Hey all!
I've been busy as heck, and haven't had a lot of time for my bikes or to log on here.
Here's a kind of combined question:
I have a '78 IT400 with a new top end. It will start cold, run with a lot of smoke, then die out like it has no fuel, and not restart. It seemed to be jetted well with the old top end, but had a wicked piston slap that turned out to be because a previous bore was done out of square to the cylinder base (I ended up buying and boring a different used cyclinder --p4, though I could have done p2 if the piston wasn't already bought).
I also have an '81 KDX175 w/120lbs of compression, which is 2lbs. over the service limit. It runs and I've been riding it, but can't quite seem to dial it in. Stock jetting is 150 main with the needle in the middle position. High altitude (I'm in Denver) is supposed to be one main smaller (145) and the needle dropped. I had a lag in the midrange stock, but good power on the pipe.
In a rush (like I said, I've been busy), before camping Labor Day weekend, I jetted 145 and dropped the needle. It seemed to run pretty good around the neighborhood (a little rich, but I had just oiled the filter), so I loaded it for a mountain weekend at about 9600 ft. elevation. Up there the midrange lag came back, so I tried the needle in every groove. Raising the needle, dropping the clip, one slot from center gave me the strongest midrange, but made the transition between that and high end flat until it kicked in on the pipe. It was ride-able, but not as responsive as I'd have liked. I bought a 140 main to try next after getting back to town, but my wife washed my pants with the jet in the pocket, and it hasn't turned up.
Anyway, here's what I'm wondering about:
Does compression interact with carburation? Could the new compression of the IT change my jetting requirements, and the low compression of the worn KDX do the same thing, in the opposite direction? I also had the best luck running my oil at 20:1 in both bikes before I did anything to them, but am now running 32:1 Klotz. Some of the 50:1 synthetics say rejetting could be required, but no one has suggested that changing to 32:1.
Any big picture thinking for me on any of this?
The IT400 is really the bike I want, but it isn't quite going and I haven't had time to sort it out. The KDX is a running substitute, and kind of fun, but not first priority right now.
I've been busy as heck, and haven't had a lot of time for my bikes or to log on here.
Here's a kind of combined question:
I have a '78 IT400 with a new top end. It will start cold, run with a lot of smoke, then die out like it has no fuel, and not restart. It seemed to be jetted well with the old top end, but had a wicked piston slap that turned out to be because a previous bore was done out of square to the cylinder base (I ended up buying and boring a different used cyclinder --p4, though I could have done p2 if the piston wasn't already bought).
I also have an '81 KDX175 w/120lbs of compression, which is 2lbs. over the service limit. It runs and I've been riding it, but can't quite seem to dial it in. Stock jetting is 150 main with the needle in the middle position. High altitude (I'm in Denver) is supposed to be one main smaller (145) and the needle dropped. I had a lag in the midrange stock, but good power on the pipe.
In a rush (like I said, I've been busy), before camping Labor Day weekend, I jetted 145 and dropped the needle. It seemed to run pretty good around the neighborhood (a little rich, but I had just oiled the filter), so I loaded it for a mountain weekend at about 9600 ft. elevation. Up there the midrange lag came back, so I tried the needle in every groove. Raising the needle, dropping the clip, one slot from center gave me the strongest midrange, but made the transition between that and high end flat until it kicked in on the pipe. It was ride-able, but not as responsive as I'd have liked. I bought a 140 main to try next after getting back to town, but my wife washed my pants with the jet in the pocket, and it hasn't turned up.
Anyway, here's what I'm wondering about:
Does compression interact with carburation? Could the new compression of the IT change my jetting requirements, and the low compression of the worn KDX do the same thing, in the opposite direction? I also had the best luck running my oil at 20:1 in both bikes before I did anything to them, but am now running 32:1 Klotz. Some of the 50:1 synthetics say rejetting could be required, but no one has suggested that changing to 32:1.
Any big picture thinking for me on any of this?
The IT400 is really the bike I want, but it isn't quite going and I haven't had time to sort it out. The KDX is a running substitute, and kind of fun, but not first priority right now.
