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Carbed 280z Bogging Issue


Italian

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Heres a brief video of the issue I have:

 


 


Engine specs:

~1500 miles since a stock rebuild

240z SUs

240z intake manifold with emissions stuff removed. Only vacuum lines are for vacuum advance and the break booster

Pacesetter header

Stock 280z dizzy

Carbs tuned with the use of an AFR meter at a local speed shop

 


You can hear the car "sputter" at around the 12 second mark, then you can hear the bogging best at the 17 second mark. The car will bog and lose all power (not electrically). I've found that pushing the clutch in and killing he engine, then turning the key to "on" so that the fuel pump is on for a few seconds then letting the clutch out and starting the car is only thing that really works to remedy it for a brief while. It happens much more frequently on the highway, which makes me think it might be a fueling issue. However, without an AFR meter, I have no way of telling if the engine runs lean when this happens. Then I was looking at the dizzy one time it happened to make sure everything was connected properly, and noticed that the dizzy cap and rotor were noticeably scored, so I'm going to replace those and see if it helps at all.

 

 

Without really know what it is for sure, my plan is to get an MSD box and eventually a 123 distributor (plus new plug wires at some point) as that should sort out all ignition issue I could have. Then I also want to redo the fuel system with AN lines and fittings, pre and post pump filters, and a nice fuel pressure regulator, which may remedy the issue should it be an issue of fuel supply. I've also noticed the fuel pump can get absurdly loud some times, so I'll probably get a new one just incase it's on its last legs.

 

 

If anyone has any ideas or suggestions on how to fix the issue completely, I'd love to hear them!

 

Thanks, 

Italian

Edited by Italian
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What's the throttle situation when it bogs?

 

What's your current fuel system like? I'm guessing an electrical fuel pump of some sort was installed?

 

Yes, you should probably replace the scored units.

 

Yes, it is quite difficult to know exactly what is going on without an AFR gauge. How did the shop tune the carbs? Happen to have a picture to show us of the AFR vs rpm?

 

Honestly, for the SU carbs, I think all AN lines would be overkill. The fuel pump only needs to push 2-4psi if memory serves, and there's a built in restrictor in the the fuel rail so some residual pressure remains. I would lean towards maybe changing out the fuel filter, and pre pump filter if your tank hasn't been cleaned. Maybe pop off the fuel line and see if you are getting a steady stream of fuel, and then go about checking the fuel pump connections to make sure they are nice and tight.

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Seattlejester,

 

Throttle position is maybe 20%, but thats just a shot in the dark. It'll happen when I'm just maintaining a steady speed on the highway. It can be running totally fine and just seems to happen out of the blue. RPMs are between 3000 and 4000 on the highway, but I have had it happen back country roads at low speed as well. Also, the car can run fine for an hour on the highway, then all of a sudden decides to bog. It can have sporadic and frequent bogging then for a period of 20 minutes or so, then decide to run fine again for a while. 

 

Yes some electric pump has been put in there by one of the previous owners, so I don't know anything about it other than that it isn't stock. Then there is a "Mr. Gasket" fuel pressure regulator maybe two feet before the carbs, which I've thought may be part of the issue. Either it isn't consistent or it restricts the flow just enough to where it'll occasionally cause the engine to run lean.

 

Yep, they'll be ordered soon.

 

The shop had an O2 sensor in the collector of the header to read AFRs. They guy that tuned them said that they were around "perfect" 14.7s just cruising around, but were in the 11s at full throttle. Could the inconsistency be a needle issue of some sort? Or an issue related to the pistons?

 

Yeah it'll be a bit of over kill for the SUs, but it'll allow me to expand in the future. Plus I'd like to every bit about the fuel system and eliminate any sketchy work. The post pump filter in the engine bay was changed when I rebuilt the engine last summer, but not the pre-pump (if there even is one). I did get a steady steam of flow when I drained the tank last summer, but I suppose that could have changed. I know the connections are tight as the whole car was re-wired from scratch a few weeks ago.

 

BluDestiny,

 

I have no idea what the pressure is when the bogging happens, but I've thought it might be something related to it.

