Request for advice re Holley 4150

Mark

Bronze Supporter
#1
Hi everyone. It's been quite a while since I last posted on GT40s.

I have just purchased a Cobra replica, with a 427ci stroked 351W in it.

The car drives like a train, but I think it's running extremely rich.

In trying to acquaint myself with the car/motor, I just noticed that both the idle adjustment screws are missing on the secondary side of the carb leaving the holes open to atmosphere, which strikes me as odd.

Also, the vacuum advance on the distributor is blanked off. As this is a road car, would I be better served by having the vac adv connected?

I am attaching some pics to show the above.

I would really appreciate any input you fine chaps can offer. I am a fish out of water.

Thanks.

Mark
 

Attachments

Brian Kissel

Silver Supporter
#3
Hi Mark ,

The Holley 82750, does not have secondary idle mixture screws. The secondary jet blocks are not drilled completely thru, if it is as delivered from Holley. As Walt says, hook up the vac advance and tune the distributor accordingly. You would have initial advance, plus vac advance, plus mechanical advance. Those three added up would give you your total advance. Here is a good tuning guide for your Holley carb.

https://www.jegs.com/InstallationInstructions/500/510/510-0-82750.pdf

Regards Brian
 

Mark

Bronze Supporter
#4
Brilliant. Thanks guys. So the holes I'm seeing are not holes! Well that's a relief!! :)

Thanks also for the link to the pdf, Brian.

I knew you GT40 guys would come to my rescue :)
 
#5
Yeah, those "holes" in your metering blocks couldn't be just wide open. I would think you'd have gas everywhere if they were. And definitely hook up the vacuum advance.
 

Mike Pass

Gold Supporter
#6
Is it running rich at idle, part throttle or wide open?
Is fuel pressure 5-7psi? Check distributor advance is correct . Check that the vacuum port is sealed up with the rubber cap if not being used. Check no leaks in vacuum pipe to brake booster or sealed up if not used.
Set float levels. Remove the brass sight plugs at the side of the float bowl. Switch on fuel pump. Have a small container or a rag handy to catch any fuel spills. Loosen the big slot screw and then turn the nut until the fuel just seeps from the opening. Retighten the big slot screw and replace the brass sight screws.
Adjust idle mixture screws both together 1/8 of a turn at a time until it runs best (or connect a vacuum gauge and adjust to max vacuum.) Start with them at about 1.5 turns from fully closed as a start point.

This carb has adjustable air bleeds for idle and running for primary and secondary which can be changed in the same way as jets. The way it works is that a bigger fuel jet - more fuel - richer mixture. The bigger the air bleed - more air - weaker mixture. Because they are more adjustable it is a bit trickier to get them spot on. The best way is to get the car on a rolling road and get some accurate readings of air/fuel ratio at all running conditions. I have always found that this is the cheapest HP you can buy. The other alternative is to fit an Air/fuel meter such as Holley or AEM which will need a boss welding in the exhaust to take the Lambda sensor. This will then display the instantaneous fuel/lair ratio. They usually have a digital and analogue readout.
The main jets and power valve are what control the mixture when running down the road and are easy to obtain and change. The air bleeds are available also if needed. If you can get a good idle and smooth transition with slow throttle opening that would be a good start.
Sudden throttle opening means the accelerator pump squirters will shoot in a shot of fuel which will prevent the engine bogging down due to a sudden lean condition as the throttle is snapped open.

Cheers
Mike
 

Randy V

Administrator
#7
Good to read you again Mark!
You’ve gotten some good advice here...
I also run the vacuum advance off of manifold vacuum rather than ported vacuum. This will give you much better throttle response off idle.
 

Mark

Bronze Supporter
#8
Thanks guys and Hi to you all.

Thanks to you chaps, I will have an enjoyable weekend for sure.

The car runs great, it just gases me at idle! :p

Driving into the garage and parking it, leaves the place feeling like a gas chamber for hours! My den sits adjacent to the garage and the other evening, after returning from a drive, I thought I was going to die before I finished my first beer! LOL
 

______

New Member
#9
From your comments it sounds like its real rich at idle, check your idle manifold pressure ( Vacuum for non aviation people ) and reference it to your power valve number, it may be low enough that the power valve wont remain closed at idle therefore enriching mixture.
Looks like it has electric fuel pump, if the problem is due to fuel pressure after shut down you could install a pump switch to kill pressure by turning pump off prior to shutdown.
A thicker insulator between carb and manifold might also help prevent excess heat soak to carb after shutdown.
 
#10
Check and compare manifold vacuum at idle and the number on the power valve. Power valves can leak as well, nobody likes a leaky diaphragm.
Check for leaks in floats.
A good investment is the sight glass for the float bowels

How much cold weather do you get? No choke on that one !!
 
#11
Mark,

One idea for tuning of your Holley - install a wide band 02 sensor and gauge. You can monitor AFR on the fly and really see what's going on with the mixture across the rev range and under load. It's pretty straight forward - weld a bung in the exhaust and install the gauge somewhere tasteful around the dash. They're typically about $200 USD. Very helpful I've found.
 
