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TT500: advice on BigBore and Carburator

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Hi all...

I have been a lurker on this forum for a year now, but registerd today. First off i want to thank you all for making my intro to the vintage MX allot easyer by posting on this forum.

First a little about myself: I am no mechanic, this is my first project, and I'm a noob (hint, don't expect much regarding technical terms, I will have to look them up and translate). Yes, i will use 3rd party mechanics if there is something i cannot do myself. I have also bought the Haynes manual.

Main use of the bike is road use, and maybe a vintage race and gravel roads now and then.

If anyone could please try to explain the compression ratios on big bore kits 90mm.

I know there is a 9:1 compression (if i understand this correctly it is the same as i have today on the standard bore size. There is also one 11:1 kit.

So here I go: What is the main difference between the 9:1 and 11:1 regards to effect, response, heavier to kick start etc?

Does anyone of these require anything else then boring the cylinder?

I think I did see something about a cylinder head gasket that I will have to replace; do I need to do anything to the cylinder head? etc?

Next thing is the carb. Before I put it in storage for the winter, I had an issue with my carb. I was at the gas station, filled it up, and then when I was going to start the bike, I noticed that it was pouring gas out of the overflow tube. The other thing I experience all the time is that it stops on idle, and i hate it when this happens when I stop on red light. I do think that the carb needs a rebuild or should i replace it with a 36mm carb (hint at big bore kit) ? If so, are there any recommendations on type that will fit?

Thanks in advance for all advices !

John

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For the use described, I would stay away from a high compression piston like a 11:1. The TT and XT 500's are solid motors but going to 11:1 compression on any motor is going to make it harder to start and add additional stress on the kickstarter and gears. In my opinion, it's not worth the gain.

In fact, I'm not a big fan of the big bore kits in general unless the cylinder is marred or out of round and needs to be bored anyway. It's money and a lot of effort for an incremental gain. From the description of the state of tune on your bike, a proper tune up (new points, new spark plug, set timing, etc.), new VM36 carb with correct jetting, and supertrapp exhaust will likely get you as much or more power than a big bore kit.

My vintage race bike is running a once over (not exactly sure the diameter) wiseco piston with 9:1 compression, torque cam, carillo rod, VM36 carb, and down pipe with supertrapp muffler (16 discs). It's a pretty mild state of tune with plenty of power for vintage MX and best of all, it will last forever.

BTW, the gas pouring from the overflow was a stuck float or dirt in the float needle seat. Your carb should be rebuilt or replaced.

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I'm no expert but from what I've gleaned there may not much point putting a Carillo rod on a torquer motor as the only thing wrong with the standard rod is that it can stretch if subjected to high revs.

The 36 carb is simpler than any of the standard ones but again I wouldn't bother to spend money on a carb just because the present one leaks. The float bowl needle valve is easily replaceable without a full carb rebuild. If not used for a while the carb's float needle can stick. You just tap the carb body with a spanner and it should unstick, works for me. If the problem doesn't fade away with use then you may need to replace the needle. That may be your idle problem. You can check by turning off the gas and seeing if the carb starts happily idling before it runs out of fuel. If not stuff like the external idle adjustment screw and the pilot jet adjustment may easily fix the idle problem if you follow the manual. Mostly the reason folks put the aftermarket 36mm on is that that it's a really simple carb. Some Aussie tuners use the standard carb as it makes better power, something that was data checked over 30 years ago. However if you change a lot of stuff in the motor the 36 may well work better. Standard carbs were I think either 32 or 34 and the USA carbs got a few features that the euro carbs did without and that aren't on the 36 and are hard to fix once they wear out

One UK website reported that Yamaha were selling the piston for the early smaller intake valve motor for the later engine resulting in the valve cutaway on the piston being insufficient. Apparently Yamaha had amalgamated the part numbers so that you automatically got the early piston whatever you ordered Folks were very unimpressed. Another reason for not increasing the compression ratio is that modern fuel is lower octane than it used to be even on supposedly the same octane rating so you could get preignition if you up the compression ratio. I get that on a 30 year old kawasaki single when I race. I've had to buy octane booster.

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I had a SR500, the street version of your bike. I put a 38mm Mikuni on it, and a Wiseco piston, .5mm over, not big bore. I think it was 10.5:1. It wasn't much harder to kick than stock. It is all technique, anyway. The carb made it run very well. I sold the bike almost ten years ago, so I forget the specs, but I got my carb from the now-defunct White Brothers.

I'd see what Mark over at Thumper Stuff (website of the same name) is recommending these days. He's a great guy to do business with and a great source for anyone having one of these singles.

The stock rod is strong enough to take whatever you should do with this engine. Note I said should. I used to run my SR very hard, redline each gear on acceleration, etc. It didn't care. Keep in mind that length stroke sort of enforces the redline they have set. Its easy to float or bend valves on these with a free flowing exhaust and a K&N air filter. They are 1/2 of an old Virago engine. Resist the urge to run thick oil in it. It may be air cooled, but I would suggest Mobil1 10-40 Racing 4T. Clean the strainer at the bottom of the front frame downtube, it likes to collect stuff that restricts oil flow.

I wish Yamaha would bring back an updated version of this engine with DOHC, FI and LC. Mounted in an aluminum frame like the WR250R, it would be very impressive.

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I agree with Cloggy. My Carillo rod is overkill for a road going bike and not even necessary for a race motor like mine. I didn't mean to imply you should build a motor like mine. I was in the motor freshening up everything, so I went ahead and installed one. I wasn't needing a stronger rod. But I did want a stronger upper end bearing. That is a weak point on the stock rod. Now my motor will easily out last my Vintage MX "career".

The stock carb is fine and cheaper to just rebuild. However, I personally got better performance (good throttle response and better mid-range) when I switched to the properly jetted VM36 on my then stock motor. So, I suggest that upgrade to any one willing to spend the money. Besides the VM is so damn simple, field stripping and cleaning it on the side of the road/track is easy.

If your bike is not idling and fuel is pouring out the overflow tube from time to time, address those problems first. As Cloggy said, rebuilding/cleaning the carb should fix it. Get a manual and follow the directions to set idle speed and the idle mixture/air screw.

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