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Adding Weight to XR200 Flyheel

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I put a late flywheel (and electrics) on an 82 XR200R motor and the almost 10oz of extra flywheel weight made a difference in engine smoothness. So when I started the 218 build, which would increase compression pressure, I wanted more flywheel weight. My plan was to use a TLR200 crank because of the heavier flywheel but Powroll tooling couldn't accommodate the larger taper on the TLR crank.

Since the build I felt the engine tended to "flame out" easier than a stocker, which I attribute to the 11:1 compression ratio although there may be other factors.

72FBR93 posted (https://thumpertalk.com/forum/showthread.php?p=10721957#post10721957) that he added a weight to a CRF150RB, which got me started again on adding a flywheel weight. The following pics are of a ATC200 flywheel showing OEM added weight, an 86-02 XR200 flywheel, and a TLR200 flywheel showing its much larger hub and thicker rotor rim.

Any input and/or suggestions would be appreciated, my research to date:

81-83 XR200R 53.8oz

86-02 XR200R 63.3oz

........ ATC200X 52.3oz

......... TLR200 84.4oz

First two pictures; ATC200X vs 86-02 XR200R, notice the OEM added steel disc weight on the ATC rotor.

RotorsATCvsXR200R2.jpg

RotorsATCvsXR200R1.jpg

86-02 XR200R vs TLR200, the TLR (on right) has a much beefier hub and in the last pic you can see it also has a thicker rim.

RotorsXR200RvsTLR1.jpg

RotorsXR200RvsTLR2.jpg

The problem is the TLR flywheel has a larger taper than the other two valve motors so that leaves adding a weight to the XR flywheel, perhaps like the ATC flywheel, but I want to avoid the problem 72FBR93 encountered with his CRF150RB.

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Chuck, I know that you ride trails, but are you looking to build a Trials bike? Have you fitted the auto-clutch like you were hoping to?

What is the crank weight that you are working with?

When I took my XL350/486 magneto from external 6 3/4lbs. down to internal 2lbs. I didn't have any flame out problems even in tight trails with a standard Barnett clutch and BIG cam and carb and pipe... That was a MAJOR change/decrease in flywheel weight for my bike. Maybe it is a timing problem in conjunction with your stock timing and the stroker crank configuration?

Swiss

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Thanks for the input. Your Trials bike comment hit home because I ride technical trails and also have a 2T Trials bike. But the Trials bike has a great clutch that can be used for very slow speed maneuvers, the 200 clutch is a bit grabby and hard to use at slow speeds (I just change oil type and will see if clutch performance improves).

Crank weight is stock XR200R (the crank is a 94-02 Powroll stroker). The performance of the 218 motor has been a bit perplexing, the thing has so much more torque that I've geared up and run one gear higher and still have more pull than a stocker; but the flame outs occur on stair steps often while in first gear. Cam timing is right on, ignition timing is 9 initial and 29 total. Idle is much smoother and about 200 rpm slower than stock (probably because of increased air flow thru the carb from the stroker). One kick starts when warm. I know if I reduce initial timing below 6 the motor begins to feel lethargic. I've considered using a constant velocity carb to smooth thing out, maybe I need to reduce the CR down from 11:1. That is why I thought about increasing flywheel weight but tuning solutions would be much easier.

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OK, lets look at what you have done to help create the flame outs.

First you increased the compression ratio, yes that would probably contribute to causing the flame outs.

Second you raised your gearing, that would contribute to putting you at lower rpms for any given situation and that would contribute to causing the flame outs.

Third you dropped your idle speed, that would contribute to causing flame outs.

Fourth you added the stroker crank which technically alters your timing relationship with the crank rotation and piston position.

From your descriptions those may be the main contributors to your problem without regard at all to your flywheel weight. If you look at what the major manufacturers have done to their current ultra-high compression race bikes to try to eliminate the flame out problem it is typically to RAISE the Idle Rpm, not lower them. With idle speeds on some of the 250cc MX bikes up around 2,000+rpms they are getting the engine spinning faster in hopes that it will not slow down enough to do a flame out stall! And they even run the acc. pumper FCR carbs!

