67 Tri-Power idle issue

[Eddie H.]

I've been fighting this problem for several years. The issue is that after raising the RPM of the engine above about 1600 RPM (either sitting still or driving), the idle RPM stays high (about 1300 RPM) after the throttle is released for at least 30-40 seconds before it will return to idle RPM of around 700. The engine has 13,200 miles since a total bone stock rebuild was performed. It still has the "143" cam and 11:1 compression. I'll add some back ground on what I've tried to determine what's causing the problem.

At first I thought the centrifical advance weights in the distributor were hanging up after being at higher RPM and leaving the timing too far advanced for awhile until the advance springs finally won out and returned the weights to idle position. I put a timing light on the engine and found out this wasn't the situation. The distributor had recently been restored by Dave Fiedler anyway so this likely wasn't the problem. The distributor advances and retards just as it should. The vacuum advance works too.

The carbs were the next thing to check. Pools of fuel on the intake manifold after a drive forced the issue of a rebuilding of the carbs (yes, I checked the fuel lines and they were okay). When the carbs were disassembled, the gaskets were pretty much like jelly due to the ethanol in the gas so the rebuild was necessary anyway. I bought a Tri-power rebuild kit from the Daytona Parts Company that had ethanol resistant gaskets and rebuilt the carbs thinking this would also solve the idle problem too. After reinstalling the carbs with new base gaskets, the float levels were set and the center carb idle mixtures were set. The secondary carbs linkage was set as per the instructions with the rear carb linkage rod about one half rod diameter short of the carb throttle lever hole and then the front carb linkage rod adjusted to just slide into the it's throttle lever hole. I tried to set the idle screw on the center carb for about 750 RPM but the engine either wanted to idle at 700 RPM or lower and about die or at about 1200 RPM. I settled for 700 RPM and retried the idle mixture screw settings but they were pretty sloppy. I did get an RPM "roll off" when the mixture screws were screwed in too far. I checked for vacuum leaks around the carb bases with started fluid and found no vacuum leaks. The idle vacuum fluxuates between 12-14 inches of HG as most 435s do. the whole throttle linkage moves freely with no hang ups. The idle problem still persists. I couldn't find any discussion of this problem in the archives so I'm asking for any ideas. Thanks.
Ed

[Leif A.]

Re: 67 Tri-Power idle issue

Ed,

I know in your second to last sentence you state that "the whole throttle linkage moves freely" but just out of curiosity did you check the linkage bushing near the firewall where it hooks to the link from the accelerator pedal? I had a similar problem where the linkage moved freely but i found a worn bushing and replaced it...solved my problem. Mine is not a Tri-power setup but I'm thinking linkage is linkage.

[Duke W.]

Re: 67 Tri-Power idle issue

Are you saying that at 700 if you begin to turn the idle speed screw in revs stay at 700 and then suddenly jumps to 1200?

How many turns does it take for the speed to jump to 1200?

L-71s originally have ported vacuum advance and a MS 201 15 VAC. Was yours converted to full time vacuum advance?

If so then a 12" B26 VAC should be installed.

Either way, what is the ID of the installed VAC? Does it meet spec?

Where did you connect your vacuum gage?

What kind or readings do you get if you tee it into the VAC signal line?

Duke

[Eddie H.]

Re: 67 Tri-Power idle issue

Are you saying that at 700 if you begin to turn the idle speed screw in revs stay at 700 and then suddenly jumps to 1200?

How many turns does it take for the speed to jump to 1200?

L-71s originally have ported vacuum advance and a MS 201 15 VAC. Was yours converted to full time vacuum advance?

If so then a 12" B26 VAC should be installed.

Either way, what is the ID of the installed VAC? Does it meet spec?

Where did you connect your vacuum gage?

What kind or readings do you get if you tee it into the VAC signal line?

Duke

If the idle screw is turned 1/8 of a turn, the speed goes to about 1200. There seems to be no in between speed.
I don't remember for sure what the vacuum advance number says but I know it was a 16. I think it was like a 203 16. I can't read it with the ignition shielding around the distributor. I'm sure it's as close to correct as Dave Fiedler could make it. The vacuum for the advance is still ported and not straight manifold vacuum. It comes off the base of the center carb as the stock configuration.

