2 cycle exhaust
- Thread starter Komet99
- Start date
Pete_Muller
Moderator
Questions:
Are you running a carb trigger?
Was the winner running a carb trigger?
Did you experiment with pipe length and gear ratio for each pipe you ran?
Did the weather change from Saturday to Sunday?
I have many, many more, but that would be a start.
PM
Are you running a carb trigger?
Was the winner running a carb trigger?
Did you experiment with pipe length and gear ratio for each pipe you ran?
Did the weather change from Saturday to Sunday?
I have many, many more, but that would be a start.
PM
I do not have a carb trigger. The winner had one but said he didnt use it.Questions:
Are you running a carb trigger?
Was the winner running a carb trigger?
Did you experiment with pipe length and gear ratio for each pipe you ran?
Did the weather change from Saturday to Sunday?
I have many, many more, but that would be a start.
PM
I did not experiment with flex, because when I borrowed the pipe, he said to use the whole header flex pipe package. They had found that to be the best package. Did not try different gears.
The south wind was 15-20 saturday. Wind was 15-25 sunday and turned from the west. Practice wind and race wind was different.
The walbro feeds fuel from the low side through to the high side. Not sure how to explain it. So I typically run hi at 1/4 and low at 2. This gets me a conservative tune to start the race with. With a trigger, would i run the low at 2-1/2 because the hi closes? Then never have the trigger pulled and lift for a corner at the same time.
With the trigger the low 2 - 2 1/4 + - the Hi about 1/2 just so when you pull the trigger it closes the hi ( some set it to leave about 1/8 open) mine is set up to close the hi all the way. That should be pretty close you can't set it in stone will need to adjust accordingly. My first time out with the trigger I took a builders settings which I never checked( my fault should have known better) and really didn't do the engine any good. So I would say if it's you're first time out with the trigger be prepared to do some hand tuning on the low side just in case. Ps with the settings above you may need to turn the low side in to start in cool weather or just pull the trigger in. later Chuck.
flattop1
Dawg 89
( the peak RPM change to higher or lower will tell you which way the pipe is going.)
Are you saying that will tell which way to adjust the flex or that the power band has moved up or down ?
michael collins
Member
It will tell you where the pipe is working better, clutch slip goes up that pipe makes better bottom, just the opposite is clutch drops pipe has less bottom. You can manipulate the pipes power band with flex but you won't have big gains from its main design. Just my opinion on pipes
michael collins
Member
Yes and also Give me your address and I will send you a trigger, I have one left you can have it just pay the shipping. A trigger is a must in road race.
Pete_Muller
Moderator
I had to laugh at this - (not at you at all, but at the situation). This is exactly what I would tell everyone if I had a trigger on my setup.
To possibly add a bit of info to some of the other items discussed in this thread...
Regarding the Walbro carb: the thing to realize about it is that the carb is essentially a weed-eater or chainsaw design. They are designed to operate correctly at two points: idle, and full throttle/load. That's it. Anything we do with them in karting is a bandaid to get it to "somewhat" meet our requirements. There are a number of ways to get to an "adequate" amount of mixture control through the rev range, but none will turn that "sow's ear" into a "silk purse" -- especially when running the exhaust pipes we typically do in karting with a wet clutch. (sidenote: the Walbro actually works much better with a low-geared direct-drive setup while using an exhaust pipe made specifically for direct drive).
Over the years, I have run everything from 6 psi popoff pressure to 16 psi popoff pressure (the "Steve O'Hara easy-tune setup), arm heights up and down a decent amount, jet settings of wildly different Hi-Low combinations.... and had success (and failures, haha) with them all. Road racing puts a different demand on the carb than a short, low-gear sprint track. Very different.
Not looking at the carb, I have found that the 3 things that that influence the overall performance of a Yamaha (well, pretty much any engine, but for now let's stick to Yamaha) are the exhaust pipe, the clutch, and the combustion chamber shape. All need to work together.
Clutch: it's not just at what rpm the clutch slips to, but *how* it engages. This becomes very important with tall gears (think something like a laydown road race kart at Daytona).
Pipe: Way too complicated to be able to state specifics, but the best way to think about this is probably to consider that the exhaust pipe needs to be optimized to perform the best in the rpm range where you spend most of your time on a given track. That's a really general statement, but it usually applies. The pipe also needs to work very well at clutch slip rpm. What rpm?.... could be up or down 500rpm from what most consider to be ideal. The pipe and clutch definitely need to work well together.
