I have recently rebuilt an L20b and have some issues with how its performing. Any help would be appreciated.

Specs: Series 2 bluebird L20b bored 40thou, New pistons, A87 port matched head, S2 bluey dizzy, chrome rings, 272 cam, new bearings, new timing chain kit, new water pump blah blah blah....

Timed the cam as per spec with the cam on #2 hole the dash lines up with the v on the backing plate. The timing chain looks a little loose on the slack side (the tensioner is out around 10mm) see photo

Timing light shows 14 btdc at idle and 34 btdc over 3500rpm

Here's the problem....The engine is stumbling on hard acceleration. I can push slightly on the pedal to get it to rev smoothly but that's no fun at all.

I was running weber 45's to fire it up. It ran fine while revving it stationary in the garage. Went to take it for a spin and it bogged down and wouldn't accelerate under load. SO took the webers off and threw the twin hitachis back on(these were running fine on the previous L20b) and it ran better but not what I would call great.

The plugs look black (rich) so tried to lean the hitachis out to cure the stumble but no dice.

Please help

New plugs?

that timing chain looks very slack. You are going to wear that tensioner very fast.
How much was the head decked?
You need to slot the holes in the curved guide and slide across

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Circuit Race 1600 in the build

Thanks for replies..
Yes new plugs installed.
Head wasn't decked. Block decked 8 thou only. Not enough to cause that amount of slack I would think. Possible wrong chain?
Hooked up vac advance today. No improvement. Still bogs down on acceleration.
When I move dizzy to get faster idle it gets a rattle in the top end.
Strange that engine revs perfectly when sitting in driveway(no miss no stutter) but when driving at low rpm it sputters until around 3000rpm
Tested coil and its within spec. Not sure what to check next.....
Any more ideas? Its got me stumped.

Have you given it a compression test? Start with all the basics. Can you try a points dizzy? You have to try to eliminate as much as possible before looking at the only annoying issue of the timing chain being slack on one side. Changing the carbs should have ruled a a vacuum leak anywhere but trust nothing. I don't have my Manual at hand right now but isn't the stock setup for the cam gear on #1 hole? Is the battery fully charged? Not using old, stale fuel? Good quality plug leads? Swap them out anyway, just to be sure. Try 10deg on dizzy instead of 14. Did you double check the dizzy drive shaft from the oil pump was lined up correctly when you assembled it? Been caught before on that one. The "L" is good, basic and strong engine with few flaws.....all the basic things like cam (valve) timing, dizzy timing, compression etc have to be right to get the best from it. The symptoms sound a little like the valve timing is a tooth out if all the other basics are spot on. Keep at it and keep posting - hope I've been of some help.

Thanks for the support Dattonut. All help is much appreciated.

I was running points dizzy at the beginning. Thought it might be the points gap. Re-gapped it and it had the same symptoms of hesitating acceleration. Then I slapped the S2 bluebird dizzy on. Same same.

Haven't done comp test as yet. Will do it asap.
Was told by head work guru to set it up on cam hole #2(this is factory setting I believe)
Battery reads bang on 12v.
Fresh 98 octane fuel.
Leads are eagle 9mm. Will try new leads, rotor and cap asap.
When I set timing anywhere under 12 deg the engine won't idle. Smooth at 14 and rough again at 16.
Dizzy shaft at 11.28 as per the infamous HAINZ VIDEO.

Spoke to an L6 guru Z racer about this problem and he suggested moving the dizzy drive cog one tooth either way. Tried this(both 1 tooth clock and anti clock wise) Same same.

It feels like a timing problem to me too but all checks seem spot on.
If the mechanical (cam)timing is off one tooth, what does it look like?
Would it still idle and rev nicely in neutral? It does.
Would the tight side of the chain be a little slack? It is tight.
I have noticed a rattle noise when backing off on the throttle. I'm guessing this is the loose chain causing this.

Thought of replacing the new chain with the one I pulled out before the rebuild. They both had the same amount of pins/links. Will try elongating the holes on slack side guide too but I think only 2mm is possible and not enough to pull the tensioner into spec.
I would have thought new guides and chain should need no modification, never-the-less chain is loose....
Any specific brand of chain kit I should look for?
Would a clogged fuel filter cause symptoms like this?

I know I have lots of q's but I really want to solve the issue and get out on the highway.....

Well Uteboy, looks like you are eliminating the usual causes one by one. Hope I can give you a few more things to look at - it's possible that the Hitachi's simply need to be retuned to suit the 272 cam grind.
What valve clearances did you set? Engine hot or cold? Exactly how did you set these clearances as I was doing it incorrectly the first time & ended up making things worse? Was the cam a fresh grind cam or a second hand one when you got it? Did the supplier give you the valve settings to suit it?
Any rattle on a rebuilt engine means something is out of spec. I've had old, worn L's that still didn't rattle on overrun. When I rebuilt it, the new cam chain wasn't as loose as your picture shows. You've had the engine out - check for strained wires on the coil, badly crimped spade terminals, loose coil nuts and remake every earth connection you can find. Sand the connection points - trust nothing. The wiring in these old cars has done a lot of work in the heat and can give really had to find faults. I could go on forever about things like wire broken inside the plastic which looked ok but caused a miss on right hand turns when it flexed etc. but it only goes to show a point - it drove a mechanic to distraction and he had to admit defeat. I found it but simply pulling on each wire until one let go!
"Always return to the scene of the crime" which means this fault wasn't there until you rebuilt the engine, so something changed and we have to find it. This will make you a better detective and improve your diagnostic skills for the rest of your life.
I still have a few L series parts that I will probably never use as I am running a CA18DET in my 910 - where are you located in Qld?
The differences between a loaded and unloaded revving engine is basically the former gets more air/fuel mixture and takes a while to use it as it slowly builds revs. It has to cope with more than it needs and do it smoothly, which is where your problem lies.
To answer your question, if the marked links on the chain line up with marked gear teeth, then the cam isn't a tooth out.
I'm sure an L will idle on 5 deg advance - it won't accelerate as well as set to 10 deg but it does go.
Keep posting what you find - happy hunting.

