GT190 petals

[byron]

a mate has a GT190 kit on his Special
goes really well, and he thrashes it mercilessly
but he does seem to get through reed petals more frequently than is ideal
is there an alternative that can take the flexing better ?
or has anyone added a stop to the block or barrel ?
not knocking the kit at all, I really rate them, I think he is giving it a hard time, but that's how he rides.

[Muttley McLadd]

Is there something wrong with it? Mine gets treated like a ginger step child, and the last lot of reeds lasted 8000km.

[Rich_T]

Hi,

The petals are from 100cc group B kart engines from the late 80's and 90's. These things reved to 18,000 rpm without reed stops, I doubt a Lambretta engine will kick them that hard by comparison. Please send me a picture of the petal damage and I'll have a look.

Petals do break on GT kits for two reasons; first (by far the most common) is a faulty ignition or dirt plug and the second is fitting an expansion chamber but not raising the exhaust port. Essentially what happens is that the cylinder pressure can't get out of the exhaust port in time and effectively charges down boost port hitting the petal directly in front of it.

The amount of force can be quite large, strong enough to break the petal off at the base and stuff it back up the inlet manifold, so a reed stop would not stop it. Prevention is better than cure so the first point to resolve is the ignition fault, this could be iffy CDI, pick up, source coil wire rubbed through by flywheel or a dirty plug.

In the case of expansion chambers a small increase in port duration is needed to help increase the "blow down" period and the effect of the expansion chamber. Most people who intend to fit expnsions lift the exhaust as a matter of course so do not encounter this problem.

Once the root cause is resolved one additional measure than can be taken is to double up the petals on the side of the reed valve that faces the boost port. BTW all these points are covered in the build manual that can be down loaded on the web site, the link is on my profile.

BTW, you should never consider metal petals and carbon fibre still breaks. By far the best solution is a glass petal if only for the reason that if it gets injested by the engine it gets chewed up and does very little damage (in fact, I've not seen a glass petal damage a GT kit). Carbon fibre petals are a bit more abbrasive and a metal petal would be catastrophic.

[Pepot]

Another reason is a non faulty varitronic...in my case one of the reeds was pushed into the reed block the first time I tried my new kit just after some meters.Fitted a BGM ignition and no problems with the reeds by the moment.

[byron]

thanks for all the informative replies
the bike does have a varitronic fitted... which has been giving some lighting issues too, maybe that would be a good thing to change next...
Rich ~ I'll ask for some photos of the broken petals

[byron]

here is the reed

[Rich_T]

Yep, that is not damage from wear. That is a broken petal from the type of fault I described earlier. I sent you a PM

[wack 63]

Funny you should mention varitronics as I have a Polossi 130 in at the moment and it has a habit of shattering the V Force reed block much the same as Rich T says ,they appear to have been hit on the petal side breaking the reeds and shattering the cage but the manifold is case mounted. Any more info regarding?

[Darrell Taylor]

racing engines are high maintenance and power comes over long term reliability ,performance reed petals ive found benefit from being as thin as a given engine size will tolerate and the block angle,stops,petal length,petal material,petal weave and lay up on carbon , block construction alloy,plastic,vulcanised all play a part ive found below to be effective ranges on motors ive been involved with over the years
50cc 15,000 rpm 0.25/0.28mm
70 and 80cc 13 to 14k 0.3/0.32mm/0.35
125cc 11 to 12 k 0.4 to 0.45mm
175 200 250 cc 10 to 11k 0.4 to 0.55mm

ignition faults especilly or rich/lean based stuttersmisfires destroy petals as well as running into the overrev area too often causes petal flutter