Since last fall I've been fighting to tune a lean spot out, carb wasn't adjusting properly so I've since rebuilt the carb, replaced all ignition components except the dist itself and the fairly new MSD wires. The engine just doesn't respond to any changes I make to the carb. instead the symptoms seem to get worse.

The symptoms are
lack of power, particularily at part throttle acceleration
backfireing through carb
chugging as if running on 6 or 7
stalls if i punch it from idle

then today.
tinkering around I richened and swapped spark plugs.
gave it half throttle in 2nd gear, the engine hardly making power made a few good backfires through the carb now all of a sudden I have a sporatic light knocking sound coming from what sounds like the bottom end!

Can a lean back fire actually damage engine components or is this most likely an unrelated phenomenon..
This engine was built to handle 150hp shot of nitrous. forged pistons, arp bolts, fully balanced etc.

Any thoughts on what could be causing my such tuning woes?

 

Hmmm, maybe all the pinging and backfiring finally hammered a bearing down...

 

Before you go any further, adjust the valves. Do it WITH THE ENGINE RUNNING. Pay close attention to all of them, especially the exhausts; be sure they are all moving about the same amount.

If that doesn't fix it, it's compression test time. Expect to find 2 adjacent cylinders with low readings.

 

yea i just readjusted all my valves as it sounded like a bent pushrod. everything seems in order there however, and the knocking sounds like it is coming from deeper within the engine. will see what compression test looks like

 

well I finally did a compression test on it and all cylinder are within spec
my lowest cyl was 140 between two 150 cylinders.
all the rest of my cylinder are 150

I also did more tinkering, swapped out plug wires with a new(er) set and i still get the same symptoms.

low rpm dead spot, hesitation, lack of power, bogging and moderate pops through carb.

the engine does not respond to carb tuning at all it seems, plug colour remains same even if i go way rich.
The knock i heard the other day is still there but it seems to go away when the engine is warm. i cannot pin point it with my stethescope.

 

Camshaft may be going flat. Poping thru the carb is usually a sign.

 

even if its a rollar cam?

everything seemed in order when i had the valve covers off. perhaps I will look again more closely.

any other thoughts? thanks guys

 

mine did exactly that, about 2 weeks ago. I followed sofa's advice there, and adjusted valves with engine running, to find 4 "missing" lobes.

With a roller cam it's definately more rare, and if you can see all rockers moving the same amount, then you should be ok.


try clamping your timing light on each different plug wire, to make sure they're all sparking...

 

Thanks guys,
well I decided to dive into my dist today because my gut is telling me its an ignition prob. I isolated the MSD and MSD coil and tryed the stock setup, that didn't make any difference but then Lo and behold I found something. A broken advance weight for my mech advance. Hmm that sure doesn't help does it.
So i'm going to try to find another dist or at least the mech advance to see if that solves it. some how I doubt it though because I dont think a slightly retarded timing would cause backfires like i'm experiancing.
If anything it acts like it's grossly advanced, but at idle the timing is where I usually set it. perhaps with just one weight flying around it might cause the timing to swing dramatically? the timing light didn't seem to show that at idle
and it still idles like junk...

 

take a flat edge and check if some of the rocker studs pulled out.

 

well i got the brand new msd dist installed but i still have the same issues. This time I watched more closely to my timing marks as i aggressively reved the engine. from what it looks like, the timing does not advance unless i rev it up nicely. If i hit the gas quick the timing will actually retard momentarily. if i rev it up agressively it doesn't seem to advance until i let off at which point it shoots off the scale. when i rev it up slowly it seems to advance normally
This has me thinking its a loose timing chain.

 

hmm, timing chain eh? That's kinda rare, and would give you cam timing symptoms as well... How about seeing if TDC is matched with your timing mark on your balancer? I mean, just with a screwdriver in the head or something, if it's wayyy off then it could be timing chain, but i'm kinda leaning towards the distributor... still..

 

dude advance your timing to 36 degrees total.... your timing is too retarded... now for the sound that it is now making listen to these guys thier right... but it really sounds to mee like you need to advance your timing

 

thanks guys, well the mark on my balancer is a bit obscure so perhaps I am reading it incorrectly? So I found TDC, remarked the balancer and timed it from scratch. It seems to run way better now (about 10deg more advanced than it was before) however that made for some extremely hot exhaust temperatures, within minutes my headers were red hot. I richened the carb from the old setting which worked well before, i stepped up 2 stages rich to cool things down. it seems to still like that, so i will try that on the street tommorow. The engine however still makes an awful rattle. I cannot pinpoint where it's coming from.

 

I'd say as others have said... cam and/or valvetrain is damaged.

 

yeah, it sounds like you have a pretty rigid bottom end, no reason for anything to let go.