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


Knox

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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

 

My electronic dizzy seems pretty tight, has about the same amount of play as the oil pump drive.

 

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 ran the same bosch oil filled coil I originally used with the points ignition, I ran it with electronic dizzy as well.

 

Even with the points distributor the previous owner had fitted, he said he it always seemed to lack top end. I guess this car has never properly 'worked'.

 

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?

 

So maybe the electronic pump idea shouldn't make much difference anyway?

 

Went out and checked, all the grub screws were tight against the venturis and lock nuts done up reasonably tight.

 

 

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.

 

Its an all Bosch distributor, it had an original Bosch control module that fried itself, so I replaced it with some 'unknown-brand' replacement module off ebay.

 

I've tried running nearly the entire timing adjustment slot in the dizzy with no changes with the top end.

 

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???

 

I read somewhere that some Bosch control modules can actually adjust their dwell time to suit the coil charging characteristics. But I'm not sure if the module I got off ebay has this.

 

 

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

 

I haven't tried a coil suited to an electronic dizzy, but I did try another oil filled coil, I haven't tried another set of plug wires yet, and I've dropped my spark gap from 0.7mm to 0.5mm without any improvement  :hmm:

 

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.

 

Its hard to tell whether its misfiring or backfiring, if I hold it at the limit rpm for a more that 5 seconds it begins to sound worse and worse until it starts misfiring/backfiring and in some cases the rpm drops and I can't rev it out at all for a small period.

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I ran the same bosch oil filled coil I originally used with the points ignition, I ran it with electronic dizzy as well.

 

Even with the points distributor the previous owner had fitted, he said he it always seemed to lack top end. I guess this car has never properly 'worked'.

 

 

Points coils have a higher primary coil resistance.

 

Get a coil suitable for 12V electronic ignition.  (.8 - .5 Ohms.) I've had good luck with the MSD Blaster coils

 

 

 

Its an all Bosch distributor, it had an original Bosch control module that fried itself, so I replaced it with some 'unknown-brand' replacement module off ebay.

 

 

This would be my next suspect. 99% of bosch on eBay is fake chinese grey market crap...

 

If OEM Nissan modules are NLA, maybe look into the GM HEI ignition mod...

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if running a point coil with out a ballast resisitor will heat up. Have you fell the coil when this happens.

 

you can still run the point coil but has to be in series with the ballast resisitor. except the HOT start wire which will bypass the ballast so the coil gets full battery voltage as the starter loads whole system down.

 

what Rick Rat says on the valves is correct also. But assume the other head you had was set up correct as it had a different set of parts or more correct parts.

 

 

pull distributor out and spin it just to make sure the weights didnt blow up and jammed. I had this happen once also

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Hey guys sorry for the long wait, been a bit busy with other problems with the car;

 

Good news and bad news:

 

the good

 

- I finally got the points dizzy running, turns out that the electronic dizzy is actually 180 degrees out from the points dizzy, so the rotor points in the complete opposite direction. I had no idea this could happen, I don't even remember running into this problem when I first installed the electronic setup. So to get it running I just swapped the plug wires over two cylinders on the dizzy cap and it fired right up.

 

the bad

 

- THE WALL IS STILL THERE!!

 

 

if running a point coil with out a ballast resisitor will heat up. Have you fell the coil when this happens.

you can still run the point coil but has to be in series with the ballast resisitor. except the HOT start wire which will bypass the ballast so the coil gets full battery voltage as the starter loads whole system down.
 

 

Is this when running the points dizzy or the electronic one?

 

 

what Rick Rat says on the valves is correct also. But assume the other head you had was set up correct as it had a different set of parts or more correct parts.

 

Yeah it feels like this problem should be something related to the valve train, I thought it was from the stock valve springs being too soft for awhile, but since I swapped to the other head with aftermarket valve springs, etc, I noticed next to no change at 5k rpm.

 

- Still on the topic of ignition, could it have anything to do with poor engine ground at all? I also heard that a capacitor run from coil+ to ground can help stabilise high rpm ignition, anybody heard of this?

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pull distributor out and spin it just to make sure the weights didnt blow up and jammed. I had this happen once also

 

both dizzys seem to spin freely. mechanical advance feels smooth as well

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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! 

 

 

 

Try these...

 

Also is the tach the problem? What if you have a 6 cylinder tach? When reading 5K you are really at 7K .

