Holley Carb tuning

[thistle3585]

I swapped a '72 AMC 304 in to my CJ5 and am using the Holley carb that came with it. I adjusted the two idles and it is running a bit fast but not too much. I also adjusted the accelerator pump. The problem I am having is that it performs fine when initially accelerating then it begins to stumble and bog down. I have been looking for a manual but can't find one. Anyone have any suggestions? I had been running a MC2100 on my last one, and I can put it back on but was hoping to make this one work.

 

[Fastatv]

I swapped a '72 AMC 304 in to my CJ5 and am using the Holley carb that came with it. I adjusted the two idles and it is running a bit fast but not too much. I also adjusted the accelerator pump. The problem I am having is that it performs fine when initially accelerating then it begins to stumble and bog down. I have been looking for a manual but can't find one. Anyone have any suggestions? I had been running a MC2100 on my last one, and I can put it back on but was hoping to make this one work.

I will try to help you if I can. First, the Holley idle system is different than most carbs, the amount of adjustment for the idle air mixture. They usually run the best at about one turn out from seat, and usually more than 1.5 turns out is about all you can get...that makes a difference that is. The accelerator pump needs to be adjusted as to where the pump arm is "just touching" the pump diaphragm when choke is off, and in warmed up idle position....just touching. There is also a wide open adjustment, but we wont go there yet. There are several things that can cause the "stumble" and "bogging". Can you give a little more info? Does it only bog or stumble upon acceleration...for instance, runs good at cruising speeds...then if you try to accelerate from there, it stumbles or bogs? Or does it stumble and bog upon taking off from a dead stop? Rick

 

[thistle3585]

Put it in gear, accelerate up to speed and it does fine, but once I back off to "cruising speed" it stumbles. That happens in any gear. The tach isn't working right now but I think it is idling fast. The screws are 1.5 turns out. Maybe getting too much fuel. My biggest problem is I haven't an exhaust on it, so its been a bit hard to tune simply because of the noise and exhaust. I hope to remedy that at lunch today.

Edit: I think I may have found the problem. The spring on the carb is worn and doesn't have enough tension to pull the throttle back all the way. That should resolve the curb idle. I also found the manual online and it looks like the float may be set too high. Will be making those adjustments this afternoon.

 

[gert]

Holley has alot of info on their site to cover their carbs
Holley Performance Products Technical Library

 

[harleydragon]

once the throttle butterflies open then the idle mixture screws are taken out of the equation..to set the idle mixture,start at about one turn out and then adjust them for max vacuum..if you reach max vacuum at say 1 1/4 turns out,then it stays there at 1 1/2 turns out then set it at the least amount out

 

[Fastatv]

Holley has alot of info on their site to cover their carbs
Holley Performance Products Technical Library

Not trying to hijack the thread but hey Gert...nice link. I can use it as well because the info is more up to date than my very old Holley books. Rick

 

[gert]

Not trying to hijack the thread but hey Gert...nice link. I can use it as well because the info is more up to date than my very old Holley books. Rick

Very cool, I printed all the stuff off they had for the 4160 and put it into a binder not to long ago for reference on a rebuild I just did I enjoy having a paper copy on hand when I need one

 

[thistle3585]

Yes, that is a great resource. Question.... How does the float level affect how the carb runs? I think I'm running too rich so I wasn't sure if that was a float bowl adjustment or not? It acts like its getting too much fuel at cruising speeds and is a bit underpowered.

 

[Torxhead]

The info that Gert sent in contains info on how to adjust holley floats, a high float level will cause a rich condition and low level will lean it out. Another way to adjust the fuel mixture would be to change the jets. N.A.P.A. is one supplier of these. I usually go by the spark plug deposit to inspect the fuel mixture.

 

[Scooter402]

I'm suprised no one has mentioned the power valve circuit yet.

If you have a power valve that has a torn/cracked diaphragm, it will "see" no vacuum from the engine, enrichening the fuel circuit. During acceleration, this is fine, because engine vacuum signal drops under load. As soon as you level off to speed and your vacuum picks back up, the power valve should close, letting your main jets be the only source for fueling.

Problem with my suggestion, however, would be that it would be rich at idle, too, unless you do have your idle air screws fairly tight (turned in close to seat).

