1980 R100 Idle Problem

This bike was supposed to have LOT's of work done to it including the rebuild of the carbs. The bike starts fine and idles fine cold with and without choke on. Once you ride it a few miles it will not idle down and seems stuck around 2000 rpm. I have heard lots of opinions but I want to come to the "source of all knowledge"! HELP!!!!

I will watch this thread closely. A friend has an R75/6 with the exact same problem. We have tried a lot of different particulars and so far none of them have fixed the problem. I'll wait a bit to list what we looked at and checked since we didn't get a solution yet. I'll add anything that appears to be missing once others have started to point you in the right direction.

My first thought is that I'm wondering if after the carb rebuild, the carbs were synched with the engine warm. Do adjustments on a barely started bike with the carbs will result in a high idle. Also, it could be that the advance unit is stick, resulting in an advanced ignition. You can test for this. When you get the situation where the RPM is elevated, slowly let the clutch out to drag the engine RPM back down to more normal. If the RPM stays there, then it's the carbs. If the RPM rises again, it could the advance unit. Still I wonder about the carb synch after rebuild.

The 1980 has the "points in a can".

High idle that seems not to drop down is usually caused by one of two things:

1. an induction air leak
or
2. a sticking ignition advance mechanism

One test that often will provide a good clue which of these is the problem is to gently load the engine while stationary by slightly engaging the clutch while holding the brake. If the idle drops and stays down that points to a sticking advance mechanism. If the idle drops but then goes back up this almost certainly signals an induction air leak.

The usual sources of such air leaks are the rubber connectors between the carburetors and head spigots, or leaking throttle plate shaft O rings.

On an unmodified '80 model the advance mechanism is in the points "bean can". These can be opened and lubricated but it is a bit of a pain to do. But the how-to is all over the Internet if you do a search. I am sure there are several folks here who can expand on this post.

Update on Idle Problem

First - thank you to all that replied to my problem. I have not checked the ignition advance, yet. However what I think seems to be the cause is that the right carburetor is slightly higher idle adjustment. On half turn seems to have fixed concern. But, just running with a fan in front of engine is not enough so I need to take the bike for a ride but huge rain storms in the area.

One comment presented was the question or fact about balancing the idles. Since I don't know the history and the fact that the right side is off I would conclude that is the next step. Here is my problem - I know the procedure from dual carb tuning on old Volvo's. But here is my problem - lots of YouTube video's showing how to set with a manometer but I cannot find a vacuum port to connect to. How do you connect to balance the carbs?

MinnCanoeGuy said:

But here is my problem - lots of YouTube video's showing how to set with a manometer but I cannot find a vacuum port to connect to. How do you connect to balance the carbs?

Click to expand...

On my '78, there's a port that sticks out the side. I've heard that on some carbs that port is on the underside of the body.

Barring that, there is a plug shorting method that works well. It involves using a screwdriver to short across spark plug extensions to the engine fins to shunt the spark energy to ground instead of letting it go through the plug, jump the gap, and then to ground. Done properly and firmly in terms of making the screwdriver short will cause no harm to the ignition system.

An alternative method is to use the idle mixture screw to temporarily disable a cylinder. These screws control an gas circuit...turning it in to seat it will shut off gas and shut off the cylinder. Just remember EXACTLY how many turns it takes to turn it in to seat. Once you've made your adjustments on the running cylinder, unseat the mixture screw. Then repeat on the other side.

idles fine cold with and without choke on. Once you ride it...seems stuck around 2000 rpm

Click to expand...

This symptom could also be caused by an always halfway-open enricher (aka choke).

/Guenther

When that happened to me, it ended up being a float that was out of adjustment. IIRC, the bowl was overfilled and the fuel was getting into the enricher circuit without the choke lever being deployed.