So got a 390 that I rebuilt with like 800 miles now. 10.2-1 compression, Cam specs
.618/.601 lift, duration at .05 243/251
locked out timing at 35 degrees
Holley Street Avenger 770

so my problem is I step on it hard just before WOT and I get a hesitation but accelerated fairly hard then at any point I mash the pedal engine goes silent then backfires through the carb.
I’ve played with the timing back and forth a few degrees but issue keeps occurring. I’m extremely frustrated at this point,
thanks in advance guys

 

Check fuel flow ,check float levels. The old drag racers rule of thumb is a 12 second car needs a pump that will provide 150 gal. per hour at a minimum of 5 psi. or about 5 pints in 15 seconds.

 

I agree with cropduster. It's going lean. If I take off in the morning before the engine is warm and try to accelerate too hard it will hesitate and back fire. If I feather the pedal to keep the accelerator pump pumping it won't.

 

First let's talk about your ignition: Why are you running locked out timing? Is this a racecar or are you on the street? Do you have vacuum advance? 99% of carb problems are ignition problems.

Next can you give more detail about what RPM, speed, and approximate throttle opening % you experience this at?

How does it accelerate from a dead stop using different throttle openings - just cracked to WOT?

 

no vacuum advance, I’m running locked out because I couldn’t go any lower then 24* initial without the engine wanting to die and I didn’t want to chance detonation because I was struggling with reducing the actual mechanical advance to less then 15. I’m sure I could get it so I have 7 or 8 mechanical but I spent a whole afternoon frustrated and locked it out. It also has ran better locked out. And at any point I step on it I get a hesitation before acceleration and at any point I mash the pedal I get the back fire. My fuel bowls I’ve gone from just below half to above have and haven’t seen a difference. Fuel pump is a year old and is suppose to run at about 6 PSI so I don’t think it’s that but I haven’t tested it.
road speed, gear, RPM it all acts the same. Hesitation with any slap of the pedal and when it’s to the floor backfire through carb.
if at a dead stop and I go less then 1/3 pedal no hesitation but anything else super hard hesitation

 

How do you plan to use this truck? What RPM do you expect to be at MOST of the time? Locked out timing is for racing/drag cars when they spend 95% of the time at WOT. If that's what you're doing with your truck, cool, but if this is a street truck you need base timing, mechanical advance and vacuum advance.

Ok so you have fairly lumpy cam but no reason it shouldnt idle around 750 rpm with ~12-16 degrees initial timing. Then add in the 15* or even 20* mechanical advance you'd be perfect. Say 14* initial, 20* mech would be 34* all in at XXXXrpm depending on your advance springs. Then your vacuum advance would give you extra for cruising. Honestly, and I don't mean to offend, but what I'm hearing is that you bandaided it and locked out the timing instead of actually figuring out what is going on with the engine.

Can you tell me how you were trying to adjust the mechanical advance? Also what is your idle rpm? Lastly if you're running locked out at 35* how does the engine start? I imagine an engine that kicks back and you can break the starter or flexplate.

All this is telling me your engine is not happy with your setup right now - sounds pretty undriveable to me. Did you mess with the curb idle screw on the holley when fiddling with it? That could be one problem, among others but really you need to get your timing set before messing with the carb.

I'm happy to help you figure this out but I recommend starting from square one.

 

okay I will put mechanical advance back in it. And yes I did bandaid this issue because I didn’t want to run any chance of detonation and needed the truck the next few days. Also it doesn’t hard crank. It cranks super smooth and without any issues. It’s an old school unilite so I’ll attach an image to show how it adjusts. It won’t be 95% strip so I’ll make up for that.
I can not decrease initial timing below 24 maybe 23 without the engine running extremely rough if not dying. Idles when warm around 1k rpm any lower and truck runs great until I punch it. It’s very frustrating

There’s a nut under the screws that I loosen and adjust so the mech weights get stopped at a certain point to limit the advance

 

So first thing I would do:

-Get #1 on TDC of compression stroke. Use a piston stop. Then check the balancer to make sure the timing mark is at zero. Stop here if it's not.

-Assuming the balancer/timing marks are good with the engine at TDC W/ a piston stop, back up the engine until you are at ~14 BTDC. Then rotate the distributor so the rotor is pointing to the #1 spark plug wire on the cap (lift the cap to see the rotor). This will get you close to 14* initial timing at start up and you wont have to try twisting the dizzy all over getting to 14*. Ensure your mechanical advance is working. I'd also hook up vacuum advance to manifold vacuum (NOT PORTED). The extra advance can do wonders for long duration camshafts at idle.

-I dont know what you may have done to the carb at this point but I'd also suggest putting it back to base setting. I'm mainly concerned with the primary throttle butterflies being open too far if you adjusted the idle screw. What this does it expose the transfer slots and will cause off idle hesitation. We can get to this later, but IF the engine/cam require more throttle opening at idle one trick is to open the secondary blades just a tiny bit. I believe Holleys have a screw on the side of the carb for this.

-Reinstall carb (could be a good time to put a fuel pressure gauge in line if you want). REMOVE THE PISTON STOP and replace with spark plug :-) Try to start it and report back.

 

Okay awesome, only issue is my distributor doesn’t have any vacuum advance. Still move on with this plan it without?

 

You might need it to make the cam happy at idle and reap some benefits of better mileage on the highway. Do you have a buddy with a vacuum adv distributor who'd loan it to you for testing?
Also I know it's really easy to spend other peoples money ....but it might be a nice time for you to upgrade to an electronic ignition and adjustable mechanical advance.

Here's some good reading for you as well: https://www.hotrod.com/articles/7-ti...d-cams-street/
https://www.hotrod.com/articles/carb...e-big-cam-sag/

Drilling holes in your throttle blades may be necessary - but be warned it's not something you can easily undo.

 

I’m curious about the duration of the problem. You stated the engine has 800 miles since rebuild; have you had the issue since the rebuild?

 

yes it is.

 

If the problem has been there the whole time, then I am inclined to circle back to what folks initially suggested. A lean condition most likely due to a vacuum leak. The FE engine is tricky on the intake manifold. If heads are decked on the block surface or ground to be true on the intake face, then the machine shop needs to work the intake to make it all match up. That’s my guess. What is your manifold vacuum? With that cam profile it should be around 10 inches Hg at 750 RPM.

 

It's a good point that all my advice above assumes an otherwise healthy engine with no vacuum leaks or dead cylinders etc

 

I'll back up SixtyFour and 66 on that- check thoroughly for vacuum leaks if your manifold vacuum isn't consistent with expected readings regardless of a lumpy cam. Pull your spark plugs and check them for soot. The link below will help on tuning with a vacuum gauge. A fuel pressure gauge at the carb probably wouldn't hurt either.

https://www.onallcylinders.com/wp-co...lcmqcYmVw2wfb4

Like SixtyFour was saying too, you can always base it to TDC, advance it to say 20 BTDC on the balancer and drop your distributor in with the rotor pointing dead at #1 cylinder and give yourself some mechanical from there. That's exactly what I did to break in my cam.

Can you run without a vacuum advance? Sure but driveability issues like what you have are harder to tune out without sacrificing something else.