Need your help/advice on low compression

[xlint89]

Heu guys, I haven't stopped tinkering with my Yerfdog. Just having many problems since the engine modifications.

At first I had a failing coil that showed up right as I did my BBK, ported head, and 28mm carb. Didn't figure out that it was the coil until I had re-installed all the stock parts again. (when you have a problem, you go back to when you knew it was good, and methodically eliminate parts)

So I've been running the stock set up until I seated the rings on the new piston. (58.5mm)

I did numerous heat cycles at first. Then started riding the kart for short periods of time. Riding it longer and faster progressively.

So tonight I changed the break in oil and let my buddy take his kids for a few laps. Letting him know he can briefly open her her up (short bursts of speed) Which he did do. Then it happened. It came to a stop (engine still running), but started to pop, sputter, and miss when given more than 1/4 throttle. Just like it did when the coil was failing.

Swapped the coil and did the same thing. Has GREAT spark, fuel tank of gas, couldn't figure it out.... Then i did a compression test and had only 90 psi


Tore it all down. Found a couple small issues, but nothing that really shouts, "I'm the problem!!!".

So, let me know if you guys see something I don't, or need more pics of something.

[xlint89]

As you can see, the base gasket was saturated with oil. But that shouldn't affect compression. Obviously something wasn't right, though.

Head gasket fine. I use Permatex copper spray on alot of my gaskets that I want an xlint seal on.

Piston edge looks like oil was getting on top of it? Would that be your guys' guess?

Black mark on cylinder was the exh gasket leaking. What happened to that piece of gasket material, I don't have a clue???........

[xlint89]

Here's the pics of the piston, cyl and rings

[metalstudman1]

Just a question- Did you get the ring end gap correct (per the instruction/spec's)

[xlint89]

You see the carbon by the top ring only 1 spot on the piston.

The piston skirts are fine. No scarring at all.

Rings look good, no damage.

A circle of carbon at the top of the cylinder except in 1 area. I'm thinking oil??? It matches up with the clean area on the piston dome. However, the piston's clean area is much longer than that clean area on top of the cyl.

As you can see, there is some discoloring going on inside the cyl. It looks worse in the pics, but you cannot feel any scratches, or catch any with a fingernail. FYI the cyl is Nikasil plated.

There are 2 of those discolored areas. 1 at 6 O'clock and the other at 12 O'clock. But remember there are no marks on the piston skirts at all.

[xlint89]

Originally Posted by metalstudman1

Just a question- Did you get the ring end gap correct (per the instruction/spec's)

Honestly, they were larger than should have been.

I read around 9 to 10 thou ring gap. Mine ended up being 12 thou without me removing anything.

NCY ceramic and Nikasil plated cylinder and piston kit.

[xlint89]

Cyl pics

[xlint89]

So would you guys get new rings and run with it since the cyl only has around 1 hour of run time on it?

Am I screwed and need to get another piston cyl combo?

Can that Nikasil be lightly honed for new rings?

This is where i need some advice please.

[x-bird]

have you miked the cylinder and piston for clearance as well as any potential taper or out of round in the bore?
hone pattern looks clean, no vertical scoring. might just be the ring gap . I have yet to do anything with one of these, so i don't know the various specs. I take it the piston was clean to start with--seems pretty loaded for 1 hr of run time? Another thought as to the exhaust carbon on the gasket would be the tightening sequence on the head bolts.
Before dumping $s into another cyl and piston, i'd mike it for piston to wall and try another set of rings.
rings may not have taken a good seat yet either, and I'm not sure if that chunk of exhaust gasket missing could cause the popping/misfire, thought i read somewhere that a leak there could cause some grief ...

[xlint89]

No, I need to get my hands on an inside mic. I am wondering about it being out of round.

I did do a compression check prior and had 130 psi. Kinda low, but figured it should increase once the rings were fully seated.

Head bolts were tightened in a criss sross manner with gradual increases in TQ. Unless there's a specific order to follow which I never read about?

I was hoping to just get a new set of rings, Prob worth getting the cyl checked for roundness before I buy anything I suppose.

As for the carbon build up, I've been running it on the rich side to help keep temps down for break in. Or maybe it was oil getting past the rings from the beginning?

Thanks X

[x-bird]

Originally Posted by xlint89

No, I need to get my hands on an inside mic. I am
As for the carbon build up, I've been running it on the rich side to help keep temps down for break in. Or maybe it was oil getting past the rings from the beginning?

Thanks X

If you've been running rich, you've probably been washing down the walls with extra fuel--big no no, especially on a fresh build. wipes out the oil film the piston/rings ride on. check and see if the oil smells like gas. if so, maybe find an engine shop to do a quick mike on the cylinder and check your piston OD so you know your wall clearance. if the bore is round, you may be able to get away with a light hone and new set of rings.

