Weird problems, help greatly appreciated

5:04 · 10:53 PM

Hey guys I'm having a bit of a problem with my car. Engine is a 76 9.5:1 comp 2.7 single plug with webers and factory permatune and pertronix

So tonight I decided to take my car to a local auto-x like event to give it a shake down before a track day on Sunday. I took it there, and it was running a little funny at redline but I thought nothing of it since it pulled great and thought it was just the limiter.

Then after the car say idling for maybe 10 minutes while I was talking with some friends getting ready to go eat. Then we get driving and the carbs start popping and there is a hesitation. I give it a bit of gas and the car still accelerates. I limp it back home and I know it's been running rich I wanted to check the plugs for fouling. I pulled the #2 and it's cool and smells like gas. I figured sweet I found the problem. I replaced all of them with some plugs I had recently purchased and took it out for a test drive. She drove a ton better and pulled well, however it still has a popping under slight throttle application (10-15%). Under load it pulls great, but there is a slight hesitation from a stop and then it just goes. The only other thing I did today was to lower the timing since it was set to 8-10 degrees advance from the P.O. and per his advice I reduced it to the standard 5 degree mark however the vacuum line to the distributor is plugged.

Before I start randomly buying parts to get me ready for Sunday does anyone have any ideas as to what it can be? I'm going to pull the jets and check them out, although I just put some newer jets in. Also I pumped in 3 gallons of gas along with changing the plugs. I'm thinking maybe I got some bad gas? Could that be the reason?

T77911S · 11-17-2010, 04:51 AM

factory prematune? dont like them. i would put in an MSD, wait, i did.

could be carb adjustment or dirt in the carb, bad cap, plug wires, rotor. changing the plug was fixing the result of what ever is wrong. you could run it again and see if the same plug fouls again.

just curious, when you remove the vac retard, how much does the timing advance?

where is the timing when all the advance is in? i would try setting the timing at 30degrees full advance, see how it runs, bump it up a few degrees, see how it runs. carefull, stop when you hear spark knock. i would also try removing the retard and see how it runs. this is a pain because you have to reset the idle. dont use the 2 throttle stop screws, the idle needs to be reset with the air bypass screws. the 2 throttle stop screws are a mechanical adjustment. if they are not set properly, it can cause off idle running problems.

are you familiar with adjusting or working on carbs?

Grady Clay · 06:59 AM

5:04,
First, a belated welcome to the Forum

New Guy with some show and tell and some questions and pics!
As you have discovered, you will find a lot of help here.

Since the car is relatively new to you, I suggest you approach this as if it were an MFI running issue: start from the basics.

Do the Check & Measure parts of the MFI Check, Measure, Adjust (CMA) procedure.

There are some components that you must know are new.
You have done an oil change; do another so you know there isn’t gas (from running too rich) in the oil.
Change the filters (oil, gas, air).
Make the all the high voltage ignition components new (cap, rotor, wires, plug connectors and sparkplugs).
A agree about installing the MSD in place of the Permatune and perhaps a new Pertronix so you have a spare.
Make a ‘map’ of the actual ignition advance (advance vs. rpm) of your distributor. (You can paint-mark your pulley or use an ‘adjustable’ timing light.)
Make sure you always set the timing at 6000 rpm and let idle advance fall where it may.

What is the ‘coil’? These aftermarket CDI systems need a regular ‘coil’ and not the Bosch CDI ‘ignition transformer’.

Next is the fuel supply system.
How is the system plumbed?
Your 911 had the fuel pump on the front cross member (good).
What is the fuel pump? Original CIS pump with a pressure regulator?
Is the system circulating the fuel past the Weber inlets or is it a ‘dead end’ system?
Where and what is the fuel filter?
I recommend cleaning the fuel tank and the fuel outlet filter screen and replacing the fuel filter.
Also clean the screens at the Weber banjo fittings.
Measure the fuel flow just like MFI.
Monitor the fuel pressure during driving where the problem appears.

Buy fresh high (100) octane fuel for testing.

Have you adjusted the valves yet?
Simply check that none are tight and measure the cam timing at the same time.