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I can't believe that you guys are still talking about this, it's an L20b, they all hit a wall at 4500-5000RPMs, you might be able to run an L16/L18 up that high, but an L20b isn't going to run up that high except in first or neutral, and it's going to sound awful when you do it, the only way you are going to get it up to 7000RPMs is if you put a radical, I repeat RADICAL cam in it like dirttrack510 did, he races circle track and spends a lot of time at 7000RPMs, ask him how he does it, here is a link to his race car thread.

https://www.google.com/url?q=http://community.ratsun.net/topic/41615-1969-510-race-wagon-video-heavy-dead-unless-theres-a-miracle-from-god/&sa=U&ei=MHwAVaTbIZGQyASeg4KIAw&ved=0CBIQFjAGOBA&client=internal-uds-cse&usg=AFQjCNGPhbSJjtAHjluZ9llNnnDrJ9Vq_Q

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Maybe this will help,my L20B will rev to about 6,000 if i push it.This motor has a .488 lift 252 duration cam a ported A87 peanut head and dual mikuni 44's.Also the proper valve springs for that cam.I cant imagine pushing it to 7,000 rpm's.

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I can't believe that you guys are still talking about this, it's an L20b, they all hit a wall at 4500-5000RPMs, you might be able to run an L16/L18 up that high, but an L20b isn't going to run up that high except in first or neutral, and it's going to sound awful when you do it, the only way you are going to get it up to 7000RPMs is if you put a radical, I repeat RADICAL cam in it like dirttrack510 did, he races circle track and spends a lot of time at 7000RPMs, ask him how he does it, here is a link to his race car thread.

https://www.google.com/url?q=http://community.ratsun.net/topic/41615-1969-510-race-wagon-video-heavy-dead-unless-theres-a-miracle-from-god/&sa=U&ei=MHwAVaTbIZGQyASeg4KIAw&ved=0CBIQFjAGOBA&client=internal-uds-cse&usg=AFQjCNGPhbSJjtAHjluZ9llNnnDrJ9Vq_Q

 

Wayno.... It revs to 5K  where it stutters and breaks down and won't go higher. Like a rev limiter. Not that it doesn't pull much after, as mentioned here....

 

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.

 

 

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.

 

 

Its hard to tell whether its misfiring or backfiring, if I hold it at the limit rpm for a more that 5 seconds it begins to sound worse and worse until it starts misfiring/backfiring and in some cases the rpm drops and I can't rev it out at all for a small period.

With the cam mentioned and multiple carbs it should rev higher easily. My stock L20B will hit 6K, agreed that it's probably better to shift sooner.

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Also is the tach the problem? What if you have a 6 cylinder tach? When reading 5K you are really at 7K .

 

I've been slightly suspicious of the tach, though it is the original 180b sss tach and it does read around 900-1000 rpm at idle and when it hits 5k in second gear its doing just under 70km/h (44mph) (this is with 3.7:1 diff and a non close ratio 5 speed), I think most normal cars that rev to 6500-7000 rpm can do something around 90 km/h (55mph) in second. I'd say the tach is fine, despite the needle having some slight momentum when hitting bumps and quick gear changes.

 

Maybe this will help,my L20B will rev to about 6,000 if i push it.This motor has a .488 lift 252 duration cam a ported A87 peanut head and dual mikuni 44's.Also the proper valve springs for that cam.I cant imagine pushing it to 7,000 rpm's.

 

what size chokes are you running in your 44s? Also what size ports? 6000 rpm sounds right for a 250 degree cam. I think a 290 cam should have a power band ranging from 3500/4000-7500/8000, depending on overlap, timing etc.

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I've been slightly suspicious of the tach, though it is the original 180b sss tach and it does read around 900-1000 rpm at idle and when it hits 5k in second gear its doing just under 70km/h (44mph) (this is with 3.7:1 diff and a non close ratio 5 speed), I think most normal cars that rev to 6500-7000 rpm can do something around 90 km/h (55mph) in second. I'd say the tach is fine, despite the needle having some slight momentum when hitting bumps and quick gear changes.

 

 

 

 

Tach check...

 

Transmissions come in all flavors of gears but the one thing in common with all is that 4th gears are all one to one straight through. A 610 with  a 22.5" diameter tire, 3.70 differential at 80km should read 2,700 and 2,8002 RPMs allowing for a one inch variance in tire size. Just drive at 80Km (50MPH) and look at the tach.

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hey, just wondering how you set your ignition timing?

have you got a timing digital timing light you can delay so you can set your timing 32-36 deg advanced at 4000 rpm?? and if so, is the timing mark stable at 4000rpm if the timing mark is erratic th those rpms, you have probably have excessive play in your dizzy shaft and probably rotor button aswell, which isnt hard to fix.

 

also regarding stock mechanical fuel pumps, i have one on my 2.3L twin 48mm webered L20b on E85 and it has no problem running that, so the pump shouldnt give any probs.

