Big valve heads - worth it??

[ar011511]

Wondering if I’ll get a noticeable increase in power from a big valve head. GY6 50cc with generic 50mm overbore, 20mm carb, foam filter and de-baffled exhaust. I get around 40-42mph at 7,500rpm. Stock cam, tried an A9 but didn’t notice any improvement. I’ve seen a few big valve heads on eBay for $50ish, are they worth it? Hoping to get closer to 45-47mph. Also, do I need to worry about piston/valve clearances?

[GrumpyUnk]

I am not too certain the valve diameter is larger, but if the valves were large, they could possibly flow more in and out. Increasing flow in an air pump motor should increase power output.
Most heads are the same, so you would have to check the bore the heads were designed to fit, and then find the actual valve head dimensions. Ask the vendor.... good luck. Most do not know how to operate a ruler, much less a micrometer.
All the 'big valve' things I have seen refer to the valve stem length, but the actual valve size and seat size are the same even with different stem lengths.
If you do swap in a 'big valve head', you may want to check valve:piston clearance... I do not know if the vendors or their suppliers would do so, as all they want to do is sell product.
tom

[ar011511]

This is the head I’m looking at. It’s 50mm like my BBK and the valves are visibly larger. Not sure how it’d be with valve clearance though.

www.ebay.com/itm/185029737612

[claydoh]

I have that head, the valve clearance should be fine. I had a clearance problem that ended up being a crappy cam - the "A9" moniker doesn't seem to mean anything consistent, and whatever one I had was out of spec somewhere. I have an SSP-G one now.

But don't expect to get an extra 5mph out of it. You may see some, but you may want to make sure your carb really is a true 20mm to be able to feed the potentially larger flowing head. There is a long thread on the topic here, somewhere.

I recently replaced my Airsal all-alloy BBK kit/STG head/20mm carb with a normal steel-lined BBK as the Airsal cylinder didn't quite line up 100% perfectly, and I was bored. I ran the stock head/19mm carb for a bit, just for comparison before swapping to the STG head. There was a difference but not outrageous. Def. worth it, though.

I am quite a very large (fat) dude, and that is a factor, and I have been sick lately and have not really fine tuned things much as of yet. Plus I can't stand the fart noise of an open pod filter, so I may be missing out

So, it may not get you the full 5mph, but is worth doing imo, especially if you have a good carb, and probably a good variator etc.

[jackrides]

Adding to claydoh's comments: bigger valves in a stock bore will reduce velocity, so add a larger bore (not the biggest) for best results. Engines are a balanced compromise on all factors.

[ar011511]

Thanks for the info. I’m going to go ahead and give it a try this summer. My carb is definitely 20mm, measured with calipers to make sure it’s genuine. Stock measured 18mm. I’ve got a 50mm (82cc) overbore, so this head should match it a bit better than the stock head. Variator itself is stock, but my scooter seems to have slightly higher gearing than most. I only run out of gearing going down hills at around 50mph. I’m running 5g weights and a slightly stiffer contra spring for ~7,500rpm.

[jackrides]

Cool! Seems a good balance. Measuring a carb maximum venturi should be in the center, over the needle jet, both vert and horz. That place has the highest velocity (minimum diameter). For best info on weights and other cvt issues, look in index. Matching port meetings is important and must be done carefully to blend diameters together without expanding diameters or steps in size. Smoothness, not polish, is important. Piece of mind at this time is important. Take clean mind/work breaks during this. Keep us posted please!