Throttle Response Issue - Brainstorming

I've come to the point with a problem I'm having that I feel like I'm out of ideas on how to solve. So, per the title I thought I'd make another post and see if I can't get some ideas from others on possible solutions.

The issue I'm having on my 05 yz144 is bad throttle response. I've had this problem since I bought the bike 3 years ago but did not realize how severe the problem was until my friend got a new 09 yz125. Once I was able to ride his bike and directly compare I knew something was not right and set out to fix it. However, I have not been able to come up with a solution. It's occured since I've purchased the bike and still occurs now after the rebuild with the EG 144 kit.

The best description I can give is that the bike has very slow throttle response. To directly compare to a new yz125, 0-1/8th throttle is much slower and 1/8th-1/4 is slightly slower while anything from roughly the 1/4 throttle range on is exactly the same. It's very noticeable while riding the bike with actual load on it while it does not seem as bad if you're just sitting their trying to test it, like in a parking lot for example. The bike will do a uhhhhhhhhWAHHHHH with the uhhh being a slow climb through the rpms until it finally picks up. The 09 will simply fly through the rpm range with almost instant response.

Things I've done so far:

Bike has a EG 144 kit now with 5 hours on it so fresh cylinder and top end. The top end was leak tested 2x with the second test just 2 hours ago and showing 0psi loss after 5min at 6psi pressure.

The engine has a brand new OEM reed cage with reeds

Fresh spark plugs are constantly run through the bike for testing or after rides with trying to problem solve.

The problem occurs with the stock carb AND my lectron. The lectron just makes it SLIGHTLY better.

I checked the timing and set it per manual specs.

Air filters are always changed after every ride.

Powervalve is working correctly.

I ran 93oct gas on the stock engine at 32:1 and I run 90 oct/110oct at an effective 95oct at 32:1 for the EG 144.

I do not lose coolant nor do I lose tranny oil.

The only things I can think of that I haven't tested that some how MIGHT be causing the problem is the coil and cdi. I plan on swapping those off my friends bike during a quick test to rule them out and well as checking the timing one more time just to do it. So after all that does anyone else have an idea on something I may have missed? =]

Edited by kan3

Just throwing an idea out there, not sure how relevant it is:

Is your head / squish set correctly? I'm always reading around on forums and lots of guys say having their head cut / bowl cut and squish set properly makes a big difference.

Anytime you can swap parts from one bike to another for testing, that is a good thing.....especially electrical parts.

As long as you're running good gas and your jetting isn't on the razors edge, I would try advancing the timing a couple degrees and see how it responds.

+1 on the advancing the timing

After I check the cdi/coil and confirm the timing I'll try advancing it to see what happens.

Is your head / squish set correctly? I'm always reading around on forums and lots of guys say having their head cut / bowl cut and squish set properly makes a big difference.

It's happened with the stock head and after EG cut the head with the 144 kit. So I don't believe that is the issue.

I got an aftermarket crank shaft in my converted bike and tweaking it towards advanced, all I could go without filing, woke up that engine.

Swap out the plug boot. Just went through this on one of my Yz's directly following a rebuild. Replaced the plug boot and problem solved.

Sad part is that I tested and spent a bunch of money on parts I did not need. Always test the easy stuff first!

+1 on the squish. I notice a faster throttle response when i set mine.

Still could be squish - just because it has done it with both heads doesn't rule it out, just means they both could be on the large side. Have you checked squish clearance on the 144? Also, what's the squish area?

Ignoring a TON of variables, in general, a larger bore will require slightly more timing advance. Might be 1 degree, might be 8 degrees. All has to do with burn rates and getting peak cylinder pressure to occur at the optimum time after TDC.

Not running a flywheel weight are you? That was the one reason I ended up selling mine, it made the bike much slower to get on pipe, required more clutch work, or being a gear lower, coming out of corners. I found it less work to go fast without it. Seemed similar to what you are saying, without fww = BRAAP, with fww = uh...BRAAP. Not a bog or even hesitation really, it just took a second to get things moving.

I'd also add that often being a little rich on the bottom can help snap and pickup, but since you've been through this with two very different carburetors I'd guess that's not the issue.

Edited by adam728

I'm not currently running a fww. I think what I'll do is next time I get a couple hours of testing I'll check on the entire coil assembly, then cdi and then timing advance. If none of those produce a solution then I'll move on to checking the squish on the head. I would be pretty annoyed though if it was something stupid like a spark plug boot.

The issue I'm having on my 05 yz144 is bad throttle response. ... It's occured since I've purchased the bike and still occurs now after the rebuild with the EG 144 kit.

Was the bike a 125 when you purchased it?

What is your gearing?

Was the bike a 125 when you purchased it?

Ya

I've owned it for around 3 years or so.

What is your gearing?

Current gearing is 14/49. On the stock bike I used 13/48 and 13/49. On the 144 I tested 13/48 and currently 14/49.

I believe there were some improvemens in the yz125 engine in 2006, and then the only performance improvement after that was a reed spacer redesign around 2008 I believe.

Can we get a list of what was changed in 2006? I know the cyl head was one thing. Not sure how it differs though.

Is the carb the same? How about the ignition mapping?

Also, since you mentioned that the difference is all between 0 - 1/4 throttle this strongly suggests jetting. What jetting were you using when it was a 125? What is your friend's 09 using?

Also, since you mentioned that the difference is all between 0 - 1/4 throttle this strongly suggests jetting. What jetting were you using when it was a 125? What is your friend's 09 using?

It could be jetting but the same problem happens with the stock carb and my lectron. The 06+ carb was changed slightly internally as far as I know and got a slightly different needle. My friend has the stock jetting except a 37.5 pilot. My stock carb has been tested with 35-40 pilot and clip 2-4 as well as with a -81 needle. I kept thinking it was a jetting problem but the lectron has the same issue even after my PV has been fixed which makes me think it's somewhere else. I could take his carb and swap that for a test as well.

I'm hoping to get a chance to test the electrical parts and timing this week but it may have to wait.

Edited by kan3

I could take his carb and swap that for a test as well.

If your friend will let you swap stuff you have an ideal situation to fix this. I would swap everything I could until the 05 wakes up and runs right. Try everything. First thing: piston/cylinder/head/powervalves. That's the biggest variable you've got now. Eliminating this says a lot. Next, pipe/intake boot/reed block/carb. Next CDI/coil/plug wire/stator. Eventually when you have his entire bike running on your frame you will find the one part that made the difference

almost IMO, the only thing remaining would be timing which I would check, or rather advance first.

Small update

Swapped the coil, plug wire, plug and cdi over today. No change to the slow response. We both noticed that the midrange seemed a bit more powerful with a touch more over rev. I assume it's the small changes that they made to the cdi in 06. Going to test the stator and mess with the timing next.

Edited by kan3

Tested the stator and it checked out ok. Advanced the timing roughly 1.5-2 deg and it did not help with the issue. I do believe I've exhausted my options.

uff not too good, have you tested the compression of both bikes?

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