TR2/3/3A Measuring manifold vacuum on TR3A

[TuffTR250]

I just installed 1/8 inch brass hose barbs in the hard plastic spacers between the carbs and manifold in my TR3A similar to the way someone else posted on the BCF quite a while back. I attached my vacuum gauge to the two hose barbs with a T-connection then a single hose going up to the gauge. See attached pictures. I just put the carbs on and have not tweaked them yet to get them better tuned. However, I wanted to go ahead see what vacuum I'm getting. The vacuum gauge fluctuated between 12 to 13 pounds of vacuum pressure. My engine is from a TR4 and I have no idea what cam it may have. The question I have is, what vacuum should the 4 cylinder engine have? Thanks!



Regards,
Bob

 

[Fat_Dog_Garage]

Here's a site that gives good guidance on vacuum gauges: https://www.secondchancegarage.com/public/186.cfm

Scroll down the page and you see buttons that allow you to see how a gauge will perform. Vacuum gauge measurements are taken in inches of mercury and a normal engine will hold the gauge steady at 15 - 22 inches of mercury. Your gauge is reading low and it shouldn't be fluctuating. Your's sounds like Scenario 6 with leaky valves.

 

[TuffTR250]

Thanks for the link to the gauge info. My vacuum does not fluctuate very much and slowly. I know I have the distributor quite retarded and I have not yet tweaked the carb mixture. So I'm thinking I may have scenarios 11 and 15. I did not know if the 4 cylinder engine should also have between 15 and 20. I was also not sure I'd get sufficient vacuum through the 1/8 inch hose barbs, although I've seen them used in a similar manner on other cars that don't originally have vacuum ports. I'll try tweaking the timing as well as the carbs to see if I can get the vacuum reading higher.
Regards,
Bob

 

[dklawson]

Keep in mind that some non-standard cams will produce "less" vacuum at idle. For example, the S2 cam in our GT6 produces an idle vacuum much closer to 13" than the 17-20" you might expect with a stock cam. That being said, you may see vacuum increase as you increase the idle RPM. In short, don't get too upset about the value of the vacuum readings as long as you eventually get the reading to be steady and are otherwise happy with the way the engine is running.

 

[TuffTR250]

Thanks Doug! Per recommendations from the 6-Pack forum I used vacuum readings on my TR250 to set the distributor timing. I had much better success using vacuum readings than using a timing light. My 250 has a non-stock cam therefore, as you noted, I get lower vacuum readings than "normal". On the TR3, I am planning to attempt to set the timing using the vacuum readings since I had good luck doing that on the 250. I want to tweak the vernier on the TR3 to see if/how that affects the vacuum readings, hoping for more vacuum than the 12/13 reading I got previously. As mentioned before I don't know what cam is in the TR3 although I do know the engine is a TR4 engine. For the next few days it's too cold to open the garage door to run the car so I'll be working on other things. Fortunately my garage is heated so I can still work in there even though it's miserably cold outside.
Regards,
Bob

 

[CJD]

I am not sure setting your timing for best idle vacuum will give you the best timing for driving. It makes the assumption that you have the perfect mechanical advance curve and the best possible vacuum advance curve. If the curves are not perfect, you are optimizing your idle timing while compromising the best timing for driving.

 

[martx-5]

Setting the timing using a vacuum gauge works pretty well for getting the timing in the ballpark. However, do not cinch down the distributor when you reach the highest vacuum reading, as your timing will be too far advanced. Once you get to the highest reading, back off 1-2 inches and start from there. Since all engines are different, final correct timing for your engine can only be established by road testing or a dyno run. Listen carefully for any pinging, and if in doubt back it off a little more.