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I 2nd Travel'n Man's comments on David Premo. I called David looking for help with a heat soak issue I've been fighting for years and not only did he immediately nail the fix he also suggested a jet change that really helped the top end power. This guy clearly knows how to tune Mikuni's.

I do have a fuel return line to the tank with an electric pump. I ran it that way to help prevent fuel boiling but apparently that is not enough to prevent boiling from impacting the pilot operation. I would be willing to run a drag race style cool can inline if that would solve the problem.

Thanks for the response John. I have noticed fuel being in the filters when removing them but to the point of dripping. Is that caused from the boiled fuel vapors settling in the filter or do you sustpect fuel slosh? The carbs were modified by the PO (came off a dedicated autox race car) to prevent fuel from spilling into the jet blocks. Install longer float bowl vents? Is that so the sprayer water doesn't get in or some other purpose? I'll ditch the aluminum heat sheilds and make a new one of stainless. BTW. I LOVE the Giken LSD you sold me! It totally transformed the cornering abilities of the car. I can precisely steer the car with the throttle now and go full throttle out of turns. I'm very impressed with it. Thanks again.

Hey guys. I would like some advice on what I believe to be a Mikuni jetting or tuning issue. Sorry it’s long but here goes… The issue is as consistent as clockwork. At our local AutoX events, we do 3 runs in the morning (day 1) and 3 runs in the afternoon for (day 2). On warm or hot weather events the following occurs: 1st Run – Car runs excellent 2nd Run – Have trouble launching off idle at the starting line, once going 10+mph the car runs excellent 3rd Run – Slightly erratic idle, hard to launch, once going 10+ mph the car runs excellent Lunch Break (Cool Down) 4th Run – Car runs excellent 5th Run – Have trouble launching off idle at the starting line, once going 10+mph the car runs excellent 6th Run – Slightly erratic idle, hard to launch, once going 10+ mph the car runs excellent Trouble Launching – The engine sputters when attempting accelerate off idle (like the pump jet system is inadequate). Heavily revving the engine before launch seems to help. Car Details: L28, mild cam, Mikuni 44s, ceramic header, good exhaust, MSD, aluminum radiator, electric fan. I assume heat soaking is the root cause but I can't seem to overcome the impact. Heat Soak Prevention: Ceramic Header + Exhaust wrap, home-made double-walled (with air gap) aluminum heat shield covered in DEI heat shielding, & a home-made DEI wrapped aluminum heat shield mounted to the bottom of each carburetor. The top of the carb gets hot but is not too hot to touch when the issue occurs. I can drive the cars for hours in 100 degree Arkansas heat and hot lap it at the drag strip and this issue never occurs in those scenarios. It only happens on my 2nd and 3rd runs on track day. Unfortunately my wide-band broke so I can’t use it to troubleshoot but I assume the car is going Lean. I have done extensive tuning with a wide-band, G-tech, and large pile of jets combinations and I’m pretty sure the tune is good but obviously it’s not perfect. I think I need to modify the pump system or step up to the next larger pilot jet. Before I order jets I thought I would create this post to get some input. Any help would be greatly appreciated.

Here is an pic of my linkage setup which is a little different than the one above.

I took the car for a spirited 10 mile drive today and the carbs remained bone dry before, during, and after the drive. It was only 40 degrees outside so the carbs never got hot and actually felt cool to the touch when I got home. I did a quick balance check at idle and they were all the same. At this point I’m starting to suspect that some debris may have jammed the float for a while and then dislodged and passed through the jets.

Thanks for the balancing procedures. I'll do that as soon as I get a chance to drive it again. The carbs have remained dry after a few days of sitting so the leak must only occur during or after driving.

The idle is consistent but I figured it would be more accurate to sync at a higher RPM so I was attempting to do it around 2500. I had a few too many clicks with the zip tie and eneded up closer to 4500 RPMs.

