[fst73mav]

I have a 302 that I am trying to tune and it has me a bit stumped right now. Been tinkering and haven't been able to get it dialed, hoping someone might have some thoughts. Here's my specs:

-302
-E-street aluminum heads with roller rockers
-Dual-plane aluminum intake
-4-barrel 670 cfm Holley (vac. secondary) with #65 jets
-Ready-to-run distributor, 8.5mm 50 ohm wires, 8-deg base timing, 24-deg at idle with vac advance, 40-deg total advance
-Champion plugs gapped at .040
-MSD 6AL
-31-242-3 Comp Cam
-Full-length headers with 2.5" dual exhaust

It idles fine and off-idle acceleration is fine but I have a hole shot stumble (launching at about 1500-1800 rpm) that I can seem to fix and at about 3000 rpm it will start breaking-up and backfire through the exhaust revving it in the garage. Haven't had a chance to try it at the track to see if the breaking-up improves with the scondaries coming in. Plugs look like it is running super rich no matter what I do.

I have tried changing main jets, timing, plug gap, mech. advance springs, accelerator pump nozzles, accelerator pump cam, and hooked up the vac advance which was previously unhooked. Nothing has resulted more than a minor improvement so far.

 

[galaxiex]

The air inside the distributor cap is ionizing and causing a misfire/crossfire….. maybe…..

Drill vent holes in the cap.

 

[danv]

I think it's your timing. Alloy heads are usually around 32 degrees total.

 

[Briansshop]

Close the plug gap some and try again.

 

[cuthrell]

have you got a spare (smaller) carb you can test?

 

[fst73mav]

The air inside the distributor cap is ionizing and causing a misfire/crossfire….. maybe…..

Drill vent holes in the cap.

Honestly wouldn't be surprised if it's something strange and obscure like that

I think it's your timing. Alloy heads are usually around 32 degrees total.

Previously had it with much less timing in it, wasn't much different on top end and the launch stumble was significantly worse. That said, timing is going to be on the 'constant fiddle with it' list while I work on this.

Close the plug gap some and try again.

I think I had settled on that for my next step, started at 050 and 045 and 040 were each a slight improvement.

have you got a spare (smaller) carb you can test?

I had the same issue trying a 570 last season but don't have it anymore to try it now unfortunately. If I continue to be stumped, I may try a 550 or 600 again.

 

[Space]

Been around a similar issue that ended up being a leaky intake manifold gasket. Apparently that one didn't like supporting the entire engine's weight while getting pulled and swapped. Where you at? Im in Puyallup area.

 

[fst73mav]

Been around a similar issue that ended up being a leaky intake manifold gasket. Apparently that one didn't like supporting the entire engine's weight while getting pulled and swapped. Where you at? Im in Puyallup area.

I did put a smoke machine on it and saw no leaks but still might be worth a try. I'm up in Everett but I grew up down in Puyallup!

 

[cuthrell]

I think I had settled on that for my next step, started at 050 and 045 and 040 were each a slight improvement.

this is strange - that MSD box should fire a .060 gap....wait...
you have a ready-to-run distributor AND a MSD 6 box?

with the ignition breaking up and the smaller gap helping...i'm thinking something's wrong there.
how are you triggering the MSD box?

 

[galaxiex]

Coil wired backwards?

 

[Monroepitt]

Coil wired backwards?

Coil is where I would start.

 

[cuthrell]

sometimes i need more coffee before posting...
yeah- swap your coil out. make sure its MSD-box-compatible coil.

if that doesn't do it...i'd start isolating components.
step 1. unhook the MSD box and use the ready-to-run dizzy by itself
if that doesn't do it, my next move would be:
step 2. plug the MSD back in, but remove the ready-to-run and use a 'normal' distributor

 

[crwalker54]

Have you looked at your float levels? Float levels to high will cause this.
I agree with Cuthrell get rid of the R2R distributor.
Side note: This is a Drag Racing discussion, why are you launching at 1500 RPM? you want to launch at 200 RPM above your max torque
and shift 200 above your max horsepower.

 

[66GT289]

fst73mav, make sure the coil is compatible with a 6AL box.......

https://documents.holley.com/techlibrary_coil_compability.pdf

 

[fst73mav]

this is strange - that MSD box should fire a .060 gap....wait...
you have a ready-to-run distributor AND a MSD 6 box?

with the ignition breaking up and the smaller gap helping...i'm thinking something's wrong there.
how are you triggering the MSD box?

I have it wired up per the 6AL wiring diagram from MSD.

sometimes i need more coffee before posting...
yeah- swap your coil out. make sure its MSD-box-compatible coil.

if that doesn't do it...i'd start isolating components.
step 1. unhook the MSD box and use the ready-to-run dizzy by itself
if that doesn't do it, my next move would be:
step 2. plug the MSD back in, but remove the ready-to-run and use a 'normal' distributor

I tried to bypass the MSD box and it ran even worse (although it was a bit of a 'temporary' bypass, will likely try to bypass with better connections to rule that out as an issue). That said, I used to run an HEI distributor and had the same problem. Coil is brand new Summit brand that is supposed to be compatible but may try to find an MSD coil incase the Summit just isn't up to snuff.

