[mlstang]

I have a 67 coupe with a new built 289. It has about 800 miles since the rebuild. I finally took it out to see what it would do. At wide open from a stop it cut out like it was cold and backfired. Tonite i adjusted the holly carb and it did not cut out but still backfired.It runs great otherwise this has only happened at wide open throttle. I'm guessing it needs a timing adjustment. What do you all think.
Matt

[shagadoodle]

you either need a new accelerator pump on your carb or your carb needs to be cleaned good. I had the same problem and the reason mine did that was because the Accelerator pump a. Wasn't pumping efficiently b. The lines its supposed to pump gas through were slightly clogged. Now that i cleaned it and rebuilt my carb i can take off the air cleaner move the throttle and watch and even hear gas squirting down the venturis. The purpose of the accelerator pump is to prime the engine for the oncoming fuel as obviosly air is going to move faster than gasoline can be taken from the jets. Thus if you have a faulty accelerator pump or clogged lines inside your carb the fuel will be stopped or slowed and the mixture will be super lean causing backfire.

Although before you take your carb off and attempt to clean it and rebuild it. adjust the time and make sure you get the time right.

[mlstang]

Thanks for the response. Its a new holly carb that was put on when the engine was built. I also replaced the gas tank that was rusted before the rebuild.

[shagadoodle]

im sure holleys are the same but when you move the trottle look down the first 2 venturis and watch and see if fuel squirts down there. on edelbrocks there is one thing in the center with 2 small nozzels that stick out about 3-5 mm and squirt fuel down there. If its squirting fuel then i'd look more toward ignition and stuff like that.

no problem for the help i rebuilt my carb from top to bottom lol 3 times cause i missed something both of the first 2 times ...lol i got a good understanding of how they work now lol. i even got an extra rebuild kit for mine just in case...lol good luck

[buzzboy]

I had the same issue with my 289 and it turned out to be a points issue. Cleaned them up with some sandpaper and it ran fine. Now it sports a pertronix unit and I have no such problems.

[Print Dad]

Hello mlstang,
When the car back-fires, is it through the carb or out the exhaust?

As mentioned there are several possible causes such as
1. - Timing off
2. - Points not set right or a coil issue
3. - A lean mixture in the carb. If the back-fire happens when you first stomp on it, it could be an issue with the accelerator timing/adjustment.
If the back-fire happens after the motor is revving high, you could be running low on fuel. Have you checked the height of the fuel in the bowls? You may have one of the new Holleys with no bowl sight screws.

I would try the timing and dwell and then I would attempt to adjust the carb.
What size Holley and is it single or double pumper.

Just for kicks, make certain the bolts on the carb are tight so there is no vacuum leak.

let us know a few more details and we'll see what other ideas we have.

Final thought...at 800 miles on the rebuild, I wouldn't be pushing her too hard just yet. You don't want too many back-fires or you could waste the power valve even though they have some blow out protection. Print DAd

[mlstang]

Sorry if i did not give enough details. I have a new Mallory Unilite electronic dist. I cant remember the model # of the carb its a Holly 650 cfm dual bowl but only has adjustment screws on the front and single fuel line, I hope that helps. Also it backfires through the exhaust after it revs. Also Im still taking it easy on the new rebuild but I was dying to put my foot in it . I'm going to check the timing this weekend. Thanks

[buzzboy]

Running slightly rich and an exhaust leak can cause afterfires. I've had that problem as well but on a different car.

[Print Dad]

Originally Posted by mlstang

Sorry if i did not give enough details. I have a new Mallory Unilite electronic dist. I cant remember the model # of the carb its a Holly 650 cfm dual bowl but only has adjustment screws on the front and single fuel line, I hope that helps. Also it backfires through the exhaust after it revs. Also Im still taking it easy on the new rebuild but I was dying to put my foot in it . I'm going to check the timing this weekend. Thanks

Hello mistang,
Sounds like you have a single pump Holley. Does it have a brass screws on the pass side of the bowls. The screws are about 1/4 in diameter. They are sight caps to check the height of the fuel in the bowls. The newer Hollies don't have the screws and also don't have adjustments for the needle and seat.

If the backfire happens on a rev......then i would guess that it is not fuel related. There should be enough fuel in the bowls for a rev. If however you are driving and mash it....and it speeds up and then backfires....I would think it is gas issue.

I would check the timing first. Because the motor is newly rebuilt it may be a good idea to pull the valve covers and check that the rockers are all adjusted properly. Do you hear any lifter noise at idle?

Check the base of the carb to be sure it is tightened correctly (4 bolts). Double check the vacuum lines etc. It is probably a small thing that needs adjustment...


Print Dad

[mlstang]

Thanks to all who responded. I checked the timing and it was a little off. I adjusted the carb several times and the backfire issue seemed to get worse every time i drove it. I was going to pull the plugs and check them when I discovered the problem. The spark plug wire on the #2 cylinder was loose Duh! I should have checked all that sooner and not drove myself crazy. It running better than ever now with the few adjustments i made. Thanks again!