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My son bought a low mileage 1987 GL 1200 Interstate. He removed the carburetors and thoroughly cleaned them. He reassembled them and reinstalled them. Bike starts and runs great but has no roll on power above 70 mph. Its like it goes flat. He told me he can apply some choke at speed and that helped but no fix. I originally had thought maybe a problem with slide diaphragms. He inspected those and found no problems. He pulled the spark plugs after a fairly long ride yesterday and plugs looked lean. He also ran a compression test and found all of the cylinders even at 175. Just wanted to see if anyone had any ideas on this. Thank you.
 

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Ole Guy
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I probably could have said all this with fewer words, but here it is. Check your manifold gaskets if you have not. My 1885 GL1200, 99,000 miles runs good. I ride often, but not very far, so the mileage is going up slowly. Mine did not have top-end power problems, just poor idling and bumbling up to 2000 rpm. I needed to apply full choke when cold. Mine is cold natured, warms up slow going down the road. I removed and cleaned the carbs. I poked some deposits out of the idle jets (tiny holes, not easily seen if clogged) - this was probably ten or fifteen years ago. If I remember one side had a few more jets, I think for the choke function. I did not dismantle them, just cleaned the idle jets and sprayed in some carb-cleaner. Put it back together and it idle somewhat better after balancing the carbs, but not as much as I hoped for. Since it only affected the low idle and ran smooth enough when fully warmed up, I left it at that. A few years later I repeated the cleaning (jets not clogged) and this time replaced the flattened O-rings under the manifold castings going into the cylinders. They were compressed flat and I thought it odd and was careful not to break them. They should have been replaced the first time because they were actually leaking air making the idle mixture lean. After new gaskets it ran much better and I did not bother to balance the carburetors, because the idle was much improved. For all I know, the old O-ring gaskets were original. They were hard and flat. The new ones were soft, round, pliable and sealed the leaks.

One other thing that seemed to boost performance - I had a scorched head light connector. When it finally melted all the way, the new connector helped it run smoother and a little peppier. May the electrical drag was contributing to the poor performance of the Pulse Generators that were also later replaced. It was weird that they started acting up, but I always managed to get it to start a short while later after they cooled. It would start and stay running. When that finally got bad enough I replaced the pulse generator about 10,000 miles ago. Today it runs nearly perfect. That's all for now, good luck!
 

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My son bought a low mileage 1987 GL 1200 Interstate. He removed the carburetors and thoroughly cleaned them. He reassembled them and reinstalled them. Bike starts and runs great but has no roll on power above 70 mph. Its like it goes flat. He told me he can apply some choke at speed and that helped but no fix. I originally had thought maybe a problem with slide diaphragms. He inspected those and found no problems. He pulled the spark plugs after a fairly long ride yesterday and plugs looked lean. He also ran a compression test and found all of the cylinders even at 175. Just wanted to see if anyone had any ideas on this. Thank you.
Seems like you have figured out what is wrong. The bike is lean. Now th question is why. As Dennis mentioned many have issues with float height and the 1200's are very fussy about float height. Did he invert the carbs to set the float height? That is not right but a common mistake. It is also possible there is a vacuum leak. If the bike sat for a long time the low speed ports often varnish shut and are very hard to get back open as they are so very tiny. When that is an issue usually low end is the symptom but if just a single port was plugged maybe. Last thought is the spring in the needle valves working nice and free if you didn't replace them. I have a feeling when you get the bike right you will realize it runs better at all RPM.
 
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