'67 Holley 3810 advice?
09-15-2017, 07:26 PM
#1
'67 Holley 3810 advice?
I need some Holly 3810 advice.... I think.
My car is a '67 with the original 300HP engine that has a huge stumble if the accelerator pedal is rapidly (not slammed) floored at engine speeds below around 1700RPM.
The engine is pretty fresh with less than 700 miles, however the cam is not a stock 300 HP cam. The previous owner admits to having the engine assembled with a "slightly higher performance" cam, (whatever that means) but no longer has the specs. (groan).
The distributor was a mess. It's the correct 1111194, but the centrifugal advance was gummy and sticky as was the breaker plate. I disassembled distributor, noticed that there was no side play (I could tell that the bushings at once upon a time been replaced), dug all the junk out of the lube well that's under the breaker plate, thoroughly cleaned everything, packed the lube well with gauze and soaked it with 30W, installed a new gear, set the end-play to around 11 thou. (it was 17), lightly lubed the weight pivots with moly-graphite grease, installed a new B26 canister and Pertronixed it. Yes, I used the 12V side of the resistor to power the Pertronix. After installing it, I am now able to get a steady and reliable idle and the car runs a whole bunch better (cooler too) ....HOWEVER the doggone stumble (although not QUITE as severe is still there.
Best idle is never going to be the 300HP's 500 RPM. ("slightly higher performance cam"), but it idles well at 650 – 700, more like an L79. Best idle vacuum is about 15.5Hg which is why I installed a new B26 canister that is 'all in" at just under 12Hg (the 2" rule).
So now, I think it may be the carb. The highway gas mileage at 60-70 MPH cruise is around 18MPG with the 3.36 rear axle, around town driveability is excellent, and the pipes aren't very sooty so I think the jetting is probably correct here at close to sea level. No, I haven't gotten into the carb yet, but according to the specs for the 3810, the power valve is a 6.5. Hmm, The 'rule' for the proper power valve is to divide max idle vacuum in half and round up to the next .5Hg which means that an 8.5Hg would be the 'right' power valve? ...I think.
I am also thinking that the secondaries might be opening to soon, but I have NO idea which spring to use for the secondary valves diaphragm ...or how to identify which one is which.
Anyone's experience or advice here would be greatly appreciated.
Thanks
My car is a '67 with the original 300HP engine that has a huge stumble if the accelerator pedal is rapidly (not slammed) floored at engine speeds below around 1700RPM.
The engine is pretty fresh with less than 700 miles, however the cam is not a stock 300 HP cam. The previous owner admits to having the engine assembled with a "slightly higher performance" cam, (whatever that means) but no longer has the specs. (groan).
The distributor was a mess. It's the correct 1111194, but the centrifugal advance was gummy and sticky as was the breaker plate. I disassembled distributor, noticed that there was no side play (I could tell that the bushings at once upon a time been replaced), dug all the junk out of the lube well that's under the breaker plate, thoroughly cleaned everything, packed the lube well with gauze and soaked it with 30W, installed a new gear, set the end-play to around 11 thou. (it was 17), lightly lubed the weight pivots with moly-graphite grease, installed a new B26 canister and Pertronixed it. Yes, I used the 12V side of the resistor to power the Pertronix. After installing it, I am now able to get a steady and reliable idle and the car runs a whole bunch better (cooler too) ....HOWEVER the doggone stumble (although not QUITE as severe is still there.
Best idle is never going to be the 300HP's 500 RPM. ("slightly higher performance cam"), but it idles well at 650 – 700, more like an L79. Best idle vacuum is about 15.5Hg which is why I installed a new B26 canister that is 'all in" at just under 12Hg (the 2" rule).
