Hopper12

Hi, I've suspected that I have a intake manifold vacuum leak since we acquired our '68 L71 nearly 3 years ago. The idle has been a bit choppier than I would expect, the headlights/wiper door are really slow (I have to get the rpm's up to make them move), etc. I'm going to fix whatever is wrong, and I started it up today after a winter nap and found it was a bit worse - didn't want to idle until it was warmed up, etc. So I got out the vacuum gauge and did some checking and could use your help. A little background may help.

Background on the car/engine

• '68 L71 roadster. About 38k miles.
• Engine was 'refreshed' about 20 years ago (I've spoken with the owner who had it at the time). Refresh included bearings, Cloyes timing chain, valves, hone/rings, other 'light' stuff.
• Car has only been driven about 1600 miles in the last 20 years, including about 600 by me.
• A little blue/black smoke on start up, but I figure I need to replace valve guide seals due to lack of much use over several years. No smoke after start up or when driving.
• Oil changed regularly by me and previous owners - high zinc Mobile 1 15-50 and zinc additive.
• I've completely redone the cooling system (runs nice and cool even on the hottest days); and restored everything under the belt line - including new off-road exhaust.
• Put about 500 miles on it last summer - didn't use any oil at all, and the oil looks great.
• In the two summers I've had it, the few times I've gotten after it I don't feel like the other 2 carbs are coming in - might be due to low vacuum and/or the few miles put on the car over the last 20+ years so the front/rear carbs may be a bit, shall we say, buggered up?
• We live at 5100 feet altitude.

Vacuum test today

• Hooked vacuum gauge to fitting on back of the intake manifold.
• At idle it reads 4" HG - quite steady, just jumps between 4-5" as the idle chops.
• Rev the engine to 1500 rpm or so and it reads 6" very steady. Let off and it quickly goes up an inch or two and drops right back to 4"

I'm figuring I likely have an intake gasket leak, but would like some more experienced opinions. I have not done a compression test or leak-down test. I can do those tests if you figure that would help diagnose the issue. I also found a gas leak on the rear carb while I was doing the test. So, I'm going to have the carbs rebuilt too.

Please let me know what you think. I'll tackle it myself, and I know I'll get some great help here (I have Lars papers - thanks Lars!).

Thanks very much, Paul



This is the port I hooked the gauge to. Just making sure it's a correct one.


Reading at idle. Bounces around 4" just a little as the idle chops. Steady at 6" @1500 rpm.


Rear carb - drivers side. Fuel appears to be leaking from the large screw on the side (I had wiped in just before I took the pic).


Rear carb - pass side. Leaking from gasket(s). I'm going to get them rebuilt.

lars

Your reading and numbers aren't possible. Pro Stock race cars idle with more vacuum than that. Your engine should be pulling vacuum at least in the low 'teens. You either have a very serious engine/tuning issue, or you have a bad vacuum gauge or an incorrect gauge hookup. Pull your PCV hose off the manifold vacuum port and run your gauge into that manifold vacuum port with a reducer - take a new reading.

Fuel seepage out of the bowl fuel level plugs is indicative that your float level is way too high. The fact that you're referring to the bowl sight plugs as "large screw on the side" indicates you are unfamiliar with the purpose of the plugs. Have you set your floats correctly..? If you haven't set your floats, have you done any other proper tuning and setup on the engine? Sounds like you have a multitude of overall engine tuning and setup issues that need to be methodically resolved. A manifold vacuum leak will not cause manifold vacuum to run at 4 inches, but really badly set timing can...

Low mileage and low vacuum has no effect on the operation of the secondary carbs. Your car sounds like a car that needs some serious setup and tuning by someone who knows how to set it up right. I doubt your carbs need "rebuilding." They just need to be set up right after your ignition system is properly set up.


Lars

Hopper12

Hi Lars, thanks very much for your response and ideas.

