Low Compression

AndyW

AndyW

Hero member
On the way back from Phillip Island to Adelaide the Mirage started to pop/fart a bit at low revs towards the end of a 500km ride to Ouyen in eastern Victoria. The following day started ok though still with some audible dysfunction at low revs when cold. I spent most of the day at the 110-120kph range though into a heavy headwind (as if I was doing 150+ on a calm day). By Murray bridge the loss of power was noticeable and by the top of the Lofty Ranges it was onto 2 cylinders.

Was expecting to find the valve clearances at zero and all the valve seats recessed away as has happened to my early 3Cs but was surprised to find all but one of the clearances as they should be. Ex#2 had halved to ~5 thou. suspecting a sticking or, worse, burnt valve I took the engine out yesterday and this morning found #2 ex valve to be a little out of shape...

So.. What might have caused this? I haven't altered the jetting since... 10 years? Valve to guide tolerance is fair and the same on all valves (not new but not rooted either) Compression on outer two was ~115psi which seems reasonable for 8:1 compression ratio.

I might pull the barrel off tomorrow and see what's what: could be time for a rebore though I do have a new set of OEM rings to fit if the bore measures to within spec

What pistons are people using in 1200s? I'd like to keep the compression as close to std as possible though I expect 9 or 9.5:1 would be ok..
 

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Float heights is one, bad fuel, water in the fuel tank are two more, wrong ignition setting another one... Or the filters in the pet-cocks are blocked. Or an air leack. or...

Check the tank with a light (LIGHT, not lighter, please... :oops: ;)) and see if you can see a large water bubble around the fuel taps. If so, starvation might have come from that... You have the Bosch engine, but I am not sure which one. If it has the removable alloy intake manifolds, remove them and check if the contact surfaces are bent or the O-Rings are broke maybe.

Good luck!

Oh - and btw: what millage has your bike done? Maybe a good Idea to go for a full overhaul at Red's workshop... He has according pistons etc.
 
hooksey said:
Suspect Quality of valves maybe Andy?
Have you found broken bits in the oil ?
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Nah to both....

Valves have done upwards of 30,000 miles and both they and the guides are still in pretty reasonable fettle. Mixture looks to have leaned out for some reason and cooked the centre pot's ex-valve. Not entirely sure why though suspect Piet is onto something with his theory above (just have to get my head around it a little better yet). Will likely just throw it back together after a light hone with new rings + a valve.

No sign of metal bits which will likely still be mouldering away in the bottom of one of the silencers...
 
AndyW said:
Nah to both....

Valves have done upwards of 30,000 miles and both they and the guides are still in pretty reasonable fettle. Mixture looks to have leaned out for some reason and cooked the centre pot's ex-valve. Not entirely sure why though suspect Piet is onto something with his theory above (just have to get my head around it a little better yet). Will likely just throw it back together after a light hone with new rings + a valve.

No sign of metal bits which will likely still be mouldering away in the bottom of one of the silencers...
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Yes, well be interesting to hear the results of carby strip down.........whats the top of #2 pistona look like?
Picked up a batch of shit fuel out the back of Ouyen :rolleyes:
 
hooksey said:
Yes, well be interesting to hear the results of carby strip down.........whats the top of #2 pistona look like?
Picked up a batch of shit fuel out the back of Ouyen :rolleyes:
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The tops of all the pistons look to be a bit on the white/cooked side so it's just as well I didn't burn one though (lucky the old Mondial Slugs are solid aluminium). Bad fuel isn't beyond the range of possibilities: that said the garages do move a lot of fuel out that way and I do use 91 to avoid the pitfalls of country 95 syndrome... I had been adding a squirt of Castrol R as a sweetner (and also for the hell of it) at each fill up...
 
AndyW said:
pitfalls of country 95 syndrome
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???. I keep hearing 95 is the go-to instead of 98 even with our mainly high-compression engines and mandatory use of 5-star or as high octane as possible back in the day. 95 is pretty hard to get at remote fuel stations anyway so I try for 98 anyway. BTW these octane numbers don't translate world-wise as there are 2 systems used plus a combination of both.
 
Country 95 syndrome’ refers to the variable quality of intermediate fuels you can find in the country due to low demand which can mean the fuel’s been in the bowser tanks for months (mb years😳) accumulating condensation and deteriorating generally..
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Out in the sticks you may be better off putty 91 in the tank, at least it'll be fresher and possibly far less contaminates due to not sitting around for eons.
 
Legs said:
Out in the sticks you may be better off putty 91 in the tank, at least it'll be fresher and possibly far less contaminates due to not sitting around for eons.
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It’s true. The same applies for Mcdonalds egg and bacon muffins.

Too early in the morning and you get a fresh muffin half on top and a yesterdays half on the bottom, until they are all out of yesterdays.

PS: My tip, just don’t go there.
 
Vince said:
???. I keep hearing 95 is the go-to instead of 98 even with our mainly high-compression engines and mandatory use of 5-star or as high octane as possible back in the day. 95 is pretty hard to get at remote fuel stations anyway so I try for 98 anyway. BTW these octane numbers don't translate world-wise as there are 2 systems used plus a combination of both.
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Go to is anything you can find.
I've been using E10 more and more. No adverse effects.
Paul
 
I will wager that the fuel supply to the carb on the infected cylinder is the key to your failure, one way or another I doubt that the float chamber was full at all times and (from the pistons) the defect is causing the same to happen on the other two cylinders but to a lesser effect, check everything starting by removing the pet cocks from the tank, and make your way forward checking every little bit of any fuel flow item. Consider fitting the RGS type fuel cross rail pipe, it helps but not much, the real cure for good fuel flow, especially when the tank level is nearing reserve switch time, and then only when using WOT or getting close, is, to use 300 float valves and seats, plus 3 fuel taps, but so far I seem to have been the only one who ever did this, and even I am not using it on my current fuel tank. 300 float valves remain though.
CLEM
 
Thanks Clem,
Some good points there: will do as you suggest and start from the petcocks and onwards from there (inc the float valves and heights)
AndyW
 
Is that so?? I have a set of Lemon’s repop taps fitted, are they the same internal design (with same fault)?
 
Hi AndyW,
The plastic tube is there to control when reserve is required. When the petrol drops below the top of tube thats when you switch the tap to reserve.
Lemon's I dare say work on the same principle . Take one off the tank , there will be a filter, and inside the plastic tube. Remove the tube and compare the size of the two holes. Its not a design fault.

With my current race bike, I did sit and time how long it took the petrol from a standard tap to fill a 1 litre container. With the tap on reserve it was quicker. With racing in mind I decided to dispense with reserve facility i.e. the tube and rebuild them after drilling out the holes and passage ways of the tap to flow even more petrol. I need to feed a bank of 37mm Keihin carbs coupled to an engine pumping out bhp approaching 120 at back wheel.

For the road (1200 Mirage) I run std taps , std Dellortos and never had a problem with fuelling, but thats my personal experience.

Andy
 
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