I connected the vacuum gage at the "tee" on the back of the intake where the power brake booster is fed. I haven't tried hooking the vacuum gage to the port where the vacuum advance can is hooked. I do know that at idle when the vacuum advance is hooked back up after setting the initial timing there is no change in the initial timing which would indicate that no vacuum signal is present with enough force to activate the vacuum advance can. I'll try that tomorrow and see what I get.

Another post suggested I check the firewall bushing where the throttle linkage pivots and that's good. The throttle always returns to the idle set screw stop. Thanks for responding.
Ed

[Gary P.]

Re: 67 Tri-Power idle issue

I just went through this and fixed it today. At idle there is no vacuum advance from the carb. so it would not change your timing when hooked back up. I put in a B26 adjusted total timing around 38 find tune carb sound great. my front and back carb butter flies were sticking some times so it changed my idle.

[Eddie H.]

Re: 67 Tri-Power idle issue

I just went through this and fixed it today. At idle there is no vacuum advance from the carb. so it would not change your timing when hooked back up. I put in a B26 adjusted total timing around 38 find tune carb sound great. my front and back carb butter flies were sticking some times so it changed my idle.

Thanks for all the responses. I found the problem thanks to Duke's question on the vacuum signal out of the carb but I'm not sure how I'm going to fix it. The vacuum advance can on the distributor is a MS 163-16 (I know it should be a MS 201-15). I checked the crankshaft advance with this vacuum advance by using a hand operated vacuum pump with a gage hooked up directly to the vacuum advance can while the engine was running at idle. There was no advance on the vacuum advance can until just past 8"-9" of Vacuum and then there was 1 degree advance for every 1" increase in vacuum up to the 16 degree maximum for this advance can.

The next thing that was checked was the vacuum "signal" out of the ported vacuum at the center carb base with it disconnected from the vacuum advance can. At idle (it will now idle at what ever speed you set on the idle screw without the advance can hooked up) there was about 6" of vacuum. At 900 RPM the vacuum signal was 8". At 1000 RPM the vacuum signal was 9". At 1200 RPM the vacuum signal was 11". at 1300 RPM the vacuum signal was 12". At 1500 RPM the vacuum signal was 13".

Now back to the original problem of the engine not returning to the proper idle speed after running the RPM up to a higher RPM. It would only return to the 1200-1300 RPM range and the reason is that the vacuum signal applied to the vacuum advance can at that RPM is enough to advance the timing and keep the idle RPM too high.

The next step was to verify these findings by applying the same vacuum with the hand pump to the vacuum advance can as was present at the base of the carb at 1300 RPM. When this 12" of vacuum was applied when the engine was at idle, the RPM jumped up to 1300 RPM. This RPM duplicates my problem with idle speed.

What I don't know is why this situation has come up. I didn't have this problem several years ago and I don't believe I've changed the vacuum advance can or the center carb causing it to be different from before. Either the vacuum advance can now pulls in advance too early in the vacuum advance curve or the vacuum "signal" out of the center carb is now higher for a given engine RPM than what is was in the past. Another question would be at what vacuum should the MS163-16 start pulling in advance and is that different from the MS201-15? Any thoughts on how to solve this idle problem?

Ed

[Leif A.]

Re: 67 Tri-Power idle issue

Ed,

I, by no means, am a vacuum advance expert but I've been doing a TON of reading and research regarding issues with my own car. From what I've read, at idle with ported vacuum you shouldn't be reading any vacuum at all at the carburetor. You state that you're reading 6" of vacuum at idle, I think there are other issues if this is the case. Others (Duke) with far more knowledge on this topic will continue to help.

[Duke W.]

Re: 67 Tri-Power idle issue

The reason I asked all those questions is that I had a hunch that as you began to turn the idle screw you were getting enough vacuum signal to start pulling the diaphragm, and you get a gold star for intuitively understanding what I was getting at and doing the tests you did.

You could probably "solve" this problem by refering to the carb adjustments in your CSM. I'm not that familiar with the 3x2 system, but I think some idle air is admitted through the end carbs, and there may be an adjustment to slightly increase the amount, which means you would have to close down the center carb idle adjustment, which will get the throttle plate farther below the vacuum advance port and delay where it begins to be uncovered. Get out your CSM and do some homework.

However, the best way to fix it for good is to convert to full time vacuum advance, which has been discussed numerous times, and everyone who as done this simple conversion is very happy with the results. The simplest way is to tee into the choke vacuum break hose, and it's easy to make the change invisible by plugging one passage in the carb and drilling a small hole to lower the vacuum advance port to below the throttle plate, but you have to remove and partially disassemble the center carb to do this.

