'70 911T poor engine performance

[Mi59]

My 1970 porsche-hard starting, doesn't idle, runs real rough, miss fires, etc...until the engine reaches 4,000 rpm's.
At 4,000 rpm's the engine runs smooth, climbs in rpm's rapidly and has good performance. It has a complete tune-up, went over the ignitions system several times, checked and adjusted valves( which seem to be normal). It was pointed into me that the timing chain tensioners are a known problem and could be the cause of the problem I have. I removed the timing covers and noticed the standard tensioners are in place and they look original. Open to any solutions on how to fix this problem are greatly appreciated.

[mikensam]

Have you pulled your idle jets?If not,pull each jet while idleing,if the RPMs drop,screw it back in and do the next one.If the idle stays the same after removing a jet,check for an obstruction.Use copper wire strand to unblock,then hit with carb cleaner.Sounds like your idle circuit could be dirty,and when you move onto the main circuit(more like 3k RPM then 4k though)it runs good.

[Henry Schmidt]

Originally Posted by Mi59

My 1970 porsche-hard starting, doesn't idle, runs real rough, miss fires, etc...until the engine reaches 4,000 rpm's.
At 4,000 rpm's the engine runs smooth, climbs in rpm's rapidly and has good performance. It has a complete tune-up, went over the ignitions system several times, checked and adjusted valves( which seem to be normal). It was pointed into me that the timing chain tensioners are a known problem and could be the cause of the problem I have. I removed the timing covers and noticed the standard tensioners are in place and they look original. Open to any solutions on how to fix this problem are greatly appreciated.

I would look to issues with the carburetors. The original carbs were Zeniths and the air by pass system for smog control often malfunctions.

[ARCSinAK]

I too am having trouble getting my Zeniths to run correctly. I disassembled and soaked them in a gallon of carb cleane, installed the re-build kits ect. I would like more information on how to block off all of the smog control tubes or if you can with out effecting another system. On full enrichment mine work well but down low they are very challenged. I have a good Synchrometer and have tried numerous times with the help of a long time Mercury marine mechanic to tune the carbs to no avail. We achieved balance with the air flow by opening the air by-pass screws but this only made her run rougher than having them all screwed in. ?? The carbs seem to run best with the mixture screws 6 to 7 full turns out. I have missing and popping down low and occasional backfires on deceleration. Any advice would be greatly appreciated.

Regards-

JS

[piggdekk]

Check if the shafts are tight. If they aren't You may be pulling some air, that would explain why in deceleration it pops, and You need 6 or 7 turn on the mixture screws.
luca

[otto_kretschmer]

ok, my two bits

having not seen the car or knowing anything about it

assume you have done something wrong and have missed something

I would;

do a leak down test and see what kind of shape the engine was in

since I have the tools, I would verify the cam timing, it can't hurt and it gives you some information, check the valve clearances

I've never fiddled with a 911 with carbs but I can't imagine its all that much more difficult than my 75 BMW R90S with Delortos.

take the carbs off and take them apart, blow out all the passages and jets with air

set the floats, if you don't know where to set them, just set them parallel to the top of the carb. Check the floats too, maybe they are leaky

this is how I set the carbs on my beemer

warm the engine to operating temp

give each cable about a tenth of an inch of slack

play with the idle air screws

screw it in all the way and then unscrew it until the engine idles THE FASTEST

when its lean, it will slow down, when its rich, it will slow down,

its normally about 2 1/2 rotations from bottom

do this to both carbs

now get out my mercury monometer and plug it in to the taps I put in myself between the carb and the head

the key is you want the levels to be the same at idle and during transient (as you open the throttle)

you want the carbs to be even

the mercury will rise as you open the throttle, you want both columns to rise about the same, the more even you can get them, the better

set the idle screw (this screw sets the height of the slides) so both columns are equal

then play with the cable slackness until both columns rise equally

keep playing with both until you are happy with it

if you can find a mercury monometer with six columns, BUY IT

but if you can't find one, just make your own that will work with just two cylinders

and balance 5 cylinders to one (it doesn't matter which one, just start with balancing two cylinders and balance the rest of them to one of the cylinders you started with)

eventually you will have them all balanced

if your manifold doesn't have ports for a monometer, put them in yourself like I did on my airhead

the carbs on a 911T work, more or less, the same as the Delortos on my BMW, there are just more of them,

there is a linkage instead of cables but the principle is the same

DIY 6 column Manometer for carburettor tuning - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z

I don't like the balance tools things because you can only check one throat at a time

you can't see the transient when the throttle is open

[gamin]

If you can afford them, buy and install PMO carbs. You will not be disappointed.