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Discussion Starter · #1 ·
1988 gl1500 Aspencade, 93K miles. Was running great this week, smooth as glass, carbs rebuilt, oil change, new plugs, flushed fuel tank, new fuel filter, all vacuum lines checked and replaced if cracked or collapsed. Had been sitting for 2 years in a barn. Was running and up to operating temp, revved it up a couple of times (not red lined, no more than 3,000 rpms, made a sound like pressure bleeding off and started idling rough. Started pulling plug wires while running, left side wires made engine worse. On the right side, any wire, no change. In fact, I can pull any two wires, and no change, pull all three and the engine finally is affected and dies. Compression when I started this project, 175 to 185psi cold. Compression right side now 145psi warmed up, all cylinders. No compression bubbles in radiator, no antifreeze in oil, no oil in antifreeze. No visible external leaks. Added oil to each cylinder on right side, no change in compression. Getting ready to check compression for left bank, as well as check timing belts.

Also, engine still revs up (hesitation) and seems to smooth out (strange), then returns to rough idle. Rich exhaust smell. Plugs clean, no exhaust smoke. Plug wires sparking good, plugs sparking good. White spark, very little blue.
 

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I take a WAG at it . i am thinking you had the right hand belt jump maybe just enough to touch the valves , causing the loss of compression . My next step would be to pull the belt covers and check the cam timing . I hope I am wrong .
 

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1987 GL1200 Interstate
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Maybe the right side carb got some crap in the idle circuit.
That seems more likely if it's fine when it revs.
 

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I'd recheck the compression. It should be done with a fully charged battery, warm engine and choke off with the throttle full open. It's possible a belt could have jumped and bent a valve it the belt was really loose but usually that doesn't get all the cylinders. If the belt has jumped a tooth the timing could be far enough off to prevent the valves being closed during the power stroke.
I'd recommend first checking the timing belts to make sure the timing on both sides is correct. Next would be to take the rocker cover off and compare the valve travel on one side with that of the other to make sure all the stems came up about the same amount as you hand turn the engine with the crank bolt on the crankshaft timing pulley.
 

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Pretty much like Paul said.... Recheck compression (on all six to eliminate variables).. If still shows left/right differences, I think a belt could have jumped a cog or two on that right side.. doesn't sound like it got far enough to bend valves though if compression is even on that side.. just timing differences will affect compression.. and each cylinder on the same side will be affected similarly...
 

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I'll go +1 on comparing total valve travel & checking the timing marks ... but...

How many miles and oil changes has this had since sitting? Loose crud in the oiling system (possibly liberated by the RPM pulses and/or fresh oil change) could effect the pump/bleed rate on the lash adjusters... it'd be interesting to slow turn the motor to re-compress the adjusters to see if smooth running returns
 

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1987 GL1200 Interstate
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:?
but how does a clogged carb circuit cause a loss of compression ?
I don't know, but I don't know how cams that far out of time would fix themselves when the engine is reved. And, the fact that all cylinders had dropped to 145, sounded a little odd too.
Now if you're done quizzing me, can we get back to the problem at hand?
 

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I would check the compression on all 6 cylinders. I'd probably hook a large booster battery up to it first to prevent battery drain while doing this. If there is a significant difference between the right and left side, where the right 3 cylinders is low, and the left 3 are normal, then it almost has to be valve (and probably belt) related. The belts and cams are about the only things that are completely separate from side to side.
 

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I'd also recommend alternating from side to side when checking compression so that any reduction in battery output wouldn't show up on the last three on one side. I don't mean to insult but just in case, were all the sparkplugs removed first? It sounds like there might be a timing error because even the high side compression seems pretty low.
One other point is that I can't see a head gasket causing three cylinders to read low but the same.
 

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Could be worth trying the "rear wheel slowly backwards" fix to see if any lifters are sticking. Plugs out, key off, in gear and very slowly turn the rear wheel backwards for a few minutes.
 

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Discussion Starter · #12 ·
All timing marks line up, no missing teeth on belts or gears, correct tension. Will run more tests this weekend. Putting a remanufactured engine in a 2001 Buick LeSabre, as well as trying to fix my Gl1200 (nothing major, just "little things" choke cable broke, fixed it, clutch leaked, fixed it, front brake master cylinder leaked from sight glass falling apart--how do I fix that?)
 

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Leaking reservoir sight glasses have been covered in the forum several times. It would be worth a search on the site. I've forgotten which sealant or glue works best.
 

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Discussion Starter · #14 ·
Went through the carb again, blew them all out, still the same problem. When I rev to 2k and hold rpms go down then back up to 2k then down again then up to 2500 rpms, like a vacuum switch is turning on and off. With engine cold and choke on, engine sounds pretty good, after it warms up, idling at 750 rpms, not so good. I'll check rocker travel in the next few days.
 

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It aint rocket science
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1988 gl1500 Aspencade, 93K miles. Was running great this week, smooth as glass, carbs rebuilt, oil change, new plugs, flushed fuel tank, new fuel filter, all vacuum lines checked and replaced if cracked or collapsed. Had been sitting for 2 years in a barn. Was running and up to operating temp, revved it up a couple of times (not red lined, no more than 3,000 rpms, made a sound like pressure bleeding off and started idling rough. Started pulling plug wires while running, left side wires made engine worse. On the right side, any wire, no change. In fact, I can pull any two wires, and no change, pull all three and the engine finally is affected and dies.
My money for your problem would be riding on a decel compensation (shot air valve) problem. When you were down under the carbs before a brittle plastic connector may have been bumped, weakened, and eventually broke.

That would be anything with the yellow color hose(s). Pay attention to any green colored hoses which might be tied in from the secondary air system also. Start by checking in the area of the one way valve.

With the air cleaner out, in the right front floor of air box is a hose connection it should make a gurgling sound sucking in air with engine idling. If not doing this there is a definite problem with secondary air supply and related plumbing underneath.

Shot air solenoid valve 2 is under left pocket. Shot air solenoid valve is by coils and the shot air valve itself is the white part to the left of water thermostat.
 

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