TR 3 timing and carb adjustments

[RonR]

I have just helped a friend rebuild his carbs on a TR 3, which has a Cramer electronic ignition. As far as we can tell, there is nothing to adjust with electronic ignition.

The car pops, or misfires, I guess, sometimes upon acceleration.
We need help in determining the correct setting for the timing. The service manual says 4 degrees BTDC but does not say if this is static or at some rpm. The service manual also does not list a maximum vacuum advance. I know some British car owners prefer to set the timing for the maximum advance and let the timing at idle be whatever it is.

Finally, the gas pedal will not allow the butterflies to open completely when the gas pedal bottoms out on the stop bolt. Will changing the length of the throttle linkage just below the point of attachment to the front carb throttle shaft adjust this? If so, should it be lengthened or shortened?

Thanks,
Ron Ray

 

[Moseso]

I can't help with the ignition thing, other than to say that the 4 degree setting spec. (I set mine at about 6) is static.

There are three places to adjust the throttle linkage:
1) the link you mentioned.
2) the link from the firewall to the bottom of the bellcrank on the manifold
and
3) the pedal stop screw you mentioned.

Shorten either of the links to get more throttle opening. Watch for clearance issues.

 

[TR3driver]

The maximum vacuum advance changed over the years and for different markets; plus it's doubtful that your friend still has the original advance module. If it is a genuine Lucas advance module, you may be able to find the specifications stamped on it, as three numbers separated by dashes, eg 7-14-10. The first two numbers are vacuum, in inches of mercury, where advance starts and stops respectively. The last number is the maximum advance in camshaft degrees. So the above module would give 20 degrees advance at the crankshaft, at 14 inHg.

But I wonder if you aren't thinking of mechanical (centrifugal) advance rather than vacuum advance. It's much more important (vacuum advance is basically only to improve fuel mileage at part throttle); and it's common in performance circles to set the total advance (= initial plus full mechanical).

Two more things to check for the pedal adjustment:
1) There is a joint in the shaft that runs across the compartment behind the engine. If it is loose, the pins may be about to break.
2) The lever where the linkage attaches to the front carb just has a pinch bolt to locate it on the shaft. It may have slipped.

I've never heard of "Cramer" ... could it be "Crane" instead? If so there is an adjustment, which is the location of the pickup within the distributor. I had a terrible time getting that right when I installed a Crane on my TR3A. I also eventually discovered that my rotor had a "counterweight" molded into it, that was hitting the pickup assembly and apparently jarring it enough to cause problems, only sometimes.

 

[RonR]

Thanks for the information.

I will check each throttle adjustment point.

I failed to mention that at some point, someone removed the vacuum line that connected the distributer to the carburetor. I assume we need to order one and reinstall it.

I believe Crane is the correct name. Sorry for my error. I did notice an adjustment screw for the pickup within the distributer but I was not sure of how to determine the correct position. Would this simply be placing No. 1 cylinder at TDC and ensuring the pickup aligned with the space in the rotating plastic disk?

As far as advance, what information I have came some time ago from an Austin Healey owner who sent me the following information from the Healey email list:


It is important to set ignition timing at an rpm where maximum static plus centrifugal advance (vacuum advance not connected) is all of the way in. Depending on the particular distributor, this could be anywhere from 4,000 to 5,000 rpm. Most engines will tolerate between 32 & 36 degrees of total maximum advance, depending on engine tune, fuel grade, & altitude. In other words, set for maximum advance & let the idle advance fall where it may.

It seems that many folks still insist on setting advance at idle & letting the maximum advance fall where it may. This method is not precise. The distributor's initial centrifugal advance curve is very steep (advances quickly) at low rpm. Only a small error in the rpm that the advance is set at can make a very large error in the total advance at high rpm.


My understand is that the Austin Healey shop manual list a specific maximum advance at the crankshaft (35 degrees BTDC for the Healey 3000 Mk. I engines) and that my friend sets his timing at this position and never bothers to check the static or idle timing...

Your thoughts?

Ron

 

[6TTR3A]

I'm at sea level; I set mine (TR3A, 87MM, mild cam, DM2 #40698 distributor, Petronix LU142A) at 31 BTDC degrees at 3000 RPM. That was 20K ago with no problems!!
FRank

 

[TR3driver]

The Crane instructions actually say to cut a hole in a distributor cap, and use a timing light to verify the rotor position. I didn't like that idea, so instead I timed the distributor with points, then installed the pickup and moved the pickup location until the timing was right (without turning the distributor or vernier).

As far as timing, I've always followed the book procedure, which is to use the 4 BTDC static as a starting point, then set the timing so the engine will just barely ping with heavy throttle at 1500 rpm. I prefer to play safe, so I usually back off a couple of degrees from that point.

I'll agree that the advance curve isn't all that great (especially if it's the original 50 year old one); but the maximum advance is set by a big steel stop. The relationship between full and no advance is pretty precise, IMO. At any rate, I've been using my method for well over 200,000 miles (not all on the same engine) and I've not seen any problems with it.

I did break a piston once, but that was a different problem that was causing inaudible (to me) knock at light throttle and freeway speeds.

 

[RonR]

What linear distance around the outside diameter of the crankshaft pulley represents 4 degrees BTDC?

Or, does anyone have a pulley off the car and could accurately measure the diameter for me?
I can calculate the distance for any degree if I knew the diameter of the pulley.

Thanks.
Ron

 

[TR3driver]

I'm not sure that "accuracy" belongs in the same sentence with 50 year old sheet metal ... but I just checked a pulley half (the side with the TDC hole) from the parts bin, and 6 or 8 different places were all somewhere between 5.520" and 5.530".

That makes 4 degrees be about 3/8", which is what I remembered.

 

[Geo Hahn]

The Crane instructions actually say to cut a hole in a distributor cap, and use a timing light to verify the rotor position...

 

[Moseso]

:lol:

Is that a picture of your distributor cap with the rubber plug removed, Geo?