Anyone Here Good With Holleys?

orange150

orange150

Well-Known Member
Joined
Mar 20, 2005
Location
Fairfax City, VA
The PO of my Jeep installed an Offenhauser intake and a Holley 390 on the 258. I am having a time getting the off idle stumble and poor fuel economy tuned out of it, and am just about to the point of installing a 2150 in it's place.
Figure I'll give it one last chance before I totally give up.
After a full rebuild, all I've played with are the accelerator pump nozzle sizes, everything from .21-.31, still a stumble.

I think I'm going to try the 50cc accelerator pump and bump the power valve to 9.5, then go back to playing with nozzle sizes.

Holley says if you have more than 13" of vacuum at idle then the 6.5 power valve (which is in the carb now) is fine. I'm pulling 20-21" at idle so that puts me at either 9.5 or 10.5. Why would I not try that out?

I'm going to bump to the 50cc accelerator pump to compensate for the long runners on the Offy intake, which I read to do on some Holley articles, and was described in a YouTube tutorial from Summit Racing.

Anyone have any insight, or pointers to give me? If I can't get this tuned right I'm going to try a MC2150.
 
More info on the Jeep that might help with tuning.

258, AX15, D300, axles are still geared with the factory 2.73, hoping to regear to either 3.50 or 3.73 early next year.
31" tires
 
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I'm not great with Holleys Im old enough to have worked on just about every carb made. With engine and air cleaner off look down in top off carb with choke open and open throttle . Do you see a squirt of gas everytime the throttle opens? I also would double check timing. Do you have a vacuum advance on distributer ? A while back somebody had a problem with a Holley cause somebody left out check valve for pump
 
Not sure if this applies on the Holley but on my old Rochester it did the same stumble. I finally got an old school mechanic to take it apart and see what was wrong. In the accelerator pump there was an aluminum ball that metered fuel when accelerator pump was supposed to be doing its job before the vacuum advance did it's job. That ball was worn so it was going too far down into the hole where it sat and would get stuck. We replaced that and it worked fine.
 
You did set the fuel level by the sight plugs right? If not, could very well be filling a tad too much in the bowls?
The check ball R Q mentions for a Holley I believe is straight down one of the center vents and is a pain to get out. Either need a strong skinny magnet or turn carb over and tap bottom plate with a brass hammer and have your hand ready for you wouldn't know it came out it's so small.
 
Accelerator pump is operating correctly. I'm getting a strong pump out of it.

Distributor is vacuum advance.

The 'check ball' is in place and operable. I checked it when I rebuilt the carb in February.

Fuel level is set correctly, if I remove the sight plug I will get a little trickle if I rock the vehicle. I am going to pull the bowl to double check the needle just to be sure everything is operating correctly there.
 
The 'check ball' for the Holley system is a big needle BTW. It's a little easier to keep track of ;)

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I was wrong, it's in here. Need flat head to remove plug. What you are referring to orange150 is the "needle and seat assembly" that controls float level. The check ball meters fuel somehow.
 
Is stumble any different when engine is cold compared to when engine is warm or hot?
 
Thing runs like crap when it's cold, I compare it to an old diesel truck when it's cold, I've never been able to get a good cold start tune out of it.

After it warms up it runs great except for the immediate off idle stumble.
 
I would rule out ignition problems before condemning carb. A marginal spark or bad wrong plugs could affect drivability. Verify timing to include timing mark on balancer. Check vacuum advance for proper operation. Does the choke work properly? First thought of cause for stumbling is too lean but poor mileage is too rich or other problems like low compression or wrong timing or bad plugs
 
I'll check it all, but I did a full tune up with the Team Rush upgrade just last year with new plugs and wires. I did check the vacuum advance a few days ago and it is operating properly.

The very first thing a Holley tech told me to do was increase accel. pump nozzle size to decrease the stumble, and if you read any of the write ups on accel. pump tuning that's what they say to do.
I've also read that the Offy intake can cause stumbling to due the long runners causing the fuel to be delayed in getting to the combustion chamber.

I may have done too much reading..
 
Can't do too much reading but sometimes not enough thinking. Poor running on start up has me concerned I would try more choke to richen things up. Even some choke at normal temps to see if stumble changed
 
I'll try to tune in some more choke to see what happens.

You do need to fiddle with the timing on the 258. You buy the Mr. Gasket #925D advance curve kit to tune the mechanical timing advance of the distributor.
 
If you dont have a computer controlling timing you can advance timing until pre ignition occurs then back off a degree or two. I would do a compression check to evaluate rings, valves and cam timing
 
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