Mid-range rpm stumble on 78 R-100

[btsmayden]

Anyone have any ideas?

Recently gone through a minor overhaul (seals, gaskets, updated rocker shafts and one cylinder head). Engine ran very well after all adjustments were finally dialed in. After about six hours of riding, the engine began to stumble at mid range RPM's (around 2,500 - 3,500). It still idles quite well and if I can manage to get it past that RPM range, it begins to pull strong again. I rechecked all ignition and valve timing, valve adjustments, etc. I rebuilt the carbs after this began, with no change. The spark plug on the right side shows some carbon, but with grey tinges at the electrode. The left side plug comes out clean and slightly damp (but doesn't really smell like gas (the left side has the new cylinder head). Any help would be greatly appreciated.

[rocketman]

If I remember correctly Mid range fuel mixture is controlled by the needle as it rises from its seat in the main jet, might need to recheck what notch they are positioned in. Also make sure the advance is working correctly, it should be at full advance at around the top of the range your having the stumbling in, so if its sticking it could effect that range because its not advancing smoothly.

RM

[brickrider]

In addition to what Rocketman stated, I've also found that for some reason crud accumulates often in the area around the jet needle. I believe the moveable piece around the jet needle is called the needle jet. The main jet must be removed in order to access/clean this passage.

Ride Safely,
BrickRider

[btsmayden]

I just swapped spark plugs from side to side and the miss followed tha plug. If I can find a spare plug, I'll let you know if it takes care of the problem. Seems too simple though...

[rocketman]

I just swapped spark plugs from side to side and the miss followed tha plug. If I can find a spare plug, I'll let you know if it takes care of the problem. Seems too simple though...

not unnecessarily, the plug could be cracked or otherwise fouled or defective in a manner not obvious by observation. Hope it solves your problem thou.

RM

[btsmayden]

Well, the problem is much better with new plugs, but it feels like it will come back. I will pull carbs and look at main jets. It's always possible I installed parts wrong while rebuilding.

[rocketman]

Well, the problem is much better with new plugs, but it feels like it will come back. I will pull carbs and look at main jets. It's always possible I installed parts wrong while rebuilding.

Check the needle positions as well and also choke, make sure that when the choke lever is off the choke cams on the carbs are all the way off, push down on them (on the carbs), sometimes if they are just a little sticky they don't shut off completely and make sure the cables have some slack, even a slightly open choke will cause poor running.
Also check that both throttle cables have the same amount of free play at the carb ends and that when you pull on up on then and take up the slack there is no acceleration on that side. Sometimes even when the slack is the same if the cable is kinked just pulling up on them to take up the free-play that side will cause that side to accelerate a bit. Also, as noted in recent posts on carbs, there are parts that are “handed” and each carb assembly is therefore opposite of the other, so do each separately and compare each step with the diagrams you are using. Look over the other recents posts on carb in this section for some other things to consider.

RM