Brain fart question 914-6 Options

[Root_Werks]

Check plug wires, probably a few crossed.

[Rufus]

It'll idle but run like shit with the dizzy 180 degrees off.

Don't ask me how i know ...
(IMG:style_emoticons/default/dry.gif)

Not sure I understand with the distributor 180* (I assume we’re talking cam / distributor degrees and not engine degrees) out unless there’s something else really out of whack.

Ignition “timing” would be before TDC near the end of the exhaust stroke, about where intake valve opens, and valve overlap begins. With both intake & exhaust open, there’s little to no compression. Any left over fuel ignited should be heard in the exhaust.

[infraredcalvin]

Spitting back through the carbs is a lean condition. Have you put a wideband O2 on the exhaust and checked afr throughout the rev range?

[Retroracer]

So I am helping ...The owner decided he was done with his sniper setup and asked me to put carbs on. I did that and for the life of me we can’t get it to run right. Starts pretty easy and idles decent. When I go to throttle up by hand on the cross bar it doesn’t want to accept all the fuel and shoots quite a bit back up through the horns. My suspicion is a major timing issue as in 180 off. The carbs are new and setup up by Dave Chenny so very confident in them. Seeing as the car is 1.5 hours away each visit I am looking for a bit coaching here. What else should I be looking at? The engine is a fresh 2.7 not built by me with about 1000 miles on it.

Help with some ideas please.

Ben - you don't state what fuel pressure / pump / regulator set up the car is running. Presumably (I've never used one, but) the sniper EFI needed a much higher fuel pressure feed than the carbs (probs at least 45 PSI or higher) - did you somehow reduce this to <5psi for the carbs? If not, likely the floats are probably being overwhelmed at idle and too much fuel is sloshing in.

Also: if Dave C rebuilt the carbs I'm guessing they are Zeniths - so will require 3.5 - 4.5 PSI for decent running. Hopefully he bored out the Venturis and rejetted for the 2.7 too...

Hope the above is useful,

- Tony

[mb911]

So I am helping ...The owner decided he was done with his sniper setup and asked me to put carbs on. I did that and for the life of me we can’t get it to run right. Starts pretty easy and idles decent. When I go to throttle up by hand on the cross bar it doesn’t want to accept all the fuel and shoots quite a bit back up through the horns. My suspicion is a major timing issue as in 180 off. The carbs are new and setup up by Dave Chenny so very confident in them. Seeing as the car is 1.5 hours away each visit I am looking for a bit coaching here. What else should I be looking at? The engine is a fresh 2.7 not built by me with about 1000 miles on it.

Help with some ideas please.

Ben - you don't state what fuel pressure / pump / regulator set up the car is running. Presumably (I've never used one, but) the sniper EFI needed a much higher fuel pressure feed than the carbs (probs at least 45 PSI or higher) - did you somehow reduce this to <5psi for the carbs? If not, likely the floats are probably being overwhelmed at idle and too much fuel is sloshing in.

Also: if Dave C rebuilt the carbs I'm guessing they are Zeniths - so will require 3.5 - 4.5 PSI for decent running. Hopefully he bored out the Venturis and rejetted for the 2.7 too...

Hope the above is useful,

- Tony

Running 3 psi with PMO regulator


As for wideband 02 no this car is so far away from requiring this and definitely is not lean the plugs are pure black

[mb911]

Check plug wires, probably a few crossed.

Checked and rechecked all good

[flyer86d]

Ben,

Did I read that when the MSD was removed, it was not replaced with a cdi? When I installed the 2.7 in my 68 911, I initially started it without a cdi and it did start and idle. In subsequent starts it would do exactly what you describe. It would idle but not take any throttle and backfire and spit. I chased the same ghosts that you are verifying both cam and ignition timing, fuel pressure, float level, initial carb setting, tried pulling what is left of my hair out and finally installed an MSD with the stock 1975 Bosch points type distributor and it runs like it should.

Charlie

[ClayPerrine]

What year is the motor? The SC motors turn the distributor backwards from the others, and if you time it like an early motor, it will idle but won't do squat otherwise.

[mb911]

What year is the motor? The SC motors turn the distributor backwards from the others, and if you time it like an early motor, it will idle but won't do squat otherwise.

It’s a 74 2.7.

[mb911]

Ben,

Did I read that when the MSD was removed, it was not replaced with a cdi? When I installed the 2.7 in my 68 911, I initially started it without a cdi and it did start and idle. In subsequent starts it would do exactly what you describe. It would idle but not take any throttle and backfire and spit. I chased the same ghosts that you are verifying both cam and ignition timing, fuel pressure, float level, initial carb setting, tried pulling what is left of my hair out and finally installed an MSD with the stock 1975 Bosch points type distributor and it runs like it should.

