Looking for opinions on pistons and cylinder condition

[ampzip]

My bike is running very rich. I've cleaned the carbs multiple times, the jetting is all factory specs. Replaced coils, condensers, wires. The intake was replaced with cone filters which I would think would make it run lean if anything, but in any case it's running very rich no matter what.

I noticed when it's running the left cylinder feels much warmer. The left spark plug comes out black and dry, the right spark plug comes out with wet oil on it still. I get exhaust out of both exhaust pipes though.

I tested compression (borrowed a tusk brand, not sure if it's a good one) and it read around 65 psi in both sides so assumed something with the top end is wrong. All I have is a digital caliper but it 54.2mm at each measurement I top and bottom of each cylinder.

I'm guessing I should take the pistons and cylinders to someone who knows what they're looking at? A hone job and new rings, or do the pistons look like they should be replaced?

[1976RD400C]

You could take the rings off, stick them in the cylinder, and measure the end gap to get an idea of wear.

[m in sc]

yeah thats what I do as an initial check myself. from the pics nothing looks terrible. do the carbs have the stock bassinet thrm? you still have the stock needles in the slides? were the jet tubes removed completely and cleaned?

[SoCal250]

As the other guys have said, nothing alarming in the pics. Piston skirts show a little wear but not bad, and there really isn't much sign of blow-by from worn rings. Measuring the ring end gap as suggested above is easy.

To measure the cylinder correctly you would need a bore gauge and micrometer. Not sure where you're located but you can puck up something inexpensive like this to do the job:
HF bore gauge set

Are you sure you're testing compression correctly because if the bike was running previously it should have more than 65psi. Make sure you're using the spark plug adapter that matches the size (dia and length) of the spark plugs. Also make sure there is an O-ring on the threaded end. Thread it in by hand and snug it finger tight. Fully open the throttle and kick the engine over repeatedly until the needle stops climbing. Where it maxes out is your compression number.

[Brad-Man]

You say you are running cone air filters - if they are the ones with a flat top opposite the carb that is a problem...

[ampzip]

Thanks for the input

You could take the rings off, stick them in the cylinder, and measure the end gap to get an idea of wear.

Ring gap measured between 0.97-0.98mm

Edit: Just noticed the manual says ring gap should be 0.15-0.35mm, so I guess it's safe to say they are long gone.

yeah thats what I do as an initial check myself. from the pics nothing looks terrible. do the carbs have the stock bassinet thrm? you still have the stock needles in the slides? were the jet tubes removed completely and cleaned?

Previously I had removed all jet tubes and cleaned thoroughly, I took them out again and they were still all clear.

Interesting that I never noticed when cleaning before, the needles are definitely different, and I think there is damage on the choke side carb needle, and there's no working spring in it. I will have to get new needles to make sure I'm using the right ones.

I also measured the float arm measurement - manual says 15mm, I measured 14mm on right carb, 13.5mm left carb (one with damage).

As the other guys have said, nothing alarming in the pics. Piston skirts show a little wear but not bad, and there really isn't much sign of blow-by from worn rings. Measuring the ring end gap as suggested above is easy.

To measure the cylinder correctly you would need a bore gauge and micrometer. Not sure where you're located but you can puck up something inexpensive like this to do the job:
HF bore gauge set

Are you sure you're testing compression correctly because if the bike was running previously it should have more than 65psi. Make sure you're using the spark plug adapter that matches the size (dia and length) of the spark plugs. Also make sure there is an O-ring on the threaded end. Thread it in by hand and snug it finger tight. Fully open the throttle and kick the engine over repeatedly until the needle stops climbing. Where it maxes out is your compression number.

I tested compression after warming engine up and doing exactly as you mention, making sure plug adapter is correct size, there was an O-ring on it, tightened by finger, throttle open.  I had removed both spark plugs as I thought they were both supposed to be out for compression test. Should I do it with the other plug still in?

You say you are running cone air filters - if they are the ones with a flat top opposite the carb that is a problem...

Yes, not doubting, but what problems could it cause?

I also did notice on the back of the right carb it looks kinda gnarly, not sure if it's an issue to have what looks like those holes plugged on the left side...

[SoCal250]

Thanks for the input

You could take the rings off, stick them in the cylinder, and measure the end gap to get an idea of wear.

Ring gap measured between 0.97-0.98mm

Edit: Just noticed the manual says ring gap should be 0.15-0.35mm, so I guess it's safe to say they are long gone.

That's pretty big! Time to take some other measurements and see how involved of a top-end rebuild you need to do. With that large a gap there's probably a lot of wear in the bores, but who knows you could get lucky and find out that you can get away with just rings.

yeah thats what I do as an initial check myself. from the pics nothing looks terrible. do the carbs have the stock bassinet thrm? you still have the stock needles in the slides? were the jet tubes removed completely and cleaned?

