Hi can you advise on VHSH30 carburettor combination
Dp 266 140 max min 62…cd1 u12 valv 60
What fuel/oil/mix are you running, and what are your normal atmospheric conditions?
Also is this for long circuit racing?
The U12 is an unorthodox needle that I don’t have any experience with. It’s geometry could make sense with a DP tube, where as we are usually using DQ tubes with the K-type needles. Your main jet is probably in the ballpark for a starting point, depending on your conditions, as that needle combo should require a smaller main jet than the standard needle
That needle is going to provide very aggressive fuel delivery, so if you find you’re having a hard time modulating the power delivery then you may try something more conventional like a DQ268 and K28 needle.
Are you tracking air density?
You’re detonating and getting too hot at high rpms.
Are you sure you have a 178 main jet ? What ambient temperature are you running at?
K98 / 178 main seems like a very rich combo…
I would have suggested under normal operation K98, 3rd position, DQ 269,170 main, B52 inner and 60 outer, but this would clearly not work if you’re running this lean with a 178.
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The ambient temperature is 25-28 degrees. What combination would you advise me to try?
I think the key here is to understand when the detonation occurred…if it was happening with Dp 266 140 max min 62/cd1 u12 valv 60 OR with your current K98 DQ268 178.
If it’s the latter - it seems strange but if that’s the case leave everything the same and move to K9 on the needle (awfully rich) and then lean from there to - K96 (very rich) - K23 (rich).
If you are not solving with that amount extra of fuel, means you are fighting something else. Check float height (may be wrong and limiting fuel/running out of juice on the straights), check/clean or remove the inlet carb fuel filter, fuel line for pinching, or you may have an air ingestion issue in the engine (e,g blown seal).
Checking EGT temps in relation to position on track may give you the clue to the above.
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Do you find it strange that the detonation is all on the intake side? I feel like I’ve more often seen exhaust side or center of the piston.
EGT is 630 degrees 52-53 degrees water
There’s a lot of weird going on. I think you need to isolate some variables when you replace the piston.
Run a more “typical” carb setup with nozzle etc as above. Verify Squish clearance and ignition advance, fuel etc.
The ignition is at 1.55-1.60mm
Really ? I’ve actually almost never seen detonation take place anywhere else other than the front of the piston, unless it’s absolutely catastrophic and it starts to move towards the sides.
That also makes sense, as the detonation occurs where the fresh charge first hits the piston (intake).
What is your lowest EGT value on track?
I’m running this on my 10C. DQ269, 175main, k23 3-4 clip, cd1. Always runs like a champ. Ofc I adjust for temp and air density.
Yours look like it had a rough time at high rpm
What are you adjusting when you adjust for air temp and air density? Main jets?
Always main jet first, then clip position. If I can’t adjust needle any further I change atomizer up or down. Always pulls trough the whole register
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I’m still developing my understanding of combustion, so I may have used incorrect terminology. What I’ve seen on the center of piston and exhaust side might instead fall under “pre-ignition”, i.e. excessive heat from a lean condition (as an oversimplification). This was observed when I crossed a particular EGT threshold, at which point I’ve gone richer with the main jet and the problem goes away.
I believe we’re still waiting to hear what fuel and mix ratio he’s running. That’s still where I’d start.
The fuel we’re using here in the US (typically VP MS98L, or VP C12) seems to be pretty resistant to this type of detonation pictured.