Budman II

iTrader: (1)

Finally getting around to firing up the 489 Chevy I have been working on. Set it up on a test cart before it goes to the dyno on Thursday. First attempt gave me a big backfire out the carb - went straight to the distributor to find that I had the damned thing 180* out of phase. Doh! Fixed that and tried starting it again. This time it idles briefly before BOOM! Had a loud backfire out the starboard exhaust - enough to blow off the corrogated hose I had clamped to the header to keep water from spraying all over the back of the cart. At this point I wrapped it up for the night.

I've been thinking through a few scenarios for this issue. #1 is that the timing might just be backed off too far. Didn't have it running long enough to get a light on it. Also thought about the possibility that I adjusted the valves incorrectly. They are Morel HR's. When I adjusted them I took them one full turn past zero lash, following the firing order. Cam is not excessively radical - fairly mild duration on a 114* LSA. It is running the LS firing order, but I checked this multiple times, along with the plug wire routing. I guess it's possible that I was smoking the same herb the night I set the valve lash that I apparently smoked when I dropped the distributor in.

Anybody want to toss in their two-cents for anything I might have overlooked? Going to start with timing, but if this doesn't fix it I'm not looking forward to pulling headers and valve covers. Have to get this b!tch running before Thursday.

Thanks!

abones

start with a verification of base timing! and priming fuel system carb/injectors, I think you will be fine.

Budman II

iTrader: (1)

It's an 800 Holley. I am having some issues with low fuel pressure with my cart - only seeing 3 PSI at the carb. Might be pump going away, and might also be the fuel tank I have that has a small pickup diameter. Either way, once float bowls are full, I would expect it to idle OK. Exhaust backfire has me thinking about timing being backed off too far - unburned fuel being ignited late in the power stroke, and spent fuel is igniting in the pipes. Anyone concur?

Throttle Fever

iTrader: (9)

Cam timing
Valve adjustment to tight on one or more
And as you stated Ignition timing

Budman II

iTrader: (1)

TF, cam was degreed on installation, so unless I was on the herb during that too, hopefully cam timing can be ruled out. I'm going for the low hanging fruit and play with the initial advance first, if that doesn't do it then I will look at valve adjustment next.

I'm also aware that it is quite possible that the backfire thru the carb could have blown my power valve. I would expect rough idle from that, but would that allow it to backfire out the exhaust too?

Why couldn't this damned thing just fire off and purr like a kitten for me?

fbc25el

Sounds like a ignition problem. What dist. and coil do you have?

Budman II

iTrader: (1)

Stock Tbolt IV, and I think it has the 22* module in it.

fbc25el

Do you have a 4/7 swap cam and forgot to switch the plug wires?

Budman II

iTrader: (1)

fbc, yes, it is a 4/7 2/3 swap cam (LS firing order), and I checked the plug wire routing twice. I think I also verified that it is in fact a 4/7 2/3 swap when I adjusted the valves, but maybe that's the bong talking again.

Budman II

iTrader: (1)

BTW, thanks for all the input guys - all good suggestions. As of now, I think my issue is either initial timing or improper valve adjustment. I set them up one cylinder at a time, and rotated the crank 90* for each cylinder in the firing order. Adjusted intake and exhaust on each successive cylinder, with one full turn after zero clearance. Maybe I need to back them off a quarter turn. I just recall reading that the Morel's seem to work better at one full turn to center up the plunger.