Throttle stumbling

[Lenbo211]

Hi All, I did a little searching and while I'm guessing my issue is common, I can't find an existing thread.

Our car starts fine regardless of temp outside. But we are having trouble with the throttle response. As in, if we slowly move the throttle, it stumbles but eventually comes up and if we blip the throttle, it almost dies and then runs up after we let off the throttle.

For a while we thought it was a vacuum leak because no matter what we did, we couldn't get the idle above 650. Found a leak and fixed it. Can get it to idle at 800 now. Using carb cleaner spray test, there is now no vacuum leak we can find. Now the idle seems fine, but still falls on it's face when we blip the throttle.

Big ISKY Cam 718149 (Link Below) so I have about 10 vacuum off the intake at 800 rpm idle. goes up to 15 when I run up the RPM.  (To set this at 800 rpm Idle, I had to back the Idle set screw out a decent amount. Further than I figured I would need to)

HERE IS MY PRIMARY QUESTION: I checked vacuum at the input to the canister and exit of canister and it shows the same vacuum as at the intake. I checked the vacuum line that T's to the throttle body and distributor. When I pull both of them off and plug one with my thumb, the other has 10 vacuum.  When I plug the vacuum line into the throttle body, the line going to the Disti is 0 vacuum. (Is this normal for vacuum loss in throttle body?)  I did a mouth test on my disti vacuum pot and it moved the internal advance.

Have set the timing at 10 BTDC at 800 idle.(Left the vacuum line on disti. Not sure if this is proper, but when I took it off, it made no difference in how it ran or where the timing mark showed.) And when I run up over 3,000 RPM I see the advance mark go up to about 30 advance.

I checked the TPS per FSM and idle position shows correct value and Full Position shows correct value.

Still need to test AFM per FSM, but was able to make the rpm change by putting my finger in and changing flap position.

Checked the Air Bypass valve. It is open about 1/8^th inch when cold and closed at operating temp. My understanding is this is correct. Checked continuity on ECU plugs for Air Bypass valve and it shows .6. Also can tell the heater is working through touch and visual inspection of flap. Checked voltage at the plug and it is over 12.

Just an FYI, this is a new professional rebuild.  I checked valve lash and they are all at .08 intake and .10 exhaust cold. One of the intake is at .07 and 1 of the exhaust is .11, but I couldn't get the locking nut to release without the bottom nut moving as well so I left them where they are since my Engine builder said he would do the final adjustment after it is running in the car.

Isky Cam 718149 http://iskycams.com/shop/index.php?main_page=product_info&cPath=127&products_id=960&zenid=52643cca0f4c18f0d1f5bc90bb2c6b9e

.030 flat top pistons

N42 head

[Zed Head]

40 minutes ago, Lenbo211 said:

HERE IS MY PRIMARY QUESTION: I checked vacuum at the input to the canister and exit of canister and it shows the same vacuum as at the intake. I checked the vacuum line that T's to the throttle body and distributor. When I pull both of them off and plug one with my thumb, the other has 10 vacuum.  When I plug the vacuum line into the throttle body, the line going to the Disti is 0 vacuum. (Is this normal for vacuum loss in throttle body?)  I did a mouth test on my disti vacuum pot and it moved the internal advance.

Can't really tell which line's you're talking about and if they are connected correctly.  But the distributor's vacuum advance canister runs off of PORTED vacuum.  In other words, it only gets vacuum when the throttle blade is off of idle.  At idle it closes so that there is no vacuum, so no vacuum advance.

The Nissan EFI system is known for "hating" big cams.  There have been some comments about in just the last few days.  If I find the thread I'll link it.  The ECU and AFM are designed to run well with a certain set of engine parameters.  Big cam isn't one of them.

[Lenbo211]

13 hours ago, Zed Head said:

Can't really tell which line's you're talking about and if they are connected correctly.  But the distributor's vacuum advance canister runs off of PORTED vacuum.  In other words, it only gets vacuum when the throttle blade is off of idle.  At idle it closes so that there is no vacuum, so no vacuum advance.

The Nissan EFI system is known for "hating" big cams.  There have been some comments about in just the last few days.  If I find the thread I'll link it.  The ECU and AFM are designed to run well with a certain set of engine parameters.  Big cam isn't one of them.

In my '78, there is a line that runs from cannister across to the distributor and T's to go to distributor and to the inboard side of the throttle body. Makes sense on the ported vacuum.  the fact that the timing advances when I add throttle indicates that the ported vacuum (only having vacuum when rpm goes up as blade opens) is working as it should.

A friend suggested it could be Fuel Pressure related and may need to get an aftermarket FPR. I have a new Factory FPR. It shows 33 at idle and jumps to 40 when I blip the throttle. Is that normal? I always thought it should drop when you blip, but not entirely sure on this car.

If I need an aftermarket FPR, is there one you'd suggest?  This '78 has a very odd shaped FPR compared to other Z's.

[Zed Head]

17 minutes ago, Lenbo211 said:

A friend suggested it could be Fuel Pressure related and may need to get an aftermarket FPR. I have a new Factory FPR. It shows 33 at idle and jumps to 40 when I blip the throttle. Is that normal? I always thought it should drop when you blip, but not entirely sure on this car.

The fuel pressure behavior is normal.  40 is higher than spec., which is 36.3.  The mixture would be rich which might cause some problems.  But the vacuum leak should have helped that.

Why is "blipping the throttle" important?  It looks like everything is working right.

