b_aero_eng

L28 won't Rev past 4500 with stock base timing

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Hey Guys


My Z has been parked for 7 years but I'm about to have it going again. I stopped driving because the heater core valve under the dash started leaking.



The car is actually running great, great throttle response but will cut out and misfire at 4500 rpm, even with no load, unless I change the base timing from 10btdc to 15btdc. At 15btdc it seems to rev out fine. I need to pass smog. I'm pretty sure I can set the base timing to 10 and take it in to pass but I would like to have this working correctly.


I can't seem to find a distributor, I know I can send mine out to be rebuilt but mechanically there's really nothing wrong with it. I replaced the vacuum canister which was leaking and got great engine stability and throttle response. I have verified that the mechanical advance it free. I tested the mechanical advance with my timing light by taking off the vac hose and watching the flywheel mark move under throttle. It gets to 10 or a little more from the base timing easily but there's where things start to break up. It even appears to retard from the 10 under further throttle which is very confusing. I doubt that the weights on the advance are moving back in so it feels like I have an electrical issue.


I replaced the cap and rotor before even trying to start it again but I'm not discounting the possibility that they are causing issues anyhow but I would like to see if anyone has ideas before buying any more new parts. The distributor doesn't actually have that many miles on it since it was remanufactured. The vacuum advance was sticking but have since fixed that. I also replaced the coil. Seemed to run stronger through the low rpm ranges after this but didn't solve this particular problem. I haven't replaced the ignition control module, looks like I can get one for $100. Would like to know if anyone has known an IC module to cause this kind of issue.


I don't think fuel is the issue. I did have to have the tank cleaned, put on a new fuel pump, rigged a pressure gauge in the windshield when I was out for a drive and the pressure was rock solid around 40psi even at the highest rpms. Also, rigged the cold start valve on a switch to supply additional fuel, while fuel pressure gauge still attached, under this condition and still didn't rev out. Also, it will rev out with the base timing set high so I'm confident that fuel, air, exhaust flow are not issues.


Any advice would be greatly appreciated.
 

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I helped a friend who's Z had a faulty distributor.  An internal spring was missing and the weights were sticking. If you disassemble yours. inspect and grease it may help.

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Thanks Bud!  Have already had the distributor apart, lubed and put back in.  Springs are there and not sticking.  The plate is labeled 8.5 which should provide an advance of 17deg.  I'm thinking about extending the slots but it doesn't seem like this is causing my current issue.

 

 

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Well it could be the TVS.  It is a switch on the side of the throttle body.  It sends a signal to the ECU when at idle and when at WOT.

The if the RPMs are high and the TVS is at idle (foot off the gas) the ECU cuts the fuel.  If the switch is stuck at idle, it will act as a rev limiter when the rpms reach a certain point. Usually corrosion holding the contacts together or an incorrectly rotated switch can cause this problem.

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I checked TPS contacts very early on and they are working fine.  I have eliminated fuel, AFM, Sensors as an issue.  Since it will wind up just fine when I advance the base timing I think the problem is isolated to the ignition system.  I don't have experience with a bad Ignition Module so I as wondering if folks have had this kind of issue with one as opposed to the typical hot shut down and dead tach issues that most people write about.

I am actually thinking the mech advance springs may be too loose allowing the weights to fly out a bit at idle which would cause me to set the 10degbtdc using part of the initial advancement range and not giving me enough range to wind up.  This is what I will be checking next.  If that is the case I will just need to find a set of springs.

 

 

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That's a good one Phillip. I had a D21 pickup that broke up at about 3,000rpms. The cap was causing it to jump spark. Drove me crazy for a year before I spent that $10 on a suggestion by a friend that had been through the same.

 

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20 hours ago, b_aero_eng said:
The car is actually running great, great throttle response but will cut out and misfire at 4500 rpm, even with no load, unless I change the base timing from 10btdc to 15btdc. At 15btdc it seems to rev out fine.

This is the weird part.

Edit - never mind my last post.  My logic only works if the 10 degree is better than the 15.  More advance is different.

Edit - so with the extra 5 degrees the spark would happen earlier in the rotation.  

Edited by Zed Head

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New thought.  Turning the distributor body to change timing is disrupting the ground.  The ignition module grounds through the body of the distributor.  Usually there's an extra wire to the body also, to assure good ground.  Check your ground, I'd say.  It's critical for ignition module performance.

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What is the ignition advance at 4500 when the idle advance is 15? Why not leave it at 15? 15 sounds more like a reasonable idle advance to be anyway.

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I wrote something about this earlier but erased it.  But now that I've thought about it more it might actually be a possibility.  Rotor phasing.  The lower initial timing might cause the rotor to spark on the terminal before or after the proper terminal.

Rotor phasing is weird because the the mechanical advance moves the shaft in relation to the pickup coil.  But the rotor is on top of the shaft so its phasing moves also.  centrifugal advance moves the phasing, vacuum advance does not because it moves the breaker plate.

I could probably work it out in my head but it's already taken some effort just to pull that out.  I remember spending some time trying to get the burn pattern more centered on my rotor back when i was messing with timing and high energy ignition.  It was hard to get more than hall of the rotor terminal burned though.  Not the center just one half of one side.  If less than one half of your rotor terminal shows a burn mark the phasing might be off and its sparking to the wrong terminal.  

A thought.

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Hey Guys

I appreciate all of the thoughts and ideas.  I did take the distributor back down to look at the mech advance again.  The Springs were a bit mushy so I bent the tabs out a bit to tighten them up.  I think this should allow me to set the base timing at idle and hopefully have the entire slot used for rpm dependent advance.  Don't know if it worked yet because I went to start it back up and it didn't fire.  Did some checking and didn't have voltage at the coil or IM.  Checked Fusible Links and one is burnt.  That's as far as I got before dark tonight.  Will be at it some more this weekend and will update on progress.  I'm happy to be finding these issues and getting them resolved before making this my daily driver again.

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Hey Guys

Thanks for all of your helpful comments.  I managed to get it going and producing power nicely up to redline.  I did end up replacing the module.  I can't be certain which fix was actually causing my problem.  I suspect that both conditions(mech advance springs & IM) were contributing but the IM was at greater fault.  I've never known a timing advance issues to have a discreet cutoff point and pop and crack in the manifold with the throttle wide open just sitting in the driveway.  Will take it in for smog tomorrow.

 

 

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