Help with 1972 LT-1 ignition
Thread Starter
Advanced
Joined: Aug 2019
Posts: 75
Likes: 1
From: Germany
Hi guys,
we tried out best today, but it wasn't good enough
.
So, we did the following:
1. checked the OT moarking using a dial gauge with spark plug hole adapter. Showed the OT marking to be in the right ball park, but I'm not convinced this is accurate enough. I have a piston stopper, but can't find it ATM. Also, how do you realistically measure between the two "new" OT markings on the balancer with the engine in the car?
2. reset the distributor to "Corvette Specs" using Lars' papers
3. put the old cap and rotor back on just to exclude those
4. put the old coil from my 71 in just to exclude that
5. measured the spark leads - they're between 12kOhm (short leads) and 22kOhm long leads, so should be OK.
6. Plugs are new
What's the car do? Well, it kind of starts but makes me wonder if it's running on all cylinders... Won't idle below 1500rpm or so. When I try adjusting the timing even ever so slightly towards late it does it's best to impersonate a flame eater!
What now? I still think the OT marking may be off, but other than that I'm stumped!
Cheers
Es
we tried out best today, but it wasn't good enough
.So, we did the following:
1. checked the OT moarking using a dial gauge with spark plug hole adapter. Showed the OT marking to be in the right ball park, but I'm not convinced this is accurate enough. I have a piston stopper, but can't find it ATM. Also, how do you realistically measure between the two "new" OT markings on the balancer with the engine in the car?
2. reset the distributor to "Corvette Specs" using Lars' papers
3. put the old cap and rotor back on just to exclude those
4. put the old coil from my 71 in just to exclude that
5. measured the spark leads - they're between 12kOhm (short leads) and 22kOhm long leads, so should be OK.
6. Plugs are new
What's the car do? Well, it kind of starts but makes me wonder if it's running on all cylinders... Won't idle below 1500rpm or so. When I try adjusting the timing even ever so slightly towards late it does it's best to impersonate a flame eater!
What now? I still think the OT marking may be off, but other than that I'm stumped!
Cheers
Es
Old Pro Solo Guy
Joined: Dec 2017
Posts: 8,095
Likes: 4,449
From: Marlton NJ
Two questions:
- Does or did the LT-1 have Transistor ignition originally? They need a specific coil. And the modules can fail.
- Are you sure you are on the firing stroke and not 180 degrees out? The distributor only turns once for every two crankshaft turns.
Le Mans Master
Joined: Nov 2004
Posts: 7,850
Likes: 142
From: Tampa, Fl.
Two thoughts:
1) Has anybody used ether starting spray? One word, DON'T!
2) An engine that displays abnormal results from normal adjustments needs to be examined for the possibility that the timing chain, (as previously mentioned), has jumped a tooth, which usually occurs as the engine is turned off. When this happens, a person can get it to run, but it'll run really poorly.
Best guess.
1) Has anybody used ether starting spray? One word, DON'T!
2) An engine that displays abnormal results from normal adjustments needs to be examined for the possibility that the timing chain, (as previously mentioned), has jumped a tooth, which usually occurs as the engine is turned off. When this happens, a person can get it to run, but it'll run really poorly.
Best guess.
Last edited by Chuck72; Feb 22, 2020 at 05:17 AM.
Thread Starter
Advanced
Joined: Aug 2019
Posts: 75
Likes: 1
From: Germany
No start spray used or engine washing done!
What's an easy way of checking if the timing chain has jumped? I think it's unlikely though, as the engine was running OK prior to starting working on it....
I've found my piston stopper btw ;-)
Cheers
Es
What's an easy way of checking if the timing chain has jumped? I think it's unlikely though, as the engine was running OK prior to starting working on it....
I've found my piston stopper btw ;-)
Cheers
Es
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
You can check to see if the cam is clocked correctly (jumped timing chain) as follows:
1.---Pull off the driver's side valve cover. Turn the engine to TDC, and check #1 cylinder's valves. If both of them are closed, you are at the ignition point. Turn the engine over 1 time, back to TDC, and both valves should be off the seat.
