Please Help Me!!!

[SanDiegoMike]

Trying to get this bike ('87 Rebel 450) running right before we give it to our daughter for her birthday in two weeks. Has a really bad pop/misfire from right cylinder. If I pull the plug wire from that cylinder while its running, there's not a lot of difference on how it runs. Switched caps...same deal. pulled the plug and there's good spark. Here's what bugs me...removed the air cleaner cover to see how much air is being drawn in from each carb. At idle, if I cover the left carb intake with my fingers, I can feel a strong vacuum and then the engine will die because of the obvious airflow restriction. When I try to do the same to the right side carb...nothing. I don't feel the vacuum, and the engine keeps right on chugging. ANY ideas?!? PLEASE HELP ME!!!

[mro]

If no "suction" on that carb with it running then you have no compression. Sounds most likely that you have a valve(s) stuck open. Check that the carb butter fly opens and do a compression test.

No compression gauge, with plug out and just cranking with starter and finger over plug hole should feel pressure from piston going up and down. Can check aginst other side for comparision.

Would be really weird for exhaust to be plugged, but $h1t happens.

mro

[SanDiegoMike]

Plug out...engine running, lots of pressurized air coming out of plug hole...

[mro]

But nothing at the carb?

Drop exhaust that side, then see if suction at carb.


mro

[mro]

btw,
with one valve stuck can normally still feel some presure at the plug hole, but will not build compression to fire.


mro

[fredintoon]

re your cri de coeur

Hi Mike,
What those guys said. Here's a Q&D trick that works on most vertical twins to narrow down the search:-
IF the 450 is a 360º twin (pistons go up & down together) & IF it has a single coil with 2 plugwires (throwaway spark iggy), Swap the plugs & wires over. IF the problem moves to the other cylinder it's the iggy. If it don't, it ain't.

[79XS11F]

I'm with MRO on this .. check your compression. With the plug out you will alway get air coming out of the plug hole because the air in the cylinder would simply take the easiest way out and the plug hole is sooo big that no back pressure would be present.
So do a compression test. If that cylinder is low put about a tablespoon of oil down through the plug hole and spin the engine several times and test again.
If compression goes up a lot then the rings are TOAST. ... if no change there is a valve issue.
If it's a valve issue check valve clearances to make sure the valves can actually close fully. Correct if necessary.
If valve clearances are good you may have to pull the head and see what's going on. You may however want to try a chemical carbon removal first and make sure the head gasket is not leaking.
Rob

[prometheus578]

So far everyone has mentioned all the usual stuff.
Check the valve clearances. If you have one that's REALLY loose, you probably have a bent valve.
Yes, these things will run fairly smoothly on only one cylinder. had one in the shop last summer with a blown head gasket. It was either that, or a warped head. The owner declined having work done so I never found out which.
Was doing the same thing. No compression on #2. Compression from #1 firing was blowing out #2's plughole.
If you have a compression tester and a compressor... bring #2 up to TDC. Use a tire valve core remover and remove the valve from the screw-in end of the compression tester. Screw it into the plug hole, and hook the other end of the line to your compressor. You will be able to hear where the air is leaking from: Intake valve, exhaust valve, down into the crank(bad rings), or across the head into the other cylinder. You can also spray the outside of the head to see if a blown gasket is venting to the outside.