Hi,

I have just been sorting out a K100 that I bought AS-IS and which was running like a pig.  After replacing a bad HT lead and a split inlet manifold it running much better: a good slow tick-over, though slightly erratic. Probably pretty serviceable.

However, on the vacuum gauges cyl 3,4 are notable lower than 1,2  ( each pair pretty much the same).

I have tried squirting auto-start around all the inlets and don't find any effect on the tickover speed, so I'm concluding that I've got the inlets air-tight now.

The bypass screws on the inlet don't really make much difference except that if I screw 1 &2 all the way home they rise a fair bit. Anyway, can't even it up with the screws.

I'm wondering whether this indicates leaky vales or messed about butterfly synchronisation. The inegality  tends to remain as I raise the revs ( in neutral ). The marker paint is gone on the sync screws so I guess they've been messed with at some time in its life.

I know the wise folks NEVER touch the sync screws but if this has already been messed up, is it possible to get it set closer by shutting the bypass screws firmly home and adjusting the sync screws?

It seems to indicate that the butterflies are slightly more open on 3 and 4.

Any tuning experts care to comment on possible interpretations?

TIA.

I did mine on the sync screws. Who told you that you should never touch them? It is your bike and there are screws there so use them. Mine runs like a dream.

Thanks Bob.

The received wisdom I have generally seem here is that these are factory set and you need special flow equipment to readjust them. The idea being that are not something which goes out of adjustment so should not be touched.

( It may well be that a running K100 provides a fairly suitable flow test too )

If they are out of sync, I suspect the situation can at least be improved.

On what basis did you adjust yours?  What were you measuring/testing to tell whether a movement improved or worsened the synchronisation?

thanks.

I did it exactly as per the manual. I hooked up a four column to the connections on the throttle body, with a t piece on the front one and adjusted them all til they were perfect. I turned the idel adjuster up to around 2000 rpm and closed all the brass screws first. 
Just take your time and do things slowly and it will all fall into place.

Thanks Bob. I've found this thread which gives several accounts on how to do this. Pretty much as I outlined initially.

https://www.k100-forum.com/t1526-how-to-balance-the-throttle-bodies-solved

By: Rob Lentini
June 1995
Here's a real simple way to ascertain whether your K's throttle plates are in synch--the adjustment we are admonished to never touch!

It did point out one thing I'd overlooked: checking the valve clearances which I have not done on this motor yet. Even if the valve clearances are good, I think there remains one uncontrolled variable which is slightly less than air-tight valves seating. That could lead to ( incorrectly ) adjusting the butterfly valves to balance the effect of state of the valves.

The bike has to have the valves precisely adjusted. You're going to be using the engine as an air-pump for a flow bench. Without the valve adjustment it just isn't going to work.

The fact that there were two up and two down does lead me to suspect a misalignment of the centre screw. It may even have been bumped in rough handling since this motor came from a breakers and was swapped over by someone who did not have the time or motivation to do it carefully.