Hi,

I was wondering what the pros and cons are to running the single downdraft setup? Also anyone out there who actually runs that system, just whether they like the setup. In the past I had run a dual downdraft weber setup and had issues with the car surging and was told that the issue was the gas pooling in the base of the intake manifolds and then when I stepped on it, it would suck up all that extra gas. Since switching to the dual su setup I haven't had the issue, but I'm getting tired of them not running right after the car sits for a while.

thanks,

Brian

SU's are not magical. Like any other carb once adjusted there is little or no reason for problems. You need to give details as to what is happening with your engine. As for the downdraft, it would work but depends on the carb and manifold arrangement. You'd probably be $$ and time ahead to work out the problems you're experiencing. A SU fitted and tuned L6 is a reliable and fun powerplant.

The problem is that after the car sits for a while it runs like crap. If it only sits for a week or two then it takes 5-10 minutes of warming up to get rid of the little backfires/sputters and then it runs ok. After it sits for much more than that the car usually runs like crap. I put seafoam in the gas tank if it's going to sit for a while.

Tune your car up including valve adjust, ignition, timing, fuel filters, etc. As mentioned above a good set of SUs, properly tuned, run great.

And the guy that told you fuel pooled in the intake manifold under the Weber DGVs was full of crap. If there's so much fuel in the intake track that its pooling somewhere you would see clouds of black smoke behind the car.

Sounds more like ignition to me. A tune up goes far to eleviate such issues. It could also be a sticking distributor too. As for the pooling, well, John is mostly right but I've seen fuel pool in the bottom of an intake when the engine was cold and the throttle was pressed too many times while starting. Especially an intake that dropped down like the conversion for a Datsun L6. I always recommend using coolant to warm the intake, if a provision is provided, on a daily driver.

Brian,

Nissan engineered a single carb arrangement for their passenger cars. Cedrics, Laurels, Kenmary's and Skylines all had them, if not for their domestic market, then for their export ones.

A single carb, in standard configuration doesn't make huge amounts of power and less than a standard twin carb setup. Modified, however, substancial increases in both power and torque can be had from using a larger cfm carb, either from a big two barrel or small 4 barrel carb.

I have run a single Holley carb for years. Two barrel 350cfm then a 4 barrel all the way to a 650 spreadbore vac secondary. On my cruiser, a 465 4 barrel is on there atm, and runs great!!

I really like a small 4 barrel the best, each barrel is small and gas velocity through the carb is kept high for superior fuel atomisation. When all 4 barrels crack open, there is enough cfm to satisfy the engine.

There are obviously factory Nissan manifolds available that a two barrel and even a 4 barrel with adapter plates will work with. The Clifford 6=8 manifold has a massive plenum under the carb base that slows the air velocity down heaps and throttle response suffers. For maximum power, the Clifford is king, however. Its a dedicated 4 barrel manifold as is the Arizona manifold. Entirely different in design, the AZ mani is built for low to mid range torque which suits my application perfectly...ie street cruiser.

The differences with the stock manifolds is mostly to do with runner length and internal diameters. This will move the power and torque curves around a bit but nothing gigantic.

Most of the poor reputation is because some guys don't take the time or have the patience to set these up properly. True, I spent quite a bit of time sorting mine as there is no FSM that tells me how to set it up unlike the twin carb factory setups.

Fuel economy is very good and it starts hot or cold, summer or winter very well, has an electric choke and the hot water stove in the manifold is NOT connected. This manifold is poor in this regard, unlike the Clifford which has a huge stove at the bottom. On the coldest winter mornings, it may take longer to warm up but not like 5 minutes, maybe a 1-2 min. warm up, that's all.

Check out youtube. My 280C (DAT28C) is there, with lots of performance vids all with a single carb. for you to watch.

Good luck with your project

OK I'm agreeing with what people are telling you about a tune up, I think your problem is not going away with a carb change. Don't get me wrong a single four-barrel carbs can be a nice setup, but if the engine is not tuned and working properly your just throwing money out the window.

My question for you has any one showed you how to properly tune these engines like doing valve lashing, adjusting carbs, testing compression, timing the distributor, replaceing points . . .? I think you have a great resource in your back door for this the Florida Z car club is a very active bunch and a lot of members to this site. I've heard how much they help out, why don't you contact them and see if someone would help you tune the car, or recommend a good specialist to tune her up.

Changing out carbs and trying to get around the problem by changing things up gets people into trouble. I hear the new guys doing this with changing and getting rid of points before the car runs right. Why add more things to the problem. I'll tell you that these cars can sit more than two weeks and run well, unless they are out of tune.

By the way what type of driving do you do and what gas mileage are you getting, does your car rev smooth through the power band?

Tune your car up including valve adjust, ignition, timing, fuel filters, etc. As mentioned above a good set of SUs, properly tuned, run great.

And the guy that told you fuel pooled in the intake manifold under the Weber DGVs was full of crap. If there's so much fuel in the intake track that its pooling somewhere you would see clouds of black smoke behind the car.

What John said.

We have a Tune Up video that'll walk you thru about all these issues, if you'd rather do the plug and play. Fixin' stuff isn't nearly as much fun a bolting on new shiney stuff but for what you are describing, needs to be done.

What John said.

We have a Tune Up video that'll walk you thru about all these issues, if you'd rather do the plug and play. Fixin' stuff isn't nearly as much fun a bolting on new shiney stuff but for what you are describing, needs to be done.

sorry to wake up this thread, but where is this said video. i think i could learn a thing or two maybe!

I'll jump in here for a second. I'm the guy who bought DHoneycutt's GC10 Skyline in Las Vegas 25 years or so ago. The L20A Skyline Came stock with a Hitachi 2bbl downdraft carb. It was an okay setup. The installation of a Weber 32/36 DGV made a huge difference in that car's performance. Docile in light to light traffic, but somewhat of a beast when the seconds were dumped open. It'd be alot of fun to talk about the good old days, but I guess Ol Dave's so *****whipped these days that he's not allowed to talk to me... I'm absolutely shocked... NOT!!! Cheers, Scott

spdcrazy, call Bruce Palmer at ZTherapy, the video/dvd is available through them. You WILL learn a thing or two.

Bonzi Lon

ahhh right on! thanks guys. its wierd cuz i read alot of bad things about 4bbls on z cars. but there is usually not much solid info as why. but then every once in a while i read a thread like this that gives me hope!

either way i'm gonna run with this little holley and see how she performs!

I've seen maybe 4 or 5 Zs w/4bbls on them that actually preformed well and many that didn't. The main problem is getting the jetting right. Take a lot of trial and error. Out of the box, a 390 CFM Holley 4BBL, which is generally the one used on Zs, is too much carb. It was designed for engines like a larger displacement V6, 3.8 litre, 4 litre, etc. or small displacement V8s. I think people just give up on them before they get it right.

Edited March 3, 201114 yr by sblake01