83cc BBK A9 cam problem
 

Hi everyone,

I'm new to this group all the way from Australia, I have an issue I hope someone can shed some light on.
I have just fitted a 83 cc BBK and big valve head to QMA139 scooter. When fitting the A9 cam I struck a problem with the inlet valve hitting the Piston once the valve lash was set when trying to wind over by hand. I was confident valve timing was correct, I have put standard cam back in with no problems. Has anyone encounter this problem before?
The engine runs well but has very minimal power increase, I would think using the standard cam is to blame.

Thanks UU4444

Hi, and welcome to the forum.
I hope you can read this all the way from Oz. The fonts might get blurry...

First off, the power increase is minimal on any BBK install. You are basically raising torque more than HP. What makes these cylinder kits worth the effort is on hill climbing ability, and take off from a dead stop. You usually have to pair these kits with lighter rollers?sliders in the 5.5-7 gram range, and a stiffer control spring along with heavier fly springs in the clutch.
Those mods allow the engine to rev higher before beginning to roll, and allow the engine to kind of down shift into a better ratio on hills.

Now for the interference problem.
Those big valve heads can be one of two different heads. It can can mean a 69mm valve stem (vs most OEM 64mm stems) or a larger diameter valve crown.
You have to know which version you have.

Most likely they sent you a 69mm stem head.
All is good tho because you can correct the problem by buying a set of 69mm head rocker arms.
If you have the larger diameter crowns you can correct that by shimming the cylinder base with 1-2 additional base gaskets. There is enough slack in the timing chain to allow for this.

Another thing that MUST be considered is carb. jetting.
You will have to increase the main jet from the normal 97(ish) to around 100(ish).

I know this will all lead to more questions so please feel free to ask.
There are some very experienced scoot owners here, and I'm sure they/we will have you smiling in no time.

Thanks for the reply

Didn't think to measure length but was sold as 64mm and come with rockers, on eBay from scooters to go, kit with 20mm carb, A9 and drive gears.

Is the A9 upgrade worth trying to make it fit from the stock cam?

Carb was supplied with 80 jet factory run fine but seemed sluggish. Kit also come with 90 jet had trouble tuning too rich, have since put Uni pod filter on and is ok, thinking may now be too lean with the increase in air?

Have variator kit,1500 rpm Spring and 1500 rpm clutch springs still to install have 5,6,7,8 rollers to tune I'm 80 kg.

Thanks

LOL!!!
There is ALWAYS more to the story.
I recently bought an "A9" BBK that was supposed to be the 64mm head, A9 cam, and all the gaskets/oil pump.

What I got was a 69mm head (which is better if you're gonna port the roof), no cam, and the 69mm rockers along w/ the gaskets, and the oil pump.

What happens is the sellers of MANY of these kits also sell jewelry, garden hoses, spare tire patches, and tooth brushes. A lot of these sellers don't really know what they are selling.

What I would do is look at the tappet head. Look where the rocker meets the valve stem. At the holes up (timing marks) position, the rocker arm surface should sit SQUARELY on top of the valve stem. There should be no apparent "wedge" shape between the two surfaces.

That will tell you if your rockers, and valves are matched.
IF that's all good then either your head is out of spec or your piston/crank/cylinder are.
No biggie tho. These are mass produced castings made by people that have many more problems in life than most of us. Whatever went wrong on your combo CAN be fixed.

Have you tried looking at your carb slide, and needle?
Most needles have a series of slots to allow you to move it (needle) up for more fuel or down for less fuel.
It's sort of a fine tunning to compensate for in-between main jet sizes.

Back to the head/valves... Open the valve cover, and sight across the flat surface. 69mm valve retainers will stand above the valve cover surface, and 64mm are well below the sighted line.
Now if all above can be verified, and you have the matching head/rockers, and the valve still hits the piston it can be corrected by adding base gaskets to the cylinder (with the A9 cam)

I would try raising the needle in the slide with the fatter jet along with all the rest...

Thanks,
Turns out I was given 64mm head with 69mm rocker gear, changed with standard rocker gear (64mm) and a9 cam went in fine.
Engine feels a lot better now, did a plug chop and running lean with uni filter setup. Waiting for jets to turn up so I can tune carb.

Cheers

That's great to hear. I'm glad you found the culprit.

One thing on "plug chops".
Four strokes are not really going to show much, if any color when they are tuned correctly. What the chops will tell (on a 4t) is oil consumption.
I ALWAYS start fat on my main, and work down until you feel the improvement. Then fine tune the needle position/shape/taper untill it is at its best.

Another way to approach it is to use torch tip cleaners. They are tiny round wire files used to clean slag from torch tips. (DUH)
If I don't have or want to order a jet, I'll use the tip cleaner to open up the jet I have. One wire size at a time.
If I go too large I fill the jet with solder, and start again.
Basically this is a one jet fits all approach.

Anyway... Glad to hear you're on the right road.