l20b hits wall at 4500 - 5000 rpm, won't rev past help!

[Knox]

Hey guys I've been holding off making a topic about this for quite awhile now but at this point I am completely out of ideas:

 

I have a mild l20b in a 180b SSS that seems to run out of breath at around 4500 - 5000 rpm (depending on what gear I'm in). I've owned this car for roughly a year and I can honestly say I have never seen the tacho needle go past 5000 rpm, no matter what I throw at it. It has decent throttle response and power picks up around 2800 - 3000 rpm but already starts to fall off at 4500 rpm, begins to struggle and wont budge past 5000 rpm, if I hold it there the engine begins to sound sick and starts popping and misfiring as though its at its limits. The motor has a 290 degree cam with 0.495" lift and valve train gear to suit, U67 open chamber head ported to 38mm, stock valves, stock l20b pistons, 4-2-1 headers, electronic dizzy with a new module and a pair of DHLA40 Dellortos running 36mm venturis. It's no race engine but it should have reasonable top end and should at least rev out to 7000 rpm without too much trouble, it just feels like I'm missing out on a good 2000 rpm of power band.

 

Here's a bunch of stuff I've tried/checked to fix/diagnose the problem:

 

- advance/retard timing nearly the entire dizzy adjustment slot in small increments

- spark jumps about 15mm if I hold a spark plug near the valve cover in the open

- Installed a wideband setup with a gauge, AFRs are in the ballpark around 13s at the limit rpm

- replaced stock exhaust with a 2 1/4" pipe and a straight though muffler

- I've actually tried two different heads, the first one was an a87 closed chamber with a 280 degree 0.550" reground cam, but with stock valve gear (rocker arm geometry was out the window). I changed to a U67 that already had a 290 degree crow cam with springs and lash pads to suit (the head I'm running now). I originally thought the first head had valve bounce at the 5k limit, but the problem was still there with the second head.

- I currently have the cam in symmetrical timing, and have tried all dowel positions and also skipped a tooth both retarded and advanced, neither improve the rev limit.

- haven't checked the compression lately, but I don't think it's much of a rev-related issue (compression is basically stock l20b 8.5:1 anyway)

 

might have forgotten some other stuff I tried but it feels like I've run through most of the common rev-related issues, at this point I'm open to any ideas, any possible causes I could have overlooked. It feels like no matter what I throw at it that same wall is there and it won't break through it,

 

thanks in advance

 

EDIT:

 

The problem currently is no longer a rev limit, I've ironed out several other issues that were causing the limit like problems with carb frothing and slop in the distributor drive causing spark jitter. The problem is now basically an early hp redline at 5000rpm, which could be head/cam related.

[Z-train]

1)Timing

2)Airflow

 

This assumes all other parameters are equal.

[datzenmike]

.... begins to struggle and wont budge past 5000 rpm, if I hold it there the engine begins to sound sick and starts popping and misfiring as though its at its limits. The motor has a .... electronic dizzy with a new module .

The new matchbox module... does it say E12 93?

 

If so this may be all of the problem. You need an E12 80 module.

 

This leaves the distributor/electrical. Try another cap/rotor maybe the coil? even a points one

[banzai510(hainz)]

a 280 degree 0.550"............valve gear (rocker arm geometry was out the window).

maybe this is the biggest issue.

 

 

dont the springs stack up ar about 480"

 

1st gear should be able to go to 7k by then the diistributor cuts out what I notice on my old L20 with Mikunis.

 

2.1/4 exhaust ia abit much but should be ok

 

you got dist set up to run a vacuum adv or not. Most time duals mikunis dont run them.

 

try another dizzy?

 

I also found the loose venturies inside carb(stick finger in and see if wiggles) can cause a cut out at higher RPM or the jets back out on top of carb when you open the plate up

[Knox]

The distributor is an Australian bosh type, I think the module is called BIM 024. I got the electronic dizzy later down the track, when I ran the points I don't remember it running much better, it lacked throttle response and some low end power, and seemed to have the same wall at 5k. I have some other points dizzy's lying around I could try though.