The above modification on L71 must also be accompanied by installing a 12" B26 VAC in order to meet the Two-Inch-Rule. The final step of this modification is to go through the idle speed mixture adjustment procedure, and I recommend 900 idle speed on SHP big blocks.

The result will be better idle quality, less heat rejected to the cooling system at idle (which has solved many "overheating" problems on these engines), and better fuel economy, which is something to consider on a big block.

The 163 VAC is a "boat anchor". Years ago GM consolidated VAC part numbers and the 201 was replaced by the 163. As you found it starts at 8-9" and does not provide full advance until 18-19", so it doesn't even meet the Two-Inch-Rule on a base small block that pulls about 18" at 500 idle speed.

The current replacement for the 163 is the B1, but I don't recommend it for any Corvette engine. One of the following three: B22 (15"), B26 (12") or B28 (8") is appropriate for all Corvette engines with OE full time vacuum advance or OE ported advance applications that are converted to full time. Once you know idle vacuum (in neutral with a manual and Drive with an auto and with either type of trans with AC, the compressor must be engaged) select the least aggressive VAC that meets the Two-Inch Rule.

Duke

[Eddie H.]

Re: 67 Tri-Power idle issue

The reason I asked all those questions is that I had a hunch that as you began to turn the idle screw you were getting enough vacuum signal to start pulling the diaphragm, and you get a gold star for intuitively understanding what I was getting at and doing the tests you did.

You could probably "solve" this problem by refering to the carb adjustments in your CSM. I'm not that familiar with the 3x2 system, but I think some idle air is admitted through the end carbs, and there may be an adjustment to slightly increase the amount, which means you would have to close down the center carb idle adjustment, which will get the throttle plate farther below the vacuum advance port and delay where it begins to be uncovered. Get out your CSM and do some homework.

However, the best way to fix it for good is to convert to full time vacuum advance, which has been discussed numerous times, and everyone who as done this simple conversion is very happy with the results. The simplest way is to tee into the choke vacuum break hose, and it's easy to make the change invisible by plugging one passage in the carb and drilling a small hole to lower the vacuum advance port to below the throttle plate, but you have to remove and partially disassemble the center carb to do this.

The above modification on L71 must also be accompanied by installing a 12" B26 VAC in order to meet the Two-Inch-Rule. The final step of this modification is to go through the idle speed mixture adjustment procedure, and I recommend 900 idle speed on SHP big blocks.

The result will be better idle quality, less heat rejected to the cooling system at idle (which has solved many "overheating" problems on these engines), and better fuel economy, which is something to consider on a big block.

The 163 VAC is a "boat anchor". Years ago GM consolidated VAC part numbers and the 201 was replaced by the 163. As you found it starts at 8-9" and does not provide full advance until 18-19", so it doesn't even meet the Two-Inch-Rule on a base small block that pulls about 18" at 500 idle speed.

The current replacement for the 163 is the B1, but I don't recommend it for any Corvette engine. One of the following three: B22 (15"), B26 (12") or B28 (8") is appropriate for all Corvette engines with OE full time vacuum advance or OE ported advance applications that are converted to full time. Once you know idle vacuum (in neutral with a manual and Drive with an auto and with either type of trans with AC, the compressor must be engaged) select the least aggressive VAC that meets the Two-Inch Rule.

Duke

Duke, What is the Two-Inch Rule? Also, what is the brand name of the vacuum advance cans you are referring to ie. B26 etc.
Ed
PS. I do have a brand new 201 VAC from Paragon that I could put on the distributor but I tested it and it starts advancing about the same inches of vacuum that the 163 does so I thought it wasn't worth the labor to change it out.

[Timothy B.]

Re: 67 Tri-Power idle issue

Eddie,

The #163 vacuum advance control is the GM replacement for the #201 when it was discontinued.

If you use full manifold vacuum for the advance on your engine it's a good idea to make sure the control is pulled full at approx. 2" vacuum less than the idle vacuum of the engine.

[Duke W.]

Re: 67 Tri-Power idle issue

Duke, What is the Two-Inch Rule? Also, what is the brand name of the vacuum advance cans you are referring to ie. B26 etc.
Ed
PS. I do have a brand new 201 VAC from Paragon that I could put on the distributor but I tested it and it starts advancing about the same inches of vacuum that the 163 does so I thought it wasn't worth the labor to change it out.

