[pjkaaihue]

I have a 1970 CL350 with an 1972 SL350 engine that FINALLY started running after a complete rebuild. I tuned it the best I could following the rebuild (timing chain, valve adjustment, points, and carbs). It idles great once warm. All electronics are new, except the battery which is a year old but holds 12v just fine for days.

The problem I have been having is vibration. At idle, it's smooth, but above ~5k rpm, the footpegs are painful to have your feet on and the mirrors are a complete blur. I've tried to do the recommended stuff for motor vibration- All 5 motor mount bolts are tight, carbs are synced as far as I can tell.

SO, do SL350 engines inherently vibrate more? I know I should expect some vibration from an older bike, especially when it hits harmonic tones, but this seems excessive.
and would the engine go "out of tune" after being ridden 40 miles, if it was tuned before it ran? in other words, should I re-tune it?


My suspicions are:
1. The aftermarket CB exhaust sucks and is somehow contributing. I suspect this because the passenger footpegs (which have brackets holding the exhaust) are even worse than the driver pegs.

2. After engine has been slightly broken in, it needs to be tuned again.

3. SL engines are just extra shaky.

Is there some other balancing mechanism that I misaligned? Motor mounts need to be loosened then retightened? Can they be too tight?
Any advice would be great as I am stumped on this..


Other facts:
15:38T sprockets
Compression when cold: 130, 120
New points
New coils
New plugs
New wiring harness

 

[doode]

What was replaced/done for the rebuild?

My 350 vibrates too... it's part of the charm.

 

[shadetree]

I think I recall reading somewhere that if there is too much end-play side to side on the camshaft it can cause similar problems. Good luck.

 

[Bill H]

The 350 I had in the late 60s would put your butt asleep. I know this doesnt help but I think its the nature of the beast.Does it smooth out above 5000 rpm or get worse ?

Bill

 

[pjkaaihue]

What was replaced/done for the rebuild?

My 350 vibrates too... it's part of the charm.

The engine was completely disassembled, soaked in cleaner, cylinders reboared, valves re-cut. New piston rings, headers, points, points advancer. That's all I can remember having done with the engine, as much more was done with the rest of the bike.

The 350 I had in the late 60s would put your butt asleep. I know this doesnt help but I think its the nature of the beast.Does it smooth out above 5000 rpm or get worse ?

Bill

It definitely gets rougher the higher the rpm

 

[TexasMark]

That does not look like a 72 SL350 engine as they did not have electric start. Engine serial number will tell you what you got.
HONDA350K sl 350 honda SL350

 

[pjkaaihue]

That does not look like a 72 SL350 engine as they did not have electric start. Engine serial number will tell you what you got.
HONDA350K sl 350 honda SL350

You're right, it's actually a 69 but it's been retrofitted with parts from other models, like the side cover and electric start. The majority of the engine is from an SL

 

[pjkaaihue]

Update:

I did a tune up today- cam shaft chain, valve adjustment and timing. The timing had actually moved about a quarter inch early on the crank in just ~40 miles after running again. While doing so, I saw the right spark plug looked a little wet (gas), so the carbs have definitely been out of sync. After trying to syncing them again, the bike was running more like an old bike and less like it was going to rattle into pieces! That's the good news.

I had to re-sync the carbs several times after that. I'm finding that the non-stock handlebars are slightly wider, causing more tension on the throttle cable and it's messing with the tension of the wires going to each carb.

In the next couple days I will probably cut the ends of the handlebars so that the throttle cable wont be so tight. I will keep ya'll posted.

I also saw that the right muffler shakes a lot more than the left. I assume its because of the same carb issue, but if it isn't, what causes muffler shake/rattle?

Let me know if you guys have any other advice

 

[Yendor]

I don't know all that you did to the bike.
But I suspect that most of you vibration is from one cylinder cutting in and out at the higher RPM.

Lots of things can contribute to this.

Points Ignition can be tricky if you are not experienced in working with them.

The condition of the condensers may contribute to what you thought was the timing moving.
They suppress the arcing that occurs when contacts open & close.
Points open and close at 1/2 RPM
(IE: at 5000 RPM they open and close 2500 times per minute).

