Briggs 5 hp tuning question??

[smwtnbndr]

Put a 5hp back together. ARC billet rod, new rings, shaved the brows and cleaned up the porting. Valves are lapped in. Running a Briggs downdraft carb. Governor is removed. Going to order a ARC flywheel this week. When I run the bike towards the end of its revving limits, I let off the gas I get a rumble, gargle, backfire? I assume the valves are floating?? I am running stock valve springs.
What is the right fix for this or is this the norm? If heavier valve spring are the answer, what weight??
Any help is appreciated.

 

[125ccCrazy]

don't sound like valve float if it only does it when you let off the throttle after full throttle run, that sounds more like a tuning issue or exhaust leak and the exhaust is sucking air.. What cam are you running?..

 

[5horsebriggs]

Mine does the same thing but its whenever i let of the rhrottle from mid ir higher im asuuming thats the vales for me

 

[smwtnbndr]

Running a stock cam.

 

[125ccCrazy]

with a stock cam you can run 2 exhaust springs or run a set of 1.360 Dyno stock springs and you should be good...

 

[Venom Engines]

put a dyno 003 cam in, and update the springs to 1.420-1.450. then your should be pushing 6500-7000 rpm before it even thinks of floating the valves.

 

[midtown mini]

it is possibly an intake or exhaust leak, typically this will cause a lean condition, clean up your intake and exhaust seals first. A cam or springs will magnify the problem.
Spray carb clean around the intake and exhaust manifold while its running if you hear the tone of the engine go up ....there is your problem

 

[The Restore Kid]

Yeah it's probably just unburnt gas, being the engine is already turning and pulling like a bitch, then ya shut off the air to it, it loads..

Those briggs Flowjet carbs aren't exactly designed for Indy Car style applications either.. Pretty well anything will do that that isn't fuel injected or running metering rods or something..