So now, I think it may be the carb. The highway gas mileage at 60-70 MPH cruise is around 18MPG with the 3.36 rear axle, around town driveability is excellent, and the pipes aren't very sooty so I think the jetting is probably correct here at close to sea level. No, I haven't gotten into the carb yet, but according to the specs for the 3810, the power valve is a 6.5. Hmm, The 'rule' for the proper power valve is to divide max idle vacuum in half and round up to the next .5Hg which means that an 8.5Hg would be the 'right' power valve? ...I think.
I am also thinking that the secondaries might be opening to soon, but I have NO idea which spring to use for the secondary valves diaphragm ...or how to identify which one is which.
Anyone's experience or advice here would be greatly appreciated.
Thanks
09-16-2017, 05:55 AM
#2
IMO, you should look at the accelerator pump circuit for your stumble, start with the pump cam for wear and the pump check valve adjustment located above the diaphragm. You may end up changing the squirter nozzle size from the stock .025 to something larger but make sure it's adjusted right first.
The Holley recommended method for choosing a power valve simply assures that the valve will not be open during street driving when the carburetor is running on the main circuit. The main circuit starts at rpm's above 2000, it's when the opened throttle blades allow air to flow through the booster venturi creating a fuel signal.
The best method selecting a power valve is to drive the car at 25, 35, 45, 55, 65 mph and using a vacuum gauge record the lowest vacuum at those cruise speeds, down hill up hill etc. Select a valve that opens approx 2" vacuum less than your lowest vacuum reading, that assures that you will not have a rich mixture while cruising on the main circuit and power enrichment is provided when the vacuum level drops to the point where a richer mixture for power is needed, the 8.5 or 9.5 valve may be good.
Good luck on your stumble, these problems can drive you nuts because for a carburetor to perform right all the circuits need to be working correctly because they all overlap each other.
The Holley recommended method for choosing a power valve simply assures that the valve will not be open during street driving when the carburetor is running on the main circuit. The main circuit starts at rpm's above 2000, it's when the opened throttle blades allow air to flow through the booster venturi creating a fuel signal.
The best method selecting a power valve is to drive the car at 25, 35, 45, 55, 65 mph and using a vacuum gauge record the lowest vacuum at those cruise speeds, down hill up hill etc. Select a valve that opens approx 2" vacuum less than your lowest vacuum reading, that assures that you will not have a rich mixture while cruising on the main circuit and power enrichment is provided when the vacuum level drops to the point where a richer mixture for power is needed, the 8.5 or 9.5 valve may be good.
Good luck on your stumble, these problems can drive you nuts because for a carburetor to perform right all the circuits need to be working correctly because they all overlap each other.
The following users liked this post:
bowtyebob (09-16-2017)
09-16-2017, 08:26 PM
#3
Drifting
"I am also thinking that the secondaries might be opening to soon, but I have NO idea which spring to use for the secondary valves diaphragm ...or how to identify which one is which."
Bow -
Secondary springs, Holley kit (20-13) are color coded w/tiny paint dab on small end coil.
White = lightest
Short yellow = lighter
Tall yellow = light
Purple = medium light
Plain = medium
Brown = medium heavy
Black = Heavy (secondaries never open)
My '66 300HP runs best with purple, B26 can, 13.5 in Hg vacuum, but I'm at 7500' MSL. Your 15.5 seems low if you're ~ sea level, but maybe it's the cam or timing a few degrees off.
I have a few extra secondary springs from a Holley kit. Examine yours, find a color, I may be able to give you a alternative spring to try.
Bow -
Secondary springs, Holley kit (20-13) are color coded w/tiny paint dab on small end coil.
White = lightest
Short yellow = lighter
Tall yellow = light
Purple = medium light
Plain = medium
Brown = medium heavy
Black = Heavy (secondaries never open)
My '66 300HP runs best with purple, B26 can, 13.5 in Hg vacuum, but I'm at 7500' MSL. Your 15.5 seems low if you're ~ sea level, but maybe it's the cam or timing a few degrees off.
I have a few extra secondary springs from a Holley kit. Examine yours, find a color, I may be able to give you a alternative spring to try.
