lamune engine build (can engineer guy build a motor?)

Optisparks are dog simple distributors. I think what people hate is how hard they are to get to on F-bodies and Corvettes. As long as the bearing and bearing seal doesn't wear out, you can use the body and the optical sensor forever, just replace the cap and rotor every 100k miles. That 4-pin harness is where I suspect the most crap gets sucked in. Replacement harnesses are available cheap on ebay and elsewhere.

Search "DR473" on ebay and there's a place selling OEM-quality cap and rotor kits for under $50. It's not pictured, but it also comes with all new bolts too.

Very interesting indeed, I will check mine today by just putting a finger over the fresh air inlet that attaches to the duct.

I would think that flat top cap is not very strong against an atmosphere of pressure on the outside of it. Rotor may actually be hitting on the contacts or something.

I'm pretty sure the optispark I have installed currently is almost hermetically sealed! I need to re-check my vacuum harness setup as well. There should be airflow through the thing, and not really any buildup of vacuum. The excess vacuum would probably explain the rough running/stumbling I was noticing as well. My assumption would be that the cap was sucked in and distorted, but for some reason only makes noise when hot.

Sherlock thanks for the tip, wish I saw that earlier. I ordered a MSD cap/rotor set and figured I'd give that a try.

By the way there's quite a lot of ozone generation inside that thing. When using the handpump you can really smell it.

The MSD cap/rotor is a good setup. How much better over an OEM Standard Motor Products cap/rotor, I couldn't tell you.

Just make absolutely sure there's an unrestricted air path from the intake elbow to the optispark, and then the vacuum hose containing the restrictor and one-way valve is properly connected to the driver's side of the intake manifold.

Well the MSD cap and rotor is either built to a high spec for MSD, or it's the same crap you can get off the internet molded in red. It's hard to tell these days. MSD at least used to make good stuff, but who knows today.

I did double check and I have the positions of the check valve and restrictor reversed when I assembled the replacement vacuum harness. I have no idea if or why that would matter, but that's what I see.

Very interesting. It appears the my engine did not have the restrictor on it when I got it 8 years ago as hose was directly to the Opti with only the checkvalve in the center of the hose. Reading a bit it appears that the restrictor is also a filter, probably for the metallic vaporizing from the spark arcing in the Opti.

I think the filter needs to be first so the checkvalve works properly and doesn't get plugged up and doesn't get delayed in closing when back flow at shutdown.

I am surprised that my system seems to be running OK with no restrictor in it.

You might also have a plugged up filter.

Harnesses hard to find and filter/restrictor not around as a loose part, so either junk yard and add your own hose or one guy on ebay selling new (allegedly) GM harnesses for $100.

Booster I want to correct myself here- I made some mistakes and have some more info.

First - I was measuring the vacuum in the distributor wrong because of where I inserted the gauge. I was just measuring engine vacuum.

Second - I did pick up one of those "alleged" (I like that) GM hoses. It came in a bag with a GM label, it at least looks legit and says made in Mexico. But since I only have the one 20+ year old example on hand I needed something to compare it to, and it was helpful.

The restrictor is a filter with a glued-in orifice in one of the holes. I never noticed that. The orifice should face the vacuum source so it doesn't get plugged up since the filter is on the other side. I might have had it in backwards. My old restrictor was much more restrictive than the new one, but after pushing some air through it, it seems to be about the same as the new one. It was probably plugged up a bit.

The orifice is smaller than .040 since that's the smallest drill bit I have on hand. I can get a better measurement later once I dig up some smaller bits.

That being said, I ran the motor without the restrictor in place to see what would happen, and that made no difference in the noise. I also ran it with the vacuum lines completely disconnected, and that made no difference either. This leads me to believe that if the noise is indeed coming from the distributor as it appears, the problem isn't likely in the cap/rotor but probably the bearing. So the plan next is to swap the whole distributor out and see what happens.

I picked the distributor I was running in the car previously as I know that one worked ok. (Looks like I have 3 spares!) and swapped the MSD cap on. (Which I kind of regret since I used RTV to seal the cap on that distributor and when I took it apart it looked almost new, but it wrecked the seal. I decided not to RTV this one)



So it certainly looks nice in the package. The rotor appears to be brass, or at least brass-colored.



