infoleather

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gonzilla

I LOVE those seats. And great color combo!

MongooseGA

I got my car back on the road this week. Been running great until today. At cruise, in 4th or 5th and around 45mph, there was a quick loss of power and then a loud backfire POP from the exhaust.

It did it a few more times before I was able to get it home. A few times it actually completely died while driving. I checked the backfore valve in the airbox to make sure it hadn't come unseated. It was tight, no issues there. Nothing seemed to be out of place, no fluids seemed to be leaking within the engine bay...

My first thought was fuel delivery. When it's under a load, the engine runs smooth and strong. That, to me, rules out a spark issue. Idle is pretty smooth as well. When cruising at a constant around-town speed, the power loss occurs. I didn't change my fuel filter so that is first on the list to check since I already have one sitting at home. I'm thinking there's some sort of clog in the filter that is overpowered (?) when the RPMs are up.

Other than that, I'm at a loss. Any thoughts?

MillenniumFalcon

Check vacuum advance mechanism to see if it's sticking- could also be mechanical advance- don't rule out spark just yet... ask me how I know. Even a little too much Ignition advance at the wrong RPM on these engines causes them to backfire.

MongooseGA

Would you mind describing the location of the vacuum advance mechanism?

As far as timing goes: It was advanced a little by the mechanic who went through the car before I bought it. I put probably 4-500 miles on the car in the few weeks after purchase and had no issues. I take the car down for about 2 months and this problem comes up. The curious bit is that I didn't mess with the timing at all.

I would think that if it were a timing issue, it would have been apparent in the first few days of driving it again and not all of a sudden.

I'm by no means an expert, so I could be completely wrong.

Either way, thanks for the input

MillenniumFalcon

The vacuum advance mechanism is on the distributor(the big round metal diaphragm with the vacuum line going into it- the vacuum line runs up to the throttle body.

The advance/retard plate in the distributor can hang up or seize for a number of reasons, as can the mechanical advance system that uses the weights that is under the plate.

You can do a little testing with a timing light- by manipulating the throttle and watching the timing advance and retard with RPM, and then by sucking on the vacuum line for the vacuum advance and watching the timing move.

Do not set the timing on your engine to anything other than the factory spec at idle- advancing your engine can lead to backfiring. I'm not a Porsche guru, and and I don't understand why this is true, but it is- this engine doesn't like to be advanced. The spec for my '75 is 950rpm idle speed with a 5 deg ATDC (yes, not advance, but 5 degrees of retard) at idle with the vacuum advance connected.... grab a manual and check it out. Set timing and idle at factory spec, then check your mixture setting.

Also: timing can change with points that wear or with an unlubed distributor cam- if the distributor guts are beginning to stick, that changes the timing also. I'm not saying that ignition is your issue, only that you have to be certain that the ignition is functioning properly before looking at the injection system.

I had these distributor problems in my '75, so I'm just sharing my experience with you. Hard to say what is really going on without knowing more, but if the timing was advanced beyond stock, put that back to stock setting first and proceed from there. Good luck!

MongooseGA

Thanks a ton. My parents' neighbor was explaining timing adjustment to me and would probably be able to help check it out on my car.

MillenniumFalcon

Find someone with a timing light... once again, there are guys on here who know heaps more about these engines than me- hopefully they'll post something here on Monday. One other thing- my car was doing the same thing yours was when the coil wire was loose in the coil tower- it ended up frying my CDI ignition box. The car would start and run fine, but for some reason it didn't like to cruise- it would start misfiring and backfiring. Hope this hasn't happened to you.

One more thing: never connect any wires from any testing device to the terminals on the ignition coil on this car (a big, expensive no-no).

MongooseGA

Just checked all the wire points on the distributor cap. All wires are snug and secure.

I've had lots of other suggestions on the vacuum lines as well. Will get on that ASAP. Unfortunately I don't have any time for the next week or two to do anything.

theiceman

Does it have a cat ? ... If it does it sounds like it might be clogging or breaking up.

MillenniumFalcon

SO many age-related things to go wrong on these older cars... stuck distributors, wrong spark plugs, bad wires, rotted vacuum lines, clogged exhaust, crud in the fuel system, rotting brakes, etc... and all of them mostly from neglect from PO's... I've had every one of these problems so far.

2.5 months of combing through each system on mine to finally get it healthy...

MongooseGA

I guess I'll start with the easy stuff. If I can get the engine pulled this winter with my parents' neighbor, I plan on going through a lot of it to replace or clean what I can. Fuel lines, boots, etc...

Sigh. We'll see. I did just start the car up after sitting for 5 days. It's cold outside (maybe 50F) and she fired right up. Smooth idle, just the slightest bit of hesitation when I blipped the throttle. It's always done this until the engine's been running for a few minutes.

I'm going to drive the car from my house to my parents' tomorrow night (2-3 mile trip). If I don't experience any trouble, would it be safe to assume that the issues I had last week were heat related? If this is the case, would that allow me to rule out possibilities and focus on others?

I get how diagnostics work, I just don't have the best experience on knowing what's what...

Jay Gratton

This thread was a great read! Nice find on the 911 and it sounds like you are enjoying it and learning a lot. Congrats!

MillenniumFalcon

Hard to say GA.... if the car stops misbehaving, then it's certainly possible. Good luck

MongooseGA

I can say that in my 5 minute drive tonight that nothing bad happened. I did make a slight effort to keep the revs up as I had in the back of my head it would stall at steady cruising speed. There was a small backfire right after I left my neighborhood but smooth sailing after that.

The car will likely remain at my parents' house from here until Spring. My complex tagged it for tow since I don't have a front plate or inspection sticker on it. I'm also putting together my list of items to replace for when I drop the motor this winter. I guess I won't be driving it for a good while.