 

WEBEZEEed,

 

I only have the rear carb choke hooked up, but I will try that next time. Great idea! Especially since the cruising AFR is so close to lean.

 

 

I'm going to definitely look into getting an AFR meter soon as it definitely seems it'll help to know AFRs when it happens.

Edited by Italian
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Yes, the regulator should be after the carbs on the return line. As long as it isn't a high pressure pump, and the gph is more than 15 you should be good for the L24. I had a mr.gasket 25gph 4psi pump in mine and it worked fine when I finally figured everything out. With the stock 240z fuel rail, I didn't need the regulator to keep the carbs happy.

 

If this is at steady cruising then you are most likely running lean all of a sudden. By turning it off, and letting the fuel pump run, you are allowing it to fill up your carbs a bit. Which indicates that the carbs are running dry, not necessarily a tuning problem. The regulator will maintain the pressure ahead of it. So right now you are maintaining the pressure in the line in front of the carb and after the fuel pump. The carbs are being filled with whatever the regulator is allowing to pass. Relocate it after the carbs if you have a non stock rail, or use the stock rail with the restrictor fitting and it should generate enough pressure to fill the bowls before returning.

 

Honestly I think if you removed or relocate your regulator, you may fix the problem. If not, then you may be running into some crud in your tank clogging the filters or something of that nature. 

 

For SU's they ramp up from what I have seen and encountered. Different needle profiles will change that, but the stock needles will lean out the higher rpm it goes.

 

If you are getting a cruising of 14.7 and 11 at full throttle (high rpm?), then that's pretty good, a bit on the rich side, but safe, if you are hitting 11 at high rpm at full throttle, then I think there's some tuning to be done there. I think mine was idling at 10-11, cruising at 13-14, and under full throttle it would go to 11 then work it's way up to 13 near my self imposed redline of 6000.

Edited by seattlejester
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It should? Everything thing I've found on it has shown it before the carbs. Plus, once the return lines were removed the car ran soooo much better. This is the regulator to clear up any potential confusion:

 

http://www.jegs.com/i/Mr-Gasket/720/9710/10002/-1

 

Heres the set up as well:

 

Fairlady-19_zpsa679f8c5.jpg

 

I'm not sure what fuel rail it is. I did some quick googling and it doesn't look like any of the stock S30 fuel rails.

 

I think I'm going to get one of the little inline gauges to be certain about the fuel pressure and go from there. It definitely sounds like one way or another, the car is leaning out and the pressure regulator is a possible culprit.

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Someone has cut up that rail. Put the regulator AFTER the carbs...otherwise you're not regulating the fuel pressure to the carbs. You're regulating the output pressure of the pump...but the carbs are downstream of the regulator.

 

The stock fuel rail runs around the front of the head, back to the back of the head, then back up along the side and around the front again. It has a restrictor placed in the return side that is sized to work with the stock mechanical fuel pump.

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So from the post pump filter to the inlets of the carbs (The banjo fittings), then return lines from the nipples on the top of the float bowls into the regulator? Or a loop type set up where the fuel goes into a loop that is connected to the inlets, then the regulator inlet is also connected to part of the loop?

 

I was starting to figure as much. Whoever did so did a good job, the cuts are clean and the line ends have had the little 'bumps' put in them. Plus the parts that aren't painted still have all the nice zinc coating.

Edited by Italian
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Yea that looks pretty chopped up.

 

I might be missing it because of the focus, but do you have a return line?

 

Basically, you should have a loop, from your fuel tank and then back to the fuel tank. Right next to the outlet of your fuel tank, there should be a fuel pump which pushes fuel throughout the loop. To feed the carbs you need to add a splice into the line. On the stock fuel rail there are two fittings on the loop, one for each carb. It is a single line, basically the splice is open so the fuel flows into it, once it fills the carb bowl, the fuel inside the bowl exerts a pressure back on the splice location, so the opening is filled and the fuel continues on its route. The regulator should be behind the fuel using device, so that it maintains pressure in the line feeding the carbs. Once the bowls are full and the line is full and exerting a pressure of 2-4psi, or whatever you have your regulator set to, the regulator opens up a bit and allows the fuel to flow through. The whole time, your fuel pump is happily buzzing away, you will only notice is picking up speed if you are really on the throttle. The pressure in the fuel rail would drop and the pump would have an easier time filling up/pressurizing the line.

