Randomly lose power when driving

[CaptainMajestic]

Hi everyone,

I've got a 74 260z with an L28 and round top carbs that I've recently restored and rebuilt. The engine is timed and I color tuned the carbs recently,and up until a few days ago everything was running brilliantly. However, on my latest drive the engine started sounding muffled and I suddenly lost power. I figured it was a fuel issue, so I went head and swapped out the fuel filter and checked for fuel pressure and everything was spot on and the engine started nicely. Then on the next drive, same issue. 

Could it be the valve lashing? 

 

Will try to get a video of it later today.

Thanks!

[jonbill]

It's always good to have your valve clearances correct, but I'd check the colour of your spark plugs.
Also what did your old fuel filter look like? Any rust in it?

[Zed Head]

1 hour ago, CaptainMajestic said:

suddenly lost power.

Complete loss, partial, sporadic...?  Did it die and refuse to start back up?  Details will help.

If your car is stock it will have the original electronic ignition module.  Things can get weird when they start to go bad.

[Mark Maras]

2 hours ago, CaptainMajestic said:

Hi everyone,

I've got a 74 260z with an L28 and round top carbs that I've recently restored and rebuilt. The engine is timed and I color tuned the carbs recently,and up until a few days ago everything was running brilliantly. However, on my latest drive the engine started sounding muffled and I suddenly lost power. I figured it was a fuel issue, so I went head and swapped out the fuel filter and checked for fuel pressure and everything was spot on and the engine started nicely. Then on the next drive, same issue. 

Could it be the valve lashing? 

 

Will try to get a video of it later today.

Thanks!

 When it lost power while driving would the engine still rev briskly with the clutch in?

[siteunseen]

I'm thinking with jonbill on this. Sounds exactly what my 240 did when first bought. The tank was full of crud. After it all settled back down it ran great for about 15 minutes. Buy a fram g2 for a couple of dollars and next time it happens put it on and see if it runs better.

[CaptainMajestic]

Hi everyone, 

thanks for the quick responses! 

20 hours ago, Zed Head said:

Complete loss, partial, sporadic...?  Did it die and refuse to start back up?  Details will help.

If your car is stock it will have the original electronic ignition module.  Things can get weird when they start to go bad.

@Zed Head The car drives normally for a few minutes then suddenly the engine sounds like it's filled with cotton balls. Like @Mark Maras asked, I can rev with the clutch in, but it's still muffled. As soon as I let off the gas, the engine quiets down and dies. I have been pretty lucky to get it started again after 5-10 seconds of turning over, but the issue persists.

[gundee]

1 hour ago, CaptainMajestic said:

 

The car drives normally for a few minutes then suddenly the engine sounds like it's filled with cotton balls. Like @Mark Maras asked, I can rev with the clutch in, but it's still muffled. As soon as I let off the gas, the engine quiets down and dies. I have been pretty lucky to get it started again after 5-10 seconds of turning over, but the issue persists.

From your description, almost 100% certain its running out of fuel. I had the exact same symptom you describe.

[CaptainMajestic]

51 minutes ago, gundee said:

From your description, almost 100% certain its running out of fuel. I had the exact same symptom you describe.

How did you solve it? Right now I am using the stock fuel rail as the fuel regulator. Am going to check the breather line when I get home.

 

[Mark Maras]

 My experience with a clogged filter was on I-5 doing 70 + mph when the car slowed to 45 mph. It would still rev normally with the clutch in but wouldn't go over 45 mph no matter what gear I was in. Thru the entire experience the engine sounded normal, no cotton ball sound.

[Zed Head]

A pressure gauge will tell you if it's a fuel issue.  Since the problem is repeatable you should be able to put the gauge where you can see it while the problem happens.

Ignition module issues usually show signs through the tachometer needle.  It will read high and jumps around as the extra sparks screw things up.

You're down in hot Austin so you could also have the vapor lock issue, especially since you have a different engine, maybe with some hacked up fuel system parts. Find out where your fuel lines are running.  What kind of fuel pump are you using, mechanical or electric?

[gundee]

Did you ever change the filter inside your fuel pump?

Most are not aware there is one in there. Assuming you run the factory 260 pump.

[CaptainMajestic]

Hi guys,

A bit of background. I am running a brand new electric fuel pump, cleaned out tank with the vapor chamber and no carbon canister, custom made 5/16" copper nickle fuel and return lines, and a 3/16" breather (if i remember correctly). I was using the inline fuel filter with a hacky fuel canister back when i was testing the system and had installed it in the final layout temporarily. I had the system dead ended for a while, then put on the factory fuel rail and routed a line to the return because I ran into a lot of vaporlock issues in Austins' demonic temps.

Do you guys know if there is a way to test the ignition module for correct operation? I have access to a signal generator at work, or i could fashion something up with a MCU.