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Triple Webers or Dual SUs?

Chino 240Z

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This weekend at the track, the Dual SU carbs had a problem, after a 1/2 hr. session on the track and a 45 min cool down period, tried to start car again and then started fuel flooding from both carbs at the same time. Shut off fuel pump, check press. regulator, and also check floats & needles all okay. Dribbling fuel from both carbs, too flooded to start. These SU carbs had lots of work put into them before I got the car, needles raised, jetted, polished, ect. and dependable for previous owner. Could both diaphrams or something internal go bad in both carbs at the same time? No gas additives, 76 Cool Blue 100 octane fuel, and just tried using Marvelous Mystry Oil for the Carb oil. Always fired first time, everytime, but this time. I've owned car for about 4 to 5 months. Been debugging car and the rest of the car is dialed in, few quick spins around the block now and then getting ready for the track. This was the first solid run on the car for us.

I have a set of triple webers, manifolds & linkage, and another set of stock SU carbs on a manifold, and also the set of SU carbs that are on the car now which are giving me problems. I'm headed next week to a shop to go over options.

Question: Invest in rebuilding and tuning up the Webers (Which were once on the car, I don't know why previous owner took off?) or should I rebuild existing SU carbs and also the second set of SU carbs for a backup set?

This is a track, road car only, no smog, and looking for performance and dependability?

Input and comments are welcome, cleaning things up getting ready for the shop next week.

Thanks in advance Craig & Cindy.


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From what I understand, the tripple Webers are extremely difficult to keep adjusted properly, and require a lot of patience. However, the gain, of course, much outweighs the stock SU's. I'm staying with the stock carbs. for that reason -- currently converting from the flat-top to the bell-top, however, my 240Z is not being used as a track car. If you don't mind the extra hassle and effort, then I think the Webers are the way to go for a track car. Anyway, that's just my 2¢. :)

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Sounds normal to me :).

Motor and carbs were probably very hot. Boiling fuel in the carbs may be causing the flooding. It shouldn't be a problem with the throttles wide open. It should start right up after a few seconds accompanied by a little black smoke out of the tail pipe. It happens with my Webers too. I think I have heard from other Weber owners it happens to them too. Do you have heat shields for the carbs?

I also wonder if your fuel tank is vented properly?


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but there are certain persons using the dual su's extremely well as in low 12's in the quarter

the su's will make great power if they are tuned properly.

as will the triples

you need to make sure that you have the car to run the triples.

if you have cam some engine and head work i would go with the triples if not stay with the su's

ease of use and tunablity with them will be your friend.

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Thanks for the replies people! I agree about the Webers, the benifit of more Air/Fuel. Very few people ever get triple webers dialed in right. I use to run dual 48s in a VW sand rail years ago and at times talked back to me. I do have confidence in my help with the carb problem. Just want to go in half way educated before ya know what I mean. The other thing about Webers is they may need re-jetting when there is some change in altitude, but I don't think I need to worry to much as the tracks we have lined up are in So. Cal and San Francisco area.

Heat Shields? Yes we have put some tin heat shields below the carbs and also have the headers wrapped too. Boiling fuel may be a short term issue, but these silly things are still spitting back at me today.

Vent? We have the vent hose coming off the top of the "Fuel Safe" fuel cell vent and to the outside of the car. Has been the same for years.

I believe the cam is an Isky cam so it has some lift to it, Nissan rockers & valve train and the head is an E31 but not milled. I like the idea of these Webers and the flow that will come from them, and I agree that though touchy, Webers should be okay since it isn't daily driven. On the other hand a back up set of SUs ready for just in case, is security for expen$$ive track time.

On the Electrical side of things we had just installed the MSD6AL ignition system, MSD High Vibration Blaster coil, NGK Iridium plugs, and Taylor 8mm Spiro Pro plug wires, so I feel we have that side covered.

Oh-well part of the fun is the search for the answers to problems like these. And part of the pain is this gotta fix it NOW addiction! ha ha

We'll keep ya updated on how it goes.

Thanks, Craig & Cindy

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Just one more thought.... since you have a fuel cell and I presume an electric pump, do you have a pressure regulator in the line? I'm thinking that you possibly are running fuel pressure too high and that is what has caused the problem with the SU's... perhaps you haven't done any damage other than just flooding the float bowls....You only need about 5-7 pounds of fuel pressure if I remember correctly.

Just a thought... one that is quite often overlooked too.. :ermm:

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Excellent thought, but we do have a press. regulator. From the fuel cell it goes to a large Fram filter, a "Red" Holley pump, then a Holley press. reg. and it's set for about 4.5 to 5 lbs, I have a guage and an idiot light watching the pressure.

After sitting on the patio last night and talking about what's in the SU carb that can go bad, and there is not much! But. . . the float needle seats are rubber, like O-rings and can dry out as it ages, perhap they cracked after the run in the heat, and it has been sitting for someting too. So tonight I'm hitting the Nissan Dealer up for some SU carb rebuild kits, and we'll take a look.

One reason why the previous owner went from the Webers to the SU carb is that in a hard long turn on the track he notice bogging and loss of fuel from the Webers. The fuel bowl is more square and flat on the weber carb and has a problem supplying fuel in a constant turn. The fuel bowl on a SU Carb has its fuel bowl seperate from the carb and is more of a bowl, round, with its fuel supplying from the bottom of the bowl then to the carb. Drag racers like Webers, no turning.