 

p.s i think i saw you today (im guessing it was you because i havent seen a red p-plater driving a datto in well,,,,,,, actually i dont think ive ever seen a red p-plater drive a datto)

on the central coast hwy going about to head up kariong hill??

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Tach check...

 

Transmissions come in all flavors of gears but the one thing in common with all is that 4th gears are all one to one straight through. A 610 with  a 22.5" diameter tire, 3.70 differential at 80km should read 2,700 and 2,8002 RPMs allowing for a one inch variance in tire size. Just drive at 80Km (50MPH) and look at the tach.

 

Driving along in 4th at 50MPH (being a '73 model, the speedo is already mile an hour before aus changed to km/h) the tach sits at around 2800. The tyres are 205 60 r13.

 

 

hey, just wondering how you set your ignition timing?

have you got a timing digital timing light you can delay so you can set your timing 32-36 deg advanced at 4000 rpm?? and if so, is the timing mark stable at 4000rpm if the timing mark is erratic th those rpms, you have probably have excessive play in your dizzy shaft and probably rotor button aswell, which isnt hard to fix.

 

I've set the ignition timing statically, this was easy with the points dizzy but the electronic one is a bit rougher to set, because you have to line up the magnetic pillars with the rotor points visually. I've set it at around 15 degrees idle, and 35 degrees all in. At the moment I don't have a timing light to confirm this, our old one seems to have shat itself, but I will try and get hold of one during the week. The dizzy shaft has a bit of play, when I was setting the timing statically, I found out that the shaft play was equivalent to about 7 crank degrees, this isn't rotor button play, but the play of the actual rotor shaft. So I'm not sure if this really is the play in the oil pump drive gear, or whether its the play in the distributor drive slot. Is there a way the fix any of this play?

 

also regarding stock mechanical fuel pumps, i have one on my 2.3L twin 48mm webered L20b on E85 and it has no problem running that, so the pump shouldnt give any probs.

 

Wow, stock fuel pump must be more than adequate for my motor to keep up with anything on e85, (doesn't that need like twice the amount of fuel to burn the same as petrol?)

 

p.s i think i saw you today (im guessing it was you because i havent seen a red p-plater driving a datto in well,,,,,,, actually i dont think ive ever seen a red p-plater drive a datto)

on the central coast hwy going about to head up kariong hill??

 

Yes that was me! Didn't know anyone on this forum was from around here! You wouldn't have been driving your 1600? I don't remember spotting one. Are you from the central coast as well?

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its probably play in between the oil pump drive spindle tongue and the where it slots into the grove in the dizzy.

 

i had play in mine in that area, i also had play in the rotor button. when you take the dizzy cap off and try and turn the rotor button you shouldnt be able to move it more than the built in advance, you will definatelly be able to tell what is extra play and what is the dizzys advance curve when you spin it. and if youthink you cant tell, you have play in it.

 

when i was trying to set my timing at 4000rpm the timing mark was really erratic, not good at high rpms.

 

i used the steel from the lid of a ring pull tuna can, i cut a bit, put a bend in it and sat it over the tongue on the oil pump drive spindle. then put the dizzy on (you wont be able to push the dizzy down by hand)

then put the bolt in and tightened the dizzy down, going a turn each time on each bolt keeping pressure even.

 

i just used some doubled up electrical tape over the rotor button locating slot when i put the rotor button on.

 

after doing all that the dizzy is solid as, there is not play when trying to turn the rotor buuton.

 

and the timing mark is stable when checking with the timing light at high rpm.

 

 

and those pumps certainly keep up  ive got 205 main jets and f2 emulsion tubes in the 48mm webers running the e85 thats alot of fuel to move.

 

 

yeah im from the central coast, ive seen you driving around a few times. ive never been in my 1600 when ive spotted you but.

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Driving along in 4th at 50MPH (being a '73 model, the speedo is already mile an hour before aus changed to km/h) the tach sits at around 2800. The tyres are 205 60 r13.

205 60 r13 are 22.69" tall so well withing the stock size estimate. 2,800 RPMs would have to be a 4 cylinder tach.

 

 

its probably play in between the oil pump drive spindle tongue and the where it slots into the grove in the dizzy.

 

i had play in mine in that area, i also had play in the rotor button. when you take the dizzy cap off and try and turn the rotor button you shouldnt be able to move it more than the built in advance, you will definatelly be able to tell what is extra play and what is the dizzys advance curve when you spin it. and if youthink you cant tell, you have play in it.

 

when i was trying to set my timing at 4000rpm the timing mark was really erratic, not good at high rpms.

