Brain fart question 914-6 Options

[mb911]

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.

[Rufus]

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.

[mb911]

What make & model carbs?

Has any attempt been made to balance them?

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

https://www.dccarburetorrestoration.com/

[technicalninja]

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.

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.

[sixnotfour]

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

[sixnotfour]

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

[mb911]

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.

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.

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.

[mb911]

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

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

[Rufus]

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.

[Rufus]

Duplicate post.

[Rufus]

“Starts pretty easy and idles decent.”

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

[mb911]

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.

[Rob-O]

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.

[Rufus]

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

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

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

[mb911]

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.

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

[technicalninja]

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.

[mate914]

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.

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 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 (IMG:style_emoticons/default/flag.gif)

[SirAndy]

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)

[mb911]

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)

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

[technicalninja]

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)

Now I'm intrigued! (IMG:style_emoticons/default/popcorn[1].gif) (IMG:style_emoticons/default/popcorn[1].gif) (IMG:style_emoticons/default/popcorn[1].gif)

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...