I think I will stick with the SUs for now and rebuild em. I'll let you all know how it goes and what we find, and I say we because my wife warned me not to do it without her. Get this, she is an Reg. Nurse and is into cars as much as I am. Decorates the home in NASCAR, and demands seat time at the track too, Cool Deal huh!

Later Craig


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  • 2 weeks later...

Okay, so here's the deal with the SU carb problem. Finally had time to get back to them and after talking with Z-Therapy about the problem of both going bad at the same time, and I purchased their tape about SUs (Just SUs) I can't believe how cool these carbs are.

The results: Both floats where way-outta-wack, flooding or as Cindy my wife said "drownding" in fuel.

But how? Well it was a very hot day, and just after a 1/2 hour of pushing it hard on the track, the heat from the engine, headers, and the outside temp., the fuel expanded enough to push the fuel floats to their limit and I not knowing where they should be I could not get the flooding to stop.

What can I do about it?

1.) I learned that they should be about .55" or 9/16" from bottom of lid to float while holding upside down and the float gently resting of the needle valve or grose fuel valve. Bend the metal tab slightly to bring to proper measurement.

2.) I have a heat shield and wrapped headers but that may not be enough to keep the heat away from the fuel bowls. So before shutting off the car, turn the fuel pump switch off first and let the engine idle and bring down the fuel level in the fuel bowls. Therefore if it gets too hot it won't heat up the fuel in the bowl so much that it pushes the floats out.

3.) Some people even cool down the carbs with fans, cool moist rags on carbs. & bowls or what ever with hood up after they have parked the car. I need to find a program that works best for me. But knowing how the carb works and how to drain the fuel out of the drain plugs on fuel bowl, and adjust float, fuel mixture nut, nozzle jet and main needle will get me out of trouble and back on to the track.

Well, I feel a little smarter now, which is rare, and I think these dual SU carbs are what I will go with and put the triple Webers back on the shelf for now. Oh and Steve at ZTherapy nailed my problem on the head 1st time. They do sell beautiful refurbished SU carbs, rebuild kits, and individual parts that you might mess up while tweaking on these carbs. The SU video tells it all!

Thanks Craig

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Yep, It's what you said! But it's not until after I build the whole puzzle that I can see the picture, but it all makes sense now. I need to know what happened, why it happened and how to keep it from happening again. It's amazing how many people at the track that work on cars look at these carbs and say "what the. . . ", wish ya luck and walk away scratchin their head. Oh well, hope I can pass some info and help on to others someday.

Cheers :classic:

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Interesting read think I took it all in.

I can understand the fuel expansion issue I did a physics course and remember that fuel expands alot in heat.

One of the reasons I installed a vented bonnet on my Z was because I thought it would help with dispersing the heat and I wanna build a heat shield and have considered header wrap. It's really amazing how much better my car runs on a cold wet night compared to a hot/mild sunny day.

I tried to purchase Scott's video but I don't think I can without a credit car where did you go to get it? I would also like to rebuild my SU's and get a kit for it as I'm in Australia and can't send my carbs easily enough to him and it would work out very expensive for me.

I was beginning to give up on the old SU's and try tripples myself been having trouble with detonation in my car especially on warmer days.

I think I'll give them a go now anyone know where I can buy just Scott's video?

Or possibly the rebuild kit aswell.

Thanks and good to hear you sorted out the problem.:cheeky:

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Gavin, from reading your entries, I know you can handle rebuilding these SUs, no problem. Steve has been racing for many years at ZTherapy and has alot of knowledge to share to us all. I hunted and purchased 3 good books on Z carbs, shop manuals, and they still leave alot of questions. But when you listen to Steve discuss the history, good & bad points, problem areas, tune-up hints, and watch him take the SUs apart, rebuild, install and tune on the car. . . Dude, we were out in the garage before the end of the 1st tape and had the job done. The SU carb is a funky design but it works, (you do need the 3 major components of the carb set up proper to perform at it's best) and is often put onto larger engines.

If you purchase a set of his carbs, you have 30 days to send back the cores (your old SUs)without a core charge. If you want machine polished carbs with reanodized gold hardware buy his set. They look sharp and run better than new. If you don't care about stained, dull carbs you should buy his rebuild kits and keep your old ones. He also offers a selection of main jet needles for a more hot-rod tuneup. Oh if you buy his SU carbs or rebuild kits, they come with the videos. If you already have the videos he will drop the price of the carbs or rebuild kits.

Gavin, I once had 2 custom Fender guitars built here in California and sent them to a mate in East Ballarat, Victoria Australia, and it wasn't to difficult. So I think you can if you really want to. Let me know if you need help.

Good luck, talk to ya later. . .


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Hey Phred,

When I was talking to Steve @ ZTherapy over the phone he mentioned 3 1/2 was more than enough. And I'll have to look at the video again, but I think they talked about the fuel pressure being from 3 to 3 1/2. . .

I recall some old Webers I had once didn't need alot of fuel pressure either.

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John has a good point..is your fuel tank vented properly?

In another life I had a 200b SSS with twin su's and had the same thing happen(a few times)..and it turned out to be that the tank breather was blocked(which only played up when the fuel cap sealed properly)....

it used to flood like a bitch!!!!

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