new guy on the block

[townsendfm]

I just purchased my 1979 Porsche 928 5 speed which I was told is a rare find. I have one small problem with the car it backfires when I shift gears. any answers on what may cause this problem?

[Dean_Fuller]

Welcome...I would look at cam timing first.

[AirtekHVAC]

Welcome. Please find some time to throw up some pics of your find.

As Dean said, cam timing, perhaps plugs, cap, rotor, wires....

[stepson]

Town,
Where are you located?

[SanDiego928]

Wow, congrats......so cool, so fun....after all the work.....we put into them.....they are fabulously fun.....

pics please.....

[townsendfm]

there are pics in my profile

[townsendfm]

cam timing how do I check that? or just check the timing w/ a timing light?

[AirtekHVAC]

Nice pics....got to do something about that under hood disaster.

[townsendfm]

Right, that engine bay looks crappy. I don't know why someone would do that

[garylewa]

^^^^^^What Dean said in the first response to your ?

[Dean_Fuller]

I believe in the 79 car...timing is done very much like any other V8.

Guys....if anyone have a 78 or 79...please confirm timing procedure.

[townsendfm]

That what I was think haven't owned a car in awhile that I have to actually set the timing with out a computer programmer

[BoiseShark]

There are marks on each one of the cams and you rotate the engine clockwise until both marks are lined up with the small engraved dotes on the pulleys themselves. They are both at the twelve oclock position. Then you take off the cap and make sure your rotor is pointing to number one. It should be the front-most plug on the driver side (If I remember correctly). The rest can be done with a timing light for fine tuning.

I hope this helps.

I have a 79 5 speed as well and made sure timing was dead on so I could rule that one out.

Is it only when shifting or will it do it on deceleration? If you simply rev it up in the parking lot, will you get backfire?

And is backfire coming from the engine compartment or the exhaust at the rear?

[BoiseShark]

Correction to my previous post. The timing marks are at about 11:00 oclock and number one is the front-most plug on the passenger side.

Here is a picture of the passenger side cam first and the driver side second with the grove in the cam pulley

[townsendfm]

It also does it deceleration and rev up from the exhaust.

[metr0p0lis1927]

Is that electricians tape on the plug wires? if it is I'd start with the wires and plugs something looks fishy about the plug wires. Also is that an unhooked length of hose laying under the distributor?

[townsendfm]

Everything is hooked up. Plugs and wires are new. Just need to check out the vacuum lines.

[Rupesy]

I have same the same car. I've had that happen when the cold start injector mounting block on the front of the manifold cracked. Changed the fuel mixture and I was getting the same symptoms. Founds the leak, replaced the block and it went away. There are many other places to have a leak on this engine. It may be that you are just having an air fuel mixture problem and nothing too complicated. I would try to find a source of leaks first before moving anything else.

If you don't already have this pdf, do a search on Google for:
bosch k jetronic fuel injection system

It is essentially the same system as on the 928. It will explain a lot of the different components on the engine and the theory of operation.

Worse case scenario will be to mess with the mixture screw. Just make sure you know where you started.

[john 928]

i have the same problem but i set up the timing like 3 timing to make sure. I know that you have to turn the crank pulley to zero then look on the cam gears. there are small notches on the back of the gear. line up the notch with the marks in the pic that bioseshark posted. that pic is the one on the passenger side. the drivers side looks like a end of a triangle. If it doesnt line up you have to losen the tensioner and take the belt off then put the belt on then tighten the tensioner and check that the marks are right on because when you tighten the tensioner the marks will move when you tighten the belt.

[townsendfm]

Thanks I have the weekend off so I will look into it