carb backfire - YOINKS!

[jwinkler]

I just replaced my 650dp main body with a proform 750cfm main body, and tried to start the car today. The car fired a couple of times, but while goosing the throttle (probably 1/4 throttle) trying to catch and keep it running I got a BIG fireball out of the carb (didn't have the air cleaner on). That was enough to make me stop trying until I got a little more info on what either I'm doing wrong

That's the first time I've had that happen - is it a big deal, or to be expected when starting a cold motor with a fairly big set of primary barrels?

I don't know what size the shooters are (no clear marking, but I'll call ProForm later), but the primary shooter was dumping a nice shot of fuel.

Thanks,

JLW
(contemplating an Edelbrock )

[MidOHasp]

Are your sparkplug wires connected to the right posts on the distributor? Dump enough fuel in there (goosing the throttle a little) and have a spark plug finally catch hold of some gas while the intake valve is open, that'll do it every time.

On the other hand, if everything is hooked up right and you don't have any shorts or anything, then I have absolutely no clue.

First paragraph is from personal experience, after 37 straight hours of not even sitting down during my engine install, Mr Smart Man (me) hooks up the spark plug wires......sequentially.

The garage sounded like a 12-gauge firing range.

Good luck, keep us posted!!

[jwinkler]

Man, I'd love it if I could trace it to something I'd done other than to the carb, but I haven't touched a thing on the car except to replace the main body since I last drove it.

I just got off the phone with Summit tech support. The tech guy thinks maybe the air bleeds may be too big. He also directed me to Proform Parts tech (closed for the day, unfortunately) to get their recommendations on air bleeds for my motor, and added that he'd gotten a lot of calls about the main body, so it's possible that the problem is with the main body itself.

Any one else tried one of these?

Thanks,

JLW

[392cobra]

Did you fill the bowls before you tried starting it ??

How about the pcv connection on the base.
Was it connected or plugged ??

A new carb to intake gasket ??

[jwinkler]

Thanks for the reply - I cranked the motor until a tap on the accelerator caused the squirters to squirt (my standard drill for refilling the bowls), and I did remember to connect the PCV hose.

I didn't check the carb-to-intake gasket. It's one that's been on the car for a while, and is a thin paper-feeling piece. I also didn't put a whole lot of muscle into bolting down the carb.

Is a carb backfire due to air leaks above the manifold?

Thanks,

JLW

[392cobra]

It can sure cause a lean condition.

It's always a good idea to replace that gasket.

Don't overtighten the mounting bolts/studs.
Besides stripping the intake manifold threads,it can crack the base plate on your carb.
Snug is enough.

[Mr.Fixit]

Cold start procedure for motors that like the catch the carb on fire:

Mash gas pedal to floor once, release.
Then crank it without touching the gas pedal until fire off.

[Bruce Edwards]

A fireball out of the carb can be caused by a lean mix or to much advance for a cold engine. If you are not running a choke the engine will be lean at start up and take a bit before it warms up and makes good use of the fuel you are feeding it.
Another name for a air cleaner is spark arrester. Now you understand why and it did not take loosing you eyebrows!

[jwinkler]

Ah, now I'm starting to feel a little better.

I know I *can* get the motor to fire by following Mr. Fixit's advice, but it won't idle at all until it's warm, so I typically keep it running with moderate throttle. I guess the trick is to keep the throttle open so that I'm not dumping multiple squirter shots down the barrels.

Also, if I understand Mr. Edwards' reply, I should not be as worried about fireballs (and black spots on the garage ceiling I have to explain to the wife) if I put the air cleaner on before I try this again.

Thanks again (and more updates/corrections/suggestions are always appreciated ),

JLW

PS - For those keeping score at home, I'll post an update when the car's running again (hopefully breathing through the choke-horn free, smoothed bowls of the proform!)

[427Aggie]

Be careful with the air cleaner...you don't want a lean misfire and that thing catching...its would be a mess.

Matt

[jwinkler]

Called Proform tech, who forwarded me to Quick Fuel Technologies. The tech guy there was very accomodating, and explained to me why carb backfires happen.

(As probably everyone else on this forum besides me knows) carb backfires are the result of a lean idle condition, in this case brought on by the increase in airflow through the proform body. His take was that given that the motor was running fine with the old 650dp, the problem wasn't a case of a timing issue. Since the dp doesn't have 4-corner idle adjustment, the proform is sending more air into the secondary idle circuit. There's a quick test that I'll do that involves changing the rear air bleeds around, and I'll also make the primary circuit a bit richer.

