Carbs boiling possible?

[TBK1]

tripple weber setup , prob is when running car for a long time lets say over 45 mins, IT WILL NOT IDLE, just dies when at a stop, does fine starting and even when it comes up to temp but after it runns and everything gets hot is when it starts.

have header and it is wraped, but no heat sheild.

Could this be a gas boiling in carb prob, if so would the heat sheil make a diff even with the headers wrapped, and if so what material would you manufacter a heat sheild from?

Thanks

[2-4-T-Z-MAN]

Try making and installing a heat shield. Use aluminum as it dissipates heat faster and that should solve your problems.

Good luck.

[240ZX]

Have you checked the state of tune on the webers lately?

[TBK1]

Sorta, have never got them to sink right and havent found anyone experienced enough to help sort it out, it's close, runs great except the idle prob, all plugs about the same burn !

[jmortensen]

If you don't have the carbs synched that will cause idle problems. Get yourself a synchronizer tool and learn how to use it. It's pretty damn easy on triples, you basically just adjust the linkage on each carb to open or close the butterflies until they're pulling the same amount of air.

Stainless steel would be a better choice for a heat shield. Aluminum dissipates heat faster because it conducts heat better. The idea of a sheild is to block something, not to dissipate it. Aluminum makes a good heat exchanger (radiator) but not the best heat shield.

[zcarnut]

Make sure you have the 3/16 inch thick plastic gasket spacers in between the intake manifold and the carbs. Otherwise the heat from the head conducts right into the carb bodies.

I had vapor lock problems with my Webers until I installed these spacers. I also made a heat shield to install below the carbs, but I never needed to use it.

[240ZX]

If you don't have the carbs synched that will cause idle problems. Get yourself a synchronizer tool and learn how to use it. It's pretty damn easy on triples, you basically just adjust the linkage on each carb to open or close the butterflies until they're pulling the same amount of air.

Stainless steel would be a better choice for a heat shield. Aluminum dissipates heat faster because it conducts heat better. The idea of a sheild is to block something, not to dissipate it. Aluminum makes a good heat exchanger (radiator) but not the best heat shield.

Hey Jon, just a thought on the subject of heat shields. As you mentioned about the aluminum....it heats up and cools down quicker than SS but the SS will retain the heat longer than an aluminum will ever though it may take longer to heat up. Eventually the SS will heat up and create a heat radiator. So the question is, which material will keep the radiated heat from the carbs the best? I would think that the aluminum may be a better choice since it does dissapate heat quicker. What I'm suggesting is that any airflow, form a fan or from the air turbulance in the engine bay, across an aluminum heat shield will carry away heat better than the SS and consiquently provide less radiated heat to the carbs. Just some thought for discussion! Of course, a nice CF heat shield would even be better!!?

[jmortensen]

I'm not a heat shield engineer, but I've always thought that stainless is the way to go. If the stainless doesn't get as hot in the first place because of its weak conductivity, then it won't matter if it retains that lesser amount of heat for a longer period of time. In the How to Modify book there is a picture of a heat shield on a set of Mikunis and it has two pieces (p110, top left). The piece between the header and the bottom of the intake and carbs is stainless. Another piece that connects to the carbs behind the air horns which doesn't get direct heat from the manifold is aluminum. If you look at racing brakes the caliper housings are aluminum to dissipate heat and the pistons are stainless to block heat from transferring into the caliper. I guess what I'm saying is that I'm not convinced that there is enough heat coming off the manifold to get the stainless that hot in the first place.

[jmortensen]

I just recalled that a friend of mine made a nice shield with aluminum. He used 2 sheets of aluminum and sandwiched a header blanket in between.

[TBK1]

I have the sync tool but when I have attemped to use it always seems I have 1 carb I cant get close think it is the back one! The way I was told to sink was as follows:

back idle speed screwout till it doest touch linkage, then screw in till they just touch.

turn the mixture screw in till seats (carefully) and then back all six out 2 turns

then take the inspection screw cover off and adjust each speed screw till the buterfly just barely covers the 3rd hole (closes to the intake) and dont touch them after that.

