Engine mis

[youztheclue]

My 74 260z has dual Weber 32/36dgav with emissions and mechanical fuel pump delete. Car runs good at idle and on throttle. The problem is cruising around at a constant speed it has a "mis"/bubble spot the it seems a cylinder isn't firing. I have new plugs wires and distributor cap and rotor. I noticed taking off the plugs the NGK caps for the wire connection unscrewed itself from the porcelain.(possible fix) The cylinder 6 has a dark plug but the rest are a dusty clean grey. The cylinder 6 exhaust bolt is snapped in the block and definitely has a leak. Brand new fuel filter put in last week. I also have never owned a carb car before so my tune is iffy and I'm going to look into fixing that, by redoing it the third time to procedure by Weber. Oil pressure is great and steady. The alternator is weak and I believe the regulator is bad now because lights get brighter with rpm. I'm wondering if the electrical fuel pump is affected but the fuel bowl should smooth that issue out. I can hear the fuel pump get faster when the headlights are turned off. Any help with cruising speed would be extremely helpful as I'm going on a drive with my club in STL Saturday.

Edited March 22, 2019 by youztheclue

[munters]

Driving around is mostly the transition between idle and main jet. Gray plug means to lean mixture. Bubbling/shooting means also to lean mixture. Do you have an afr meter?

[youztheclue]

No AFR sensor unfortunately. I'm gonna be buttoning it up today and drive it around again to see if I can pin it down.

[Zed Head]

14 hours ago, youztheclue said:

dual Weber 32/36dgav 

The problem is cruising around at a constant speed it has a "mis"/bubble spot the it seems a cylinder isn't firing. I have new plugs wires and distributor cap and rotor. I noticed taking off the plugs the NGK caps for the wire connection unscrewed itself from the porcelain.(possible fix) The cylinder 6 has a dark plug but the rest are a dusty clean grey.

Sounds like it's lean in that RPM range.  There's probably an adjustment possible.  Don't know how the Webers work.  Should have put Weber in your title.

[jonbill]

Sounds like there's something odd about cylinder #6.

If the overall engine is lean, is 6 so lean its missing and fouling, or is it oil fouling?

Where does the master vac take its vacuum from on this setup?

 

 

[youztheclue]

So I put my car on jack stands and put it in gear. First, second fine. Then third started clunk and fourth made me scared. Crawling under the car it's definitely shaking and clunking in the transmission case:( I add the last .4qt from the flush I did and it quited down and actually lasted 100 miles in one day. It helped a lot but seems it could be the transmission messing with the engine and driving feel. I definitely need someone that's driven old Z around to actually nail this down.

[Zed Head]

Are the wheels hanging or are the stands under the control arms?  If the wheels are hanging the u-joint angles can cause binding and clunking as the wheels spin.  You started with a miss and now you have a transmission problem.  Doesn't really follow.  Move the stands to under the control arms to get the wheels back to normal ride height.  Only need to do it on the back of course.

[7tooZ]

Can you post some photos of your fuel/carb system. It is amazing what people here can see with pictures

 

[Mark Maras]

 Getting back to the first problem, was the loose spark plug connector on #6? If the compression is good on #6, the reason for the misfire lies in the plug, plug wire, cap or rotor and or any of the elec. connections.

[youztheclue]

the loose spark plug male tip was on cylinder 6 and 4 i believe and i had a intake vacuum hose open to the rear then to the gas tank filler neck loosely. here are my engine bay pics. the video has the on off i was talking about and also the tire skip from the welded diff.In the first pic you can see the evap charcoal canister that i deleted and add the red caps. Also you can see the second pic the cylinder 6 exhaust doesn't have a bolt, because it was snapped inside the head. it has exhaust sealer goo on it as well as using that to seal the firewall.

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Edited April 4, 2019 by youztheclue

[Mark Maras]

Tighten the plug ends, clean and swap #6 for one of the others that looks good and try it again to see if the problem is gone, still #6 or moved with the plug.

 Does the engine miss, at steady throttle, occur within a particular range? Say 3000-3500 RPMs but not at 2000-2500?

 I know little about DGAV carbs but I have a Haynes Weber book that may give us a clue.

Fault diagnosis, Engine lacks performance; incorrect tuning adjustments, incorrect float adjustments, excessive sediment in the fuel, acceleration pump seized and throttle valves do not fully open. 

 My gut feeling is tuning and float adjustment will clear it up. The DGAVs are very adjustable, much like DCOEs. It would be a good idea to get a good manual for them.

[youztheclue]

I just bit the bullet and took the head off the block gonna do a whole top end service and see if that fixes it.