Could anyone please tell me the standard jet sizes (primary and secondary) for the 715cfm Holley on a stock '67 GT350 4-speed?
My car smells like it's running very rich and I want to try some smaller jets in the primaries. I'm not sure if what's in the carb is correct for sea-level tuning.
Many thanks
David
If this is a recent occurrence then you need to confirm that the power valve is still good. If the diaphragm ruptures you will leak a reservoir of fuel directly into the intake manifold. It's a VERY common failure.
I'm pretty sure the primary jets are 67's. I can't remember what the secondaries are though. Secondaries may be 78's.
Yes I did wonder about the power valve. Do you happen to know which power valve should be in the carb from the factory?
To be clear, the car starts and runs great. It's only the smell that has alerted me to a potential problem. I've pulled the plugs and they look ok (sooty round the edges but clean/light colour on the tip). I realise that plugs can't be read on modern fuel like we did in the old days so this might be misleading ! 😁
Thanks for your help
David
You probably know this, but jet size will have little to no effect on idle richness. Of much more importance to idle mixture is fuel level - which usually needs to be lowered from original specs due to the "lighter" quality of todays fuel. Of course power valve leaks and vacuum leaks can mess up your idle too.
Kurt.
Quote from: Skidado on July 12, 2019, 05:52:37 AM
Yes I did wonder about the power valve. Do you happen to know which power valve should be in the carb from the factory?
To be clear, the car starts and runs great. It's only the smell that has alerted me to a potential problem. I've pulled the plugs and they look ok (sooty round the edges but clean/light colour on the tip). I realise that plugs can't be read on modern fuel like we did in the old days so this might be misleading ! 😁
Thanks for your help
David
There can be various degrees of a leak. It depends on the size of the rupture of the diaphragm. You can only check that with a power valve testing tool. Out of 12 new ones, I had six that were bad out of the package.
A strong running engine will mask a small leaking power valve. The original in that carb was a 7.0 but a 6.5 is fine.
The issue with float levels is not only the ethanol but too much fuel pressure. The Holleys were designed to run on 4.5 to 5.5 psi. If you run more then that then you need to lower the floats to compensate and as a result you will not get enough float drop.
Diagnosing from afar is not a good idea. My experiences with the current state of power valves would immediately send me in that direction.
One other thing, you CAN NOT let a Holley sit for a month or so and let the fuel evaporated in the bowls.
For one thing the inlet valves will stick open. The other thing is the diaphragm on the power valve will dry out and crack.
67 jet 8.5 power valve. Original bowl gaskets were cork and easily over tightened , warping metering blocks and bowls that create idle issues as well. Modern coated "paper" gaskets don't always seal well on warped metering blocks.
Randy
Wow - thanks for all this great info! This is why I love this forum.
My rich running problem is not so much at idle. I've got a good steady idle at about 750 rpm and although it's a bit smelly it's not bad. The problem starts at cruising speed and is really bad if I nail it.
I agree that leaving the float bowls to evaporate isn't good, but this happens overnight and doesn't in itself appear to cause any problems.
I will pull the carb and see what jets and power valve it's running and work from there.
Thank you as always
David
Try a drop to 65 jets in the primary and a 6.5 power valve. IF your idle mixture screws are "responsive" to change now , the metering block is OK and likely the power valve too. If the jet change causes a "surge" at cruise speed , you have gone too lean and the 67s will need to go back in. One last thing is to check the air cleaner element. If it's paper and you can see light through it , it's probably OK. if you have a K&N style , you could have a serious restriction despite it being clean. I had that problem recently. Car was running bad and when I pulled the air cleaner lid off the engine smoothed out. Set the lid back on and it ran rough. Pulled the element and put the lid back on , all OK. Put a new paper element in and was still good.
Randy
For what its worth: Rebuilt by Pony Carbs in January of 2002.
Primary 71
Secondary 80
10.5 power valve.
Runs great so save your comments......
