1980 GS850 G Power dropping off between 3500 and 4500 revs

[hermann]

I know similar subjects have been mentioned but thought you might be able to help on a couple of questions. Just bought a 1980 GS850 G for £600 (about $1143) Its tatty but runs. just been to a rally which was 160 mile round trip and identified a couple of niggles. Thought people might be able to give me some advice. Its the first time I've had a shaftie. How hot should the drive be at the back wheel? The main problem was while riding it between 3500 and 4000 revs the engine seems to have a bit of a flat spot, where the power seems to drop off. Get up to about 5000-6000 revs and it seems fine. This only seems to happen when the engine is under load. Now I haven't replaced the spark plugs yet which I'm going to do tomorrow. I'm hoping that a fouled plug is the problem cos it felt like it was dropping onto three plots with the fourth one kicking back in when the revs got higher or lower. Just wondering if anyone has encountered anything similar with the gs850.

[mark]

A little more info. would be helpful. Stock airbox? Stock exhaust? Or has the bike been modified in anyway?

[hermann]

The bikes standard apart from 4 into 1 exhaust. There's a picture here

[mark]

Your exhaust will probably require some minor carb adjustments in order for your bike to run right. I have a stock airbox with 4-1 exhaust and it took a little work to get it right. Sounds like you have a lean spot.

A couple of things.
Make sure that your air box is absolutely sealed with no leaks. Check your intake manifold boots for cracks and your manifold boot O-rings for air leaks.

The other thing that would help you is to run your bike at the rpm where the flat spot is happening and do a "plug chop". Run it for a minute or two at that exact rpm, then shut off the bike with the kill switch and check the color of your plugs.

Let us know the results. Good luck.

[Guy]

Sounds like carb issues. Give them a good clean and balance. The bevel box gets quite hot after a long run. Replace the oil with EP90 and it's good for 6000 miles or so. The front end of the shaft has an oil filler hole too (it often gets neglected). The 850 isn't the most powerful of engines out there, but it should pull clean and linear if it's in good fettle.