Darth Linux

Hi Forum

I've got a pretty bad vacuum leak that I just can't find no matter what I do. I've sprayed carb. cleaner and starting fluid at all the connections and fittings and found nothing. I've stuck a long piece of hose in my ear and listened around everything and can't find the source with that method either. I'm totally out of ideas for things to check or use to trace the source.

Here's the details of the situation:

car is a 73 RX-2.

I'm using the 73 carb and intake manifold. all the smog valves are removed and i have two block off plates that cover those two holes that remain. Each has a gasket.

the engine is a 1980. engine runs and has good, even compression. I used dimes and JB Weld to block off the holes where the thermal reactor would inject air into the exhaust stream.

the EGR valve on top of the center housing is removed, and a block off plate is installed. Only using RTV at the moment, no gasket, but spraying it with ether doesn't change the idle.

the exhaust manifold is an 84 cast iron.

I just got the engine running a week and a half ago and have had idle problems since the get go. I completely disassembled and cleaned the carburetor out since the idle circuit was kind of varnished and it wouldn't idle what-so-ever. The cleaning fixed that so it does idle finally. I traced down some small vacuum leaks where my block off plates were and replaced RTV with real gaskets.

currently I have to use the throttle **** on the dash to keep it idling at about 800 RPM. If i push that in, it goes to 500 and then stalls. My vacuum gauge goes from 10 in/Hg at 1100 down to 5 in/Hg. at 800 rpm, so there isn't much vacuum being generated. The car shudders very badly at 800 as well, and takes about 2 seconds to rev up to 2000. It's almost like one rotor isn't working, but I've verified spark, compression, etc.

Things I've checked out already as a possible source and found to be ok:
power brake booster
block off plates on intake
egr valve block off plate
various hose nipples on intake (all capped)
4 bolts that hold intake manifold to engine
PCV valve - cleaned and operational

What am I missing? Does the 73 intake NOT work on a 80 engine? Are my Thermal Reactor ports sucking air due to a possible failure of the dime and JB Weld block-off?

I have a 80 intake manifold - I've been hesitant to use that since I'd have to buy a block off plate for that, and go through the whole drain the antifreeze thing, which is a pain without a radiator draincock . . .

thanks in advance for any advice
Dave

black_sunshine

Vacuum leak? Is is "bobbing" at idle? If so, I'd re-check those dime-and-JB plugs. The 12a manifolds SHOULD be interchangable (I think 72-73 RX-2's had a 12a anyhow). I'd make sure that the manifold is correctly attached and that there aren't any pieces of old gasket preventing it from making a perfect seal. To remove old gasket(s), a short and sharpened-on-one-side putty-knife works great; gasket scrapers are good too. Also, check the gasket to the carburetor, and make sure no chunks of a previous gasket are there.

Sometimes vacuum hoses leak in other places and Cab. cleaner doesn't always pinpoint the problem. I assume that since you don't have emissions that you don't have a lot of vacuum hose(s) under the hood. Try replacing them if you have your doubts. Oh, and when you've got the carb. off, check and verify all the vacuum nipples are plugged/connected.

It's pretty easy to tell if you've got a bad rotor; it'll sound waaaay different than normal. Try leaving a rotor housing's spark plugs disconnected and see what happens.

Hey, if you're going to have the intake manifold off, why not cut-out a block-off plate for the '80 manifold and put it on? I'm not exactly certain that the manifolds are interchangable, so it never hurts to check. Just make sure the carb matches.

Now, if you're certain it's a vacuum leak, what leads you to believe so? Is there an audible "HISS" anywhere? If you've checked everything for vacuum leaks and have come-up empty handed, I'd look at the fuel system. Check your fuel pump and filter (or at the very least pressure-test it and flow-test it); a good and easy way to flow-test is to take a 2-litre and see how full it gets within a minute. If it gets 1/2 way or more, then you're good.

If that still doesn't fix your problem, take the carb. off and rebuild it. Make sure to blow ALL the passages (fuel and air) out with carb cleaner. ESPECIALLY the emulsion tubes and the accelerator-pump passages. Other than that, I'm out if ideas

Hope it helps,
Matt

Darth Linux

thanks for the detailed response. I was *very* thorough when I put the various componants of this engine together. I got the basic short block for $75 and then fitted all the relavant 73 parts to it for compatibility with the original car.

I'm absolutely positive there is no gasket material stuck anywhere - I used a razor blade to scrape the surfaces.

I've only got 3 vacuum hoses on the whole engine - two for the distributor and one for the PCV, and all the hoses are new, so that rules that out. Fuel filters have been replaced. Fuel levels are fine. Carb has been completely apart and rebuilt already (i've done about 20 carb rebuilds in my life so that was the first thing I did).

The thing that makes me think it's a vacuum leak is the reading from the vacuum gauge. It gets only 5 in/Hg. at 850 RPM. That's about a third of what it should get.

I've been through a bunch of things and at this point I'm thinking that compression is leaking past either the apex or water seals, and that's why it lopes at idle. It seem do just fine above 1500 RPM, where centrifugal forces creates a better seal at the apex and there is significantly less time on each compression stroke for the fuel/air mixture to leak past the seals since things are happening so much quicker.

I'm gonna take it to a radiator shop as soon as I get it licensed and have them pressure test the cooling system and see what happens when I crank the engine over. I'll probably see a spike in pressure, indicating leaking past the o-rings

But I only paid $75 for the engine, so I'm glad it even runs at this point.

hugues

I would check the engine compression and go from there. Low vacuum at idle (assuming it can even idle) is not a good sign.

hugues -

black_sunshine

Yeah, check the compression... if it idles at ~800rpm...it should be okay.

The only other thought I had was that the carb. may be running really rich and need more air to compensate. Other than that, I'm out of ideas.