Back fire... big time

springer · 03-12-2003, 06:52 AM

Just as a little bit of background... after a rebuild I've been trying to get the thing to start up for a while now. Had a little trouble with a leak in a fuel line, then the fuel distributor, then it was issues with it not firing. Got all that figured out but now my problem is when I was trying to start it up it back fired. It happened twice and scared the hell out of me both times. Here's my theory as to what I think it could be:

1. There is a lot of fuel in the engine from cranking it over when it wasn't firing and now that fuel is being burned off and causing the backfire.

or

2. My timing is off (in other words I've set it wrong).

So my question is should I keep trying to start it up and see if it's just excess fuel in the engine and hope it starts? If not what would you suggest.

Thanks for the help.

yelcab1 · 03-12-2003, 07:08 AM

your timing IS off. Don't try it until you get it dialed in again. Check the dizzy, that is the easiest thing to get wrong.

targa80 · targa80's Garage

I don't know if the 78 has the same connections on the back of the engine like my 80 has but if it does watch out. I crossed two connectors on the back of my engine after a rebuild. They both go to the CIS. Timing and distributor and spark plug wiring were correct. Tried to start the engine and had a massive backfire with flame come out of the air box thank heaven for the pop off valve. Scratched my head and looked at the CIS diagrams. Found the two connectors and swapped them around. Engine fired right up. In total I believe I have three connectors on the back of the engine. One connector is green and goes to the green plug. The other two looked like they were both red connectors and the plugs looked red. I took a wild guess about what hooked up to what and got it wrong the first time. My engine is out of my car now so when I get home I will take a closer look and post back some more details.

springer · 03-12-2003, 08:50 PM

Pat,

Let me know about those connectors. Sounds about like what's happening on mine but I'm not to sure which connectors you are refering to. I reset the timing of my distributor and it still backfired. So I'm looking for the next posibility.

targa80 · targa80's Garage

I had reversed the two green/aqua connectors which caused the backfiring. The cables were long enough to make the mistake. Once I reversed them to this position the car started right up.

springer · 03-13-2003, 07:21 AM

Pat, thanks for the help. Tried it and it back fired immediately.

Okay so let me run this theory by you guys. I think that I have sent so much fuel to the engine prior to getting it to fire that the cylinders are full of fuel and won't allow the engine to fire up. When one of the cylinders does fire it creates a huge explosion due to the amount of fuel firing.

I think this based on two things:

1. I have fuel seaping out of the top of the cylinders.

2. Prior to realizing the engine wasn't firing I had crank this thing over so much that the engine was beginning to bog down from the amount of fuel in it. You could actually hear the engine struggling. That's when I realized it must not be firing.

So I believe at this point I am going to try to rid the engine of this fuel and start fresh. To do so I believe I need to pull all the plugs, unhook the coil, stick some rags on each cylinder opening, unhook the fuel pump and crank over the engine until the fuel is dispelled.

Let me know if you guys think this is the logical next step.

VincentVega · 03-13-2003, 07:25 AM

If you think it's flooded, let it sit and maybe change the oil, if it's that bad.

springer · 03-13-2003, 07:41 AM

Does it make sense that I would have fuel leaking based on what I described?

McDaniel · 03-13-2003, 08:25 AM

Take the plugs out. Unplug the coil so you don't have spark any
where near the engine and crank it over. This should blow all
the gas out of the cylinders, and also give you a chance to dry
out the plugs. Be careful of spark and fumes.

targa80 · targa80's Garage

The cold start valve comes into play with a cold engine and that provides a large amout of fuel on startup.

Dan Owens · 03-14-2003, 06:15 AM

Hey Bob,

If it's flooded to the point of leaking gas from the cylinders then I would think you definitely have a fuel related problem and not just a timing related issue. Not that the timing may not be off too but even with it off you shouldn't be pumping that much fuel into the cylinders under normal circumstances. That much gas in the engine can do some real damage to the bearings and cylinders as it's probably saturated the oil as well. I'd take a hard look at the oil and change it out if you have gas in it. It seems to me there was a similar thread here recently about this same issue being related to a cold start valve needing to be rebuilt. (I have a DME car so I don't have that issue). That won't fix the timing issue but I'd solve the fuel issue first if it were me so I didn't continue to wash all the oil and lubricant from the cylinders and bearings in the engine. If I'm not mistaken you have a fresh rebuild here don't you? I'd post a new thread to John Walker and Wayne and see what they have to say.

Dan O
84' Targa 3.2

springer · 03-14-2003, 07:09 AM

Thanks Dan... I'll post the question on the rebuild board.

stormcrow · stormcrow's Garage

Can you smell gas? Pull a plug and see if it's wet. If so, pull all plugs and dry them out. Check your timing. If your timing is off, it's going to fire when the valve is open. That's where the backfiring may be coming from. Hook up a timing light and check it out.

Steve

"A Porsche does more then just go fast in a straight line"