This is a continuation of my other thread, but the problem the OP was about has been resolved, I figured it was time to start fresh.

Since the weather has gotten cold, my problem has gotten worse, as they usually do. Basically, the issue is that my cold starts are real rough, pumping the gas like crazy and cranking. Once it starts, it backfires. While its cold, it usually kills the engine but a few starts gets it to a healthier idle. Eventually it gets to where its idling decent, so I get the car moving. It does okay from here on - but it backfires and stalls for several minutes, which is scary as hell in traffic.

Once it warms up the backfiring stops, but not all is well. It seems like my low end has a lot of power - like, a lot of rubber burning, tire screeching power. This is the 1,000-2,500 RPM range. But as soon as she revs to 3,000rpm the fun stops - and there's no power gain. I'm not real familiar with the power curve of these cars, but I seem to recall them showing a steady gain until 4,500-5,000rpms. Mine seems to peak at 2,500rpm and then fall flat on its face. Getting to high speeds its rough. 0-60 times are twice what they should be, and once it gets there, going faster is no easy task. When I bought it, it did not seem to perform this way...

Its a '73 240z, but has a round top conversion. Up until a few days ago, it had a Pertronix Ignitor, but broke and was just changed back to points, so that and the condenser are new. The problem existed with the Pertronix as well. The timing is currently set to 12, and the dwell is at 40. It idles around 8-900 on a nice day at operating temperature. The backfiring is not present if I advance the base timing, but the flatlining performance remains. Spark plugs look good and fresh and compression is strong and even, and the vacuum advance seems to do well too. I been able to identify any issues with the dizzy so far. I installed a new fuel filter a few tanks ago, and its still clean and full, so I don't really suspect problems with the fuel itself, but the fuel pump isn't out of the question, but it doesn't seem to be the main issue.

The fact that advancing the timing fixes the backfire makes me thing that something is forcing the timing to retard, especially when cold, causing it to backfire.

As for the performance, I'm not real sure... Valves out of adjustment, timing not advancing far enough, vacuum leak, something else?

I know that was a novel, but if you guys have any suggestions I'd greatly appreciate it. If you need any more info, let me know.

Don't bother "pumping" the accelerator. The carbs are not equipped with power pumps like American carbs have.

Additionally, you need to investigate Z-Therapy's video on SU's to get a good understanding on the carbs, and while you're at it, check your vacuum advance mechamism, it may be sticking.

This should give you a starting point.

E

I've tripled checked the vacuum advance and it seems fine from testing - not to say that there isn't something causing it to stick when driving that isn't being observed when I check it. As for Z therapy, I've looked at the site, and I'm not real sure where to start. I'm new to carbs as far having a carb'd daily driver, but I do know a thing or two, and spent much of my childhood at my dads auto shop. I'm by no means an expert or even real experienced, just a beginner with a head start.

I'm not saying that you don't know anything, but I AM saying that the SU's are a different animal than the typical Downdraft or even side-draft carb. Even experienced mechanics have been known to scratch their head with them. Z-therapy has been in the SU business for years and have done a LOT to improve their reliability, fame and responsiveness.

As a suggestion, check out Z-therapy's video on the SU's. From what others have said, that ALONE is worth gold. That's a great starting point.

Do searches here on just about every string of words that you can think of. Lastly, ASK your question and try to be as specific as possible, or if possible add pictures if they help describe what you're looking at.

Hope this helps

E

Do the carb pistons rise and fall smoothly and both at the same rate?

Do the jets rise and fall smoothly when the choke is pullled and pushed?

Are the needles straight and set to the correct depth in the piston?

How many turns down are the jets?

What is the fuel level in each bowl?

How is the fuel pressure and pump?

Does the car run the same if you lift the front carb piston vs lifting the back carb piston?

What colour are plugs?

Edited October 19, 201212 yr by Blue

Thanks E, I'll take a closer look at their stuff.

Blue,

I'm honestly not sure on most of those questions. I've been reluctant to dig that deep into the carbs.. But I'll inspect each of those things and report. Thanks.

As for the plugs, they were a little brown on the tips, but looked practically new, that aside. The #1 cyl plug was a little blacker, but it was the only one, and not significantly.

Everything Blue mentioned is in the video. Best money for my Z and brain I've ever spent.

Bonzi Lon

Everything Blue mentioned is in the video. Best money for my Z and brain I've ever spent.

Bonzi Lon

Okay, I get it guys! ;D - Buy Ztherapy

Haha. I'll watch the video and learn 'er up.

Thanks everyone.

EDIT: Oh - forgot to mention - Today the engine seems to have started Loping - looping, whats the word? Its Idle RPMs are wavering all over, and it seems to do it in drive as well, less noticeably at higher speeds. When moving, it kind of feels like someone is lightly tapping on and off the brake pedal consistently. When that happens all the sudden, its an indication of a vacuum leak, no?

Edited October 19, 201212 yr by ninjazombiemaster

Oh - forgot to mention - Today the engine seems to have started Loping - looping, whats the word? Its Idle RPMs are wavering all over, and it seems to do it in drive as well, less noticeably at higher speeds. When moving, it kind of feels like someone is lightly tapping on and off the brake pedal consistently. When that happens all the sudden, its an indication of a vacuum leak, no?

Cant say why, but it didn't lope on the way home yesterday or today. It was hotter when it happened, if that makes any difference, though today was rather warm as well. I'll keep my eyes on it, and order that video sometime in the next few days.

Did anyone mention that you should buy the Z-Therapy DVD? It's the best $35 you'll ever spend on your Z.

Did anyone mention that you should buy the Z-Therapy DVD? It's the best $35 you'll ever spend on your Z.

I might be mistaken, but I believe it may have been mentioned. Haha

Lots of content on this web site too that compliments the video:

http://www.google.ca/search?source=ig&hl=en&rlz=&q=su+site%3Aclassiczcars.com%2F&btnG=Google+Search&aq=f&aqi=g-v1g-b5&aql=&oq=

This is painfully stupid. The lack of power was from a kink in the fuel hose from the bowl to the dome of the carb, so tiny I didn't even see it. I shortened the hose and reattached it, and its literally twice the car it was before. I was driving around a 3 cylinder.

On another note, I installed the new fuel sending unit today. Glad to finally know where my tank stands. I've yet to see if its going to backfire come morning when its cold, and it loped today hot (before the repair) but progress is being made. There seems to be something funky with my choke now though, I'll investigate that further when I can.