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Jeff G 78

Help needed ASAP

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I am at my wit's end and need help on my 260Z race car. Ever since I got the car two years ago, I have had issues at high RPM and after trying lots of things, I'm pleading for help. Here's the engine background...

  • L26 engine just rebuilt with new rings, valve job, and a mild cam
  • Thermostat guts removed leaving only the ring and cage for a restriction
  • Prior to rebuild a few weeks ago, it was 100% original
  • 3 screw SU's with SM needles, floats set to spec and chokes working smoothly and carbs synchronized. N36 intake
  • Previously used OE needles in carbs
  • New mechanical fuel pump with 260Z electric pump removed
  • Previously tried another new mechanical pump and an electric pump as well
  • Stock exhaust manifold with a 2-1/4" straight pipe and a single glass pack up front
  • Clean '75 fuel tank with new filters
  • Previously ran a POR-15 lined '74 tank with new filters
  • ZX distributor with E12-80 module. 10°BTDC at 800 RPM and 37° total advance at 2500 RPM
  • Previously ran stock magnetic pickup distributor

So, the car starts right up and idles like a champ. It free revs cleanly and runs well at low RPM. On the track (car is not street legal, so all testing done on road courses), the exhaust note goes totally flat at about 5000 RPM and the car won't rev any higher. It makes no power at that point. Sometimes, the RPM limit before it goes flat is much lower than 5k. It's slightly better at part throttle, high RPM that at WOT, but the problem is still there. It doesn't always happen though. Some days, it runs fine for a short or even extended length of time and other days, it runs poorly right off the trailer. It happens at all temperatures and weather conditions.

In the past, we had some success pinching off the fuel return line, but yesterday that didn't help at all. I tried moving the needles up slightly to richen it up and the problem got worse. We ran out of time to try moving them the other way. In the past, we have tried running with no air filter, we also tried running it with the hood off. We have the fuel rail thermal wrapped and we have extra heat shielding between the intake and exhaust manifolds. Nothing helps at all.

The issue is exactly the same pre and post engine rebuild, pre and post fuel tank swap, pre and post distributor swap, pre and post fuel pump swap, and pre and post stock to SM needle swap.

We have a 25+ hour endurance race in 12 days and we are out of ideas. Does anybody have a clue what's going on?

EDIT: One thing I just thought of is that the temp gauge doesn't work (reads ~130°) even though I have swapped in another used sensor and tried a different gauge. The engine and radiator get hot to the touch like they should, but is there any chance it's running too cold?

Edited by Jeff G 78
Added info

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Hi Jeff,

I'm sure you know already... "It could be lots and lots and lots of things", right? But from what you've described, I would guess fuel delivery issues. Sounds like you're getting enough fuel under all conditions except at high RPM full throttle conditions when, of course, you need the most of it. Only thing that seems to contradict that is this part:

Some days, it runs fine for a short or even extended length of time and other days, it runs poorly right off the trailer. It happens at all temperatures and weather conditions.

In the past, we had some success pinching off the fuel return line, but yesterday that didn't help at all. I tried moving the needles up slightly to richen it up and the problem got worse.

Could you put a pressure gauge on the fuel rail so that you can see it as you drive, and take a run with it installed?

Have you checked to make sure that the inlet filter screens in your SU's are clean? You might also pull the needle valves and make sure crud isn't partially plugging them too. I've seen paper towel fibers get through the fuel lines and gather like a wood pulp wad in the needle valve. They built up gradually and can act intermittent.

As an aside... Wow! I'm surprised you don't have fifty responses with help by this point. I would have expected a post like yours to draw in all the experts from world around!

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Hi Jeff,

As an aside... Wow! I'm surprised you don't have fifty responses with help by this point. I would have expected a post like yours to draw in all the experts from world around!

Thanks for the help Captain! Yeah, I thought I'd get lots of responses too, but I guess I just didn't catch the expert's attention. Maybe next time I'll ask what the biggest tires I can run or if anybody has ever tried a turbo on a Z. LOL

I have taken the carbs apart many times to check the inlet screens and float valves and they are always clear. Last night, I talked to a British car mechanic and he too thinks it's a problem getting fuel to the carbs, not the tuning or carbs themselves. He suggested ditching the mechanical pump and the return line and installing an electric pump and pressure gauge. We already tried that, but maybe the electric pump we bought was a dud. Who knows...

I also talked to a Z guy from Atlanta who races and he just went through exactly the same issue last weekend on his L28 powered 510. He solved the problem with an electric pump. He told me that the new metal bodied fuel pumps don't flow as much fuel as the old cast bodied pumps. I have never heard that, but I guess it could be true. SOMETHING is obviously wrong and it might just be that the pump can't keep up.

He said one other thing that he said that made sense. I told him I tried to slide the needle up a bit to richen the mixture and the car ran even worse, cutting out at 4k rather than 5k. He said that with a richer mixture, I'm draining the bowls even faster which is why it cut out sooner. I was thinking that it was too rich at that point, but his theory sounds plausible.

I plan to bypass the mechanical pump and install a new electric pump and gauge as well as block off the return line. The British mechanic will take a look at the car as well and give the carbs an expert tune.

