Holley 750 carb adjustment

[oldbee]

You can try it but if it was me I'd do a rebuild && blow out everything hole/passage with air pressure. From all the crap in picture I'm sure something got into that carb somewhere you can't see. You may have found your main problem though with the rubbing & sticking float needle/seat.

 

[Wietse]

Yes, i will order a rebuild kit and strip & clean it top to bottom.
Just wondering why using a blank power valve?
Does this mean the fuel supply is purely controlled by the jets only?
As mentioned, the power valve chokes with "higher" vacuum, but with a blank it just gives the full wack of fuel any time right?

 

[oldbee]

No, a blank of blocks off those fuel ports so then you have to increase main jet size to compensate. Did you check choke operation yet?

 

[Wietse]

Yeah, then that's why the jets on the secondary are bigger then on the primary.
My carb has no choke equipped.

In the afternoon i pushed it even to kick-down and so far so good.
But i **** myself when he went back for 2nd gear and took off, i am used to some acceleration with my other cars but this thing is mental!!

 

[coloradodave]

Some good advice is given in the posts above. Concerning the low vacuum, you might try checking your timing before trying to adjust the carburetor. Again, advance your timing a couple of degrees at a time to get the highest vacuum. If the car will 'hot' start at that setting and doesn't 'ping' on acceleration, leave it there. If it cranks slow or pings, retard the timing 2 degrees at a time until it doesn't. Then attempt to tune the carburetor, doing basically what you said you did. Again, the purpose of the power valve is to give you some fuel economy by running smaller main jets and when the vacuum drops by opening the throttle, the power valve enriches the mixture. With that much sediment in the filter I would recommend pulling the tank and sending unit and thoroughly cleaning both (sending unit has a filter sock attached to it) along with blowing out the fuel line. Use spray carburetor cleaner with a 'straw' to blow out all the little passages in the main body and metering blocks, followed with high pressure air. As far as the secondary power valve block off, I probably wouldn't mess with that or the secondary jetting until the engine is running right through the RPM range. Jet sizing and power valve/no power valve (secondary) are things you can mess with later. I've only blocked off power valves in race motors where I wanted exact fueling determined by the jetting. A street car, in most cases, should run them.

 

[Wietse]

The timing was set not long ago at the shop i bought the car when they installed a new ignition system, i am not sure where it is set at but their mechanic does a good job.
The more i find the more i get the feeling this car is made for drag racing, pumped up engine, 4.10:1 ratio axle etc.
Now the carb also seems to be set for it.

All advice is noted from all you guys, thanks for that!
For now it's driving ok, when winter comes i have planned several big jobs incl. engine removal for cleaning and painting the engine and engine bay.
At that time i will also pull the fuel tank, renew the fuel lines and rebuild the carb with a rebuild kit.
I will also have to invest some more money in a good vacuum gauge i guess so i can check it properly to get a good vacuum.

 

[coloradodave]

The timing was set not long ago at the shop i bought the car when they installed a new ignition system, i am not sure where it is set at but their mechanic does a good job.
The more i find the more i get the feeling this car is made for drag racing, pumped up engine, 4.10:1 ratio axle etc.
Now the carb also seems to be set for it.

All advice is noted from all you guys, thanks for that!
For now it's driving ok, when winter comes i have planned several big jobs incl. engine removal for cleaning and painting the engine and engine bay.
At that time i will also pull the fuel tank, renew the fuel lines and rebuild the carb with a rebuild kit.
I will also have to invest some more money in a good vacuum gauge i guess so i can check it properly to get a good vacuum.

Sounds good. My best advice to you is to read everything you can about tuning the ignition and carburetion. NO-ONE will care about getting it right more than you. Enjoy your car and remember; Timing THEN carburetor...

 

[PurpleBeeper]

may want to see whats in the gas tank too...damn

Good point. You can put a large magnet or two on the fuel tank itself to hopefully collect any metallic debris & keep it out of the pump. I've used old speaker magnets with success.

 

[PurpleBeeper]

The timing was set not long ago at the shop i bought the car when they installed a new ignition system, i am not sure where it is set at but their mechanic does a good job.
The more i find the more i get the feeling this car is made for drag racing, pumped up engine, 4.10:1 ratio axle etc.
Now the carb also seems to be set for it.

All advice is noted from all you guys, thanks for that!
For now it's driving ok, when winter comes i have planned several big jobs incl. engine removal for cleaning and painting the engine and engine bay.
At that time i will also pull the fuel tank, renew the fuel lines and rebuild the carb with a rebuild kit.
I will also have to invest some more money in a good vacuum gauge i guess so i can check it properly to get a good vacuum.

I think you've solved your problems, but I'd still suggest you keep swapping out fuel filters for a while until all that junk in the tank clears out. GREAT JOB finding the float hanging up & needle/seat issue in the back half of the carb. Your stock fuel pump should just barely keep up with a 750 double pumper.... but work OK. Yes, you should have a "dummy" in the back metering block.

 

[Wietse]

The fuel pump is of the brand Carter, probably not original.
From what i can see on Summit is that the Carter pumps are high flow compared to stock.

I will order a bunch of filters and keep changing them every now and then, at some point i will lower the tank and clean it out.

Still one thing remains unclear to me and hopefully someone can send me in the right direction.
As it is running quite rich, how do i supposed to lean it out with the 4 idle screws, after an initial 1-1/2 turn, start to screw them IN or OUT to get the best vacuum reading?