 

The 280 degree 550"(?) lift cam was in the a87 head I was running before, the springs did actually bind, and because it had stock lash pads the rocker arm pivot was wound up a lot higher, moving the wipe pattern almost way to close to the pivot end of the rocker arm and it basically ended up cheating the valve lift. I thought this whole setup was responsible for the wall at 5k, but when I ran the U67 head with aftermarket valve gear, it did the exact same thing.

 

I haven't run vac advance in a long time, I'm not sure if Dellortos run it either. 

 

I put my old SU's on yesterday to if it was somehow the carbie's fault, but the same problem was there, could budge past 5k.

 

But yeah I'll try another points dizzy and see how that goes

[Knox]

*Couldn't budge past 5k sorry

[DanielC]

First, check the compression.   Check the timing, at idle and up into the RPM range.

Then put a vacuum gauge on it.  It should have good vacuum, even if the cam is radical, it should develop a decent vacuum at 2,000 or so RPM, even if the vacuum is not as good at idle.

[Doctor510]

Set the total timing to 36* BTDC @ 2,800 RPM, all mechanical do not hook up the vacuum diaphragm (leave it on the dist)

[datzenmike]

So... different heads and cams, different carbs and intakes. This only leaves the electrical.

 

This leaves the distributor/electrical. Try another cap/rotor maybe the coil? even a points one

[Knox]

Could spark breakdown be a main factor? I'm not sure whether it's a common thing with electronic dizzys though, I thought it might have been a fault in the car's internal ignition wiring, so I ran a wire through a switch that completely bypassed the factory wiring and went straight from the battery to the + coil terminal, but the wall was still there. What else could I try apart from trying the points unit?

[Z-train]

No-Electronic distributors either work or they don't

[datzenmike]

The distributor is an Australian bosh type, I think the module is called BIM 024. I got the electronic dizzy later down the track, when I ran the points I don't remember it running much better, it lacked throttle response and some low end power, and seemed to have the same wall at 5k. I have some other points dizzy's lying around I could try though.

 

 

 

 

But yeah I'll try another points dizzy and see how that goes

So... different heads and cams, different carbs and intakes. This only leaves the electrical.

This leaves the distributor/electrical. Try another cap/rotor maybe the coil? even a points one

The distributor is an Australian bosh type... If it isn't a Nissan electronic ignition then I have no faith in it whatever.

 

Heads have been replaced, intake and carbs replaced. Only thing left is something in the ignition. Are my posts not showing up out there???

 

The cap, rotor or coil may be carbon tracked and breaking down at high RPMs. Loose wire on coil or dizzy breaking contact due to high RPM vibration. Bad ground on dizzy to block. Disconnect the tach and try it. If you have a ballast resister place a jumper wire around it temporarily and try it. Jumper 12 volts directly from the battery to the coil + to eliminate the ignition switch. There's lots you can try and only one thing that's causing this. You have a points dizzy... swap it in! 

[DanielC]

Again, do this.

First, check the compression.   Check the timing, at idle and up into the RPM range.

Then put a vacuum gauge on it.  It should have good vacuum, even if the cam is radical, it should develop a decent vacuum at 2,000 or so RPM, even if the vacuum is not as good at idle.

 

Engines are simple.  Compression, Ignition, Fuel. Supply all three things and the engine runs.  Millions of engines prove that every single day.

Check the basics.  If the rings are worn, or the timing is off, it does not matter what carb you put on it, it is still not going to run good.

[wayno]

To me it sounds like an L20b, they fall on there face at 4500 to 5000rpms, I actually don't go there anymore because there is nothing there, I shift between 4000 and 4200rpms if I am in a hurry, otherwise at about 4000rpms.

I suppose with the right cam/cam timing you could get more out of it, but you will be giving up your torque to get that higher RPM stuff, which in my opinion is only good at the track.

[Stoffregen Motorsports]

What fuel pump are you using? Are you using an in line fuel pressure regulator? If so, what type (Holley, trans dapt, etc).

 

It sounds like a fule delivery problem to me.

 

Have you verified TDC and checked the cam timing? There is a simple procedure that does not require measuring valve lift or using a degree wheel. I could explain it to you if need be.

