Cam Timing

[Don Jones]

Pulled the heads off the 71T motor without pulling the distributor or disassembling the heads. Checked the valve lash after I was finished and it was fine - no adjustment needed. I take this to mean the Cam Timing went back together right on the money?
Got fire spitting out both carbs and as the engine hadn't ran in 20 years, there is a good possibility the problem actually is the Carbs. Need to eliminate some of the other possibilities first......

[Flat6pac]

Dial guage to spec is the only way to know. I have a doubt you hit spec using SWAG system.
Brie

[Don Jones]

Used a Dial Guage and spent lots of time on the Cam Timing. Thought process was that if the valves remained in time with the Crank then that somewhat confirms the Cam Timing to be pretty close....

[lindy 911]

Valve lash has little to do with cam timing. What matters is the initial travel the intake valve has on #1 and #4 at top dead center (TDC / Z1). The intake valve has just begun to open at TDC by a specified amount know as you cam spec.

[lindy 911]

Correction; lash must be set before measuring valve travel at TDC.

[Eagledriver]

Originally Posted by Don Jones

Used a Dial Guage and spent lots of time on the Cam Timing. Thought process was that if the valves remained in time with the Crank then that somewhat confirms the Cam Timing to be pretty close....

If the valves remained in time with crank then the cam timing is correct. The only way to check if the valves are still in time with the crank is to do the valve timing procedure. If you didn't do this procedure then you have the sproket pin in a random place and you are on a random link of the chain. You don't have to spend alot of time. You just need to time the cams.

-Andy

[Walt Fricke]

Don

The valve lash will be the same throughout the entire base circle part of the cam. Look at a cam profile - isn't the base circle about 180 degrees of the cam? So having the same valve lash just means your cam is somewhere within that 180 degree area.

Either you are using valve lash to mean something other than what the rest of us know it to be, or you are one lucky SOB if you were able to put the chains back on the cams after you had pulled the heads/cam carriers/cams off as a unit and then reinstalled them without either the crank or either of the cams having moved a fraction of a degree.

It is conceivable that, if you put the crank at TDC when you removed the cam gears, and then the head assemblies, and put the crank back at TDC during reassembly, that you might have hit the mark. The cams might not have budged. I once removed a cam gear to tighten the three 10mm headed bolts holding the seal in place (ooops, that's why it leaked). Put it back on, tightened the big nut, and checked the cam overlap timing. Wow - right on the money. Which means nothing moved. But I didn't count on that, and it might have been a fluke (though a happy one), and that was done with the engine in the car and as a much lesser procedure than you went through.

So if you measured intake TDC overlap on #1 and #4 after setting the rocker lash at 0.004"/ 0.1mm, and it was within the specified tolerance, then your cam timing is set.

[Don Jones]

Walt,
I spent lots of time with the dial indicator on the cam timing - picked the hole behind the nuts, ect. My logic here could be flawed. When the crank is at Z1, the distributor points at Cyl #1, and the Cyl #1 valves are both closed as proven by the valve lash which wasn't touched. At 240 degrees, rotor towards Cyl #2, valves closed exactly as previous cam timing/rotation, at480 degrees, Cyl #3, ect.
If the Crank matches the old Valve Lash, then it also matches the old Cam Timing?

[Walt Fricke]

Don

Buy Wayne's 911 engine book. Follow the instructions on how to time cams. It seems you are making up your own system, but you don't understand quite how it all fits together.

Nobody who assembles motors pays attention to where the distributor rotor points to time cams. You have the valve covers off, and can thus easily tell when #1 is at TDC firing (both valves closed, because both rockers are loose) and #4 is at TDC overlap (both valves slightly open and rockers tight - because the exhaust cycle is ending and the intake cycle is starting, which is why it is called overlap) and when it is at the other TDC, which is the one you want to measure intake valve lift at overlap.

The rotor mark on the side of the distributor is there so you can reinstall the distributor with the rotor pointing to about the right plug wire. To make sure you don't have the rotor 180 degrees off, you need to do the feel the rockers trick mentioned above. If you are doing leakdwon testing, or checking your valve lash, you can use the rotor position after you have had the motor running to tell wiich of the two Z1s you are on. But that's it.

The only importance of valve lash is to set it at the proper figure so you are using the same apples to apples measuring as the factory (or cam grinder) did in determining what the cam timing setting/intake valve opening at overlap should be. Increasing or decreasing the lash will move the effective valve timing a bit one way or the other, because the cam will hit the rocker a bit sooner or later. I have a motor where the lash is 0.008, and old VWs all were set at 0.006. But nothing, zero, nichts, nada, niente to do with precision setting of the cam in relation to the crank.

