Middle cylinder, looking for advice, consolidating threads

[smoketriples]

Consolidating my threads as I got this going in other topics during my poor attempt at troubleshooting.

Here's the current state:

74GT750. Middle cylinder usually is not combusting.

Troubleshooting so far:

Compression check: 120
Center plug evaluation: clean and gas wet. Fires well out of cylinder and tested good in another cylinder.
Re-timed, it's dead nuts on with dial gauge and real close with strobe.
Replaced points
Removed condenser (just to see)
Swapped carbs from good running '74.
Ran with middle plug out of port and HT lead connected, shows healthy consistent spark.

None of the work has had any effect.

While running I used a long (insulated) screwdriver to hold points for middle open, then let them shut and operate normally. A couple of times this resulted in combustion 'a couple of times', (middle cylinder picks up, idle goes up, then within seconds drops back down and no more pop sound from middle). Did this ten times, two of those times the idle picked up for a couple seconds, the other 8 times, no change.

WTF is left? Before moving to this odd space time dimension I always found that fuel/air, spark, and compression meant 'bang'.

-Eric (the frustrated one)

[H2RICK]

Eric, if that plug is "gas wet" then I'd bin it and put a brand new one in there. Once the gas gets up inside the plug in the top of the insulator it may fire outside the cylinder....but won't fire under compression.
Before installing the new plug, I'd leave the plug hole open, key off, throttle wide open and kick the kicker 5 or 6 times with my thumb blocking the open plug hole.
The crank chamber of that cylinder MAY have a lot of raw gas in it from the non-firing and that has to be dried out before you put the new plug in. It may not hurt to leave the plug hole open and dry that crank chamber out for a half hour or so, as well.

[oldjapanesebikes]

I didn't notice if you had traced back through the harness and cleaned/checked the connector blocks ?

[smoketriples]

Ian,

No, have not, sounds like a good exercise for this afternoon and I have a brand new can of crc electrical contact cleaner just begging for use.

Hey on another note folks, after I do that I'm doing the relay for life walk tonight and tomorrow. If anyone feels moved to donate here's the link (shameless I know, but there it is).

http://main.acsevents.org/site/TR/Relay ... r_id=33466" onclick="window.open(this.href);return false;

[jabcb]

Hopefully it runs correctly after you dry out the cylinder & try a new plug.

If not, you could swap the ignition circuits as a test:
1) switch left & center spark plug wires
2) run temporary wires switching the left & center points.

If the left cylinder doesn't fire & the center does then you know its electrical. (Still could be a problem with the points.)

Could check the voltage drop across the coils when the points are closed. Use a metal paper clip or thin wire that fits in the coil's electrical connector.

If the voltage drop across the center coil is lower than the others, check the primary resistance for that coil -- should be about 4 ohms.

If thats ok, compare the voltage on the coil hot side (orange wire) with the points closed. That will point to a problem before or after the coil.

Could be a secondary problem. Spark plug cap may be bad. How old are your caps?

[smoketriples]

Edited:

JacB was right on for testing, eliminated electrical when I thought that was all that was left.

After verifying middle coil could run left, and left could run left, and neither would run middle, I was stumped. While it was running I shot a little ether in middle, it picked right up and ran a few seconds.

After posting that I was totally clueless.......I went back out and got a clue . Since my jets showed up today, I went back into the carbs. Cleaned everything in middle carb that I had cleaned before, made sure all the passages shot clean through, then raised the middle needle and put my 120 jets in (pods and large chambers, 3 into 3).

It runs like a two stroke again. Winds up fast, winds down slow. Middle is firing (even at idle).

Only downside was my (used) water pump gear off a parts bike broke. That's two that have broken just by me rotating the crank backwards to set timing. Since I've proven that the indicator on the bike is correct by dial gauge, I'm just going to set timing by light now.....cheaper than waterpump gears .

Also got some walking in for Relay for Life, will be back there tomorrow morning to walk some more.

[Cliff]

Try running the bike with the exhaust pipes removed and see if the center cylinder fires. She'll run like crap and be loud as hell, but you'll be able to see if the exhaust is the same from each cylinder. Check each for colour, flame, smoke and temp..
A quick check for crank seals, albeit not the most technical, is with the air box removed and the engine running, partially cover the throat of each carb. with the palm of your hand until that cylinder almost dies. Each one should have the same amount of suction. If the center cylinder isn't drawing air in throught the carb., it must be getting it from somewhere else! Choke off each cylinder, one at a time, and each one should act and feel the same. Just be careful doing this! There's enough suction at higher RPM's to damage your hand! Not to mention the amount of raw gas that's going to be in the carbs. and on your hands!
Cliff.

[Cliff]

Oops! Looks like smoketriples found his problem! His last post was edited between the time I read it and my reply!

[H2RICK]

It runs like a two stroke again. Winds up fast, winds down slow. Middle is firing (even at idle).

Ahhhh.....the old "dirty carburettor" problem again. Well, I'm glad you solved the problem.....and learned some more about how to properly clean carbs.

[smoketriples]

Can be right frustrating though. Especially when my 'good' (and very pretty), carb set turns out to need a cleaning too....luckily that bike is a moon or two from needing them, I'm still stripping it down so I can have the frame powdercoated.

[TheMarkedMan]

Crazy! I am having this exact same problem with my left cylinder on my 74 Buffalo.....120 compression on all 3, great spark on each one, but the left cylinder keeps misfiring, giving a few pops here and there....but of course, when i'm haulin A$$ through town, it fires great! The plug condition is a nice butterscotch color like the others, but it will eventually foul up solid black and stop working.....i'm gonna try cleaning the carbs like you did....seems the gas might not be getting in there properly, and the cylinder is running on a heavy oil mixture, resulting in the smoking and eventual fouling of that plug. AND! When the bike is on the kickstand, that particular carb overflow leaks, but stops as soon as I level the bike (ie: sitting on it)....this makes total sense!!!! Something is blocked, and it's choking on oil! I love this board! I'll keep you all posted on my results! I have a couple days off this week, so no better time than now.....

[tz375]

I'm sure you fixed the leaky float valve / low float level measurement (high fuel level) / hole in a float on that left carb by now. What was it?