Hybrid WR/YZ426

I am new to this forum but already found so much useful information on the project I'm busy with. Thank you!

My 2002 WR426 had a heat seize and I got a 2002 YZ426 engine at a good price to replace it with.

I swapped the 2 engines out and installed the YZ CDI.

Valve clearances are fine, valve timing correct and spark test good.

The engine starts and idles perfectly but is slow at picking up revs and also start to backfire and run unevenly at higher rpm.

I did not get a carburettor with the YZ engine so is still using the WR carburettor.

Will this WR carb work as is or should I do modifications?

I am also still using the old sparkplug.

Edited by RooiBoer

Does the engine just stumble a bit in response to opening the throttle and then otherwise accelerate normally?  You can usually tune that out.  Your carb should be usable, although a carb from any YZ450F would be a big improvement.

Does the engine just stumble a bit in response to opening the throttle and then otherwise accelerate normally?  You can usually tune that out.  Your carb should be usable, although a carb from any YZ450F would be a big improvement.

Initially the engine ran unevenly at higher revs but that is all sorted out now. I think it just needed some running in since the intake valves were new and there might have been too much air in fuel.

The engine now just stumbles when I open the accelerator fast. How do I tune this out?

I get much more backfire when I close the throttle compared to the WR engine. Not really a big problem by itself but can that be a symptom of a bigger problem?

Engine has good power!

Edited by RooiBoer

"stumbles when I open the throttle fast"...

 

Does that happen when you are actually riding, or only sitting at a standstill revving up off idle?

"stumbles when I open the throttle fast"...

 

Does that happen when you are actually riding, or only sitting at a standstill revving up off idle?

Both conditions but more so when standing still

Does the engine just stumble a bit in response to opening the throttle and then otherwise accelerate normally?  You can usually tune that out.  Your carb should be usable, although a carb from any YZ450F would be a big improvement.

I've considered doing this to both my WR and YZ. The WR has a bit of a stumble when I open it quickly at speed (I don't know what it does at idle, I don't do that), and I haven't been able to tune it out. Would I want the same jetting as now? I need to start looking for carbs for the right price...

 

edit: what years are best? How are the quad carbs?

Edited by toten

The quad carbs are like the WR carbs.  They're set up with an air cut system to be compatible with the air injection systems,  and that makes them a pain in the butt.  Any '03-'09 YZ450F carb is a near direct bolt-on fix, and the stock 450 jetting works great as a starting point.  The '05 - '09 carb is preferable because they are slightly more sophisticated internally and also have a leak jet that adds another tuning tool to your bag of tricks. 

 

You will need the hot start assembly, including the cable and lever/clutch perch, and the 450 throttle cables.  Try to get as much of that with the carb as you can.

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