Only firing on two cylinders

JamieDemos · Post by **JamieDemos** » Thu Jun 19, 2014 2:56 pm

Hi guys, need some help. So I recently bought this bike (nc30) and I noticed a tapping noise coming from the bottom of the engine, like a metal tapping on metal kind of noise, sounds like about 4 taps a second maybe, it seems to be most audible at 1500rpm and then above 2500rpm the sound of the engine seems to drown it out so I cant tell if its still there. So I assumed it could be a rod knock.

I decided easiest way to determine if this is the situation would be disconnect each spark plug lead one at a time and see if the tapping quietens down. After removing the one rear cylinder lead I noticed there was no change in engine town. As if the cylinder wasn't firing to start with. Then I tried the other rear cylinder and same thing, no change in engine tone. But when removing the front cylinder leads there is a definite change in engine tone and the engine dies after a few seconds.

So I assume the rear cylinders aren't firing? What could be causing this? And could the tapping noise be a rod knock?

Advice much appreciated

richyrd5 · Post by **richyrd5** » Thu Jun 19, 2014 4:50 pm

Could be a number of things making tapping noises.you can usually tell if its terminal..but check with a spark plug in each of the rear leads to see if coil is working...ground plug on motor then spin it over to see if plug is firing..

JamieDemos · Post by **JamieDemos** » Thu Jun 19, 2014 7:36 pm

Okay I will try that, I also noticed there is a slight fuel leak, fuel droplets seem to be collecting under the carbs.

arsey30 · Post by **arsey30** » Thu Jun 19, 2014 8:32 pm

Did it run right when you bought it, if not check valve timing.
If rear 2 not firing, then it will knock, do not panic, not a problem.

Could be both plugs/leads /cap [[not common] but could be coil, CDI, ignition pick up.
Swap the coils to prove.

JamieDemos · Post by **JamieDemos** » Thu Jun 19, 2014 9:01 pm

The guy I bought it from said he recently replaced the cdi. Its running the same now as when I bought it. I will try swapping the coils tomorrow and report back.

richyrd5 · Post by **richyrd5** » Fri Jun 20, 2014 1:08 am

JamieDemos wrote:Okay I will try that, I also noticed there is a slight fuel leak, fuel droplets seem to be collecting under the carbs.

Watch it with the plug then coz if it does spark you might go kaboom!!

JamieDemos · Post by **JamieDemos** » Fri Jun 20, 2014 3:56 pm

I swapped the coils and unplugged the connectors to the coil on the rear cylinders. Started the bike and its running the same on only the front cylinders (with the coils swapped) and wont start with the rear coil plugged in and the front coil unplugged. So now I'm going to try swapping the front and rear spark plugs and see if there is any change.

Post by **skinnydog0_0** » Fri Jun 20, 2014 10:28 pm

Check you have a supply to the rear coil feeds.

JamieDemos · Post by **JamieDemos** » Sat Jun 21, 2014 12:44 am

Good point, can I do this by measuring the volt drop between the two input terminals to the coils and seeing if they the same for both coils?

Post by **skinnydog0_0** » Sat Jun 21, 2014 7:27 am

Yes make sure the rear is the same as the front should have (I think) a 12v supply across across the terminals.

Only firing on two cylinders

Only firing on two cylinders

Re: Only firing on two cylinders

Re: Only firing on two cylinders

Re: Only firing on two cylinders

Re: Only firing on two cylinders

Re: Only firing on two cylinders

Re: Only firing on two cylinders

Re: Only firing on two cylinders

Re: Only firing on two cylinders

Re: Only firing on two cylinders