Google duke williams san diego corvette, and download the pdf of my 2012 National Convention presentation. Study it as if you have to take and pass a test to get out of jail.

The Corvette repro houses will sell you any VAC number you want, but all will likely be the 163 boat anchor with a fake stamp, so they will cause problems on most Corvette engines. SHAME ON THEM!!!

DO NOT BUY A VAC FROM ANY CORETTE PARTS VENDOR!!!

All VACs available in various "brands" - Delco, Standard, Echlin, Niehoff, Airtex... - that you can buy from any parts store for 10 bucks are all manufactured by Standard Motor Products, and the B-numbers are the way to ID them based on actual their performance specs since the parts themselfs all look the same on the outside other than the B-number stamped on the mounting bracket.

This has been discussed on the TDB a zillion times including a recent thread that discussed how to find a B28, which is getting rare as they are no longer manufactured by Standard and apparently the only remaining stock is in the Airtex brand.

Duke

[Eddie H.]

Re: 67 Tri-Power idle issue

Thanks for all the information. I'll do the Google search and watch the video. Hope I can find the right vacuum can. If the carb tuning doesn't work then I'll convert to direct manifold vacuum assuming the right vacuum advance can be found.
Ed.

[Eddie H.]

Re: 67 Tri-Power idle issue

Duke, if you're still following this thread then I want to tell you I looked at your San Diego presentation you told me about. It was awsome. I see why others deferred to you on my issue with the idle speed. Also, thanks for tolerating my ignorance on the VACs that had been so very thoroughly discussed in this technical section. I didn't know I needed to investigate them until you started discussing using a "B26". After I looked them up on the Technical Discussion Board, I see you're almost always a responder and provider of good information on the subject.

I've ordered a VC-1765 (hopefully a B26) from NAPA and I'm going to convert to manifold vacuum advance for the L-71 and do away with the ported vacuum advance. I've been trying to locate the thread that discusses modifing the center carb base by plugging one hole and drilling another to access the direct manifold vacuum but my inexperience with using the Technical Discussion archives seems to be "biting me". I can't seem to come up with the right combination of search words to find the thread on modifing the carburator. I have an extra carb base so I can modify it and still keep my original the same configuration. I think I see how to do it but I would sure like to see what's already been done successfully before I go for it. If anyone can find this thread in the archives, please copy and paste the link to this thread and I use it. Thanks to all for their help. I'll consider this thread closed unless I get a link to the thread in the archives for modifying the center carb.
Ed

[Richard M.]

Re: 67 Tri-Power idle issue

Ed,

Here you go.......It's buried inside a thread I started years ago when I built my first L71....
There is much info in there, but at post# 11 & #13 Tim B. shows the details.

https://www.forums.ncrs.org/showthread.php?83918-Guidance-Please-1967-L71-Engine-Reinstall-amp-Run-In&p=533433#post533433

In the above thread we discussed the carb modifications, but in my case I connected the vac line the the choke pull-off circuit source as we decided not to drill out the carb base.

Recent thread also here about the conversion to full time with photos of the hoses connected to the choke pull-off circuit.

https://www.forums.ncrs.org/showthread.php?106675-Wanting-to-switch-my-1967-L71-to-manifold-vacuum&p=712440#post712440

For reference, below are the DW Tuning and Vacuum Advance spec documents for future reference.

...and BTW, after you convert to manifold vacuum, everything will get better!

Rich

[Duke W.]

Re: 67 Tri-Power idle issue

I've ordered a VC-1765 (hopefully a B26) from NAPA and I'm going to convert to manifold vacuum advance for the L-71 and do away with the ported vacuum advance. Ed

...suggest you obtain 2-3 feet of 1/8" vacuum hose and a 1/8" tee. Remove the original choke vacuum break hose and make a new harness using the tee to connect both the choke vacuum break diaphragm and VAC to the manifold vacuum source.

Then go through the idle speed/mixture adjustement procedure. Once you have the set up dialed-in properly you should be pleased with the results, and then you can proceed with modifiying the carb base if you like - reinstall the original choke vacuum break hose and no one but you will notice the mod unless they put a timing light on the engine at idle and that's not part of the judging process.

It's also a good idea to take a Mighty Vac to the parts store and check that the VAC starts and stops within on inch of spec, (which is 6 and 12 for a B26) before you accept and pay for the part.

Duke