First thing that I would try is raising your idle back up. Then split the difference between stock gearing and your current raised gearing to see if that will help. Third would be playing with the timing to see what effects it would have on that specific problem without causing the power to soften. That might include looking at different advance springs to alter the advance curve and modifying the advance while adjusting the "max" advance by altering the advance limiter tabs on the mechanical advancer.

CV carb is a possibility, I used to run one on my old 150cc race/play bike back in the '70s. Go a bit bigger than the stock size if you try one because there is a flow penalty with the butterfly shaft in the air stream. I know, I have flow tested and compared CV with slide throttle on a flow bench. You can improve the flow if you flatten and smooth both sides of the shaft around the butterfly but you will still lose some flow. Same with the Keihin style choke shafts, they drop airflow. Main thing is to have sufficient flow for your top end power in your larger displacement engine. Problem with the CV carbs is often the larger size of the top of the carb. I ran a modified smoothed-bellmouth +1mm bored 28mm on my little XL145 and it would let me bump start it and pull off smoothly from idle rpms through 12krpms in 4th gear.

Last option would be to add flywheel weight which would tend to reduce the flame outs but also slow down the crank acceleration when cracking the throttle open. Accelerator pump carb is also an option if you can find one close to the size you want to run. Make sure that your jetting is spot on for all of the above suggested options.

I would not decrease the CR, this is what is helping your engine to be more efficient and make more power. Use the other options.

Just some possibilities to help you look at.

Swiss

Forgot to mention that you are running a larger diameter, heavier Trials tire than the stock knobby. Maybe your Tubliss system equalled them out?

Adding weight is always a tough thing for me to do! HA!HA!

Edited by Swiss
added text

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Gearing is a big one, and switching tires changed overall gearing. The stock bike was OK with stock gearing (47/13) but with the TT I needed to use 49/12 for similar overall gearing. With the Powroll motor & TT I geared up to 49/14 but liked 49/13 better. This is only 4.4% taller gearing than a stock bike with a 17" knobby, but oh so much more traction.

I'll fuss with idle speed and mixture this afternoon to see if I can improve things. A ride on Thursday will be the test. Next maybe fuss with ignition timing but previous research indicated the engine needs at least 6 degrees of initial for reasonable throttle response at low speeds. Right now I have 20 degrees of mechanical advance (stock) and a Powroll recommended limit of 29 degrees total for my build, which is why initial is 9 degrees. I have another advancer that has 24 degrees that would put initial timing at 5 degrees for a much softer bottom end.

As far as a softer bottom end I love the way this thing pulls from the bottom, back to back comparisons to a stock XR200R with stock gearing is eye opening. This thing pulls harder in 3rd on the bottom than a stocker does in 2nd gear.👍 Sort of like the torque of a XR250 but in a 50lbs lighter chassis.

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Chuck,

Another option is using a G2 style cam throttle. You can either buy one of make one like I did. It will soften your throttle action off of idle and still give you full throttle when you need it.

Swiss

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One of the downsides to stroker cranks is rod/stroke ratio and rod angularity. The same crank angle (say 6 degrees) on a stroker motor gives a much more acute rod angle than the original configuration. That will make it more sensitive at low rpm. More flywheel would help pull the crank up to TDC. I'd machine a weight to bring the riveted plate up closer to TLR standards even if you have to turn the outer diameter for a shrink fit. Every stroker I've ever used was sensitive at low rpm as you describe. I can relate to you wanting to use the tractorlike torque of the stroker and also see the only viable option to be a heavier crank assembly.

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On my 218 modded engine I have not had one flame out. I was thinking of going with a lighter fly wheel for better off idle response. I run stock ignition timing with a web cam and a 30mm carb. This engine will pull very high gearing but the RPF (rev per foot) is to high with stock or higher gearing for technical stuff that your trying to get over. Even on big bikes that will pull very high gearing you still have to match the gearing to the speed of the terain.

