Good evening, I have a 360 with j heads, headers, edelbrock air gap intake, msd coil and distributor and an edelbrock vrs 4150 carb 850 cfm. I am having the typical holley flat spot issue, if you gently roll into the throttle from a stop, it bogs/hesitates. If you are already rolling around 40 and punch it it backfires through the carb. I have adjusted the idle air screws to the highest Idle/vacuum per edelbrocks instruction, I have properly adjusted my accelerator pumps, yet the issue remains. I have been told to try changing my pump nozzle size up and down, so that is my next step. Does anyone have advice? My timing is set properly and vacuum is good.
Edelbrock vrs 4150
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Professor Fate
Push the button, Max...
By your description, it sounds more ignition-related than carburetion.
What exactly is "properly set timing"? Are you running a vacuum advance or is it mechanical only? If you have vacuum advance, is it hooked to ported or manifold vacuum?
What exactly is "properly set timing"? Are you running a vacuum advance or is it mechanical only? If you have vacuum advance, is it hooked to ported or manifold vacuum?
Cal Tonsley
Well-Known Member
I feel that some of what you’re seeing here is very similar to the issues you mentioned last year when you were running the 650. Back then you found the timing marks were off, and once you set the timing by feel and richened the carb slightly the engine behaved much better. The hesitation on tip‑in and the backfire you’re getting now point to the same underlying thing — a lean transition and timing‑curve issue rather than a pump‑nozzle problem.
The VRS carbs look like nice pieces, but they’re really calibrated more like a race‑style 4150. On a mildish 360 they don’t always get enough signal at low rpm, which can exaggerate the same symptoms you had with the smaller carb.
PF’s comment about ignition is on the money — “properly set timing” can mean different things to different people, so some actual numbers would help. Initial timing, total timing, when it comes in, and whether the vacuum advance is on manifold or ported all make a big difference to how these engines behave.
I’d suggest checking transition slot exposure, initial timing, and the mechanical advance curve before chasing pump nozzles. Getting those right usually clears this type of behaviour regardless of carb size.
The VRS carbs look like nice pieces, but they’re really calibrated more like a race‑style 4150. On a mildish 360 they don’t always get enough signal at low rpm, which can exaggerate the same symptoms you had with the smaller carb.
PF’s comment about ignition is on the money — “properly set timing” can mean different things to different people, so some actual numbers would help. Initial timing, total timing, when it comes in, and whether the vacuum advance is on manifold or ported all make a big difference to how these engines behave.
I’d suggest checking transition slot exposure, initial timing, and the mechanical advance curve before chasing pump nozzles. Getting those right usually clears this type of behaviour regardless of carb size.
"Timing is properly set" is not an answer. If you truly want help, we need numbers. We need initial (idle) timing as well as total timing and what RPM that timing is all in by. A timing curve graph would also be helpful. When I see "timing properly set" or some such, and we do quite regularly, it tells me two things. The owner doesn't know where timing is and might not understand it. We can help with confirming "timing is set properly", but we need numbers. We don't need "I'm away from home", "my girlfriend ran off with my timing light and my best friend" or whatever. We need numbers. If you're serious about help, we need specifics.
No, I dont. I have set the motor to tdc on #1 hole, and according to my timing gun it has been advanced 12°. The idle is set around 750 I have no vacuum on my distributor
Regarding whatever static compression is and camshaft size, that could be a really low initial timing setting.
Ok so probably stock compression. They routinely blueprint either a FLAT 8:1 or down into the high 7s. That's just how Chrysler did it, even though they rated them at 8.4. I would pull in 18 degrees initial to start. It's very likely going to want more. If that's the case, and it likely will be, then you're going to have to limit total timing somehow. The old school way is to disassemble the distributor, weld the governor slots and file them back out to the appropriate length for the given total you want to run. There is a chart all over this site regarding slot lengths and distributor degrees if you decide to go that way. But you can try it without limiting the total first to see how it responds. I wouldn't leave it like that, as it will ultimately have too much total timing. Timing is easy. Because if it doesn't respond better, you can put it right back where it was. I got bank it will though.
Total timing is how much total advance the distributor has in it at the RPM at which it stops advancing. Total is "whatever" initial is, plus all the mechanical advance as RPM increases. To make math easy, lets say you're running 20 degrees initial. That means you'll want no more than 14-16 mechanical, for a total (initial plus mechanical) or about 34-36 degrees. As it is now at 20 initial, you'll likely be around 40 total, which is a bit too much. You can tell how much more you have by revving the engine looking at the timing marks with the light until the mark on the balancer stops advancing. That's where the total is. I would "guess" it will be somewhere around 3500-4000 RPM on a stock distributor. Only one way to tell.
I am running an msd pro billet ready to run distributor and their blaster 3 coil. Is there something I can do apart from getting in there and welding and filing the distributor?
Hell yeah. That makes it easy. Do you still have the instructions? If not, you can download them. You will likely need an advance bushing larger than MSD offers and they're easy to find. That's kinda puttin the cart before the horse though. You need to see where the total is and at what RPM before you go any farther.
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