Front cylinder low compression + blow-by in rocker cover + pushrod tubes

[panama86]

Thanks for taking the time ratedspeed to put that together.

I'm going to head down stairs in a minute and measure what I am getting. Just a quick question, that BDC measurement of 5 1/6", my measurement yesterday at the cylinder wall at BDC was only 3 1/2". How are you getting that 5 1/6" measurement? Same with the 4 9/16" measurement. Also is your motor stock cam 74"?

[Ratedspeed]

Panama,

Sorry for the confusion, BDC = 5 1/16"
The piston sits 1 1/16" below the spark plug top thread, so there's approximately 4" or 3 31/32" piston travel.

[panama86]

Ok so i've gone down to measure everything. Also using a 1/4" straw up against the D-ring, my D-rings have a lip on them so its hard up against that.

I took all my measurements with 0 being the highest point of piston travel (which is actually 31/32" from top thread of plug hole with dot in the window).

That stroke is still a bit of a weird one, shy of the 3 31/32" I expected but again that could be due to the method I'm using to measure it. I'm not sharp enough to figure out what degrees they would correspond to.

TDC = 0
BDC = 3 13/16"

Combustion stroke
Exhaust on ramp = 1 13/32" ATDC

Exhaust stroke
Exhaust at full lift = 3 1/8" BTDC
Intake on ramp = 3/4" BTDC

Intake stroke
Exhaust lash = 1 11/16" ATDC
Intake full lift = 2 15/32 ATDC

Compression stroke
Intake lash = 1 21/32" BTDC

[Ratedspeed]

Panama,
Can you verify if all your measurements are direct readings from top thread to piston or did you subtract the 31/32" space between the top of piston to top thread on some measurements?
The way it stands now you have way to much overlap if your intake is on the ramp at 3/4" btdc ( early ) and your exhaust is still open until 1 11/16" atdc.( late )

Also, if your intake is actually closing at 1 21/32" btdc there isn't any time to build compression.

Lastly, if BDC is 3 13/16" you could be running a EL bottom end 3 1/2" stroke ( 1000 cc ). Years ago I ran a 1948 EL with 74" jugs and had no problems.

Calling Nifty !! What are your thoughts?

Mike

[RooDog]

Lastly, if BDC is 3 13/16" you could be running a EL bottom end 3 1/2" stroke ( 1000 cc ). Years ago I ran a 1948 EL with 74" jugs and had no problems.

Mike

Mike....
On the 61/74 mix up: Did you use stock 74 pistons, and how was the compression height? How well did it run?
I know that 61 jugs are about 1/8" shorter than 74 jugs, so I was wondering if the pistons came up high enough for adequate compression....
....RooDog....

[Ratedspeed]

Roo,
I wish I had more specifics, but this was in the 70's and I bought the bike already modified, and I'm sure you know the 70's was more about riding and only wrench enough to keep riding. The only changes I made was a sifton cam and a shovel oil pump, from what I recall anyway.

[panama86]

Ratedspeed,
All those measurements are with the 31/32" subtracted. Looking at how long my exhaust was hanging open for made me go back to confirm and it really is staying open that long.

On the 61/74 side of things. I think my stroke will actually be 3 31/32 I just can't measure it accurately enough through the plug hole. At least I'm hoping so. The bike has been running alright since I've had it on the road but I've had persistent issues with the top end. Something is up with it.

I doubt it's of relevance but when I had my heads done a little while ago I noticed they used kibblewhite 80" shovel valve springs. Not sure why they did this. The specs match up with that of the stock springs as far as I can tell. I was wondering if they were catching or hanging up somehow but there was no signs of that when I had the head off. Other thing I'm considering is that swap I did for the sifton 412. I wonder if the cam I was given was for something else. This is all a bit over my head.

[Ratedspeed]

I think the next step would be to perform the same check on the rear cylinder since it has good compression and lets compare.

[panama86]

Rated I just went down again to double check my measurements from yesterday and also measure the rear cylinder, I wont bother posting all of those figures because the one that says it all is the front intake closing vs rear intake closing.

I decided to make things a little more precise I would set up a dial indicator on the intake of both the front and rear so I could see exactly where in the stroke the valve was completely seated.

The front intake only fully seated at 13/16" BTDC on compression!
The rear intake fully seated at 3 3/8" BTDC on compression

I can't believe the bike started and ran as well as it has with it as it is. I just rode it a good 1000 miles on it and while it was hard to start it seemed to run pretty well and didn't notice much out of the ordinary. This definitely explains my compression issues.

I guess from here it's a matter of checking the cam timing, I'll head down now and rip the cam cover off to see what's going on. Could it be that it's actually the wrong cam for the rest of the internals and/or out of time? I am sure I had everything lined up as it should be when I had it out last. And it was timed correctly when I pulled it out after the builder had first installed. I have always had dramas with this front cylinder, makes me wonder if this has been a long standing issue with this bike.

[Ratedspeed]

Loosen or remove the pushrods before you remove the cam cover to safeguard against loading the screw on the idler gear. If you forget and try to rotate the flywheels with the cam cover off and the pushrods installed the load on the cam will bend the idler gear screw.
If you pull the cam, take a pic of the lobes.

Mike

[panama86]

So popped the cam cover off. This how everything looked when I pulled the cover with front cylinder at TDC compression.

With front at TDC compression this is how it looks

IMG-20221216-WA0000.jpg (237.04 KiB) Viewed 157 times

And if I roll it over some everything lines up but at that point the exhaust valve on front is at top of it's travel.

IMG-20221216-WA0001.jpg (338.21 KiB) Viewed 157 times

[RooDog]

The timing, actually alignment, marks on the cam, etc, have no relationship to the other event timings. It would be so much easier if it did, and one could go ahead and set the spark, but it ain't, so move on. Can you pull out the cam and see what the maker's nark is?

[panama86]

looking at this I can see that the pinion gear may have been clocked incorrectly on the splined shaft. That gear has never been removed so it would have been in that position the entire time this motor has been in the bike.

Wondering what other issues this may have caused. I've had issues with oil leaking from the top of the mag drive shaft. As well as a lot of other leaks and also pretty quick build up of carbon in the combustion chamber and heads.

What is the correct orientation of this gear on the shaft? The cam in there is H grind which I understand to be stock specs.

[PanPal]

perhaps the cam gear is clocked wrong.

[panama86]

Yeah Panpal, I can see now the pinion gear can only go on one way so it can't be that it was clocked incorrectly.