• I'll take another reading from the alternative vacuum source tomorrow afternoon.
• I haven't touched anything to do with timing or carbs since I bought the car 3 years ago. Your idea that the engine has tuning and set up issues rings true with me because the owner that had the car 20 years ago said they 'had a really tough time tuning the engine after the refresh'. My guess is that they didn't know what you, and some others on this forum, do. And, while the engine runs pretty good, it just feels like it's holding back and won't cut loose like other cars I've had.
• When I take another vacuum reading tomorrow I'll also remove the ignition shielding and see what distributor is on the engine. I have the original TI distributor, but they put a different distributor on it years ago. I think it has a red cap, so probably an MSD? I'll check.
• Your assumption is correct that I know very little about carbs, or how to really set up timing. I have a good place to work on it (lift, tools, etc.) and I don't mind tackling new stuff. I'm good with body/paint, I figured out how to do the entire suspension, etc. on this car last year, I'm comfortable replacing intakes, removing/installing heads (been a long time on those, but I can do it), etc. - - but I'm out of my element on when it comes to properly setting up carbs/ignition. It's just something I haven't learned, and I'll pay to have someone do it who knows how to set it up right - I've put a lot of $ into the car and I want to make sure it's set up correctly.

I'll circle back after I have more info tomorrow. Thanks again, Paul

Hopper12

Some additional information from this evening.

• The distributor is a Mallory Unilite. It has to be at least 25-30+ years old. I've done a bit of research and it sounds like there are better options.
• The coil is noted in the pic below.
• I pulled the plugs and all of them are sooty - carbon fouled, look like they've been running quite rich. I'm guessing that's from ignition issues and/or carb(s) not adjusted correctly?

My questions are:

• Should I consider a different distributor and, if so, what are the best options (I still want to use a tach drive, and I'd like it to fit under the shielding)? I have the original TI distributor. If I use that, it will need to be sent out for restoration. I'm okay spending some $ to do it correctly. And I'm okay with going one way now and restoring the TI for use at a later time.
• I've read and re-read Lar's papers, and they're making a lot of sense. I also think the carbs need adjusting, etc. Lar's suggested that the float is set too high and I believe the plugs are an indication of that? I've been searching for info and am starting to understand the carbs a bit better - but still not certain how to adjust them.

I'll fire it up tomorrow and hook the vacuum gauge to the pcv side as Lar's suggests.

Any ideas on the distributor and carb adjustments (and other thoughts) would be greatly appreciated. Thanks, Paul



I can almost read the model number, but some of it is worn off.

3X2

My thoughts: ditch that distributor and install the T.I. piece. Does the car still have the rest of the T.I. parts (harness and module)? More importantly, do you have a real GM T.I. coil, not a repro? If not I'd use the T.I. distributor to work with a GM h.e.i module. That's what I'm going to do if the module craps out again.Follow Lar's papers on distributor set up, timing, and converting to manifold vacuum for the vacuum advance. My L71 idles around 16" of vacuum. Verify float level as mentioned.

Hopper12

Thanks for the push - I called Mike Zamora this afternoon and will box up the stuff and send it off to him tomorrow. The harness is still there, as is the amp - but they're old and untried. So Mike is providing his updated board and stuff in my box, a new harness, the right coil, a rebuilt OEM VAC for the 435, etc. I'll convert it to manifold vacuum, timing, etc. per Lar's papers, and should be ready to go! I'm also going to set the float levels (watched several youtube vids), set idle mixture, etc. and see if the carbs act a bit better (it'll probably be hard to tell until I get the ignition sorted). If I'm not satisfied (not really sure what's going on inside of them since it was driven very little over the last several years), I'm going to send them out to be restored/rebuilt. Appreciate your help! Paul

Rescue Rogers

I had the same reading on my setup. I did 2 things. Changed the vacuum advance line to make sure it was on manifold vacuum and made sure my carb bases were flat. It went from 4 inches to 16 inches. Ild be your base timing is around 6 to 8 degrees as per the factory emissions setting. If you set the distributer to get 36 degrees with mechanical advance plus your timing advance as per Lars papers. Then plug the vacuum advance back in you will get a higher reading. As for your carb rebuild. I suggest Allstate.

Hopper12

Thanks RR. I'm setting the floats tonight - I'll see what I have going on there because the plugs were uniformly carbon/gas soaked. After I get the floats set, I'll refire it and check the timing - my guess is it's a bit messed up; I haven't checked it since I bought the car 3 years ago. I don't know much about the Mallory dist that's in it other than it's really old and has no vacuum advance. I haven't been able to find anything about it via google searches. I'm sending my TI dist and amp out to Mike Zamora tomorrow - removed the amp and got it all packed up earlier this evening. Then I'll set that up per Lar's papers, as Mike is specking it with the right VAC and coil (not one of the lousy aftermarket ones I've been hearing about. I actually bought one to have on hand whenever I decided to put the TI back in, and it's leaking oil like crazy). Mike said he can turn the stuff in about 2 weeks.

