Holley Setup

[MartinGough]

Just fitted a Holley carb to my GTD40. Working great bar a little intermittent stutter while setting off, think that could be cam related and can live with that. The main problem is a slight surging while at a steady speed around 30-50mph. Think it may need bigger main jets. Any ideas?

[Sandy]

Martin -

Lots of things to check :-)

Ensuring your fuel pumps and pressure regulator are all in order, start with the float levels and ensure they are correct.

Another thing to check is the spark plugs, if the carb is jetted incorrectly your plugs will be the best way to tell that.

Listen for vaccum leaks, and check you base gasket to ensure that it is correct as well as all unused vacuum lines on the carb are plugged. A water spray bottle can help as the engine speed will change as you spray a mist into anything sucking air.

Don't know what you had on before but if it didn't have the same problem you can at least be sure it's not something else.

You didn't mention if the carb was brand new or a used one, which could also be an internal problem to the carb.

For the starting off stumble you might need to check the primary accelerator pump and related settings. If a used carb most people mess with accel pumps (Always on the more side) and the jets. You should be able to find some online help in setting up this, or pick up one of the holley tuning books to set things stock. Most things are pretty simple unless someone butcherd the carb and sold it off, I have one on my Mustang that I got off EBAY and I'll likely have to have it gone through as I can't quite get it right with my holley hacking skills

BTW What size carb and motor are you running? Mechanical or Vacuum Secondaries?

Sandy

[MartinGough]

Sandy
Thanks for the reply. The carb is brand new and fitted by myself. I have set the float levels and set the four corner tickover mixtures which has helped the surging at a steady speed. The only thing now is to correct the stumble while setting off. The primary accelerator pump seems to have an effect on this so I think its just a case of small changes to get the best response. I will get some mallory pumps fitted when I visit Frank Catt soon and let him cast a critical eye on my work.

[jnielsen]

If you have a cam with a rough idle you will have reduced vacuum. This can result in a lean condition. Many times people adjust the fuel mixture screws out to get idle mixtrue correct but when the vacuum is low often the screws are adjusted out to far. The screws are meant to achieve correct idle mixture within a certian number of turns. If you go further you can get a stumble on transfer from the idle fuel circuit. If your screws are past the recomended max munber of turns you should remove the fuel metering block and enlarge the idle circuit feed. The best thing you can do is get a book on carb setup. They usually have great photos and information. Another thing to check if you have a large cam is the correct power valve is installed. Hope this helps some.

Jim

[Kevin M]

Martin,

Check this website:

Carburetor Tuning the Scientific Way

Kevin

[Doug Sainlar]

It sounds like you have a good grasp of all things Holley, but I haven't yet seen anyone mention the plastic cam that drives the accellerator pump. Holley markets a kit of various cam designs that will alter the amount of fuel delivered by the accellerator pump as well as where in the throttle "travel" the fuel is delivered. Sandy is right--in the past I've been guilty of using the wrong pump cam and putting more fuel into the carb when what it really needed was less. The engine was almost flooding out with the huge accellerator pump cam I foolishly installed, when what it really needed was less fuel. It felt like a low RPM stumble, though, which it was.

Good luck. As I plan for my build, my experiences with the Holley are the main reason I think I'll go with an EFI sytem. On the Holley there are a multitue of things that can and do go wrong--for example, one "hiccup" and the power valve needs replaced, etc.

Doug

[MartinGough]

Had another play tonight and have I think got the tickover mixture correct. I have played with the primary accelerator pump and got it pretty close. If I set off slowly the car moves off then stutters after 10-15 yards then clears. If I use a little more revs I dont have the problem. I think a little tinkering with the primary pump and the cam(s) may do the trick. Thanks for everyones input, greatly appreciated. The car certainly feels more responsive with this carb and has more torque.

[Sandy]

Definitly get the book that will get a good starting point. I think you said the carb has 4 corner idle setting which is a good thing as many of the holley hp carbs are done that way (Not sure if an HP version). If an HP version it likely has power valve protection in the base plate (2 ball berings), if not, easy to add on the next rebuild.

The pump cam and nozzles are tricky as Doug mention because you have the tendancy to go the wrong way. I ended up going with a Pink cam which is I think the smallest of that batch I had after using the big one that made worse and hard to start due to way to much fuel.

Get a vacuum gauge also that will tell a lot about your idle vacuum and can help understand if the stock power valves are ok or if you have other issues. If you do have a very hot cam you might need to mess with the power valves. But get the book and it will give loads of things to do to make the carb work good. Also don't forget thing like checking the timming as that can have a big impact on the idle vacuum.

Oh, never be within reach of a hammer when working on the carb, you may have an 'incident' that will make you feel better until you have to order a new carb

Sandy

[Doug Sainlar]

Originally Posted by Sandy

Oh, never be within reach of a hammer when working on the carb, you may have an 'incident' that will make you feel better until you have to order a new carb Sandy

Been there, did that--really!! It felt almost as good to feel that brittle throttle body crumble under the force of that 6 pound sledge hammer as it felt bad to have to write the check. Of course, in my youthful exuberance I ordered a larger carb with mechanical secondaries and that made the problem even worse. I eventually sold the carb and manifold and took a factory manifold and a Q-Jet in trade and the GTO ran much better. Go figger....

Doug