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But you are saying it falls over as soon as you hit 5 grand right? Doesn't seem to me you'd drain the floatbowls in that short a period of time. Steve had a starvetion problem on the E-prod car but it was at the end of a mile and a half front straight at wide open pulling hard throttle. The second he let off for the turn, the fuel pump caught back up and no more issue until the next lap.

If you'd like to talk to him call me and I'll give you the number at the shop. 503-587-9800

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Hey Bruce, I just asked my teammate (I never drove the car on Sunday) if the car accelerated OK when he first pulled onto the track and he said no. His run in the car was after I raised the needles a bit to see if that made any difference. The car wouldn't rev above 4k for him even right out of the pits after it idled for a bit. I should get more feedback from the other two who drove the car prior to me adjusting the needles.

I did not check the bowls on Sunday, so I guess it's possible there could be some debris in the valves, but there never has been anything there in the past. We run a filter near the tank and another before the mechanical pump. Both filters are 3/4 full of fuel when we check them in the pits, but I don't know what they look like on the track at speed.

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Bruce, I have a quick question. What is the ID of the fuel hoses between the bowls and chokes? I need to buy some clear hose at lunch today to double check the fuel height in the bowls.

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Update: I got feedback from another driver and she said that when she drove the car (prior to my needle test), the car ran fine for the first few laps and then wouldn't rev past 5k. After her session, I raised the needles and re-balanced the carbs and it was the next driver who reported the issue occurring right away and at 4k rather than 5k.

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This is a long shot, but Duffman, on this site, was having a similar problem with his L-28 with mechanical fuel pump and a fuel pressure gauge installed inline between the fuel rail and the SU carbs. We went though many of the same things you did with no success. We set and re-set floats, checked for obstructions in fuel delivery, rebuilt the distributor, etc. The fuel pressure gauge fluctuated periodically, indicating an air suction issue somewhere. That led us to look more closely at fuel delivery from the tank to the carbs. The fuel tank had been out of the car recently which was a further clue. What Duffman discovered was the short fuel hose from the tank to the metal fuel line was sucking air where the clamps were. The car never leaked fuel but was apparently sucking in air and creating a lean condition under hard acceleration. As soon as the hose was replaced (some cracks were seen around the end) the problem was gone. Before you lose the mechanical fuel pump, install an inline fuel pressure gauge and watch the pressure under all conditions from idle to WOT. If you notice a 20-25% change you may also be sucking air somewhere. By the way Jeff, I am an ex-Detroiter, moving from Milford, MI to Scottsdale, AZ in 1994. Used to head to the Northville Dairy for ice cream when I was a kid.

Edited by Randalla

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I wouldn't rule out ignition problems yet, especially if the fuel pressure isn't wandering around. The 240 distributors had shaft bearings fail quite a bit. Not sure about your ZX dizzy. A bad electrical connection, coil, ignition wires, plugs, dizzy bearing problem could take your top end out.

I'd do a compression test as well just in case something got missed during your engine rebuild. Improperly installed timing chain or broken chain tensioner will also do that to you. Hopefully you put in a new chain during your rebuild.

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Just my two cents worth:

100 micron filter coming out of the tank (or stronger for racing - one the size of an oil filter would be best) to Electric fuel pump to Fuel Pressure regulator to 20 micron fuel filter to fuel line. Sounds like a pressure of fuel supply could be running short with the mech pump - I belive I have read that the mech pump can only supply 2.5 lbs of pressure. Sounds like you need a little more - keep us posted.

What pressure setting do you have on your electric fuel pump? - I would take it to at least 3.5 to 4 psi and give that a shot (monitor your carbs while you are reviewing - I would hate for your to force out fuel of your SU's on to your hot header - fire safety equipment is a must at this point!

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Over on the 510 forum there is a saying..... If you think it's fuel, it's electrical and if you think it's electrical, it's fuel........

Oh, and a question if I may..... Why do the needles get adjusted up and down rather than the nozzle which is all about adjustment? I don't think this is the first time I've heard of this being done on here. Needles move an RCH where the nozzles adjust a half mile up and down so to speak.

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Over on the 510 forum there is a saying..... If you think it's fuel, it's electrical and if you think it's electrical, it's fuel........

Oh, and a question if I may..... Why do the needles get adjusted up and down rather than the nozzle which is all about adjustment? I don't think this is the first time I've heard of this being done on here. Needles move an RCH where the nozzles adjust a half mile up and down so to speak.

Bruce, the needle adjustment was a trackside test just to see what it would do. I didn't really expect it to fix anything, but I wanted to see what direction it took us.

As for electrical vs. fuel, I certainly agree, but I have tried three different distributors and three different coils. In every combination, the symptoms didn't change at all. I will add another ground strap, but I don't think that is a problem either.

Randalla, I have read about fuel lines sucking air elsewhere as well. All of the hoses are new and the hard lines are in good condition, but I will verify once I get a big gauge connected that I can watch. You must be talking about Guernsey Farms Dairy. I live within walking distance of it and go there often in the summer for ice cream.

