Uneven Acceleration

[fatcharlie]

Hi all..

Stock motor on a 86 Aero 50, DDX variator with 41.1 grams of weight total, Leo Vince pipe - 90 jet and 90/90 tires front and rear. The belt measures around 15.3mm. Bike usually takes off and runs relatively smooth up to top speed.

In the past few days, I've started feeling the bike dropping RPM through the full range of acceleration, it isn't a smooth subtle drop, but more like a few hundred RPM at a time.

I removed the driven pulley (rear) and inspected it... didn't notice any outstanding issues except for worn clutch pads and a scored clutch bell... I cleaned up the pads and clutch bell and the bike seemed to run great for about 50 miles.

Since this has started again, I decided to try swapping the entire pulley/clutch assembly with another one that I have... this one seemed to have good pads and the clutch bell was not damaged. Both the original pulley and the "new" one seem to have a similar feel to them when I squeeze the pulleys by hand (testing the center spring).

Now with the new rear pulley, I'm noticing my RPMs jump up (beyond what I'm used to from when it ran well), then drop down significantly as the bike starts moving... through the acceleration range, I'm noticing the RPMs are dropping, sometimes significantly (a few hundred RPM at a time) and sometimes smoothly.

From what I have read, this can be an issue with the center spring (AFAIK - both pulleys are stock). Should I try swapping the center spring with something heavier? How much should I go up? I'd like to swap out the clutch pads while I'm at it.

Ideas?

[bonesv]

Sounds like the contra spring is either binding, or it has lost it's spunk.
Replacing it with a mild upgrade seems appropriate.
1000 rpm over stock is mild, but you might like a new stock spring better.
The new clutch "shoes" could help if they are really worn down, as well as the little springs.
You could try some new stock ones there, see if it helps. Especially if they are all 25 yrs. old.
Heavier contra spring might need heavier roller weights, so maybe get some of those too.
This is all trial and error with these springs, so have fun with it.

[Wheelman-111]

Greetings:

Contra spring going soft wouldn't make the RPM "jump" lower but only drop gradually. A dry pulley might, but I'm betting notched rollers. Also check that the rear pulley spins true.

[bonesv]

You have 6.7g. rollers in a DDX vario. Nice L/V pipe and a 90 main jet. Otherwise stock AF05.
First try the heavier rollers. If it makes it worse, put the 6.7g. rollers back in and use a stiffer contra spring.
This is assuming that your motor is running in tip-top shape. If there are other mods you have not mentioned, or the compression is low, etc., we will not be able to arrive at a solution. Check compression. Be sure there are no intake or exhaust leaks. Check the roundness of the roller weights. What is riders weight.

[fatcharlie]

Greetings:

Contra spring going soft wouldn't make the RPM "jump" lower but only drop gradually. A dry pulley might, but I'm betting notched rollers. Also check that the rear pulley spins true.

Some very very minor flat spotting on the rollers... can barely feel it but its visible when closely examining them in bright light.

I have another full set I can swap out to test them.

[fatcharlie]

You have 6.7g. rollers in a DDX vario. Nice L/V pipe and a 90 main jet. Otherwise stock AF05.
First try the heavier rollers. If it makes it worse, put the 6.7g. rollers back in and use a stiffer contra spring.
This is assuming that your motor is running in tip-top shape. If there are other mods you have not mentioned, or the compression is low, etc., we will not be able to arrive at a solution. Check compression. Be sure there are no intake or exhaust leaks. Check the roundness of the roller weights. What is riders weight.

Motor is in great shape, no noted intake or exhaust leaks... performed a compression test earlier today and hit 118psi. I weigh 215 lbs.

My biggest concern is that this just started... bike has been running very strong up until a few days ago.

As I mentioned in my reply to Wheelman, I'll try swapping out rollers with another fresh set (same weight)...

[bonesv]

Good on the rollers, give it a try. That 90 main jet sounds a little rich, are you pre-mixing your fuel?

[fatcharlie]

Good on the rollers, give it a try. That 90 main jet sounds a little rich, are you pre-mixing your fuel?

Not premixing...

Was running an 88 and found the plug to be a tad lighter in color than I liked to see, and was seeing CHT (on a TTO) around 375+ on a mile plus WOT run.

[maddog]

That 118psi tells me you just blew a crank seal

[fatcharlie]

I like to blow guys!

d*** dude... TMI.

[Wheelman-111]

Greetings:

118PSI isn't a bad compression ratio. Works out to about 8:1. Shouldn't affect performance at all.

[fatcharlie]

Ok... so I've changed the rollers out to a brand new set, tried dropping my jet from a 90 to an 88 in case what I was really noticing was 4 stroking and retuned. (added a tach as well)

The issue is still happening. Here is what I notice. If I'm stopped for a while, or the bike isn't completely warmed up, I'll accelerate at about 7500RPM, as the bike gets to the top end, I'm cruising around 6500RPM.

Once the bike is fully warmed up, I'm accelerating around 6500RPM and cruising at the same.

The 88 jet puts me around 380 - 385 degrees at a WOT run.

Today, I'm climbing a hill, and couldn't get over 5500 RPM and ~20 MPH. This morning, i climbed the same hill at 6500 RPM and ~27MPH. I stopped for a short moment on the side of the road and took off again. This time I was able to get to 6500 RPM and about ~27 MPH.

I also felt the bike do the same thing i've been noticing, and watched it drop about 200 RPM on the tach.

[bonesv]

Well, what's the roller weight? You want some more weight if it's spinning and going nowhere. Also,

What is rpm rating of the center spring in the clutch?

[fatcharlie]

Well, what's the roller weight? You want some more weight if it's spinning and going nowhere. Also,

What is rpm rating of the center spring in the clutch?

Rollers are 7 gram. (42 total grams of weight). Contra spring is stock.

When the bike gets up to 7500 RPM, it does great... it just seems to fall out of powerband and drop to the 5500-6500 RPM range. This is not a constant thing... its intermittent.

[bonesv]

Well, what's the roller weight? You want some more weight if it's spinning and going nowhere. Also,

What is rpm rating of the center spring in the clutch?

Rollers are 7 gram. (42 total grams of weight). Contra spring is stock.

When the bike gets up to 7500 RPM, it does great... it just seems to fall out of powerband and drop to the 5500-6500 RPM range. This is not a constant thing... its intermittent.

Ok, let's start over here. How many miles on the ddx variator? I have had the bushing go bad on a ddx in just 500 short miles. Check for excessive play. It should have only a very little.