 

[Cornpatch MO]

The fuel pump is of the brand Carter, probably not original.
From what i can see on Summit is that the Carter pumps are high flow compared to stock.

I will order a bunch of filters and keep changing them every now and then, at some point i will lower the tank and clean it out.

Still one thing remains unclear to me and hopefully someone can send me in the right direction.
As it is running quite rich, how do i supposed to lean it out with the 4 idle screws, after an initial 1-1/2 turn, start to screw them IN or OUT to get the best vacuum reading?

You might want to use the "see through" gas filters Maybe you can monitor how clean the gas is.
I looked into a metal filter , and was surprised to find that one, only had a fine screen inside. I know old tractor sediment fuel bowls had a screen so fine ,they would filter water out of the gas. ....................................MO

 

[Wietse]

Yeah, i had a see through type filter installed as well, this was installed inline after the original filter.
There i installed a new K&N filter as backup.

I don't really fancy these plastic filters for obvious reasons, damage by impact or heat are easily done and the result is a fuel leak so i rather stick to the metal type.
They are not expensive so i will by a bunch and replace them a few times and see what comes out.
I guess most dirt that fits through the suction screen ended up in that filter, either the tank will be as good as clean or the remaining dirt is too big to pass through the screen and maybe got stuck there.

 

[oldbee]

I think turning in on screws leans out but it only can do so much. Best vacuum could be in or out, but all 4 should be real close to the same setting.

 

[Wietse]

Yeah, i pushed it once on screwing them back in bit by bit and at some point it start to run like ****.
What else could be used for this, a sensor on the exhaust right?
Like a lambda probe on modern cars to monitor the remaining oxygen in the exhaust gasses??

Every time i pay close attention on doing the same amount of adjustment on a 4 screws indeed.

 

[oldbee]

Lambda probe would work, I don't go high tech just a sensitive tach or vacuum gauge and go for highest reading. Then reset idle speed.

 

[Cornpatch MO]

Yeah, i had a see through type filter installed as well, this was installed inline after the original filter.
There i installed a new K&N filter as backup.

I don't really fancy these plastic filters for obvious reasons, damage by impact or heat are easily done and the result is a fuel leak so i rather stick to the metal type.
They are not expensive so i will by a bunch and replace them a few times and see what comes out.
I guess most dirt that fits through the suction screen ended up in that filter, either the tank will be as good as clean or the remaining dirt is too big to pass through the screen and maybe got stuck there.

Not un-common for the filter "sock" to be off the fuel sending unit. I actually saw a piece of sock tied on the end of the pickup tube. .........................MO

 

[beanhead]

To see if your idle mixture is in the ballpark...while it's warmed up good and idling, give a little push on the accelerator pump arm. If the RPMs increase momentarily, the engine wants the extra fuel so turn the screws out to richen your idle mixture. If it bogs slightly/slows down, the extra gas was too much so lean it out by turning the screws in. When you bump the pump arm RPM should stay constant, then you know your real close. Do the primary side first, then the secondary side and remember a little goes a long way..turn the mixture screws only 1/8 or even 1/16 at a time and wait 30 seconds or a minute before turning them again. This has worked well for me as I don't have access to A/F meters or analyzing equipment. (I'm currently running an 850 speed demon mech. secondary/no choke carb on a 440..has no option to add a PV to the secondary side)

 

[Wietse]

As a part of the troubleshooting i've decided to have a closer look at the spark plugs to see what story they tell.
As i keep having the engine stumble every now and then, knowing the fuel supply is good there might be more going on.
Spark plugs are NGK BP6 S, so heat rating 6 to me and as per stock should be a gap of 0.035".
I checked the gap and found most of them accepting a 0.030" feeler gauge, which makes sense as from what i read is that with higher powered engines the gap should be reduced.
Knowing the engine has been modified and puts out more power then stock i am wondering if this type of plug with that gap is a decent setup for this engine.

See the pictures of the average spark plug, the core is not fouled with carbon so it does burn clean and shows a quite reasonable color, on the outer ring there is fouling and some carbon build up but nothing major to me.

I've cleaned them all up/checked gap and reinstalled them, the random stumble at proper throttle remains.
Could there be some temperature issue that happens once the engine get high loads and the spark plugs start to get hot?
So once at higher operating temperature in the combustion chamber the spark plug fail to give a good/strong spark or something?

My ignition consists of a MSD distributor (ready-to-run) w/o vacuum advance, MSD Blaster SS coil, and MSD 8.5mm plug wires.
Coil has a direct feed from battery through a relais.
Battery has been in overcharge conditions for some time and has been boiled before but have seen no weird issues with it and still works well i guess.
Makes me wonder when the engine get higher revs the alternator also will put out quite some voltage which could cause issues? (just far guess here?)

 

[oldbee]

With all your fuel problems I would try a new set of plugs,maybe the same to start with. They look like ur running rich a little bit, might be from the bad fuel,sorry didn't go back to start of thread. At least a .035" gap on plugs, and idle mixture might be too rich at least from the pics. They should be almost tan/white on the insulator part. Might be too "cold" of a plug?

 

[coloradodave]

Not that it's your problem solver, but with the MSD ignition you should open the plug gap up to .045". And, I agree, the next hotter range of plugs or leaner mixture. Start with new plugs and do a resistance test on each plug wire. Look for (+-) 5,000 ohms per foot. Check the contacts on your rotor and distributor cap as well.
Good luck.