Consider this Touch a HOT wire to Ground and you see a tiny spark.

That is what your points are doing.

Now think ARC Welder.
When they Open and Close tiny amounts of metal from the ARC get removed and deposited from one contact to the other.
Tiny little mountains and valleys get created on the surface of the contacts.
As this occurs the GAP get changed when the gap changes the timing changes.

The Condensers purpose is to suppress this Arcing.
If you are running on 40 year old points and condensers it may be as simple as replacing them.

That is the reason that many choose to convert to aftermarket ELECTRONIC Ignition Systems.
They use a Magnet to pass by a PICKUP coil.
The Energy generated by that action is passed to a Transistor, (which is nothing more than an electronic switch) that serves the same purpose as the points but with NO moving parts or contacts.
This is a highly simplified explanation but accurate.

Also Old Points have weak springs and they BOUNCE at higher RPM.
They may not actually CLOSE long enough to fully charge your coils.

If you are running original coils did you remove the Spark Plug Caps (they screw onto the ends of the wires) and cut back approx. 1/4" or so to get to fresh wire?

Have you tested the Caps themselves ? Do this while you have them off to cut back the wires.
The have a built in Resister (to suppress Radio Interference instead of Resister PLUGS)
They should test at 5K.
Significantly - More or Less replace them.

 

[pjkaaihue]

Update:

The tuning was still slightly off, and right side carb was running rich from a high float.

Thanks Yendor for the response. The right cylinder fires intermittently at higher RPM.

As far as the rebuild, the points are new, as well as the ignition coils, wires, caps and spark plugs. The caps tested at 4900 Ohms and 4600 Ohms, which seems good.

If the condenser failed, wouldn't it affect both cylinders? I am only having issues with the right cylinder.

It will idle pretty good now, but as I rev it, you can hear the right cylinder fire off and on. You can especially notice while riding and feeling the random its of power. Compression is decent (120 on left cylinder cold, same when wet. On right 100 psi cold, and about 102 psi wet). Carb may be a little rich, but tuned the same as the left side which runs fine. I opened the ignition points cover today and noted that the left set of points look slightly browned (maybe slight carbon buildup or something), whereas the right set of points looked very clean. Does this help with determining the origin of this problem?

Something else I noticed was that while the bike was off (and battery was slightly drained), the left points were getting 10.2V and the right points 9.5V. Is this voltage drop significant enough to cause worry? If so, I can test it with the bike on to see if the right side points are close to 12V. It does seem odd that they would get different readings, since the hot wire goes from the same source, through each respective coil, then to the points/condenser..

I am open to any suggestion... at this point, I will probably start switching parts from the left half of the bike to the right half until something seems to work.

 

[pjkaaihue]

Update:

I decided to try swapping things on either side of the bike to see if it would make a difference. So I plugged the left ignition wire into the right spark plug and vice versa, then switched the leads going to the points. When I kicked it over, the right cylinder was running good and the left was not so great! So either the ignition coil, wire, or cap is bad!

Huge relief, as I was starting to consider having to redo the valves again or something.

I just noticed what looks like a pinch in the bad ignition wire just outside the ignition coil. Maybe I'll just cut that part shorter.

I'll post some updates as things get better.

 

[outobie]

Minimum spec for compression is 150 psi
a healthy motor will have a tad over 170 psi

are you measuring compression properly with the carbs off or at least fully open?

if those compression numbers are accurate no amount of tuning will make it run properly.

in your notes you say the cylinders were bored and new rings installed. If you bore the cylinders you need to replace the pistons with the next larger size and accurately bore for that size with proper clearances

 

[pjkaaihue]

Minimum spec for compression is 150 psi
a healthy motor will have a tad over 170 psi

are you measuring compression properly with the carbs off or at least fully open?

if those compression numbers are accurate no amount of tuning will make it run properly.

in your notes you say the cylinders were bored and new rings installed. If you bore the cylinders you need to replace the pistons with the next larger size and accurately bore for that size with proper clearances

Hey thanks for the response! Yeah, the compression was tested with throttle completely open. And the cylinder boring was done professionally through the previous owner, so I assume it got new pistons as well as rings. I agree though, the compression does seem a bit low.

