So close

[nb1914]

Hi All,

I have been battling with a poor running right cylinder for quite sometime and now I'm after a few ideas, below is where i am at present:-

After some analysis i replaced the points with a Newtronics electronic ignition.

found that there was way too much oil so emptied and refilled to minimum level, previous to that oil was appearing out the exhaust.

So where am i , bike starts first/second kick (on the left cylinder) the right cylinder takes about 15seconds to run smooth with a slight backfire. After a minute or so it runs quite well on both cylinders ( white smoke has gone) right exhaust is pumping nice hot gas as the is the left ( every now and then i see some small embers out the right exhaust very small amount ) bike tick over nicely and accelerate very well. Still a bit of misfire on right side.

I have advanced and retarded the timing to try to eliminate the backfire and slow start on the right side.
i have swapped the coils and plug wires from left to right to see if i have a bad coil but the backfire stays on the right cylinder.

I am running off the battery only i.e. no charging circuit but i would assume a low batt would affect both pots.
I have let the bike run for awhile to burn off any oil in the right pot but at present i can't see any plug fouling but i will try again later.

i have cleaned out the carbs, but had to repair the right darn diaphragm as 2 holes were present but as i understand a backfire is usually unburnt fuel in the exhaust suggesting the carb is supplying fuel.

Any ideas welcomed, as i say once the right pot start running its almost spot on and accelerates very well.

 

[nb1914]

quick update , i gave her another run. started again easily right cylinder running quite early this time but still with a backfire. left her running and blipping the throttle. again accelerates easily but after sometime again saw embers from right exhaust and then actual flames even though it was running well ?

 

[rainycity]

Bad spark plug maybe?

 

[nb1914]

i thought that might be the culprit so i bought 2 new plugs and gapped them at 0.6mm BP7ES NGK, i will try an older plug i have which is a BP8ES. on the last run the exhaust cleaned up nicely and both cylinders nice and hot.

 

[rainycity]

Sometimes you just get a bad one and its just something you dont expect to have happen.
It`s happened to me on more than on occasion.

 

[nb1914]

thought i would do a quick and dirty compression test on the right cylinder by removing the left plug and see how much resistance to kick over, it was fine. i know thats a bad test but i don't have a compression tester

 

[rainycity]

Any port in a storm. I`ve used a broom handle before.*lol*

 

[nb1914]

tried an old plug and it ran much worse, below is the new plug from the right that has just been removed after a run, it actually seems in better nick that the left.

 

[nb1914]

just thinking……i noticed the right pot runs much better when i give it some throttle i.e. backfire seems to stop or lessen considerably. Could it be that with more throttle the ignition advances meaning the timing is still off. Or is it that the pilot he right c is blocked ?

 

[5twins]

Have you checked your valves? You might have a tight one. I would also verify that carb float levels are correct and that the carb sync is good. If you have the BS34 carbs, there is a tiny o-ring on the mix screw that needs to be in good shape for it to function properly.

 

[nb1914]

hi 5 twins , thats something i missed in my post. when i dismantled the carbs i couldn't see any o rings on the mixture screws so i used a bit of amalgamating tape to make some temp seals and also plugged the top as no nylon plugs were present, maybe the right one is no good. i did check the float levels to be around 27/29mm. I'm also wondering about valve stem seals, i set the pilot screws at 3 turns out.

 

[nb1914]

How do you check the valves for sticking ? is it just a matter of watching to see that it springs fully up when closed. I was thinking about what you said above 5 twins a sticking exhaust valve could be a possibility. When the engine was cold i disconnected the left spark plug lead and kicked it over a few times i got at one point a puff of smoke from the right pot in sync with a pop, i wonder if thats blow past the exhaust valve ? as the exhaust header can't have ignited any fuel due to being cold.

 

[5twins]

The o-ring is at the bottom of the stack of parts on the mix screw. It may remain stuck down on the screw hole. There should also be a small thin washer between the o-ring and spring. That keeps the spring from digging into the o-ring and destroying it .....

I'm assuming you have 1980 BS34 carbs with brass floats. If that is so then the float level spec for those brass floats is 27.3mm ± .5mm. 29mm would be too lean of a setting. That could be contributing to the backfire.

 

[nb1914]

Yes they have brass floats, when i pulled the mixture screws out i had everything except the o ring. They might have been stuck in the recess but i couldn't see them. I will pull the carbs again and reset the float levels.

 

[5twins]

If you need to get replacements, the mix screw o-ring is 1mm thick x 3mm I.D.

 

[nb1914]

cheers 5twins, i guess i should also really check the valve tappet gaps.

 

[5twins]

Yes, do check them. Like I said, a tight one can cause misfires and backfires.

 

[nb1914]

what do you think about this quick and dirty test to eliminate the right carb, i can move the whole card assembly to the right so the left carb is on the right pot and the right carb is in free air. The left pot spark lead i would disconnect, if i can get the right pot running fine on its own using the left carb then i know its the right carb. Could i cause any damage doing this to the left pot i.e. simply having an open inlet ? the bikes in a clean area so there shouldn't be any dirt ingested by the left carb but i could fix an air filter on it.

 

[nb1914]

UREKA at last, i swapped the carbs as per below (maintained the original right in-let manifold), and the right cylinder ran perfectly on its own, tick over and on the gas. So clearly i need to strip down the right carb. heres the evidence i patched 2 tears in the right diapraghm also botched the mixture screw o ring. Does anyone now of a good carb rebuilder in the UK ?