More Questions - Rough running, What am I missing? Options

[NotLance]

My new engine is in, but I continue to have issues with rough running - and its gotta be something that was retained when i swapped the short blocks, as the behievor is similar to what I witnessed with the other engine too. Engine is good above 3000rpm, so I think the main circuit of the dual webers is good, but at lower rpm it surges on light-med acceleration, consistent on foot down acceleration. backfires out of exhaust on decel and at idle. All of that said, when I pulled one plug after 300 miles, it was very carbon fouled so leaned out that one to hopefully fix... it did not. Resyched and tried tuning carbs per factory weber procedure but still cant get it happy. I also can't verify running conditions at higher than 3500 rpm becuase I'm still breaking in the brand new block.

Fresh plugs and ignition coil, advance on 123 distributor set properly, theoretically timing set to the mark on the fan housing (27* i think), I do not suspect vacuum leaks due to the engine not picking up speed after spraying everywhere with starting fluid... STE reads synchronous airflows on all carbs

Plugs I pulled seemed out seemed okay, not super carbon, oily, or anything.

Maybe I just can't tune dual carbs to save my life? Or they need a rebuild or something (IMG:style_emoticons/default/confused24.gif) I've got it going to a shop friday just to attempt to tune the carbs for me becuase I don't know what else to do...

Consulting the 914think tank to see if there is something obvious I'm missing because despite pretending to know a bit about fixing my car, im just a kid (IMG:style_emoticons/default/biggrin.gif)

I dont have a timing light, can you rent one from somewhere so I could verify timing? I trust the people who installed my engine but starting to pull out my hair so trying to eliminate any variables possible

Its a -4 cyl GA case, dual weber 40 idf carbs and a 123 distributor set to curve 1. BR6ES spark plugs

[emerygt350]

My new engine is in, but I continue to have issues with rough running - and its gotta be something that was retained when i swapped the short blocks, as the behievor is similar to what I witnessed with the other engine too. Engine is good above 3000rpm, so I think the main circuit of the dual webers is good, but at lower rpm it surges on light-med acceleration, consistent on foot down acceleration. backfires out of exhaust on decel and at idle. All of that said, when I pulled one plug after 300 miles, it was very carbon fouled so leaned out that one to hopefully fix... it did not. Resyched and tried tuning carbs per factory weber procedure but still cant get it happy. I also can't verify running conditions at higher than 3500 rpm becuase I'm still breaking in the brand new block.

Fresh plugs and ignition coil, advance on 123 distributor set properly, theoretically timing set to the mark on the fan housing (27* i think), I do not suspect vacuum leaks due to the engine not picking up speed after spraying everywhere with starting fluid... STE reads synchronous airflows on all carbs

Plugs I pulled seemed out seemed okay, not super carbon, oily, or anything.

Maybe I just can't tune dual carbs to save my life? Or they need a rebuild or something (IMG:style_emoticons/default/confused24.gif) I've got it going to a shop friday just to attempt to tune the carbs for me becuase I don't know what else to do...

Consulting the 914think tank to see if there is something obvious I'm missing because despite pretending to know a bit about fixing my car, im just a kid (IMG:style_emoticons/default/biggrin.gif)

I dont have a timing light, can you rent one from somewhere so I could verify timing? I trust the people who installed my engine but starting to pull out my hair so trying to eliminate any variables possible

Its a -4 cyl GA case, dual weber 40 idf carbs and a 123 distributor set to curve 1. BR6ES spark plugs

This could be carbs, I am not at all familiar with those on a type 4 so I can't be of help there, but I would say it kinda feels like timing to me. Maybe a combination of the two.

I would verify TDC on the #1 cylinder, I just put the car in 5th and roll it till the piston is at the top of the bore on the compression stroke. Once you have it at TDC (recently I have gotten into using the TDC mark on the flywheel to time my car) check to see where your mark is on the fan (and the flywheel). I find the flywheel way more convenient for timing, and on my car TDC on the fan is just a smudge of paint.

I wouldn't trust any mark on the fan due to the different fans that can be added to these. I replaced my fan with one off of a bus 2.0 because I had some broken fins on the original. The replacement doesn't have any marks at all. I verified my flywheel mark and just go with that.

When timing, if you don't want to rev the thing all the way up to 3000 rpm to set the timing, you can just set it at 7-8 BTDC at 900 or lower rpm.

[Superhawk996]

Need info on all carb components:

Venturi size (most important - defines how everything else will be tuned)
Main jet
Idle jet
Emulsion tubes
Air bypass screw settings - prefer all closed but may need to trim 1 or 2 slightly open to get all barrels to match.
Idle mixture screw setting - how many turn out?

Based on symptoms - my guess is your Venturis are too large not allowing a good vacuum signal. You would be best served running a 28mm Venturi for anything around 2.0L - 2056 size motor.