Then use the mixture screws to adjust youre idle speed and flow

obviously I am not understanding things or was told wrong!

Can someone elaberate?

Here is my setup

77 280Z

2600 from 260Z

Triple Weber 40 DCOE two barrel carburetors. with K&N Air Filters

Bored 0.030" over

New Wiseco pistons and rings

ARP rod bolts

New main & rod bearings

New oil pump etc.

E-88 head with all new guides; all valve seats replaced with hardened steel

inserts.

Erson Cam and new rockers/followers (I forgot the cam specs but have them

in a file)

Mallory Unilite distributor - Centrifugal advance

MSD Blaster II Coil

Morroso Plug Wires

Holly Bypass Fuel Regulator

Optima Battery

6:1 header

2 ½” Exhaust, W/Resinator and Turbo Muffler

Hyperflex Urethane Bushings Thru out

Alternator, fuel pump, starter etc.

Centerforce dual clutch.

5 speed attached ('83 ZX)

White Face Guage Modification

MSA Areo II Body Kit

Tokico HP springs and Struts

[jmortensen]

I have Mikunis, but I don't think it should really be any different. I definitely didn't follow the same procedure you did.

What you're really trying to do is get the butterflies open an equal amount so that all 6 cylinders draw in the same amount of air. Whenever you get stuck, remember that you just want the carbs to draw the same amount of air.

The first thing I noticed when I originally installed my carbs was that one of the linkage pieces was too long (I pieced together my linkage from several different setups). This caused the other carbs to be stuck a little open all the time, and the one with the linkage that was too long was completely shut. So first thing I had to do was shorten the longest link so that all of the carbs would sit on the stops, with the butterflies closed all the way.

In your case this probably isn't an issue, but what you can do is set all the idle speed screws fairly evenly (just eyeball it and make sure you have available adjustment in both directions). Then loosen the linkage bars that connect the carb to the long rail, and let them move around until the idle speed screws touch the stops, and there is no pressure on the linkage bars. That should be the last time you have to adjust the long bars. Everything else should be done on the idle adjust screws on each carb or the idle speed adjust on the main linkage bar.

Start the car. You'll probably have to open the idle speed adjuster up a bit to get the car to a comfortable idle speed. Once you have it idling at a normalish speed, you can start using the synch tool. Unless your throttle shafts are bent, each barrel of the same carb should flow the same amount of air, so what you're doing is adjusting the front, middle, and rear carb. You might want to check the throttle shafts by checking one barrel of each carb quickly against the other just to rule it out as a problem. When that is done, pick one carb and check it, then jump to the next and compare. If one is much higher than the other, you can either turn down the one the idle adjust screw on the carb that is higher, or open up the one that is low. Once you have them relatively even, then move to the last one. Do the same thing until the last is relatively equal to the other two. Once you have them roughed in then you can go back and really dial them in so that they're all even.

I only ever adjusted them at idle, but you can also open the throttle a bit and do the same at 2000 rpm if you want.

What I would really stay away from is playing with the idle mixture screws. You do NOT want to play with them in order to get the carbs synched, and you always want the idle mixture screws to be in exactly the same position. On Mikunis, that position is 1.5 turns out.

That's all there is to it on Mikunis. I haven't synched my carbs in about 2 years because the car has been torn apart, but I don't think I'm forgetting anything there. If I am I'm sure someone else will post.

After writing all of that I checked the How to Modify book and it has only a very brief description of synchronizing that basically says "adjust the throttle plates and/or linkage stop screws". It goes on to say that the mixture screws should be one half to one turn out. If you need to turn them out farther, your idle jets are too small. The idle mixture screws should be adjusted until you get a smooth idle (should be all set the same). Check synchronization again after setting the mixture. Then raise the idle about 3 to 400 rpm and if you have the idle mixture correct, turning the idle mixture screws either in OR out should lower the engine speed.