Now comes the really important part: measure the cranking compression and cylinder leak.
If there is any significant ‘scatter’ in measurements or measurements out of spec, go on a ‘sporting’ drive (‘Italian’ tune-up) and re-measure.

Finally, clean the carburetors.
A little dirt in an idle jet or a flake of something setting on an air correction jet can cause your symptoms.
Check to see if there are any internal modifications.
You need to know that the carbs are functioning as designed.
If modified, you need to confirm the mod is appropriate.
There is a common issue with the accelerator pump check valve and diaphragm that causes transitional running problems.

Are there insulating spacers between the intake manifolds and the cylinder heads (I use two on each cylinder, matched to the ports)?

It is not a good idea to let the engine idle for any extended period of time.

Great care must be taken at events when restarting after ‘heat soak’.
It is not uncommon for there to be a carb fire inside the carb and air filter.

At events in hot (even warm) weather, I ice the manifolds between sessions.
This prevents the gas in the carb from boiling, fowling the sparkplugs and poor running on re-start (the major cause of fires).

Tell us more about the configuration.
What engine fan & ratio?
Front oil cooler?
What engine/transmission mounts?

Best,
Grady

john walker's workshop · 11-17-2010, 07:28 AM

might try cleaning the idle jets. takes 5 minutes. unscrew, pull out of the holders, blow through, eyeball the orifice against a light source.

5:04 · 11-18-2010, 07:54 AM

Thanks for the excellent responses guys. I've had a bit of progress. Since my last post I've changed the cap and rotor and cleaned out all of the jets. The car is running a lot better and I think the clogged jets were what caused it. I haven't had a chance to get her warm and drive her to see if the highway cruise pops are still there. When I get home I'll be cleaning the air correctors and spraying some carb cleaner.

t77911S- I bought and read a book on webers, but these past couple months have been my first experience with them. I think I have to retune the carbs while I'm at it and set the timing at 6k. I haven't checked timing when I pull the vac. I'll double check with how it runs without the advance.

Grady- Thanks for the warm welcome

Now everything except the wires are new. The fuel system is the cis with the pmo regulator. The coil is just a regular stock one.

Fuel flow is a little over 3psi. Is this too low?

Thanks guys,
Amir

john walker's workshop · 11-18-2010, 08:56 AM

cleaning idle jets once in a while is something you will get used to doing. to locate the dead cylinder at idle, loosen one idle jet at a time about 1/2 turn and listen for the idle speed to drop. if it drops off, then that cylinder was doing ok, so retighten that one and continue until you find one or more that don't respond, or actually speeds up just as you loosen the jet a fraction of a turn. if that jet appears clean, the plug may be fouled instead.

5:04 · 11-18-2010, 10:48 PM

Thanks John you've been a great help to me since I've joined. The progress I've made seems to have taken a step back. Now when cold the car won't hold idle unless I give it a bit of gas until the car is warm and then the idle is at 500rpm.

The problem has switched from the left bank now to the right bank so I don't know what to think. I was driving through a dirt parking lot too at that event and I think that may have not help too. I also just added some Wix filters to replace the fram ones that came on the car. I tried the unscrewing the idle jet, and on cylinders 4 & 5 it seemed like the idle rose a little bit. I'm going to pull the plugs and see what they look/smell like.

Again the car is showing a pop under light application of throttle and running fairly normal at 1/4 plus throttle. The transition feels a bit rough, almost like the car did when I had the jets from 5000k altitude in it.

Also since it was a CIS system I'm assuming the CIS fuel filter is still there. Would a bad filter from the original system cause this problem while still holding 3psi of pressure or would it be able to block enough fuel while the pmo gauge still shows 3psi

john walker's workshop · 11-19-2010, 04:28 AM

you can't adjust webers at 500 rpm. there will barely be any response when you turn the mixture screws. get it up to 1000, it should idle at about 1000 anyway, when it's hot. they don't have any chokes and unless you have a hand throttle, expect a real slow idle when cold.
use a syncrometer to balance side to side and individually before attempting the mixture adjustment.

