I recently bought a Holley 4160 600 cfm carbueretor for my 82 302.
The carb has a new rebuild kit in it and the bowls and air fuel mixture is set just right.
The problem I'm having is it hesitates when I slam on the gas from idle. It backfires and almost dies until I let off the petal a little bit. It doesn't hesitate at all at high rpm's though.
What do yall think my problem is??

 

Sounds like it could be timing. Also, how do you know the bowls and air mixture is set just right? I'm not doubting you, but some more information on how you came to that determination would be helpful.

What is your timing set at? It sounds a little too retarded and that is why your getting backfiring out the tailpipe.

Another consideration is, you might be running too rich. What jets are in the carb? Unburned gas will also cause backfiring as it leaves the exhaust and could flood the engine which is why it almost dies when you floor it.

 

I am going to follow this thread. I have the same issue w/ my 85 F-350 w/ 460 auto. I cannot remember my Holley Carb (600cfm) model, but it is was replaced about 10 yrs ago, when the factory carb was shot. I had the backfire issue until a local mechanic installed a new power valve (which got rid of the back firing issue), but did not fix the hesitation exactly as you describe. I have never been able to resolve this on my own. I had a new intake manifold gasket installed several years ago, which lessened the hestiation, but did not git rid of it. I have seen several posts about checking the accelerator pump so, when I my truck inspected this monday, I am having them look into the hesitation issue. The only thing on my engine that is not factory is the Holley. Timing chain was replaced about 5yrs ago when I needed a new water pump. This issue has been driving me crazy for about 10 yrs, but so long as I let the truck warm up for about 5 minutes before driving it, the hesitation issue reduces, but it is still there which makes me wonder if once the engine is at operating temp, a small leak around a gasket somewhere is partially sealing it self, but I don't know. I am not in any way, more than a toal 'novice' on engine realted matters. Good luck.

 

Coney, you bring up a good point that I forgot about. OP, when you say the carb was rebuilt were all the parts replaced (i.e. the power valve)? If the power valve is blown then it would cause a rich environment.

Also, the vacuum leak is another good place to look. Vacuum testers are fairly cheap and a great investment. I get about 17-18 Hg on my vacuum when I test my truck. It shakes just ever so slightly but that's because I have a slightly higher cam than stock.

Really, that's the only way to check for sure if the carburetor is set for your particular vehicle. Holley's especially are not the set it and forget it type of carb. Many on here will tell you that.

If you check your timing, vacuum and know the jets to be the right size then I would say you're looking at a power valve but don't rip the carb apart until you check those others first.

 

In my experience, a hesitation off idle is usually caused by not enough fuel from the accelerator pump. If it is backfiring through the carb at the same time, that pretty much confirms it in my book.

 

Could be a maladjusted accelerator pump or the wrong spring in the secondaries too...

 

Gotta agree. Have a look at the accel pump linkage and see if the gap is set correctly. Just because there is a nice stream of fuel being shot in as throttle opens, doesn't mean it comes in at the right time.

 

As stated, backfiring through the tailpipe can be a number of things. These include incorrect ignition timing, burnt up valves, sticking valves, lean fuel mixtures and problems with the carburetor itself.

Check to make sure your accelerator pump is working (do this by looking down the throat of the carburetor with the truck off and hit the accelerator a few times. You should see gas squirting and you might even be able to hear it too). Did you change jet size at all? Did you tune the mixture by ear or did you do it with a vacuum gauge?

My advice, go buy yourself a vacuum gauge if you don't have one already, set the carburetor mixture screws with a vacuum gauge, and set the timing with a vacuum gauge.

I'll bet it's a combination of lean mixture and timing.

And, like RW says, it could be there needs to be more adjusted with the carburetor.

 

The Holley info about adjusting the accelerator pump is not real clear. The way I do it
is to adjust the pump arm to move as soon as the throttle does. You can check this by
very slightly moving the throttle arm and looking at the pump arm while you do it, and
see if the pump arm moves as well.

It should, if want the best performance. As mentioned, above, make sure the idle mixture is adjusted by a vacuum gauge for best idle as that will also affect the off idle performance and transitionary performance.

 

Dalton - I hope you have luck resolving your issue. (I hope I am not interfering w/ your post, but I too want to get this issue resolved) I am taking my 85 F-350 in for Pa inspection this Monday. I spoke to the owner of the shop, he does work on carb's and I mentioned the same issue you have described, except I no longer have the back firing (only did a few times when it did do it). He basically said pretty much the same things the good folks here on FTE have mentioned "1983... and Old Hickory" - I took off the air cleaner today after work and I cannot see or hear the gas squirting in the carb. doing the procedure you described. I should have taken this to a good mechanic yrs ago, but I don,t even put 1k miles per year on the truck and once warmed up, the truck has less of the hesitation issue under normal driving conditions, but if I am idling and hit the gas to rev it up, I always get the hesitation. If I slowly press the gas pedal, this condition does not appear.

 

The Holley adjustment is very clear if you understand how it works. You have to have a little play at WOT between the lever and the screw. You check this by holding the pump diaphram lever in the full down position and measure the clearance with the throttle wide open between the spring loaded plunger and the lever. You can get a real close setting by holding the throttle open and loosening up the adjustment screw until the head is just clear of the arm, then tighten it about 1/2 turn. It really needs to be checked any time the bowl has been off as that changes the relationship.