How long was the carb sitting before you put it into use again? Could be gunked up fuel passages in the metering blocks/main body. The fuel passages are ~.030" in the emulsion tubes, and really only about ~.120" or so through the boosters in the main body (all depending on model of carb).

 

[thistle3585]

Thanks. The idle screws are 1.5 turns out per the manual. I don't know how long it sat and actually was surprised it fired up on the first try. It sat at my place for six months and I don't know how long it sat at the PO's. What jets should I be using? I opened the float bowl window and fuel came pouring out so I lowered the float level flush to the window when not running. It seeps out if it is running. I think I'll back the float down a bit more and check the jet size. It came from a local guy so I would expect them to be the right jets. I'll look up the power valve in the manual too. I'm also going to replace the flange gasket to make sure it isn't leaking.

 

[Scooter402]

The jets that are installed would probably be ok. Typically, Holly carbs are jetted a little rich from the factory. The jet numbers would either be found in a parts manual under your specific carb model number found on the choke horn. Depending on its age, they likely have been changed at some point. If you were so inclined, after resolving your issue, you could pull your float bowls (I'd already have a set of float bowl and metering block gaskets on hand, unless you can tell before disassembly that you have the blue re-useable gaskets installed) and find the jet number stamped in the side of the body of the jet. Most people go down 2-3 numbers for street use.

As far as the power valve goes, I'd start by replacing it (if need be) with one of the same Hg vacuum that is currently installed. If you really want to get into tuning that circuit, you can find some great information on the Holley website for power valve tuning (it is different for manual vs. standard Transmission ). You can check your current power valve(s) (you didn't happen to say if you have a single or dual metering blocks, did you?) by removing it, holding the diaphragm side up to your lips (if you don't mind the taste of gas), and attempt to pull air through the diaphragm similar to drinking through a straw. (Sorry, was trying to avoid using the word "suck." LOL) If you can pull air through the diaphragm, it is torn. Otherwise, it's fine.

 

[thistle3585]

Just an update. I got the float adjusted per the manual. I also dialed in the idle screws. It did smell like it was running really rich but that has mostly gone away with the adjustments. I still need to change the gasket out and I want to install a fuel pressure gauge. It runs a bit rough and underpowered as if the choke is on. It gets worse as it warms up. Does that sound like a vacuum leak?

 

[Scooter402]

Sounds like it needs a full cleaning/rebuild. I had an Edelbrock carb do that to me years ago. Stripped it down, soaked it in carb cleaner, then put back together to factory specs, and it ran like a top. A vacuum leak would tend to have a miss and/or a high idle.

I'm still leaning toward power valve(s) being shot.

 

[thistle3585]

Yeah, I was thinking a rebuild is in order too. I have an MC2100 that came off my old engine, so I'm going to rebuild it then put it on and see how it runs. I'll go ahead and rebuild the Holley then sell whichever I like best.

 

[thistle3585]

Rebuilt the carb last night and I'm worse off now then I was. I can't get the float bowl to fill. I can start it and it runs rough if I press the accelerator but it wont idle and just dies. I adjusted the fuel valve assembly that adjusts the fuel level in the bowl with no success. Any thoughts before tearing it apart and doing it again?

 

[Scooter402]

My knee-jerk reaction to that is a faulty fuel pump…but rebuilding the carb would not make the fuel pump suddenly quit working properly.

How long did you crank the engine over before it would sputter and die?

Did you happen to fill the fuel bowls through the bowl vents before you tried starting? (I'm asking because I'm assuming mechanical fuel pump) Without fuel already being in the bowls, and a mechanical pump, it can be a …chore, lets put it that way…. to start.

 

[thistle3585]

I pulled the bowl assembly off this afternoon, reviewed the diagram and noticed I was missing the fuel float spring. I'm guessing that prevented the bowl from filling. I had to order a new one along with the sight plug. I dropped the plug while trying to adjust the carb. I'm hoping that is all it is.

 

[Scooter402]

Either that, or somehow the needle and seat assembly was stuck closed somehow. I really think there's only one way to install them, but is there any chance that the needle/seat assembly got installed incorrectly?

 

[thistle3585]

It looked like it was installed correctly. Should I manually fill the float bowl before trying to start it after I put it together?