Topic: '78 CB550 Weird engine rattle when i let off the gas (only at 20+ mph)

[jcorrill]

Hey guys, i know i have been blowing up the forum lately, but it is only because i just recently found out about it - and everyone on here has so much knowledge about vintage bike.  I am new to the the vintage bike so everything bit is helpful.

REcently i have noticed a strange rattle when i let off the gas.  It only occurs when i am moving at 20+ mph or so.  It sounds like it is coming from the engine.  Are there any common issues that '78 CB550s are known for having or does this seem to be the type of issue where ill have to pull it apart and search for the culprit. Any input will help.  Thanks

Jake

[flybox1]

might be a little soon to jump INTO the engine.  lots of other things you can check.
check all the fasteners you can find to see if something is loose and rattling...like the chain guard or a cover.
is one of your cables rattling against the headlight?
i'd also check for a correctly adjusted cam chain
could it be chain slap?  is your axle properly adjusted for your drive chain?

[jcorrill]

I will definitely check out those issues.  Could it also possible be the timing chain?

How do i figure out what the proper adjustments/setting are for my axel in relation to my drive chain?

Thanks for the speedy reply.


Jake

[calj737]

Two very common causes of rattling in these engines are: carbs not synch'ed and the cam chain out of adjustment. Can you attest that these are both properly adjusted?

[2wheels]

Had a similar problem with my 78 750.  Kind of a scary sound when down shifting.
I thought maybe the cam chain?
But it turned out to be the gas tank rubber mounts.  I put soft rubber on the  rear tank mounts.  Problem solved.

Point is to check the easy stuff first.

[78 k550]

I had a rattle i could not figure out. It ended up being the license plate.

Paul

[jcorrill]

Calj - i can not attest that either of those are done.  In fact, i would bet that they both need done.  How in depth of a procedure are they?  Do i have to pull everything apart, is a bit more simple than that?  Thanks for the insight.

The reason i do not think that it is an easy fix is because it only occurs when i let off the gas and i am mocing at 15+ mph.  It does not happen when i rev the engine, or manually shake the bike.  It also has a couple other rattling noises in which i have already diagnosed, such as the ones in which you guys have presented. 

[calj737]

A carb synch should be done using a vacuum gauge. Either a buddy or a shop...

Cam chain adjustment you can do, but follow the procedure per the manual. This is a very important adjustment to have checked as improper adjustment can cause catastrophic engine failure.

[jcorrill]

The more I listen, the more I think it's chain slapping, which like you said seems like real bad things can happen lol. Thanks again

[jcorrill]

Cam chain tensioner adjustment: so ive read a couple different ways to do this.. Does the tappet cover screws and spark plugs need to come out in order to do the adjustment?  Or can i just adjust it with the screw on the back of the motor?  Can anyone give me the procedure that they have used successfully?

Thanks,

Jake

[LokisTyro]

http://manuals.sohc4.net/CB500/HSM500550_1.pdf

The procedure for adjustment is in there. It only mentions removing the tappet cover for the No. 1 cylinder. As far as tips go, I don't even own one yet 

If you don't have the service manuals you can find them... http://www.sohc4.net/cb550books/ ...there. Hope this helps

[trueblue]

The 550's like to wear out the primary chain and chew into the case, you can check this out by dropping the oil pan.  Not hard to drop the pan and check.  This is the sort of damage that occurs when the chain starts to slap.

Images courtesy of Dave500.

[jcorrill]

So if i drop the oil pan, and that type of damage has occurred - what is the next step for me to take.  I EVENTUALLY plan on doing either a 650 swap or using a 650 top end (cam, chain, pistons) - however, i do not have the 650 ready to put in yet, and i have been itching to ride.  Ideally i can get it running good, as is - then do the swap once i have my 650 beefed out a little bit. 

[calj737]

If that damage has occurred, the only remedy is to replace the primary chain, which involves a complete teardown of the motor.

[trueblue]

If the engine is otherwise in good condition, you can simply flip it over, split the cases, pull the crank out and replace the chain.  Reuse the bearings, they'll be fine.  The only thing I would do is open the bearing shells out a little so they have a little more crush in the rods and main caps.  Leave the pistons in the cylinders and the cylinders attached to the top case.