Hitchcocks Forum

Evening all, Just lately my 05 bullet (500)has suffered from a lack of compression. At first it came back after a few turns of the kick starter. Now though there is no compression for several cycles, and then only minimal comp, which seems to occur when the kick starter is at the eight o'clock position, without fail. I've checked the tappets and found both pushrods spinning freely at TDC on the comp stroke, but after a couple of revolutions the inlet goes tight. I slackened things off a tad which seems to have sorted this, but still the compression issue is there. I've removed the carb and held a rubber glove over the inlet manifold and found the inlet valve to be leaking when I operate the kick starter. The really odd thing is that the bike starts easily (no need to use the de-compressor) and runs great, cruising at 45/50 mph with a top speed of 60 plus(true speed).Should I just ignore the "problem" and ride on? Or could I be letting myself in for some serious spannering in the future? All comments greatly appreciated.

P.S. Bike has done 19,000 kms, mostly by me and oil has been changed regularly. Bike never thrashed. No signs of smoking/ excess oil consumption.

Decoke time?


Aside for a sticky valve; carbon deposits on the stem etc. If the bike has the OEM piston fitted, pinched rings due to crown and skirt compaction as the piston slowly collapses on its self is an ever present possibility...


Check the piston!!! Tight or badly worn rings? Replace The piston before if decapitates itself as many Thousands of other 500 owners have discovered.

Valve seat recession, maybe ?? ..........In any event it's "Off with his head"...plus barrel and piston and have a good old root around etc........and no, don't ignore it but get stuck in ASAP .... or at least before the Winter cold arrives and that comfy armchair seems oh-so erm, comfortable !!

As TIM NZ indicated you may well have an OEM piston fitted so now would be the time to DUMP IT !!!!!......and opt for one of the forged versions....

BIG TIP !! Remember when trying to lift the head away from the barrel that you might well have original Enfield washers fitted on the head studs.....These are soft and tend to grip the threads thus appearing to lock the head.....TAKE GREAT CARE if using prize bars not to break a fin off......I'm planning to start mine at the end of next month..........the bike is new to me and I MUST find out what piston it has...Good luck and keep us posted please ??

Hi Reg, before getting over excited and rebuilding the whole engine with a mountain of new parts, check your decompressor and its' cable are working properly - the cable might be dislodged or fouling on something causing it to open slightly. As for the oem pistons, some fail, I've had one do it myself and they are pretty crap, but there are plenty more still standing up to the job.
Regards, Paul.

Definitely check out the simple stuff first off...


My experiences with the Stock 500 piston here in NZ is that no less than 20% of them fail inside 10,000 miles. Similar stats from Aussie...


Another 20% will fail ins1de 20,000miles...


I have now replaced over 60 of the barsteward OEM Pistons! They are potential Grenades!


If you want one to last (fingers crossed) the Top Land dia must be NO LES than .028" smaller than the nominal bore dia or incipient piston slap compacts the top-land ring area down, the Oil ring eventually fails, (oil consumption rises) and the crown separates from the skirt.


https://image.ibb.co/n1YX6v/Dead_piston2.jpg


One of many...

Morning all------Here's a present for you Tim}-----------

Just a thought but the late, great Bert Munro would be in his element with all this scrap piston material....For those who don't know of him have a look at this}------------- https://en.wikipedia.org/wiki/Burt_Munro ---------- RIP matey !!

What's getting me is the intermittant compression loss. It goes then comes back.



I'd expect compression loss from a collapsed piston to be consistant?



Also a pushrod being tight on revolution then free on the other.



Gotta be a valvegear problem surely?



I'd agree with others, you're going to need to start pulling it apart. I'd start by taking the rocker covers off and having a damned good look at everything. Then I'd check the pushrods are straight. THEN I'd whip the head off, it would make sense to inspect the piston while you're in there (although I now consider bullet pistons to be a service item).



The evidence of an intermittant compression loss and a tightening/loosening tappet mea I suspect the root of your problem won't become clear until you strip the head and have a look at the valves and valve seats. Check the valves are straight (I gently clamp them in a drill chuck and spin them, any deviation becomes immediately apparent). Check the seat area for damage from carbon deposits (looks like a mouse has been nibbling round the edges) and check the seats themselves are still firmly inserted in the head (they can come adrift).



I had an exhaust valve seat drop out on my 350 bullet and it did the whole intermittant loss of compression thing, accompanied by popping and banging on the overrun. Not sure if this is such an issue on the 500 heads. I'm not sure what the symptoms of a loose inlet valve seat would be, I suspect relatively mild but if it's been going on for a while, could certainly result in a dgree of "leakiness" like you describe.



I'm sure all will become clear when you get in there anyway, I do like to hear what happened in the end too so please do let us know.