Topic: Valve kissed piston after 100 miles....

[samm_j2]

Scottlt, there is a foto of just the 1st piston. Did you take a look at it? I will post some pics of all of them tomorrow as well with my findings on if the cam lobs are equal.

I am really hoping all this is can be chucked up to not degreeing in the cam. But I am still very interested in everyone's opinions on the matter.

I am still very confused on why it took 100 miles and did not happen immediately and why it only happened on one cylinder. Don't get me wrong im glad it only happened one haha but still want tonunderstand this better!

Thanks!

[scottly]

There is only a pic of one half of one piston.

[samm_j2]

Scottly, here are some hastily taken pics. Let me know what you think

[Ellz10]

Tracks,

The bike ran incredibly well before power loss. Better than ot had ever run!

No idea why valve lash was changing, probably due to valve and piston contact.

All pistons pr weisco 836, and yes they are oriented correctly. Unless I totally installed them wrong haha but I am sure they are in there correctly.

Yes, this is the carbon from only 100 miles. It seems like a lot to me? I thought this was strange as evey time I check the plugs they were lean. I was also trying to get the carbs with a good AFR so I was stepping up the jets and needle positions.the carbs are mikuni rs34.

Does this information give any ideas??

Thanks!

The 34 on that 836, or most any 836, is way too big. Unrelated to your issue, just thought I'd chip in my 2 cents.

[samm_j2]

Ya people have said that. If its true I can't tell. The carb is great, less expensive than the CR, fairly easy to tune, and it has an accelerator pump. So, maybe on a dyno you would get a few more hp? I don't get why people say its too big though, I get incredible throttle response with it... if your saying they are too big as i mentioned i was lean, this was not the case with a few other cams i was running in the past, it has definitely run too rich to the point of fouling a plug. if  you have a more specific reason other than  its just "too big" I'd like to hear it, but otherwise to each their own...

[PeWe]

Dyno with air fuel measuring will tell more about the carbs and their jetting.
From idle to top if the dyno operator will check that, not only WAT, torque and hp curves.
This is very important and you will save lots of time at a good dyno shop that can tell you how fuel screws, pilot, needle heights and main jet need to be changed.

The power curve will tell if there is dip when the fun acceleration should occur depending on what the cam will produce.

It is very interesting with good running examples of carbs like RS34.

I have spent years to adjust my Mikuni TMR32 on a moving target of rebuilding my bike. 
First 836cc, 900cc followed by 1005cc and finally 970cc from end of last year (a quick test of an 890 setup with its head just before the 970).

Different cams and exhaust systems. And 3 ported heads
My carbs needed less fuel screws for clean idle, more pilot jet up to 4000 rpm lift steady cruise. Needle and mains followed thereafter.

Main jet safer to test on dyno, not on the road that means cruising in 150-160kmh and WOT which will end up in 200kmh.
I have had too close moments testing that.

I'm close to a complete working setup from idle to full.  Before always too rich and too lean somewhere in the throttle lift area.
My nearby dyno shop was fully booked so I could only get one late run.

[scottly]

Are the valves stock size for your head? You mentioned something about sinking the valves into the seat due to the lift of the cam? Do you know how much that was? How much did you have to adjust the tappet when it loosened up?
You definitely need to clay the pistons. If you have proper clearance and it's about the same on all four cylinders, there is only one thing I can think of...
I rebuilt the engine in my family's 1959 VW bus in high school auto shop, including a valve job and replacing the valve guides using a driver I made in the metal shop. The motor fired right up, but in a short amount of time, a tappet got very loose, so I tightened it up and carried on. Soon it got noisy again, so I adjusted it, and after that it began to miss on that cylinder. It turned out the guide was tight, and the valve was stuck a tiny bit open, and adjusting it again only made the valve more open. Once I reset the clearance it quickly loosed up with only a few miles and some top end oil for good measure, but there was no chance of a valve hitting a piston even if it was stuck fully open on that motor....

[Tracksnblades1]

Sam,

When you refer to  100% leak down, you're meaning 100% sealed...? 🤔

In our area we like 3% leakdown or less.

Around 10% leakdown we're thinking about losing to much HP.

Usually by 15% leakdown there will be signs...

I think we're on the same page just on the inverse. 🤔

[samm_j2]

Tracks,

Yes sir, we are. I think I usually say it backwards. When I say 100 I mean 100% of the air is staying in. Like if I put 100psi of air pressure it holds 100psi. So 0% has leaked out.

[samm_j2]

I checked for difference in the cam lobs and everything seemsnto be dead nuts on. I am going to degree the cam in this time but am thinking I will also smooth some clay and double triple check things. Should I smoosh the clay with how I had the cam set up last time? Or degree and smoosh the clay and see where I am at? Does it even matter? I plan to do this with the old head gasket as to not waste the new one...

