Hitchcocks Forum

Hi All , I have just changed the silencer on my classic 500 2015 model and am now getting loud popping on when closing the throttle, it does not happen every time, I have put sealer around where the silencer and pipe join and have also changed the gasket where pipe enters the engine, the silencer part number is 91080 from Hitchcock,s and the gasket is also from Hitchcock,s any ideas what might solve this problem.

This is "reversion" where cold air is sucked back into the exhaust on the over-run where is meets hot exhaust and expands rapidly with loud retort or even ignites unburnt fuel. I have the same 91080 but with the entire system 91130. I did not find that sealant goo worked. There are two points of air ingress, the head and the silencer join. At the head there are two options Exhaust Ring 510804 or Exhaust Gasket 582638. The former cement biscuit crumbled away to nothing whilst the latter is a copper ring but may need two in place. When I went to replace the non-asbestos cement thing there was nothing there to remove, it had been crumbled and spat out.

You may have disturbed the head seal fitting the silencer. The head seal may need jiggling into place until it seals and then retightened. Don't gorilla the nuts, they're only small studs but they do expand and loosen when hot. The silencer to pipe seal I found best if the clamp was over the closure of the split so it contracts the pipe. I've got mine sealed but it still pops a little.



Where you get air in the exhaust, the O2 sensor sees it as a weak mix and promptly injects more petrol, which ignites in the exhaust.



After sealing the exhaust there are a number of solutions; cheating the fuel map on the Power Commanderto shut off the injector at high revs and zero throttle, or to use the Autotune AT-200 to do something similar in real-time with the air/fuel ratio.

Neither of which are much help if you don't have the Power Commander overriding the ECU.

One trick I found fairly successful before fitting the PCV was to increase the idle speed on the inaccessible brass screw by maybe a quarter turn.



With the PCv alone you can enter data into the normally unused 0% and 2% throttle columns. This was partially successful. The Hitchcocks PCV fuel map is pretty good in the area it covers, the main power band but does not cover the "cruise" region to the lower left of the map so well (an area set to zeros, "use the ecu map" which is now way-out of kilter due to the more open silencer etc). Hence my use of the Autotune to cover that area, whilst keeping H's dyno mapping intact.
My last attempt with the AT-200 went horribly wrong, even more popping, but this serves to tell me it needs adjusting in the other direction.



The 918080 has adjustable baffles. I ran it stock then opened it up. Opening it may have improved high revs marginally but reduced mid-range where the torque is. So I closed it and the increased back pressure noticeably improved midrange, and seemed to make the exhaust note louder when opened up. This motor is more about midrange thump than top speed anyway.

Squeezing the baffles shut until the points just touch may reduce air being sucked in from there.

If all you have done is replace the standard silencer then you'll probably always get some popping because the base map cannot compensate in the same way as in days of yore when a larger main jet would be needed. As has been said, you'll need to fit a PCV to tune it out. FWIW a freer-flowing exhaust on my Triumph Thruxton has the same effect; it's a trait of EFI systems. Although a leak in the exhaust system would be a likely cause on a carb I doubt that is the problem here.

This is another tragic case of creating a problem where none existed before. More money for nothing.

So true Leon, the buyers of these bikes seem to think that they are still 1950s throwbacks when in reality they are now modern retro styled bikes and don't take kindly to armature meddling.

Predictive text again, amateur meddling.

Not this owner; mine is completely standard mechanically and electrically. For me the C5 looks like a fifties bike. As standard it performs like one too, which suits me fine as it is what it is. One reason I didn't like the CGT is that one has to chuck serious money at it to make it go as quickly as it looks it should do. That is why I bought a Thruxton. While no fire breather it does get a move on if you wind it up and IMHO upgrading that is likely to be more rewarding and cost effective than doing the same to an RE. One of the problems with EFI bikes is that many don't understand how the fuelling works, so they proffer solutions that are appropriate for carb engines but the exact opposite of what is needed to rectify a fault on an EFI eg air leak in inlet can cause rich mixture, not weak because ECU tries to compensate.

May be of use:-
http://www.enfieldmotorcycles.in/2010/0 ... -easy.html

Thanks for three constructive comments ref popping on new silencer , as I am not particularly mechanical I think I got the gist of your advice ( I am obviously an amateur meddler who should not be allowed to own a pretend classic bike) is the PCV you refer to the power commander unit from Hitchcocks and would this solve the problem as are they worth the money

Mike W - there are plenty of us on here looking for constructive advice. I am surprised and irritated that two regulars felt it appropriate to post cynical comments. I would dismiss those comments. After all, you bought a hosts silencer and one would assume that would work, albeit a bit of meddling is needed, which is part of the enjoyment. It is not a big problem to resolve so press on and be assured there are plenty of us on here who do not have degrees in motorcycle engineering and tuning. Keep us posted on your progress.