popping through the exhaust at 4500 rpm or so.. HELP!

[d240zx2]

Having read most, if not all of this thread's posts, I notice a similarity of sorts with what I experience with my Mikuni set-up. But only when everything is sorta cold. Once the engine and carbs are fully heat saturated, the stumbling goes away... I'm sure that's NOT the issue you have, Stephen, so take it as what it is....

[Diseazd]

Zedy.....I'm coming in late on this thread so I hope this hasn't already been checked. Most popping through the exhaust in my experience has been caused by air being drawn into the intake or exhaust manifold. My green car had a popping because I had replaced the 3 studs in the downpipe with the wrong studs (from the head instead of the right ones ). They bottomed out before sealing the little downpipe gasket tightly. Took forever to find that one, but completely quiet now. Also, headers are notorious for poor matching of the flat area that snugs up to the intake/exhaust gasket. An intake or exhaust gasket leak will cause a popping......oxygen burns! I would start by checking your downpipe gasket (make sure it seals), followed by pulling the intake manifold and exhaust header and checking the intake/exhaust manifold for leaks (replace the gasket). All IMO......Good Luck Guy

[Zedyone_kenobi]

Thanks very much Guy.

UPDATE:

I am about ready to throw these freaking SU's in the river. After I shoot them.

THe 20wt oil did absolutely nothing to the problem.

If I gradually push the throttle about an inch, the car pops loudly through the exhaust. If I floor it it will pull hard through all 3 first gears.

Guy, I have a thought that you may be on to something. There is just no reason these SU's should not work the way they are...

What the hell would cause popping and a complete loss of acceleration at part throttle but works when at wide open throttle.

I am going to put the SM needles back in and call it quits until I get my wideband.

I have had it.. screw these. Maybe I need a time out.

spending 1499 dollars on webers or 1650 on mikunis is sounding like a far better option at this point!

HAHA

[Zedyone_kenobi]

Guy, I will check out my exhaust tonight to be sure.

[Diseazd]

http://www.classiczcars.com/forums/showthread.php?42465-Proper-vacuum-readings&highlight=

Zedy.....You can check for vacuum leaks (which would cause your problem) with the info that BLUE provided. I would just pull the exhaust and intake manifolds off and install new gaskets. It's a 1 1/2 hour job.

[Zedyone_kenobi]

I have not checked for vacuum leaks around the exhaust guy. I did check for manifold bolt tightness, and all the nuts on the studs were as I left them. All the carbs are tight. I will run an extensive battery of vacuum checks when I get home with my vac gage. That is easy enough. I did that earlier, and I think I got around 20 " if I recall. I wrote it down in my Z car journal.

Still it cannot hurt to check.

[madkaw]

I can understand the exhaust causing the popping, but not the loss of acceleration at part throttle. That would be a huge leak that he would hear. Vacuum leak is a possibility but his idle is great?

Is the power there at WOT?

Do you think your engine is making it's normal power when revving out or is it just running up the rpms with less power?

[Zedyone_kenobi]

Man, that is tough, I have no idea, it seems to pull okay, but I have only ever driven one other classic Z and it was stock. I would say it feels like a stock Z madkaw. It revs pretty nicely at WOT. Of course, the tach go bazerk when I get close to 6000 rpm

[Zedyone_kenobi]

Vacuum leaks is my next check. It just makes sense. I think it is safe to say we have ruled out SU oil as a cause. It popped with

2 wt

12.5 wt

20 wt

It popped with SM needles and stock OEM needles

Pistons are moving up and down in the domes at the same speed.

Car has a great idle.

Car ran noticeably better with SM needles

Here is what I have NOT done:

I have checked fuel pressure from my brand new fuel pump, which was good, but I have not checked volume. OR pressure while the car is being driven. But I ruled that out as if I can rev to 6500 rpm at wide open throttle I am getting fuel.

I have NOT checked if the fuel level is right at the top of the jet opening in the carb body, I just assumed the float bowls being properly adjusted was okay.

I have not done a proper vacuum check of my power brake line for leaks (will do that today)

I have not checked for a lose connection of my header to my exhaust pipe, but have checked most of the bolts on my header to my cylinder head, but not all.

I have not installed a wideband O2 sensor yet, but it will be here tomorrow which should GREATLY help narrow down things in a direction of goodness.

[Zedyone_kenobi]

again, webers or mikunis

haha

[motorman7]

After experiencing what sounds like the exact same popping issue, as I noted earlier, I can say that I did very little adjusting to my SU’s to fix the issue, other than minor mixture adjustment. I even had the popping at idle at one point and then again at 3K. My SU mixture nuts are now at about 1.5 turns. I could adjust these either way ¼ turn and not make much difference in the way it is now running, which is just about perfect. I think the fuel delivery/fuel filter was the issue. I thought what I had installed originally was pretty new and clean, but apparently it was not. I installed the new filter (made in China) and did the 20 degree advance on the timing and that seemed to do the job.

As I mentioned before I also punched some holes in the air cleaner housing. Not sure if this was a contributor; the car just seemed to pop at higher RPM’s when I tested with the air cleaner off (when I was having the popping issue). Another problem I had with the air cleaner housing (a couple months earlier) was I had somehow accidentally flipped the cold/hot baffle on the front of the housing to “cold” (I live in SoCal so never need this setting). Anyway, the Z was really dogging and popping at about 3.5-4K in this condition. Flipped this back over to hot and made a big difference. Anyway, Just thought I’d pass this on. Definitely a very frustrating issue.

[madkaw]

I'm still stuck on float adjustment.

The jet needles are being set flush against the piston with the jet collar flush??