Jump to content

Recommended Posts

Hi All,

90 days of preparing the 1973 240Z (L28 engine) to breath again, asleep for 17 years.  Read all the posts on 3 screw SU have the Ztherapy video as well. With all that said, the car has not cranked over in 17 years, little more complexed then a baseline tune up.

Completed the basic fuel mixture setting on the SU's, 2 1/2 turns down. Car backfires out of both carbs. With minor advancements to the setting (richer) the car will progressively start quicker but still backfires (more from the front carb then rear). After an hour of stop, restart, minor adjustments to the carbs the engine now backfires (carbs), stalls and then revs at high rate, repeats the process, until the revs get too high need to shut down. Carbs just rebuilt and I reset the floats to spec. Is this a needle, jet, timing, other issue or operator error?   

Car specs, 1973 240Z with a L28 with a bunch of transplanted L24 parts. Distributor, L24 single point with a vacuum advance, rebuilt and installed. N42 head, B cam. I believe the 3 screw SU carbs and manifold (N33) are from the L24 engine, not sure? 

Any advice would be wonderful . Thanks.

 

Share this post


Link to post
Share on other sites

I think you've got a few things need fixing. First up is a big air leak that's making the mixture lean and causing revs to rise. Either the throttles or chokes are open, or the carb to manifold or manifold to head gaskets aren't sealing.

  • Like 1

Share this post


Link to post
Share on other sites

All good stuff and a task for today. Need to deal with the high rev issues first, can't play with the timing or carbs adjustment before the need to shut down the engine. Will hunt for air leaks. Air cleaner / housing is not installed any issues with the vents on the carbs?

Tried to keep the engine mods to a minimum until running but did replace the 73 split balance tube with an earlier version (single tube) and removed all emission, ERG and PVC valves. I retained all should I reinstall?  New tube is fluid (old tube was fluid delete) and emission delete. California car. 

Thanks again for all the advice. 

Share this post


Link to post
Share on other sites

  The only way an engine will rev is more air. I agree with the air leak theory. The backfiring out of the carbs confirms a lean mix but I'm going to play Cap'n O here. Have you installed both throttle return springs? They hook onto the heat shield below the carbs. I forgot them one time. Your description jogged my memory.

Share this post


Link to post
Share on other sites

Picture worth a thousand words.  Not sure on the linkage. I'm fully advanced on the distributor (only position the car will start), same position when purchased. Thanks again.20190313_080547_resized.jpg 

20190313_080527_resized.jpg

Share this post


Link to post
Share on other sites

The spacers in between the carbs and the manifold are a lot thinner than my '72.  Would that have an effect on the length of the rod with the plastic marbles?

Do the new spacers have the holes for water?

image.png

 

Share this post


Link to post
Share on other sites

Hi All,

Yes, springs are attached. Not sure about the spacer and water response?. Spacer between carb to manifold? Not sure, I'll check.

There are zero threads to adjust the ball linkage, would need to bend to shorten the length. Purchased that linkage, not original to the engine. Had to reinstall the old throttle linkage rod off the split balance tube on the early single tube. The single tube linkage rod was too short would not reach the linkage rod coupler from the firewall. 

It was my understanding on the balance linkage between carbs, only one balance adjustment screw (lower screw) was needed.

Will disconnect the choke cables before cranking again. 

Thanks again for the info and advice. 

  • Like 1

Share this post


Link to post
Share on other sites

 In the first post, you mention "completed the basic fuel mixture setting on the SU's 2 1/2 turns down". The 2 1/2 turns down is a secondary, fine tune, adjustment. The float height adjustment is the primary mixture adjustment. 

Share this post


Link to post
Share on other sites

Hi All,

You all are brilliant. The 240 roared back to life after a very long sleep.

Adjusted all the linkage to neutral, high revs gone. I'm out about 3 3/4 - 4 turns on the air mixture nut. Backfiring and stalling gone, engine will hold a 700 - 800 idle.

New issue, can't rev the engine, front carb backfires with the introduction of air and fuel. Rear carb reacts fine. Engine is now dieseling at shut off. These SU devil's are tricky to tune.

Thanks for all the advice, Cheers. 

  • Like 1

Share this post


Link to post
Share on other sites

Blitzed,

I recently went through many of the same issues getting my '71 240Z back up and running after sitting idle in the garage for 28 years.  As Mark just mentioned, the mixture adjustment nuts are just for fine tuning the mixture, you've got to get the float bowl level set correctly first for the adjustment nuts to work properly.  If you're down 4 turns and still backfiring out the front carb, it sounds like the front carb needs the float level checked and adjusted to "richen" it up.

I noticed your location says Placentia, is that Placentia, California?  If so, I am just a few miles away in Orange.  Let me know, I would be happy to stop by and give you a hand.  I am by no means an expert, but two eyes may be better than one to help get it sorted out.

  • Like 2

Share this post


Link to post
Share on other sites

Hi Nomad,

That would be awesome. Love to see your car and your vision moving forward. Give me a call this weekend if you have the time. 

send me your contact info we'll and set a time. anwhiteford@att.net

Thanks,

  • Like 1

Share this post


Link to post
Share on other sites

Hi All,

Big "Thank You" to Nomad, took the time to help another Datsun nut. 10 mins in my garage looking at the engine and discovered my throttle linkage was still slightly engaged, fixed. Pulled the distributor and adjusted the plate bolt so we had additional advancement on the timing and showed up with a set of SU, so he could explain the internal issues and working of the carbs. Engine is purring and we took the car out for it's maiden voyage (around the corner). 

Still need minor adjustments to the float levels as engine is running slightly lean but it's running. 

Where can I locate a new/used (working) vacuum advance for the distributor? VA is currently moving but too weak to advance the plate. 

Thanks to all who have given great direction on SU's. 

  • Like 3

Share this post


Link to post
Share on other sites
Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use. We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.