go with 260 to start with. My steed has pattern pipe on, pinned power valve and some boysen dual stage reeds(not sure if they did anything really) any advice would be apprieciated. Thanks Sam
That's very interesting, if you get a chance upload some photos I'd be interested in seeing what sort of job you've done there.
More air is more fuel so yes it'll be wise to upjet it. But best thing to do is to plug chop it. Jets are widely available and cheapish. So start large and work your way down. You could get away with 240 i'd imagine with no problems. But as soon as you take the snorkel out it'll need upjetting.
My advice (by no means an expert on the matter) would be to take it as it is for a plug chop. Have a look at the colour of the plug. If it's starting to go white then upjet it. If it's still brown then keep it standard and periodically check the colour. If you notice it starting to go white then upjet it. I'd say 260 is a good starting to place if the blistering of the white is mild. Although as said it's better to go too rich than too lean. So if it's severe start at 280 and work down.
Have a look at needle clip position too. It may need raising. As well as the idle mixture screw.
You sound like you know what you're doing but just make sure that air isn't circumventing around the throttle slide.
That's all I can think of ATM. Be interested in some photos if you get time.
Finally managed to take some photos of my carb will upload more at a later date showing you the slither iv taken out using the wirer and with the standard carb and modified one next to each other. For now i have taken some photos of the sodich wire eroder and the small jig i used to hold the carb and a few photos of the bored out carb, i have also fettled the unpleasant lip the carb has on the front edge of the intake off and given it a little polish (couldnt resist) what do you think anyone want thier carb boring out lol :P to be honest im surprised no one else has put any pictures of fettled carbs up that lip that comes on them is terrible its got to be disturbing smooth flow into the carb like mad would think this would be more benificial then the intake manifold mod (which as many have said and il agree with i dont believe the intake rubber is a restriction rather a design feature to maximise optimal flow into/onto the read block. every other 2 stroke i have seen has had these guides to some degree)
-- Edited by sheddy5912 on Monday 27th of January 2014 10:18:40 AM
-- Edited by sheddy5912 on Monday 27th of January 2014 02:19:28 PM
Slapped carb on my bike last night after and was very happy to find she started up and runs fine with the opened up carb with a 250 main jet. Was very surprised at how different it sounds on idle and low revs, sounds obout the same up top end really....loud :) seemed keener to me in midrange and when it hits the band (goes nutty at 6500) and now sounds very differnt on deaccelleration (guesing because it is always sucking in more air then usual?) couldnt test it properly as my lights dont work at the moment. So only tested on a small stretch of road will be going for a good blast on the moors saturday and i cant wait to see how shel go on a big rocky straight and up the bumps, hopefully be abit more competitive with my mates kdx200 lol :) Next on the list to do is lighten a flywheel i have laying around did it to my last dt and liked it, sadly have no photos or proof of my last lightened flywheel went with my bike when it got stolen :( lucky begger probly doesnt even realise its custom. Anyhow when i do the flywheel i will take many photos this time so you guys can see how i do it (and its not with a cutting dick thats for sure :P no offence intended NORFE) Thanks Sam
Cheers AndyYam il kepp checking my plug, im going to put the needle clip up one position aswell for abit more peace of mind. Was well capped this morning went in my cupboard of bits to grab the spare flywheel to take to work and skim down and..... eh? 2 flywheels :) forgot that when i lightened my flywheel for my last bike i did a tester that came loose when machining and dented the ring that holds the magnet onto the inside of the flywheel ever so slieghtly. so i didnt fancy/dare use it on the bikes so bought another and reapeated (without the **** up) it may be fine but didnt want to cahnce it. anyway have both flywheels at work now so i thought id show you all some photos. Forgive the dull sandblasted finish regretted it as soon as i did it lol
-- Edited by sheddy5912 on Friday 31st of January 2014 03:11:30 PM
Well went for a good thrash on Saturday the bike defiantly goes better when on song only bad thing was iv still got slicks and couldn't put the power down very easily as the wheel was just spinning up, no problem though as iv got some maxxis IT nobblies on the way this week :) kept checking my plug throughout the day and it looked ok could of been abit darker brown but I never got round to putting clip up a position (lazy) iv also got a 260 jet on the way for safety. Cheers mr carnage always nice to get a compliment :) Shotti iv seen and read Norfes flywheel reduction thread its actually what made me do mine last year :) iv also read about the fact that a lot of people use flywheel weights on larey mx bikes to give them more centrifugal inertia weight to keep the bike pulling smooth and steady up hills and to stop the rear wheel spinning up so easy in the soft stuff. I don't really think this makes much difference on a dt it went well with the lightened flywheel on my last dt the only thing I did really notice is it is noticeably easier to lock up and stall the engine when braking down a steep hill. I mainly want it lightened so it flys up the revs easier when it hits the band, have you seen the size of the yz125 flywheel :O, I am not too bothered about a smooth delivery (that's why I love 2 smokers). My mate looked at his kdx flywheel the other day...."hello whats this" there was a flywheel weight on it which was quickly removed and he prefers it,bit more nutty delivery :P on another note was thinking this morn on a easy was to raise comp and optimise spark location by simply dropping the spark pulg 0.5 or more into the combustion chamber would be easy peasy what you guys think.
