Author Topic: Jet sizes  (Read 470 times)

Skidado

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Jet sizes
« on: July 11, 2019, 04:17:38 PM »
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

shelbydoug

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Re: Jet sizes
« Reply #1 on: July 11, 2019, 07:04:49 PM »
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.

Skidado

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Re: Jet sizes
« Reply #2 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. Itís only the smell that has alerted me to a potential problem. Iíve pulled the plugs and they look ok (sooty round the edges but clean/light colour on the tip). I realise that plugs canít be read on modern fuel like we did in the old days so this might be misleading ! 😁

Thanks for your help

David

67350#1242

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Re: Jet sizes
« Reply #3 on: July 12, 2019, 07:26:20 AM »
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.
67 GT350  SJ 02/01/67
67 Coupe  SJ 11/16/66

shelbydoug

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Re: Jet sizes
« Reply #4 on: July 12, 2019, 08:57:40 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. Itís only the smell that has alerted me to a potential problem. Iíve pulled the plugs and they look ok (sooty round the edges but clean/light colour on the tip). I realise that plugs canít 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.

gt350hr

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Re: Jet sizes
« Reply #5 on: July 12, 2019, 11:16:07 AM »
  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
Celebrating 44 years of drag racing 6S477 and no end in sight.

Skidado

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Re: Jet sizes
« Reply #6 on: July 12, 2019, 02:00:24 PM »
Wow - thanks for all this great info!  This is why I love this forum.

My rich running problem is not so much at idle. Iíve got a good steady idle at about 750 rpm and although itís a bit smelly itís 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 isnít good, but this happens overnight and doesnít in itself appear to cause any problems.

I will pull the carb and see what jets and power valve itís running and work from there.

Thank you as always

David

gt350hr

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Re: Jet sizes
« Reply #7 on: July 12, 2019, 03:44:09 PM »
   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
Celebrating 44 years of drag racing 6S477 and no end in sight.

roddster

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Re: Jet sizes
« Reply #8 on: July 12, 2019, 04:02:31 PM »
  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......

mygt350

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Re: Jet sizes
« Reply #9 on: July 12, 2019, 05:49:08 PM »
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
Continuous caretaker of 5S228 since May 1967

shelbydoug

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Re: Jet sizes
« Reply #10 on: July 12, 2019, 09:39:41 PM »
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.

Skidado

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Re: Jet sizes
« Reply #11 on: July 13, 2019, 11:43:18 AM »
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

67350#1242

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Re: Jet sizes
« Reply #12 on: July 13, 2019, 12:21:08 PM »
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
« Last Edit: July 13, 2019, 12:23:58 PM by 67350#1242 »
67 GT350  SJ 02/01/67
67 Coupe  SJ 11/16/66

mygt350

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Re: Jet sizes
« Reply #13 on: July 13, 2019, 01:57:35 PM »
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?

Continuous caretaker of 5S228 since May 1967

67350#1242

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Re: Jet sizes
« Reply #14 on: July 13, 2019, 02:07:30 PM »
Quote
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?

At first glance that seems right, but the opposite is true - vacuum would have to get down to 3 or4 before extra fuel added.
67 GT350  SJ 02/01/67
67 Coupe  SJ 11/16/66