Topic: It's running, kinda.... need some help please?

[mlinder]

My guess is he's hitting all the way around.

Shoulder is too proud of the cylinder deck.

[BrianAdair]

I was wrong, the Rocky pistons are dead flush at the edge with the deck of the cylinders.




I measured the stock pistons from top of pin hole to top edge of piston deck, it is 16.83mm
I don't have a picture of the rocky pistons but they are 17.25mm from top of pin hole to top edge of piston deck.
The Rocky pistons measure 63.40mm in diameter, the dome on the stock pistons is roughly 50mm in diameter, the chamber in the head is 59mm. So if the pistons hit center of the chamber it is roughly 2.2mm in from the edge of the Rocky pistons to the edge of the chamber in the head. at that point on the piston from top of pin hole to top deck of piston is around 17.70mm, and this is a rough measurement cause my calipers won't sit flat since the piston has a slight dome to it.


I tried this with the head gasket but I know its not accurate since it's not compressed, came up with .054in.




My base gasket is .020in thick, was thinking of doubling this, or maybe tripling.....? just a thought

[MRieck]

Those pistons look fine in regard to deck height. That gasket is thick...they are usually .043 or so. Those pics are with the base gasket? I would not start stacking base gaskets.

[BrianAdair]

Those pistons look fine in regard to deck height. That gasket is thick...they are usually .043 or so. Those pics are with the base gasket? I would not start stacking base gaskets.

Yes that is with the bae gasket, and I had also had the head on before that picture so they had been seated all the way down. I also was wondering that, cause the gasket I have is the Rocky S12 head gasket... And if it was made as a kit back in the day, why would I be having prroblems?

[crazypj]

Show us a pic of combustion chamber.
 If pistons are flush with top of cyl it has to be the crown touching.
 It won't turn over with just head on so it won't be a cam problem.
 Realistically, you have three options
from easy to hardest
1. Get a copper base gasket, available in whatever thickness copper sheet is available.
2. Machine tops of pistons to clearance in combustion chamber
3. Machine combustion chamber to size and angle to get most efficient shape
 Personally, I would go with option 3, I've done it before and found it works best

[mlinder]

waitaminit...

Shoulder height is flush with cylinder deck...
Cylinder has very little dome.

I don't see anything that can be hitting, piston and head-wise.

[754]

Not sure but I think Cometic? can make say a .060 base gasket....would that not solve it?

OR
 a cut straight in at the top outer piston edge, for about a 1/4 inch, should remove the offending portion, dome then should have slight bevel on the new edge.
(could cut straight across, and loose compression, hope dome is thick)
Seems to me the chamber is nor round, using the tape method would shoe how far the flat part of piston needs to be.

 What year head BTW?

[BrianAdair]

Not sure but I think Cometic? can make say a .060 base gasket....would that not solve it?

OR
 a cut straight in at the top outer piston edge, for about a 1/4 inch, should remove the offending portion, dome then should have slight bevel on the new edge.
(could cut straight across, and loose compression, hope dome is thick)
Seems to me the chamber is nor round, using the tape method would shoe how far the flat part of piston needs to be.

 What year head BTW?

 I tried the tape method, couldn't get it to show up on the tape and was scared to really put force on it to indent the tape. It is 1977K model.    I talked to the guy that bored my cylinders and he said he could cut a ridge around the pistons so they would clear, the only issue with this is time and more MONEY.... LOL    I estimated that at the point of contact on the pistons to the edge of chamber in head, to have the same clearance as stock pistons, about .055in would have to be removed. That is at the highest point at the edge of piston it was about .020in.

[BrianAdair]

my only concern with anyway I go is lowering CR I know it will lower it some either way, but I want to go the way that would be the least amount. Stock it had 9.2.1 I think, I know the pistons won't do it but it would be nice to get it to 10 on the CR.

[mlinder]

You have to figure out whats hitting before you do anything. Go get some clay and find out.

[BrianAdair]

Here you go, as I thought, about 2mm - 2.5mm around the edge of the pistons hit the deck of the head. I'm thinking now it maybe fine with just the head gasket.


Clay or play dough before.



After #1



#2



#3



#4

[mlinder]

Maybe I misread... did you not have the headgasket on before when checking for clearance?

[BrianAdair]

No, I didn't, just deck to deck no gasket.

[BrianAdair]

I have another question,


The cam chain tensioner, the larger wheel at the bottom A in the picture, and the sleeve B that goes inside it. How tight is this suppose to fit?
Reason I'm asking is I have a NOS one and it is tight as hell, wont twist or rotate I had to drive it out, The one that was in my engine spins and rotates freely, but the rubber is chewed up and hard as a rock. Wouldn't it have to spin freely since that little clip on the end of the sleeve B, holds it in place in the main body? If not  I screwed the NOS one up, took my dremel with a sanding drum and honed it out to fit and spin, other wise I would have had to drive the sleeve in.

[crazypj]

I wouldn't worry about compression ratio, the head was skimmed too much (or not enough, depending on point of view  )
 The combustion chamber is smaller than it was so machining pistons will probably bring CR down to 10:1, your only guessing unless you've actually cc'ed everything?
 Opening combustion chamber 2mm to clear pistons will give beast performance gain, if it's done right.
 I would have another 0.010" off head then machine chambers 1.5 degree difference to piston and have a squish band all around piston crown. (even if I had to machine pistons first to get them shaped right)
 set clearance at 0.038"

[mick7504]

Here are several different piston designs for the 750 Big Bore applications as a reference.
From Axl's site (Satanic Mechanic)
http://www.satanicmechanic.org/bigbore.shtml

You can see some are fully domed and some have a ledge cast/forged.