[NotLance]

Heres some more info -

Venturi Size: 30
Main Jet: 130
Idle Jet: 57
Emulsion Tubes: f11
Air Bypass Screw Settings: All closed except for one which is slightly opened to balance the barrels
Idle Mix screws: 1/4 turn out
pump jet: 50

with these settings it runs as described (poorly but better than not at all).

I'm in SLO so pretty much sea level.

I can't remember what the stock sizes were, but I still have them at my parents house in the bay

[NotLance]

Also, do you need a timing light to set timing or just check it? I've never set timing on a car before but willing to give it a go. Can't really afford the tool but can ask around my machine shop buddies

[fiacra]

Lance, not everyone tuning in right now knows the back story. You should give a bit more detail about how you got to this point. It might be helpful in getting trouble shooting insight.

Did you drive the car from the Bay Area to San Luis Obispo? That's a good long drive to shake it out on. I'm assuming it was running well when you got it back from the shop after the engine install? Correct? I can't imagine that they let it leave the shop running like it is now. If all that is correct, something had to have changed in the interim, so it is unlikely (in my opinion) that the carbs are improperly set up. That should have shown up rather quickly, and before it left the shop. I don't know where you had the work done, but your dad was pretty clear about their level of expertise and having a long history with them.

So...have you pulled the jets and cleaned them? Even just a little bit of debris in the jets will give you trouble. Since everything has been apart recently the chances of some contaminates getting into the fuel system are pretty high. I'm for sure no expert on carbs, so I'll let other more competent folks weigh in, but that's probably where I would start. After that, I'd check the timing and valve adjustment. You can get a timing gun reasonably inexpensively. I invested in a good set of tools when I was about your age and other than the cost of my marriage license it was one of the best investments I ever made.

I know you can figure this out! Keep "pretending" you know how to fix your car and pretty soon you will know how to fix your car (IMG:style_emoticons/default/biggrin.gif)

[Jack Standz]

Find and mark TDC on your fan (or flywheel if you prefer). Set timing using a dial back timing light.

Here's one ($55):

https://www.amazon.com/gp/aw/d/B000EVYH7W/r...X0DER&psc=1

Although, a pro version of this timing light is better (has a tachometer built-in that is very helpful when tuning carbs and a built-in dwell meter too, if you're still using points), but costs twice as much.

An accurate tachometer can be used to set your best lean idle mixture by watching what happens to rpm when adjusting the carbs. Timing and carb adjustment of the idle circuit are your likely culprits.

Best wishes for the fix!

[Superhawk996]

Timing needs to be set with a light. Guessing won’t suffice.

See what the shop can do for you but if it were me I’d be 28 Venturi’s and more like a 50 idle jet.

The fact that your mix screws are only a 1/4 turn out suggests idle jet is too large. It is also suggestive that your closed throttle position isn’t correct - meaning that a portion of the idle jet transition port is being exposed and flowing fuel which is why your mix screws aren’t more like 1 turn open. (See page 10)

Remember that for all cruising and partial throttle conditions (ie 80% of your driving) is on the idle jet circuit.

Spend some time reading and understanding the Weber tuning guide to understand how the idle circuit and the progression port holes work at part throttle.
https://www.lainefamily.com/images/WeberTuningManual.pdf

[Jack Standz]

(IMG:style_emoticons/default/agree.gif)

1/4 turn out on mixture points towards the idle speed screw opening the throttle blade too much and/or the idle jet too big.

[NotLance]

Good call fiacra - so to recap the story,

My engine ate a bearing and I got a new one from fiacra - the engine is good and can feel it wants to run good, but suspect something else is amiss, and was with the previous engine too. It wasn't what made it go kaboom, but it never quite ran exactly right. Fresh plugs always made it happy, for a while.

As for the shop that did the work installing the new engine, I trust everything they did was competent work. Long story short, they grew up with my dad, and have been taking care of his 911 for decades. It'll cost but they do it right. They swapped the short block, and reused everything from my old engine on my direction. I'm interested in FI in the long term, but for now would like to make the carbs work for the next year or so until I hopefully move once I graduate.

As they knew it was my intention to mess with it, I decided it was running properly enough to send out the door and avoid hours of fine tuning my wallet could not afford. Now its time for the futzing around

On the drive from Martinez to SLO I noticed these issues

Maybe I'm just being picky, but it just doesn't feel 'right' and idle up to about 3000rpm - and while I'm not convinced its the carbs, I am convinced the motor has more to give to make me want to drive it, not worry about it running rough, hurting it. Most importnantly to me, I want this new engine to last longer than the first so hearing/feeling it unhappy is not conducive to the daily driving I used to do with it. (only car (IMG:style_emoticons/default/biggrin.gif) )

[NotLance]

Thanks for the advice so far guys! Going to buy a timing light then...

[Superhawk996]

Based on story above. - continues to point to carbs

[emerygt350]

Yeah, carbs but do get a timing light. Super easy and you don't want that messing up your work with the carbs. Timing has a huge effect on vacuum and vacuum has a huge effect on carbs. Takes a couple seconds to check and a light is cheap.