Hotspot on YICS tool - advice needed please.

After doing the valves and cleaning the carbs (incl. clunk-test) I put together a home-made YICS tool from plans on the net.

Thoroughly cleaned the YICS passage. New plugs. Inserted the tool and balanced the carbs, not pefect and not as easy as I thought, but much better.

Put in a Colortune plug, and saw the bunsen blue. Somehow, someway I badly damaged the mixture screw, it's now all chewed up both brass and alloy.

Eventually got that back to 2.5 turns out, the base setting.

Decided to balance again since fooling with the mixture might have knocked it all out of whack.

At that point I noticed the first rubber on the YICS tool, the one to the left as you face forward. It had in one spot burnt away. I replaced that rubber, and balanced again. Took YICS out and it's damaged in the same way, in the same place (the top side of the rubber as it sits in the passage). The other two rubbers are fine.

The whole reason for doing this job was to try and eliminate a stumble and slapping sound when the bike is warming up.

One more thing - I noticed with the Colortune that when the bike stumbled, the bunsen blue totally went to a very obvious orange.

So I suppose there are two questions -

1. What caused the damage to the YICS tool ?

2. The stumble seeming to synch with an orange flame in the cylinder ?

On the open road the bike is absolutely a screamer, good throttle response and engine braking.

I will check the new plugs tomorrow, and maybe Colortune the other pots to see if I get the same symptoms.

Would really appreciate anyone's thoughts though.

Thanks. in advance

 

What caused the damage to the YICS tool is your "tool" was likely not properly sealed off from the outside of the passage. Most of the DIY YICS tools on the web miss the mark by not properly sealing off the outer end, where the passage is open to the outside. You not only need to isolate the cylinders' passages from each other; the main passage needs to be sealed tight as well.

OR-- you could have an intake valve issue allowing hot gasses to "blow back" into one or more cylinders' intake ports.

2: Yes, that follows. Do you have a float level issue? If the floats aren't right, you could be "dumping" fuel occasionally causing a "rich" stumble.

Did you "wet-set" your float levels individually using fuel and clear tubing?

What method/tool were you using for the vacuum sync?

 

If the cylinder "misses" a beat - the next fire ends up being rich enough, due to unburnt fuel, to clearly be the bright orange/yellow. It's only four hundreths of a second later( at 1,500 RPM) your brain processes it as the same event. See the sheep game . . .

Tape a bit of tissue to a stick, search for an exhaust gasket leak making funny noises.

Pack strips of oily cotton rags into the YICS passage until you get that sorted. In fact, you should be doing a compression test and valve check next. The "hotspot" is a symptom to a possibly serious problem.

 

Thanks to you both for sharing your thoughts.

Bigfitz, you're right, the homemade YICS is not sealing correctly at the outside. I will redesign and go again.

I don't know if it's an intake valve issue - I checked them and got 0.15/0.16/0.15/0.15.

I don't think it's a float level issue either. I set up a rig for the carbs and levelled them carefully with little bullseye spirit levels. Used the clear plastic tubing and got them all within half a mil of the 5mm spec.

Used a Morgan Carbtune for the synch.

This evening -

Used the oily (2-stroke) rags instead of YICS tool and balanced again.

Put the Colortune on all four cylinders, and they're all exhibiting the same characteristic. Will make a video and post it up, maybe tonight if I can.

Sprayed the plug leads with water and fired it up - no sign of any arcing or flashing. So no point in messing with coil swapping ?

Pulled the new plugs, and they're looking quite lean actually. Which reminds me - the mixture screws were set much further out when I stripped them out to clean the carbs, 4 turns iirc.

What next ? Is there still value in doing a compression test ? I didn't bother doing it when I measured the shim clearances since they were all looking good and the bike has great power.

Thanks

 

Do a compression test by all means. You might have a burnt/damaged valve, having good clearances won't tell us how well the valve(s) are sealing.

If you get good numbers, you can at least cross that off the list.

 

OK, these numbers seem good to me -

140 / 140 / 128 / 130

Specs given in Haynes are 142 as standard; max. 171; min. 114; and max. difference between cylinders of 14.

 

They're decent; #3 bears watching. You're sure the valves are in spec?

Back to the stumble...

Are those Mikunis on that bike?

 

They are indeed the Mikuni BS36.


Inlet Valve range is given as 0.11 to 0.15 in Haynes.

I have readings of 0.15, 0.16, 0.15, 0.15


Exhaust Valve range is given as 0.16 to 0.20.

I have readings of 0.22, 0.21, 0.21, and 0.19.


I know they are, unusually, out of spec on the high side. I used a very good set of feeler gauges, using a 'go/no go'' system. Also, I rotated the cams say, 20 degrees each side of what I visualised as being the perpendicular.

I will be taking off the carbs again, and rechecking the float levels. I'm pretty sure they're good though.

I'm moving away from the idea that it's a single carb/cylinder issue, since the Colortune showed the same behaviour across all four.

I forgot to mention earlier that the exhaust gaskets are new, though I haven't yet done the ''tissue on a stick'' test. Will do so tomorrow when I can get the bike indoors.

Have I missed any potential causes ?