My modded 66 build

[gary55]

This engine has been mounted in my bike now and has me confused a bit,I did a compression check and it only reads 140 p.s.i when the previous one read 180 p.s.i !?The only difference being I didn't trim the piston skirt on the intake side yet,And I had to raise the cylinder a little with a extra thin base gasket to get the head clearence needed!So I assume that I lost some compression there but am now currious if the piston skirt being cut to allow more duration has that much effect on the charge volume because I also found that with out the cut skirt my carb that had an 82 jet on the previous engine with the skirt cut,This one required an 8-10 jet size drop It has a 74 in it right now and it's still a little fat on top!The only other thing that I didnt do to this engine that was done to the previous is that I had filled the keyway for timming on the crank and re cut it 7 degrees retarded and I dont see that having any effect on the compression or charge volume,What I did notice from that is that the midrange power is not as strong and it fights the top end a bit!

I have noticed about 1.1 lb. per .001" variation in lowering deck height at the top. I got this by averaging gain over .030". Your differences will be greater cause of that dense Canadian air . Sense I don't know the physics of exactly what is happening with the compression this might not be a good meter as it seems to me things would be increasing exponentially at the top of the stroke.
Don,t know about the charge thing, but with no skirt trimming and raising the jug you could have something there.
It makes sense that some reduction in comp. would be seen with the smaller jet providing 10% less liquid.
Assuming you corrected the squish after raising the jug the only other unmentioned factors I'm seeing is less volume to compress due raised exhaust and possibly not as good of a seal at the rings yet. If it wasn't you posting this I would go right to the extra base gasket. with the other variations compounding as minor contributors.

 

[Street Ryderz]

I have noticed about 1.1 lb. per .001" variation in lowering deck height at the top. I got this by averaging gain over .030". Your differences will be greater cause of that dense Canadian air . Sense I don't know the physics of exactly what is happening with the compression this might not be a good meter as it seems to me things would be increasing exponentially at the top of the stroke.
Don,t know about the charge thing, but with no skirt trimming and raising the jug you could have something there.
It makes sense that some reduction in comp. would be seen with the smaller jet providing 10% less liquid.
Assuming you corrected the squish after raising the jug the only other unmentioned factors I'm seeing is less volume to compress due raised exhaust and possibly not as good of a seal at the rings yet. If it wasn't you posting this I would go right to the extra base gasket. with the other variations compounding as minor contributors.

Yes I did get the squish right thats why I needed the extra thin gasket and to have the dome clear the head!It's quite probable that the raised exhaust timing is the culpret here but it's only a half mm difference from the previous one!The intake charge volume without the cut skirt must also be reduced for such a change in jetting and if that volume changed then what gets pushed up the tranfers and into the cylinder top to be compressed would be less also.

 

[Street Ryderz]

Four stroking on the top is the fault of the C.D.I., the little black box that comes with the kit is a four stroke engine C.D.I.. They do that to cook the engine and get more parts money. Damn Chinese. Slightly different piston dome?

I dont use the stock C.D.I and It's not 4 stroking it's a slight rich signal on top not fully cleaning out!

 

[gary55]

Yes I did get the squish right thats why I needed the extra thin gasket and to have the dome clear the head!It's quite probable that the raised exhaust timing is the culpret here but it's only a half mm difference from the previous one!The intake charge volume without the cut skirt must also be reduced for such a change in jetting and if that volume changed then what gets pushed up the tranfers and into the cylinder top to be compressed would be less also.

Ya I figured you got the squish right. It must be the reduced charge and the reduction of fuel with the smaller jet along with the higher exhaust. They are all taking a little bite, but all together it's a big bite. I don't know how it would effect the combustion characteristics of the head, but their is a lot of room for a longer plug to to rob some of the volume. The more I think about it the more I realize how little I know about it. It sure seems like the reduced volume i.e. skirt was the instigator making you reduce fuel being the only thing that can't be compressed.

 

[Bike_Builder]

I dont use the stock C.D.I and It's not 4 stroking it's a slight rich signal on top not fully cleaning out!

Is your exhaust port a good 2mm bigger than the carb size? It may not be drawing enough to run that carb.
My carb is 21mm and exhaust port 23mm. that's working great for draw for me. If the carb is too big the top will never tune.

 

[Street Ryderz]

Is your exhaust port a good 2mm bigger than the carb size? It may not be drawing enough to run that carb.
My carb is 21mm and exhaust port 23mm. that's working great for draw for me. If the carb is too big the top will never tune.

So on That engine the exhaust port is 27mm across oval shaped and 15mm in height witch makes an effective port dia of 22.5 mm and my intake is 23mm across oval shaped and 14mm in height witch makes it's effective dia of 20mm. I use an 18mm intake and 17.5mm carb so no there is no issue with draw it's all steped down to get the max volume through the venturi and carb!Though I am still thinking about boring the carb body out again to 18mm should work but then that would be as big as I could go with that one!

 

[Bike_Builder]

So on That engine the exhaust port is 27mm across oval shaped and 15mm in height witch makes an effective port dia of 22.5 mm and my intake is 23mm across oval shaped and 14mm in height witch makes it's effective dia of 20mm. I use an 18mm intake and 17.5mm carb so no there is no issue with draw it's all steped down to get the max volume through the venturi and carb!Though I am still thinking about boring the carb body out again to 18mm should work but then that would be as big as I could go with that one!

You may want to try an intake tract with an I.D. smaller than 17.5mm to speed up the mix between the carb and intake port. I run an 18.5mm itake tract to speed it up.

 

[Street Ryderz]

You may want to try an intake tract with an I.D. smaller than 17.5mm to speed up the mix between the carb and intake port. I run an 18.5mm itake tract to speed it up.

Well it's stepped the way it is for a reason!The port being the largest volume drawing through the smaller intake and then smaller carb keeps the metered a/f maxed through the carb but as that a/f charge travels from the carb to the port it slows down as it hits the larger volume intake and again at the port,when the a/f 's flow rate see's an area with a larger volume the presure drops reducing the flow rate!

 

[butre]

Yes I did get the squish right thats why I needed the extra thin gasket and to have the dome clear the head!It's quite probable that the raised exhaust timing is the culpret here but it's only a half mm difference from the previous one!The intake charge volume without the cut skirt must also be reduced for such a change in jetting and if that volume changed then what gets pushed up the tranfers and into the cylinder top to be compressed would be less also.

the half millimeter higher exhaust port hurts compression big, but not quite that big. I suspect you're leaking compression somewhere, likely the rings or around the head mating area, possibly at the crank seals. I did a big write up on the difference between dynamic and static compression a while back, including the formulas needed to calculate what your potential compression ratio should be.

if you post your crankcase compression ratio, static compression ratio (or head volume), and the exact distance from the top of the deck to the highest point in your exhaust port I can tell you what your theoretical cranking PSI should be.

 

[butre]

also you may be losing compression from bad transfer port geometry, but that's not tremendously likely if you haven't touched them