Forum

[stinkwheel]

Probably paranoia because I've been working on a 500 bullet most recently. I was out on my 350 bullet at the weekend and it seemed down on compression. Highly subjective and it's not long had the last collapsed piston replaced (maybe 3-4k miles ago). It has a "higher" compression piston fitted and a Hitchcocks tuned head.



Anyway, I've been meaning to get myself a compression tester for some time, more for my multi-cylinder bikes but I've got one and done a test. On a cold engine, I was getting around 40psi. This increased to 70psi following introduction of a little oil. When it was parked up it was after a 150 mile ride so the oil will have been pretty hot and runny. (unlike my fingers!).



I've got no baseline to go off but I'll start logging compression when I do a service. Do these readings sound reasonable or is it new piston time? Or is the reading itself fairly meaningless? Oil consumption has been relatively normal of late, it normally skyrockets when the piston goes.



I'm away to do the points and clean the carb (it's been a bit latchy starting recently, mostly due to timing I suspect) so now would be a good time to pull the top end off. Equally, I've got the Dragon rally next month and would preferr not to have to be torquing down head nuts in a snowy laybye.

[Bullet Whisperer]

Does the engine start and run well? I don't use compression testers myself and other factors can influence how easy or hard an engine is to kick over, including close or wide ratio gears altering the kickstart ratio, as with engine sprocket size and kickstart lever length. Another thing to consider is cams and their timing - I like to retard the inlet cam by 1 tooth in many of my tuned engines and this makes them easier to kick over, due to the inlet valve shutting later - do you have performance cams in yours? Otherwise, if it is down on power, perhaps it is time to take a look, but make sure the pushrods have no weight on them at all, when the valves are meant to be shut first of all, it could be something as simple as that1 Regards, Paul.

[Alan R]

Hi Guys----- I tend to use a compression tester in the same manner in which you read a barometer ie}---- by way as a comparator...With a multi-cylinder you can see if one particular cylinder is down on compression and needs further investigation ( along with spark plug readings, manifold pressures, tappet settings, exhaust pipe colours etc) ..........With a single it's not quite so straightforward as you have nothing to act as your comparator---hence the question asking for a "baseline", which normally would be the compression test figures supplied by the manufacturer.......BUT, as you now have a Higher Compression piston fitted then those figures will now mean little as the mep ( Mean Effective Pressure ) or average pressure will now have gone up anyway.......

So, what to do ??.....Yes, log pressures at regular intervals is one way....make sure that they are all done in the same circumstances eg}--- at the end of a long ride through varying traffic, but in any event the engine must be at NORMAL RUNNING TEMP each time....One occasion might be when doing a "Plug Chop" test-- use the Compression Tester immediately you have removed the plug................. Another idea would be to find a fellow 350 owner with identical set-up ( The HC piston, Tuned Head etc.) and compare notes.........In any event before you do all that I would take Paul's advice and just check the basics....Those push-rods can be a real pain with the steel ends coming loose or they develop a slight bend which shows up very easily by just rolling them on a flat surface --- a modern day kitchen worktop is good enough for that...As Paul says --their setting is Zero which means that at TDC Compression stroke ( Engine cold ) they should spin freely with no up-down lift.........PS}--- Many years ago a friend of mine attended the Dragon Rally on a 250 Ambassador Twin which made it all the way from Reading, Berks. and then got as far as Bridgnorth, Shropshire before breaking down....But then that's a Villiers 2T for you, I'm sure you're be OK..............................................probably.

[stinkwheel]

No worries about the Dragon, the bike knows the way better than me. I'll have a look at the tappets as a routine although I've never had an issue with them to-date and I did take the precaution of fitting the big adjusters.



The more I think, the more I reckon the compression is probably ok. It just seems low after repeatedly horsing over a freshly rebuilt 500 and I have sorted the clutch recently so it's not got the same tendancy to slip as you try to go over TDC now.



The engine on this bike has never seemed all that poky though. It is kind of flat with a tendancy to progressively slow down unless you keep prodding the gearbox. Bit like a C90 rides in top gear. Even with all the "tuning" parts fitted (head, alloy barrel, big MK1 amal, higher comp piston, balanced rockers). Nothing ever seemed to make much of a difference to overall performance, although I've got the oil consumption down to a more sensible level.



I'm not aware of it being in any way quicker now than when I was riding it round with the iron barrel, standard carb and a loose valve seat flapping in and out of the head. In fact, this is borne out by the standing quarter times I recorded at straightliners in two consecuative years (best was 19.56s @ 64mph with the mirrors and rear subframe removed). I've put it down to just being what a 350 bullet does since all the significant engine parts have been replaced at some point. I've never ridden another one for comparison though.



In any case, it'll manage to haul itself, me and a PAV trailer to Conwy again this year I have no doubt, ropy piston or no. Usually makes it up to 55 on the motorway (unless there's a hill or a headwind, in which case 40-45 is more realistic) With the trailer, technically I'm not supposed to exceed 50 anyway (and you wouldn't want to, the subframe isn't the most ridgid structure in the world!)



One thing I did notice last night is that when I blow down the fuel inlet of the carb with it upside down, the flow rate is pretty poor. There can't be more than a fairly slow trickle of fuel getting in there. The float doesn't exactly "lift" the needle out of its seat, more eases it away slightly but this appears to be how it was designed to work. I'd have expected a lack of fuel flow to result in stuttering and missing at higher revs like it's running out of petrol rather than a generalised lethargic wheeziness?

[stinkwheel]

I think I'll leave the piston for now. I doubt it's collapsed but now's a good time to start logging compression readings.

On the carb front, it appears the float height is a good bit lower than it ought to be so I've ordered one of those new-fangled unsinkable floats with the bendable tangs and a lightweight needle.



Getting it hooked up to a funnel and fuel line to set the float height has also highlighted how sluggish the fuel flow into this carb is. Some fiddling about shows most of the resistance to flow is coming from the plastic gauze filter on the carb inlet banjo. I'm going to snip the gauze off so it's just acting as a spacer. It's redundant because I use a paper element inline filter anyway which has both a larger surface area for filtration and a finer particulate trap.

[simon]

40 to 70 psi is very low but I imagine you've kicked it over without opening the throttle. This makes an enormous difference to the registered compression. Just try kicking the bike over without a carb on to see the affect it has.

[Leon Novello]

Compression testers are just another tool to keep you awake at night, staring at the ceiling. Unless you are competition riding, leave well enough alone and just ride the bike. Compression can vary greatly from day to day on some engines.

[PeteF]

Too right Leon. Almost as bad as oil pressure gauges

[Alan R]

---------- and talking of which, guess who has one for sale right now ?? ....Hi Stinkwheel, as per Peter and Leon above, in my last posting I assumed you had read the operating instructions and were using the tester in the correct manner....

[stinkwheel]

It had the carb off at the time I took those readings.