NC30 running on 3 cylinders (Mostly)

[st22041967]

Ok

Bikes been running fine however now C1 (Rear left as you sit on the bike), Seems to be "bogged down" under 6000rpm but then seems to pick up again when at higher RPM. I have swapped plugs with know good plugs and I'm getting a spark. However with the air filter off when I put my hands over the belmouths of the other cylinders there is LOADS of suction when revving. C1 Not so much :0(

The carbs have been stripped and cleaned within an inch of there lives so I'm not thinking fuel etc.. Or valve clearances either (that's been done)

Obviously I think it may be time to swap over ignition packs and such like, just looking for "The simple" solution first.

Suggestions to the usual address please Oh kind 400greybiker's.

Thanks

[CMSMJ1]

make sure the carbs are seated and sealed..... the clamps tight and the rubbers not split

[st22041967]

Thanks

Hmmm.... New manifold rubbers and on as tight as a gnats chuff.

Anybody else?

[magg]

Valve adjustment and carby clean was before or after problem began?

Lack of suction could be many things, can only suggest loss of compression due to valve leak, head gasket or ring failure or possibly carby slide sticking. If carby slide action ok, try a compression test.

[dobbslc]

+1 on the compression test.
Worth checking you haven't scrunched up the inlet rubbers doing up the clips to tight?

[st22041967]

rubbers look OK and are new.

[BB_158]

Has bike been laid up not started for a while prior to you working on it?

If it was ignition pack then you would prob have 2 cyls not working, which is not the case.

Even though u say carbs done, it still smacks of that C1 carb is not functioning properly.

A ballache I know to take them off again but that's where I'd be looking. Re-check anything particularly the pilot jet as that's what it sounds like.

Before you do though do these checks to know what to focus on when in there:-

1. With engine off & air box removed, push carb slider up with your finger & see that it returns immediately (compare with the operation & sucking sound that the others make). If it sticks at all then carbs off to investigate slider, diaphram & needle assembly. If ok go do check 2.

2. Start engine and run it at low revs (don't rev it onto main jet at all), then feel exhaust downpipe on right side coming from C1 cyl to see if it is getting warm or not (compare it with the other rear exhaust). Then shut it off and take out C1 plug to see if it is wet or not.

If exhaust is cold - no fuel or no spark;
If plug is wet - you have fuel, but no spark - verify good spark, clean or swap plug/lead for known good as required;
If plug is dry/normal then you have no fuel - stick pipe on float bowl drain & open to see if any fuel, if yes check jets, if no then look at float & valve operation.

Remove carbs (I'd check everything again whilst in there), in particular the pilot jet & fuel ways are clear.

If you find jets are blocked, also check the fuel filter on the fuel tap is present and in good condition. This can sometimes be the root cause of carb blockages as you don't want crap from bottom of the fuel tank going over to carbs.

Hope this is of use, let us know how get on / what you find.

Cheers

[st22041967]

Thanks for the "workflow" for fixing the carbs. I will be trying this in the next few days. However I do have a spare carb so I will try this first to let me know "quickly" if its the carb (Which like you I think it is). Then I'm going to go over the original bad carb with a very fine tooth comb.

Thanks for the time it took you to type it all out it is very much appreciated.