Updates to follow (for those that are following this breathlessly )

JLW

[Doug I]

We're all fired up to see what's cooking. Having a blast finding out all the info ?

[HSSS427]

I'm going through almost this exact scenario now. Had Holley 4150 - 9379 (no choke, mechanical secondaries, 750cfm, annular flow) replaced the main body with ProForm and also replaced the throttle plate with the ProForm unit as well.

Having a heck of a time getting it to idle. I came close and it would idle at about 1200 rpm. This was with the idle mixture screws on the primaries 2 1/4 turns out from the seat (vs the suggested 1 1/2), secondary mixture screws were 3/4 turn out, I had the idle speed / throttle stop screw turned almost all the way in, so probably exposing the transfer slot too much. However, I also have drilled small 3/32 size holes in the primary throttle plates. This main body also has a idle speed screw for the secondaries and I've now got that opened up some as well.

Base timing is at about 20 degrees and it certainly seems to like more advance in order to idle.

This is on a 427 side oiler with an Isky cam 250 duration .525 lift.

Not sure what to do at this point......
Thanks!
Scott

[jwinkler]

My fix was almost too easy - along with pulling the secondary idle air bleeds (and at the time I reasoned through why that helped the lean problem, but can't recall now), I also richened up the idle mix by turning the screws on both sides out 3/4 to a full turn, if memory serves.

Went to start up the car, fearing that I'd burp another fire ball. I pumped the gas twice, counted to 5, and then turned the key, ready to lunge for the fire extinguisher ... and the dang thing started up like it was warm and fuel-injected!!! That freaked me out so much I shut it down. It took another few seconds for it to register that the car had actually started instead of imitating godzilla.

I did start it again, and put the air bleeds back in, and it still started and ran just fine, right up until the HyFire died. Now I've got holes drilled in the firewall, an MSD 6AL on the workbench, and no idea when I'll get time to get back to her.

Best of luck with your car - all the time I was trying to figure out the problem, I always assumed everything was too rich, and it turned out the problem was it was too lean. It may be that if you take a look at iyour issue from the opposite end, you may see something that will help!

JLW

[blykins]

I don't think the secondary idle bleeds had anything to do with your solution.....I think adjusting the mixture screws probably did it.

The secondary bleeds are fast idle bleeds.....probably not at all used when starting the car and letting it idle. Also, removing the bleeds totally would make a more lean condition. The orifice in the air bleed meters how much air enters.....when you make that hole big, you're allowing a lot more air to get in. That's getting leaner.

Glad you're up and running though. Little problems like this tend to get under your skin....

[SSSnake]

We have basically replaced our entire holley 650 DP with quick fuel parts and ended up with an entirely new carb. The car runs better than ever and sucks more fuel than ever. We have the proform center body, the red anodized throttle plate, and the anodized metering blocks. The metering blocks were the only thing that was basically different from the old holley because of a power valve in the secondaries. The carb looks awesome and runs equally well. Quick Fuel is also a great place to order any holley carb parts.

Glenn

[HSSS427]

I finally got it to idle around 1100 w 18 degrees advance. However, it doesn't want to run very well. Took it for a drive and it was not transitioning well at all and didn't want to run very smooth. I'm concerned that I have a vacum leak somewhere, but have taken the carb off, put new gaskets between the throttle plate and turkey pan, and turkey pan and intake. The other thing I was thinking is that if the vacum is too low at idle, then the Power Valve could be opening and causing it to go rich, thus the need to open up both the primary and secondary plates. Don't know so I'm limping over to a local carb guru tomorrow to see if he can work some magic on it. Hope it's just a matter of getting the screws set right.
Thanks!
Scott

[914fan]

Definatly look listen for an intake leak. That would cause a lean idle condition that WILL cause backfire through the carb. Also check that the corect size idle jets are in the carb (i usually play with webers but i'm sure that a holly is simular in function) Also you could have no problem other than you gave it tooooo much fuel when you tried starting. Now that it fired through the carb you have one or more cloged jets. You may also have burnt / melted whatever. Pull the carb and look at it close. You probably do not need to re-rebuild it but look at it. Blow it out. check the float setting. check your fuel pressure. Basic stuff.
But if I were to put money on it I would say intake leak.