73 L48 tune up

[Douglas C.]

Re: 73 L48 tune up

Unplugged vs plugged in will have zero effect on driveability if you are running direct vacuum and bypassing the TCS solenoid on the intake.

Ok, thank you for clarifying that. Doug

[Duke W.]

Re: 73 L48 tune up

F

If all ok I can then change advance weight springs. Factory stock says all in at 4,200, so try to get that all in at the 3,000 to 3,500. Now, total timing of 36 to 40 degrees I assume would require pulling distributor and having the stop bushing changed to get about effective 24 crank degrees.


Doug

I think in general you are on the right track, but the only way you are going to get into the ideal total WOT advance range of 36-40 is to increase max centrifugal to 30. That means 6-10 initial, and with 16 degrees vacuum advance at idle total idle advance is 22-26 degrees, which is optimum for base engines with the 3896929 camshaft.

So sooner or later if you really want to optimize the spark advance map the distributor has to go under the knife, but it's a pretty simple DIY job.

Once set up as above your coolant temperatures, especially in low speed traffic, will go down, fuel consumption will be lower, and the upper end of the usable rev range will have new found power.

Put the following phrase into a search engine including the quotes so it will look for that specific title and pay particular attention to the page on emission controlled engines.

"tuning vintage corvette engines for maximum performance and fuel economy"

Duke

[Douglas C.]

Re: 73 L48 tune up

Thanks Duke. I now have my reading assignment. Doug

[Douglas C.]

Re: 73 L48 tune up

I think in general you are on the right track, but the only way you are going to get into the ideal total WOT advance range of 36-40 is to increase max centrifugal to 30. That means 6-10 initial, and with 16 degrees vacuum advance at idle total idle advance is 22-26 degrees, which is optimum for base engines with the 3896929 camshaft.

So sooner or later if you really want to optimize the spark advance map the distributor has to go under the knife, but it's a pretty simple DIY job.

Once set up as above your coolant temperatures, especially in low speed traffic, will go down, fuel consumption will be lower, and the upper end of the usable rev range will have new found power.

Put the following phrase into a search engine including the quotes so it will look for that specific title and pay particular attention to the page on emission controlled engines.

"tuning vintage corvette engines for maximum performance and fuel economy"

Duke

Ok, I did the following and maybe I'm missing something.

First checked and have 30 degrees dwell. Stock setup shows correctly per manual 12 degrees initial, unplug TCS vac hose and remains at 12 degrees. So, TCS is working. Idles at 950 rpm, manual calls for 900, so seems ok.

Put vac gauge on the port running to TCS, top front of carb, and reads 17" vac. So I will use this port for full manifold vac.

Run vac hose from top front port to vac can. Disconnect TCS wire, plug hose that previously ran to TCS. Back initial down to 10 degrees, hook up vac hose and idle timing is at 26 degrees. B25 can rated at 16 degrees at 14". So below the 2" rule, and 10 + 16 = 26, so ok there.

Issue I have is full manifold vac only increased idle rpm by about 300, so idle now at around 1,250 and will not decrease. See pics; choke lever free, not on cam, idle solenoid and idle speed screw not touching throttle arm. Is there something else I'm missing.

Took for a ride anyway, runs crisper but previous temp of 230 now maybe about 220. 73 vette1.jpg73 vette2.jpg73 vette2.jpg

[Jimmy P.]

Re: 73 L48 tune up

Hi Douglas,
Not related to your current issue, but something I noticed in the recent photo you just posted. Looking at your EGR valve I see it has a paper GL broadcast code label on it. I can see what appears to be the correct part number stamped into your valve beginning with the broadcast code letters MU which is correct for your L48 4 speed, and should also be the same letters on the EGR sticker if it would have survived. The GL label ( sticker) should be on the front of the round area on your Diverter Valve. Just a suggestion.

[Douglas C.]

Re: 73 L48 tune up

Looking at my post # 34 I think the temp not lowering is my fan clutch. I noticed when shutting engine off warmed up the fan just spins with little resistance. I can get a replacement clutch. I did get my idle down to around 1,050 by backing off fast idle screw under choke pull off. But can't get idle any lower. My 17" of vacuum is very steady, vacuum gauge does not flutter even a hair.

Tks. Doug

[Duke W.]

Re: 73 L48 tune up

Changing from ported to full time vacuum advance increases idle speed, so going through the idle speed mixture adjustment procedure it required.

I don't know why your Q-jet will not allow further closing of the throttle plates. Hopefully someone more familiar with that car will be able to help out.

You should eventually be able to get the engine to idle at 450-500 at 18-19" Hg very smooth and steady.

Duke

[Douglas C.]

Re: 73 L48 tune up

Changing from ported to full time vacuum advance increases idle speed, so going through the idle speed mixture adjustment procedure it required.

I don't know why your Q-jet will not allow further closing of the throttle plates. Hopefully someone more familiar with that car will be able to help out.

You should eventually be able to get the engine to idle at 450-500 at 18-19" Hg very smooth and steady.

Duke

Thanks. I'm happy the vacuum gauge shows not a hint of fluctuation. I'll check all vacuum lines, hope to find one that is causing high idel. Otherwise I'm not sure if any other solenoid or electrical item effects throttle plates and idle. Overall it runs smooth and strong. Doug

[Douglas C.]

Re: 73 L48 tune up

I think in general you are on the right track, but the only way you are going to get into the ideal total WOT advance range of 36-40 is to increase max centrifugal to 30. That means 6-10 initial, and with 16 degrees vacuum advance at idle total idle advance is 22-26 degrees, which is optimum for base engines with the 3896929 camshaft.

