I installed a new Edelbrock 600 CFM, electric choke, Non-EGR Carb on my new Chevrolet Performance 290 HP 350 c.i. motor and have a both an occasional carb backfire, (sneeze) and a defined dead spot on acceleration. Used a vacuum gauge to adjust air bleeds, timing light to set timing. Motor starts great and runs at crusing speed perfect. Any suggestions for a cure? Thanks.....
If you have a back fire, its sparking when an intake valve is open. How did it run with the old Carb before the new Carb. flat spots and stumbles are usually because of lean conditions, Oh and don't forget this old saying: "90% of Carb problems are electrical in nature"
It sounds like a ignition issue to me. What did you set the timing at? Also check the distributer cap for cracks of carbon build up.
I've always found they are happier with less pressure than a stock fuel pump has. . They tend to act rich without a regulator, imho.
Below are the recommended timing settings for that engine. Set the timing with the vaccum advanced disconnected. Than reconnect to a vaccum port that has vaccum all the time. This info is from the GM installation guide. Once the engine is warm, set the total advance timing to 32° at 3000 RPM. .
If you are running a dual plane intake such as a performer than you need to get an adaptor that Edelbrock sells part number 9266. This adaptor helps seal the carb to the intake. I had vaccum leaks on mine before I installed this adaptor. I also had a slight dead spot upon take off.
Specs, I have a Victor Jr. manifold, a new GM HEI distributor, 5 psi fuel pressure, new plug wires, new plugs....
that sounds like a race intake for 3000 rpm up did you try changing stroke on accel pump for more shot?? LIKE THE OTHER GUY SAYS GOT IGNITION????? THAT WILL FIRE YOUR PLUGS CHECK THAT OUT FIRST its a true statement
also whats your plug gap i run .065 and thats a minimum, you may be able to go bigger, need a bigger flame for lean mixture, fire goes out and puts raw fuel out tailpipe
The Adaptor the I was talking about is actually a Heat insulator gasket. The part number for the Victory Jr. is 9265. It may not solve your problem but it will help seal the carb to intake better than the standard square gasket and it helps keep heat down in the carb. You might check to make sure you don't have two spark plug wires crossed. I had this on a flathead I owned. Engine would run great until you put a load on it.
54 Chevy, I am using a 1" open spacer under the carb now, I have a 4-hole 1 inch spacer also. do you think changing the spacer would improve the problem. I didn't have the problem with the old motor and old carb. The old carb was a 600 Edelbrock, manual choke. I have about 12 years on the old motor and decided it was time to replace everything with new parts.
I'm inclined to think you have two different problems caused by 2 different things. I agree that the back fire is more likely electrical......or a sticking valve. I would hook up a vacuum gauge to manifold vac. Here is a link to troubleshooting engine problems with a vac gauge. Flat spot....carb...unless you have an advance problem...but unlikely if your HEI is fairly new.
The spacer that you have got is probably ok. I would check for vaccum leaks. Did you check the timing? I agree with ddawg16 I think you have 2 problems. In my experiance a backfire is always a timing, valve, or distributer issue. I know from personal experiance with that engine that you have to set the timing with the engine RPM at 3000 and the timing at 32 degrees. Make sure that when you set the timing to unplug the Vaccum advance and then plug it back in to a full vaccum port afterwards. My 290 ran like crap until I did this. You might look at the GM performance website and download the install instructions if you do not have them. My engine did not come with them.
I'd be interested in knowing what the initial timing is. SBC like 10 degrees or more. I've also had engines sneeze because of lean mixtures. If the car runs all right at cruise speed and it has a flat spot or sneeze accelerating, I think I'd try the inside hole (richest) on the acc. pump arm. Also might look at heavier power valve springs, raises the metering rods faster. Or you might have to change you're jet/metering rod set up to one that is a little richer at idle and the same at WOT. About the only way to figure that out is to look at the chart in the Owners Manual. Larry T
What do you mean by backfire? Backfire, to me, comes out the exhaust and is caused by unburned fuel in the exhaust. Some people refer to popping out of the carburetor as backfire. That can be caused by a lean condition or the engine firing with an intake valve open. Since you say your previous engine ran well, I'd try that carb. If it cures the problem, you know it's in the new carb and you will have to make some adjustments to the new one.
Thanks for all the help and good advise. I found the problem,,,I got a different (new) timing light and found my initial timing was about 1 or 2 degrees set with my old timing light. Once I set the timing at 12 degree advance the motor runs great, no more sneezes and runs nice and cool.. Thanks to the H.A.M.B. guys.....
The Jalopy Journal
