Tach 3k limit. Crane xr700

[maton_acoustic]

Hello, the tachometer on my 78 260z has a 3k limit and picks up rather slowly. I'm running a ps92 coil. Are they supposed to work with this setup? I'm not familiar with the wiring but I know I have a wire doubled up on the positive of the coil and the installation instructions for the xr700 show tachs coming off the negative.

I'm not sure if the tach worked before i installed the xr700 unfortunately.

many thanks.

[maton_acoustic]

Just realised the tach acts normally when sitting, it's only under load that it hardly moves. I can blip it to 5k rpm on the drive but can't get near that on the run. I'm in the process of trying to get it to run good at the moment.  So far;

new plugs (gapped to 1mm) and cap and leads

xr700 ignition and fireball coil

tank clean and seal, new fuel filters

new su jets and needles and floats and valves

I've set timing to 15btdc and it advances up to 33 . Colourtune shows correct mixture. Compression is also good. So I'm running out of ideas. Im thinking of picking up a cheap electric fuel pump just to try.

It runs ok when up to speed, I've only had it pop out the carbs once, it'll cruise nice in 5th. Just poor acceleration through the gears.

any help appreciated

 

Edited April 16, 2017 by maton_acoustic

[Mark Maras]

   With the engine up to temp, while you're accelerating in third or fourth gear, gradually pull the choke on. If it pulls better with a little choke, the mixture is lean. It's an easy way to check for a lean mixture under load. How many turns down are the nozzle mixture screws?

[maton_acoustic]

I tried it today with 2 more flats down on each carb. They're both 15 flats down now. It did improve with the choke pulled all the way on also. Checked the plugs when I got back and they're more tanned to sooty now. I had the engine screaming in 4th and the tach was just sat there on 2000 rpm.

i must mention that these aren't original carbs, they're hs6's. I'm not sure if the po ever got them running right. It does have a slight miss at idle and when I blip the throttle it never settles right, and drops to below 500rpm and I have to stop it from stalling.

[Mark Maras]

 Improved performance by adding choke indicates a lean mixture at the RPM that the engine was turning AT THAT TIME. I'd check fuel pressure and volume first, just to eliminate that as a possibility. Then I'd look at the carbs. Have you been using tech. info for Hitachi SU's or British SU's? HS6's  were used on TR6's, among others. They had a similar engine size so I'm assuming they would be suitable for a Z. Re-check the float level.  Fifteen flats down sounds about right for Hitachi's but I don't know the spec for British SU's. The new jets and needles could also be causing the lean condition if they have a different configuration than original.  A comparison of the old needles and jets and the new ones may be a good idea. Did you have any trouble tuning and balancing them previously?

 Any of you electrical Guru's want to comment on the tach operating differently under load? That's a new one for me.

[Chickenman]

Ummm... I don't think an XR700 box can handle the amperage that a PS92 coil draws. A PS91 which pulls less amps might even be iffy with an XR700.  I'll have to check my Crane  technical documents.

PS92 coil is a High Output race coil is only rated for Crane Hi-6 CDI boxes and higher. Even an MSD 6AL is not recommended to be combined with a PS92 or LX92 coil. The 92 coils are CDI coils and are not meant to be used with an Inductive box like the XR700. 

Edit: Yeah, you can't use a PS92 on an XR700. It will burn it out. Even NSD requires a MSD 7 series or higher for a PS92 coil. Ther's mor info than just this link available. But a PS92 will burn out an XR700 pretty quickly. 

PS91 you can get away with on an XR700, but it does need a Ballast resistor attached. 

http://www.cranecams.com/uploads/instructions/9000-1091_.pdf

Quote

COIL APPLICATIONS PS91 ignition coils can be used with most original equipment electronic ignition systems including GM HEI and Ford TFI. PS91 coils are recommended for use with Crane HI-6 and HI-6S ignition systems. PS91 coils can also be used with most other aftermarket ignition systems including Accel, Holley, Jacobs, MSD 6, and Mallory Hyfire. When used with a Crane XR700 or points, the PS91 requires an additional 1.2 to 1.9 ohm automotive type ballast resistor (such as Echlin ICR23 or Wells CR107). PS92 ignition coils are intended for use with Crane HI-6 and HI-7 capacitive discharge (CD) ignition systems. The PS92 can also be used with MSD 7, MSD 8, and Mallory Hyfire IV CD ignition systems. INSTALLATION FOR PS91 AND

 

Edited April 17, 2017 by Chickenman

[Chickenman]

I've been using Crane Hi-6 CDI boxes and there coils since about 1998. Pretty familiar with the products.

Crane recommends the PS20 or PS40 coil for the XR700. I think that the PS91 compatibility with the XR700 was be old info as well.  later coil recommendation catalogs do not mention the Crane XR700 as being compatible with PS91 coil even with a ballst resistor. The PS91 is also a CDI coil;

http://www.cranecams.com/404-405.pdf

Edit:  More info on PS91 and Ps92 coil. XR3000 box recommended as minimum required for a PS91. ( XR3000 is much more heavy duty than XR700 ). And a  PS92 Race Coils much stronger than a PS91.

 

Word to the wise. Either of these coils can stop your heart and kill you. Never check spark gap quality with your hands. Always use a spark gap test tool. Seriously folks. I got laid out on the ground and thrown 3 feet from car when I got zapped from a Crane Hi-6 CDI box and an PS92 race coil . Fortunately current went down the right side of my body, as I always handle HT leads with my right hand only. 

Edited April 17, 2017 by Chickenman

[maton_acoustic]

My mistake, it is a ps91. No good?

im looking into getting new jet needles. At the moment I have BBR which I can't find specs for. I recall reading somewhere that a BAX needle works well with hs6's on these engines.