running on two

[cbb]

Was running fine last time out . Refuelled and came home. Next day started but one cylinder didn't fire went round block and it didn't fire. Tried wd40 to no avail.  Next day tried again and fired but still down, checked downpipes, only 1 & 2 hot. Next day changed plugs for new -started but again only 1 & 2 hot. Checked spark on 3 & 4 - OK, Compression seems OK, seems to be fuel on plugs but not soaking when removed. Is there a splitter from the fuel feed to 1&2 / 3&4 ?? which could be blocked. Any ideas? Don't think its bad fuel as still runs on two. Could it be flooded as been starting from cold with no choke. Although left for one whole day before trying so it should have evaporated. Any help appreciated.

[unfazed]

Sounds like blocked pilot jets, common on carburettored bikes left on the side stand for very long periods.

[cbb]

Parked on main stand every night & done nearly 2000 miles in last 5 weeks. Do you still think that could be it. Carbs out to fix?

[unfazed]

The fact it is 3&4 rules out electrical as the coils work 1&4 and 2&3.

It sounds like the most likely problem, might be just a bit of sediment picked up from the bowl.

Has the fuel filter ever been changed? If not change it and in the meantime drain the float bowls as it may be enough to fix it.

Put a bit of white toweling under the bowls when draining to see if there is any sediment in them

[cbb]

The fact it is 3&4 rules out electrical as the coils work 1&4 and 2&3.

It sounds like the most likely problem, might be just a bit of sediment picked up from the bowl.

Has the fuel filter ever been changed? If not change it and in the meantime drain the float bowls as it may be enough to fix it.

Put a bit of white toweling under the bowls when draining to see if there is any sediment in them

Will give that a try next Thank you.

[Jules-C]

Double checked you haven't swapped the plug leads when changing the plugs or checking sparks.

[Bretty]

As said, check that the leads to 3&4 are the right way around.

Then Swap the spark plug leads around so swap 1 & 4 with each other, and 2 & 3 with each other. These pairs actually spark at the same time, although being a 4stroke engine only one of the cylinders has petrol in and fires.
See if the problem moves with the leads.

Problems with cylinder 3&4 are quite common as water and crap sits around the plugs and caps when the bike is outside on its sidestand. Especially in the crap weather we've had.

If you think it's the carbs try some redex before taking them off/to bits. It's a ballache of a job that rarely solves the problem.

[cbb]

Thank you for the messages.


Removed carbs today, really clean no sediment etc. Blown though everything anyway.


I then checked the pilot screws and No 4 o'ring was snapped. So that looks like the problem.


I think No 3 was just not getting as hot as 1 & 2 because of No 4 problem.


Fingers crossed a new o'ring will sort it out.


Put it all back together running on four now and then fuel leaking out of both overflows!


Out they came again and checked float heights and needles all good.
Back together and fuel still coming out of both overflow pipes?


Got set of needle seats with new o rings - this has fixed it.


It turns out that one pilot jet o ring and two needle seat o rings (one one each bank!) all failed at the same time.


Moral of the story - if you remove the carbs its worth getting a set of o rings etc and changing the whole lot.


Good news is, it is running better than ever now with much smoother pull from lower down the rev range

[bikemad]

Id be prepared to replace all rubber parts inside just had loads of problems getting mine running it was stood for nearly a year the modern 5% ethanol in petrol ruins carbs if ones gone the rest will follow!



I brought a blown up donor bike to get spare parts ended up cleaning out the best carbs swapping out the knackered float seats & o rings that where ok

[sgpwwg]

Carbs can be real pain to fit back on. After so many times of doing this myself I have found that fitting the carbs to the air box first and ensuring that they fitted properly guarantees no air leaks then fit them to head.

[bikemad]

I slide mine into place them fit front 1st then the airbox as I've done them 4 times bit of a dab hand at it now lol you are removing the battery & loosening the bolts either side to slide it back allowing enough room.


My airbox engages ok but in the past have owned older bikes with hardened rubbers which where a real pain