The 715 metering block has a small ridge I assume is designed to help the gasket to seal the parameter. If a metering block is warped, can it be "fixed" by milling it a couple thousandths until it is "flat"? Understand this will eat the ridge off, but modern billet plates are milled and do not have the ridge.
Thanks
you need to put a straight edge on the metering block. Usually all that you need to do is use a flat bast ard file on them. It will show right around the blot holes if it is warped.
Sometimes the "air corrector" get plugged. You need the check them and if need be clean them out with a numbered drill and a pin vise.
Keep in mind that the 715 is a 427 carb. It is super rich to begin with on a 289.
Ok so I got some time to work on the car today - with curious results -
First I drove about to warm the engine. All working well as normal. Then I fitted a vacuum gauge to the manifold and found it to be running at about 10in Hg. This was the first surprise as the engine is pretty much stock. I don't know what cam is in it, but it's a pretty mild flat tappet cam.
Based on 10in Hg I would expect a #45 power valve. I pulled the primary side float bowl and the power valve was... unmarked! I remembered that I had rebuilt the carb many years ago and fitted the power valve that came in the rebuild kit. I was young and inexperienced and just assumed it would be correct. Don't hate me... :-)
So I dug out the power valve that had been in the carb when I rebuilt it and found that it too was unmarked! Now what...?
I decided that since the carb was apart (and I had noted the primary jets were 68s), I would refit the 'old' power valve. I noticed that this old valve had a much stiffer spring than the currently-fitted valve. I'm struggling to think whether this makes it open sooner or later - depending on whether the spring holds it open or closed.
Anyway, I fitted the old valve that I had removed years ago (the one with the stronger spring) and I think there is a subtle improvement in smell, throttle response and smooth running at cruising speed. It's so slight that it might just be my imagination but I don't think so...
One other observation - with the engine running at idle, I disconnected the vacuum gauge allowing extra air enter the manifold. The engine speed went up noticably! I assume this to mean that it is running rich at idle, but it responds well to closing down the idle jets in the side of the metering block. Not sure what to think about this...
Anyway, a quick drive around the block confirmed that the car is running a little better with the old power valve fitted. I'll see how it goes on a longer journey when I get the chance.
Any thoughts or comments gratefully received.
David
The spring opens the valve, the vacuum holds it closed against the spring. So a stronger spring will open at a higher vacuum and weaker at lower vacuum. The valve is CLOSED by vacuum at the designated value of the valve.
https://www.youtube.com/watch?v=RvlmJoeIcpw
So PV has no effect at idle, unless PV is ruptured or PV is way too small for the engine vacuum and opening way before it should. If engine pulls 13 inches of 'vacuum", PV should be a 6.5. However, if engine pulls 13" and a 3 or 4 PV is installed, lot of fuel will be pouring into engine at idle?
QuoteIf engine pulls 13 inches of 'vacuum", PV should be a 6.5. However, if engine pulls 13" and a 3 or 4 PV is installed, lot of fuel will be pouring into engine at idle?
At first glance that seems right, but the opposite is true - vacuum would have to get down to 3 or4 before extra fuel added.
Ok. At idle, engine has highest vacuum reading and power valve is kept closed by the spring and vacuum. When throttle plates are opened under acceleration, vacuum decreases and spring tension is decreased allowing fuel to get sucked thru the "small" end of PV and exit into the two holes in the metering plate that eventually go to the main jets effectually increasing jet size by like 4 numbers?
Am I getting there?
Way too low of idle vacuum. Check the power brake booster. The diaphragm eventually rots out.
QuoteOk. At idle, engine has highest vacuum reading and power valve is kept closed by the spring and vacuum. When throttle plates are opened under acceleration, vacuum decreases and spring tension is decreased allowing fuel to get sucked thru the "small" end of PV and exit into the two holes in the metering plate that eventually go to the main jets effectually increasing jet size by like 4 numbers?
Am I getting there?
Yeah - that's the ticket. Except that spring tension doesn't decrease, the lower vacuum isn't enough to keep spring from opening valve.