The distributor rotor pointing is no where accurate enough to play any role in setting cam timing. It can tell you which 360 degree point you are at with the Z1 mark, but that is it.

Wipe your mind clean of thoughts of lash measurement telling you anything.

You need to know how far the #1, and then the #4, intake valves have opened when the crank is at Z1 overlap for each respective side. Period. Nothing else. if that measurement is within spec (Wayne shows it as 2.2 to 2.7mm or the equivalent in thousandths of an inch since inch dial indicators are inexpensive, and metric ones for some reason are not?) after you have torqued the big nut down, you are set.

If you are worried that you have timed #1 correctly, but got #3 exactly wrong, first off you can check the dots on the ends of the cams. When one is on top, the other should be also. Or you can set #1 to TDC firing, when intake and exhaust valves will be closed and the rockers loose. Then turn the crank 120 degrees and go to the next cylinder in the firing order to be sure its rockers are loose. And so on. Fact is, if you get your cams 180 degrees off, I think you can't rotate the motor because valves on the "wrong" side will hit the piston, so it kind of saves you from such a mistake.

So - to cut this short: what did your dial indicator read when #1 was at TDC overlap?

[Eagledriver]

Walt means number 4 and number 1 in the above discussion.

-Andy

[Don Jones]

Guys,
Last fall I spent a week setting up the Cam Timing using Wayne's book for a guide. No one wants to have to get back into it after the engine is back in the car. I was very careful and ultimately it seemed that everything fell back together the way it should have ie; #1 intake valve wasn't found to be half open at Z1 timing mark.
Car is backfiring through the carbs (Zeniths) so I'm trying to decide if I focus on ignition or induction ??????

[lindy 911]

Well, if you're sure the cams are right, I would first check the ignition timing and see to it that it's set correctly (around +5 degrees at idle and +27-30 at 4,000 rpm. / check your specs).

Backfiring through carbs is a sign of being too lean. Do you know how to setup the carb adjustments? Do you have a carb air flow meter? Performance Oriented has a nice write-up on how to set up and fine tune Webers.

[Don Jones]

Well, I spent a lot of time on the cam timing following the Dempsy book to the "T". Even now as I reason my way through the trouble shooting, I'm still confident that the internal timing is not the cause of the carb firing.
I've regulated and filtered the fuel supply but beyond that, the jetting isn't giving me the control they should. The jets are clean but I could have missed a passage somewhere. I put my hand over each carb feeling the airflow and most of them spit fuel on to my hand. They seemed flooded more than starved (which is what steered me back towards the timing). Unlike other parts of the rebuild, carbs seem to be considered more of an Art than a Science. :~)

[Don Jones]

I forgot to share that the Timing light showed things to be pretty close. It won't set and idle but the timing falls towards 5 deg. and advances towards 30 when I rev it up. Not ready for the racetrack but probably close enough where it should run a little.

[greenspeed]

bent valve?, air leak in inlet?, plunger in accelerator valve stuck (happened to me on a webber dcoe once)?, water in the carb bowl?, plugs fouled?
did you do a compression test?

[Walt Fricke]

Carbs spit fuel back up into the air filter because cam overlap reverses the flow of air. Big race cams with big overlaps (my race cams have settings from 5.6 to 6.2mm, which means the intake valve is around half way open at TDC) do this at low RPMs, where you don't much care.

I've no experience with a T motor or its cam, but someone out there does and can say if this is or is not typical.

[lindy 911]

I think you might be a little confused. A GE 60 cam has almost .500" lift total. The TDC overlap spec is 5.2 MM or .204". With 46 Webers I can get a nice smooth idle with no popping through the carbs. It's all in the idle setup of the carb. If it pops back through the carb, it's lean and you can adjust the mixture screw out to add more fuel. But you need to have the cabs in balance first and the air correction screws set. Again, Performance Oriented has a great write up on this procedure and its free on their site.

[Walt Fricke]

I'm often more than a little confused. I must have been thinking of reversion generally. My 2.7 race motor with Elgin 315 cams and Weber 46s created fuel stains on the underside of the watershields. I assume that was from a cloud of fuel traveling upward and hitting it.

But, come to think of it, the motor idles fine when warm. So do my 2.8s with DC80s. And doesn't pop or spit even when cold.

[lindy 911]

Sorry Walt, I didn't mean you were confused. I think the OP was confused as to his symptoms. Usually though, with big overlap, the exhaust scavenging effect has created a vacuum in the cylinder before the intake valve starts to open so by the time it's at TDC the intake charge is on it's way in.

[Walt Fricke]

At least we hope it is. Searching for the holy grail of VE>100%.