choppa wrote:I had a similar problem with an engine i built using High speed conversion parts. And it was the flatest engine i ever owned. I took it back to them and they offered no help.
It could be your cam? As HSC. used to do alot of aircraft engines which are only meant to rev to 4500 i think?
And just what are "high speed conversion" parts???
Just for info, the VW engine when used in aircraft is usually set to cruise at about 3200-3300rpm and max rpm is around 3500rpm. Any faster than that and you have to have a smaller propellor which reduces it's efficiency and so you don't get any extra thrust anyway (thrust is the important measurement in aircraft - not horsepower as such).
Even that rpm is quite high for an aircraft piston engine - the smaller "direct drive" Lycomings rev to about 2850 max or so, and the larger ones to less than 2600 in most cases. Once again that's a propellor efficiency issue - long slow revving propellors are more efficient than short high revving props (the Wright Bros had propellors of about 66% efficiency spinning at about 1000rpm through reduction gearing, and modern Cessnas etc have props of about 75-80% efficiency at 2500-2800rpm so things haven't improved a huge amount in 100 years!)
The huge paddle blades on Orion aircraft are about 85% efficient and spin at about 1000rpm (geared of course - the turbine driving the prop is spinning at about 20,000rpm).
ANYWAY - on to the "advance" issue.
The 009 has about 25 degrees of internal advance, so when it is set at about 7.5BTDC idle, it provides up to about 32 degrees total advance.
But they vary - cheap build - so you set them at 28-32 degrees at 3000+ rpm and let the idle fall where it may - max advance is more important than idle advance.
They START to advance at about 1200-1300rpm and it's all in by 2500-2600rpm, which is why you set tham at 3000+rpm - just to make sure you really do have all the advance in place
Flat spots are commong with the 009. To accelerate and engine, you need a shot of fuel and some extra dadvance. The carby provides the shot of fuel, but the 009 has no vacuum line so can't sense the throttle movement, so the engine misses out on the extra advance (the 009 doesn't BEGIN to advnace until about 1200rpm remember).
So to reduce the flat spots - you have to compensate with MORE fuel, so the carby must be set to run a little rich and the accelerator pump must be set for max squirt.
Setting the 009 at 32 rather than 28 helps a little too (a bit more advance at idle that way), but you can only do that if it doesn't ping/detonate at 32 degrees - some engine need to be set at 30 or 28 to avoid pinging/detonating. NEVER use less than 28 degrees though - use a higher octane fuel if it still pings at 28 degrees.
For info, if you know where TDC is on your pulley - 28 degrees is 43.5mm to the right around the rim, and 32 degrees is 49.5 mm to the right of TDC around the rim.
Rob
Rob and Dave's aircooled VW pages
Repairs and maintenance for the home mechanic
http://www.vw-resource.com