Author Topic: Detonating or Alternator Rattle? (Read 4078 times)

736cc · « **on:** December 10, 2012, 12:55:15 PM »

My 1978 Supersport has a really noticable rattling that sounds like REALLY bad pinging while under load at low 1500- 3500 RPM. Now I'm thinking it sounds like its coming from the alternator but its really difficult to pinpoint. There are no spots of melting pistons on the plugs which is the 1st thing I checked, and this noise occurs when the bike is fully warmed up. Doesn't do it in nuetral, only under load at low rpms. Bike runs like a raped ape, no complaints there. At some point in its earlier life, there are various clues the motor was pulled and there's allen head screws holding the stator to the outer cover. Has an electronic ignition which I haven't messed with. Are alternators (or something else mechanical like the camchain) prone to rattling under low rpm load? My K2 is smooth as glass and quiet under same conditions (and doesn't pull nearly as hard at any rpm comparison).
Thanks in advance
736cc

TwoTired · « **Reply #1 on:** December 10, 2012, 01:16:34 PM »

Have you tried using higher octane gas?
What gas ARE you using?

Any compression test numbers?

If you retard timing does the ping/rattle go away? Have you checked the timing?

Care to tell us what spark plug numbers the bike has? Tune up history?

736cc · « **Reply #2 on:** December 10, 2012, 01:24:31 PM »

Always run premium, haven't checked compression, D8EA's new, haven't touched timing. If its pinging, the plugs would have spots, right? So what rattles?

TwoTired · « **Reply #3 on:** December 10, 2012, 02:41:29 PM »

If you want to know if it is pinging or mechanical rattles, then retard the timing, or at least check to see if it is advanced.

A possibility is that since you always run premium and the bike normally calls for hotter burning regular, there may be a deposit build up, which would up the compression ratio and build to where the even premium can't prevent pre-ignition. The fact that it occurs under load condition lends credence to the "ping" theory. If you have occasional ping, plug deposits, if new, may not show signs, particularly if the engine wasn't pinging the entire time the engine was running.

What DO the plug deposits look like? ...and under what throttle settings?

If you can't localize where the actual offending noise is coming from, you can do the basic maintenance checks or start taking things apart hunting for more definite clues.

A ping rattle is where the mixture spontaneously combusts from being compressed before the spark ignites it. This behaves as early timing would and starts pushing the piston down before the crank has reached TDC. In essence, some of the pistons may be starting to push the crank backwards. Depending how bad this detonation is, you can break or bend rods and damage bearing, etc.

Cheers,

bjbuchanan · « **Reply #4 on:** December 10, 2012, 05:11:15 PM »

Maybe its not the case but to me it seems like its time to be more involved. Or have your mechanic delve in to it

Periodic maintenance like valve lash and cam chain. Maybe run some seafoam to possibly clear any deposit build ups.

Plug gap and condition. Its tough to diagnose with little to work with. Im not ragging on you just pointing out what would help

camelman · « **Reply #5 on:** December 10, 2012, 05:36:51 PM »

I bought a CB750 a while back that had a loose bolt holding the alternator rotor in place. I expect the reason the bike was parked is because the starter gear was no longer held tight to the crankshaft. If you think you hear noise in that area, then it wouldn't hurt to check that bolt.

BobbyR · « **Reply #6 on:** December 10, 2012, 05:53:35 PM »

I think Lloyd gave you a good starting point to narrow this down. These are non invasive tests so rather than tear things apart you may find it is carbon build up. That can be fixed with some water, it also can be the reason it pulls so strong is your compression ratio could be way up.
What you are looking for on the plug is soot.

Just a thought do you have standard gearing?

754 · « **Reply #7 on:** December 10, 2012, 08:05:45 PM »

Doesitsound like more than one cylinder ? Doesitgo away when you back off ?..then it sounds like a rod.
Norubbing parts in the alternator...If the alt rotor is loose it willscore the taper, if its a rod and youdont treat it soon enough it will take out the crank

scottly · « **Reply #8 on:** December 10, 2012, 08:38:42 PM »

If the alternator rotor bolt is loose, the electric starter won't work. If it only happens under load, it's either pinging or rod-knock. In either case, the noise should be reduced by retarding the timing.

736cc · « **Reply #9 on:** December 11, 2012, 10:09:41 AM »

I added a couple ounces of Startron and went for long ride today. No pinging under load, so the culprit appears to be the lousy gas available nowadays. But I remember when it was hot and in traffic the detonation sounded like the worst rattling as if the pistons, cam chain and and valves were loose! But in true CB750 tradition, none the worse for wear (I hope

)

TwoTired · « **Reply #10 on:** December 11, 2012, 10:20:47 AM »

At what setting are your pilot screws? The exhaust change will make the stock carbs provide lean mixtures.

I really don't understand why you won't even check the timing. ...Or explain the spark plug deposits.

The rattling from detonation is like a death from 1000 cuts. Each "ping" hammers on the rod bearings, wrist pins, etc. Hammer on something long enough...

Good luck!

