ownor

iTrader: (5)

hi there guys,

i know this is not a vette forum but it's all the same GM family right? anyways my buddy and me built this 350 engine in an 84 CFI Vette. it's got a rebuilt bottom end, patriot heads, comp cams 280h, weiand x-celerator single plane, BBC TBI and an EBL flash along with a lot of other goodies.

first break-in try was in january. we couldn't immediately get it to start up, which i think was a problem of us not setting up the TBI & EBL the right way. had put some MoS2 and other cam lube stuff on upon cam & lifter install. after some starting tries we realized we had bent some pushrods after they fell off because we had by accident got non-self aligning rocker arms and weren't using guide plates.. ordered & mounted guide plates, re-lube'd lifters (didn't remove intake, fiddled around with magnets through the pushrod holes in the heads to pull this off). then at some point finally got to break it in for 20 minutes and was all looking so far so good. when restarting a couple days later, we got some intake backfiring, which btw also happened sometimes during break-in startup tries. went to measure lift at the spring retainers and found #6 exhaust dead

second try after getting new cam & lifters. also ended up buying 1.3 roller rockers but they wouldn't fit. so installed some old stock L69 springs to keep load on the lobes lower. lubed it up even more and also put ZDDPlus in the oil. again had troubles getting it to start. lots of intake backfiring again, some times also "hydrolocking" it with too much fuel (i.e. cranking would stutter briefly). so again had troubles with dialing in TBI & EBL and nailing down distributor timing. at some point got to fire it up and broke it in for 10 minutes (local SBC engine builder's advised around 5 minutes would suffice..). seemed to be fine safe for a tick. measured lift and it all seemed good next day so we decided to change springs. trying to start it, no success. lots of intake backfiring and throwing fireballs out. not much of a change with messing around some more with EBL and distributor timing. eventually would fire up with like 30ish% TPS and obviously get into the 3-4krpm range, then die from reducing TPS. compression checked out ok on all 8 and also compressed air didn't leak much when changing springs, both checks done before and after. removed valve covers several times to see if anything was wrong, reset lash, etfc.

don't know what else to try right now. the cam might as well be dead again by now feel like still got a lot left to learn here. i've been working EBL on some stock and one modified engine so far so i'm not new with this one either, but the combination with the flat-tappet break-in just makes it so much more crucial. not sure if this is a mechanical issue or tuning issue now any more, or maybe a combination..

please guys, i know there's a lot of knowledgeable folks on here. i'm asking you for some constructive input, thanks in advance!!

Night rider327

iTrader: (13)

Sounds like timing out to me. The timing off will send fireballs out the carb/tb, backfiring and even cause the starter to drag when trying to crank it.

ownor

iTrader: (5)

you talking about valve (cam-crank) timing, right? did a dot-dot alignment. i guess an open valve cover and #1 spark plug out to check TDC should be enough?

86LG4Bird

iTrader: (1)

No, I'm pretty sure he's talking ignition timing.
Dot-to-dot on cam to crank is plenty close.

ownor

iTrader: (5)

Ok.. set #1 to 10ish BTDC at the crank pulley tho.. would the ignition/distributor timing typically be heavily retarded or advanced if it backfires through the intake? i guess too late (retarded) would shoot into the exhaust system, and too early (adv) into the intake?

ownor

iTrader: (5)

ttt for further ideas please

ddeennis

Well if you had it running then swapped springs and won't start now maybe lash to tight hanging open valves cause backfire and no start, try setting lash at 1/2 turn.

steve87ta

iTrader: (4)

I agree, maybe do a compression check and see if the valves are set right. While there is a lot of flat cam break in talk the last few years, and they are tricky, my experience is that the milder the profile and spring pressures they are not as crucial. I probably would have broke it in with a known carb and ignition and then started tuning. Of course I have the stuff just laying around here... Good luck and report back on what you find.

ownor

iTrader: (5)

i think denis' approach is the most straight-forward and since as he states the springs were the only thing swapped, lash might be the culprit. buddy set it up with half a turn when swapping the springs, then we checked and reset it later to 3/4 turn but both didn't really change things.
the profile is not that mild (i guess).. this one:
http://www.summitracing.com/int/part...67-4/overview/
http://www.compcams.com/Company/CC/c...?csid=122&sb=0
but given the mild old stock springs it should have been a bit more mild, yeah.
don't have a known good carb kicking around, feel more comfortable with EFI however this recent experience here is kinda uncomforting
compression check was done and checked out ok but need to ask if it was done with the old springs or new ones.

steve87ta

iTrader: (4)

Yeah that's a healthy cam. I would normally think higher pressures than they recommend, but they built the cam. Maybe see if you can time it.
I would think it may have bent some valves but you say the compression is good?

ownor

iTrader: (5)

thanks for hanging in here steve. compression check was done with the old springs, right before checking valve lift at the retainers and then swapping in the new ones.
could any case of too weak springs lead to this backfire, assuming valve timing is ok? or other spring-influenced parameters, coil bind or whatever?

steve87ta

iTrader: (4)

Not in my opinion. Increasing valve spring pressures with performance cams is done to keep the valve train stable with the steeper ramps and higher lift. In my experience if they are to soft it will break up at rpm, or float the valves,but I don't think you are running it that hard yet.
I would check it all out if some push rods were bent. Easy to hurt the valves.


Many years ago a friend bought a malibu that had an old 327 trans planted in it. It had a bad valve and the carb looked like old faithfull with it spitting gas all over. Still would burn rubber though.

A lean condition will cough in the intake at tip in or with no ignition advance.
Good luck.

ownor

iTrader: (5)

Yeah, that's kinda what I thought on the softer springs. It shouldn't cause any trouble for idle or low-rpm operation, say up to 2500 or maybe 3krpm for a break-in, without and load (just P/N) for now. Would coil bind maybe cause a valve to not fully open or something? Even then it should be kinda fine concerning the problems we see, as long as it seals properly.

As said on first attempt with the first cam, we had some bent pushrods (well actually half of them snapped) as we were stupid enough not to recognize the difference between self-aligning and non-self-aligning rocker arms, which is what we were using but weren't running guideplates back then.. you learn something new every day.
Since compression always checked out good (for fresh rings, FWIW) and compressed air leakdown rate when swapping springs seemed acceptable as well, I don't think it would have valve sealing problems.. any more thoughts on how to prove this?

It might as well be a lean condition but I remember that at some earlier point we pulled some fuel since it sometimes wanted to lock itself with too much fuel, in a way so that the starter would briefly interrupt cranking during start attempts. Checked plugs and most of them were wet with fuel and you could also smell it was too rich. Maybe it's just real hard getting the cranking fuel right at 10°C ambient temperature, with this large single plane intake without any heat to it, on a cold start.

On the topic of ignition timing, we set it up around 10° BTDC at the balancer and timing tab, with the EST disconnected using a timing light. EBL calibration is set up with the same amount of initial timing and using a modded '113 headed, same cam, similar combo's timing tables (EBL F 3001 template binary).

Btw would a good amount of intake backfiring and fireballing scorch the intake gasket or any other gaskets that could cause vac leaks? Could also cause a lean condition with vac leaks I guess, or would that be compensated by the measured MAP value?

Thanks again.