Fireball out of Carb

OhBuoy · #1 (**permalink**) 06-07-2002, 05:48 AM

I have this posted on CobraForum, and have had great input there, but looking for more ideas...

I have a 302 that was rebuilt, it has a Holley 1850S carb, MSD distributor, MSD ignition and Edelbrock Performer RPM intake. First attempts to start were close but futile with a fireball comming out of the carb. Yesterday, we discovered one vacuum port on the carb was open. I plugged it, and subsequent attempts to start were easier, but some yeilded a fireball still. However, I was able to get one good start by depressing the pedal and backing off to about 1/4. After a good 30 second run, I knocked the pedal free (it was stuck, will have to work out that kink) and it died. No fireball, but it died. Subsequent attempts yeilded the fireball.

I have verified that it is at the #1 plug at TDC. Could be 180 out, but then it would not start, right?

I have triple checked the wiring according to what the builder stated the timing order should be.

I do have two holes in the back of the heads open, apparently for the air pump, I was originally told they could be left open, but the forum consensus, along with my gut is they need to be closed.

I have not set the float levels, and quite honestly, with flames coming out of the carb and open headers, I am a little cautious of doing this at this point. Something tells me that adjusting them to the point where fuel barely comes out of the site holes at this time would take care of all my worries with the car, the only thing is, it would not be the way that I want to take care of them (up in flames).

The only vacuum source on the Performer Intake that I was aware of is the port at the neck where the carb attaches. Am I missing something there?

Also, I have a manual choke. If I close the plate, it will not start. Half open seems to be a key, but will backfire occasionally. Full open is definately great balls of fire.

The neighborhood kids are having a blast watching this. I quite honestly, am getting a little nervous.

BTW, engine builder will most likely not be much help on this unfortunately.

CobraEd · #2 (**permalink**) 06-07-2002, 06:00 AM

Backfiring thru the carb is typically ignition related. Bad timing or duraspark module.

Hotfingrs · #3 (**permalink**) 06-07-2002, 08:06 AM

My guess would be timing......

Pete C · #4 (**permalink**) 06-07-2002, 08:10 AM

I had a similar problem, hard start, fireball and it turned out to be a lose contact on my MSD rotor. It slipped out of position in the slot and on cursory inspection it looked just fine. If it slips around, you get variable timing but not in any predictable way! Have a look and make sure that the contact is not sliding in the rotor. Good luck, Pete

PnRCobra · #5 (**permalink**) 06-07-2002, 12:59 PM

Try to run the motor without any vacum lines hooked up. Try to get it started and running so that you can set the running time to 30 degree BTD with the timeing light. my crate motor did the same thing. I now run it without any vacum advance , all mechanical. Works great !!!!!!

dscott · #6 (**permalink**) 06-07-2002, 02:16 PM

When I put an engine on the dyno and it doesn't start within 3 or 4 revolutions there is something wrong. It is critical that you find the problem if this is a new engine or you may wind up with a flat cam. I wouldn't keep trying to start it until you check everything. This is how we start a new engine. Find TDC, you can do this by sticking your finger in the spark plug hole and turning the engine until your finger starts to blow out. You then know you are on the compression stroke. Look in the hole and run the crank back and forth until you are sure the piston is at TDC. Then remove the dist cap and make sure you are pointing at #1 on the cap. You also want to make sure the firing order is correct, there are 3 different firing orders for the 302. You need to check your cam card to be sure. If you have the piston at TDC then make sure you are getting proper fuel, have the float set as was described to you. Make sure you don't have any open vacumn. The engine should start almost immediatly. If it doesn't you could have the cam installed wrong. The ignition timing may be ok but if the cam timing is off it will start but it will flood easy and be very lazy. It may pop back as you say also. The cam breakin is very important. Once the engine starts you want to bring the rpm up to 2000 and then after a few minutes run it from 1800 to 2200 for a total of about 20 minutes. This allows the lifters to spin in the bore and breakin properly. If they don't spin the cam will go flat from the lifter sitting in one spot. I hope something here helps you. Good luck!
Don

