• Hi Guest! Being a Paying Supporter is NOT a requirement to participate at BCF. However, subscribing will help ensure we can continue to serve the LBC community for years to come. If you enjoy BCF and find our forum a useful resource...Please consider supporting us by **Upgrading**
    (PS: Subscribers don't see this banner)
Tips
Tips

TR2/3/3A Measuring manifold vacuum on TR3A

TuffTR250

Jedi Warrior
Country flag
Offline
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!

IMG_5243.jpg IMG_5244.jpg

Regards,
Bob
 

Fat_Dog_Garage

Freshman Member
Offline
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.
 
OP
T

TuffTR250

Jedi Warrior
Country flag
Offline
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

Yoda
Offline
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.
 
OP
T

TuffTR250

Jedi Warrior
Country flag
Offline
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

Yoda
Country flag
Offline
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

Yoda
Country flag
Offline
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.
 
Similar threads
Thread starter Title Forum Replies Date
jfarris TR2/3/3A Measuring and honing cylinders Triumph 5
K SU float bowl....measuring the height? Triumph 13
Morris Measuring Combustion Chamber CC Spridgets 14
L TR2/3/3A TR3A - measuring operating temperature w/o gauge? Triumph 12
tomshobby water pump part 2, inspecting and measuring Triumph 5
Bruce_B Measuring Crankshaft end-float questions? Spridgets 20
Darwin Measuring spring arch Triumph 4
YankeeTR I did some measuring...it WILL fit in my garage... Triumph 26
B Measuring kingpins and bushings Austin Healey 10
Ripper01 For Sale- TR-6 Intake Manifold Triumph Classifieds 0
M WANTED......Cannon Inlet Manifold Triumph Classifieds 0
9 For Sale: Thermocarb - complete setup including special manifold Austin Healey Classifieds 1
Ripper01 SOLD-TR6 Dual Port Exhaust Manifold Triumph Classifieds 6
Ripper01 For Sale TR6 '69-72 Intake Manifold Stanpart V3108 Triumph Classifieds 0
nichola TR6 Water pump to manifold adaptor threads BSPT or BSP or ??? Triumph 17
steveg TR6 Fixing the intake manifold Triumph 8
D Wanted TR6 exhaust manifold w/air rail Triumph Classifieds 2
B AH Spares Fast-Flow Fuel Manifold Austin Healey 2
HealeyRick For Sale Bare 29F Block, Cylinder Head and Manifold Austin Healey Classifieds 2
B AH Spares Ltd. Inlet Manifold No. FUL 340 Austin Healey 2
Michael Oritt Intake manifold question Austin Healey 11
Michael Oritt Manifold Gasket Dressing/Sealant Recommendations Austin Healey 6
pbraun For Sale Weber downdraft Carb & manifold Spridgets Classified 3
M Wanted Twin carb manifold for 1500 Midget or spitfire Spridgets Classified 3
GeorgeC Intake / exhaust manifold gasket install goop? Spridgets 2
D TR2/3/3A manifold studs..locktite or antiseize Triumph 5
steve00136 MGB Intake manifold alignment MG 4
AUSMHLY Gasket, Inlet Manifold to Exhaust Manifold BJ8 Austin Healey 8
Z For Sale TR-4A exhaust manifold Triumph Classifieds 0
K For Sale TR4A Intake Manifold..Put one on your TR3...improves air intake....cheap like borsht. Triumph Classifieds 2
AUSMHLY Exhaust Manifold Flange Stud Austin Healey 13
J Bj8 intake manifold to heat shield to carb gaskets Austin Healey 4
Lin Intake manifold drain pipes Austin Healey 29
R FFuel overflowing at Intake Manifold Austin Healey 10
BLong63AH Removal of Intake and Exhaust manifold. Austin Healey BJ7 Austin Healey 24
F TR2/3/3A SU 1 3/4 Carb to Manifold Insulating Block; '59 TR3A Triumph 6
AUSMHLY BJ8 Manifold, how many brass nuts? Austin Healey 2
crispy How to Remove the Exhaust Manifold?? [late BJ8] Austin Healey 27
T why would an intake manifold stud have both coarse and fine threads? Spridgets 8
doc50 Wanted H2 SU carbs / manifold for 1275 1973 MG MIdget Spridgets Classified 3
G Gasket sequence manifold to carburetor Austin Healey 4
K For Sale Curved tube intake manifold...2" Triumph Classifieds 2
Drone Dog BT7 head manifold studs Austin Healey 7
B Gasket Sealer on Intake/Exhaust Manifold Gasket. Austin Healey 7
G TR4/4A Distributor Vacuum Manifold Hard Line Triumph 11
K Wanted 100-6 exhaust manifold for two port head Austin Healey Classifieds 0
T TR6 Refinishing Exhaust Manifold Triumph 4
T TR6 Refinishing Exhaust Manifold Triumph 6
H BJ8 blew out intake manifold plug Austin Healey 10
I Tips for installing intake manifold plugs in a stock Bugeye? Spridgets 6

Similar threads

Top