scrap that last sentence lowering the spark plug is a daft and pointless idea and would probly loose performance with it being stuck out further in the combustion chamber.
-- Edited by sheddy5912 on Monday 3rd of February 2014 03:02:43 PM
Sneaky peak of my new lightened flywheel I will upload a more detailed thread on it later on with lots of pics. I didn't go as crazy as I did last time have only taken the outer of the plate on the front off and put a big counter bore 4mm deep and 46mm diameter and increased the size of the original counter bore for the nut by about 5mm. The weight is now 1.04kg down from 1.47kg :) might have to take a tad more off to get it down to spot on 1kg lol anyway heres the preview pic.
oh and what do we think on the finish? sandblast again, leave as is or polish it upish
-- Edited by sheddy5912 on Tuesday 4th of February 2014 10:12:03 AM
Norfe did a flywheel mod, it let him rev off clock and build rev's a little faster
but calum found some info saying the weight actualy helped the centripetal force
for more low end torque
I've done extensive research in Lightened flywheels. My ultimate decision was that it's a no no. It unbalances the engine putting serious strain onto the bottom end.
If you lighten the fly wheel be prepared to modify the rest of the engine. Or expect to do more often rebuilds.
It's not something I'm going to do to my DT.
Kyle's done it on the RS. Initially thought it was great. Now not so sure.
You basically lose out on that inertia. There is lots of info out there. Lots of contradictory information
Norfe did a flywheel mod, it let him rev off clock and build rev's a little faster
but calum found some info saying the weight actualy helped the centripetal force
for more low end torque
I've done extensive research in Lightened flywheels. My ultimate decision was that it's a no no. It unbalances the engine putting serious strain onto the bottom end.
If you lighten the fly wheel be prepared to modify the rest of the engine. Or expect to do more often rebuilds.
It's not something I'm going to do to my DT.
Kyle's done it on the RS. Initially thought it was great. Now not so sure.
You basically lose out on that inertia. There is lots of info out there. Lots of contradictory information
Yeah I did a bit of research too and would agree. On a race 4 stroke it will allow the engine to rev more freely but it seems to be a case of gaining somewhere and losing elsewhere. Theres a fair amount of weight on these flywheels and in fact alot of race motocross teams run their flywheels with weights added. A couple of interesting points below.
(1) Moments of inertia. A two-stroke can use a lighter flywheel than a four-stroke because it fires every stroke, compared to the four-strokes every-other-firing pattern. Two-strokes dont have to maintain momentum while waiting for the next explosion, thus two-strokes deliver a snappier and more abrupt style of power. This hyper powerband was never a problem in the two-stroke era, but with the four-strokes rise to dominance, a two-strokes rat-a-tat-tat power delivery is foreign to riders bred on torquier, slower-revving four-strokes. That is where a flywheel weight comes into play; it can make a two-stroke feel torquier, broader and more hooked up.
(2) Horsepower. You might be tempted to think that a heavier flywheel would cost the YZ250 horsepower. Not so. It just makes each gear pull further, stronger and longer.
Well now I don't know what to do :) maybe I will stay with the standard flywheel after all.
Had a few thoughts recently, the 1987-1993 yz125 has the same bore x stroke as the dtr (56.0 x 50.7) Im tempted to buy a 1988 barrel iv seen and just have a look if its at all possible to fit it on the dt bottom end. they have fairly crazy looking porting on them and also have exhaust boost ports and a o-ring head. iv had a good look around and cant find any threads or evidence that its possible but im intrieged and wouldn't mind giving it a go. Even if the bore is a no go it may be worth a shot trying the 1988ish exhaust on the dtr if one pops up cheap enough that is, or even use the 56mm single ring yz piston with the yz head for higher comp and less friction, any thoughts?
yeah I will obviously be matching the cases if I do go down that route and im not saying it will be a direct swap just because its got the same bore x stroke Cal lmao :) but it does mean if I can make it fit I wont have to machine any off the bottom of the barrel to account for the longer stroke on the later yz. Yeah been considering just sending a standard barrel away for porting and possibly new sleeve for 60mm forged piston but im just playing with silly ideas at the moment and I wouldn't mind doing something that's not common or well documented. have you or anyone you know tried the flat top single ring with yz head? you get noticeable gains from using the head with the better squish so why not give it a go? theres no harm in trying lad, that's what I reckon anyway although il probly give up after iv toasted a few top ends and go back to standard :P
-- Edited by Sam M on Thursday 27th of February 2014 03:12:21 PM
Flat top pistons run lower compression and do not direct gas flow towards the exhaust port.
