dcohen

So my 289 starts and runs great. The only problem is that when I go from idle to WOT quick it backfires through the carburetor. I nearly lost my eyebrows tonight because of it. Is this due to the timing being too far advanced? Like I said, it starts perfect and idles fine. Off idle throttle response also seems to be good.

Any ideas?

oxfordbp

I would say timing. Not sure if it means advanced or retarded though. Someone else will know.

kenash

Hi,
Assuming yours is a stock engine......and
If all is as you describe with the idle, attach a timing light (plug dist. vac Ln to carb) and note initial timing. What is it?
Then re-attach the dizzy vac line and note the timing. Did it advance?
With both the timing light and dizzy vac line attached, gradually increase your RPMs while watching the timing advance. Does it gradually advance with increasing RPMS?
Where does it stop?
performing the RPM/advance test will determine, somewhat, the overall quality of your distributor.
With the idle at (700-800 Rs) and noting the timing advancing "a little" when you "re-attach" the dist. vac hose, will determine to what some extent your vac canister is working properly.
Another test is to simply, disconnect the hose a the carb and suck on the vac line while watching the action of the advance plate in the dist. it should move a little and hold. Otherwise the vac canister rubber diaphragm has ruptured.
Now, attach an engine vac gauge to a full-time manifold port and note the engine vac at idle and the "action" of it's needle. The vac should be high 18-22" with a steady needle. While the gauge is attached, adjust your curb idle mix screws to obtain the highest reading. Wait a few seconds between each adjustment for the engine to respond. Doing this will fine tune your idle mix up through the transition ckt.
What do you know about your float level?, Point setting?

dcohen

The engine is not stock. It has edelbrock performer rpm heads, performer rpm intake, roller rockers, a cam, long tube headers, and a holley 4160 carb.

everything is freshly rebuilt, including the distributor.

The floats in the carb are set to where the fuel level is even with the sight hole.

I have a vacuum gauge hooked up but don't remember right off what it is reading.

kenash

OK, Gotta have those vac numbers and the action of the needle. Back firing through the carb, based on what you describe, can be from a sudden lean condition, late valve timing. What size carb? Was the cam degreed? What initial timing?

dcohen

The carb is a 600cfm. I will have to get check the vacuum gauge again. I also cannot remember the cam specs off the top of my head so I will have to dig out the cam card. I believe I set the timing at 8 deg BTDC

kenash

Hi,
Don't really need to know the cam card specs, only that it was installed and degree'd to insure the cam events are maximized. The vac reading can be useful in how the engine is responding to the power adders. Personally, off the bat, you can use "more timing".
After getting a vac reading, performing the advance timing test, and adjusting the curb idle mix ( use the vac gauge), if all seems normal with this, bump the initial timing to 12 deg. Then road test (under load).
A new carb will rarely work well OTB (out of the box). Without the use of a wide-band AFR source, you're relegated to road testing after each change. Which brings up an important point. Document each adjustment and it's affect (positive, neutral, negative) after a road test. Do not make a bunch of changes at once. One at a time and test.
The power adders you installed will should make the engine run more efficient and pump more air, however, the carb needs to be fine tuned to deliver based on this. The timing will need a jump in lighting the fire. More than likely, the carb is running lean.
This is how I would proceed with what you have noted.
Good Luck.

frdnut

I agree it's either timing or more likely a lean condition..Try advancing the timing and see what happens?..If that doesn't help try more pump shot or bigger squirter nozzles in the carb..Double check to be sure you don't have any vacuum leaks anywhere..