When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I'm very new to this, so I'm hoping those with more experience will chime in a bit here. As you can see in the video, I'm getting pretty low vacuum in the 10-12 range at idle (750 RPM) with fluctuation. With a little bit of carb tinkering, it can pop up to about 12-14 but again with the fluctuating needle. From what I've read, it seems I should have 18-20 with a steady needle and that vacuum in the 10-14 range indicates a likely valve issue (timing, sticky, worn valve guides, etc.).
Other likely relevant information, on cold start the engine runs a bit rough at first and needs a pretty lean choke setting before it gets warm and smooths out.
So with all this in mind, before I start tearing everything apart, what do you think is my most likely problem here? It seems the easiest place to start is to pull the valve covers and see what's going on inside there. What should I be looking/testing for in there? Or should I be looking elsewhere first?
Are you sure your initial timing is at 16-18*BTDC? That is a pretty steady needle, but a low reading. Can't really tell if you have a lopey cam from the video on my laptop. Plug it into a port on the manifold.
Are you sure your initial timing is at 16-18*BTDC? That is a pretty steady needle, but a low reading. Can't really tell if you have a lopey cam from the video on my laptop. Plug it into a port on the manifold.
Admittedly, I am eyeballing the timing with a simple timing light. The timing mark shows about 1/2" above the stock indicator which if I am interpreting correctly only reads to +12. I've got an MSD balancer timing tape on order at Summit so that will help me get a more exact reading.
I will try a reading direct from the manifold. Best/easiest place to plugin?
That vacuum gauge video looks very similar to my car, which runs with ~16" vacuum with the best tuning settings, and note in my sig line below the camshaft in my engine. It could be that your engine has an aftermarket cam and that is the "normal" vacuum reading for it.
Is there something other than the vacuum reading that makes you think there's a problem?
Your carb could be pig rich, that will affect the vacuum signal some. Add in a possible larger cam, that will also affect it, potentially a lot. I could only get 17" of vacuum with a 204/214 cam.
That vacuum gauge video looks very similar to my car, which runs with ~16" vacuum with the best tuning settings, and note in my sig line below the camshaft in my engine. It could be that your engine has an aftermarket cam and that is the "normal" vacuum reading for it.
Is there something other than the vacuum reading that makes you think there's a problem?
I guess my two primary concerns are the rough/chugging cold starts and the fact that the brake pedals seems hard, presumably because of low vacuum to the booster. Additionally, when the air cleaner is on and the car is in gear with brake applied it can come pretty close to dying, but maybe this can be fixed with a curb idle adjustment?
I've posted a video of the warmup of the engine on cold start that I did this morning before work. As I review it, I kind of wonder if my choke isn't coming off soon enough. But it's still kind of rough/chugging even after it starts to open or when I hold open with my finger. Comments appreciated.
Your carb could be pig rich, that will affect the vacuum signal some. Add in a possible larger cam, that will also affect it, potentially a lot. I could only get 17" of vacuum with a 204/214 cam.
Maybe so. Exhaust does seem a little vapor heavy and can leave damp spots behind exhaust pipes when run in place for a while. Carb A/F screws are only turned 1/2 turn out from bottom to recommended starting specs according to manual. I suppose something could be malfunctioning. I haven't opened it up at all, so I don't know what jet size it has either.
I went back and found my notes from my phone conversation with the seller and he did put in a bigger cam, but did not have specs.
Thanks for all the ideas so far. I've spent a lot of time searching and reading this site for the last few months, and have made progress with this problem but just couldn't quite get it over the hump. Was trying not to pester with too many questions, but I'm still hoping to get this one on the road before winter comes.
I'm going to say after hearing the video, your cam is causing your lower vacuum readings, the needle I would consider steady with nothing to worry about. I think you can get your vacuum a bit higher by double checking your timing, a/f mixture screws, making sure your vacuum advance is limited to 10ish* and that its connected to manifold vacuum. Don't worry about the moisture coming from the exhaust before it warms up.
You can ask all the questions you want as that's what the site is all about.
I think you can get your vacuum a bit higher by double checking your timing, a/f mixture screws, making sure your vacuum advance is limited to 10ish* and that its connected to manifold vacuum.
I agree with this.
When I had the initial timing set to 10º and advance canister connected to ported vacuum, the engine pulled ~14-15" vacuum and it was very bouncy, and the power brakes barely worked, same as moranj2 described.
After resetting the initial to ~18º and connecting the vacuum canister to manifold vacuum, the idle smoothed out immensely, the vacuum increased to a steady 16", and the power brakes functioned properly.
I'm going to say after hearing the video, your cam is causing your lower vacuum readings, the needle I would consider steady with nothing to worry about. I think you can get your vacuum a bit higher by double checking your timing, a/f mixture screws, making sure your vacuum advance is limited to 10ish* and that its connected to manifold vacuum. Don't worry about the moisture coming from the exhaust before it warms up.
