View Full Version : 2.5L Carb and Ignition Questions

06-07-2009, 08:10 AM
Hi guys,
I have a '74 TVR 2500M with the TR-6 engine, and have a couple questions the TVR guys couldn't answer. I bought it in non-running condition and have just recently got it running. The carbs were rebuilt by Apple Restorations (I think that's the name) and never run before I bought the car.
I did a complete tune up and the car runs smooth. My questions:

1. I have not been able to sync the carbs. There is a huge airflow difference between the front and rear carbs that I can't adjust out. If I check the airflow at 1500RPM, it is equal in both carbs. I tried sync'ng them with the sync tool and by ear, and wasn't successful. The engine idles and runs smoothly, has good compression and pulls a steady 18" of vacuum. I've never driven a stock 2500M before, but the car does seem a little slower than I'd expect for a car with this power to weight ratio, but that's the only thing that might point to a problem. Otherwise it runs great. Any ideas as to why the carbs won't sync?

2. The distributor has a vacuum retard, but no vac advance. The retard is disconnected.
The only change from stock is that I installed a pertronix ignitor. I'm getting 10 degrees of mechanical advance, and am running 15deg of initial advance. I don't have a lot of experience iwth these engines, but do with other types of engines. From my experience, an engine like this should be running a lot more total advance than 25 deg. I'm thinking more like 35 deg total. Looking at the advance mechanism, everything looks like it's working fine. I could change the springs and get the advance in earlier, but it seems that unless I were to modify the weights, there's no way to get more mechanical advance. So my question is, what is the appropriate timing curve for a stock 2.5L engine? I'd appreciate any insight you can provide.

06-07-2009, 04:17 PM
I don't know why you can't synchronize the carburetors but I would suspect a vacuum leak.
For the timing, reconnect the vacuum retard line and reset your timing to 10 degrees advanced at idle or wherever you think it runs best. Now when you come off of idle you will loose the vacuum which will allow the advance plate to rotate enough to get you to about 20 degrees and then at higher revs the mechanical advance will do it's thing and take you out to around 35 degrees.
I did this to my '74 TR6 and it helps the performance immensely.

06-07-2009, 07:57 PM
Dunno if your TVR would have this, but a 74 TR6 has a linkage from the throttle to the float bowl vents. That linkage might be propping one throttle open at idle.

For cars before EGR, the total advance was limited to hold down on NOx emissions. But 10 degrees does seem low (assuming you mean crank degrees). If you pull the point plate out, you should be able to see the advance number stamped into the arm that limits centrifugal advance.

06-07-2009, 10:43 PM
Total advance on my 73 TR6 at about 3K RPM is about 35 degrees which is correct.
Lack of synchronization success seems strange, assuming no glaring vacuum leaks then are you sure the valve gear is all in order ie tappet adjustment, valves ok, rocker shaft ok? I'm thinking there must be some difference in the vacuum being pulled on one carb vs the other.