Make my 650l Rev better

So I'm sure there's a thread on this topic already... I want to make my 650l Rev better and more freely. Already desmogged, removed snorkel, put full exhaust on with a uni filter. Also did Daves mods. So maybe my next step would be mill the head With stage 1 hotcam... what do you think?

Hot Cam.

 

Kiss of DEATH.

 

You have been warned...

Is there a better brand to go with?

Put a high compression piston in there and leave the cam alone.

 

It still won't rev like a multi-cylinder engine, but it improves things.

Tossing the CV carb is the best step toward more rpm. When the butterfly is at WFO and the load is high vacuum is at or near zero. The slide will react by not fully opening until the engine needs more air. So in effect you can't get full throttle. The CV is counterproductive. It was installed only to provide good low to midrange torque and response not rpm or power. Any of the other carbs is an instant 500-1000 rpm. IMO and experience Hotcam failure is highly overrated. I've installed about a dozen now with no failures at has little effect on rpm without other modifications and too high can actually decrease rpm. High compression is most responsible for quicker response and low to mid range power. For high rpm power you need more cam duration to go with higher compression. The higher you rev the less time air has to get thru the cylinder. Without bigger ports, valves, duration or some combination you reach a point where higher compression can't get all the air thru in time and you come up at a hard shift point. The engine just goes flat. It can be very strong to that point but will just stop revving due to lack of flow. The carb, cam and higher compression is what you really need but may not want because of the expense or amount of work needed. The old Baja Commanders ran so strong and had much faster top speeds because they had higher compression AND much more cam duration.

Thank you for the input fellas. I wondered about that slide on the cv keihin. As the rpms climb it loses steam exponentially and that was my theory.

 As the rpms climb it loses steam exponentially and that was my theory.

You got it. An air filter change helps, a pipe helps, but that carb just slams the door harder the more airflow you feed it. No matter how much water you put in a toilet it still only flushes so fast.

Lmao maybe a carb off a 600r or something is in store.

I know the more lift and duration a cam has the less vacuum you have. So at that point the cv would be completely worthless.

The 600 still had a CV carb. I swapped my 600r over to an fcr carb from an 05 crf250r. It's a slightly smaller version being 37mm. Most fcr being used are 39mm-42mm off a 450 or newer model 250. Even though I'm running the 37mm it has instant throttle response, and pulled way harder than the CV carb.

I'm going to look into something different for sure. I have an 41 fcr I could use. I don't really want to rob it from my quad though. Lol

After valvesrule's excellent explanation, i have to ask:

 

Has anybody done comparative dyno runs where the only difference was the original CV carburetor compared to an FCR?

What was the overall power difference at full throttle?

FCR carb swap for me made more of a difference in useable power than exhaust, high compression, and overbore combined. The accelerator pump mostly helps throttle response more than high RPM maximum power.

FCR carb swap for me made more of a difference in useable power than exhaust, high compression, and overbore combined. The accelerator pump mostly helps throttle response more than high RPM maximum power.

I know what it does to throttle response, having ridden other motorcycles that were converted to FCR.  I'm okay now with my throttle response with the original carburetor.

 

I want to know if it actually increased full throttle power in the upper half of the rev range.  You appear to be saying that it doesn't make a big difference there.

you could also try removing weight from the flywheel/starter gear, isn't that good for 3 to 5 pounds rotating mass removal?and removes that much weight from the bike itself but keep in mind there is a trade off.

I assume that would make it stall a little easier and lose some torque maybe?

Srt8605, on 19 Apr 2016 - 8:18 PM, said:

Lmao maybe a carb off a 600r or something is in store.

I just put a 600R carb on a 3000 mile new 650L a couple weeks back. Vast improvement. Get the cables and throttle too. L cables don't work well.

Edited by valvesrule

After valvesrule's excellent explanation, i have to ask:

 

Has anybody done comparative dyno runs where the only difference was the original CV carburetor compared to an FCR?

What was the overall power difference at full throttle?

You might find a dyno chart from White Brothers that showed stock versus stock. It was not just for carbs but showed about 10 hp better than stock. Years ago I did an ass dyno test on a Mugen cammed, WB piped 531cc bigbore XL500S. It still had the stock carb. I swapped on a stock XR500 carb and you could really feel the improvement in all aspects of performance. Then I put the factory Honda hop up 37mm on and saw more gain. Keep in mind the stock carb had an accelerator pump while neither of the others did yet they were both far stronger. I'll see if I have a link to the WB dyno chart.

you could also try removing weight from the flywheel/starter gear, isn't that good for 3 to 5 pounds rotating mass removal?and removes that much weight from the bike itself but keep in mind there is a trade off.

I don't think it's anywhere close to 3 lbs but it is still quite a bit of rotating mass reduction. It won't affect real torque numbers much but can make stalling more common. Won't affect peak rpm either. Of course the acceleration improvement is the flip side.

I'm not sure the CV carb doesn't pull the slide up all the way. There are a heckuva lotta street bikes with CV carbs.

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