I've got a over-bored 5K, 280/285 cam, 40mm Mikuni sidedrafts, compression, headers etc. The power picks up properly at about 4000RPM (I think it should start earlier, but the 39mm chokes may be causing the air to stall) but then plateaus pretty quickly after 5000RPM. I'm still running the stock dizzy and mechanical points with the mechanical advance and no vac advance with about 18 degrees static advance. With this cam I'd expect a decent power band out to at least 7000RPM. I'm about to build a new head to take advantage of the cams power band, but would like to sort this power-dropping off issue before I finish and install it otherwise there is no point. One of my friends with a similarly built Datsun motor also running mechanical advance has the same issue.

 

Does anyone have any opinions on what the issue is? My hypothesis is that the mechanical dizzy stops tipping in advance around 3500RPM and by the time it gets to 5000RPM its begging for more.

I'd say it's definitely a timing issue. Though, cams with that duration need a bit more a advance earlier on and a bit less over all.

For instance, my camshaft is pretty close to those specs. Static timing is 10*, it advances quickly and is all in at 35* by 3000rpm. No vac advance.

Peak is about 6500rpm but it does keep going.

 

Distributor was reco'ed & regraphed by Performance Ignition Services in Vic for $270.

Edited March 23, 201511 yr by GJM85

If you have 18 degrees initial advance plus the mechanical advance of the weights you are ending up with a fair bit of advance, assuming that you have checked to make sure the mechanical advance is working correctly it could be getting to much total advance ?

 

Not having vacuum advance is not going to effect the total advance at wide open throttle as the vacuum advance unit will not put in any more advance then anyway, as its only there to aid fuel economy at light throttle & low down light throttle driveabilty & once you get into wide open throttle stuff even if it was there it wouldn't be putting in any more advance ..

 

I would be looking at fuel delivery as it could be running lean up top, best way is to put a sniffer up the pipe or fit a wide band sensor into the exhaust or do a plug read as other wise you are only guessing ..

 

I don't know much about 5k's so my question is what RPM will they pull strongly to in stock form & if you are below that range look at what you have added to the motor as to the possible cause not the factory stuff that has always been there ..

Edited March 23, 201511 yr by lexsmaz

So you run base 10* and then +35 by 3000RPM? so a static 45* after 3000RPM.

 

That kind of leads me to my second question. How should the advance curve be defined? I understand the parameters that affect it (RPM, Cylinder filling, AFRs...) and what I want to achieve by advancing it, but how much, when and what rate? I would have assumed a curve like this would roughly characterise a curve for advance vs RPM for a specific throttle position. Which correlates with what you said, sharp early rise, then eases off, but how do you know where it should ease off at?

 

I've been working on a side project which is a programmable ignition module using an arduino for the math, mosfet (solid state relay) to trigger the coil and a current sensor hooked up to the points as the trigger. My plan was to program an advance curve comprised of a steep early section up to 4000RPM (+8 Degrees/ 1000RPM) and then a flatter section (+2 Degrees/1000RPM) from there on. Does that sound like a reasonable base curve or should actually plateau?

Lex, I have a wideband, AFRs up around 5000RPM - 6000RPM are around 12.5. A little rich, but it runs that way for most of what and doesn't both it lower down. Could try leaning it out a bit.

 

5K's have the same stroke as a 4K so it should be good to rev to 8000RPM in theory. I woulnd't think any of the modifactions I've made would cause power to plateau around ~5000. I would think too much advance would lead to a small power drop off the pinging and I'm not getting pinging.

Initial advance of 10* + 25* mechanical is 35* all in.

If you have an initial of 18* & those 4k dizzy's have about 30* mechanical advance, your heading up to 48* all in which is likely too much.

If you are getting detonation at 5000rpm you likely won't notice or hear it over the engine noise.

 

How you determine the correct rate of mechanical advance is done through trial and error. Unless you have the correct equipment, like the shops do.

Advancing a stock distributor is the wrong way to adjust the timing curve, it needs to be recurved with vacuum advance deleted.

Also your standard ignition system using points and factory coil may need changing to a performance setup. Factory valve spring

pressure may not match the cam and can cause a natural rev limit through valve float.

 

Base Timing is at idle rpm

Mechanical Advance is the advance the distributor adds

Full Advance is Base Timing + Mechanical Advance

"All in" is the rpm that Full Advance is set to

 

Vacuum Advance operates under certain conditions for economy but becomes less

effective with a recurved dizzy and adds an additional timing factor and setup, difficult

to use with twin carbs and is best deleted in a lot of peoples opinion. Base timing is done

with the Vacuum Advance off in a factory setup just for reference.

Also you may like to know that there are some slightly fancier timing lights with an advance wheel built in.

Did you dial the cam in?

I'lll try less advance then. I dialled the cam in.

My 5k used to rev up around the 7500-8000 rpm range. This was with a wade 169 grind cam, modded 3k-b carbs, running around 12 degrees static advance, stock 3k(?) dizzy and vaccum advance hooked up.

39mm chokes sounds a bit big? With Weber DCOE40 or Dellorto DHLA40 32 mm chokes might be more suitable for 1500cc.

 

Could get some ideas here:

 

http://www.rollaclub.com/board/topic/62085-5k-reving-problem/

Flywheel. You have to lighten the flywheel!

you don't say if the head has been ported?

chokes are WAY TOO BIG.

i don't see it as an advance problem, more of "lack of air" at higher rpm.

low down torque, or lack of, would probably be due to the choke sizes.

oh and get rid of the points ignition, there is some gains to be made with electronic ignition.

 

hope it helps.

My two cents...

Chokes way too big! should be more like 32/34mm. I run 12deg static, dizzy mod to limit mechanical adv to 20deg, recurved to give all in adv of 32deg@3200rpm.

With those massive chokes it will run pretty poorly low down and you will need heaps of static advance to get some drivability down low, far from ideal if the dizzy is not recurved to limit mechanical adv. I played round with a few dizzys 3/4/5k mechanical adv was between 25/35deg std