Topic: Won't Idle, dies sooner and sooner

[Harper]

Hi everybody,

Yesterday I put on my rebuilt carburetors and recharged my battery. I was preparing to tune the motorcycle, and at first the bike would start right up with the electric starter. It would run poorly and the idle was set too low, but as long as I gave it throttle, it would run. However, this morning when I tried to start it, the engine would catch and roar, but die within two seconds, no matter how much throttle I gave it. After doing this a few times it stopped catching altogether. Now it responds to neither the electric starter nor kickstarter. I think this might be related to a severe oil over-filling a few days ago. My dad put in twice as much oil as necessary, and I ran the motorcycle twice for less than ten seconds each time with the excess oil still in the tank before I checked the level and drained it.

Is there anything that could cause this gradual dying? Any diagnostics I could do?


Thanks,

Harper

[Gordon]

First of all, what bike?

Check the battery again.  What you're describing sounds very much like a dead battery.  Just because it initially took a charge doesn't mean it can hold it.  Doesn't sound at all like an oil overfill issue.   

[Nikkisixx]

Check the battery again.  What you're describing sounds very much like a dead battery.  Just because it initially took a charge doesn't mean it can hold it.  Doesn't sound at all like an oil overfill issue.   

+1

[afastman]

I am having a similar issue.  I am leaning towards gunk in the tank that i missed.  $3 filter time...

[Harper]

It's a 1975 CB 750

Gordon: thanks, I'm glad to hear that its probably not a oil issue. Is there a way to test the battery? All lights and the starter motor work well.

afastman: I think that my fuel intake is fine, I just cleaned the petcock. Also, there's fuel in the float bowls. (Or is there another place where fuel flow could be restricted? I really new at all this).


Thanks

Harper

[afastman]

new myself, evidently, little teeny tiny flakes of rust, goop, etc lodge in the jets.  those little tubes on the way from the bowls to the motor.  they are small and it does not take much.  Also, check your plugs.  if the are fouled, they will need changing as well.

[shorterdanny]

this is spooky , sounds very similar to my problem, i have just clean my carbs out and now, fires and runs rough for a bit, then dies,

diagonsis. mine only running now on cylinders 1 and 2, 3 and 4 exhausts warm up a little but certainly not firing correctly, tomorrow need to check the carb boot seals, and plugs wondering if i might be losing my vacuum, we'll see tomorrow.

but watching this tread for ideas to!!

[Nikkisixx]

I have had this issue and found it to be the battery/electrical.  After the bike has been out of commission for a week or more, the battery looses charge.  While trying to start the bike and tune it, the battery takes even more of a beating.  This is because the starter really drains it and the stator never hits the charging threshold of 3000 rpms for any sustained period of time.  The 30 year old electrical system has a lot of current leaks and resistance points that add to the problem.  So by the time the weak current gets to the coils the voltage has dropped significantly.  So the bike stutters, runs for a bit then stops, fouls plugs, and generally misbehaves. 

A multimeter is the best way to diagnose the problem by checking voltage at the coil's bullet connectors and of course at the battery.  Harbor Freight sells an inexpensive tester for $3 http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=90899I have one and it works as well as my $20 tester.  A trickle charger or battery tender is the fix - provided the battery is good.  A battery tender is nice to have if you ride infrequently as well.

[Harper]

afastman: I think my jets are clean, or at least they should be. I just replaced or cleaned all of them. Ditto plugs.

Shorterdanny: I have a 4 into 1 exhaust, so its hard to judge which are warming up. But about the vacuum: I suspect a bad connection between my airbox and the carbs. The rubber was hard, and they are not completely seated. Could this cause a loss of vacuum? Is there a way to check? I just dont think that a leak would result in the progressively worsening behavior

Nikkisix: thanks for the information, this makes sense for my symptoms. I just fully charged the battery with a trickle charger last night. Also, the starter motor still sounds strong. Would the battery be able to turn the starter motor and not be able to run the engine? IS there a way to check if it is bad?


thanks,
Harper

[Gordon]

A 4-1 exhaust still has 4 headers.  Touch them as the bike is warming up or after it has died to see if there is a significant temperature difference between them. 

Yes, a battery can have enough juice in it to start the motor but not enough left to keep it running for very long.  The ignition system and any running lights use more current than the charging system can make up for at idle speeds, so it will continue to drain the battery until it dies. 

[shorterdanny]

Harper i have a 4 into 1 to.. you can feel the exhaust at the headers this will tell you if they are all firing, all should be dam hot, mine there is a prominant difference between 1/2 and 3/4 so obviously something wrong, as for the vacuum, a leak after the carbs is prob worse than before the carbs, as this will some the fuel being sucked in correctly if at all, where as before the carb would prob just make it run lean and rough. although this is jsut my limited knowledge thinking, the way i check is to spray the whole area with wd40, then watch for bubbles coming out.........Scientific i know. will let you know what i find, might be able to help each other out.

as for the electrics could def be a problem if you run you battery down, but if you have enough current to turn over the motor then you should have enough to run the engine, i have a conditioner connected to my battery so i know it is not an issue for me.

fun fun fun

[Gordon]

as for the electrics could def be a problem if you run you battery down, but if you have enough current to turn over the motor then you should have enough to run the engine,

This is true only if the battery is in good condition and is holding a full charge. 

[Nikkisixx]

But about the vacuum: I suspect a bad connection between my airbox and the carbs. The rubber was hard, and they are not completely seated. Could this cause a loss of vacuum?