Anyone got any comments? Or is it just a waste of time?

Thanks

[david 750f]

I would degree the cam and then check with the clay. With an adjustable sprocket you have no way of knowing if you are close. I was 12 degrees off before degreeing my cam with an adjustable sprocket.

[samm_j2]

Scottly,

I missed your last reply. Thanks. I think this was a thought ken had as he ran a reamer quickly through the valve guide. I am wondering if this is part of what happened. I am also very curious to see how far I was with just dropping the cam in this last.

I am pretty sure the valves were sunk .015 and the valves are stock size for the 78 f

[Ssicbx]

Just want to say there are a lot of good points have been made, but what is the deal with the O.D. of the pistons?  There is no carbon build up.  Are the pistons that close to the top of the cylinder?

[samm_j2]

Ssicbx,

I have no idea man. Again there was only 100 miles. And the piston comes close enough with this cam that the valves needed to be sunk and pistons were close enough for that damn valve to hit the piston.

What exactly are you thinking with the carbon build up patter???

[Ssicbx]

Being I'm troubling shooting on line, the first things I would do:
1. Lay the head gasket on the cylinders, ensure there is no head gasket overhang into the bore.
2. Get the cylinder back on and see were the pistons are in regards to the top of the cylinders. 

What is the compression ratio? 

[samm_j2]

Head gasket definitely does not cover boar. And the piston goes damn near flush with the top of cylinder. Ther is a a small ring of carbon at top of cylinders where the ring reaches its highest position.

I have no idea what the compression ratio is...

[scottly]

I am pretty sure the valves were sunk .015 and the valves are stock size for the 78 f

Looking at Cyclex's site, it appears the .015 sinking is only required when that cam is used with an F2/F3. This is evidently done to prevent valve-to-valve interference with the larger late F valves, and has nothing to do with piston interference. If it was me, I would clay the pistons using the same sprocket and timing as before.

[samm_j2]

Scottly,

That is the current plan, homework pending. Do this just to see what the clearance is to get an idea of what happened right?

I am also thinking I will do the clay again and degree the cam with adjustable sprocket and thr old head gasket one more time just to cover all the bases.

I will post in what happens and if anyone has more ideas or things to try please let me know

Thanks

[samm_j2]

Ok,

I just put the old head gasket on and torqued to about 23ft-lb installed the cam with the sprocket the way indid the first time, just lining up marks with 1 and 4 at TDC. Had clay in the piston. Did 2 full revolutions...

There was an incredible amount of space. The exhaust valve clearance was  0.176" and the intake was even more... seems like more than enough space!

so the fact that the piston hit the valve is even more of a mystery to me now... I am wondering during if scotly is right with the valve guide just being a little too tight?

[Tracksnblades1]

Ok,

I just put the old head gasket on and torqued to about 23ft-lb installed the cam with the sprocket the way indid the first time, just lining up marks with 1 and 4 at TDC. Had clay in the piston. Did 2 full revolutions...

There was an incredible amount of space. The exhaust valve clearance was  0.176" and the intake was even more... seems like more than enough space!

so the fact that the piston hit the valve is even more of a mystery to me now... I am wondering during if scotly is right with the valve guide just being a little too tight?

It just means Scotty was right….😁

[samm_j2]

Ya, that's what I figured...

He is always right haha

[PeWe]

Interesting... I have not heard about too tight guides before.

But I remember how my bike sounded direct when started when I had cam chain a tooth wrong back in the 80's.

I had to wrap engine out of the bike... and set it correct.  No permanent damage.

Rotate engine without plugs. Chain tensioner in it place.

 See how the cam markings line up at 1:4 TDC. A tooth or 2 wrong is easy done.  Rotate it a few crank turns  when setting the lash on all valves and check.

Still stock sprocket?
A slotted one can be needed to time cam right for best power.
Possible to grind a stock sprocket as much as needed.

The sprockets with slots have usually way too wide slots.

[samm_j2]

Pewe, me neither but I don't know much to begin with anyways...?

The fact I had that much clearance really adds to the mystery of it all... I really don't wanna get everything back together and have it happen again hehe... 

[PeWe]

I should time cam with only 2 rockers for cyl 1, In and Ex.
When timing finally done, assemble the other.

Measure when In open and close. At lift 0.04 or 0.05" lift.
Same for Ex to see if numbers are close to cam card.
Probably better if IN open 2-3 degrees earlier than later compared to cam card. 
See the balance between in and ex. When IN close an important detail.

The cam numbers might not be as the card.
A stock sprocket can end up in retarded cam.
I guess the cam is not grinded for stock sprocket.

The lobes might need adjustable sprocket to set  correct timing.

[samm_j2]

Yes, it looks like the stock gear caused a retarding of the cam. I hope to get some time to degree in the cam this weekend. I have an adjustable gear. I will keep this on mind pewe, thanks