2.---Check the height from the top of the retainer to the spring seat, first on the intake, then on the exhaust. I recommend something like a Machinist's 6" steel rule. Make sure you write them down, because you need to see which one is closest to the head.
3.---If the cam is in the engine right (timing chain not jumped), the intake retainer will be .030" to .060" CLOSER to the head than the exhaust retainer is. If the cam is retarded, the exhaust will be closer.
Lars
1.---Pull off the driver's side valve cover. Turn the engine to TDC, and check #1 cylinder's valves. If both of them are closed, you are at the ignition point. Turn the engine over 1 time, back to TDC, and both valves should be off the seat.
2.---Check the height from the top of the retainer to the spring seat, first on the intake, then on the exhaust. I recommend something like a Machinist's 6" steel rule. Make sure you write them down, because you need to see which one is closest to the head.
3.---If the cam is in the engine right (timing chain not jumped), the intake retainer will be .030" to .060" CLOSER to the head than the exhaust retainer is. If the cam is retarded, the exhaust will be closer.
Lars
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
You can check to see if the cam is clocked correctly (jumped timing chain) as follows:
1.---Pull off the driver's side valve cover. Turn the engine to TDC, and check #1 cylinder's valves. If both of them are closed, you are at the ignition point. Turn the engine over 1 time, back to TDC, and both valves should be off the seat.
2.---Check the height from the top of the retainer to the spring seat, first on the intake, then on the exhaust. I recommend something like a Machinist's 6" steel rule. Make sure you write them down, because you need to see which one is closest to the head.
3.---If the cam is in the engine right (timing chain not jumped), the intake retainer will be .030" to .060" CLOSER to the head than the exhaust retainer is. If the cam is retarded, the exhaust will be closer.
Lars
1.---Pull off the driver's side valve cover. Turn the engine to TDC, and check #1 cylinder's valves. If both of them are closed, you are at the ignition point. Turn the engine over 1 time, back to TDC, and both valves should be off the seat.
2.---Check the height from the top of the retainer to the spring seat, first on the intake, then on the exhaust. I recommend something like a Machinist's 6" steel rule. Make sure you write them down, because you need to see which one is closest to the head.
3.---If the cam is in the engine right (timing chain not jumped), the intake retainer will be .030" to .060" CLOSER to the head than the exhaust retainer is. If the cam is retarded, the exhaust will be closer.
Lars
Jebby
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Drifting
Joined: Nov 2019
Posts: 1,370
Likes: 99
Ok everyone, I've been following along here/only had my '73 about 3 months. So I got my new digital timing light with advance/retard buttons + dwell function today. I tweaked the dwell to 30 then checked advance and found I'm at 10 degrees BTDC @ 700 ish rpms with vacuum line off distributor and plugged. The bad news is I plugged the vacuum line back on there/no change whatsoever. Then I took vac line back off and rev'ed it up and found I only get to 22 degrees adv @ 3000 rpms, also not good. Put vac line back on, same exact deal, 22 degrees adv at 3000 rpms. So I guess that means I got a vacuum problem? The question I guess is where do I start? Don't know if this matters but my headlights work fine, so there is vacuum under there somewhere.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Are you attaching the vacuum advance to a ported vacuum port on the carb, or are you hooking it up to a manifold vacuum port on the carb? If you're using ported, switch to manifold. If you're using manifold vacuum, you have a bad vacuum advance control unit - replace it. Then fix your timing so you get 36 total.
Lars
Lars
Last edited by lars; Feb 23, 2020 at 03:41 PM.
Drifting
Joined: Aug 2019
Posts: 1,339
Likes: 225
If you can get your hands on a mityvac, hook it up to the advance unit to see if the rod even moves. If it does, then make sure you have it hooked up to the right place, Don't you have the under hood vacuum decal?
Last edited by Mrvettenick; Feb 23, 2020 at 03:50 PM.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Don't use the underhood vacuum decal hose routing unless you need to pass an emissions test. It will have you using ported vacuum. Do not use ported vacuum - use a manifold vacuum source on the carb.
Lars
Lars
Last edited by lars; Feb 23, 2020 at 04:38 PM.