However, after seeing the condenser and how old it looked, I actually just switched the leads going into it (matched the blue with the yellow wire and vice versa) to see if it would make a difference and now the bike is running amazingly! It's not misfiring on either cylinder, there much less vibration overall and it's just like a new bike. Needless to say, the old condenser is in need of replacing.

So good to finally feel some power coming out of this completely restored bike! Thanks all for the input

As far as low compression goes, are those numbers for a cold engine, or hot?

And what would low compression affect mostly? Like high rpm power?

And because I didn't see any significant difference doing a wet compression test, and valve clearances are spot on, should I assume the valves need to be reseated?

 

[pjkaaihue]

FIXED

So I was having misfires on at least one cylinder and the bike wouldn't rev above about 5,500 rpm. These were exact symptoms of a bad ignition condenser, which I replaced for $13 or so and the bike now runs like new!! I should have known from the beginning because the condenser looked original to the bike.

Bike does still vibrate, but it's more of an "old bike" vibration.

Next upgrade will probably be an electronic ignition!

Thanks ya'll

 

[pjkaaihue]

Another update:

It's been a month since the ignition condenser was replaced, bike has been running well ever since.

Yesterday, I changed out the sprockets from a 15T front and 38T rear (probably the lowest gearing possible for this bike) to 17T front and 34T rear (one of the lowest possible). After doing that, the bike has 10x less vibration! From what I am able to gather, certain sprocket combinations can cause interference with each other, where the chain links may be hitting the sprocket in a less-than-ideal spot (like closer to a point on the sprocket) which causes vibration. After changing them, there is only slight vibration at certain resonating rpm's! Something to keep in mind.

 

[outobie]

Gearing is always a compromise... shorter gearing gives better acceleration taller gearing reduces rpms at speed

for reference you went from a lower than stock ratio to a much taller than stock ratio

stock cl350 is 16 38
stock cb350 is 16 36

generally speaking street bikes are geared so they max out around 80% to 90% of redline in top gear

with your gearing your essentially making 5th overdrive

how many rpms can you get on a flat road wide open in 5th?
7,000k?

can you pull all the way to redline in 4th?

 

[ancientdad]

Another update:

It's been a month since the ignition condenser was replaced, bike has been running well ever since.

Yesterday, I changed out the sprockets from a 15T front and 38T rear (probably the lowest gearing possible for this bike) to 17T front and 34T rear (one of the lowest possible). After doing that, the bike has 10x less vibration! From what I am able to gather, certain sprocket combinations can cause interference with each other, where the chain links may be hitting the sprocket in a less-than-ideal spot (like closer to a point on the sprocket) which causes vibration. After changing them, there is only slight vibration at certain resonating rpm's! Something to keep in mind.

I think, more than anything else, that you simply moved the vibration rev range down in the bike speed range. Your change, as outobie described clearly, is a big one - changing 2 teeth on the front sprocket would probably have been enough to accomplish a vibration "move". reducing the rear sprocket 4 teeth as well is almost like going up an additional tooth on the front. If your goal is vibration reduction and performance isn't important, you should be much happier... but getting up to freeway speed and keeping it there in the face of hills and headwinds might be a bigger challenge now considering the modest horsepower you're working with

 

[pjkaaihue]

Gearing is always a compromise... shorter gearing gives better acceleration taller gearing reduces rpms at speed

for reference you went from a lower than stock ratio to a much taller than stock ratio

stock cl350 is 16 38
stock cb350 is 16 36

generally speaking street bikes are geared so they max out around 80% to 90% of redline in top gear

with your gearing your essentially making 5th overdrive

how many rpms can you get on a flat road wide open in 5th?
7,000k?

can you pull all the way to redline in 4th?

I rode this morning on the way to work. I didn't try getting to redline in 4th, but it stayed close to 6k rpm (~68mph) without too much difficulty, and that's about as fast as I need to go. The speed limit here is 70mph, so it's a huge improvement compared to before, where I was redlining just to keep from getting run over.
It doesn't perform nearly as well, which was to be expected. Acceleration is much slower, but so far it seems to at least keep up.