BBK 50mm Piston Stock Head vs BBK Head or BBK with Big Valve

[jeffjohnston]

I have installed the following:

-BBK 50mm piston kit
-A9 Cam
-95 jet, stock carb and settings
-Air Cleaner
-Stock CDI 8050 max RPM, tried a cheap orange unlimited CDI but scooter was better with the stock cdi
-Advanced Variator with 7 gram rollers
-2k Torque Spring and 2k clutch springs are installed.

1. Stock Head: I have initially ran the BBK with the stock head, it had great power and torque and with my 230 pound body on it. I could go a maximum of 47mph on flat ground. Down side was it was tough on the starter and hard initially to start.

2. BBK Head with Big Valves: I installed the BBK Head with the big valves (wider than stock) and lost performance maybe 1 - 3 mph and it does not go up hills as well. Also it tends to bog when I accelerate hard, so I have to be smoother on the throttle. It does run smoother though and is easier to start. What would I need to do to make this head work well with my scooter?

3. BBK Head with Stock Valves. I have not used this head so I know nothing.

I appreciate your advice and comments in what heads you prefer and any information on how to make the BBK with the Big Valves oerform better.

Thank you,

Jeff

[Deleted]

You lost compression with Bigger Valve head. Either use fatter piston or shave around 1 mm from Bigger Valve head for more pressure.

[humanshield]

The initial compression was to high.
The Big valve head has lowered compression back to reasonable levels.

Check the actual compression.

Did you keep track of how hot the engine was getting?

[jeffjohnston]

Makes good sense, and I appreciate your help. The loss of compression from the stock head going to the BBK Big Valve head makes sense, but boy did that extra power in the stock head feel great! I did use a heat gun to look at temperature and I got 134 degrees on the cylinder head, doesn't mean I did it properly. We were comparing the heat with the new setup against a stock 49cc scooter. I really like the scooter it's fun! We picked up 3 of them and we decided to do mine as an experiment and get the bugs out before moving on to the other two. They are IceBear 50cc with a 1p39qmb motors.

Today, put on a NCY decompression tube and it did help throttle response. I appreciate your suggestions. With the Big Valve Head what would be the best way to shave off 1mm off of it (no machine shops for 72 miles).

Thank you both!

Jeff

[jeffjohnston]

I did not do a compression test. I used to have a tester. If I find it I will do. I am sure it was up there in compression. Even though I want power, I do need realizability and some longevity. I presently use scooter to commute to work in hilly NE Washington State.

Jeff

[mrkswthwrth]

Jeff, to deck the head get a nice thick piece of flat glass, glue a full sheet of sandpaper on it. Rub cyl head on sand paper in figure 8 pattern. Start with a 200 grit or so to get close to final dimension, i finish with 800ish but finer wont hurt.

[jeffjohnston]

Thank you will do!!!

[jackrides]

Hey mrkswthwrth, that's some serious DIY! How much time involved to take off say .010"?

[AtariGuy]

Shouldnt take more than a good half hour, but the more detail you put into your work the better the result. Take your time and do it right. Check it often to make sure you're taking it down level. Then clean it reeeeeeeeeeeeeeeeally well. No grit, sand, dust, hairs, or anything else that'll squirrel up the engine

[GrumpyUnk]

I just used 200 grit to clean up the cylinder heads on two. Both had low spots in the area nearest the chain case/channel. One had a bad gasket that allowed 'trails' from the chamber towards the chain case. Went through 3 sheets of 200 or 220? to get it pretty cleaned up and flat. I then washed with white until the sheets came away clean to insure removal of as much 'grind' as possible. The second head had a slight depression in the same spot. Perhaps it is a design 'feature' that I am unaware of, but my take was "FLattest is bestest", hoping to get the gaskets to seal properly. We shall see. One has been going with a 47mm kit for about 6 weeks now, and runs very well. The other, a 44mm kit was installed, but the starter doesn't want to spin it over. Starter was weak before the change, so must be investigated. Battery at full charge, and will check starter relay for wear/damage.
tom