high rpm problems L26

[slammed260z]

Hi guys,

 

My Name is Robin and i'm from The Netherlands

 

I drive a white '75  260z 2+2

 

I have completely rebuild the car except the engine. Because of futureplans.

For now the Original l26 engine is in and is het 53k Kilometers on it. In Miles i gues something between 25k and 30k.

 

It has a Janspeed 6-2-1 header and a full freeflow 2,5 inch stainless steel exhaust.

My distributor is from a '81 280zx and has the E12-8 module.

It has been set on the original L26 advised degrees.

The engine has had a complete tune up with new spark plugs etc.

 

The carburators are 3screw Hitachi SU's from a 240z with a N42 intake, velocity stacks en open air filters.

 

The carbs are synchronised and run well in low rpms.

 

I need to choke to start and let the choke on for a few miles and then it runs fine.

When the car is at worktemperature the engine will stutter as soon as i come aboven 3500- 4000 rpms and stops at 6000 rpm. At these high rpms you can feel the loss of power.

 

I have tuned the carbs a dozen time and try a few adjustments to find out why it is running bad at high rpm's.

 

What i've tried and discoverd:

 

-The carbs have been adjusted very lean. the screws are almost completely turned in. But the car drives good at low rpms.

-The mechanic fuel pump is still Original but gives good pressure at idle

-Idle is around 900-1000 rpm

-The vaccuum pipe on the left carb is blocked of.

-One vaccuum pipe on the intake is blocked of and the other one is connected with the distributor

-When tuning the carburators and lifting the needles up doesn't give difference on the left carburator. The engine has to raise and fall back to normal idle when the mixture is correct. leaner or richer doesn't make any differen when lifting the needle

-the right carb has more or less the same. But i can hear difference on it but it is not very clear.

 

 

I am thinking myself to replace the mechanial pump for a new one, same type although electric is better

I want to rebuild both carbs and give them a good clean.

 

 

I hope these discoveries could help you guys to think with me about what could be the problem.

Please scream everything you think. maybe it will help others with argueing

 

Thank you!

 

[BluDestiny]

what is the timing at? could also be a skipped tooth from the 280zx dizzy install. 

 

I would invest in a fuel pressure gauge, and possibly a wideband. 

[seattlejester]

Hi Robin,

 

Seconded to blu above, it would be very helpful to have numbers to backup the claims. A timing light, a fuel pressure gauge, and a wideband AFR kit with a gauge would make your problem very easy to pinpoint.

 

Is the car bogging at high rpm or is it pinging? Does it feel like a gradual loss of power or is it like you hit a wall and the car just doesn't develop any power? Is it a fuel overflow or a lean problem is what I am getting at, have you looked at the spark plugs? Are they dark or grey?

 

If it drives well under the 3500 rpm mark, I think it points to your vacuum advance, fuel flow, or the fuel mixture.

 

For the SU carbs you have to kind of tune for a rich idle, that leans out as the RPM rises. I would say you want to get the mixture to the point where the car seems like it wants to stumble at idle and then back off a 1/2 turn, and slowly back it out after some runs if you don't have an AFR meter. Unless you have a sticky seat or a leaky throttle shaft, tuning each carb should have some effect with the "raise the carb piston" style tuning. You may just have to add oil to the carbs or turn one carb a little more than the other.

 

If you have an AFR meter, have the car idle around 11 or 12 warmed up and see what it goes up to when you give it WOT. If it gradually goes higher then 15, then you will want to decrease the mixture so it idles around 10 or 11 at idle, any lower than 10 and the car will stumble at idle. If it gradually goes up and all of a sudden jumps up into the 20's then you have a problem with fuel delivery. If it slowly decreases as the RPM's rise, the mixture is far too rich, you may want to have the carbs looked at for a very sticky valve or an oversized needle.

 

For fuel flow, a fuel pressure gauge would definitely help with determining if the carbs are getting the right pressure. Are you using the factory fuel rail? The factory rail comes with a restrictor on the rail for the return line to maintain some pressure in the carbs. Do you have any fuel filters? Have you had the tank cleaned? How do you know you are getting good fuel flow?

 

The vacuum advance problem would be a bit harder to diagnose, with the cap off, and a vacuum brake bleeder pump you can manually generate vacuum and see if the distributor internals are rotating as designed. If you have a timing gun even better, turn the car on and have someone accelerate the motor and monitor the timing to see what it advances to. Adding additional marks with white paint/nail polish will help pick up the timing marks a bit easier. Speaking of timing, what did you set the timing to? Around 10btdc advancing to 30btdc seems to be a decent rough starting point.