Are my piston and cylinder shot?

C

carkl

New Member
I've been having trouble with my 80cc motor from thatsdax. Had a long discussion with drhofferber (aka Dennis) on the troubleshooting forum, and I think we've pinpointed the problem, but I wanted to get a second opinion and maybe a little advice for repairing. You can look at the thread "Loss of Power" on the troubleshooting forum for all the details, but here's the condensed version:

-I put together my kit and rode it happily for about two and half weeks. Probably about 150-200 miles.

-I started it one morning and noticed I had a considerable drop in power and after the motor warmed up it would not idle unless I goosed the throttle now and then. The idle improved if I adjusted the idle screw all the way in (most open position).

-I started trouble shooting.. checked electrical system, tried a new spark plug and new plug wire, checked for air leaks, went over the carb, cleaned it out, tried various c-clip positions. With Dennis' help I think I was pretty thorough. After all that, I opened up the motor.

-I found a thick build-up of carbon on the top of the piston and all over the dome of the cylinder head. It was maybe a 1/32" layer of soot coating everything.

-I found some light scoring all around the piston and cylinder wall as well as a few deeper grooves on the piston and through the piston rings, and deeper scoring, through the chrome, on the cylinder wall. (I'll attach a few pics)

So... I think the problem is 1) carbon built up in the combustion chamber, 2) bits of carbon flaked off and caused wear on the piston, piston rings and cylinder wall, 3) loss of power and poor idle are result of poor compression. Does that sound like a reasonable assessment?

My questions are: 1) If I want to get it running again as cheaply as possible, does it make sense to replace just the cylinder, piston and piston rings? 2) It seems likely to me that there is some grit and crud in the crank case. If I do rebuild the top end, how can I be sure there won't be more stuff in the bottom end to get thrown back up into the cylinder and send me right back where I started?

I'll take any input anyone's willing to give. Thanks for looking!

-carkl
 

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You bottom end should be fine. I would flush things out with a solvent chemical or kerosene. Before reassembling, coat lower end bearings with 2 stroke oil.

I'd rebuild the engine and once it was done, I would use Maxima 927 racing oil at 5 oz to each gallon. It contains 20% castor oil which will provide superior engine protection then conventional oil and its synthetic base oil (80%) is low ash so you have as much crud build up. More oil in these engines isn't necessarily good. Once your internals get coated with a desired castor oil varnish (its an incredible dry lubricant), you then should be able to decrease oil to 4 oz per gallon of Maxima. I'll buy you your next engine if you experience the same engine problems with a castor based oil.
 
A compression test would tell you how bad your piston and cylinder is looking at the piston and cylinder it don't look real good but pictures can be deceiving as low cost as these engines are and you bought it from Dax ask him if he has a reject engine maybe one with a casting flaw.They are perfect for parts even if the cylinder has a flaw( no hole into the cylinder) they run just fine.
Norman
 
carkl said:
So... I think the problem is 1) carbon built up in the combustion chamber, 2) bits of carbon flaked off and caused wear on the piston, piston rings and cylinder wall, 3) loss of power and poor idle are result of poor compression. Does that sound like a reasonable assessment?
Click to expand...


No way. While that is some decent level of build up for so few miles (?) - no way was it caused by a carbon hunk. Either something foreign entered or something broke.
 
Last edited:
donuts31 said:
You bottom end should be fine. I would flush things out with a solvent chemical or kerosene. Before reassembling, coat lower end bearings with 2 stroke oil.

I'd rebuild the engine and once it was done, I would use Maxima 927 racing oil at 5 oz to each gallon. It contains 20% castor oil which will provide superior engine protection then conventional oil and its synthetic base oil (80%) is low ash so you have as much crud build up. More oil in these engines isn't necessarily good. Once your internals get coated with a desired castor oil varnish (its an incredible dry lubricant), you then should be able to decrease oil to 4 oz per gallon of Maxima. I'll buy you your next engine if you experience the same engine problems with a castor based oil.
Click to expand...

That seems like good advice to me. I was planning to switch to synthetic oil after I get everything running again. I think I'm kind of forced to run the motor in ways it doesn't like just because of the terrain around here. I have about a 9 mile ride to work and I'm guessing that there's just under 1 mile of it that's on level ground. The rest of the time I'm either riding up hill with a lot of throttle and the RPMs staying in the midrange, or I'm going down hill and I pull in the clutch and coast. Not much opportunity to give 3/4 throttle and have the RPMs up in the sweet spot. I'm guessing it's this kind of operation that lead to so much carbon so quickly.

Norman said:
A compression test would tell you how bad your piston and cylinder is looking at the piston and cylinder it don't look real good but pictures can be deceiving as low cost as these engines are and you bought it from Dax ask him if he has a reject engine maybe one with a casting flaw.They are perfect for parts even if the cylinder has a flaw( no hole into the cylinder) they run just fine.
Norman
Click to expand...