RoninLB · 11-19-2010, 04:33 AM

Quote:

Originally Posted by 5:04

Again the car is showing a pop under light application of throttle and running fairly normal at 1/4 plus throttle. The transition feels a bit rough, almost like the car did when I had the jets from 5000k altitude in it.

how i adjust the acceleration pumps is to lean them out till the engine pops under acceleration and then adjust rich from there till it stops popping

this assumes that the 6 engine idles are adjusted and that under a steady pedal the engine runs fine in 4th gear and rpms are over 3,000. Of course other carb adjustments and linkage is properly set properly for this accelerator pump flow routine to work

why i do this is to avoid the "seat of the pants" feel where the accelerator flow is probably rich and crashing piston power. A rich mix will drop EGTs under 800F. A somewhat lean as possible flow leads to a smooth powerful acceleration keeping EGTs and piston power at max, in my humble opinion.

I've also found that when accelerating that the pedal isn't suddenly whacked to the floorboard.. ie: the pedal when pushed to what engine rpms and power is capable of produces max acceleration. It may take me 2 or 3 steps to "floor it"

I like 4th gear for checking and adjusting my carbs

..... And I know almost nothing about Webers and their nuances

5:04 · 11-19-2010, 11:17 AM

Ok, my next step is to take out all of the jets and shoot some canned compressed air into the passages to try and rid any blockages. If that doesn't work then I'll pull the carbs and rebuild them. I plan on doing that anyway, but I'd like to be able to just fix my current problem so I can attend the track day.

John, this low idle seems to be caused by my current problem since I was idling around 1000 before with no issues.

Also if I do need to adjust my idle should I do it by the air correction screws or by the screw that adjusts all the butterflys?

Again John thanks a ton.

Ronin, thanks for the info. I wasn't having a transition problem until I had this issue where the carbs were acting up. If I can get the lean pop to go away and still have a transition issue I'll follow your advice.

RoninLB · 03:53 PM

Quote:

Originally Posted by 5:04

Ok, my next step is to take out all of the jets and shoot some canned compressed air into the passages to try and rid any blockages.

lets say you have one cylinder carb fuel blockage or partial blockage

if full blockage the cylinder is completely dead ie: no fuel mix

if partially blocked the lean mix when accelerating increases EGT over 1500 deg F
ie: not good

How do you know if it's completely or partially blocked? An engine EGT and CHT monitor [that Grady once posted a list of mfg's about] would tell the tale. My experience is that it's a 1 in 20 chance as a general rule.

that crap said... if I have a clog when the jet cleaning doesn't clean up the issue I take a small diameter hose that's trimmed to fit into the idle jet holder and suck gas from the fuel bowl. Gas doesn't taste as wild as it did in the 1960s but keep water around to rinse your mouth if you try this. If that clears up the fuel flow passage the carb fuel bowl has to be cleaned imh opinion.

I know almost nothing about Webers

ps: gently blowing air through the passage doesn't work nearly as good as sucking imh opinion

roadhog · 11-19-2010, 06:20 PM

5:04
A lot of weber set ups don't have a port for vac. advance,in that case the original timimig spec won't apply.With a diferent fuel delivery system possibly different cams etc. from stock as mentenioned above look at the timing @ 6000 .You may need to do some seat of the pants work to get the timing optimal on this car.

Generally you should have every thing else nailed down before doing carb. adjustments.

5:04 · 11-20-2010, 12:57 AM

Thanks again guys, I can't say it enough!

Ok so I took out all of the carbs and the #4 had a pretty mean blockage in the idle jet and was the offending cylinder. I cleaned it again, took out every jet and the air cleaners and took compressed air and blew into every orafice I could. After that and some fresh gas the car run great.

Also Ronin the webers drain the float bowls when you remove the main jets. There was no fuel at all in the carbs by the time I was done and it took a few tries to start the car. Once it started then bam ran smoothly, no popping at all while driving (I had a bit while it was cold and right after I cleaned it out. After an Italian tune up she ran and idled and did everything better than she ever has. I'm going to adjust my timing at 6k tomorrow and then I'm hoping I'm good.