Misfire & Hesitation

[ajs350]

When I bought my 80 back in November it hesitated off idle up to 4000rpm and had a noticable misfire if I held the throttle at a constant rpm in the same range. While the bike would run strong above 4K if I held the throttle steady above this number I could still feel the mis fire. If I put my hand behind each muffler I could feel the uneven pulses on the left so the misfire must be coming from 1,2 or 3. Another observation was that by using an infra red temp gauge I found that after a long run #2 would read 157 degrees while the other 5 would range from 115 to 121 degrees. All plugs have a nice light tan appearance. Compression tests have all cylinders between 132 and 138 psi.

Since November, I have installed new plugs, caps and wires. Set the ignition timing. Shimmed the valves to 0.08 to 0.10mm. Removed the carbs and rebuild with Randakks kit and Mike Nixons book (and alot of his e-mail help! Thanks Mike). Tapped and removed the idle jets for cleaning and checked the accelerator pump before assembly for correct action.
Once all was back together I still had the same problem. Mike suggested I sync the carbs and recheck the valves before taking the carbs off again. Yesterday I completed both but still have the hesitation and missfire.

I also found a crack in one coil so replaced it from one from Louis and have since rotated them all to eliminate the coils as the problem

So before I drop the engine and do the carbs again can anyone think of anything I've missed?
Thanks Ross

[oilheadron]

This could be a toughie. First off, how easily does it start from cold and how well does it idle after warmup?

[ajs350]

It will take several attempts to start. It will fire but not all cylinders seem to catch. After 4th or 5th attempt it will idle and run up to 3K on choke. Once it is warm it idles fine at 1500rpm but gets lumpy if I lower the idle further.

[daves79x]

Hate to say, but it's the carbs. Something is still plugged in the low-speed circuitry on whatever cylinder doesn't fire at idle. You removed and cleaned the idle jets, but the passageways above may not be clean to the pilot screw and the 3 clustered holes at the butterfly. Also, you may have not gotten the air bleeds opened up - just like a gas can with no vent. I've devised a way to force brake or carb cleaner through to just the spray holes in the throat, if you get around to doing that. Let me know and I'll tell you how. When you first start the bike up, check to see if that pipe is colder than the others - dead giveaway.

As for hard starting - this has been discussed since the beginning of time. What I've found is, if you are running the bike at least weekly, and your accelerator pump is working, just pump the pump at least 12-15 times before even turning on the key. You can't flood these things. The bike should fire immediately on several cylinders, but may not catch the first time. Turn off the key and repeat the pumping (very quick 1/4 turns or blipping the throttle). Then try again. It will start faster and run on all cylinders quicker this way.

Just be aware that gas evaporates quicker out of these carbs than any I've ever seen. I contend that when you pull in the garage with a hot engine, by the time the bike cools down, at least 1/3 of the gas has evaporated out of the carbs. Add a week or 2 of sitting and there goes some more. What's left goes 'stale' very quickly. With the vacuum valve and no positive fuel flow until cranking, this presents the hard starting even more. On the '79s, just turn the fuel on a couple of minutes before trying to start (you did turn it off?). On the others, I route the vacuum valve vent hose to where I can easily fish out the end of it. Just blow into it and hold pressure on it for several seconds. This will open up the valve and allow the bowls to fill before trying to start. There are lots of other ways to do this, but I've found this to be the easiest since you don't have to modify anything to do it.

Anyway, this should help with the hard starting. If you doubt the 'stale' gas theory, try this: Drain the carbs when you come back from a ride. Then a week or 2 later, fill the bowls before cranking however you choose to (see above). Once you are sure they are full, do nothing but full choke, no throttle and hit the starter. It will be running on all 6 before you can get your thumb off the starter. Works every time on any CBX I've owned. My original '79 sits for six or seven months each winter, tank full and carbs drained. I pull it out, turn on the gas for a few minutes, and it starts immediately like it was just run yesterday. If I let it sit with gas in the carbs for even a few days, then I have to go through the drill with pumping the throttle and trying several times to get all cylinders to fire.

Sorry for the long ramble. Good luck.

Dave

[ajs350]

Dave, thanks for the detailed post. I tried your cold start trick and the bike fired right up first try! As for the exhaust temp all headers progress together form cold except for #2 which runs 20 degress hotter after 1 minute and stays hotter as the motor warms up. Not sure if that helps diagnose the problem or not?
Looks like I'll be taking the carbs off again so any procedures you might have that would free up these idle jets would be much appreciated.
Thanks Ross

[oilheadron]

What did you see the first time you checked the carb synch?? Anything unusual on #2??