Combustion chamber shape: (I'd include squish band in this, but on a Yamaha most everyone has settled on around .030", give or take a bit). But... the diameter and depth of the "bowl" can have a fairly significant effect on the rev range where the engine will run the best (which of course circles back to the exhaust pipe). Very generally speaking, my opinion is that a deeper, smaller diameter bowl/dome in the cylinder head tends to run better in the peak torque range, and a wider, shallower dome tends run run better at higher revs (at peak horsepower or slightly above even).
PM
Great post Pete, as usual. A wealth of knowledge. When i used to enduro race, i watched my engine builder do work on a yamaha KT and raised questions as i saw them. The combustion chamber shape was one question that i brought up. Reshaping. His answer was the exact same as yours. Different shape different power. Took us a whole day to rebuild that thing.
I also understand the weedeater carb issue and what we do to make it do what we want. I sure wish they would let us use a real carb!!!
Id like to hear more about the clutch you speak of and what you mean when you say "how" it engages.
One thing i did for a while is tinker with the SMC axle clutch. Balanced all pie weights to .01 grams because thats as low as my scale would go. Tested with brass weights and tungsten weights. Got it to where lock up speed wouldnt vary more than 30 rpm consistently in a 30 min race.
I also understand the weedeater carb issue and what we do to make it do what we want. I sure wish they would let us use a real carb!!!
Id like to hear more about the clutch you speak of and what you mean when you say "how" it engages.
One thing i did for a while is tinker with the SMC axle clutch. Balanced all pie weights to .01 grams because thats as low as my scale would go. Tested with brass weights and tungsten weights. Got it to where lock up speed wouldnt vary more than 30 rpm consistently in a 30 min race.
Excellent post Pete! Relative to the squish band, I've read that the angle of the squish band can affect performance - more low end or more high end depending on the angle - but I've never found or heard of any specific angle numbers. Apparently that's top secret information! Could I possibly get you to elaborate? Thanks!
Ive been told that with no angle...squish band parralel to angle of the piston....then there isnt any turbulence to push the gases to the center. Too much angle and there is too much turbulence. Either way is a power loss. Once again, meet in the middle for middle of the road performance.
So that answers an earlier question of mine. A motor CAN be built to a pipe....a narrow squish and shallow pocket would perform better with a "peaky" pipe at a track like Daytona.
Pete_Muller
Moderator
Yes, but the trouble with Daytona is, the engine also "hangs" at clutch slip speed for a l o n g time on that track (at least on a road race kart which runs a very tall gear there).
Regarding squish band shape, I have personally found that parallel squish tends to "act" similar to having a smaller diameter and deeper "bowl", and a squish band with a good amount of taper "acts" more like a more open (larger/shallower) bowl. In my opinion, the corner radius where the squish meets the bowl changes things as well, especially if the squish band is fairly "tight" and close to parallel.
PM
Regarding squish band shape, I have personally found that parallel squish tends to "act" similar to having a smaller diameter and deeper "bowl", and a squish band with a good amount of taper "acts" more like a more open (larger/shallower) bowl. In my opinion, the corner radius where the squish meets the bowl changes things as well, especially if the squish band is fairly "tight" and close to parallel.
PM
Pete_Muller
Moderator
I have an article I wrote about 25 years ago that covers, (among other things): how the engagement characteristics can be tweaked on a clutch, (especially a disc clutch).
I'm in the process of "modernizing" the article a bit so it's a bit easier to understand, and will be posting it here soon.
PM
Pete, which squish configuration requires the least amount of spark lead for best performance ?
TIA,WP
Pete_Muller
Moderator
Generally speaking, my opinion is that the lighter the "load" on the engine (lower gear, lighter go-kart), the more advanced the ignition timing can be (and probably "should" be).
Road race setup, tall gear, heavy kart -- less ignition timing.
Of course the exhaust pipe can really enter into the equation as well, but even considering that... I feel the above generally holds true.
PM
Road race setup, tall gear, heavy kart -- less ignition timing.
Of course the exhaust pipe can really enter into the equation as well, but even considering that... I feel the above generally holds true.
PM
rainman
Member
Pete, I do have a question for you. I raced a 100 cc reed PCR TSL98 on dirt last weekend on a dirt oval and other than braking a pipe everything went OK and finished 2nd. Only change I did for this weekend was fixing pipe and change the SMC axle clutch for another one I had since the other was leaking, but using same weights, and same carb setup. To my surprise this weekend the engine would die under load once at the track. Very strange since both clutches are basically the same. Tried opening the car and would still die under load. When I 1st set it and tried at home on my driveway it seemed it would pull out good, but once at the track it bogged and stopped in less than half a lap, and after that it wouldn't even pull off the grid. I changed carbs too just in case. It was on gas, small 24 mm carbs, both 3 jets with the 3rd closed, a German HHC and a Baroni. They worked great last weekend.