Is this a vacuum leak?

[3pedals]

I'm still trying to get this car sorted and running reasonably well since I bought it in May. Starting a dedicated thread here, but had another here: https://7173mustangs.com/threads/hello-from-nyc-73-mach-1.43188/. It has a Demon carb sitting on top of a custom adapter plate on top of the original 73 intake, which is a different bore pattern than the carb and has the exhaust heating channel. I suspected a vacuum leak from when I re-attached the carburetor without changing the gasket directly on the manifold flange, so I took everything off, removed and scraped the old gasket and used new felpro 1901 4-hole gaskets on both sides of the metal adapter. But, starting up after that the idle is horrible. It starts but won't hold idle and dies and giving throttle doesn't help. I presume I made it worse and there's now an even more severe vacuum leak happening between the intake and bottom of adapter plate. Given that's all that changed, a significant enough sealing issue could cause that, right?

Video of startup:

Here's how carb sits on adapter:


Here's what the original gasket looked like. It's clear that there was some RTV used underneath as I had to scrape it off and it's visible around the studs. Also, in lower left, you can see that PO, in addition to needing to drill out material for the ports to line up, they also drilled out space from exhaust heat area which was needed for the secondary linkage on the car to fit down into. I believe that's the source of the exhaust leak ticking that is heard on that side of the engine, too.


This is what the intake flange looks like under that gasket (photo not from mine) and the parts in blue are where material was removed to allow for the demon/holley style carb to go on.


So, to fix this, if the new gasket is just not sealing, should I do what the PO did and put some RTV sealant on the intake metal under the gasket? I know that's not normally the right thing to do, but this weird adapter situation I think I have no choice. I do already have an aluminum Performer intake that i'm going to swap, but I really want to just get it running decently for a little while before tackling that. Is the starting issue I have most likely explained by poor sealing here? Thanks for any guidance.

 

[Steve McMahan]

I would install the new manifold if it resolves the mismatch of the carb to intake. Since all you know is what you can see on the existing installed manifold, you don’t know what else the PO might have cooked up. So, getting a baseline you understand would be good. Especially if it’s the entire reason it won’t idle or ever start. Otherwise, you’ e got to get it to run, even at 1k rpm at idle, if you’re gonna try and find the existing issues. 2 cents

 

[Sheriff41]

I'm with Steve on this one; just install the Performer manifold and get it running with parts that match. Swapping the intake is an easy job... other than lifting that boat anchor cast iron manifold off of the engine! Make the change and get out there driving your Mach 1!

 

[basstrix]

Not familiar with your circumstances, but from prior posts, it sounds like you have a plan to switch over to a performer intake. I agree this will be the least amount of headache and fastest way to solve this problem.

That being said, the image below shows the gasket that's intended for this intake. It's metallic (aluminum coated with some fiber/paper material) and somewhat thicker/pliable than what you're trying to use. I saw, in the pic, that someone has installed a pipe plug into the exhaust passaged. If your intake doesn't have this, you should install one. Not sure what sealant will work on the plug, but I'd start with several wraps of teflon tape....teflon is compatible with relatively high temps....doubtful it will hold but best I can think of in this location...maybe the red high temp RTV? Last is the gasket between the carb and your adapter. I suggest a thicker material than what's shown. The combination of the proper gasket between manifold and plate, a plug in the exhaust port, and a thicker gasket between plate and carb is going to give you a better shot at making this work.....but as I started with....just do the intake and be done with it.

 

[Kilgon]

+1 with all of the above. Until you replace the manifold and have the proper fitment you will be chasing ghost trying to figure out where the true underlaying problem is.

 

[3pedals]

So, old intake has been removed (that was a challenge) and new intake has been painted and ready to install. I cleaned all the surfaces, applied high-tack to the head surfaces and head-side of the fel-pro gasket, placed those on, and set the new intake on to check everything fits right and seems good so far. The china wall gaps seems pretty large now as the new gaskets are much thicker than the turkey pan one that was in there before. The consensus seems to be to toss the cork front/rear gaskets and just use a thick bead of RTV, but the gap is pretty large so a little worried about filling that gap completely. I've got some black Right Stuff to do that with and I guess the gap will shrink a good amount when I torque it down. This is a first time job for me, wish me luck. Hoping this clears up other issues i'd been having.




This "hole" in the lifter valley by the distributor scared the crap out of me until I googled and saw they came from the factory this way. What the heck, Ford?

 

[giantpune]

Yep, just cake it on there. You can build the RTV pretty high. You want to make sure its not so top heavy or excess that it is falling into the engine.

