Okay, so my frustration with myself stems from trying to get my 408 set up to run properly, at first when I got it started and running, it ran fine except for a backfire that I could not figure out, I went and readjusted the valve lash SEVERAL times to only have it continue to backfire, changed carbs and some improvement but still a backfire, looking it over yesterday and realized I hadn't checked the firing order, sure enough I had crossed up the #4 and #8 wires, made that correction and it was like a total change.....but, I can now gun it and it jumps right up in RPM's just fine, no backfire, if I hold it at say 2500 RPM's (or anything above and idle) it backfires like crazy! I can remove the vacuum advance hose and plug it off and it stops backfiring at sustained RPM's, so now do you think I need to go back in and readjust the valve lash, I did leave it a bit loose (not to the point of rattling, but just beyond that point) or should I be looking at something else? only mad at myself for not checking firing order first, I don't have a vacuum gauge, but I may borrow one if I can, it seems to have good vacuum on it though, timing feels/sounds like it pretty much on, I will see what it is later today if I can find my timing light.
You didn't say if you were running a hydraulic or a solid lifter cam. The hydraulic obviously is more forgiving. In either case, I doubt that valve lash adjustment is the issue. There are two possible firing orders for the small block ford. Are you running the correct firing order based on your cam card?
It's a hydraulic roller cam and yes I am running the correct firing order, it starts with just a bump of the key, idles fine and jumps right up in RPM's, just backfires like crazy when I hold the RPM's up on it, random but crazy backfiring through the exhaust, and disconnecting the vac. advance stops it, distributor is Pertronix.
The change when you disconnect the vacuum advance is your clue. The back firing is a timing issue. The vacuum advance is not working properly.
Check the rotor in the distributor that is right at the tower that should be getting the spark. You may be getting too much advance that the spark jumps to the next cylinder and the spark finds the wrong wire.
Is your vacuum advance hose connected to the ported vacuum nipple on the carb or directly to the intake?
It should not backfire just revving the engine with no load... As already mentioned, check indexing of dist...
That sounds to me like you just have a simple timing related issue. What's likely occurring here is the result of several ignition curve factors being added together in a cumulative fashion. First is that the initial timing is likely already high enough(but how high is it?.. do you have a reliable number generated from a light?.. or maybe you just hand cranked an energized ignition to pinpoint where the spark occurs on the balancers timing mark?). Second is the centrifugal weights swinging into action(again.. what is the ignition lead number at this "troublesome rpm point" in the timing curve?) and possibly pushing the ignition lead up further than it may need to be when combined with the initial timing number. Excuse the unintended pun but, without a light.. you're always going to be in the dark here. Third is the addition of vacuum assisted advance which only comes into play under partially closed throttle positions(which is exactly why you can quickly "wing the throttle" without similar issue and is simply because that level of timing assist/supplementation suddenly disappears as vacuum drops away with those larger abrupt throttle movements). IF.. the engine is actually idling up in rpm(which in turn causes higher manifold vacuum and now needs to be idled back down in speed with the idle speed screw and have the mixture reset/fine tuned), doesn't run erratically or increase lopiness when the vac assist is run from FULL manifold vacuum source?.. then your engines current tune "likes" the additional timing at that specific rpm. If it causes any of the above?.. you may need to reduce initial timing just a tad more and also utilize an adjustable vacuum advance pot to effectively dial back the amount of vac assist that it provides during closed/light throttle angles. Swapping it over to a ported.. or "timed" source won't help you here either since the amount of vac assist that source provides will still be very similar under very high idle speeds/light throttle angles. IOW, the disty's vac pot still sees the same manifold vacuum level.. only now it's "timed" to assist/supplement well above idle speed/closed throttle angles. You inevitably just end up robbing peter to pay paul and I myself ALWAYS prefer using a FULL manifold vacuum sourcing(broken record here) due to the mechanisms and combustion physics involved under part throttle/low load conditions(very slow charge movement and much reduced density level NEEDS additional timing to increase low speed cylinder pressures). And when fully boiled down, throttle/and low load doesn't get much lower than when your car is just sitting there idling while out of gear.. or with a barely cracked throttle while cruising down main street in top gear at only 40 mph. Ported/timed does nothing there and full manifold vacuum gives you the benefit of additional supplementation when the engine can make best use of it and tolerate it the most. Plus, your car is light and your engine is big, so your combo can likely tolerate more advance than many others on those two factors alone. Not utilizing what the combo will allow will only leave efficiency, throttle response, and power on the table. Fourth and final aspect that often comes into play, is the leaning out of your fuel mixture with such high levels of ignition advance which causes all kinds of hell to break loose as you seem to be well aware of by now. My best advice would be to reflect on how much this investment is worth to you in the long run and take the appropriate and proper precautions to get everything working as it should be. To avoid having to taking it in for a dyno tune will require several key tuning tools. A timing light, a vacuum gauge, couple of basic hand tools, and simple pen and paper to plot your ignition curve at various rpm's. What disty are you running here? Can you give us full build details so we can recommend a "base timing curve" to tune from?
