Detonation, lean seizure, or....?

[RManson]

Darrell,

I think I agree with you. At the moment, that definitely makes the most sense. The whole exhaust has been modified to suit the build and start life as a YZ 250 unit. I tried to keep the dimensions as close to stock as possible but didn’t think about the stinger diameter. I did run the numbers on the pipe design and they came back susprisingly close so I figured it was a good baseline.

We never got a real power run as the revs started to tip over around 5,000. We initially thought it might be the CDI but now I’m thinking that was where the temps in the pipe/exhaust port/piston crown reached their max due to the restrictive stinger size and WOT. On the street, I hadn’t clipped it in third and then just let it rip at WOT, it was always on the pipe and shifting so the exhaust was never allowed to build up the back pressure/heat like I’m assuming it did in the dyno.

This would at least explain what happened if oil mix, coolant temp, timing, and jetting is ruled out which I’m fairly certain was the case. Looking over other examples of detonation/piston failure, there are always other signs to point to what failed whether it be timing, jetting, etc. Oh, coolant temps never rose over 180 and that’s as high as it’s ever got. Fans don’t come on until 220.

At this point, I’ll be cleaning the cylinder and buying a new piston and having a good look at things as they go back together. The exhaust will be a different deal and I may just build it to suit from scratch having gone through this. But that’s for another thread!

Thanks for all the replies! Keep 'em coming!

[garry inglis]

Something for holty to keep an eye on as he built his too suit you are both clever builders and it's been very informative to follow these builds and overall for what you have done engineering wise it's a small setback

[hendy]

Something for holty to keep an eye on as he built his too suit you are both clever builders and it's been very informative to follow these builds and overall for what you have done engineering wise it's a small setback

Agreed. This plus holty's info would make a great basis for a scootering article. In my opinion.

[RManson]

Something for holty to keep an eye on as he built his too suit you are both clever builders and it's been very informative to follow these builds and overall for what you have done engineering wise it's a small setback

Agreed. This plus holty's info would make a great basis for a scootering article. In my opinion.

Crap, I knew this would somehow tie into my day job (Editor, Classic Trucks Magazine)! You just can't escape it, can ya?! LOL

[holty]

oh dear, im sorry to see this ryan, i read your thread and you said you were getting it dynoed, i didnt expect this to be the result, as least the cylinder will clean up and a new piston will get you up and running, when you build a one off you expect to have teething troubles, lets hope that a bigger stinger will cure the problem, do you have an egt gauge fitted, this will show exessive exhaust tempratures, i retro fitted one to my 350lammy after i holed a piston, and yet plugs didnt look weak, problem was it was that powerful i couldnt hold it at wot very long,so a plug chop made me think it was fine, the day i holed the piston was a 100 mile journey and a massive headwind, with 5 good hills which it climbed, but detonated 10 miles from home, i went from 240 mains to 300 and its dandy now, but does foul a plug now and then pottering around, it likes to be in the band, keep us posted on how you get on, im going to check the size of my stinger on my rotax now, just as a matter of interest,
all the best,
holty

[RManson]

Thanks Holty!

As you said, it's to be expected and if I'm honest, I actually expected much worse when the time came for things to go South. Good news is top end replacement is already on the way for under $100. Will inspect the parts once they arrive but the bore looks good and the piston is new, so we will be back in action in no time. I agree on the EGT gauge. Gonna buy one now. Any recommendations?

[Adam_Winstone]

The stinger diameter is a very good shout as I've run into similar issues when trying to run big cc motors on clubman type pipes, with it being very evident that the clubman pipes with smaller diameter tailpipes have caused the cylinder to 'gas up' and run hot. Through exhaust swaps, I've seen plug colour go from white, through light grey, to black/light grey, to perfect brown. In each of these cases I've known the jetting, timing, compression ratios (geo and corrected), blowdown, etc. not to be the cause, even when on the reserved side of suitability, but when the pipes couldn't shift the volume required, the temp would creep up when held at constant throttle, including through the range. However, fit a free flowing pipe, without restriction and the plug colour and running temps would immediately change for the better.

The only problem with this is that my old gutless VW clubman would instantly resolve this gassing up but was SO LOUD!!! All of the other clubmans that I have tried have caused eventual overheating issues (one beyond red-hot on the dyno [U-bend] and heading towards orange yellow!) and horrible plug colours but with it being very obvious, as I progressed through the free'er flowing and bigger tailpipe offerings, that the heat dropped and the plug colours improved. Eventually, I ended up with a couple of the Ron Moss V1 clumans, which gave great performance but suffered some build quality issues (insides used to rattle loose). The RM clubman cured the overheating issue, as the gutless VW had, but was much better as far as noise was concerned. Sadly, with my RM clubman having since rattled to bits internally, I've had to return to some of the other clubman offerings and... guess what... my bike is cooking plugs and running hot again!

I really don't want to return to a curly type expansion but don't seem to be able to resolve this from what is currently on the market

Adam

[Muppet]

cud you fit a wider tail pipe to a stock clubman thin wall pipe is cheap bends easy with heat

[Adam_Winstone]

^... yes and no, as the tailpipe is only the final element, many of the clubmans are also too restrictive on the internal baffling, which also leads to gassing up. Any clubman that will stop this must be relatively free-flowing through the internals AND then get out via a bigger diameter tailpipe. This is why gutless VW clubmans are the other option that AF had when they launched the TS1.

NB: Many of the 'super' clubmans will allow you to ride B-Roads all day long or tootle on faster roads, without this becoming an issue, but it is the A-Road and motorway sitting at a decent throttle position that brings this issue to the fore. Don't think that this is a fuel flow/supply issue either, it is not. I do run a fuel pump on on of my bikes but that is a different story.

[Darrell Taylor]

if you look at pics of piston head and plug you can see theres a good dark base layer of carbon but on the piston see that the layers are build up over the top of the eroded alloy so id say this failure started long ago and kept on running until the final fail on the dyno that i wouldnt blame in any way , in fact a visit there before any road running is what i would do so you can analyse quickly whats happening , the parts look a bit short of oil ,id expect to see more and the the blackening of the traces left suggest it may have broken down ,be interested in type and percentage ,did you have a temp gauge ,what running temps are you seeing ,, make sure you check the actual vent size at the end of the baffle cone that often needs the connecting silencer tube cutting open to access and measure it,the connecting tube and silencer run as a minimum same size as the vent or increase by a few mm ,id not worry about the design belly section size as calculated , many skinny pipes work very well .