 

i used the steel from the lid of a ring pull tuna can, i cut a bit, put a bend in it and sat it over the tongue on the oil pump drive spindle. then put the dizzy on (you wont be able to push the dizzy down by hand)

then put the bolt in and tightened the dizzy down, going a turn each time on each bolt keeping pressure even.

 

i just used some doubled up electrical tape over the rotor button locating slot when i put the rotor button on.

 

after doing all that the dizzy is solid as, there is not play when trying to turn the rotor buuton.

 

and the timing mark is stable when checking with the timing light at high rpm.

 

 

 

As an electronic and a points dizzy were used and the problem was exactly the same I'm tempted to discount the distributor but interesting way to remove the slop. 

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Alright so I made a bit of progress this afternoon. I took this dizzy off the check if there was slack in the oil pump spindle gear, there was very slight slack but nowhere near 7 degrees, but I noticed the tip of the spindle drive looked worn around the edges, so I did what nad015 had suggested earlier:

 

i used the steel from the lid of a ring pull tuna can, i cut a bit, put a bend in it and sat it over the tongue on the oil pump drive spindle. then put the dizzy on (you wont be able to push the dizzy down by hand)

then put the bolt in and tightened the dizzy down, going a turn each time on each bolt keeping pressure even.

 

I dug through the recycling bin and found a tin of come sort, cut a strip from the base, I was a little too scared to mess around with it near the oil pump spindle, so instead I jammed it in the dizzy drive slot with a flathead so it formed a U inside the slot, bolted it on and the slack was completely gone! 

 

I started it up, and immediately noticed the idle was a lot smoother and throttle response was crisper. When I took it for a drive something strange happened, when I took it to 5k it still seemed flat but when I held it there it didn't start going sick and leaning out like it usually did, instead I could hold it there and it could actually crawl up to 6k, but by the time it got there, the engine sounded like it wanted to kill itself and smoke actually started to float around in the cabin. 

 

My theory is that by eliminating a lot of the slack in the dizzy, the ignition was more stable and helped the engine run smoother as a result. I think this has benefitted the carbs by reducing vibrations that might have caused the fuel to froth and go lean at the limit (which might have been why the afrs would suddenly go lean after holding it there). However the engine still seems rough at the 6k limit, and although some of the vibration is reduced, there could still be something ignition related causing the engine vibrate severely and sound sick at the limit... 

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Hey guys sorry for the long waits between posts,  

 

Okay so even after I tightened up the dizzy slop, I found that the engine would still sometimes lean out at the top of 5-6k, even sometimes at 4.5k, so I thought the carbs might still be frothing up from vibration. So I finally ordered some proper soft O ring carb mounts from redline, installed them, took it for a drive and it never went lean again, even if I held it at 5k at part throttle in second gear (the engine used to absolutely shit itself if I did this), it ran silky smooth. But I don't know if its fixed the leaning problem completely yet. 

 

What are your AFRs looking like at the absolute top of 6k?

 

Since I've installed the soft mounts, the afrs are around 12.5-13:1. 

 

But there is still a wall at 6k . I've spun the engine out to 6k a few times now, and I'll be honest, it really doesn't sound like it wants to be there, it sounds like an engine at its limits.

 

Really, it doesn't feel at home at anything beyond 4500rpm. It makes me feel sick stomping at on the throttle when it just starts to pick up 3500 rpm only to have to change at 4500 before power drops off, the engine may as well be a diesel. I have to find out what the cause of this wall is.

 

I've also checked the timing with my new digital timing light, and its spot on to how I set it statically, so I don't think timing is the issue.

 

Has anyone had similar issues with poor engine ground by any chance? Any problems with car internal wiring?

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Aside from anything that you may be overlooking, it really sounds to me like either a fuel delivery or cam timing (cam spec) issue. I don't LOVE the o-ring style softmounts as there is still a hard connection between the carbs and the manifold through the studs, but they ought to be better than nothing. I would be curious to se  if the NISMO style softmounts would help more.

 

How about valve float? I forget if you mentioned the cam spec, but if the valve springs are not up to the task, valve float could really be an issue.

 

I built a HiPo A12 for my 320 years back using a monster cam that was highly sensitive to valve lash. If it were off by only a coule thousandths, the same symptom you are describing would occur.

 

Does you engine feel like it's vibrating more than it should? Humming, whirring noises at the top of the RPM range?

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According to the write up two different heads were used. Two different dizzys. So much has been changed it's a wonder it wasn't fixed.

 

 

Did you run a jumper from the battery to the coil + to eliminat the ignition switch? Maybe loose connection?

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