What electric pump does everybody recommend? And don't say get an RX7 pump from a JY because I haven't seen a JY RX7 in these parts for decades. I'll probably look for a 3 -4 psi Facet cube pump, but I want to find something local if possible to save time.

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Okay, I'll throw in my thoughts. You haven't said-or I missed -what theplugs look like-carboned, lean??

Check your float bowl hoses. I bought some from Bruce-sorry Bruce- that collapsed or kinked, which shut off fuel obviously. They could be kinking and unkinking under heat conditions.

Float bowl adjustments-does the car bog under hard acceleration before it gets to this wall at 5k? If NOT, I would go in there and tweek those floats to maximize fuel. You'll know if you tweeked too much. It could still be running your bowls dry-been there!

Sorry I was so late on suggestions-I guess i have been sulking in my own misery here lately.

Edited by madkaw

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I forgot to ask, why aren't you using a thermostat. I would want one in there to regulate temp. You don't want a cold engine.

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I forgot to ask, why aren't you using a thermostat. I would want one in there to regulate temp. You don't want a cold engine.

The T stat is gutted simply to avoid a stuck thermostat and an overheating condition in race conditions. I can certainly put one back in. That was on my radar as a possible problem, but I figured it wouldn't be an instantaneous cutout, but rather a general running problem.

As for the plugs, I didn't have a chance to shut the car off during a stumble to check them at that moment, but after a session and cool down lap, the plugs are a perfect light tan color.

I can't say if it bogged prior to hitting the wall, but I don't believe so. I think it's fine and then terrible.

I am running new hoses from Bruce. When I installed them, I made sure they were straight and they show no signs of kinking, but how can I tell? Is there any evidence to look for of them collapsing?

Mitchell, thanks for the pump suggestion, but I need to find something local due to time constraints and the only thing I have found is a Facet 1.5 - 4 psi cube pump from Napa. I just talked to Bruce and he thinks that will work fine. The low end of the pressure range scared me a bit, but ??? They also have a 4 - 7 psi Facet cube pump, but Bruce is worried that the pressure will puke fuel out the vents.

Thanks all for the help and keep the suggestions coming. IF I ever fix the problem, I'll be sure to report my findings.

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Just as important as the pump is the fuel pressure regulator - you really need to control that fuel flow! Get Bruce's idea on what PSI should be ran for your application with your engine set up and racing requirements - he would know the best! Most of all - keep that fire extinguisher handy for your first race!

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As far as the hoses, mine were brand new and would kink. I can't quite remember how I discovered this, but I believe I was just checking that the nozzles were flush after closing the choke.

I would run a quality thermostat-IMHO

The point i was making about the bogging is that even if the float bowls were lean, you would probably see decent acceleration until they emptied out. If they were lean you would definitely hit that wall after at WOT for a while. Mine are adjusted beyond factory specs.

It would be nice if you had a wideband to see what the mixture is doing. I had SM needles in my Z and took them out and put OEM back in. The OE needles supplied enough for my mods.

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Hi jeff I believe i have a ome 83 rx7 carb fuel pump around somewhere .

Let me know if you want it i will look for it.

Only 70,000 mi on it.

Bill

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I had the problem with new hoses on my carbs as well. I went back to the old ones which were more rigid.

Do you have a spare set of carbs that you could swap in just for sh#Ts & grins? That could at least remove one variable.

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I don't have a clue about the stress of racing but...

Maybe you've got two problems going on. I understand your not wanting a stuck thermostat to toast your engine, but seriously, even with a malfunctioning gauge, if the thermostat stuck, I think you'd pit because the engine would start running badly before any great damage occurred. If the engine is too cold, and you've tuned the carbs for the cold running engine -- wait, I just read a thread on the effect of temperature and altitude on carb tuning. Running 50 -- 60 degrees below normal might not have a one to one correspondence with the effect of air temperature on mixture, but surely it must have some effect, would 1:2 be an unreasonable guess? That would give you an unusually dense air-fuel mixture.

Combine that with your first post describing the effect of RPMs on the engine, which sounds very similar to my experiences when I adjusted (another word, beginning with "f" might better describe the entire process of my first attempt) float levels. My first attempt had fuel running out the air cleaner. I decided that lowering the float bowl level was appropriate. Car would run fine around town, even accelerate normally up to and cruise at 70 mph. But run it through the gears hard and fast, hack, ack, ack or drop into 4th and run it from 70 (~3600 rpm) it would make it to about 95, then start hacking and be unable to hold speed. Might not be the bowl levels, but I'm thinking an unusually heavy fuel demand caused by low engine temp and some sort of fuel delivery problem might well be the problem.

My not even two cents.

Chris

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One more long shot, but worth considering. Do you notice any difference when the gas tank is topped off vs. 1/2 tank or less? If so you may want to check the fuel pick up tube in the tank. I don't know if this has actually happened or not but it was something we looked at on Duffman's car since he had recently had the tank boiled out and we suspected the chemicals may have perforated the tube, thereby allowing air to be sucked into the system. Admittedly this is a real long shot. You know of course, that when you find out what's causing all your problems it will be some little detail we have all missed so far.

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I think you need to come up with somekind of temp guage

Edited by madkaw

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