Mikuni jetting question

[Silky_Johnson]

Haha!  Okay okay...

 

Actually, for all of my belly aching, the tune has been pretty dang good the past couple of weeks now.  It's like the first time that I've ever really had the cam, carbs, and distributor dialed in where I like it.  That said, the engine still has a slight sound of loading up (ping, detonation, etc.) in lower RPMs.  I'd like to try an open U67 and see if that makes a difference.  Maybe I would end up running my .480 cam on a U67?  I've thought about downdraft before, but then I'd have to take the Mikuni stickers off my windows...  Not gonna do it.  Wouldn't be prudent at this juncture.  These are all options, and probably won't happen for a while, if at all. 

 

By the way, I gots 40s.  Not 44s.

[ggzilla]

Open chamber head will ping more than closed chamber for a given compression ratio.

 

 

> the engine still has a slight sound of loading up (ping, detonation, etc.) in lower RPMs

 

Loading up means "too rich" and is a kind of chugging and uneven firing. By contrast, Ping (detonation) is caused by too much spark advance. If the pinging happen at moderate or heavy throttle, retard the distributor. If pinging only occurs at light throttle, disconnect or modify the vacuum advance.

[Silky_Johnson]

I'll have to take a running video for you guys.  Maybe it's too rich, but so far, making it richer has been the only thing to make the sound not as bad.  In other words, leaning out mixture makes the noise worse.  I'll have to check and see what my timing is like with a light, because I've been adjusting it for sound (ping) and performance in small increments without a light (driving tests).  If I had to guess, I'll bet I'm running between 0 and 5 degrees BTDC right now.  REALLY RETARDED.  Retarding the timing further doesn't help the sound at all and makes the engine run like doodoo.  Taking off the vacuum advance also makes the engine run like doodoo.  Any other ideas?

 

I mentioned running an open chamber head only because I understand that the quench area only really comes into play if you're using flat top pistons.  Dished pistons don't allow full advantage of the quench area.  Wrong?  I got my info from Mike on this one.

 

You guys want a video?

[Sealik]

 If the pinging happen at moderate or heavy throttle, retard the distributor. If pinging only occurs at light throttle, disconnect or modify the vacuum advance.

 

 

Yeah...but he's only running at 5 degrees for some reason...

[ggzilla]

Maybe it's not pinging.

[Sealik]

More torque/bottom end...?...lower gears....advance sprocket to No 3 position.

I have 4.38s in my 510......vroooooom

Try some higher octane/premium fuel???

No vacuum leaks???

Confirm dizzy and vacuum advance is functioning properly....maybe swap it out for a confirmed 'runner'...?

You should be running between 12-15 degrees BTDC

 

'Power curve' is based on duration.....higher duration/higher power band

For every 10 degrees of duration.....power band increases approx. 500 RPM

My 510 has lots of torque. LZ23/U67 head/.491 lift- 260 duration/46mm SUs.....and those 4.38 gears

 

You should splurge for a wide band.......I've got one.....helps a lot when tuning

I feel naked driving without.... :D

[ggzilla]

> You should be running between 12-15 degrees BTDC

 

Only for a stock L-series distributor. If it is a Z-series matchbox then the timing is all different.

[Silky_Johnson]

I suspect my compression is just too dang high.  I noticed the other day that I have just a little coolant weeping out the head gasket on the spark plug side, among other things.  :o

 

Here's the story.  Yesterday, I finished some projects.  I put in a new heater assembly and it works GREAT!  First time I've ever felt warm in my car.  What else did I do yesterday?  Well I put new correct range spark plugs back in.  BPR6EY to be exact.  I had BR7ES plugs in for diagnostic purposes.  I didn't notice any difference.  I also retorqued the head bolts as I previously observed coolant weeping out the freeze plugs, head gasket, etc...  Lastly, I adjusted the cam gear to the No. 3 position.  Everything seems fine.

 

Funny thing though, whereas before yesterday my temperature gauge was right on (exactly on the line toward the middle), now the temperature wants to read at about the 2/3 level or above -- right on the "P" of "TEMP."  How could this be?  It's like 30 degrees out, and I am now running a heater which should be dissipating more heat and helping the engine run cooler?  I also have one of the new three core aluminum radiators.  It hasn't yet acted like it's going to overheat.  It won't go above the "P," and the engine feels as happy as it ever has.  Would advancing the cam sprocket to the No. 3 position make it run that much hotter?  I'm stumped.

 

By the way, I stuck a light on it and found out that I'm actually running 2 degrees AFTER TDC.  Runs better there than more advanced...  It's still making noise on me though.  I guess I'll have to make a running video so you guys can diagnose what this sound is.  I thought it was compression ping, but maybe not?