I run 12x 46 with 18" trials tire. and use 2nd gear 90% of the time. I run the idle RPM a little higher than stock because with the modded engine there a lot of engine braking sometimes too much. I think the clutch is marginal on this engine I don't use it that much but the adjustment goes away when it gets hot and it's very noisey no failures yet but I'm just saying.What kind of oil are you guys using

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I'm sure glad others have observed similar engine behavior. Don't get me wrong IMO the Powroll mod is the best thing you can do to a 200 motor.

Swiss: I modded a stock throttle for cam action; I relocated the barrel anchor right down to the throttle tube OD and ramped the wire groove. I also have an old Magura gear drive throttle that has been modded with a slower initial pull than the modded XR throttle that I can try.

Valvesrule: Yes the torque is nice, easy 3rd gear starts and 3rd is my trail gear. I was concerned about having only three holes to secure the weight to the flywheel so maybe a shrink fit would help.

msgod: I'm running stock ignition but with the Powroll Torque cam and stock carb. And matching speed to gearing can be a problem when riding two very different bikes. I just put on a rear disc so I now have better brake control with less need for compression braking and I could kick up the idle a bit. The engine will idle smoothly down to 1100-1200rpm but I usually set it at 1400-1500rpm, maybe I'll go up to 1700.

Yes the clutch is marginal with the added torque, I put 0.064" shims under the clutch springs but a few full throttle shifts using the clutch can heat it up and the adjustment goes away. I may be ready for new clutch frictions, maybe stiffer springs, any suggestions?

I've been running Torco 20W-50 oil per Powroll's suggestion but just switched to Mobile 1 0W-40, I'll know on the next ride.

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Chuck, I think you should first just try turning up the idle a little on the high side. I need to try again a race report-I feel I have been doing well. But back to your flame outs, I had a few flame outs while raceing H/S the first race of the fall season (still no stroker here, just a stock motor) while in a few real slow tight spots and motor would die, turned up the idea a little on the high side and no more troubles. But when I come back into the pits with a hot motor it is idleing high and I turn it back down.

Tom

ps-I have been enjoying my little XR200R.

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Went riding today and did turn up the idle to 1900rpm and had a flameout at the same spot again! A greasy downhill into a road side ditch, seems like the forks bottomed, and then up a stair step onto the gravel road. As soon as the rear wheel hit the step - flame out while the front wheel is still in the air! 👍

Later in the ride I turned out the mixture screw another 1/4 turn and that helped but still another flameout in a rock garden. During the ride throttle response off idle seemed a little rough and the cold start this morning was longer than normal; had to keep full choke on a bit rather than down to half immediately after firing. I'm thinking a carb clean may be in order.

The carb has two low speed discharge ports and I had similar problems when the one above the mixture screw became obstructed, I wonder if these slightly different symptoms are from the one above the pilot jet becoming obstructed.

In reviewing when this problem started and what changes to the bike:

1. Migrated from E-0 pump premium with a 25% blend of race gas (VP-110) to pump premium E-10.

2. Adjusted the valves for the first time since engine assembly, the intake was only about .001" loose, exhaust .002".

3. Added a Supertrap quiet core to the ISD2 muffler.

One and three also seem like possible contributors.

As a side note the Mobile 1 0W-40 engine oil made clutch engagement much smoother, I like.

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Of course if you had a starting point with modifications that brought this on then you know what you have to do! Undo the mods one at a time and see which one might be causing the problem and then engineer around it so that your mod works but without the problem!

Carb clean, Quiet Core and Race Gas are all easy enough to do and figure this out!

Swiss

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Hey Chuck,

Just got off the phone with Ken at Barnett Clutch discussing some things. Asked him about the difference between their Kevlar vs. Carbon Fiber plates and he said that the compounds are the difference and that they CF plates are used when high temps are expected. Might help a bit on heavy working of your clutch? Just a thought.