 

2.25 inch exhaust is not too big. I ran 2.5 inch on all my L powered cars.

[Knox]

Alright so I dug out my old hitachi points dizzy and  ballast resister, hooked it up to the engine only to find out it couldn't even start the car. It seemed to have spark out in the open but for some reason it wouldn't even kick, I then jumped the ballast resistor, but still no kick. I scratched my head for awhile checked the points gap, cap, leads, spark plugs, timing, etc; everything seemed fine, I eventually gave up on it (it was about 9:30 at night and I needed to drive it to work the next day), I whacked the electronic dizzy back on and vroom! started at the touch of the starter. I'll give it another shot tomorrow. 

 

At the moment I'm running the stock L series (self-regulated?) mechanical pump. I noticed the other day that if I rev it up to 4500-4800 rpm the afr's are good, around 13:1, but if I hold it there until it struggles to 5000 rpm, the afr's all of a sudden climb to around the 17:1 mark  :o . Do you think an electric pump with a regulator would fix this lean spot?

 

Cam timing should be good, I verified it by marking on the crank pulley where the intake valve opens and the exhaust valve closes (overlap period), and measuring with a flexi rule the distance between each event relative to the TDC mark, pretty close to symmetrical timing. 

[Three B's Racing]

Alright so I dug out my old hitachi points dizzy and  ballast resister, hooked it up to the engine only to find out it couldn't even start the car. It seemed to have spark out in the open but for some reason it wouldn't even kick, I then jumped the ballast resistor, but still no kick. I scratched my head for awhile checked the points gap, cap, leads, spark plugs, timing, etc; everything seemed fine, I eventually gave up on it (it was about 9:30 at night and I needed to drive it to work the next day), I whacked the electronic dizzy back on and vroom! started at the touch of the starter. I'll give it another shot tomorrow. 

 

At the moment I'm running the stock L series (self-regulated?) mechanical pump. I noticed the other day that if I rev it up to 4500-4800 rpm the afr's are good, around 13:1, but if I hold it there until it struggles to 5000 rpm, the afr's all of a sudden climb to around the 17:1 mark  :o . Do you think an electric pump with a regulator would fix this lean spot?

 

Cam timing should be good, I verified it by marking on the crank pulley where the intake valve opens and the exhaust valve closes (overlap period), and measuring with a flexi rule the distance between each event relative to the TDC mark, pretty close to symmetrical timing. 

 

You may want to check your carbs float bowl levels making sure they are correctly set. If they are check to make sure you haven't any obstruction in the valve/needles filter screen. A good stock fuel pump is fine with your setup. Does your fuel feed dead head at the carbs or is there a feed bypass back to the tank? 

[DanielC]

Take the gauge you used to check vacuum, most also check lower fuel pressures, and "T" it into the fuel line close to the carb as possible.  Check fuel pressure at high RPM, under load.

If it drops off, you have a flow problem.   Getting a higher pressure fuel pump will not help flow much.

 

A stock L-16 fuel pump should pump a quart a minute, at 1,000 RPM.  If the flow is low, check the 42 year old fuel lines.  Rubber ones could be deteriorating on the inside.  The steel ones could have rust in them.  Check for gunk in the gas tank collecting around the bottom of the fuel pick up, and restricting the flow.  

 

You have checked the compression, and the timing by now, haven't you?

[datzenmike]

- Installed a wideband setup with a gauge, AFRs are in the ballpark around 13s at the limit rpm

 

 

I noticed the other day that if I rev it up to 4500-4800 rpm the afr's are good, around 13:1, but if I hold it there until it struggles to 5000 rpm, the afr's all of a sudden climb to around the 17:1 mark

 

Well what can I say. Symptoms have changed without notice to us. Hard enough to troubleshoot, worse when it's a moving target.

[Stoffregen Motorsports]

Knox, you clearly understand the concept of cam timing so let's focus on fuel delivery.

 

It sounds like the fuel is the problem. If the fuel pump is working properly then it's probably an issue with your carbs. Could be broken or improperly set floats, needle and seats maybe too small, or...

 

Do you have soft mounts behind your carbs or are they mounted rigidly?

