Surging

[nicomkii]

The saga continues with the carbie / fuel issues with the MkII. I have just finished with a full strip, clean reassembly of the Carb module. As suggested I have swapped the choke plungers around as I believe No3 was bypassing.
As Im working 12 hour days I haven't had a chance to pull the plugs but performance has improved dramatically which I attribute to both the clean and plunger swap.
However.... a new problem has raised its head. She pulls like a train from standing start, no flat spots anywhere, cruising along and roll on the throttle and away it goes again pulling like a train.
BUT, stay on a constant speed, say 100kph @ 4k rpm and she surges. not dramatically but a definite feel that she is struggling on that constant speed. As she surges go for a squirt and away it goes, runs up without a dead spot.
Hints / guidance ????
I did hear a high pitched air leak from the aftermarket fuel cap so I thought maybe its under too much vacuum so I have pulled that apart, there seems to be too much restriction so removed the plastic thingamajig hoping its that. Will find out on the way to work early in the morning tomorrow but any other leads greatly accepted. ( yes I need to pull the plugs but not until Monday night )

So in review,

Pulls through full throttle range - no issues and great improvement
Roll on from any range - no dead spot, great reaction
Stay on constant speed - surges, enough to annoy and know its not 100% right.

[needaz1100r]

At those speeds and loads you are using mainly the idle circuit, believe it or not.

Also, surging as you describe is caused by a lean mixture.

So, I would be looking at making your idle circuit as rich as you can, while still getting a stable tickover, to see if that helps.

Once you've done that you are into the needle position, raising the needle, i.e. dropping the clip one groove, or even using a half groove spacer under the clip.

You could also try making the middle two cylinders, which run a little hotter, slightly richer, while not doing the outside two.

Look at fig 5, page 3 in this manual and you will see how much the pilot circuit affects fuel flow at smaller throttle openings.

http://wahlracing.com/Mikuni-carb-tuning-manual.pdf

[nicomkii]

Thanks for that Mark, been looking for a good guide like the one you given here.
Hopefully tonight I'll pull the plugs and get a better idea and start going through your suggestions. As long as I can minimize pulling the carbs out again, I use too many creative words apparently when trying to get them back in.
It just confused me that its only on a steady cruise speed that it surges, any throttle increase at all and it reacts as expected.

[zpower]

Also check inlet rubbers.

Look for cracks.
Check that they are seated correctly.
Check vacuum blanks on all on fully.
If they are old and have gone hard take them off and put a thin smear of heat proof sealant on them and put them back on.
Do not fully tighten them!
Tighten them so they just pinch the head and leave them for 24hrs then tighten them up fully.
Or of course buy some new ones.

Phil.