Friend looking at a 260Z with DGVs... LOTS of backfiring...

[QWKDTSN]

I test drove a 260Z with a friend today. It is mechanically very sound and fresh but it has DGVs. I've never been fond of them, even from my outside knowledge, but I've never really had a chance to work on them so I'm not sure what kind of adjustments need to be done. Anyway, the car backfires a lot and is obviously very rich. it idles fine but the exhaust fumes are simply poisonous. The reason I mention the fumes is because the muffler has already been blown out and the fumes just go everywhere under the car. A particularly massive backfire literally blew the muffler in half, and it's gaping wide on the bottom, essentially a straight pipe with no muffling (sounds pretty good, actually). While driving, every time you let off the gas, you're greeted with lots of nice poppings and bangings from under the car - even while shifting from one gear to the next. I spooled the motor up through about 4k in 2nd and let off the gas and was greeted with the loudest backfire I've ever heard - it set my left ear ringing for 10 minutes afterward.

 

So, what do you guys suggest? For starters, I know the air filters need a good cleaning. They're K&Ns and VERY dirty (as is everything under the hood). How would I go about leaning the mixture out? I'm wondering if it's an easy adjustment that I could do next time I take my friend to go look at the car (nice piece of magic there, if I could borrow a wrench and make the car run perfectly there in the driveway). As it is, if he buys it, it will get new plugs/wires/dist cap/Pertronix etc.

 

Where can I find info about tuning DGVs?

[Drax240z]

Well your problem could be entirely the dirt clogging the filters if they are really that bad... try taking it for a run with clean filters or without filters and see what happens...

[THUNDERZ]

Putting a muffler on it will probably greatly reduce backfiring. There are mixture screws on the carbs that you might try to lean it out with. I had a set that ran very rich also. The fuel seems to pool in the elbo of intake.

Jim

[Guest Al 260Z]

My current 260 has the twin DGVs, and I used to have a lot of exhaust backfires that trashed standard type mufflers like yours. I did a bunch of stuff including installing anti-backfire valve from a later model 510 that was recommended. I can't say when exactly backfiring stopped, but now I haven't had any for some time, and I've never done anything but clean, lube, and idle adjust on the DGVs, in over five years.

 

I suspect that in order of importance, factors contributing to elimination of backfire include:

1. Installation of a decent tailpipe, with all joints from manifold flange back fully welded (not just clamped). This was with Flowmaster muffler with welded construction that looks highly resistant to backfire explosion. The old flat-four Volkswagens were notorious for backfiring with even the smallest pinholes anywhere in the exhaust system. (I'd recommend a resonator in place of the pre-muffler since Flowmaster by itself is rather loud.)

2. Head job with steel seats eliminating various wierd issues due to loose bronze seats and dubious valve seating.

3. Removal of smog air pump. (My conscience isn't bothering me quite enough to make me re-install it, but I mean to at some point return the car to more/less stock daily driver w/ smog controls.)

 

The fellow that installed my DGVs was pretty sharp and obviously did a decent job in selecting jets. Now that I've installed a big cam and will soon have a substantially improved exhaust system, I'm gonna have to bone up on re-jetting the DGVs. A great deal on jet kits is from IPD Volvo (http://www.ipdusa.com), and they've got parts and a Haynes book. I generally detest Haynes, but haven't seen any alternatives for the DGV. DGV parts are also available from Pegasus Racing.

 

Consider a Colortune for adjusting idle mixture. Highly recommended.

 

Later,

Al