When you turn the crank and line it up to TDC (zero deg on crank) and the dash lines up on cam I say your 99.9% OK. Wheither there is slack on the slack side or not it wont know.

How hard is HARD accelleration?
SUs dont have accell pumps(plugers dont move fast)
Weber DCOEs do.( Might flood out here)

I noticed on my Timming chain swap or rebuilds that one manfuacture of tensioners would be flush while another would be out say abit more. Like you say 9mm or so. Matter of fact it was the Taiwan made tensioner that fit better in my case. But they looked both the same compared with the OSK jap made one.

I will assume you adjusted thetop of the slack side guide(its slotted) by pushing it right so the bottom would be flush thus pushing the tensioner in all the way.Thus making a smooth arch. I loosen the slack side guide both top and bottom bolts then with my left hand squeez the tensioer in all the way while right back of the hand pushes the tensioer to the right and tighten down with my rachet and hopefull the tensioner stays in all the way. olddatsuns.com "Datsun tech "section has my youtube vids on me doing this.
So I say your motor mech is timmed.

Now is to make sure your dist spindal is in the 11:28 position at TDC and you install the dist with the correct matching timming plate/mount and its in the middle so you have full adjustment. NOT crank to one side to get a basic idle is a sign soemthing is wrong.You should go between 0 and say 25BTDC with a timming light. I would call this full movement. Here in USA the elctric distributor has 2 type of mounts and people get them mixed up and the tang that loks down the distributor is off like 15 deg thus dist is cranked all to one side to get running. Rotor will be between 1 &3 if I remember right. BUt most like the faster you go the more it cuts out in this situation. NO HIGH END POWER as your dist advance its goes more out of time.
You have the same coil set up as was before. If point ignition I assume you still have the ballast. If a EI then you have that set up.IF ANYTHING WAS CHANGED you have to say so now. COil HOT to the touch?
So if you think this is good. I assuem you also have a low pressure fuel pump for carbs.. As both carbs seem to give the same proplem.

I would look at the distributor. I have just plain wore out about 4 since owning datsuns.Last one the vaccume advance locked up. No high end power. Most my proplems were timming related and my carbs have been more reliable than my ignition systems.But you said you tried that already

also recheck you valve lash . its simple and ez




replace the left turn signal bulb is all I can offer next. Im not a Weber DCOE or SU expert so I cant say how there transision circuts work

see a upper slack side bolt shear off causing a rattle. also the lower water housing bolt was too long pushing the tight side guide making a grinding sound cathing the top of th guide.

update:
I have a set of mikunis40s and the jet blocks would back out everynow and then giving that loaded up feeling right before it really loads up. Ran this stock cam& aftermarket cam and I never adjusted the jet sizes.

On my 521 the load up on this was timming was to far retarded with my weber 32/36.

When setting the cam timing, how are you checking for crank TDC? Is there a chance the timing marks are not accurate? Have you changed the pulley / harmonic balancer?

Mick

on the distributor proplems I would think it would get worst as you rev or load up as you increase speed. If that's not the case Im kinda lost. If this is just a hesitation proplem but it fine in the latter rpms.

My 40mm Mikunis uses a rod the pushes to levers. One end the bushing would come out hus putting my carbs out of sync. Have you tried to sync your carbs . TYhis does help out. But you did say the SUs were fine on the other motor it was on.

Hi Uteboy. Found the Nissan service manual & it states that "Camshaft should be installed by accommodating its No. 1 hole to knock pin of camshaft.". After fitting chain and making sure the marked links line up with the marks on the respective gear teeth, "Install chain guide (adjustable side) and tensioner and tighten guide so that the protrusion of the chain tensioner spindle is 0mm." So it looks like you have the wrong hole lined up on the camshaft sprocket and the chain is not up spec either.
I have replaced a chain very carefully with a helper slowly turning the engine by hand as I fed the new chain in. The chain must have a joiner link in it first. Adjusting the guide means front cover has to come off.
Sorry to be the bearer of these tidings...but it's what the book says! Keep posting on your progress.

Thank you all for the suggestions.

The stumble on acceleration must have been due to overly rich tune on the carbs.

Leaned out another 2 flats it seems to run okay.

Gonna drive it like this for a while until I can get an appointment for the twin weber tune.

Update.... So the L20b was running okay after I tuned the hitachi's BUT:

*It was still pinging on acceleration.
*It was a pain to start.
*It should have had ALOT more power considering the mods.

HAINZ, you were right! Pulled the dizzy off today and noticed slight movement in the shaft (Not the good kind )

Swapped it out for another dizzy with no shaft play. Now it accelerates sooooo much better. Like a whole new beast!! And no pinging!!

So the dizzy was the culprit all this time. Now to fit the webers and get out on the highway.

Thanks again to everyone for the advice

Im telling you!!!!!!!!the distributors is almost 40years old and they all fall apart.
I always just swap them out. 5 -10 min job and eleininates alot of trouble shooting time

My last proplem I had last week was bad idle loading down/stummbling. It was the **** coil wire just being held by the rubber boot. Just hanging inside. SIMPLE fix

Funny how its usually the last thing you check is the culprit. Well funny in an irritating kinda way.
Thats part of the glory of driving a classic car I guess.
I owe you a beer mate