Here's the thread about big cams and EFI - 

 

[Lenbo211]

25 minutes ago, Zed Head said:

The fuel pressure behavior is normal.  40 is higher than spec., which is 36.3.  The mixture would be rich which might cause some problems.  But the vacuum leak should have helped that.

Why is "blipping the throttle" important?  It looks like everything is working right.

Here's the thread about big cams and EFI - 

 

Any throttle input causes it to stumble and almost die whether it be slow input or fast input.  I may not be explaining it well, but it is definitely not normal.  trying to give it a little throttle just to let the clutch out and get moving is impossible.  

I'll try and take some video tomorrow and post it so you can see what it's doing.

[Zed Head]

Your cam is "bigger" than Jeff G 78's cam.  The odds are against you.  Jeff G knows his engines.

Here's your cam specs.

• Racing Cam
• Stage III
• Intake 36-74
• Exhaust 74-36
• Valve Lift 0.490
• Valve Lash Cold 0.010/.012
• Advertised Duration 290
• 0.050 Duration 242

Here's Jeff G 78"s post.

"I tried it - twice.  The first time was circa 1997 with a L28 at 9.83:1 CR and a street cam.  The engine was fresh and the car ran like crap.  It had low power, wouldn't idle and was simply a disaster.  Since the engine was new and the EFI a bit untested, I removed the cam and related components and returned the engine to a stock cam.  It ran great and I was able to enjoy the car.  The performance cam sat on a shelf for a decade or so.  My Z was running great and was well-sorted, so I figured that I would try again.  I installed the same cam and related parts back into the engine and got the same result.  I tried to tune it, but it didn't make enough vacuum to keep the L-Jet happy.  I once again pulled the cam springs and rockers and went back to stock.  

I then installed that cam into my SU carbed L28 racecar and it runs like a dream.  It was everything I had hoped for in my street car.  Both engines are very similar other than the fuel systems.  

You can try it, but many have the same results as me.  L-Jet simply doesn't like low vacuum.

For reference, my cam is a Web Racing Grind 91 with .450 lift and 260 duration"

 

 

[Lenbo211]

4 hours ago, Zed Head said:

Your cam is "bigger" than Jeff G 78's cam.  The odds are against you.  Jeff G knows his engines.

Here's your cam specs.

• Racing Cam
• Stage III
• Intake 36-74
• Exhaust 74-36
• Valve Lift 0.490
• Valve Lash Cold 0.010/.012
• Advertised Duration 290
• 0.050 Duration 242

Here's Jeff G 78"s post.

"I tried it - twice.  The first time was circa 1997 with a L28 at 9.83:1 CR and a street cam.  The engine was fresh and the car ran like crap.  It had low power, wouldn't idle and was simply a disaster.  Since the engine was new and the EFI a bit untested, I removed the cam and related components and returned the engine to a stock cam.  It ran great and I was able to enjoy the car.  The performance cam sat on a shelf for a decade or so.  My Z was running great and was well-sorted, so I figured that I would try again.  I installed the same cam and related parts back into the engine and got the same result.  I tried to tune it, but it didn't make enough vacuum to keep the L-Jet happy.  I once again pulled the cam springs and rockers and went back to stock.  

I then installed that cam into my SU carbed L28 racecar and it runs like a dream.  It was everything I had hoped for in my street car.  Both engines are very similar other than the fuel systems.  

You can try it, but many have the same results as me.  L-Jet simply doesn't like low vacuum.

For reference, my cam is a Web Racing Grind 91 with .450 lift and 260 duration"

 

 

Thanks,  Do you think it's an ECU thing that I could fix by swapping from Factory to a MegaSquirt etc?

[Jeff G 78]

22 minutes ago, Lenbo211 said:

Thanks,  Do you think it's an ECU thing that I could fix by swapping from Factory to a MegaSquirt etc?

Yes, a modern EFI system would solve the problem.  L-Jetronic is close to 5 decade old technology and simply doesn't have enough sensors or computer power to deal with anything beyond stock hardware.  Carbs are always an option as well.  Cost-wise, a stand-alone EFI is probably the best value.

[Reptoid Overlords]

https://zcardepot.com/products/fuel-injection-retrofit-kit-fast-efi-280z-280zx#

This is the system I have been considering when switching to a hotter cam. I haven't read anything bad about it, and it seems pretty plug and play.  I'd love to know how to tune with things like Nistune and such, but the truth is I don't.  The FAST EFI kit allegedly tunes itself after roughly 50 miles. 

Edited May 6, 2020 by Reptoid Overlords

[Zed Head]

There's an old discussion about it on the forum.  People wanted to see an engine with the system installed and the engine running, maybe even a driving video.  Never heard back.  Basically, it looks neat but there's no evidence about how it will actually do with the L6.  It might be as bad as the L-Jet system with a big cam.  Who knows, no way to be sure.  If I was marketing that system I'd install it on an engine with a big cam to show what it can do, and make a video to prove it.

Found it - 

 

Edited May 6, 2020 by Zed Head

[Zed Head]

Here are his youtube videos.  They're half off the page on my computer on his web page so I thought I'd link them here to make it easy.  It will produce an engine that runs.

8 seconds of engine noise.   No dyno sheet.

Installing...

 

[Zed Head]

Found a dyno run with numbers, in the title.  Apparently it's the factory system.  But he doesn't show the numbers with the FAST system.  Don't know why he doesn't put it on the web site page.  He has a bunch of videos on his account.  

https://www.youtube.com/channel/UCAvjCjvssZ_1BcTC1Rdx69A/videos

 

Edited May 6, 2020 by Zed Head