And it installs ok. The hardware it comes with is not like the factory hardware. For instance with Philips head screws instead of the reverse Torx, etc. Also the seals are just plain round o-rings unlike the molded channelized seals that the OEM cap uses. That was a little disappointing but it seems to work just fine.



The MSD cap has an extra boss molded into it and is supplied with this clamp that provides an extra clamping area across the top. I didn't know this was a problem that needed fixing but I suppose it might help prevent coolant from getting in there if it leaks. It's at least different than the aftermarket caps we can get off ebay though.

The orifice in the filter is about .015"

It should be noted that the restrictor and one-way valve are common parts used across many manufacturers (same with the plastic heater core restrictor and T, BTW).  I would never spend the extra money to buy a GM optispark vent harness; just go source a new valve and restrictor, and buy some vacuum line from a parts store and make your own.

Sherlock I did take a quick look around and didn't find them, but I'm sure they're nothing special- I probably just didn't use the right search terms to find them. If you have some leads that'd be useful for all of us going forward rather than spending $100 for $10 worth of parts.

Personally I just reuse the ones I have and buy new vacuum hose. Try mcmaster.com, amazon or ebay.

For check valves, I have always used the windshield washer hose valves that every parts store has. The seem to work fine with vacuum also.

Cool, thanks. I did find a company called Air Logic that makes "filtered orifice restrictors" that seem to be something that'd work. Also apparently these things are used in medical devices as well.

I swapped out the NOS distributor and swapped in the one that was on the car with the MSD cap and rotor assembly. The strange noise (so far) has not come back. I noticed that the o-ring on the water pump drive coupler was too thick and sheared in half at the block side. I did fix that as well and was thinking it could possibly explain that noise as well, though it doesn't strike me as likely given how strong the vibrations coming off the distributor cap felt.

I did also discover that the 45 degree cap boots (Taylor) are slightly too big and don't seal against the terminals. Not a huge problem now but I think I should fix that before water gets in there.

I also have been recently coating the paper water pump gaskets with a thin coating of RTV. When I took the pump off the gaskets stuck to the block nicely and upon inspection I really felt like they could be reused many times. I did replace them (and recoat them) but next time I'll see if that works or not.

It's been a while since I updated this thread. I did recently get everything back together and at that point, nothing left to do but bite the bullet and take it out for a drive. I put about 75 miles on it over 2 days and it didn't explode (yet)



Without getting too far off into the weeds, just a couple of notes here after doing all this work

* Overall impression is that the driveability issues I was hoping to fix are largely still present, so whatever was causing them is external to the guts of the motor
* The rebuilt steering box from BBB finally solved the weird vague steering/non-centering issues the car always had
* I need to start learning how to tweak the PCM

I have some data dumps from TunerCat I can share if anyone wants to take a look. Also if there's a good contact to get a baseline tune from, I'd be interested in hearing about that too.

lamune wrote:

It's been a while since I updated this thread. I did recently get everything back together and at that point, nothing left to do but bite the bullet and take it out for a drive. I put about 75 miles on it over 2 days and it didn't explode (yet)



Without getting too far off into the weeds, just a couple of notes here after doing all this work

* Overall impression is that the driveability issues I was hoping to fix are largely still present, so whatever was causing them is external to the guts of the motor* The rebuilt steering box from BBB finally solved the weird vague steering/non-centering issues the car always had* I need to start learning how to tweak the PCM

I have some data dumps from TunerCat I can share if anyone wants to take a look. Also if there's a good contact to get a baseline tune from, I'd be interested in hearing about that too.

On the tuning, you may want to contact this guy, who even though I prefer to do my own, seems to be the most knowledgeable and easy to work with that I found when I was looking for information. It was he who got me to the help I needed to get the DST to work on Buicks, so very grateful to him for that.

Be sure to let him know what you are looking for in a tune as that can make for much better matching to your needs.

LT1 tuning

Congrats Lamune; I'm just getting back on GMLR, been enjoying life too much. Send me a PM and we can talk about tuning.

Will do Sherlock - thanks!