Topic: Intermittent Loss of Power

[Rosinante]

Twice now, in as many short rides, my bone-stock 1978 CB750 has suddenly started running poorly.  The first time, I stopped, put the fuel petcock on Reserve, and the bike still ran poorly.  I parked for 5-10 minutes, and then it ran fine.  Went straight back home.  Half-hour altogether.

Last Saturday it did the same thing.  Putting the petcock on reserve (this time the gas tank was nearly full) seemed to have no effect.  Parked for a while.  Managed to get it started and headed for home.  Felt like it was running on two cylinders instead of four.  Had enough power to get up hills at 50 mph, but not much more power than that.  Then, halfway home, BOOM.  It started running properly again.

I am suspicious of ignition.  The tank is off, and I will be looking for corroded connections, perhaps at the coils.  My plan is to put a spark plug in my pocket and insulated pliers, and go for a drive close to home.  If/when it acts up again, use the spark plug to verify there is a spark from each coil.

I wonder if a condenser could be giving intermittent trouble.  Any advice/suggestions would be appreciated.

[cgrayiii]

I just posted my results to a similar issue.  I changed fuel filters, fuel line, plugs, wires, and air filter.  Only this past week did I figure out that the CDI units were all but melted and would function perfectly until they got hot and then the same symptoms you're describing - runs great for about twenty miles or so and then feels like it goes to two cylinders.  Let it sit for ten minutes or so and she ran fine again.  I looked at the CDI spark units and the black resin was melted from the back of both of them.  I bought a couple of replacements and it appears the problem is solved.  I hope this helps you.

[TwoTired]

Has the bike ever had a routine 3000 mile tune-up?

Also, sediment in the bowls may be stopping up the pilot jets, an then falling out when carb suction is stopped.

At least LOOK at the spark plug tips for evidence of fouling.

Air filter old?

[seanbarney41]

My first guess would be a marginal ignition coil failing when it gets hot.  Try testing it cold, then blast it with a hair dryer or heat gun and see what happens.  Also, previous posts are good guesses too.  Wait a minute, is a 78 electronic ignition or points?

[Rosinante]

Thanks, guys.  All data points are helpful.

The carbs were cleaned last summer, new accelerator pump diaphragm, etc.  I've been running without a filter (bad, I know.....a new filter is in my car right now).  So....I don't think it's a fuel problem.

I don't think I changed points or condensers last year, so perhaps I will do that.  I've never done anything with the coils, and the metal parts sticking out of them look very rusty.  The only screws I can see easily appear to be simply holding a metal bracket that holds a double-insulated wire.  I need to look more carefully, to find (and detach and clean) any and all electrical connections.  Corroded electrical connections create plenty of resistance.  Beyond that (connections, points and condensers), I would have to conclude a bad coil.  Perhaps I will take my timing light on my next ride...that should show which cylinders are firing and which are not.

[bollingball]

You are not running a filter and you don't think it is a fuel issue I don't know if I changed the points or condensers So maybe I will do that If you keep this maintenance program up good luck Why not try the 3000 mile tune up with out leaving ANY Thing out

[bollingball]

I'm with you TT not sure how that ignore thing works Does it block out all his post?

[Rosinante]

You are not running a filter and you don't think it is a fuel issue I don't know if I changed the points or condensers So maybe I will do that If you keep this maintenance program up good luck Why not try the 3000 mile tune up with out leaving ANY Thing out

You're right.  A fuel filter should be installed, which is why I mentioned there is one on the seat of my car as we speak.  I ran perhaps 4 tanks of gas through the carbs....gasp....unfiltered.  As far as whether my points are 3000 miles old......I know for a fact they are not.  What I said was that I don't remember whether I replaced them last summer.  And.....I've stated that I will be changing those and the condensers.

[Rosinante]

I'm with you TT not sure how that ignore thing works Does it block out all his post?

What's up with this? 

TT?  What's up?

[Rosinante]

Also, sediment in the bowls may be stopping up the pilot jets, an then falling out when carb suction is stopped.

I don't think so.  This problem is similar to others' experiences, as uncovered using the Search button.  Engine goes from 2 cylinders to 4 cylinders, and back again, while highway cruising.  Also, it's not simply a rough-running condition.  Even when the problem is occurring, the engine sounds "regular."  As if two cylinders are dropped.  Judging by what I've learned today, I'd say it's coil, condenser or electrical contact.  Those are my best guesses.

[cameron]

did you do that thing with the pliers and the spark plug?

[Rosinante]

did you do that thing with the pliers and the spark plug?

No, I didn't.  That'll have to wait until I have time for another ride.  And.....as I mentioned, I think I'll use my timing light.  The plier and spare spark plug method is suboptimal for at least two reasons.  First, it can result in my hand conducting tens of thousands of volts.  BTDT.  Also, a spark under the pressure of a cylinder is a very different thing from a spark at one atmosphere.  I've seen ignition systems that can produce a spark outside the cylinder, but not inside.  Due to the added pressure.  At 9:1, that's nine atmospheres.  130-140 psi, or so.  Weak sparks won't jump that gap, and a timing light will see that.

Also, I verified that I did in fact change points and condensers last summer.  They have about a thousand miles on them.

I still want to know what the Ignore comment was about.  Is this what we do with guys who ask questions here?

TT?  Bollingball?

[Duke McDukiedook]

How does the inside of your tank look?

