Piston Rings? Valves? Timing? All Three?

[Isoparmesan]

A bit of Marvel Mystery Oil down the cylinders is the go to for many members.

Let it soak overnight, rotate the engine daily for 4-5 days.
Then start, yes, it will be smokey for a bit.

I was thinking this, or ATF/Acetone mix, or Turpentine like another poster recommended.

Think I should drain the oil if I do MMO?

[Big T]

No, just put an ounce or two in each cylinder
MMO is ok in your motor

[Isoparmesan]

Interesting development: The bike has also had an issue with the throttle cable sticking. After lubricating the hell out of it, and seeing that have no effect, I discovered that it wasn’t the cable at all, the bolt that holds the slide for carb #3 had come completely loose and was bouncing around inside the top of the carb, and occasionally binding against the slide arm.

this means that carb #3 was pretty wide open at idle. Once I put it back in place, the bike magically started idling normally and that air hiss I was hearing in every stroke is gone. That could also explain why cylinder #3 seemed to be pulling the hardest, and won the compression championship, even with the throttle closed.

still going to do another compression test tomorrow, along with re-balancing/syncing the carbs (will also consider throwing some threadlocker on the slide adjuster bolts). I’m more hopeful for the compression results now though.

[Chuck78]

Yes your compression test was definitely biased because only one cylinder was wide open, unbeknownst to you!
Low mileage engines still suffer from severe bore wear or sticking rings, sometimes people just run them out of oil and they run extremely hot and wear things out. other times, people can't get them to run because the carbs are gummed up, and they do a really foolish thing and run them with no air filter at all, and ingest dirt, which will score of the cylinder walls if it is abrasive...
Also, cylinder walls can get rusty from humidity in the air and not being run...rimgs as well, as when the temperature drops, the moisture in the air can condensate on the cylinder walls and run down into the rings.


all that being said, I think you having one cylinder wide-open full-time was the cause of your compression test only accurately being done on one cylinder.


if you still have difficulties, I have had excellent luck with some engines in the past, that have been neglected, and no valve adjustments ever done. On first valve check, I will set the valve shim clearance at the maximum side of Suzuki's tolerances or above, give the valves plenty of room to slam shut and break/wear off any carbon that had formed on the faces of the seats and valves from the valves hanging open (due to neglect) and not shutting fully.
In conjunction with that, I will squirt automatic transmission fluid or acetone ATF mixture in the spark plug holes and cycle the engine over a few times manually with the kickstarter or with the large nut underneath the bolt that holds the ignition parts on the crank. Let that sit overnight, then go run the bike fairly thoroughly. I don't ever baby them! I have even squirted it into the intake ports before just to make sure I got some on the valves to help break up any carbon deposits. If you are exhaust bolts are able to be removed without breaking them in half, I have even done the same on the exhaust side.


When I first met my wife, her bike was the first shim bucket style that I had the opportunity to work on. I did all of the above to that, and the bike had the most power she had ever felt on a motorcycle before, and she commented heavily that she was so happy to as "THIS is how a motorcycle is supposed to ride!"
helping the valves seal better with this technique, and using plenty of ATF persistently on the piston rings restore to the compression to around 130 or 140 PSI, and the bike was very fast again after that. eventually she wore that engine out, and I got a low miles junkyard engine that tested 170 PSI on all cylinders.


Also, thanks for the re-reminder that I need to go out and coat the cylinder walls of all the bikes and spare engines in the garage with some good thick oil for the winter! You don't want the winter warm days humidity condensing in the evenings and fouling up your cylinder walls and rings!

[Isoparmesan]

Well, it looks like my compression is actually totally fine. Just redid the dry test (properly) and got 160-170 in every cylinder.

So, here’s another question. If my compression is good, why am I getting such significant blowby out of the breather?

[Big T]

The most common cause is overfilled oil, or gas in the oil
Could you post up a picture of all four of your spark plugs?

[Isoparmesan]

Spark plugs are all sooty, it’s definitely running rich (surprising with pods). I’m going to try to dial them in again next week since I need to rebalance them anyway after that slide adjuster bolt came loose.

I’m at a bit of a loss when it comes to adjusting the fuel screw on the bottom of the carbs. Can I treat it like the air screw? Turn it both directions until I see a drop in revs and then go for the middle? Or do I really have to tweak it and check the plugs repeatedly?

[Steve]

Most here will tell you that you set the fuel screw ONCE, based on your intake/exhaust equipment, then do all fine-tuning with the air screw.

While looking at the carb diagram to see how they work, I question that, but have virtually no experience with VM carbs, so I have no way to verify my theories.

In the BS-series carbs (also know as "CV" carbs), you have a fixed air jet and a fixed fuel jet to provide a set mixture. The amount of that mixture that is admitted to the main airstream is adjusted with the pilot screw (also known as the "mixture" screw).

Your VM carbs have a fixed fuel jet. Air is mixed with that fuel, the amount of air is controlled by the pilot air screw. That mixture then passes the "fuel" screw to get into the main airstream. The only functional difference that I see is that the air/fuel ratio of the mixture is variable.

VM carb:


BS carb:


.

[erki52]

After setting and re-setting and re-setting and ..... (you know how it goes) my VM slides I have screw number 3 in Steve GS Whisperer's picture set at 1/2 turn from VERY lightly seated and screw number 1 in said picture starting at 1 turn out. once the bike was well warmed up i fettled screw 1 until the revs rose and fell and left it at the highest revs. This took a bit of practice but i got there in the end. These figures worked the best on my carbs, they may not work as well on others but it's a starting point. I have a Marshall deeptone exhaust 4:1 (on the bike when I bought it) and foam filter pods.

[Steve]

Those pictures are from the factory service manuals.

.

[Isoparmesan]

After setting and re-setting and re-setting and ..... (you know how it goes) my VM slides I have screw number 3 in Steve GS Whisperer's picture set at 1/2 turn from VERY lightly seated and screw number 1 in said picture starting at 1 turn out.

From
just did the same. Here’s what’s a new plug in cylinder 1 looked like after 30 or so minutes of riding. I flipped the kill switch at high revs before pulling the plug. I was getting kind of a lumpy idle after riding a bit longer. I’m still going to rebalance (have to borrow a manometer) and also take another look at the ignition timing.