[spencer]

I've devised a way to force brake or carb cleaner through to just the spray holes in the throat, if you get around to doing that. Let me know and I'll tell you how.

Dave, I would like to know how you do this. Thanks in advance!

[daves79x]

Ross:
Not sure I've heard of the dead cylinder's pipe being hotter than the rest. It has to be firing for that to happen. Are you sure that's the cylinder that's misfiring? As Mike said, are you sure you have no tight valves? I guess your compression check would say no, but that's without the engine running. I'd make sure you had valves no tighter than .003 inch - better at .004-.005. Has the engine ever been apart that you know? Any chance that #2 got hydrolocked and slightly bent a rod? You usually hear a metallic 'clicking' of the piston wrist pin boss hitting the crank fly weight, but they will run. Compression is usually so low in that case that the cylinder does not fire.

Are you sure nothing is plugging either the intake side or the exhaust side of #2? Nothing stuck in #2 headpipe? As Ron asked, what vacuum did #2 draw when synching? If I think of anything else, I'll let you know.

Dave

[6Pipes]

daves79x wrote:
I've devised a way to force brake or carb cleaner through to just the spray holes in the throat, if you get around to doing that. Let me know and I'll tell you how.

Dave, I would like to know how you do this. Thanks in advance!

ME TOO!!

[ajs350]

What did you see the first time you checked the carb synch?? Anything unusual on #2??

These are the gauges I used and the orginal readings were; #1 at 4, #2 at 5, #3 at 6, #4 at 10, # 5 at 8 and #6 at 7.
I synced them in the order of 5,6,3,2 and 1 and they now all read 8.

[ajs350]

Ross:
Not sure I've heard of the dead cylinder's pipe being hotter than the rest. It has to be firing for that to happen. Are you sure that's the cylinder that's misfiring? As Mike said, are you sure you have no tight valves? I guess your compression check would say no, but that's without the engine running. I'd make sure you had valves no tighter than .003 inch - better at .004-.005. Has the engine ever been apart that you know? Any chance that #2 got hydrolocked and slightly bent a rod? You usually hear a metallic 'clicking' of the piston wrist pin boss hitting the crank fly weight, but they will run. Compression is usually so low in that case that the cylinder does not fire.

Are you sure nothing is plugging either the intake side or the exhaust side of #2? Nothing stuck in #2 headpipe? As Ron asked, what vacuum did #2 draw when synching? If I think of anything else, I'll let you know.

Dave

Dave, All cylinders are firing at idle. It is only when I hold the rpm's constant between 2 and 4K that I hear the bike miss a beat and hear and intermittent "put.....put.......put" from the left muffler. Being as #2 is running between 20 and 40 degrees hotter than the other 5 I am assuming that this is the one that is misfiring. I could be wrong.
All valves have been set between 0.08 and 0.10mm (or 0.003 and 0.004 inch). Compression is:
#1 132 psi
#2 135
#3 136
#4 135
#5 133
#6 138
I've had the headers and airbox off and nothing is obstructing flow. I can't help but think that with #2 running so hot that this is my biggest clue. Would restrictions in the idle jet passage ways cause a lean enough mixture to run it hot and misfire?

[EMS]

In some cases a hot header pipe (that could also turn blue) can also indicate too rich of a mixture and/or a leaking or mistimed exhaust valve. What happens is, you get combustion of fuel mixture behind the exhaust valve in the header and that causes the excessive temperature. That could also be the cause for the noise in the exhaust.
Old BMW "Airheads" often have blue headers and while some diehard BMW riders insist this is a sign of beauty and proof that the bike is ridden hard - as intended to, it is often a result of running the bike with the enrichener on for too long.

[6Pipes]

Well, this might be a silly suggestion but what if you had a broken exhaust valve spring? This might still let the valve close properly during the compression check but not while the engine is running.

[oilheadron]

Hey Ross, get your bike over here somehow and we will diagnose this problem for you (I hope :) ), no charge.

[ajs350]

Hey Ross, get your bike over here somehow and we will diagnose this problem for you (I hope :) ), no charge.

I might well take you up on your offer if I make no headway. I'm going to take the carbs off to clean them again and replace the idle jets. I did not remove the air jets last time so maybe that's also worth a try. If I have no success I will plot a coarse from Raleigh to Athens...603 miles! It should not be a problem getting there as the bike runs fine when you are accelerating or cruising at say 60mph at 5000 rpm. Its just the spluttering when holding the revs at anything below 4000. Ross