If you ever watch them build an engine on the engine power youtube channel, they do enough of them that they are masters at the RTV bead on the china wall.

One hint is make sure to cut the opening of the tube far enough back that you get a big bead. If you have a little bead, you really have to make an ugly mess of it to get it tall enough.

 

[3pedals]

Intake is on. Not entirely confident I got total sealing with the Right Stuff bead on there, but we'll see. After letting it cure for 24 hours and after torquing everything down to 22lbs, I started it up just now. Good news is it started, bad news is it's idling like crap and dying. About the same as before I swapped the intake. Anyone have ideas? Here's a video of the start. 2nd one I checked the timing and tried advancing to 18-ish to see if that would help (didn't really):

 

[Steve McMahan]

Is that an exhaust leak I hear? Any chance it has the wrong firing order?

 

[Kilgon]

Several things to check out. As Steve mentioned, have you checked your firing order? Have you checked to make sure that the balancer hasn't slipped on the hub. Bring number one to tdc using a piston stop or some other means to know that it is and then check the timing mark. Also, have you checked your float levels on the carb. Does the car have a stock cam?

 

[cjstaci]

the choke appears to be closed. Try opening the choke a little and see what happens. Make sure your float level is correct, it looks low in the video. Make sure your idle mixture screws are 1 and 1/2 to 2 turns out for initial start up.

 

[3pedals]

Is that an exhaust leak I hear? Any chance it has the wrong firing order?

Yeah, it's always had the ticking sound on the passenger side, probably exhaust leak. I checked and rechecked the firing order before starting, but will do again next time to make triple sure.

Have you checked to make sure that the balancer hasn't slipped on the hub. Bring number one to tdc using a piston stop or some other means to know that it is and then check the timing mark. Also, have you checked your float levels on the carb. Does the car have a stock cam?

the choke appears to be closed. Try opening the choke a little and see what happens. Make sure your float level is correct, it looks low in the video. Make sure your idle mixture screws are 1 and 1/2 to 2 turns out for initial start up.

I haven't checked balancer slippage. It's on my list, but i've been able to get it to idle and run pretty well before, so even if it's not true, it shouldn't be preventing a good idle since i've played around a good amount with the timing. Last time it was idling well, it showed 20-ish degrees. Float levels do look potentially on the low side so i'll raise those a bit next time. Also, I haven't reset the idle mix screws to 1.5 turns out since putting the intake/carb back on so will also try that.

Thanks for ideas on things to try, will update when that's done.

 

[Steve McMahan]

You haven’t mentioned the vacuum reading at idle. Just wondering if the two are linked? I would disconnect anything vacuum wise connected to the manifold, except the PVC valve, and verify that isn’t the issue. Just another line to pursue.

 

[c9zx]

I suggest cleaning the air bleeds and checking the power valve (requires partial disassembly), and set the primary transfer slot to .020-.030 exposed (requires removal to set/check). The secondary side just cracked from 100% closed. If you take it off, I'd check the secondary diaphragm to see if it is leaking/torn. There is also a small gasket that seals the diaphragm housing to the main body. I hope you get it sorted soon. Chuck

 

[mrgmhale]

the choke appears to be closed. Try opening the choke a little and see what happens. Make sure your float level is correct, it looks low in the video. Make sure your idle mixture screws are 1 and 1/2 to 2 turns out for initial start up.

I totally agree with cjstaci above re: the choke being closed. It is closed so hard re: choke cap spring pressure, that when the engined does try to run the choke blade does not look like it is even trying to open a little bit. Before chasing down any other issues I would definitely set the choke up to a proper choke cap index, make certain you are getting full battery switched voltage to the positive terminal on the electric choke cap, and make certain the choke cap ground terminal is actually grounded. I cover much of that in the following YouTube video:

https://youtu.be/WDcskIPfLcE

 

[3pedals]

Thanks, it was the choke! After loosening, it's idling ok again. Now need to re-dial everything in, but back in a good spot now to proceed.

 

[3pedals]

It's actually running pretty well again finally with the new intake on. The initial idle was really low RPM, vacuum was low (~9") and was blowing out some liquid from tailpipes. I leaned everything out a lot, lowering the float levels and turning in the idle screws from the initial 1.5 turns out setup. Half a turn of idle and vacuum picked up and RPM came up, another half turn and got even better, up to about 15.5 inches. Took out for drive then and she's running strong. What a relief! Those carb adjustments were fairly coarse so next step is to do some finer tuning of the idle to see how smooth I can get it and bring the RPM down from where it's at (about 1000). Timing is showing about 18 degrees.