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

[Diseazd]

I agree with the Captain........Install a set of working carbs and eliminate carbs as your problem. This thread is going to last forever.

[240260280z]

I now think it is a vacuum leak because at WOT the manifold and engine bay vacuum/pressure difference approaches 0.

However, with the partial throttle opening under light acceleration, the manifold vacuum will increase with rpm and more air will flow into the manifold down stream from the throttle valve due to the increased vacuum with RPM . This plot of the data shows it:

[Zedyone_kenobi]

Everybody breath and take a break.

I am going to install new intake gaskets next and report back in a week or two.

[Zedyone_kenobi]

PIcking up my intake manifold gaskets today.. .should have them on this weekend or at least by monday.

[steve91tt]

Before you put on the new gasket you might consider spraying a little carb cleaner around the intake ports on the head while it's idling. If you have a gasket leak the car will stumble when it sucks the carb cleaner through the leaky spot. I've used this technique before with good success. If you don't see any sort of idle change when you spray the carb cleaner you don't have a significant leak.

[Zedyone_kenobi]

I agree Steve, I would bet you I do not have a significant leak. Since my idle is so steady and my vacuum at idle is fairly strong and steady. However, I have bought into the thought I have to have a marginal leak down stream of the carbs that is aggravated by the higher vacuum inside the manifold at part throttle as RPM climbs. All the clues point to this. I can still try that trick though... It cannot possibly hurt anything.

Just to be crazy anal, i have ordered a set of new carb insulators from courtesy Nissan so every gasket in the intake system will be OEM new. I am using Fel Pro intake/exhaust gaskets, which I trust.

For all I know I have a slightly cracked insulator...so I will replace them and inspect the old ones.

Edited January 20, 2012 by Zedyone_kenobi

[Zedyone_kenobi]

Intake came off with complete ease. There is a bit of buildup on the top of the intake valves and a nice thin layer of soot in the exhaust runners nothing alarming on either side. But I was wondering if it would be advisable to scrub and clean it out. It will no doubt cause a bunch of junk to fall in the cylinders. Better to spray some sea foam while it's running instead? Any advice before I assemble it all back?

[Captain Obvious]

It sounds pretty normal to me. I wouldn't go poking that nest. You don't want that stuff to fall into your cylinders.

My (amateur) advice is to put the new gasket on and forget you ever saw anything in there. If you clean it off, it's going to come back anyway.

[Zedyone_kenobi]

I agree, but I was thinking if I wanted to take this down time to rebuild the head. I know my rings are top notch due to the compression test, but I would not be surprised if my valve guides were a little worn. Meh, forget I mentioned it.

[Zedyone_kenobi]

Will be installing the new gasket this week sometime. The felpro on bottom is far beefier than the MSA gasket on top. All surfaces are clean and I am currently running a thread straightener on all the nuts and studs. Also ordered a new set of thick washers as my set looks very tired.

[madkaw]

Could you please speed up the process a little-we need some closure here:)

Is that sealant I see on the old gasket? If so,don't use any on that new gasket

Edited January 24, 2012 by madkaw

[Diseazd]

Could you please speed up the process a little-we need some closure here:)

Is that sealant I see on the old gasket? If so,don't use any on that new gasket

Ditto that....