Swiss

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The 200 motor could arguably be described as a short rod motor, Stroking it aggravates the condition. Short rod motors are notoriously sensitive to cam duration, intake tract volume and throttle position. They usually get the biggest hp gains at mid to high rpm due to intake velocity smoothing out at the higher volume and velocity. The reason is said to be caused by reversion, the low speed pulsing in the intake tract due to the intake pulse being so short, fast and hard then ending as abruptly as it began. Changing fuel and the other things may have upset the balance you had before. Like Swiss suggested try reversing the changes. May find a work a round there. I had an 84 CR500 with flame out/start issues. Went thru all the usual fixes with poor results. Finally machined a shrink fit flywheel weight and it completely changed the personality of the bike. Went from a 8-9 kick starter to 1-2, never quit when lugging thru tight trialsy sections and smoothed out the incredible midrange hit. The weight was light too, only 5-7 ounces. I just copied a Moose weight with it basically being an L profile ring that just fit over the outer 3/4" of the flywheel. On a 500 the difference in acceleration/response was imperceptable. More flywheel is not always a bad thing.

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Chuck, barring some kind of carburetor problem I'd bet a dollar to a doughnut that tightening up your valve lash is causing your stall.

Since your running a hot cam with more valve overlap and tighter lobe centers, a minor change such as reducing valve lash 0.003 can increase low RPM stall. Think about it - three thousands doesn't sound like much, but if you degreed out the cam you increased valve overlap duration 6-10 degrees depending on the grind.

I both agree and disagree with what Valvesrule posted. I agree that intake reversion is what causes stall, but rod length has nothing to do with reversion; valve overlap duration/lift and lobe centers has everything to do with reversion. Even increasing idle speed is only a band aid; as soon as your RPM drops enough in conjuction with enough throttle opening, intake vacuum can't overcome the pressure pulse created when those valves are open at the same time too long. Just like a hot cam in a car with the rumpty idle. That rump is the motor missing periodically due to excessive valve overlap at low RPM.

I agree that a flywheel weight would help, but if it where me, I'd back off the lash setting where they were previously and try it again.

Good luck,

KDXIdaho

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Thanks guys, lots of good insight. And yes I will start backing out all changes.

I measured several cams and found the overlap and the inlet duration of the Powroll Torque cam to be 5 degrees greater than the stock XR200R cam, but inlet closing is 4 degrees earlier. The Torque cam lift is much higher @.369" vs .321", along with a higher lift rate. The engine does have a much smoother idle than stock but I attributed that to the increase in displacement causing greater air flow thru the carb but the earlier inlet closing may also be a contributor.

I'll start with a carb tear down and clean, then I'll readjust the valves (I'll make the intake a bit looser than spec). Next I'll do a test with and without the quiet core in the muffler. Last is back to E-0/race gas blend.

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The carb cleaning was interesting, first some background: As I posted earlier regarding the carb

During the ride throttle response off idle seemed a little rough and the cold start this morning was longer than normal; had to keep full choke on a bit rather than down to half immediately after firing.

Also during the ride we took a long break and I had to use the choke to restart the bike. Mixture screw was 2 1/2 turns out, indicating that maybe I should increase the pilot jet size, or I thought maybe an obstructed discharge port. The temperature during this ride was about 10-15 degrees cooler than other rides which may be why I hadn't noticed these mixture problems before.

What I found was a clean carb with no obstructions, but the float level was 1mm low. A low float level makes it harder for the air flow thru the carb to pull fuel from the bowl so that would create a lean condition which was corrected at idle with an extra turn on the mixture screw. But when manifold vacuum drops from opening the throttle the fuel flow from the idle port drops and the increase in air flow under the slide pulls fuel thru the pilot jet port and the needle jet; so a low float level could cause an off idle lean which could contribute to flame out. I raised the float level to spec and when readjusting the mixture screw ended up with a closer to normal 1 3/4 turns out.

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Curious as to your progress in finding a happy setting for everything?

Swiss

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Nothing yet, I ran out of time and put the bike in storage until next spring.

The results from raising the float level were encouraging but float level in that carb has always been a bit low, so that makes me think there are other issues.

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On 10/29/2011 at 7:07 PM, Chuck. said:

Nothing yet, I ran out of time and put the bike in storage until next spring.

The results from raising the float level were encouraging but float level in that carb has always been a bit low, so that makes me think there are other issues.

interesting thread -what did you do that eventually fixed the flame outs?

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