1981XS
XS650 Member
I have an '81 XS650, bought new, with about 24k mi. on the engine. I was a Honda auto mechanic from 1981-'87, so not a novice, but I haven't personally seen this before. About 9 years ago I began a cafe racer conversion on this bike. Did a complete engine rebuild (no bore out, no crank changes, just new guides, lapped valves, rings, bearings, seals, gaskets, w/ported and polished heads). I removed of lots of weight incl. starter and battery, added a 2-into-1 header, new pipe and muffler, and 34mm Mikuni flat slides. I've got the fuel system covered and ruled out. I replaced ignition & charging with an HHB system, and triple-checked everything. If I'm able to start the engine (rarely and only cold), It's breaking up above 2.5kRPM, with no power, and minor backfiring. I thought it was ignition, and still think it may be a bad ignition module, but I can't test that. I want to avoid ordering ignition parts as costs will add up and funds are scarce.
The other primary suspect in my mind is that the cam timing isn't correct. I did hundreds of Honda head gaskets, timing belts, and top end overhauls, so I know how to index a cam. But mistakes can happen. Perhaps a brain fart? '81's don't have points or any external index marks on the cam (that I've seen), so I'll have to pull the engine to see the cam index mark in order to check it perfectly. I DON"T want to have to pull the engine. I could use a degree tool to figure out if the valves are opening at the correct times, but IMO I could still be off by a tooth and not know it. To test the current compression I used a drill, turning the crankshaft at a couple of hundred RPM, with the throttle wide open, and the engine stone cold (since it won't start). I got 90PSI and 84PSI compression readings, which didn't seem great but high enough that incorrect cam indexing may be a longshot. My question is would I be able to get compression readings that high if the cam is off one tooth?
Appreciate any feedback. Thanks.
The other primary suspect in my mind is that the cam timing isn't correct. I did hundreds of Honda head gaskets, timing belts, and top end overhauls, so I know how to index a cam. But mistakes can happen. Perhaps a brain fart? '81's don't have points or any external index marks on the cam (that I've seen), so I'll have to pull the engine to see the cam index mark in order to check it perfectly. I DON"T want to have to pull the engine. I could use a degree tool to figure out if the valves are opening at the correct times, but IMO I could still be off by a tooth and not know it. To test the current compression I used a drill, turning the crankshaft at a couple of hundred RPM, with the throttle wide open, and the engine stone cold (since it won't start). I got 90PSI and 84PSI compression readings, which didn't seem great but high enough that incorrect cam indexing may be a longshot. My question is would I be able to get compression readings that high if the cam is off one tooth?
Appreciate any feedback. Thanks.