Machining will overcome the contact issue, and the outer edge of the piston should run a bit cooler.

[BrianAdair]

I wouldn't worry about compression ratio, the head was skimmed too much (or not enough, depending on point of view  )
 The combustion chamber is smaller than it was so machining pistons will probably bring CR down to 10:1, your only guessing unless you've actually cc'ed everything?
 Opening combustion chamber 2mm to clear pistons will give beast performance gain, if it's done right.
 I would have another 0.010" off head then machine chambers 1.5 degree difference to piston and have a squish band all around piston crown. (even if I had to machine pistons first to get them shaped right)
 set clearance at 0.038"

 ...... That sounds like it would be a good mod, but this is East Texas...... lol Im dealing with people that, are like "nah just put it together and run it"
I don't want to do that, the problem with what your recommending, is I am new to all this and have an idea of what you mean, but don't know how or what I would tell my machine shop guy. He doesn't build bike engines, I asked him a while back what he thought about opening the chambers up on the head, and he said he didn't know, but that it would lower CR. He builds high performance VW engines.
The head deck is already really close to the valve seat rings, don't know how much more I could take off before getting into them.
This is one of those things where I'm scared to even attempt, cause if I screw it up, I will be very very sad....  
If i had an extra head to play with I wouldn't care, Im just kinda overwhelmed 

I would like to CC everything, I want to know what my CR is, trying to read and learn how to do that.

Here are several different piston designs for the 750 Big Bore applications as a reference.
From Axl's site (Satanic Mechanic)
http://www.satanicmechanic.org/bigbore.shtml

You can see some are fully domed and some have a ledge cast/forged.

Machining will overcome the contact issue, and the outer edge of the piston should run a bit cooler.

Thank you for the link, I seen that a while back and forgot where I found it at.

[NitroHunter]

Have you clayed the pistons with the head gasket installed? Since that is when you want to see the .050" clearance, you may be right where you need to be as is.
A little basic math will get you in the CR ballpark, search out the cc's for that head - it's here somewhere - and go from there.
Too much clearance and you no longer have a 'squish' band - just an area of incomplete combustion due to flame front propagation issues. A CR gain/loss of .1 is easily tuned around and isn't a big deal until you get to serious mega stuff, but getting the squish right-on has far greater gains in any application. I applaud you for taking the time to work through this.

[BrianAdair]

Have you clayed the pistons with the head gasket installed? Since that is when you want to see the .050" clearance, you may be right where you need to be as is.
A little basic math will get you in the CR ballpark, search out the cc's for that head - it's here somewhere - and go from there.
Too much clearance and you no longer have a 'squish' band - just an area of incomplete combustion due to flame front propagation issues. A CR gain/loss of .1 is easily tuned around and isn't a big deal until you get to serious mega stuff, but getting the squish right-on has far greater gains in any application. I applaud you for taking the time to work through this.

thank you,
No I haven't I was scared I would ruin the gasket by compressing it, and then re-using it since it isn't a copper gasket.
The metal chamber rings on the gasket measure .055in thick, at full torque will it compress more than .010? that would be about .045in clearance.

[NitroHunter]

.045" should be fine.
About 25 years ago George Bryce told me it was safe to use .040" squish with steel rods and .050" with aluminum. I took him at his word.   
Me personally, I've run +- .040" and revved to 10.5K (and more sometimes) without issue, this with both cast and forged pistons in several different motors. So my opinion is GB knew what he was talking about. Imagine that... 
 
Just as an FYI I know a guy who's record-holding race application cut this MUCH thinner (with a different motor) but his was a result of many post-run teardowns and machining. 

Of course be sure to check valve to piston clearance when you degree your cam. Those pockets don't look very deep and may require a little Dremmel magic.

[BrianAdair]

yea, they aren't very deep at all, and with the cam I'm wanting to run I think they will need dishing out. I'm going to do the dial gauge methed on valve to piston clearance, instead of clay. May just have to buy, or see if someone is willing to trade for a milder cam.....

[mlinder]

a guage doesnt show you where it's hitting. Use clay.

[BrianAdair]

Well I got it all together and running.
I had some tapping noise in the top end, (valve lash)?
Had them set at .005 and .006 so I tightened them up to .003 and .004, now I can't get it to start, or stay running, I checked the point timing with a light and when it gets to the F mark on 1 & 4 it starts to fire, same for 2 & 3.

My carbs are not synced yet, So in a way seems like it is getting to much air and not enough fuel. I have 120 main jets 40 idle jets, bench adjusted the carbs with a 1/16 wire to try and get them as close as I could to being the same, air screw is 1 1/2 turns out from being seated in all the way.

Open carbs, no air box, filters, or stacks yet, I plan on running stacks. Kerker 4 to 1 exhaust.

Before I adjusted the valve lash, it would start right up, but ran kinda rough,(assuming the carbs were out of sync) and the tapping seemed a little to loud to me, and # 4 carb had smoke coming out after I killed it, too much fuel?

Any help will be appreciated!
I'm frustrated and don't know which way to go....

[scottly]

Before I adjusted the valve lash, it would start right up,
Any help will be appreciated!
I'm frustrated and don't know which way to go....

You answered your own question.

[BrianAdair]

Before I adjusted the valve lash, it would start right up,
Any help will be appreciated!
I'm frustrated and don't know which way to go....

You answered your own question.

So i need to adjust them back out to .005 and .006?
That's not too loose? sure seems to be loud as far as the tapping noise... I have never heard one of these bikes run, so I don't know how loud the engine (top end tapping) should sound anyway...