 

Right now, you are pressurizing the line in front of the carbs, your fuel pump probably doesn't make a noise change at all, it is constantly fighting against the fuel regulator with no release other then the regulator exit. And if you don't have a return line, basically once the fuel passes the regulator, it will build up pressure behind the regulator. At that point, the regulator has pressure behind it and I imagine your fuel pump would be pretty unhappy since now it is trying to push fuel down a line that is full, I imagine it would stall, or if it was advanced it would have a fuel flow back design/release valve, but on a cheap pump I don't imagine anything of that nature. Once you are using fuel, the pressure drops, and the fuel pump will be able to rotate a bit and push fuel until they are full again, it will still be fighting against the regulator though.

 

I guess the way you have it would work, but it seems like a weird way to do it. I usually read about regulators in front of fuel using device for guys running a non return system, it puts a bit of pressure on the pump, essentially stalls it until fuel is used. Some OEM systems now a day is indeed that setup, but it has a regulator near the pump where it will shut the pump off when you reach a certain pressure, much more different then having it stall or slow down from pressure build up. I imagine your fuel pump is pretty unhappy right now.

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Nope, no return line connected. The hard line that goes to the tank is still there, just capped off, so a few feet of line and another T-fitting should fix my issue. 

 

Thanks for all the help guys! I'll get back with how it works out and my set up hopefully over the weekend.

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  • 2 weeks later...

Alright so I was finally able to redo the fuel system up front as susggested, but there are some issues.

 

The car seemed to bog more often, but to a lesser extent that previously. By this I mean I could just let off the gas for a second or two and it'd be fine, I didn't always have to kill the engine and let the system prime. Then on top of that, the Summit fuel pressure gauge read no pressure at all. I don't know if its a faulty gauge (Very well could be, I didn't expect it to be a quality gauge, only a cheap temporary item) or is something else is wrong and there actually is no fuel pressure. I assume the first as if there weren't any pressure, I would assume there would be no fuel, and thus no running car. 

 

I'm going to get some carb cleaner and clean out all the fuel passages in the carbs, then also replace the fuel filter(maybe filters?) to try to eliminate the possibility of clogs.

 

If all of this still doesn't work, I'm going to start moving faster in my collecting of the parts needed to completely redo the fuel system from end to end.

 

 

Anyone have any more suggestions?

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Not running the carbs in series, but two seperate T fittings, and you haven't hooked the float bowl vents up to anything, right?

 

Adjust your fuel pressure regulator to produce 3-4PSI of fuel pressure.

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Interesting. Bummer that it didn't work out very easily.

 

Do you have a fuse for your fuel pump? For that matter, can you ID your fuel pump? Any pictures of the new way you routed your carbs? Couple of photo's from different angles will really be a big help here. Is the fuel filter the clear variety? Can you see if it is full or picked up any bits?

 

I think the questions now are where does the problem reside...

Fuel pump

Fuel lines

Carbs

 

If you could ID your pump and check the wiring that would be a big help.

Maybe wiring in an inline bulb to the fuel pump or something of that nature would tell how the pump is behaving when the engine bogs, I had 1 poor connection on my fuel pump relay and my AFR gauge which was getting power from the fuel pump would flicker before the car ran out of fuel/started bogging.

 

Fuel lines, if there is a restriction before the carbs then not enough fuel could possibly be making it to the carbs.

 

Carbs if the float heights are set incorrectly you could actually just be shutting off the fuel flow and using the meager amount of fuel, have you checked the fuel float heights?

 

Do you have a volt meter on hand? If you aren't supplying enough voltage and amps to the pump it could be struggling a bit, granted it would be popping low amp fuses if that was the case.

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