[SYCARMS]

The Nikasil bore is very hard, and not sure if your rings are chrome moly or cast, but if CM then the breakin time will be mush longer. Owners manual will tell you 10 hours of breakin time. With 1 hour that is not enough time. Nothing from the pics look abnormal but some pics are tough to determine the little signs due to glare ect. But overall they look normal. Ring gap seems fine. Did you do the oil in cyl comp test to see if your problem is head or cyl. related? Most times on new heads the seats will seat further in head causing too tight valve clearance which if tight enough it will hold valve open and loss of comp. is the result.

[xlint89]

X, the oil didn't smell like gas when I changed it.

Tom, how can I tell if the rings are CM or cast? They were in a package marked HK.

What do you want pics of? I can take more....

Did not do oil test. I just tore it all down once compression reading was taken.

It was the old head being used. The new one is sitting on my workbench waiting for me to install the valve spring keepers you sent me.

[SYCARMS]

The compression oil test is all important to determine what causes low compression. The head can still be the problem but it is not kown for sure what caused the drop in compression. Moly rings are usually shinny on the edges but since the engine has been run the edges will be shinny from running. Even if they are,compression should be more than 90psi. First when doing the test did you have the throttle wide open? If not your reading will be low. If so then the best thing to do is pull the rings from piston keeping note of which is up and down as well as 1st and 2nd, and do same with oil control rings. Then check your ring gap making sure the gap is not excessive. If all checks out good then take your head and spray some carb cleanner or brale clean in the runners then inspect the valves for leakage. If head checks ok then reassemble and check the compression again, then if low squirt some motor oil in the cylinder 2-3 squirts then recheck compression. It is hard to tell from pictures but from what you showed us everything looks like it should work. It ran good for an hour then didn't. I believe if in the cylinder there should be some deffinate signs of malfunction with the bore, piston or rings.

[xlint89]

Thanks for your time.

It had the shiny edged ring with the writing on it (facing up) for the top.

Then the all black ring second. (bevel facing up)

Staggered the ring gaps.

Compression I had was 130 psi. Then let my friend ride his kids around (taking it easy with very short bursts of speed) and on a lap, it just started to pop, sputter, bog, and go no where right before my very eyes.

Checked compression and it was 90.

WAIT a minute..... I did forget to hold the throttle open when i took that reading!!!!! UGH...... CRAP!!!!!!

Do you think that exh gasket missing could cause a problem such as described?

I'll get another set of rings and have the cyl mic'd for roundness in the mean time.

Crap, I totally jumped the gun!!!!! (maybe...)

[Johnny 5]

I have had a exhaust gasket blow out, exhaust nuts come loose and fall off, and our last trip my son and I were 2 miles from our camp site and the exhaust pipe on our dune 150 broke at the weld on the header pipe and it sounds like bad news. If you have gaskets I would say put it back together and re test the compression hot wot. I just put a 58.5 high compression piston kit on my dune 150 and compression went from 125psi to 145 psi. I am running a 122 main jet and it is still lean going to a 125 main. Good luck and GOD bless.

[SYCARMS]

130psi on non seated rings sounds about normal, I would put it back together and retry since there doesn't look to be a problem with the components. As I said you will need at least 5 hours of breakin time to seat the rings. I just run the strait 30 non detergent oil during breakin, or you can spend the extra money to run actual breakin oil. Running regular oil will extend the breakin time and especially synthetic. The non detergent does not have the slickers added as in reg. oil which lets the rings seat quicker. You could have had a problem elsewhere causing it to run poorly. 90psi with a slightly cracked throttle sounds very good to me. I would think you should be at or over 150 with throttle wide open. Make sure your fuel is good for that will cause many problems. In fact go to <pure-gas .org > and they will tell you which fuel stations have the ethanol free gas. I would run this fuel and eliminate the many problems associated with ethanol blended fuels. When you get on the site scroll to the bottom and click on your 2 letter state abv.. Thiss will take you to another page listing in alfabetacle order by city/community complete with name and address of station as well as all grades carried.

[xlint89]

Just to let everyone know, the compression was probably fine. I just forgot to hold the throttle wide open when taking the reading.


What I did find the problem to be.....



A $.07 piece of steel!!!!!!!!!

My $26 replacement carburetor (Rontang I think is the brand) uses an "E" clip for the throttle needle. Some how that "E" clip broke.

So the diaphragm would pull the slide up, but the needle sat inside the jet, thus starving it for fuel.


Ugh......!!!!!!

[metalstudman1]

So glad you resolved the issue- but WTF an E-ring clip? So you almost had throttle as the slide rose the metering rod just bounced on & off the seat of the main jet!!!!! That's crazy-but a great find - you tore the whole thing down for a carb issue

[SYCARMS]

Great to hear it's running.