For the carbs, I've been talking with Harold Demes and was going to send them to him. He's put up a lot of youtube videos and it looks like he does really good work - only down side is he's 6-7 weeks out so I have some down time when I wish I was able to drive it. I checked the Allstate site and they look good too. I had been looking at Holley until you gave a warning in another thread - so they're out, and thank you for that!

It'll be interesting to see how this thing is timed tomorrow night. I'll verify TDC and then see what we've got. While the carbs are out for rebuilt I'll use that time to set the valves too, as I haven't checked them since I bought it either.

Any additional thoughts? Thanks! Paul

Rescue Rogers

I like Harolds Videos. I was just watching those last night to investigate a very lean condition i have with my carbs Im trying to set up now. I'ld be interested to see what his costs are. Im very dissatisfied with Holleys attempt at restoring my carbs. They cost just under $1000. Allstate is around $1500.

I sent my carbs out to Holley and they came back un-useable, so I had some NOS carbs I was going to go through but just do a quick planing of the outboard bases and replaced all the gaskets. The center carb acts like the idle circuits are gummed up so Im swapping out the metering block. and going from there. I suspect you will need to plane your surfaces and replace all the gaskets to just start with a good base

Hopper12

Some new info from last night:

• All 3 floats were set perfectly - didn't have to adjust them at all.
• I think I found the cause for the slight leak on the rear carb. I was doing a quick check to see if all the screws/nuts/bolts were tight and found the top screw on the pass side of the rear carb is stripped. Probably been that way for years. I confirmed with Harold that he will fix that when he gets the carbs.
• Harold charges $650 plus shipping for each carb. He does everything including machining base plates, metering blocks, plating, etc, etc. Expensive? Yes. But after watching his videos and communicating with him I believe it will be worth it. He spends as much time getting the plating right as he does the rest of the carbs. Here's a vid of how he does the plating:
  You Tube
  I'll provide an update after I get the carbs done.
• I got rid of the old corn gas that's been in the tank since October - as I've found that to be an issue when I get this car out after winter storage too. I found a Shell station within 5 miles of me that sells 110 leaded with no corn so I filled up a few gas cans last evening and put that in the tank after siphoning out the old stuff and disposing of it at our local garbage dump that takes it.
• I'm still going to crank it up tonight and check the timing - I want to know where it's at - probably hasn't been checked for 20ish years, certainly not since I got it 3 years ago.

Best, Paul



This screw is stripped - won't tighten and I'm pretty sure that's causing the slight fuel leak and maybe other issues. Will soon be fixed....

Rescue Rogers

Those screws Strip easily as well as the fuel bowl fuel line connector. Sounds like i may be getting in touch woth Harold

Hopper12

I figured that would be the case; and I was concerned about all the goop that the last person who installed the carbs used on the fuel lines to carb; and that I might cause an issue trying to remove them.. Harold suggested that I just disconnect the fuel line at the junctions and send him the carbs that way and he'd take care of it. If you want to contact him, here is his email address: chucknorbid@gmail.com

Thanks to you, Lars, other forum members and threads, and youtube videos, I'm confident I can move forward and get this all done. I sent the TI stuff out yesterday for rebuild/restoration and I'll swap out the Mallory that was put in it years ago for the TI when it's back. I'm pretty good at figuring things out and have the tools, it's just been a looooong time since I've messed with distributors or carbs - and I only learned the very basics and not the detailed explanation that Lar's provided.

So, my plan is to move it back to the stock TI system; have the carbs redone by Harold; while the carbs are out (6-7 weeks :-( ) I'll remove and restore the intake manifold; I'll check the valve adjustment (stock cam/solids); replace the valve guide seals; and be ready to go when the carbs get back. What could possibly go wrong? It's all part of the adventure!