Have you looked at your float levels? Float levels to high will cause this.
I agree with Cuthrell get rid of the R2R distributor.
Side note: This is a Drag Racing discussion, why are you launching at 1500 RPM? you want to launch at 200 RPM above your max torque
and shift 200 above your max horsepower.

Float levels are good, both just below the sight glass.

Launch at about 1500 (probably closer to 1800, I typically just rev it up to where it is just barely held by the brakes) right now because I am foot braking and I need a converter but I am working with what I have for now.

 

[cuthrell]

found this on another forum...sounds similar to your issue.
(i'm underlining the part that strikes me)

if you can see the timing breaking up with a timing light...might be worth a call to MSD?

quote:
The engine is fresh, and I deceided to install a new MSD ignition system.I picked out a 8360 because it could be run off the electronics in the distributor (ready to run) or with a 6AL box.Installed a blaster 2 coil.
Upon start up of the engine it had violent popping out of the exhuast and I advanced the timming so far as 20 degrees base timming and still it popped,not as much tho. Rechecked cam timming,carbs,vacuum leaks,disconected coil tried another.disconected 6AL and still the problem exsisted.With a timming light attached I could see where the flash of the timming light would skip and as RPM increased so did the miss.
Above 3500 the engine ran on about 4 cyl hahaha (my 4 door 85 escort could out run it).LOL
Talked with MSD tech Joe at Ext 2286 and he suggested grounding the aluminum heads.Still no change. He suggested running off of just the pickup in the distributor so I used the Orange/Black,Violet/Black and the long pigtail to th 6AL.The car no longer Misses spark and the engine revs thru the RPM range all the way with no problems.

 

[Mercury4me]

I have a 302 that I am trying to tune and it has me a bit stumped right now. Been tinkering and haven't been able to get it dialed, hoping someone might have some thoughts. Here's my specs:

-302
-E-street aluminum heads with roller rockers
-Dual-plane aluminum intake
-4-barrel 670 cfm Holley (vac. secondary) with #65 jets
-Ready-to-run distributor, 8.5mm 50 ohm wires, 8-deg base timing, 24-deg at idle with vac advance, 40-deg total advance
-Champion plugs gapped at .040
-MSD 6AL
-31-242-3 Comp Cam
-Full-length headers with 2.5" dual exhaust

It idles fine and off-idle acceleration is fine but I have a hole shot stumble (launching at about 1500-1800 rpm) that I can seem to fix and at about 3000 rpm it will start breaking-up and backfire through the exhaust revving it in the garage. Haven't had a chance to try it at the track to see if the breaking-up improves with the scondaries coming in. Plugs look like it is running super rich no matter what I do.

I have tried changing main jets, timing, plug gap, mech. advance springs, accelerator pump nozzles, accelerator pump cam, and hooked up the vac advance which was previously unhooked. Nothing has resulted more than a minor improvement so far.

Not sure what your choke set up is but vacuum secondaries might mean automatic choke too? how about the choke pull-off?

 

[fst73mav]

found this on another forum...sounds similar to your issue.
(i'm underlining the part that strikes me)

if you can see the timing breaking up with a timing light...might be worth a call to MSD?

quote:
The engine is fresh, and I deceided to install a new MSD ignition system.I picked out a 8360 because it could be run off the electronics in the distributor (ready to run) or with a 6AL box.Installed a blaster 2 coil.
Upon start up of the engine it had violent popping out of the exhuast and I advanced the timming so far as 20 degrees base timming and still it popped,not as much tho. Rechecked cam timming,carbs,vacuum leaks,disconected coil tried another.disconected 6AL and still the problem exsisted.With a timming light attached I could see where the flash of the timming light would skip and as RPM increased so did the miss.
Above 3500 the engine ran on about 4 cyl hahaha (my 4 door 85 escort could out run it).LOL
Talked with MSD tech Joe at Ext 2286 and he suggested grounding the aluminum heads.Still no change. He suggested running off of just the pickup in the distributor so I used the Orange/Black,Violet/Black and the long pigtail to th 6AL.The car no longer Misses spark and the engine revs thru the RPM range all the way with no problems.

Wow yes, that sounds very similar. I actually picked up a different distributor that is not the 'ready to run' type so should be set up like that sounds like their suggestion was. Tried a different coil to no avail so dizzy is next, then I'm calling MSD. For S&G, I'll probably also ground the ever-loving sh*t out of everything.

Not sure what your choke set up is but vacuum secondaries might mean automatic choke too? how about the choke pull-off?

I have it set up with no choke, actually. That said the issue has spanned an electric choke, manual choke, and current no choke set-up.

 

[fst73mav]

Alright, new coil: no noticeable change, new dizzy: no noticeable change...calling MSD next week when I'm back in town. If I get ambitious I may try to go back to the 'starting point' plug gap that MSD suggests (.050) or try the 650 and 750 off a couple of my other engines for S&G. What's also weird is I don't really see the timing breaking up on the timing light.

 

[cuthrell]

hmmmm.
if the ignition isn't breaking up on the light, and you've swapped coils, distributors, used an HEI, etc...trying to think what else it might be.

i'd think at 3k rpm, you're moving enough air to "drown out" any sort of vacuum leak.
keep us posted as you swap carbs around.