Mr.Fixit · #7 (**permalink**) 06-07-2002, 05:05 PM

Pull out plug #1, rotate motor unto "puff" of air comes out of spark plug hole. This is the TDC that firing occurs on, not the other TDC. Timing mark should be at TDC now, if not rotate back a little bit until it is. Pull off dist cap and make sure it is pointing at lug #1. Make sure the firing order is correct it may be 13726548 or 15426378. Make sure the intake valves are actually closing, setting valves "dry" is difficult and it's all too easy to overtighten them so that they hang open a TINY BIT when they should be closed. I had that problem on my very first motor, everybody kept saying "You're 180 degrees out" but I wasn't. Figured it out by bringing #1 to TDC and putting a leak down tester on it, the air came rushing back through the carb. Rotating it over either direction didn't make the valve close, so I readjusted all of them, going easy on the preload, and it fired off nicely. If you have a really wild cam, this can happen every time you start it when cold if you don't do the following: depress pedal to floor all the way once and release, keep foot off pedal, crank motor and it will fire off, then you may touch the gas pedal again to keep it running.

OhBuoy · #8 (**permalink**) 06-07-2002, 07:14 PM

Stopped by the hardware store to pick up some set screws to plug the "AIR" holes in the back of the heads. Picked up the 5/8 18 set screws and headed on home. Had company, so was not able to accomplish much. Snuck off to the garage after they left and tried the set screw. No go. Seems to be a different thread, will try again tommorow.

Now for the other items I checked into. I rotated the engine with my thumb over the #1 spark plug. I am not 180 degrees off. I pulled the #3 plug and rotated again, and I get a puff of air as it leaves the number one and goes onto the 3 on the distributor. So, I think my firing order is correct.

My timin mark does not line up when I am at TDC. Is this easy to fix, or should I just use chalk when I time?

Any other thoughts? Going to bed for now.

Thanks for all the help guys!

JayC · #9 (**permalink**) 06-07-2002, 08:03 PM

Sounds like the timing is way off. Have you tried rotating the distributor as you are cranking it. As Don said don't keep cranking and cranking without it starting or you will have a flat cam. As far as the timing marks not lining up, is the harmonic balancer new? I've seen old ones shift, then the only fix is to replace the balancer.
JayC

SCOBRAC · #10 (**permalink**) 06-08-2002, 01:37 PM

You have a timing problem. Likely one ir more cylinder wires got mis routed. The engine will run out of firing order sequence but not well and if pushed, not long..

The worst part about the fire ball is the eyebrows.. They can take two months to grow back depending how much got singed off.. Blindness is also a concern, but belching fuel has a certain sound as it is burning back up through the manifold and out the carb.. Sort of sounds like a jet taking off.. It is almost certain by the time it hits your face your eyes are closed.. Mine were..

Lesson learned: Never let your dad help you work on your car when you are bent over the fender checking the carb for a reason IT would misfire.. Trust me.. It's the timing

Cal Metal · #11 (**permalink**) 06-08-2002, 08:51 PM

TDC on the compression stroke. Are you running a 351W cam or the original 302? If you have modified it to accept the 351W cam, you need to use the 351W firing order.

OhBuoy · #12 (**permalink**) 06-09-2002, 03:11 PM

I know what you mean about the sound. I was not over it, but I do hear it. I also see a vapor cloud right befor the fireball.

Ron61 · #13 (**permalink**) 06-10-2002, 06:12 AM

I agree with all the above posts about your timing but have one suggestion concerning the float level. I also never liked having gas splash out onto a hot intake so several years ago I got sight plugs for both of my cars and I like them. You don't have to take them out of the side as they magnify the float chamber and you just adjust as normal until you see gas come up to the bottom of them. These are for Holly Carbs and I figured from the way you stated your message that is what you are using. I got mine from Jegs and I think they were around $5 dollars a set.

Ron61