The Yamaha TZR 4HW Runs a flat top piston. From which I do have the cylinder and head. Also note that the 4HW runs a nikasil liner instead of steel bore. I do have this complete assembly.
Although do I want to run lower compression? Not really. The fact that the 4HW was a european spec and the 4DL was Italian Spec I'd go with the Italians if I want power.
Note it'll only run lower compression if the you're using a standard head. No idea how the YZ125 head is but I really can't see how a flat top piston could have more compression at TDC that a round top. Due to the round bit protruding into the combustion chamber.
However looking at those transfers on the YZ cylinder I would guarantee you that even if you did get it running you could not match the power of a 3MB.
If you increase the port sizes you reduce pressure created due to the piston moving. Since P=F/A (Pressure = Force/Area). A bigger area causes the pressure to drop. Pressure is needed to pull the fuel and air into the combustion head. For this reason is why I can't see how a 200cc 3XP0 Cylinder can be much good on a standard DT125R bottom end. Yes the Force is increased over a slightly bigger area but Pressure would almost certainly increase. You then leave yourself with the problem that the bottom end is only meant to accomodate the 125 barrel and therefore the fuel/air mixture is setup for 125cc's.
I don't know whether this helps but to give some idea According to Graham Bell there is a limit as to what you can accomodate. This same Pressure draws fuel/air through the carburetor. Which is why in the case of a small cc engine a 32mm carb can be much better than a 40mm carb due to the increased area but same pressure applied.
intresting stuff Cal about the tzr flat top setup actually having lower comp, the flat top piston and head setup on the cagiva mito actually raises compression and surely a 87 yz is going to have more comp then a standar dtr setup?
you really "guarantee" me it couldnt be made as powerful as a standard 3MB? ;)
I understand what your on about with openinig up ports drops pressure but is that not what any tuner i send a standar dt barrel is going to do, not to mention the yz barrel has exhaust boost ports to maximize flow and the rear port is split into 2 to keep flow/pressure up.
Read a few starter books on tuning 2 strokes and used to mess around with my giva alot but need to borrow that Graham Bell book off my buddy and get deep into it.
Why not. The TZR SP is a highly ported 3mb barrel pushing 36 bhp on a 125. Pipey power.
I'm not saying that it would be faster than a yz125. I'm saying you putting that barrel on, even if it worked, you could have achieved the same power that you gained on a standard dtr 3mb. By no means am i saying if that cylinder was on a yz bottom end would it be faster.
I know that is horribly worded.
I just had a horrible revelation too!
I need to confirm this with our resident yamaha lover Scrin.
But the flat top piston TZR 125R 4HW may have used a restricted combustion chamber.
I imported a 4FU cylinder head. That matches 4FU cylinder and it has that nasty combustion ring that the 3BN's have. Now I struggle to see why they would restrict a TZR from 1993 but thats my interpretation.
But it's not uncommon to see Yamaha mismatch numbers to cylinders. So will have to confirm with Scrin as to what I'm looking for.
the cagiva has a combustion chamber to match the flat to piston, it's lower and a different shape to domed piston heads
you can get your 3mb head machined to take o rings and have the compression raised aswell as having the squish at to about 0.7mm for about 70 quid, I messaged a company on eBay that do Tzr 250 head machining and that's what I got quoted without having the cr raised
the cagiva' and the rotax 122 typically are about 12-14:1 depending on the model
the dt is about 7:1
I worked it out and the dt has a combustion chamber volume of 21.5cc
to get 14:1 cr you need to reduce the combustion chamber volume by 11.5cc
0.7mm squish is a 'sweet spot' for squish clearance, but you'll have to measure your deck height
Not only that but there is a reason why Yamaha don't machine the dt down to take o-rings and have loads of compression. There is a reason why they take domed pistons and what not.
It allows for the worst possible combination of rod, rod stretch and piston height possible. So worst case scenario does not destroy the engine.
Aye Cal its because they had to restrict them, lol
No he means because of the varying sizes of con rod length, piston height and barrel when manafactured. Albeit really small differences and the chance of getting 3 all at the max size they allowed for it as the average joe wouldnt be measuring an modifiying each top end rebuild. To prevent the piston hitting the head.