736cc · « **Reply #11 on:** December 11, 2012, 10:52:33 AM »

The plugs look perfect light tan w/ no aluminum deposits. I'm running 6 plates in the Supertrapp, which is quietest level. Bike runs so good I'm not changing the timing or air screw settings. Fuel additive Startron will be on its diet until further notice, 1 oz per tankful, which is about $1/oz. Bike has 24,000 miles. BTW, bike always gets a consistent 50+ MPG and it runs w/ the big dogs, this puppy pulls on top like no other CB750 I've ever owned..

andrewk · « **Reply #12 on:** December 11, 2012, 11:20:33 AM »

Low RPM ping can be cured with octane, but can be caused by weak advance springs. Octane cures the symptom, but not the disease.

Check your timing.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #13 on:** December 11, 2012, 12:09:09 PM »

"perfect light tan" "50+ MPG"

Nothing 'wrong' with those but you may be running lean. That much loud pinging just isn't right.

"and it runs w/ the big dogs, this puppy pulls on top like no other CB750 I've ever owned.." I believe you've had some early 750's too... What's inside the engine??

736cc · « **Reply #14 on:** December 11, 2012, 01:55:58 PM »

I'm gonna check the timing. But how will I know the advance springs are weak, its a Honda, since when do things like that go wrong? As for whats inside the engine, I haven't the foggiest idea. It doesn't have a lumpy idle but it really pulls HARD upstairs in top gear. They say late F's are faster, I won't argue. I did have a particularly hot sandcast that hit 125 on the speedo while flat-out w/ me crawling under the paint.

754 · « **Reply #15 on:** December 11, 2012, 04:40:00 PM »

Check valve clearances BEFORE setting, in case its cammed.. otherwise you would have to guess at the settings..

Jerry Rxman Griffin aka MuthaF'er · « **Reply #16 on:** December 11, 2012, 08:07:23 PM »

Nice bike. Did you see the link yesterday to the repro side AND tank badges? They look really nice. Marblesmotors.com He's a paint guy. I inquired about the pin spacing on the CB900F side badges for my restore/build 750F and he got right back to me.

I'd like to think those big valves are helping the breathing on top end. Once you put on an electronic ignition you won't have to worry about the springs.

seanbarney41 · « **Reply #17 on:** December 11, 2012, 08:44:49 PM »

Quote from: Jerry Rxman Griffin on December 11, 2012, 08:07:23 PM

. Once you put on an electronic ignition you won't have to worry about the springs.

The only ignition I have heard of that doesn't use the stock advancer is the Dyna 2000...Pamco, Hondaman, Dyna s all use the stock advancer. 736, you might consider that these engines are generally pretty noisy and all that noise is gonna possibly get amplified and reflected back into your face by that fairing. I would check your timing, set to spec, along with the rest of the 3k tune up (timing chain tension, valve clearance adjust, point gap and timing, carb sync) keep a close eye on those plugs for signs of detonation (be sure your checking as far down into the insulator as possible), and if things keep running as good as you say they are, not worry about it. Also, advance springs are a known weak point on a 35 year old engine. Why not replace them or mod them as per Hondaman's recomendation?

736cc · « **Reply #18 on:** December 11, 2012, 08:49:58 PM »

It has an aftermarket ignition. I'm having a spare gas tank painted right now, all badges and stripes removed for a "shaved" finish. Black, of course. And a Don Vesco seat i acquired will look right at home.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #19 on:** December 11, 2012, 11:51:44 PM »

Good point about the advancer springs. I recently installed the Dyna 2000 in place of my Dyna III which did have the advancer. And a great point about the fairing funneling the noise back to you. My Fat Boy scared the #$%* outta me when I first installed the removable fairing. I thought the top end was coming apart. Sounded like it wasn't getting proper lubrication. Less wind noise, more engine noise. I just saw a NOS advancer on EBay. It is ONLY $150 or so.....

andrewk · « **Reply #20 on:** December 12, 2012, 06:22:25 AM »

Quote

But how will I know the advance springs are weak, its a Honda, since when do things like that go wrong?

You'll want to check timing dynamically, with a timing light. Operate the throttle, and observe the operation of the advance unit. If the advance springs are real weak, the advance will start coming in at idle or just above idle, making it impossible to hit the f mark on the advance unit. Another test is to hold the throttle to a point where the advance is working, but should not yet be fully extended. The timing light will let you see if the timing drifts.

As to when the springs wear out, I don't really know, but I've witnessed the problem a lot, especially on bikes with 25k miles or more.

BobbyR · « **Reply #21 on:** December 12, 2012, 07:39:53 AM »

andrewk · « **Reply #22 on:** December 12, 2012, 02:08:52 PM »

I gotta add that I love that vesco fairing on that bike, and am interested to see it with the vesco seat too- I'm doing a similar "sport touring" build on an F2 right now.

736cc · « **Reply #23 on:** December 13, 2012, 04:53:51 AM »

Action Fours headers circa 1972 w/ a universal Supertrapp

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Author Topic: Detonating or Alternator Rattle? (Read 4078 times)