#12
As many have stated it sounds like you have low vacuum at idle and the power value is opening. Typical if you have a bigger cam with low vacuum. You need to take a vacuum check at idle and in most cases you will find your vacuum is low allowing the standard 6.5 power valve to open making it too rich at idle. In most of my cobras I was running a 3.0 or 2.5 power valve with big cam. You want the power valve to open at high rpm low vacuum so you richen up the motor.

High Performance Engine Power Valve Selection:

High performance engines with modified cylinder heads, long duration camshafts and single plane intake manifolds may require a change to the power valve. To find out which power valve your high-performance engine needs, you can perform the following procedure:

1) Hook a vacuum gauge to an intake manifold vacuum port.
2) Warm up the engine and note the vacuum reading at idle. Automatic transmission vehicles need to be in the Drive position, while manual transmission vehicles can be in Neutral.
3) Divide the vacuum reading in half. The number will determine the correct power valve.

Each power valve is stamped with a number that indicates the correct vacuum opening point. For example a power valve with the number #65 stamped on it, will open at 6.5 inches of engine vacuum. As an example, a vacuum reading at idle of 13-inches, is divided by two and results in a 6.5 inches of vacuum. Therefore, you should have a #65 Holley Power Valve installed in the carburetor. If your vacuum is running at 8 then you would drop to a 3.5 power valve.

If you divide the vacuum reading and it falls on an even number, you should select he next lowest power valve number. For example a vacuum reading of 8-inches, divided by 2 and you come up with a number of 4. In this case you would use a #35 power valve.

Finally, if your engine produces 13 inches of vacuum or more, the stock power valve that the carburetor is equipped with from the factory, is sufficient.

Holley power valves come in a range of orifice sizes. The higher the number the more fuel is added.
 

Mark

Bronze Supporter
#15
Thanks again everyone.

So far I have hooked up my vacuum advance canister to manifld vacuum and connected a vac gauge to establish that my vacuum at idle is 15 after tweaking the idle mixture screws, so thanks to your collective knowledge and noting Mike’s mention above re the stock 4.5 PV, I guess I should install a higher numbered one, say 6.5 or 7.5?

I’ll happily pay you Mike, to take that spare 6.5 off your hands if you’d like to pm me your details :)

I didn’t get a chance to delve any deeper into floats etc, as the family were around for the extended holiday weekend, so more fun next weekend, when I won’t be answering either the door, or the phone! :)

Pip pip for now chaps :)
 
#16
Thanks again everyone.

So far I have hooked up my vacuum advance canister to manifld vacuum and connected a vac gauge to establish that my vacuum at idle is 15 after tweaking the idle mixture screws, so thanks to your collective knowledge and noting Mike’s mention above re the stock 4.5 PV, I guess I should install a higher numbered one, say 6.5 or 7.5?

I’ll happily pay you Mike, to take that spare 6.5 off your hands if you’d like to pm me your details :)

I didn’t get a chance to delve any deeper into floats etc, as the family were around for the extended holiday weekend, so more fun next weekend, when I won’t be answering either the door, or the phone! :)

Pip pip for now chaps :)
Your original post stated running extremely rich at idle so see what number power valve you have in it first and you might want to replace that first to see if it is blown. Going up in number will give it more gas at idle and usually you drop to keep the power valve closed longer so your issue may not be the number but a blown power valve that can happen with backfires etc. As others have suggested check the floats also. I really have seen many run a power value bigger than 6.5.
 
#17
Some of you guys make it hard work, anything modified with big cam or headers I just fit blanking plugs in place of power valve/s then take manifold vac reading and select suitable PV from there. Holley plus others maker em or if your a DIY guy you can block off an old PV.
 

Mark

Bronze Supporter
#18
Hi Gary.

I read a Holley pdf that said that one way to check if the PV has blown (without physically inspecting it) is to screw an idle adjuster all the way in, as the engine sits at idle and if the engine stops, then this means the PV is not blown. My engine did stall, so I guess the current valve is OK.

I am only just getting my head around this form of tech, with it being my first ever American motor, so please forgive me if I get things the wrong way around. It’s a symptom of age I think.

I am definitely going to strip the carb down further next weekend and have already accepted Mike’s generous gift of a 6.5 PV but agree with you, that this is unlikely to be the cause of my rich idle state.

I think my having tweaked the idle mixture, may have improved that, but I have a gas analyser on its way, just to make sure.

One thing that is odd, since my tinkering: the engine did not backfire at all when I first bought it and drove rhe 200 miles back home, when the vac adv was blanked off, but since I have connected that to the manifold vacuum, I am now getting quite a few backfires on extended periods of overrun and quite some pinking under high revving.

:)
 
#19
Disconnect & plug that vac advance like it was before before you ' stuff it', the extra advance is too much and your carb /manifold combo are quite small for 427ci so its likely the vac advance when hooked to manifold is adding to the mech advance under those conditions and the engine is telling you its not happy- listen to it! Your engine although we dont really know what build specs it has wont need any more than 32/34 degrees total timing advance at at 2500 rpm without the vac advance hooked up.
 
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