I will check that next, just got home from work. I have tried it both ways with no change. Going out now, I have timing light and Vac. gauge.......I have a pertronix dist. the one I have been running all along, 1994 351w block, 408 Stroker full roller, Scat crank, Keith Black (icon) Forged Pistons +30, icon Forged I beam rods, Edelbrock Performer RPM heads Combustion chamber volume: 60cc, Intake runner volume: 170cc, Exhaust runner volume: 60cc ,Comp Cams XE282HR .565/.574 Crane Energizer Rockers 1:7, 2.02 Intake 1.60 Exhaust, Cloyes double roller timing chain, Edelbrock Performer Intake, Holley 650 DP,
Nice stout street combo there with the same XE282HR grind/lobe design as I'm running on my 385 Chev(but with a hair more duration split and lobe spread). Trust me, you will surely be traction limited until the rear suspension and tire bite gets worked out. Few more questions. What deck clearance? What head gasket thickness? Reason I ask is this. IF.. you didn't control or compromise static compression with dished pistons and/or thicker gaskets and/or a bit of added deck height?.. that combo will EASILY be over 12:1 SCR and not very pump gas friendly unless the tune is dailed in fairly precisely. To give some perspective here.. that very similar cam grind in my smaller/shorter stroked 385 with flat-tops(-5cc reliefs) and a .050 thick x 4.060 diameter MLS gasket and 3 degrees of added cam retard makes 215 PSI of cranking compression(although some of that is due to the gapless top rings I run). So, your combo could be well past that and up near 225 psi if you have it installed straight up. Point is that if you get your timing wrong on a potentially highly finicky combo like this(finicky as in.. you're walking on the razors edge of pump gas usage here) and you'll surely have these types of issues regardless of how that disty ran on the previous engine combo. Which I assume was likely to have had much lower compression than this fresh mill. If any of my above assumptions are actually correct for your particular combo of parts here.. you will no doubt NEED a programmable ignition box to dial all this stuff in. You won't have knock control capability.. but the map based programming capability will make the likelihood of using Premium fuel a closer reality. On the plus side though.. IF.. and I sure don't recommend it because I spent/wasted hours and hours and lots of fuel trying to dial my barely streetable combo in with it before I finally switched to the programmable setup.. but IF you try to run that disty with a mechanical curve?.. you will leave much engine vacuum, low end torque, and throttle response on the table as you are forced to compromise with the design. It will no doubt end up being like a poor mans traction control device used to soften up your bottom end and help the tractability. You're next best bet would be to lock the timing out completely and run a cheap and simple start retard box so you don't piss your starter off trying to crank this beast over with 38 degrees of timing.
I am running KB 750 Step Dish Top .150' forged pistons ( http://www.jegs.com/i/KB-Performance-Pistons/648/IC750.030/10002/-1 ) FelPro MLS .047" Compressed head gasket (10.2occ) deck .003 IIRC, I went out this afternoon and played around on it a bit, got it pretty well dialed I think now, not backfiring at any RPM now, used a Vacuum gauge but it was bouncing around pretty good between 12 and 18hg at idle, timing as best I can tell is at 12, had no backfire with or without Vacuum advance hooked up.
I'm all for fun n games, but you lost me there, Frank. Regardless of the final CR.. this IS still a simple timing issue.