09-16-2017, 09:20 PM
#4
IMO, you should look at the accelerator pump circuit for your stumble, start with the pump cam for wear and the pump check valve adjustment located above the diaphragm. You may end up changing the squirter nozzle size from the stock .025 to something larger but make sure it's adjusted right first.
The Holley recommended method for choosing a power valve simply assures that the valve will not be open during street driving when the carburetor is running on the main circuit. The main circuit starts at rpm's above 2000, it's when the opened throttle blades allow air to flow through the booster venturi creating a fuel signal.
The best method selecting a power valve is to drive the car at 25, 35, 45, 55, 65 mph and using a vacuum gauge record the lowest vacuum at those cruise speeds, down hill up hill etc. Select a valve that opens approx 2" vacuum less than your lowest vacuum reading, that assures that you will not have a rich mixture while cruising on the main circuit and power enrichment is provided when the vacuum level drops to the point where a richer mixture for power is needed, the 8.5 or 9.5 valve may be good.
Good luck on your stumble, these problems can drive you nuts because for a carburetor to perform right all the circuits need to be working correctly because they all overlap each other.
The Holley recommended method for choosing a power valve simply assures that the valve will not be open during street driving when the carburetor is running on the main circuit. The main circuit starts at rpm's above 2000, it's when the opened throttle blades allow air to flow through the booster venturi creating a fuel signal.
The best method selecting a power valve is to drive the car at 25, 35, 45, 55, 65 mph and using a vacuum gauge record the lowest vacuum at those cruise speeds, down hill up hill etc. Select a valve that opens approx 2" vacuum less than your lowest vacuum reading, that assures that you will not have a rich mixture while cruising on the main circuit and power enrichment is provided when the vacuum level drops to the point where a richer mixture for power is needed, the 8.5 or 9.5 valve may be good.
Good luck on your stumble, these problems can drive you nuts because for a carburetor to perform right all the circuits need to be working correctly because they all overlap each other.
09-16-2017, 09:37 PM
#5
"I am also thinking that the secondaries might be opening to soon, but I have NO idea which spring to use for the secondary valves diaphragm ...or how to identify which one is which."
Bow -
Secondary springs, Holley kit (20-13) are color coded w/tiny paint dab on small end coil.
White = lightest
Short yellow = lighter
Tall yellow = light
Purple = medium light
Plain = medium
Brown = medium heavy
Black = Heavy (secondaries never open)
My '66 300HP runs best with purple, B26 can, 13.5 in Hg vacuum, but I'm at 7500' MSL. Your 15.5 seems low if you're ~ sea level, but maybe it's the cam or timing a few degrees off.
I have a few extra secondary springs from a Holley kit. Examine yours, find a color, I may be able to give you a alternative spring to try.
Bow -
Secondary springs, Holley kit (20-13) are color coded w/tiny paint dab on small end coil.
White = lightest
Short yellow = lighter
Tall yellow = light
Purple = medium light
Plain = medium
Brown = medium heavy
Black = Heavy (secondaries never open)
My '66 300HP runs best with purple, B26 can, 13.5 in Hg vacuum, but I'm at 7500' MSL. Your 15.5 seems low if you're ~ sea level, but maybe it's the cam or timing a few degrees off.
I have a few extra secondary springs from a Holley kit. Examine yours, find a color, I may be able to give you a alternative spring to try.
The 15.5Hg IS low for a 300. It's about right for an L79, ...I think. Like I initially wrote, the cam in this engine is a lot more like one for an L79 thn for an L75.
Both the 300HP and the L79 used the 3810. I don't know if both had the same parts, like the pump cam, shooter, or power valve.
Without a vacuum guage inside the car, I can only guess what manifold vacuum is a cruising speed. I'm guessing the it is around 11 to 14. Seems to me that 6.5 power valve needs too low a vacuum to open. going to open.