I got lucky because the carbs actually came with 135,140,145,150 main jets, 170,180,190,200 air jets, 52.5,57,5, & 60 pilot jets, 34 & 37 mm venturis. I installed the 34mm Venturis first. I tried probably 20 different jet combinations tuning with a wideband & Gtech to see which combo worked the best. I think I ended up getting the best A/F & HP results with the 190/145/60 combo. I don't remember what pump jet I used. I do know all the jets match though. I may check with the PO to see if he remembers the issue but they came off a race car so he may not know. They also sat in a box for 3 years before he sold them to me.

I did balance the cabs during install using an unisync. They seemed right but I had a tuff time keeping RPMS consistent. I plan on balancing again this weekend. Anyone know a trick to keep the RPMs up? I tried a zip tie but that did not go well . It was early in the tuning process and running way rich. The RPMs went too high so I turned the car off and it let off a LOUDEST shot-gun sounding backfire I had ever heard. Multiple neighbors were looking out their windows to see what happened. It was pretty embarrassing to say the least. I agree a fire extinguisher is a must. I have one mounted behind my seat. Tonight I floated the floats and they are consitant; good idea btw. I disassembled all the float valves and they were clean and functioning properly. All the float bowls and banjo filters were clean. I checked the venturis and they are tight. I reassembled everyting and let the electric pump run for a few minutes. So far everything is dry. I'll try to drive it around tomorrow without the filters and keep checking for leaks. I didn't do anything to correct the issue so I should be able to replcate it. We'll see what happens when they get hot.

I have the Mikuni service manual and adjusted them to 12mm per the instructions. I pushed on the needle valve when I was checking the floats last night and they seemed correct. I should probably remove the valve assembly to get a closer look on the inside. Maybe the "push test" wasn't actually proving anything. A stuck float or heat-soak sounds like the only possibilities so I'll keep digging until I can prove one or the other.

I bought them used and I adjusted the floats before installing them. I don't know if they have always leaked or if this is a recent problem because the fuel doesn't seem to leak out the K&Ns. Only the front carb had fuel on the filter. The others just had a drop on the inlet horn.

I do have the rubber isolaters. I was able swap jets by hand immediately after driving when I was tuning and they weren't too hot to the touch then. I'll try to get it back together tonight to do some more testing. I found some 1500 degree heat wrap tape at oreilly for $17 a roll that I may stick to the bottom of the heat shield.

I checked the carbs and the float level is correct and the needle valve seems to be functioning properly. I have never seen fuel leaking while the car is running or not but one filter created a puddle when I set it on the floor so I'm not sure when its leaking. The car runs excellent, the A/F is near stoic at idle/cruise, and low 12s at full throttle. I'd like high 12s at full throttle but I couldn't get it there with the jets I had. If the carbs were leaking while running, would there be a noticeable difference in the drivability/AF readings? Great thought on the heat soak. I have actually been suspecting a heat issue. When I AutoX it runs great during the runs, but it has trouble idleing and launching after the first run. Oddly this is never an issue hot-lap drag racing or driving it for hours. I have a ceramic coated header, header wrap, and an aluminum heat shield I built out of a double-walled aluminum baking sheet. Is there something I can add to the heat shieild to make it more effective? Would a stainless steel one work much better?

I do have a pre-pump filter and I am planning on adding a higher end filter after the pump. I'm going to take the carbs apart tonight so i'll check to make sure they aren't being stuck open. Can an overfilled bowl allow gravity to pull the excess fuel into the venturi when the engine is not running? A fuel leak above the header is terrifying. Plus, all that fuel wash into the cylinders can't be good either. Thanks for the info.

I pulled off my mikuni 44s last night to do some head work and I noticed one air filters was fuel soaked. There was also a bit of fuel on multiple air horns and possibly some on the valves. My fuel pressure is set at 3psi and I recently adjusted the floats to spec. What are some possible causes for fuel leaking into the venturi on these carbs?

According to their website, Isky has .490/290, .530/306, and .595/306 profiles available. I had them regrind my cam to a mild .480/280 profile and I was happy with their work and turn-around time. They also sell springs and lash pads.