[TwoTired]

I use the "ignore" feature when I no longer wish to read or even consider what a member says.  You'll see the button on the left side under the avatar.
Others, just ignore what people offer in print after reading.

You already rejected all my input, preferring your own theories and comments that support them.  So, I just assumed you'd rather fix it on your own.

Cheers,

[Rosinante]

How does the inside of your tank look?

It's really clean. 

[Rosinante]

I use the "ignore" feature when I no longer wish to read or even consider what a member says.  You'll see the button on the left side under the avatar.
Others, just ignore what people offer in print after reading.

You already rejected all my input, preferring your own theories and comments that support them.  So, I just assumed you'd rather fix it on your own.

Cheers,

What makes you think I have rejected your suggestions?  You mentioned a tune-up, and I have now looked into that.  My records show that new points and condensers went in last summer, less than 1000 miles ago.  The air filter went in at that time also.  Sediment in the float bowls is something I have not looked at since the carbs were disassembled and rebuilt, also last summer.  Might be worth a look.  Same with the spark plugs.  Less than 1000 miles.  So yeah, a tune-up is a good starting position.  I was unaware yesterday what had been done, until I got home last night.  Again, I’m just wondering why you thought I had rejected your suggestions.

I know what the Ignore button is for.  What I don’t know is who uses it and for what purposes.  I have nearly 16,000 posts on a particular performance car technical forum and I have never, ever used the Ignore button.  In my humble opinion, it’s childish.  Technical discussions are dialogues, or at least they should be.  I value your suggestions.  And there is a chance I will at times have a perspective that is different from yours.  I won’t share it with someone else in a PM.  I will say it right in a thread because that is “dialogue.”  Perhaps that is what your Ignore button is for?  I say this with all due respect.  I have valued your comments in other threads as well, not just this one.  I hope dialogue is your game.

Back to the intermittent problem.  Sure, it could be fuel or ignition.  Perhaps I will have a look at the float bowls.  But based on my search of other threads, and comments in this thread, my best guess is that one of the coils is not firing.

[TwoTired]

If a coil is not firing, the head pipe temps will be cold; 2/3 or 1/4.  If you still have the 4 into 4s you can feel the exhaust to find dead cylinders.

IMO, you still need to look at the spark plug tips.  The deposits are the best indicator for combustion conditions.  If you have a spare set of plugs, swap them and see if the trouble persists.  If so, you eliminated a possibility.  If not, new plugs are an easy fix.

1000 miles ago the engine ran well.  Now it doesn't.  To me, that means all the tune up items need to be addressed, as these items can easily make the engine run poorly.  You don't have to replace anything unless a check shows it needs to be done. Timing can change/slip, and points can burn if the ignition is left on without the bike running.

The tank should have a fuel filter above the fuel tap.  If your tank is clean, it's either well preserved or it has been cleaned.  If cleaned, by who, and did the retain the stock filter intact.  Clearly, clean carbs fed dirty fuel won't remain clean very long.  And, the pilot jets only need a .016" sized peice of sediment to block them.

If the paper air filter got wet, the fibers collapse, and this chokes the engine.  The plugs can get soot deposits, and short out.  One reason to look at the plugs.  White plugs are an indication of fuel starvation.  A cold engine needs fuel enrichment, a hot engine doesn't.

I was just trying to gather information to help you.  If you don't share information we've asked for, it looks as though you are just taking discussion with those in agreement with your a theories.

Best of luck!

[scottly]

This problem is similar to others' experiences, as uncovered using the Search button.  Engine goes from 2 cylinders to 4 cylinders, and back again, while highway cruising.  Also, it's not simply a rough-running condition.  Even when the problem is occurring, the engine sounds "regular."  As if two cylinders are dropped.  Judging by what I've learned today, I'd say it's coil, condenser or electrical contact.  Those are my best guesses.

I would guess condenser, points, spark plug caps, coil, in that order. Plugged carbs don't come and go, especially in pairs...

[camelman]

I would address condensers and possibly points.  The other option is a loose connection to your condensers or points.  A distant option is HT leads from your coils rubbing through the insulation to ground on the frame.  It probably wouldn't hurt to trim off a quarter inch of the plug wires and re-screw the boots back on while you are at it.

I've seen a couple of new condensers fail immediately, or almost immediately.  Don't count on those new condensers being in perfect shape.  Also, since you mentioned that you changed them last year, that would be the perfect amount of time for some oxidation to set in if you didn't get super tight connections (mentioned in the previous paragraph).  Try pulling all the connections apart and applying dielectric grease before reassembling.  If that doesn't work, then swap in new condensers.

Camelman

[Rosinante]

Thanks TT, and others.  This is all good information.  I regret leaving an impression that I am dismissive of input.  I appreciate it all.  In my view, some potential culprits are more likely than others of course.  Particularly in view of the fact that the bike runs good most of the time and then, like a switch is tripped, runs very poorly but somewhat evenly (on two cylinders, I think), then back to good running again like the switch was returned. 

I can see now that my timing light idea is unnecessary.  I started this thread hoping to learn, and succeeded.  It is helpful for example to learn that when a cylinder pumps cold air into an exhaust pipe, that pipe is likely to cool off fairly quickly.  Makes sense.

When I get time, I will be looking at points first, then various other things.  Thanks again.  And thanks, TT, for letting me get away with my value remarks by posting additional helpful suggestions.  They, and the mature dialogue, are appreciated.