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.

[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.