So I am helping a member put brakes on his 914-6 conversion and then put carbs on it. He previously has a poor working sniper setup. It ran ok prior according to him. Then they switched to a 123 dizzy and removed the MSD ignition and then took to a tuner prior to my part. It ran much worse once the 123/MSD removal. 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.

What make & model carbs?

Has any attempt been made to balance them? Even if the carbs are perfectly adjusted / tuned individually, they MUST be balanced so each of the 6 “barrels” open & supply air / fuel simultaneously once installed.

I have done color tune, balanced, and they are remanufactured carbs from Dave whom does fantastic work.

https://www.dccarburetorrestoration.com/

Maybe valves too tight?
With consistent worsening run problems, I'd look to crap that can affect timing.
I don't think it would even start 180 out. The blowback via carbs leaders me to believe either intake timed wrong from the get-go or decreasing valve clearance as the valves seat. Don't 911s have super tight initial clearances? I thought they were .004".

I'd do a compression test and leak down if I didn't find tight valves.

Piss poor seating valves could do the same...

Do you trust the engine builder?

If not then anything could at fault. It is probably the reason the sniper sucked as well.

It will run with one cam 180 out,,, check exhaust temp at head ....

Id check cam timing ,, what cam,,, maybe wrong setting at Overlap..

Absolutely no trust for the engine builder. The owner bought from someone online that had no prior 911 experience.


The thing is supposedly it ran well prior to the 123 setup that aligns with it not running well . Just very cold blooded.

But doesn’t it sound like it’s running on 3 cylinders?

Great re balancing. One obvious thing out of the way.

“… My suspicion is a major timing issue as in 180 off.“

Unless there’s backfiring thru the exhaust too, I kinda discount the idea of a major timing error. (Happened to me once on a V8 Chevy 50 yrs ago when distributor damage caused each firing to be significantly advanced for each cylinder.) But I could always be proven wrong.

Duplicate post.

“Starts pretty easy and idles decent.”

This leads me to put my money on the carbs, and not ignition or cam timing

Maybe the valves are too tight. That will be a check. I have not been inside this engine so no idea what that’s like.

Well, to me I’d look at the 123 timing. 180 out? Maybe but I’d suspect that it wasn’t set up correctly. Since the car ran well before it makes me thing internals are okay. Since carbs were done at a reputable shop I’d discount t (but not rule out) that as well. For some reason some folks have a hard time wrapping their head around installing and timing the 123.

In my experience running on half the cylinders will affect idle quality more than higher speed operation. That’s where it will smooth out due to greater firing frequency

I did check the curve and verified with timing light. I will maybe double check Resistances on the ply wires. I wonder about not running a cdi as well

I'd "baseline" that first.
I've not actually rebuilt a 911 engine yet.
I've been inside to repair but have limited experience with the H6.
What I've seen (especially the cam drive) is NOT what I'd consider "beginner friendly".

Lots of spots to screw up on.

I'd want some idea as to what the compression ratio was and cam specs (if possible).

At this point-if the comp/cam combo looked funny-I'd post on this forum for your opinions!

Then I'd start with cold compression and leak down. Safer for spark plug threads IMO.
If problems weren't revealed, go for a hot run and recheck both tests.
During the first run session I'd be messing with the timing to see what improved my problem.

I once solved a weird problem on a friend's Legends race car with staggered valve adjustments.

Cold comp 180 across 4, hot 195-200.
leak downs 92% cold 95% hot across board, did not vary.

Run great for 2 heats, so-so for 2 more, then worsening trash for the rest of the night...

I had Marty recheck both tests after the car went to shit.
Leak down 94-96% across
Compression 185 160 140 115...

The air-cooled 4 cyl motorcycle engine is installed sideways and the rear cylinders ran hotter and retained heat between races more than the front.

Loosening valve adjustment .001" per cylinder fixed it right up...
Turned a front runner cold-dead last hot vehicle into an "all night" monster.

Cost nothing but a little time and I never touched the car.
Did that one over the phone...

Marty was stoked, he'd tried a bunch of ignition and carb BS while chasing this issue.

When I have a confusing one, especially if it's been recently "rebuilt" doing full basic engine baseline cold then hot is where I'll start.

I would go into carbs. Then check camshaft timing. Some people advance them for slight performance with out know what they are doing. Dizzy did not start problem, it was changed to rectify. I would look for pecker tracks...

Matt

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

Don't ask me how i know ...

That’s my first thought as well. If I could share a video here of how much fuel it’s spitting back up the carbs you would be a amazed

Now I'm intrigued!

I want to know why?

How?

Ignition close to TDC exhaust, 180 out.

Intake valve is opening.

Combustible mixture everywhere.

I'd expect it to spit FIRE!

And not run...

Check plug wires, probably a few crossed.

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.

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?

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

Checked and rechecked all good

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

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.

Yes correct on you assuming there is no CDI

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

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.

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....

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...

For sure

Compression check for sure

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.

It just was not built by an experienced builder they “ported” the intake ports in a way that I am not a fan but used stock cams and pistons so the story goes.

Evidence of monkeys...

Yes

If fuel pressure is good and spark is good and in the correct order then-


Are all the carb jets correct? Don't forget to check the air jets and emulsion tubes those all play a big role off idle.

What RPM does the hesitation start? May need larger idle jets or smaller idle air jet if it's lean.


Also I heard you want to have 3.5 psi at the carb. I actually run a 4.5psi fuel pump and bleed a little return to get the idle pressure to 3.5psi at the carb.


Also make sure all the fuel filters are in good shape. When jumping down in pressure then you need to be cleaner than with an injection system.

Hope this helps

Question: does the 2.7 have the FI heads from a CIS? What intakes are you using for the carbs? Are they properly covering the notch in the head for the fuel injector? Could just have a vacuum leak at the intakes

I had some Alien adaptors made to allow the use of early intakes to work with FI heads. I will see if it came find a set and post a pic.

Interesting on the cam timing 180 out…. Ive read numerous tales on the bird turbo forum about cams installed 180 out.

My track car runs seemingly well but for a 3.4 with s cams, 0-2% leak down - multiple tests, multiple gauge sets. it should be making at least 100 more hp than it shows. I posted on the 911 rebuild forum, cam 180 out was one of the first and many responses to my issue…

I’ve come to belive it’s “a thing” on 911 engines…

So after some PMs from a member I think this should be added to the equation. When throttling up slowly there is no issue it’s just when you jump on the throttle. That screams sparks fuel. Plan is to get the MSD wired in, change out the 123 coil and change to a traditional spark plug and go from there.

I suggest a vacuum gauge. You can balance the individual throttles at idle and suck air between the gasket and head with the wrong intakes when you crack the throttle.that will produce the same issue you are having. This covers the injector notch without restricting the port flow.

Click to view attachment

Interesting will check that.

Update.

No vacuum leaks,changed plugs, compression check with 1,5,6@90 psi and 2,3,4 100 psi

Looking like cam timing to me or bad valve job