 

Another observation is that all of my old spark plugs were super black and sooty (rich), except for the plug on the No. 1 cylinder, which looked fairly clean and amber-ish color.  Throttle plates are even for the most part, and there isn't anything blocking fuel flow.  Is it possible that what I actually have is lean ping on the No. 1 cylinder only.

 

At this point I'm getting hacked off.  I may be putting up a wanted ad for a stock U67 head just so I can get back to square one.

[Silky_Johnson]

By the way, I'm also running 92 octane from Chevron.  Should I try an octane booster?  I may have access to some 100 octane leaded aviation fuel...

[ggzilla]

> I suspect my compression is just too dang high. I noticed the other day that I have just a little coolant weeping out the head gasket

 

Not caused by the compression ratio.

 

 

Running best at 2 ATDC? You got other problems. Octane booster won't help. Maybe you have the wrong timing pointer on the engine.

[Silky_Johnson]

Well, I thought I had heard of people blowing head gaskets more on high compression engines.  Maybe was wrong...

 

And yes, I do have other problems, I just can't figure out what they are.  Carbs seem to be flowing well, and plugs are black as night, so I'm not running too lean.  Timing is definitely not too advanced.  Valve lash is appropriately set.  Bottom end was just rebuilt and there aren't any rod knocks.  Last thing I can do is blame it on the head/compression ratio?  What else could it be, that is and ignition related ping that only occurs at low RPMs and under load? 

 

I verified top dead center with the head off when I was building the engine.  With the piston at it's very highest point, the pointer was pointing exactly at the notch second from left on the pulley, like on the early style pulleys as described on OldDatsuns.com.  Now, the light is showing just left of that notch.

[ggzilla]

Oh, so it pings? That is compression.and timing.

[banzai510(hainz)]

I think you should be more advance on the timming

 

maybe disconnect the vac adv tube and plug it and se the timming more advance and adjust the carbs to match that. idle and the speed setting.

My old Datsun SSS book show Dual SUs at 14BTDC for the timming if I remember right.

 

maybe dizzy is fucked. UYOU sure the spindal is located at 11.28 a TDC? rotor is right on?

Is this a Matching matchbox with the matching pedastal as there where 2 types and maybe the other pedastal got in there. To verify a wrong pedastal was installed the rorot will be between #1 and #3 plug wire at TDC with timming plate in center

 

 

head gaskets dont last long that I seen with flattops and closed chamber heads with the common back yard mechanic. stock with closed chamber should b fine.

[Silky_Johnson]

I think I may have found some of the problem.  When I replaced the plugs, I noticed No. 1 was lighter than the rest.  Maybe too lean?  I adjusted the idle mixture screw out another 3/4 turn, and the engine is running smoother, but there is still some ping-ish noise.  It's better though.  If I have all of the screws adjusted out the same number of turns, it idles REALLY lopey.  With just No. 1 out a bit more, it sounds like it isn't missing as much now.  But screwing around with the idle adjustment doesn't really solve the problem, right?  What could be causing the No. 1 cylinder to lean out like that?  Is it possible that the throttle plates are off?  I checked the throttle plates a while back, and I thought I would be able to see if they were off.  Twisted throttle shaft?

 

I'm going to mess around with the vacuum advance, but past attempts at disconnecting the vacuum didn't work out very well -- lots of cough, sputter, backfire.  But who knows?  Maybe better this time?

 

The dizzy should be good.  I had an old dizzy that I put Pertronix on.  Suspecting messed up advance, I then bought a remanufactured early single point dizzy and put the Pertronix in the new one.  Maybe the spindle isn't set at 11:28?  I was 95% sure I got it right on when I assembled everything, but maybe not?  I'm gonna change the oil this weekend, and while I'm under there, I'll pull the dizzy and oil pump and see what I got.  But here's a question: so long as the timing is set with a light on the crank pulley, doesn't it not really matter if the distributor gear/spindle is off a tooth?  It's all adjustable right?  It may be an extreme adjustment, but so long as the light says it's good, it's good, right?

[Sealik]

This is the cam with the unknown duration...ie....a lot of?

Maybe you timed the engine to compensate for the loopy idle and lack of bottom....<......then tuned the Mikunis accordingly, which only compounded this issue?

 

Not knowing...just a 'guess'

[banzai510(hainz)]

I'll pull the dizzy and oil pump and see what I got. But here's a question: so long as the timing is set with a light on the crank pulley, doesn't it not really matter if the distributor gear/spindle is off a tooth? It's all adjustable right?