[banzai510(hainz)]

My distributor bushing was worn on my 521 and I had a Pertronix ignition. It would cut out on the top end.

replace the distributor with another using same Pertronix and was alot better.Believed it caused a bounce. Thought a worn bushing would be that bad but it made a difference. Ck to see if dist shaft wiggles side to side

 

as for coil and points and not starting I assume you know how to hook back up to stock, use the point OHM coil and the ballast. If you use a EI coil maybe its cooking the wires inside as it gets got or points arching.

 

If you have another EI dizzy with matching coil try that or what ever was working before if it was ever working at all.

 

 

I think a stock fuel pump will diliver enought fuel. Those things pump alot. No differnt that a old ford V8 mechanical fuel pump.

 

 

like i said did you ck the venturis if loose also?

[ggzilla]

The Nissan distributors with Bosch control module are OK.

 

Timing too far advanced will cause this. It will have great power at low and mid rpm, but refuse to rev high.

 

L20B should easily rev over 4500 in first gear.

[620Turbo4X4]

At the moment I'm running the stock L series (self-regulated?) mechanical pump. I noticed the other day that if I rev it up to 4500-4800 rpm the afr's are good, around 13:1, but if I hold it there until it struggles to 5000 rpm, the afr's all of a sudden climb to around the 17:1 mark  :o . Do you think an electric pump with a regulator would fix this lean spot?

No. The gauge is operating as it should for a misfire. Contrary to what everyone thinks air fuel ratio gauges do not measure the air to fuel ratio. They measure the percentage of oxygen in the exhaust by using a oxygen sensor. Wideband or narrow band. They both measure oxygen, not fuel. When an engine misfires the contents of the cylinder is unburned and the sensor sees all the oxygen and shows a lean condition. If you were to pull a plug wire while engine is idling you would see a lean reading even though the actual A/F didn't change...

 

It's good you added this info, prior I was thinking this was a timing- pre-ignition/detonation issue since A/F ratio wasn't changing. Now I'm thinking this may be an issue of weak ignition. Possibly a result of lack of in-adequate dwell with mismatched ignitor on a coil that has too much primary resistance, bad plug wires, or excessive gap on the plugs???

 

Have you tried a different coil? Plug wires? What plugs are you running and what are they gapped at?

[Knox]

I've only had the wideband for probably couple of weeks so a lot of this is new to me, the weird thing is that when the power already starts to fall off at 4500 rpm the afrs are still in the ballpark, around mid 12s, low 13s, then if you leave the throttle wide open until it struggles to 5000 rpm and stays there, the afrs jump to 17:1 or even leaner, which may be why the engine sounds sick at this rpm. I only discovered this the other day, after I first posted the topic. Sorry about the confusion. I just don't know why the power would already start to fall off when the afrs are still good, and why it all of a sudden reads lean well after the powers dropped off.

 

When I tried the Su's the afrs didn't go as lean (around 14 or 15:1) but it still had the same wall.

 

I've set float level to dellorto spec and I've also tried raising it a good 5mm but no improvement, except the car seems to sound less sick at the limit rpm. The carbs are mounted on some pretty thick cork gasket material, but I guess you could say they're solid mount, compared to the O ring type mount.

 

Hope this info helps

 

P's haven't been able to get my hands on a vacuum gauge yet will get onto that asap

[620Turbo4X4]

I've only had the wideband for probably couple of weeks so a lot of this is new to me, the weird thing is that when the power already starts to fall off at 4500 rpm the afrs are still in the ballpark, around mid 12s, low 13s, then if you leave the throttle wide open until it struggles to 5000 rpm and stays there, the afrs jump to 17:1 or even leaner, which may be why the engine sounds sick at this rpm. I only discovered this the other day, after I first posted the topic. Sorry about the confusion. I just don't know why the power would already start to fall off when the afrs are still good, and why it all of a sudden reads lean well after the powers dropped off.

That tells me it's not a carb or fuel supply issue. When it's misfiring at 5000 RPM the engine is pumping out straight oxygen and raw fuel. The oxygen sensor sees the oxygen and reports lean. Does not matter how much fuel is actually entering your engine. If it's not burning your going to see lean on the gauge.