Drifting
Joined: Nov 2019
Posts: 1,370
Likes: 99
Are you attaching the vacuum advance to a ported vacuum port on the carb, or are you hooking it up to a manifold vacuum port on the carb? If you're using ported, switch to manifold. If you're using manifold vacuum, you have a bad vacuum advance control unit - replace it. Then fix your timing so you get 36 total.
Lars
Lars
Drifting
Joined: Aug 2019
Posts: 1,339
Likes: 225
That large diaphragm is the EGR valve. If you're happy with the car, keep it stock. That's why I recommended following the label. The label should tell you what the solenoid is.You decide.
Last edited by Mrvettenick; Feb 23, 2020 at 05:02 PM.
Drifting
Joined: Nov 2019
Posts: 1,370
Likes: 99
Now that I've had a moment to ponder this, I recall the guy I bought the car from mentioning that he had an electronic ignition type distributor put on there (by whom I have no idea, he was definitely not a wrench type dude at all). He said it didn't work right so he had original distributor put back in/ran fine after that. I bet that's when the vacuum lines got shuffled, at least that's my guess.
Drifting
Joined: Nov 2019
Posts: 1,370
Likes: 99
Last edited by Elcransonator; Feb 24, 2020 at 01:05 PM.
Thread Starter
Advanced
Joined: Aug 2019
Posts: 75
Likes: 1
From: Germany
Hi Guys,
We had another go today....
Checked TDC with a piston stop. TDC was indeed out (after TDC) by over an inch on the circumference of the damper!
We drew a new TDC marking an set about adjusting the ignition again. Started fine an ran for long enough to adjust to 36 max BTDC.
However, once it got a bit warm, it started running rough again spitting through the exhaust.. I then noticed that header #3 was glowing red......
We pulled the valve covers, and I think the valve lash is much too tight, causing fuel to be burned in the exhaust. We couldn't measure the lash stone cold, but warm and hot we had less then 0.004" (yes, two zeros behind the .) lash....
Correct lash would be 0.024 and 0.026 intake and exhaust, no?
Re the Holley: we fitted new float valves, and there's now fuel dripping from the annulars at idle. That's not correct surely?
We'll have another go at the weekend I think....
Cheers
Es
We had another go today....
Checked TDC with a piston stop. TDC was indeed out (after TDC) by over an inch on the circumference of the damper!
We drew a new TDC marking an set about adjusting the ignition again. Started fine an ran for long enough to adjust to 36 max BTDC.
However, once it got a bit warm, it started running rough again spitting through the exhaust.
. I then noticed that header #3 was glowing red......We pulled the valve covers, and I think the valve lash is much too tight, causing fuel to be burned in the exhaust. We couldn't measure the lash stone cold, but warm and hot we had less then 0.004" (yes, two zeros behind the .) lash....
Correct lash would be 0.024 and 0.026 intake and exhaust, no?
Re the Holley: we fitted new float valves, and there's now fuel dripping from the annulars at idle. That's not correct surely?
We'll have another go at the weekend I think....
Cheers
Es
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Where exactly is the "underhood vacuum decal hose routing?" Only sticker I see under there is a placard (vehicle emission control information) that is on flat spot right behind brake booster at left rear corner of engine bay. It has the timing, dwell, advance info for manual/automatic etc etc, but nothing like a hose diagram.
First year for the hose routing decal was 1979 (separate decal). Beginning in 1980, the hose routing was part of the emissions decal.
1979 Emissions & hose routing decals:
1980+ decal after the 2 separate decals were combined into one:
Lars
Last edited by lars; Feb 25, 2020 at 04:40 PM.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Yes.... .024 and .026 is a good baseline.
If you are leaking through the boosters after float is installed then the metering block gasket is bad....did you reuse it? Did you reset both floats for dribble out of the sight plug?
You are definitely on the right track now....
Replace that damper ASAP.
Jebby
If you are leaking through the boosters after float is installed then the metering block gasket is bad....did you reuse it? Did you reset both floats for dribble out of the sight plug?
You are definitely on the right track now....
Replace that damper ASAP.
Jebby
Last edited by Jebbysan; Feb 25, 2020 at 04:49 PM.