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Discussion Starter · #1 ·
Hi all: I hope everyone is doing okay.

To introduce myself, I am a Yamaha 1986 FZX700 owner who transplanted an FZR1000 cylinder block, pistons, cams, and carbs to the FZX700 transmission. For air filtering, I’m using individual K&N air filters (one for each carb body.)

My problem is this, and I’m absolutely perplexed by it:

The bike starts fine, but the air filters on the carbs frequently “puff” back, as if pressure is coming up from the cylinders occasionally and pushing back through the carbs. I’ve checked NUMEROUS times that the camshafts are installed perfectly. I’ve even removed the FZR1000 camshafts and put in the FZX700 camshafts to see if that solved the problem…but the same problem occurs.

The only variable I can think of is that I do not have the crankshaft in the proper position, but I believe that I have followed the manual’s directions.

Do any of you have any input on something I could try to fix this problem?

Thanks very much!

Elimax
 

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Interesting project!

I would suspect ignition timing (too advanced), or cam timing. A lean mixture will also tend to pop back through the carbs. Does running it with a touch of choke change anything?

Out of curiosity, did you do any work to the crank to balance things for the new top end?

Post some pictures sometime - sounds really interesting!
 
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Ace Tuner
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Wow, I know anything is possible but this one sounds like a real project.
You're braver than I. :surprise:
Assuming the cam, crankshaft and ignition timings are correct that would (?) leave you with the carb tuning probably being out of whack.
Are you using stock jetting in the carburetors?
Pod air filters make for very different tuning than what the stock air box would use and popping back thru the air filters makes me think maybe you have a lean condition?
What do the spark plugs look like after running the engine?
This is a very interesting project...
 

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Might be helpful to try to put the stock air filter box on - see if that changes things.
 

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Are you using stock jetting in the carburetors?
Pod air filters make for very different tuning than what the stock air box would use and popping back thru the air filters makes me think maybe you have a lean condition?
Yep yep. When I decided to reroute the intake on my 250cc scooter I stopped the engine from inhaling the crank vent and made it exponentially easier to change the filter. Unfortunately I also made the scooter run so bad it topped out at 40. Pod filters have basically no restriction on them. And remember, you want to be able to put out what you put in, or whatever.

I just tied a small winter hat to the pod filter. That restricted it enough for the engine to breathe how it should.

Of course, that isn't the proper way to fix this but it is a quick way to help diagnose where the problem is.
 

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Discussion Starter · #6 ·
Thanks, folks, for the replies.

I'll richen the idle mixture first, see if that helps, then pop the airbox back on to see if that helps.

I'll letcha know how it works.

Thanks again.

Elimax
 
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