MPS backfire protection. Options

[technicalninja]

Starting a thread with a quote from another.
See http://www.914world.com/bbs2/index.php?sho...68374&st=80 for the back story.

Once again, an ignition problem thread ended up involving another system during diagnosis.

If Flaca is correct there exists the possibility that a backfire damaged a rubber diaphragm.

I don't know shit about D-Jet, I've never had to diag that system and I don't know it well.

But I've had SHITLOADS of problems involving the destruction of a rubber diaphragm via back fire.

If this is possible on an MPS I know what I would try first...

OK. BIG revelation.
Turns out my MPS appears to have gone bad somewhere along this process.
I had tested it when tracking down my running-warm situation and it held vacuum fine.

Meantime - just last week - I picked up an 043 MPS that I saw on ebay for a good price. Just to have as a spare - and because my 041 had been adjusted/installed previous to me.

So today, I got back to timing the 123 and it just wouldn't hold a constant rpm no matter what I did. Would start, idle somewhat ok but i couldn't get it over 2000.

So - on a total whim, I threw in the new 043 and it's like night and day. amazing difference. I'm pretty happy right now.

I'm wondering if my 041 bought it due to one of the backfires during my initial 123.

But anyway - wow. Hopefully will have this buttoned up today.

Most Holley carburetors have a power valve that opens a secondary path for fuel into the main wells. The back side of the diaphragm is exposed to manifold vacuum via a small port from the valve through the carb base.

You get a good backfire and it ruptures the power valve. Massive rich is what results.

The solution is drop dead simple.

You drill out the passage in the base and install this.
https://www.summitracing.com/parts/hly-125-...ASABEgIROvD_BwE
Almost all new carbs with a Holley style power valve come with this already installed.

The pictures don't really explain how it works.
If you have a shop vac the "ball in the cage" to keep liquids from entering the electric motor is a pretty good analogy.

This is SO old school I would have expected someone has already applied this technique to protect the MPS.

If not it's what I would try to apply to add backfire protection to my MPS...

A simple one-way check valve WILL NOT WORK!

You need a "special" check valve that only closes when it sees a rapid pressure rise on the vacuum side of the valve.

[Superhawk996]

Not trying to bonk you (IMG:style_emoticons/default/smash.gif) since you admit you’re not familiar with D-jet or the details of the components.

The fact of the matter is the MPS diaphragm is metal (berryllium copper alloy if I recall).

That metal diaphragm isn’t terribly sensitive to backfire but I have no doubt that if you have one that’s already fatigued and has a hair line crack (but holding vacuum), a backfire could finish it off and crack it.

IMHO this simply falls into the category of trying to fix what isn’t broken. By and large adding in more components to protect for a low occurrence event (backfire) is just adding complexity and more things to fail. Proper troubleshooting of components is more effective than adding extra components to protect for rare events. These D-jet systems and MPS sensors have largely been operating 50 years without backfire prevention valves. Hard to complain too much when one fails well beyond its design life and possibly in conjunction with a backfire which may have been caused by an improperly installed distributor (again not meant to be slam on Flaca).

There is already a degree of protection in the system that exists by virtue of a small restrictive orifice that consists of the plenum to MPS hose size differential.

None of this takes away from what you’ve said about rubber diagrams, Holly power valves, etc.

[technicalninja]

Not trying to bonk you (IMG:style_emoticons/default/smash.gif) since you admit you’re not familiar with D-jet or the details of the components.

The fact of the matter is the MPS diaphragm is metal (berryllium copper alloy if I recall).

That metal diaphragm isn’t terribly sensitive to backfire but I have no doubt that if you have one that’s already fatigued and has hair line crack (but holding vacuum), a backfire could finish it off and crack it.

IMHO this simply falls into the category of trying to fix what isn’t broken. By and large adding in more components to protect for a low occurrence event (backfire) is just adding complexity and more things to fail. Proper troubleshooting of components is more effective than adding extra components to protect for rare events. These D-jet systems and MPS sensors have largely been operating 50 years without backfire prevention valves. Hard to complain too much when one fails well beyond its design life and possibly in conjunction with a backfire which may have been caused by an improperly installed distributor (again not meant to be slam on Flaca).

