L16 missfire?

[///FiveOneOh]

So yesterday was Novice Championship Tour 1 VCMC autoX, Myself and my buddy co-drove his 70 510.

 

the night before the event we found it had a bad HG so we replaced it. now for some reason though the car has an intermitant missfire.. you know the L series flat spot where it feels like your engine has hit a wall..

 

its been parked for a year didnt; do it when it was parked.

 

-Engine is an L16 with closed chamber 210 head

SSS SU carbs that run HORRIBLY rich. however if you lean them out at all the car sputters even worse.

ignition is handled by a crane cams fire ball optical pickup

timing set slightly advanced over stock.

 

1st run the car sputtered badly through the whoel track ending in a 35 second finish time, we foudn that if before we took the car out to run if you held the RPMs at about 3K for a minute or 2 the missfire would clear up nd go away. This resulted in a class winning 30.6 time (also good enough for 3rd over all in PAX time)

 

what is causing this miss fire though? I'm sure we could shave a 10ths off if the motor was running perfectly.

[Duke]

What do the plugs look like? Also, how old are they. I had the same problem with my L18 for a while and after changing the plugs it cleared right up. Also, check them to see if perhaps the valve seals are bad, or maybe even the rings are going. Excessive oil blowby can cause some misfiring as well.

 

The fact that holding it at high RPM's for a while was able to "clear it out" suggests to me that something is fouling the plugs...so that is where I would start.

 

Also, you may try moving to a hotter or colder plug, depending on what you are running in there right now.

[banzai510(hainz)]

do the usual,

ck cam timming

dist/oil pump spindal lined up properly (11.28 at TDC)

then electrical timming.

valve lash

otherwise than that, its carbs or ignition

 

I assume the cranecams ignitions(XR700) still uses the stock coil and ballst resisitor.

[///FiveOneOh]

dizzy drive shouldnt; have moved as we never touched it, cam timing is correct, we lined it up to TDC but we also marked it with paint before removal just to be exra sure it went back the same way it came apart.

 

Coil is a newerish 12V coil with no balast.

 

Valve lash was done before it was parked so theres no reason for it to have moved.

 

plugs were new when parked..seemed clean when we pulled em. they could be fouling because the carbs though.

 

The carbs have been a pain my ass since the day I got them.. they puke out black smoke so you try lean it out.. if you lean it out the car wont; run... so I'm forced to richen them up agian.

 

we have ATF in the dampers and stock tension springs...

[DISLEXICDIME]

when my car did this with my su's it was because my bottom in take manifold bolts where not tight. when tried too lean them out it would run to hot. so i had to run it way to rich. i used a extension and a bendy socket to tighten them. what i did not know was the bendy socket was binding making me think the bolt was tight.

Atf is way to thin the dampers cant work right . at least on my car but boy the Throttle Response was great with the atf.

hope this help's