Rescue Rogers

While you are restoring the intake take a straight edge and make sure the maing surface is flat and true to the post mounts. If they arent even and you use a phenolic spacer it can bind the buterflies

Hopper12

Here is some additional information from working on it this Saturday. I think I have multiple issues, but will find them:

• Running VERY rich. So rich that the idle mixture screws have no effect, even when both are turned in all the way. I know I have messed up carbs, and they're coming off this weekend and being sent to Harold. I know I have a leaking rear carb (small leak, likely due to the stripped screw - see above). But there are other things going on in the idle circuit and/or front and rear carbs that are not right - so Harold will fix them up.
• When I got the car 3 years ago it had been running an old Mallory dist for several years. I got the original TI dist with the car, but have never used it. I checked Saturday, and with the Mallory the initial timing is about 14-15 degrees at 800 rpm. Advances to 36 degrees at 2800 and is all in at that point. (checked with my Innova 5568). This is all assuming (I know) that the TDC marking is correct - I'll verify this when I put it all back together.
• The vacuum is definitely 4" at idle. I've checked it with two different gauges hooked up to the pcv side. It's steady at 4" at 800 rpm, goes to about 14" at 2000 rpm (steady, no fluctuations), and when I let the throttle go it shoots up very briefly to about 17-18" and settles back to 4". Based on my research, I think it's very likely I have a vacuum leak at the carbs, gaskets, intake, or?
• So here is what I'm doing:

The Plan

• I've already shipped the original TI dist and amp to Mike Zamora. He's setting it up correctly for a L71. I plan to install the stock TI system because I'd prefer it and - at least from my research - they work well.
• The carbs are going to Harold. I think it's quite possible he'll find several issues with them. They haven't been touched in at least 20+ years, and have sat for months at a time without being used before I acquired the car.
• While the carbs are being restored I'm going to restore and reinstall the intake manifold. It's possible there is a leak there and I want to clean it up anyway.
• While the carbs are out I'm also going to check the valve lash and make sure all is well there.

Other ideas and thoughts? Thanks! Paul

Rescue Rogers

Have you tried disconnecting an plugging all the vacuum lines including the PCV line. Have you taken any of the carbs off to check the integrity of the gaskets? On your massive rich situation, have you taken the metering block off and checked to see if its straight, checked for fuel in the power valve vacuum well, and checked the power valve to see if the diaphragm is ruptured? I had the exact opposite issue and my metering block must have a plug somewhere in it even though all the orifices produced fuel flow, veryy lean at idle but fine everywhere else on the throttle . Do your out board carbs have idle mixture screws? They should not.

It also looks like here was someone in there before you as you have a mismatch of bowl screws, maybe the gaskets they have used are failing. I had original gaskets in all three carbs and they were leaking and sucking air as well

Hopper12

Hi Rescue, I really appreciate your thoughts/help. Here are some answers to your questions:

• I have plugged both vacuum lines coming off the manifold - no change. Dang.
• The front/rear carbs have no idle mixture screws. They're the right number, but....
• You are correct. I can tell someone has been into them before. My guess is they're buggered up pretty good.
• I haven't pulled any of the carbs. Once I really looked at them and could see a few 'issues'; plus the fact that they've been sitting for months off/on over the years (and we know what the awful gas we have now does over time); plus the leak(s) I found in them; etc. - I decided to just pull them and get them restored/rebuilt by someone who really knows them; and that's not me.
• My guess is you're correct on the gaskets too. Many years old plus sitting for long periods of time plus probably not being rebuilt so well adds up to 'get 'em done'. Anyone who rebuilds the carbs and miss-matches bowl screws and knows (they had to know, I found it by just trying to tighten it and could tell it was stripped the second I touched it) they had a stripped screw and installs them anyway - isn't worth whatever they were paid to do the job.

Does 'the plan' in post #14 sound reasonable to you? Thanks again, Paul

Rescue Rogers

your plan is probably the best use of time and getting things corrected. I cant wait for you to get everything back and set up.

Hopper12

Thanks, me too! We're out of town for a few days celebrating an anniversary (well, I'm celebrating - my sweet wife probably deserves condolences!) so the carbs will be coming out Friday night/Saturday. Mike emailed me today and already has the TI done and will ship it out tomorrow. Lots to do, but I want to do it right and get it done in time to still enjoy some summer/fall driving. Best, Paul

darrentpi

sounds like you have it all under control. I had similar problems with car running too rich, stalling all kids of issues, funny exhaust smells. but I learned the power valve section was pulling fuel inside it and not vacuum, which caused all kinds of unmetered fuel to enter the throttle plate. All due to warpage and leakage around those metering block gaskets.