cylinder #4 not firing

[stratgooroo]

i recently bought an 84 gpz and after riding it around a little i realized that it was not firing on cylinder #4. it has spark and i pulled the carbs to make sure that it was getting gas and they were clean as a whistle. i dont know very much about this bike and am just going off the fact that i need gas, air, and spark.As far as i can tell it has all of that so now i am at a loss. also, i dont really want to take it into a shop and pay a bunch of money.Im just hoping that someone out there has had a similar problem and can help me out,,thanks

[GPZKrazy]

Do a wet and dry compression test of all the cylinders before you do anything. If you don't have a compression tester just remove the spark plugs and put your thumb over each spark plug hole while turning the engine over. If cylinder #4 gives you nothing, try it again with a squirt of oil in the cylinder. If you start to get something (compression of the piston trying to blow your thumb off the spark plug hole) after adding the oil, your rings are worn. If you still get nothing, its very possible that you have a burnt valve. I speak from recent experience

[Mike900]

Besides spark (at the correct timing) & air/fuel (at the correct mixture) you also need compression. It's best checked with a gauge but you can do a quick check by removing a plug & placing your thumb over the hole while cranking over the engine.

Edit - GPZKrazy beat me to the punch.

[stratgooroo]

i have done a dry compression test on only the #1 and #4 cylinders so far and it read about 60 psi on both.The #1 cylinder hasnt been giving me problems and the #4 has though. do you know how much compression they are supposed to have?... if it is in fact a burnt valve, how involved is that job and how expensive?

[Mike900]

60 is quite low. You should be up around 125 -150 maybe even 175. If all 4 show that low & don't come up with a wet check then the cam chain may have jumped the cam sprocket. However that wouldn't explain 1 cylinder not firing while 3 are. If all 4 cylinders check that low then I'd check another vehicle to see if the gauge might be faulty.

[stratgooroo]

I am looking for an air cleaner for my 84 gpz, it is the round kind not an oval but on bike bandit it called for an oval one so im not sure where to look on line. I also need the two rubber gaskets for the petcock and i havent seen those either

[stratgooroo]

okay, found the air cleaner now im back to wondering why that cylinder wont fire...i havent quite got to the bottom of the compression issue but im thinking that the rings or valves being bad wouldnt keep the cylinder from firing

[MFolks]

Did you check the compression on that cylinder? The engine needs at least 120 PSI to run.

[herk]

Check the compression at WOT (wide open throttle). It's handy to check when the carbs are removed.

I know you said that there's spark, but I would redo the spark plug wire connections at all points from the coil to the plug. The connections become poor from oxidation over time. The presence of spark doesn't necessarily mean good spark. Don't ask how I know.

[stratgooroo]

thanks i will definitely check that... i havent had time to mess with it in the last few days but today i will look into it and let you know how it goes

[scaldwellk]

I had a problem like this when I received someone else's problem bike. Turned out that mine had a bad intake cam. Even the rocker was worn bad. I guessed that the other owner tried to adjust the valves and that was why I did not get alot of noise. With the valve not opening, no fuel equals no firing. I hope yours is not the same problem.

[stratgooroo]

say i am having a valve problem, is it very difficult to adjust or repair? Expensive?

[stratgooroo]

okay here's the latest...i had previously taken off the petcock to see if the o ring gaskets were in good shape and there were no clogs...i didn't see anything wrong except the tiniest pinhole tear...i didnt think anything of it and so i kept searching for a problem.My hypothesis is that since the number 4 cylinder is the furthest from the fuel source, the fourth cylinder is starving for fuel... let me know what u think

[scaldwellk]

Originally Posted by stratgooroo

say i am having a valve problem, is it very difficult to adjust or repair? Expensive?

You will have to follow the manual.
I found cams on ebay for mine. Had to go to dealer for 2 rockers.

The gas idea does not sound like the problem, but you are thinking good. One thing you could check is if your bowl is full. If you have gas and spark, then look at the valves and cams.

[stratgooroo]

i just keep hoping that is not the problem but if worst comes to worst then i guess ill have to take a look at the cams... i just do not want to take the engine apart and spend all kinds of money on it if i do not have to

[GPZKrazy]

Doing the full dry and wet compression test will tell you a lot about what direction you'll need to go in to get the motor running good...and how much you'll need to spend. I still reccomend pulling the plugs, putting your thumb over the cyl #4 spark plug hole and turning the motor over with the throttle wide open. Repeat with cyl #1. Compare that feeling to a cylinder with good compression; you'll be able to tell the difference immedietly. Does the motor smoke at all while it's running or when you rev it? If so what color is the smoke? How many miles are on the bike? How do all the spark plugs look?

[stratgooroo]

the bike does not smoke at all even if i rev it. the bike had about 25,000 miles on it when it had a big bore kit put in and now it has a little over 30,000 after the rebuild. the compression tester i had read 60 lbs on the dead cylinder and 60 on the running cylinders so i know that even if the compression is low the cylinder should still fire because the others do with that compression. the engine doesnt smoke like i said and i figured that if the rings were bad it would be smoking... as far as the plugs go, all the cylinders' plugs that fire seem slightly rich but the one that doesnt fire is the perfect tan color...something interesting also that i found today was that when the bike was running of course the dead cylinder's exhaust pipe was not hot but when i pulled the plug wire off it ran worst so i didnt know what to think of that

[kopcicle]

not to sound like a jerk here but why is the cam cover still on the bike , and why have the valve clearances not been checked ? I'm known more for my diagnostic abilities and somewhat less for my patience and people skills ...

~kop

[Mike900]

Originally Posted by stratgooroo

...something interesting also that i found today was that when the bike was running of course the dead cylinder's exhaust pipe was not hot but when i pulled the plug wire off it ran worst so i didnt know what to think of that

When the coil fires the voltage goes down one spark plug wire (let's say #1) jumps that spark plug gap & travels through the cylinder head over to #4 spark plug, jumps that gap & continues up #4 wire to the coil to complete the circuit. So removing the plug wire from #4 will also kill the spark at #1.

It sounds like #4 spark plug isn't wet & is getting spark. You may have a carb problem on #4.

[StarGate]

Originally Posted by Mike900

When the coil fires the voltage goes down one spark plug wire (let's say #1) jumps that spark plug gap & travels through the cylinder head over to #4 spark plug, jumps that gap & continues up #4 wire to the coil to complete the circuit.

Where did you find this theory?