A leak on that side of the carb will not cause the problems you mention.  It would just let unfiltered air into the system and perhaps cause some turbulence that will have an imperceptible effect on performance

Nikkisix: thanks for the information, this makes sense for my symptoms. I just fully charged the battery with a trickle charger last night. Also, the starter motor still sounds strong. Would the battery be able to turn the starter motor and not be able to run the engine? IS there a way to check if it is bad?

You may want to pull a sparkplug and test the spark.  Here is how (not trying to insult if you already know this) pull a plug, reconect it to the wire, touch the trheaded part to the engine, hit the starter, check for a bright blue sprk.  It helps if this is done in the evening, or in a garage.  Hard to see in bright sunlight.

[Harper]

Gordon: right, the headers. I'm Dumb. But, The engine stays on for such a short period of time that nothing gets hot (or warm). I just have a second or so at a time.

Update: I dont think that it is a battery problem. I just went out and gave it a try after letting it sit for a couple hours, and I got a burst of engine action (using the kickstarter). Previously I had had no life. It seems to me that if the battery is drained to the point that it can't operate the motor, a few hours rest will not recharge it. Is this true?
And just as a description, in case this means anything to anyone: the engine almost started with the electric starter, and then started completely with the kickstarter. It revved high at first and then backfired and quit. Total duration was less than two seconds. After this, the electric starter would produce nothing. The kickstarter would not catch the engine either, but there was a strange spinning  sound (higher pitched, faster, lighter than the engine turning over) that would sound after each time i tried it.

Nikkisix: thanks, I'll try the test. (And Thank You! for the explanation. I need all the information i can get, never am insulted by instruction)

Any Ideas?

Thanks,

Harper

[Gordon]

Update: I dont think that it is a battery problem. I just went out and gave it a try after letting it sit for a couple hours, and I got a burst of engine action (using the kickstarter). Previously I had had no life. It seems to me that if the battery is drained to the point that it can't operate the motor, a few hours rest will not recharge it. Is this true?

And just as a description, in case this means anything to anyone: the engine almost started with the electric starter, and then started completely with the kickstarter. It revved high at first and then backfired and quit. Total duration was less than two seconds. After this, the electric starter would produce nothing. The kickstarter would not catch the engine either, but there was a strange spinning  sound (higher pitched, faster, lighter than the engine turning over) that would sound after each time i tried it.

Everything you're describing sounds exactly like the symptoms of a bad battery.  Even the part about it getting back some life after sitting for a few hours.  You need to be able to check the battery voltage in order to accurately diagnose this.

[Harper]

Gordon: Great, I'd much rather it be a bad battery than some mechanical problem. I'll try to check the voltage (I'm bad with the multimeter). What voltage reading do I want to get?

Thanks

Harper

[Sporkfly]

Check your fuel levels in the float bowls using the clear tubing method? It sounds like a fuel problem to me.

[Harper]

Sporkfly: What's the clear tubing method?

Update: I got the engine to start a number of times, but there was no consistent pattern. Three times in a row I could start it and keep it running as long as the choke was engaged. As soon as I disengaged it the engine would die, no matter how much throttle I gave it. Then, it started (with the choke) but was searching. It would die down to 800 or 700 RPM, almost go out, and then catch up with the throttle I was applying and jump to 3 or 4 K. Most recently, I could only get it to catch, roar once and then go out.

It seems to me that beyond the possible battery problem, the engine could be running lean (because it liked the richened choke mixture but would die without it).

Any Ideas?

Thanks

Harper

[Nikkisixx]

One lead on the + one on the - set on DCV 20 (ish) you should get 12 volts or so.  This bike has been sitting for a week with no charger, it typicaly shows 12.3.

[Nikkisixx]

Sporkfly: What's the clear tubing method?

Here's the thread
http://forums.sohc4.net/index.php?topic=1404.msg9884#msg9884

Basicaly, you put clear tubing on the drain tube at the bottom of the bowl, open the screw to drain the bowl, hold the tube up agianst the bowl, and see where the level is.  If I were you, I'd make very certain you have good spark first. Then move to carbs.  Here's why - the bike will run with a bad air fuel mix.  You've got four carbs, so you have a good chance that one or two cylinders will be ok.  It will run poorly, but it will run.  Only if the fuel stops at the petcock (and it doesn't?) would you get the problem you describe under the conditions you've explained.

edit: link

[Harper]

Nikkisix: Awesome information! thanks for the link and picture.
Your reasoning makes a lot of sense to me, I will check for spark tomorrow
The fuel does not stop at the petcock.

About spark plugs: The PO had had NGK D7EA in, the manual suggests D8ES, an auto part store computer said that "stock for 1975 CB 750 K" is D8EA, and I now have D8EA in. Could this affect the way the machine is running? What do the spark plug code numbers indicate about its performance (are higher numbers hotter)?

Thanks


Harper

[Nikkisixx]

Hotter/Colder plugs eh?  Dang man, what next?  The meaning of life?  42. 

NGK has tons of good stuff on their site, worth the time to read.  For a quick answer, hotter plugs will make the engine run hotter, colder plugs will make it run colder.  "The spark plug heat range has no relationship to the electrical energy transferred through the spark plug. The heat range of a spark plug is the range in which the plug works well thermally.  The heat rating of each NGK spark plug is indicated by a number; lower numbers indicate a hotter type, higher numbers indicate a colder type." 

http://www.ngksparkplugs.com/tech_support/spark_plugs/p2.asp?mode=nml

http://www.ngksparkplugs.com/tech_support/spark_plugs/installation/index.asp?mode=nml#heat