[pcoghlan]

I was trying to trouble shoot a rough idle on a freshly rebuilt 350 SBC. I lifted the door on the side of the distributor and noticed the platform upon which the points etc are mounted is moving?! It is rotating from side to side about 1/4-1/2 of an inch in a rhythmic fashion. I would think this would mean the timing is inconsistent (whic the timing light confirms!). Any reason it would do this? new dizzie required?

[T-bucket23]

Something in there is worn or broken. open it up and take a look. Are you sure the plate isnt moving due to teh vacuum advance kicking in and out due to some other problem. you may want to pull the vacuum hose off and take another look.

If you have room and you have to replace it an hei is a nice easy replacement and can be had fairly cheap.

[pcoghlan]

thanks

I removed the vacuum hose (and plugged it) and looked again, no movement whatsoever!

I reconnected the vacuum hose and sure enough thte rhythmic movement was back. I clamped the vacuum hose and as I clamped it the movement stoipped immediately. What does all of this mean? Should the vacuum advance cause this to happen or should it still be stationery?

The engine was running fine but with an intake oil leak. I removed the manifold and replaced it with a fresh gasket and now this. The intake now appears to be sealing but it idles like a dog and drops to almost stalling when I put it in Drive. If I keep the car stationery in Drive for more than a few seconds it does indeed stall.

[pcoghlan]

An update to this. I took it out on the road for 5 or so miles and it is definitely running rough. Once you get the engine above 1500 or so RPM it runs smoother but acceleration is practically non-existent and it just generally is rough.

Upon coming back there is an oil leak around the base of the distributor but the intake is holding good.

I am starting to think I should remove the distributor and put it back in with a new gasket. Maybe removing/replacing it will resolve whatever is causing it to not seal properly.

Ho hum, 2 steps forward, 1 step back...

[scholman]

pcoghlan,

You have a vacuum leak someplace. Because it pulses I would say it is a carburator gasket leak. Check all intake bolts to be sure they are tight just to be safe then check the carb hold down nuts. Maybe spray some WD-40 around the gaskets and see if the engine RPM picks-up. When it does that is your leak. Also if the transmission is an automatic check the modulator hose to see if it came off at the transmission.

Scholman

[Classix_Lover]

You did torque/retorque everything correctly right?

[pcoghlan]

I have to say my suspicion had been a vacuum leak.

I clamped the vaccum hose that feeds the distribuotr and auto as it leaves the carb, no change. So I think that leaves me with the leak either at the intake or the carbs.

I am running a tripower setup so there are NUMEROUS places to look.

I sprayed WD-40 onto a few places and had no change. I will try again tomorrow when it is daylight and easier to see what is happening.

Is there a way to test the vacuum and confirm without doubt that there is a leak? That way I could see when it is fixed.

[pcoghlan]

Classix_Lover,

Yes I torqued the intake and used the proper sequence. The carbs never left the intake and were fine before. Nothing else was removed other than the distributor.

[MI2600]

I think I would pop the distributor cap and test the vacuum advance can with a vacuum pump to see if it holds vacuum first.

[T-bucket23]

Originally Posted by pcoghlan

I have to say my suspicion had been a vacuum leak.

I clamped the vaccum hose that feeds the distribuotr and auto as it leaves the carb, no change. So I think that leaves me with the leak either at the intake or the carbs.

I am running a tripower setup so there are NUMEROUS places to look.

I sprayed WD-40 onto a few places and had no change. I will try again tomorrow when it is daylight and easier to see what is happening.

Is there a way to test the vacuum and confirm without doubt that there is a leak? That way I could see when it is fixed.

Put a vacuum gauge on it. If it is pulsating I would bet you have a bad seal on the intake somewhere. Did you clean both surfaces really well before re-installation. If it was a bad hose going to the modulator or dist or whatever else you have it would be constant low, not fluctuating generally, depending where the vacuum is taped from.
If it was running ok before you fixed the manifold leak I would be very suspicious of the manifold to head seal. If you take it apart again make sure you use good gaskets like felpro or mr gasket brand. Some of the cheap ones really are garbage.
When you set the intake you need to have as little movement as possible, a helper is always good just to get good preliminary alignment. You dont want to be moving the intake arround on the gaskets or you are looking for trouble.
Good luck with it.

[pcoghlan]

OK, I bought a vacuum gauge this morning.

The reading is indeed pulsating, between about 10 and 15 in a completely rhythmic fashion.

I sprayed a ton of WD-40 all over the intake manifold edges, carbs, base plates and the carb bodies where there are ports and pltes etc, no change in the reading whatsoever.

Sounds like the manifold is coming off again

If it is the intake I am surprised I have vacuum problems but no oil/water leak?

[Jmark]

Personally, I don't think a vacuum leak will cause a pulse in the advance hose. If you hook up a vacuum guage to ANY manifold port and it pulses, i'd be much more inclined to look for a burned intake valve if it were an older engine, or an over tight intake rocker arm adjustment, since its a fresh engine. That would allow pressure to get back into the intake system and give the pulsing readings. Might be time for a quick compression check before you tear it all apart.

Also, a very aggressive cam and low idle speed with give uneven vacuum readings. What cam do you have?

Mark

[pcoghlan]

sorry, I don't have the cam spec with me but bear in mind the vacuum issue is new.

When the engine was rebuilt I had an oil leak on the intake manifold but other than that it ran like a champ, super smooth.

I replaced the intake manifold after about 150 miles and now have this issue. Using logic I am pretty confident it is either a leak at the intake or carbs.

I just went to the local speed shop and bought a Felpro gasket set with raised silicon seals. Fingers crossed!

[pcoghlan]

This is REALLY turning into death by a thousand cuts!

Removed the intake and spent about 45 minutes with a razor and solvent ensuring the mating surfaces were VERY clean and dry.

Put it back on and it fired up in less than a second and sounded very smooth. Hooked the vacuum gauge to it and now have a steady 16'ish on the gauge

Delighted with myself and cleared the tools away thinking that once it was up to temperature and the tmiing was set I might even be able to road test it.

Set the timing and then sat in the car, blipped the throttle, and...BANG!

Pretty large back fire through the exhaust. Did it again and BANG, same result?!!

If I slowly increase the revs it revs smoothly and sounds great, if I try to increase revs at more than a leisurely pace I get a back fire? Firing order is fine and timing set to 8 BTDC with vacuum plugged. Seem to be getting the back fire regardless of timing though.

At least the vaccum issue was resolved!

I posted the new issue at :
backfire upon blipping throttle

[Guy Hiltz]

What about your distributor drive gear?If this is worn it could be causing the symptons you are experiencing.Check it out!