So sooner or later if you really want to optimize the spark advance map the distributor has to go under the knife, but it's a pretty simple DIY job.

Once set up as above your coolant temperatures, especially in low speed traffic, will go down, fuel consumption will be lower, and the upper end of the usable rev range will have new found power.

Put the following phrase into a search engine including the quotes so it will look for that specific title and pay particular attention to the page on emission controlled engines.

"tuning vintage corvette engines for maximum performance and fuel economy"

Duke

Ok, I had my 7043203 carb rebuilt and modified with idle restrictors to help get idle speed down. Stock carb is designed to idle with throttle plates closed and no vacuum advance. I plan on getting distributor rebuilt and recurved, and running full manifold vacuum.

So, as this modified Qjet still wants to idle no lower than 700 rpm at 20 degrees (no vacuum advance) I'm thinking of targeting initial 8 + limit vacuum pod to 12. Have centrafugal of 30 degrees at around 3,000. Giving me idle timing of 20 degrees, all in of 38 @ 3,000, and cruise timing of 50 degrees.

Thoughts? Doug

[Joe L.]

Re: 73 L48 tune up

So the B25 is 16 crank degrees at 14". I'm not sure if it's currently available, but it's close enough to the 16@15" B26 to be within production tolerance. As long as it's reasonably in spec it's okay to use.

It looks like the distributor needs the uniset under the shield, which is okay.

Those centrifugal springs look REAL stiff. What are the centrifugal specs from the CSM and AMA specs?

Excellent chart of the VAC specs! Where did you find it!

Duke

Duke------


With the Uniset the shield is unnecessary. GM advised to remove it when Uniset points are installed.

[Bob I.]

Re: 73 L48 tune up

Ok, I had my 7043203 carb rebuilt and modified with idle restrictors to help get idle speed down. Stock carb is designed to idle with throttle plates closed and no vacuum advance. I plan on getting distributor rebuilt and recurved, and running full manifold vacuum.

So, as this modified Qjet still wants to idle no lower than 700 rpm at 20 degrees (no vacuum advance) I'm thinking of targeting initial 8 + limit vacuum pod to 12. Have centrafugal of 30 degrees at around 3,000. Giving me idle timing of 20 degrees, all in of 38 @ 3,000, and cruise timing of 50 degrees.

Thoughts? Doug

I had the same high idle speed problem, when I change to full time vacuum on my 1970 L46. I blocked the idle bypass air, which provides additional air for the idle circuit. Not all quadrajet carburetors have idle bypass air, the motor now idles around 750 rpm.

Bob

[Douglas C.]

Re: 73 L48 tune up

I had the same high idle speed problem, when I change to full time vacuum on my 1970 L46. I blocked the idle bypass air, which provides additional air for the idle circuit. Not all quadrajet carburetors have idle bypass air, the motor now idles around 750 rpm.

Bob

Yes Bob. The stock 73 L48 Qjet has idle bleeds, etc. and from the factory idled at 900 with no vacuum advance. I since have had it rebuilt with idle restrictors so I'm trying to stay around 20 degrees idle timing with vacuum advance to get it to idle around 800. Tks. Doug

[Duke W.]

Re: 73 L48 tune up

Ok, I had my 7043203 carb rebuilt and modified with idle restrictors to help get idle speed down. Stock carb is designed to idle with throttle plates closed and no vacuum advance. I plan on getting distributor rebuilt and recurved, and running full manifold vacuum.

So, as this modified Qjet still wants to idle no lower than 700 rpm at 20 degrees (no vacuum advance) I'm thinking of targeting initial 8 + limit vacuum pod to 12. Have centrafugal of 30 degrees at around 3,000. Giving me idle timing of 20 degrees, all in of 38 @ 3,000, and cruise timing of 50 degrees.

Thoughts? Doug

That should be okay, but if you can get it to idle down without limiting total idle advance, the full 16 deg. vacuum advance will likely be best.

Duke

[Douglas C.]

Re: 73 L48 tune up

That should be okay, but if you can get it to idle down without limiting total idle advance, the full 16 deg. vacuum advance will likely be best.

Duke

I tested by setting static timing at 20 degrees and got smooth idle around 750 - 800. So figured 8 + 12 vacuum would be reasonable. I could try at 22 static to test, if idle speed is still around 800 I could shoot for 6 + 16 vacuum for idle timing of 22. Then centrafugal of 30 would be an all in of 36 @ around 3,000. Cruise timing of 52. Again, trying to do best overall function while using the stock carb as it's a Top Flight car. Tks. Doug

[Douglas C.]

Re: 73 L48 tune up

So the B25 is 16 crank degrees at 14". I'm not sure if it's currently available, but it's close enough to the 16@15" B26 to be within production tolerance. As long as it's reasonably in spec it's okay to use.

It looks like the distributor needs the uniset under the shield, which is okay.

Those centrifugal springs look REAL stiff. What are the centrifugal specs from the CSM and AMA specs?

Excellent chart of the VAC specs! Where did you find it!

Duke

Been a while and I've gone thru everything and with all emissions in place I'm surprised it's running at around 180 warmed up. I think part of this good temp is I've set initial at 16 with lighter advance springs, factory calls for 12, and when Qjet restored I had the idle restrictors altered.
So current specs:
16 initial, idles smooth at 800.
TCS switch eliminates vacuum advance until third gear.
36 all in at around 3,000.
Engine at idle has 19" vacuum and gauge shows dead steady.
Now, the B25 vacuum pod is not holding vacuum, only picks up about 7 degrees additional advance, also checked it with Mityvac.
I have a B26 on the shelf. Is that a good choice for this dead stock engine?
That should give me around 52 degrees cruising.
Tks. Doug