There is already a degree of protection in the system that exists by virtue of a small restrictive orifice that consists of the plenum to hose size differential.

None of this takes away from what you’ve said about rubber diagrams, Holly power valves, etc.

(IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif)
Yep, I agree, much ado about nothing.
If Holley made the power valves from the same material, we'd not have the repair kit.
I was responding to another's mention of a backfire being the possible culprit and I thought "Wow, sounds just like the Holley bullshit".
I did a quick google search on "MPS backfire protection" and didn't find anything.
So, I posted...
Incorrectly!

I was trying to help.

[Superhawk996]

I was trying to help.

Totally understand that. (IMG:style_emoticons/default/beerchug.gif)

Not like my opinion is 100% correct and I welcome disagreement.

FWIW - I just ordered a new diaphragm from Tangerine to rebuild a MPS with a cracked diaphragm and to get more of experience tuning a MPS. In the past, I’ve just replaced leakers. Here’s to hoping I learn more.

[technicalninja]

@Superhawk996
Hit me with the OBVIOUS and I'll agree pretty quickly. (IMG:style_emoticons/default/ninja.gif)

In my first post I used two "ifs" and your second sentence destroyed them!

I was a moron, I thought I was dealing with a rubber diaphragm and my lack of experience with D-Jet shows bigtime...

Truth be told, the only car I'd put up with the D-Jet limitations on would be a car that NEEDED to stay concours. A nice 73 LE might be a candidate.

As I have an unquenchable "need to improve" that car would be very hard for me to own.

I'd not enjoy "stock" and would end up selling it to a better caretaker.

I REALLY should just pass on the D-Jet threads...

Replacing a metal diaphragm, are we?

That's a thread I'd like to "READ ONLY"...

[Superhawk996]

Replacing a metal diaphragm, are we?

That's a thread I'd like to "READ ONLY"...

Here’s the Tangerine diaphragm replacement that mimics OEM in material and shape.

There are a lot of guys here that already have significant experience rebuilding and tuning these. I’m late to the game.

[ChrisFoley]

That metal diaphragm isn’t terribly sensitive to backfire but I have no doubt that if you have one that’s already fatigued and has a hair line crack (but holding vacuum), a backfire could finish it off and crack it.

^^^
This

I've rebuilt many units which held vacuum ok but didn't perform well in use. The diaphragm was always cracked already when I disassembled them.
The diaphragm is under spring pressure when the engine is not running and while at WOT. The perimeter support surface has a sharp corner that creates the fatigue which eventually cracks the diaphragm.
A sudden transition to positive pressure could be the last straw.

[FlacaProductions]

I'll work through testing this "bad" one and report back asap....

[JeffBowlsby]

The LE cars are all from 1974.

An ancillary benefit of the stock decel valve is that it softens abrupt changes in the intake vacuum signal to the MPS, offering a modicum of protection to the MPS diaphragm.

[technicalninja]

That metal diaphragm isn’t terribly sensitive to backfire but I have no doubt that if you have one that’s already fatigued and has a hair line crack (but holding vacuum), a backfire could finish it off and crack it.

^^^
This

I've rebuilt many units which held vacuum ok but didn't perform well in use. The diaphragm was always cracked already when I disassembled them.
The diaphragm is under spring pressure when the engine is not running and while at WOT. The perimeter support surface has a sharp corner that creates the fatigue which eventually cracks the diaphragm.
A sudden transition to positive pressure could be the last straw.

@ChrisFoley
I have two questions that I would appreciate your experience regarding.

#1 Is the metal diaphragm failure the common issue with these or can they fail in other ways?

#2 What do you see as the performance limits of the D-Jet system?

Thanks
Rick

[ChrisFoley]

I have two questions that I would appreciate your experience regarding.

#1 Is the metal diaphragm failure the common issue with these or can they fail in other ways?

#2 What do you see as the performance limits of the D-Jet system?

Thanks
Rick

1) Diaphragm cracking is far more common than other failure modes.

2) Camshaft that provides strong/steady manifold vacuum at idle.