Topic: '78 550 engine knocks at idle

[yozzie]

Hi there, I have a '78 550 and I have been having trouble with an engine knock at idle.  If I rev the engine slightly above idle it goes away.  Also, if I pull the clutch in the knock changes tune.  I have been reading a bit about problems with the primary chain and/or the clutch basket making knocking noises.  Otherwise the engine runs fine.  Any help would be appreciated.  Thanks

[Deltarider]

First of all: mistrust your tachometer. On later 550's I've seen ND's that showed 500 rpm more than the rpm I counted on my DMM. Check with a good electronic tachometer and set your idle  around a true 1100 rpm. I bet there will be less knocking.

[Gordon]

When the carbs are out of synch the cylinders are fighting each other, which causes the clutch basket to make a lot of noise at idle speeds. 

[yozzie]

I've heard that carb synch can make these engines noisy.  I don't think it's a connecting rod or anything else.  The idle does seem to hunt a bit as well.  Maybe it's time to pull the carbs and go through them.

[yozzie]

Had a chance to pull the carbs today and give them a cleaning.  Did a bench sync on them and wow, no more knocking!  Runs like a clock now.

[Gordon]

Had a chance to pull the carbs today and give them a cleaning.  Did a bench sync on them and wow, no more knocking!  Runs like a clock now.

Nice!

Now do a true carb synch with a manometer and you'll be even more amazed at how smoothly it runs.

[Deltarider]

Just check with a manometer. Maybe your benchsynching did the trick already. You wouldn't be the first one.

[1timduke]

First of all: mistrust your tachometer. On later 550's I've seen ND's that showed 500 rpm more than the rpm I counted on my DMM. Check with a good electronic tachometer and set your idle  around a true 1100 rpm. I bet there will be less knocking.

Is it possible to set the stock tach at the true reading, or does one need to just live with it reading 1500RPM at idle (but it's really about 1000).

Oh, does that mean I could get a little more before I actually redline!

-Tim

[Gordon]

Just check with a manometer. Maybe your benchsynching did the trick already. You wouldn't be the first one.

Well, obviously if you hook up the manometer and the cylinders are already reading equal then you wouldn't make any changes.

[Zaipai]

First of all: mistrust your tachometer. On later 550's I've seen ND's that showed 500 rpm more than the rpm I counted on my DMM. Check with a good electronic tachometer and set your idle  around a true 1100 rpm. I bet there will be less knocking.

Question, what is ND's and what is a DMM and where does one connect an electric Tach? Where would one even get one.? I know but I am a novice so please pardon my ignorance on this.

[Bill/BentON Racing]

Nippon Denso,makes your gauges,Digital multi meter,to spark plug wire,any auto parts store. Bill

[Zaipai]

Nippon Denso,makes your gauges,Digital multi meter,to spark plug wire,any auto parts store. Bill

Sweet, thanks for the info. I will have to look for that DMM cause I suspect mine is off..

Scott..

[Deltarider]

Well, obviously if you hook up the manometer and the cylinders are already reading equal then you wouldn't make any changes.

That's why I emphasized the word "check" in italics.

[Bobby Lee]

First of all: mistrust your tachometer. On later 550's I've seen ND's that showed 500 rpm more than the rpm I counted on my DMM. Check with a good electronic tachometer and set your idle  around a true 1100 rpm. I bet there will be less knocking.

Is it possible to set the stock tach at the true reading, or does one need to just live with it reading 1500RPM at idle (but it's really about 1000).

Oh, does that mean I could get a little more before I actually redline!

-Tim

I found this quite useful as well.  I have to set mine right around 1500 in order for it run at its smoothist.  Anything less than that, and it seems like its fighting to stay alive.

[yozzie]

just thought i'd follow up and post my results using a manometer.  Yup, the throttles were still a bit out.  Synched them up and it really runs nice now.  Was using a colortune plug to get the mixtures right but can't seem to get a nice blue flame on all four cylinders.  I am assuming that backing out (CCW) the mixture screw is leaning it out, but it seems to work the opposite on two carbs.  I am running a 4 into 1 exhaust with stock airbox.  Anybody else have this experience?

[BVCB650]

Well, obviously if you hook up the manometer and the cylinders are already reading equal then you wouldn't make any changes.

That's why I emphasized the word "check" in italics.

In actuality, bench syncing only gets it close. Sometimes very close but rarely right on.