Charlie

Yes correct on you assuming there is no CDI

[Ishley]

Check distributor…. Plug wires… change the plugs and
Make sure they are gapped correctly.

[mb911]

Check distributor…. Plug wires… change the plugs and
Make sure they are gapped correctly.

Fresh plugs and gapped correctly though he is using iridium plugs not sure if that matters

[technicalninja]

What year is the motor? The SC motors turn the distributor backwards from the others, and if you time it like an early motor, it will idle but won't do squat otherwise.

Right there! That is the type of info I don't know yet as I haven't had years of 911 motors...

Thanks for posting. I'll always keep that in the back of my thoughts when jacking around with 911 ignition. Some of these puppies rotate the distributor BACKWARDS!
Verify before light up!
That would be a wierd set up. 2 cylinders good and 4 cylinders at 60 or 120 degrees out.

One thing all these posters should remember is that mb911 is a MASTER!
He really doesn't need to be told "clean plugs".

As I have shitloads less experience on these as he does, I adjust my responses accordingly.
My baseline suggestions are what I do when I'm stumped. They may not work for him.

I still think he has internal problems. He's had a wizard do the carbs, he, himself is competent with ignition systems.
CDI vs ID shouldn't make a rat's ass of difference regarding his issue. Both should work fine at idle and tip in.

I'm interested in what the root is in this case.
I ALWAYS hunt the root!

Iridium just means they are so brittle that you have to be damn careful when checking/adjusting gaps. I would guess 100K life span in a 911 motor.
In a Camry I tell my customers, "No PM plug changes, wait till it bitches, 165k+ is where I see IRs fail in modern vehicles.

[mb911]

I posted a video on FB 914 world page and some have mentioned timing of cams. Possible but from what I understand it ran ok but not great prior to the 123 dizzy and CDI delete

[Root_Werks]

I posted a video on FB 914 world page and some have mentioned timing of cams. Possible but from what I understand it ran ok but not great prior to the 123 dizzy and CDI delete

You might be chasing your tail until you can establish some things. Cam timing being one of them.

Yikes, if that's off....

[technicalninja]

Compression test will show if intake cam timing is off.

You know the wife's tail about you can NEVER have a perfect comp test across an engine...

That's total bull shit!
As long as the rings are properly bedded, and monkeys have not been into the engine, I can almost ALWAYS get a perfect compression test across all cylinders.

All you have to do is get the intake valve adjustment PERFECT! Exhaust being off doesn't change shit.

First time I did it, I didn't believe it!
133k Datsun 260z, normal car, nothing special.
Hit perfect after a "blueprint" style adjustment, did comp test 4 times and every time was perfect.
I didn't believe even still, so I tested my test equipment on another car and got different readings.
I thought my comp tester was broken...

I've won a couple of thousand dollars betting on this phenomenon!

The thing that varies compression tests the most is when the intake valve closes during the compression stroke.

Comp test will show intake mis-timing. Nothing will show exhaust mis-timing IMO.

If the problem is poor intake valve sealing the leak down will show where the comp test might not.

I just do both...

[mb911]

I posted a video on FB 914 world page and some have mentioned timing of cams. Possible but from what I understand it ran ok but not great prior to the 123 dizzy and CDI delete

You might be chasing your tail until you can establish some things. Cam timing being one of them.

Yikes, if that's off....

For sure

[mb911]

Compression test will show if intake cam timing is off.

You know the wife's tail about you can NEVER have a perfect comp test across an engine...

That's total bull shit!
As long as the rings are properly bedded, and monkeys have not been into the engine, I can almost ALWAYS get a perfect compression test across all cylinders.

All you have to do is get the intake valve adjustment PERFECT! Exhaust being off doesn't change shit.

First time I did it, I didn't believe it!
133k Datsun 260z, normal car, nothing special.
Hit perfect after a "blueprint" style adjustment, did comp test 4 times and every time was perfect.
I didn't believe even still, so I tested my test equipment on another car and got different readings.
I thought my comp tester was broken...

I've won a couple of thousand dollars betting on this phenomenon!

The thing that varies compression tests the most is when the intake valve closes during the compression stroke.

Comp test will show intake mis-timing. Nothing will show exhaust mis-timing IMO.

If the problem is poor intake valve sealing the leak down will show where the comp test might not.

I just do both...

Compression check for sure

[mb911]

I also wonder if the builder put pistons in upside down which would drastically effect the whole deal

[mepstein]

I also wonder if the builder put pistons in upside down which would drastically effect the whole deal

We rebuilt an engine after a well known builder rebuilt it with upside down pistons and a half dozen base gaskets under each cylinder. Without the additional base gaskets, piston would have hit valves, with it, it had super low compression and ran poorly. Hack job rebuild from a well known (and infamous) shop.