Would seem engine with somewhat lower vacuum and a "large" PV would have bit too much fuel due to PV spring keeping valve in open position.
Sorry about the PV discussion but it relates to jet sizing.
Quote from: mygt350 on July 13, 2019, 07:10:27 PM
Ok. At idle, engine has highest vacuum reading and power valve is kept closed by the spring and vacuum. When throttle plates are opened under acceleration, vacuum decreases and spring tension is decreased allowing fuel to get sucked thru the "small" end of PV and exit into the two holes in the metering plate that eventually go to the main jets effectually increasing jet size by like 4 numbers?
Am I getting there?
The spring and the vacuum surely work in opposition to each other? Doesn't the vacuum act on the PV diaphragm to pull the valve closed and the spring tries to force it open? When the vacuum drops the force of the spring overcomes the vacuum and the valve opens. I don't know that this is right, but it seems logical.
However, counter to this logic is the fact that I swapped a PV with a light spring for one with a heavier spring (suggesting that it should open at higher vacuum) but the mixture seems leaner than it was. I'm beginning to suspect that the PV with the light spring was faulty.
Two other questions remain -
1) while the idle adjustment screws remain effective, why does the idle speed increase when I introduce more air into the inlet manifold, suggesting I'm running rich at idle?
2) why is vacuum so low at idle? I don't think this is a brake booster issue as that was rebuilt by Jim Cowles just a few years ago and the brakes work as they should. I suspect there's a leak in the rubber pipe I use to connect the vacuum gauge as I'm sure I've seen 14" vacuum in the past.
Lots more investigation to be done when I get a moment to spare!
Onward and upward!
David
I just went thought a problem with my dual quad FE. If you have more than 6lb fuel pressure at idle it bumps the needle off the seat. You wind up with a rich condition. Holley had two different size air bleed holes drilled as well as the throttle plates where in two different locations so it couldn't be set correctly. I sent my carbs to Drew Pojentic and they were straighted out and ran OUT OF THE box from him. I have never seen that happen, ever. I think there are a few that don't understand their operation and a leaking PV can absolutely cause a very rich condition. Ask me how I know.
I can put you in contact if need be.
Here are my corrected carbs running on his 427 Galaxie.
He's on FB. Tell him Capt Keith sent you.
https://www.facebook.com/AirFuelSParkTech/videos/1231150237067129/
Quote from: 1967 eight barrel on July 24, 2019, 05:42:41 AM
I just went thought a problem with my dual quad FE. If you have more than 6lb fuel pressure at idle it bumps the need off the seat. You wind up with a rich condition. Holley had two different size air bleed holes drilled as well as the throttle plates where in two different locations so it couldn't be set correctly. I sent my carbs to Drew Pojentic and they were straighted out and ran OUT OF THE box from him. I have never seen that happen, ever. I think there are a few that don't understand their operation and a leaking PV can absolutely cause a very rich condition. Ask me how I know.
I can put you in contact if need be.
Here are my corrected carbs running on his 427 Galaxie.
He's on FB. Tell him Capt Keith sent you.
https://www.facebook.com/AirFuelSParkTech/videos/1231150237067129/
I agree. It's fuel pressure you may be fighting.
I managed to get some time with the car yesterday and after some small adjustments to the idle mixture screws and idle speed screw I drove it about 60 miles.
Idle is definitely smoother and throttle response is crisper than it was. I didn't get the chance to pull a plug to see if the colours have changed.
I'm still not clear why a PV with a stiffer spring makes the car run leaner, but while I try to work that one out I will consider this result to be a 'win'!
Thanks for all your help with this.
David
Very possible that fuel leaking through pinhole in diaphragm or that the valve itself isn't sealing well. Holley has had quality control problems with PVs last several years.
An indicator of a leaky diaphragm is evidence of wet fuel in the pocket in the carb body where the diaphragm end of the PV lives. Fuel is sucked through the diaphragm and directly into the manifold bypassing the boosters.