Previously I had removed all jet tubes and cleaned thoroughly, I took them out again and they were still all clear.

Interesting that I never noticed when cleaning before, the needles are definitely different, and I think there is damage on the choke side carb needle, and there's no working spring in it. I will have to get new needles to make sure I'm using the right ones.

I also measured the float arm measurement - manual says 15mm, I measured 14mm on right carb, 13.5mm left carb (one with damage).

Those float needles should be replaced. Do the valve seats have a marking stamped in them "2.0"? Probably best to get them as a set and just replace them all. When buying new float valve sets be sure to get size #2.0 (Mikuni Type VM26/26). The DS7 & R5 run a #2.0, while the RD models run a #2.5.

As the other guys have said, nothing alarming in the pics. Piston skirts show a little wear but not bad, and there really isn't much sign of blow-by from worn rings. Measuring the ring end gap as suggested above is easy.

To measure the cylinder correctly you would need a bore gauge and micrometer. Not sure where you're located but you can puck up something inexpensive like this to do the job:
HF bore gauge set

Are you sure you're testing compression correctly because if the bike was running previously it should have more than 65psi. Make sure you're using the spark plug adapter that matches the size (dia and length) of the spark plugs. Also make sure there is an O-ring on the threaded end. Thread it in by hand and snug it finger tight. Fully open the throttle and kick the engine over repeatedly until the needle stops climbing. Where it maxes out is your compression number.

I tested compression after warming engine up and doing exactly as you mention, making sure plug adapter is correct size, there was an O-ring on it, tightened by finger, throttle open.  I had removed both spark plugs as I thought they were both supposed to be out for compression test. Should I do it with the other plug still in?

Shouldn't matter if plugs are out for the test. I usually run my tests with a cold engine.

You say you are running cone air filters - if they are the ones with a flat top opposite the carb that is a problem...

Yes, not doubting, but what problems could it cause?

https://www.2strokeworld.net/forum/index.php?topic=6462.msg53591#msg53591

I also did notice on the back of the right carb it looks kinda gnarly, not sure if it's an issue to have what looks like those holes plugged on the left side...

That right carb definitely needs to be cleaned up. Make sure all the passages are clean/clear.

[m in sc]

aside from those float needles looking rough, i mean the jet tubes and needles.

https://www.2strokeworld.net/forum/index.php?topic=6624.0

[ampzip]

Those float needles should be replaced. Do the valve seats have a marking stamped in them "2.0"? Probably best to get them as a set and just replace them all. When buying new float valve sets be sure to get size #2.0 (Mikuni Type VM26/26). The DS7 & R5 run a #2.0, while the RD models run a #2.5.

Yes, 2.0. I noticed yambits just sells them as sets, so I'll go that route.

I will replace those air filters as well.

aside from those float needles looking rough, i mean the jet tubes and needles.

https://www.2strokeworld.net/forum/index.php?topic=6624.0

Gotcha, although it is clean through the jet tubes, the tubes themselves weren't removed, I'm not sure how I would do that. The needles appear to be in good condition and are clean.


Other than that I will get the rest of the top end measured properly and figure out what needs to be done. Hopefully these are all of the remaining pieces of the puzzle.

[m in sc]

take the main jet & washer out, turn carb upside down, gently tap out towards the top. I guarantee they are not clean

[SoCal250]

When tapping the jet tubes out be careful. Brass is soft. I keep an old plastic chopstick in my toolbox for this. A wooden pencil can also be used as a punch for driving it out.

And when reinstalling them make sure that the groove in the end of the tube is aligned with the small locating pin in the bore.

[ampzip]

Pencil worked to push it out. They don't look too bad but do need some cleaning.

I took the cylinders to a local guy who does work for bike shops in town and he measured them. He said they were fairly wore and a .25 bore is necessary, so I will be replacing the pistons and rings.

[Striker1423]

Those cone filters cause weird reverberations when running at certain rpm ranges. Basically the pressure wave that comes back out of the carb when the reeds close hits the hard back of the filter and bounces through the carb and the reeds again causing them to flutter. It'll feel like a chopped up ignition and a loss of power.

Swap those air filters for the stock airbox of you can, or replace it with a foam UNI filter.

[SoCal250]

Swap those air filters for the stock airbox of you can, or replace it with a foam UNI filter.

Simplest and cheapest would be to swap out the filters for a pair or UNIs. Alternatively, a Y-boot with a UNI or K&N could be used.
https://www.economycycle.com/product/uni-up-4182ast-2-stage-pod-filter/
https://www.economycycle.com/product/economy-y-boot-filter-kit-for-yamaha-rd250350400-r5-ds7-rz350/

[2steve]

Just a side note: There is a difference in the main jet if you run a UNI with the red sock on as opposed to taking it off.