I got the new MSA balancer installed yesterday and it's wobble free. The bad balancer and the new balancer look the exact same. I guess you can't always tell the condition of one just by looking.

I pulled the balancer off to inspect it and I don't see anything wrong with it. I have a new one on the way but I did some test fitting on the original to see if I can find an installation issue. Should the front of the balancer center section be flush with the end of the crank? Can someone measure the distance between the blancer and timing chain cover seal using a feeler gauage on their Z? The belt looks straight but i'm concerned the balancer isnt getting fully seated. I can't find the specs in the FSM or other to verify.

I replaced the key, and bolted it down making sure it lined up properly, and then torqued it down to 100ft lbs. It appears to be fully seated on the snout and the belt is strait. Is there a different procedure I should have used? The inside of pulley and the bolt don't seem to wobble, only the outside section wobbles a slight bit. I will replace the balancer next month I just don't have time to do before the races.

I hesitate to even ask this question because I probably know the answer and will be razed for being cheap, but here goes. I recently obtained a slightly used euro balancer with the upgraded 35mm bolt and washer. I installed the balancer and noticed it has a very slight wobble to it. At 1200 RPMs it looks perfect but at 800 it appears to be a bit off. The engine feels smooth when driving. Here are my questions: 1. Could this be within the tolerance or should it appear to spin absolutely perfectly? 2. Will temporarily running this pulley cause imminent damage to the rotating assembly? The main reason I ask is I plan on taking the car to the drag strip an hour from my house with some friends next weekend and I don't have the time to get install another pulley but I really want to run it. The car is probably driven less than 1000 miles a year but they are usually hard miles. I’d like the engine to last a few more thousand miles until I can put together a stroker.

I installed a 4.11 R180 out of an early 80s truck into my 72 240 and it bolted right in. I have a 5-speed transmission and 205/60/R14 tires. My speedo cog isn't correct so i'm not sure of the exact speed, but I think it turns around 3500rpm at 70mph. Gas mileage may be a concern if it's a daily driver, but the overall drivability seems just fine to me. Acceleration is definitely improved IMO.

I think I have the understeer problem pretty well sorted out now. After originially installing the springs I ended up with a bit of positive camber which was the primary cause of the understeer. I have since installed adjustable lower control arms and slotted the holes on the strut tower brace to end up with 2.5 degree camber. I think the camber is about right now and the car handles much better. I would like the car a bit lower and a bit stiffer on corner entry so I'll probably go to coil-overs down the road. For street use the suspenion feels perfect to me, I just notice the short commings on the track. To John's point above, I agree that you should build track experience before modifying your car. I know I will shave far more off my lap times with experience rather than car improvements, but I'm a gear head and live for improvements. The primary function of my Z isn't just to race but to have fun modifying it. Plus, I would look pretty dumb drinking beer in my garage every night if I wasn't wrenching on something

Thanks for the input guys. I hadn't taken the spring travel into consideration. I'm a little nervous about cutting up strut housing to change to coil-overs but i'll just have to get over that and do it right. I plan on switching to larger wheels and tires in the near future. Will the tire diameter need to be factored into the spring selection or cut point of a coil-over install? I'm trying to figure out if I need to wait on the coil-over conversion until I know the final tire size. I'm leaning towards going from a 205/60/14 to a 225/50/16 which I believe is about 1" larger diameter.

Last year I installed Eibach springs and Tokico Illumina struts on my 240z. The ride hieght looks nice and it rides and handles great springs on the street. I finally got my car road and track worthy this year so I've started doing autox events. Now that I have learned to navigate the cones more aggressively, I think the spring rate is too soft for my taste. I would also like to lower it another 1/2" or so to get more camber out of the front end. Is there a linear coil higher rate spring available for a stock strut assembly (non-coil over)? If I cut the progressive section of the Eibach spring to lower the car a bit more, would that have any effect on the spring rate? I was thinking about cutting 1/2 coil off to see what happens.