Idling on one cylinder? Compression?

[RBN]

Anyone had compression low enough to make the bike run on just one cylinder?

My bike's running on one cylinder when idling but if I give it some revs the other cylinder comes in. As I ease off the revs it drops back onto one.

I've gone through everything else I can think of and am left with just possible low compression (though I have no gauge to test it).

It's not electrical (huge spark on both sides, have tried swapping the coils/plugs over just incase).

It's not fuel as my bike only runs a single carb with a 2-1 carb manifold (which isn't leaking air).

The valve clearances are fine and when the head was off (about 200 miles ago) there wasn't any obvious damage to the valve seats etc.

The only thing I did notice was when rebuilding the engine (after doing the head gasket) the cylinders did seem to slip over the pistons without too much resistance, a slight wiggle and they just slid right on.

[The_big_dill]

Originally Posted by RBN

Anyone had compression low enough to make the bike run on just one cylinder?

My bike's running on one cylinder when idling but if I give it some revs the other cylinder comes in. As I ease off the revs it drops back onto one.

I've gone through everything else I can think of and am left with just possible low compression (though I have no gauge to test it).

It's not electrical (huge spark on both sides, have tried swapping the coils/plugs over just incase).

It's not fuel as my bike only runs a single carb with a 2-1 carb manifold (which isn't leaking air).

The valve clearances are fine and when the head was off (about 200 miles ago) there wasn't any obvious damage to the valve seats etc.

The only thing I did notice was when rebuilding the engine (after doing the head gasket) the cylinders did seem to slip over the pistons without too much resistance, a slight wiggle and they just slid right on.

Exactly what you have tried doing so far would be more helpful.

I think your best bet, if you think its a compression issue is to just get a compression testing kit. If you do some research, you might be able to find different methods of testing this. I haven't tried it myself, but i heard you can cover your exhaust pipe, and if the bike stalls, then there isn't any compression.

[quarterliter]

Wait what? U run a 2:1 carb? Why on earth would you do that? And I think its a fueling issue maybe the manifold is directing the majority of the fuel to one cylinder? Or maybe the pilot is too lean and by revving it you get adequate amount of fuel for both cylinders to run. Try adding choke and see if it improves.

[quarterliter]

And you rebuild the engine without new rings? Or were the bores oversize?

[RBN]

Thanks for the replies

Originally Posted by The_big_dill

Exactly what you have tried doing so far would be more helpful.

I think your best bet, if you think its a compression issue is to just get a compression testing kit.

Well so far I've checked all things electrical, it's running a race style loom so it's pretty easy to check the cables for defects and switch things from one cylinder to the other to rule out issues. So far I have swapped all electrical things from one cylinder to the other, one by one but the problem persists. I have also.

I have also checked the valve clearances and 200 miles ago when the head was off I did a quick visual inspection of the valves.

Compression wise I now have a gauge (borrowed from a mate) but it's showing 100psi per pot, at fist I thought this was certainly the problem but having tested the gauge on my other bike my other bike (which runs perfectly) it shows a reading 100psi lower than book standard?!, leaving me feeling the thing is disastrously inaccurate. But what it does suggest is that they are the same psi, which is really annoying as i had expected to find them to be very different.

Originally Posted by quarterliter

Wait what? U run a 2:1 carb? Why on earth would you do that? And I think its a fueling issue maybe the manifold is directing the majority of the fuel to one cylinder? Or maybe the pilot is too lean and by revving it you get adequate amount of fuel for both cylinders to run. Try adding choke and see if it improves.

Why not run just one? I much prefer as it's easier to diagnose faults, i don't have to worry about them delivering unequal mixtures or being out of sync, I have half as many potential items that can fail and I find it easier to tune (I tune for MPG not MPH). My crusier is a twin that runs one carb as factory standard, as is my nighthawk 250 (it's cousin the CD250 runs twin carbs on the same engine) so I saw no reason to run twin carbs on the EX. As for fuel imbalances they only occur if you're using a defective or non symmetrical manifold, I always make sure I make mine as short as possible, don't leak are smooth flowing and exactly symmetrical.

Originally Posted by quarterliter

And you rebuild the engine without new rings? Or were the bores oversize?

Yes at the time of the replacement head gasket we did not have enough time to source them.

On that note I am still attempting to find a UK stockist of the rings if anyone can suggest one? Or perhaps alternative rings of the same size? Anyone had success fitting rings from a different bike? No idea why there're so damned hard to find here especially on a bike that can still be bought new.

[tc.young]

Originally Posted by RBN

Thanks for the replies

Compression wise I now have a gauge (borrowed from a mate) but it's showing 100psi per pot, at fist I thought this was certainly the problem but having tested the gauge on my other bike my other bike (which runs perfectly) it shows a reading 100psi lower than book standard?!, leaving me feeling the thing is disastrously inaccurate. But what it does suggest is that they are the same psi, which is really annoying as i had expected to find them to be very different.

did you have the throttle wide open, that's the only way to take compression readings, with it WOT, otherwise you will get inaccurate readings

Why not run just one? I much prefer as it's easier to diagnose faults, i don't have to worry about them delivering unequal mixtures or being out of sync, I have half as many potential items that can fail and I find it easier to tune (I tune for MPG not MPH). My crusier is a twin that runs one carb as factory standard, as is my nighthawk 250 (it's cousin the CD250 runs twin carbs on the same engine) so I saw no reason to run twin carbs on the EX. As for fuel imbalances they only occur if you're using a defective or non symmetrical manifold, I always make sure I make mine as short as possible, don't leak are smooth flowing and exactly symmetrical.

dunno about running 1 carb but i would think it would effect both cylinders counting everythings equal and balanced

[quarterliter]

IDK why you would "rebuild" an engine but not put new rings in but to each his own. Also how many miles are on this engine? Has this bike run before on the single carb? How did you make your manifold? What carb? Have you adjusted the needle pilot and main jet for the ex250?