 

just put motor to TDC Zero and look at the spindal position. then drop in the dizzy nd it shound be on #1 plug wire. and you should have alot of adv and reatrd on the timming plate.

If its cranked to one side all the way then soemthing is wrong.

 

if throttle plates are bent you would see this in the Unisync I would think. adjust the speed up once you think the idle mixture is Ok.

[Sealik]

But here's a question: so long as the timing is set with a light on the crank pulley, doesn't it not really matter if the distributor gear/spindle is off a tooth? It's all adjustable right?

 

 

Correct...as long as there's enough adjustment in the dizzy

 

Actually...set the motor at 12 BTDC, rotor should now point to No 1 spark plug wire...exactly

 

Re-tune Mikunis

[Silky_Johnson]

This is the cam with the unknown duration...ie....a lot of?

Maybe you timed the engine to compensate for the loopy idle and lack of bottom....<......then tuned the Mikunis accordingly, which only compounded this issue?

 

Not knowing...just a 'guess'

 

 

Other way around actually.  I messed around with the Mikunis long before I even knew I had a reground cam.  I used to have the timing set at 12 like everyone, including the factory, says.  It wasn't until I found out about the cam that I considered retarding the timing.  It sounds a lot healthier now.  And yes, the cam seems to have a lot of duration.  Those lobes are pretty dang thick.

 

I'll pull the dizzy and oil pump and see what I got. But here's a question: so long as the timing is set with a light on the crank pulley, doesn't it not really matter if the distributor gear/spindle is off a tooth? It's all adjustable right?

 

just put motor to TDC Zero and look at the spindal position. then drop in the dizzy nd it shound be on #1 plug wire. and you should have alot of adv and reatrd on the timming plate.

If its cranked to one side all the way then soemthing is wrong.

 

if throttle plates are bent you would see this in the Unisync I would think. adjust the speed up once you think the idle mixture is Ok.

 

I hear you on the spindle.  Adjustment is pretty far off to one side.  I just don't think it would mess anything up but the adjustment.  Maybe not.

 

My Unisync shows the No. 1 cylinder pulling substantially less vacuum than the No. 2 cylinder.  Both 3 and 4 are the same.  Does less vacuum equate to a leaner condition, or am I just making this up? 

[ggzilla]

haha, yes

 

Have you done a compression test to see #1 and #2 are equal? Also inspect the intake manifold for open or cracked vacuum hoses.

 

and always adjust idle mixture *after* changing the idle speed.

[banzai510(hainz)]

get the exact spec on the cam but cams want more advance.

 

otherwise put a weber DGV on ther and run it just to mak sure your motor is fine. then you can assume the carbs are finiky

[ggzilla]

???> cams want more advance.

 

A cam wants a specific advance. Start with the stock timing setting. And the timing will not cause one cylinder to run poorly while the others run good.

[Silky_Johnson]

I've messed around with the idle mixture, and already things are getting better, but it's not a long-term fix.  Fixing the uneven vacuum issue with idle mixture screws is also messing up my higher RPM mixture.  The good news is that I think I have ruled out compression, because now I can run more advance without any change in ignition noise.

 

Yeah, I'm going to do a compression test this weekend, but I think I already know that the carb is outta whack.  I guess my question is, what should I look for if one venturi is pulling less air than the other?  It could be uneven throttle plates (bent throttle rod), air leaking out the side seals, and what else?  Is there anything else that could be causing this assuming nothing blocked?

 

After the compression test comes back good (newly rebuilt block, and leak down test performed on the head while it was in the shop, so I'm 95% sure of this), I'm probably going to buy a new throttle rod and plates from Todd Walrich at Wolfcreek.  I already have rebuild kits with gaskets, so I will put in new side seals (O-rings) with the new hardware.  I doubt it's the side seals though.  I sprayed carb cleaner around the side seals a while back and it didn't make a bit of difference.

[ggzilla]

> what should I look for if one venturi is pulling less air than the other?

 

* bad compression

* bad gasket

* bad casting (air leak)

* bent throttle plate

* twisted throttle rod

* air leaking out the side seals

[Silky_Johnson]

> what should I look for if one venturi is pulling less air than the other?

 

* bad compression

* bad gasket

* bad casting (air leak)

* bent throttle plate

* twisted throttle rod

* air leaking out the side seals

 

THX.  Time to do some diagnostic work.

[banzai510(hainz)]

Kinda off topic but I had a loose inner venture. would rattle around.

 

I assume you made sure the jets are screwed down good. Mine would alwasy work loose and always carry a screw driver to tighten them up.