I don't have a compression tester so I can't really be precise about it, but I did think about that last night. I put everything back together and tried to plug the spark plug hole with my thumb then turn the crank. It was hard to make a good seal with my thumb, there was definitely a little air leaking around my thumb, but I'm sure I heard a little hiss coming from inside the cylinder as well. Tried plugging the intake tube also, that was little easier. Can't say I felt much of a vacuum there, more like a slight pulsing and also heard a hiss inside the cylinder. Never tried any of this before I started having trouble so I don't really have a baseline to compare to.

Not sure I understand about the reject engine. I would need to get something that, at least, has a good cylinder, piston and rings right? I think the head and gasket look just fine I was going to reuse those. I made a new base gasket out karropak (sp?) gasket paper. Anyway, I'll write to Dax and see if I get can get any kind of deal on the parts I need. Thanks!

-carkl
 
Pablo and bikeguyjoe, I know what you're saying. Carbon is too soft to make gouges like I've got. Maybe some burs around intake or exhaust ports, or piston skirt left after casting? I don't know... When I rinse out the crankcase, maybe I'll try to dump out the solvent through a screen... see if I can find any offending particles.

Another question, donuts31 said I should oil the crank bearings after I clean out the crank case. It'll be easy to put a few drops of oil on the tie rod bearing, not sure about the bearings on the crank ends. Are they accessible?
 
spray some kind of oil even wd 40 or light machine oil to keep bearing etc from forming any rust. You can sometimes pick up an engine from Dax that has a good piston,cylinder it might have an out side casting flaw that someone didn't like the inside of the cylinder etc is fine. You can sometimes pick up one of these engines for $40.00 lots of extra spare parts. Dax might not be doing this anymore. Your engine could have done many things to score the cylinder, rings, piston. Dirt inside is one, small parts of the chrome on the cylinder could of flaked off, left over metal partials from the manufacturing of the engine. You might of over heated the engine or ran it a little too lean on the oil mix. This is one of the things that sometimes happens. You could even have a main seal that is leaking or letting air into the crank case causing a lean fuel/air mix which will eat them up pretty fast. Any air leak can cause a lean run condition and over heating.
the cylinder base gasket or the crankcase's main gasket can leak if you have ever seen a black oily smear on the engine around a gasket area that is a sign of an air leak and needs to be fixed.
Bottom line your engine took some kind of a hit and has lost its power to run properly.
You need to decide if you want to spend the money on parts or replace the engine or parts of the engine. My way of thinking is these are cheap throw away engines parts can soon be more than a whole new engine kit. You will have to decide which is more affordable to you.
 
So I'm planning to rebuild my top end, and for reasons I won't go into, I'm not going to go to my original vendor for replacement parts. Can anyone comment on how interchangeable parts are for these motors?
 
There are a few factories and several sizes of various parts, if you are going to try to mix and match parts, good luck.
 
no one has ever just made the parts fit? there all the same relitive anamial right some metal gone here a diffrent pice there hit it whith a big hammer till it works kinda thing?
 
Exept for the 6 verses 8 millimeter studs the piston and jugs are quit universal. Someone might correct me? I got to replacement jug kits here in 8 mill studs. I can't tell the difference From each. One from Thatsdax the other from Gas bike. Before Kings turned into Gas Bike as well I once got a piston with three rings on it. That motor ran great until the piston clip came out.There are two styles of piston clips out there. One of them assuredly bites the dust the other quit casual and easy! http://motorbicycling.com/f34/piston-clips-6307.html It was a fun distraction! I lost that piston and jug! It seams a great many vender's don't stock the clips I checked at the time. If they have that certain style of clip its not reusable. So when they part the motors out it goes in the trash. One style of these clips is brutal the style I found on this thread was casual as eating pie! I would hate too see someone anyone else go through the mischief that I saw. Hope this helps!!
 
If you have the bushing stick to the bushing. From what I could tell a jug kit from a needle bearing set would work with a bush. Its the other way around that wont work! I don't know how they worked the evil version of those clips cryogenics to put them in or what but they are evil little bugers.
 
all depends on the stroke of the crank and the cylinder studs where the wrist pin on the piston is and the rod if it is needle bearing for the wrist pin or bushed wrist pin. you can't run needle bearing in a bushed rod for the most part. doing the change won't make a monster engine either just a runner so don't buy a couple of engines thinking your going to build a stroker or big bore.
 
The way i diagnose a bad piston is i will have my bike off, clutch disengaged if your bike is easier to push forward than what it was before it is it is a bad piston if you dont have any airleaks/gasket leaks
 
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