You can ask all the questions you want as that's what the site is all about.
OK, this sounds like a good place to start. And just so I am clear on the concept, when you say limit the vacuum advance to ~10 degrees, do you mean limit the total advance supplied by the distributor or just the amount supplied at idle RPMs?
I made a limiter using a small bushing similar to the pic below. The advance canister originally gave 24º advance and the bushing cut that down to 16º.
I later made one out of a small piece of metal similar to the pic below and it is adjustable, so I now have it set to 10º.
Ok, so I popped the distributor cap and it looks like I already have a limiter of some kind in there. I assume the 18 stamped in the metal over the gap means it allows for 18 degrees of vacuum advance? So theoretically finding a way to close off half of that gap should then only allow for about 9 degrees of vacuum advance?
You don't have a limiter on there, and that vac advance should be adjustable for rate. This is the stop that MSD sells for that distributor. https://www.summitracing.com/parts/msd-84281
You might want to try the adjustable PCV valve made by M E Wagner - I can attest that it does give engine vacuum a boost, I'm running one on my modified 350. A bit pricey at approx. $129 but it's a well made set-and-forget piece.
You don't have a limiter on there, and that vac advance should be adjustable for rate. This is the stop that MSD sells for that distributor. https://www.summitracing.com/parts/msd-84281
You might want to try the adjustable PCV valve made by M E Wagner - I can attest that it does give engine vacuum a boost, I'm running one on my modified 350. A bit pricey at approx. $129 but it's a well made set-and-forget piece.
I have a stock 1971 350 with 204/214 cam as only internal change. My vacuum is 18 in. hg at 750 rpm idle with 19* initial advance plus 12* from manifold vacuum.
Listening to your cold start video it doesn't sound like the engine is in "fast idle" initially. On cold start, the choke flap should just be barely open as yours appear to be and should idle at about 1400 rpm. After about 30 seconds a blip of the throttle will drop the idle lower. Quadrajets have two fast idle steps (approx 1400 and 1000). I don't know about the Holley.
Tell us what the idle speed is at cold start and when warm.
Yes, I know his carb is not a QJet. But wouldn't his pre-Holley/Demon have a fast idle feature when cold? My only experience is with a QJ and modern Street Demon, both have a fast idle when cold and both are adjustable to the extent that there could be no fast idle if not adjusted correctly.
Listening to your cold start video it doesn't sound like the engine is in "fast idle" initially. On cold start, the choke flap should just be barely open as yours appear to be and should idle at about 1400 rpm. After about 30 seconds a blip of the throttle will drop the idle lower. Quadrajets have two fast idle steps (approx 1400 and 1000). I don't know about the Holley.
Tell us what the idle speed is at cold start and when warm.
John
I found a lot of the starting issue was that I had the choke set well too rich. It was not coming off soon enough. I leaned it out and that seems to have helped. I will have to check the RPMs. I'm usually under the hood when I start it, so I haven't noticed specifically where the fast idle is currently set. The carb has two steps down off the fast idle when warming up.
I got my timing tapes and vacuum limiter yesterday. Installed both. I double checked my initial timing by unplugging the vacuum line from the distributor. It was about 18* advanced. Plugged the vacuum line into the manifold vacuum port on the carb and started it again. Timing now reads about 30* advanced at idle. As Fun71 mentioned, the idle definitely seemed smoother. There seemed to be a small improvement in vacuum, but not a lot. I haven't fiddled with the A/F screws yet because it was late and I didn't want to drive the neighbors nuts (this thing came with flowmasters).
Now, do I leave the timing set as is, or do I set it back down to about 18* advanced WITH the distributor plugged into the manifold vacuum?
On a side note, when I removed the cap on the manifold vacuum port on the carb, a little fuel dribbled out of the cap. Normal?
You leave it where it is. You have 18 initial+12 vac. So doing the math theoretically, the initial of 18 + 21 mechanical that's advertised built into the distributor gives you a total advance of 38. Then you add your 12 vacuum your total advance with vacuum is 50.
Since you have a tape you can check how much mechanical is built in. Disconnect and plug the vacuum line, rev the engine until the mechanical advance stops advancing. Note the amount and at what rpm this happens, rev all the way to 4000 to make sure its not adding any more. Then subtract your 18 degree setting from what you measured and that's how much mechanical advance is built into your distributor.
I have a stock 1971 350 with 204/214 cam as only internal change. My vacuum is 18 in. hg at 750 rpm idle with 19* initial advance plus 12* from manifold vacuum.
I realize this is an old thread, but maybe someone can help me. I "also" have a Stock 1971 350 Olds with a 204/214 cam. Did you install the cam straight up, OR 4 degrees advanced or retarded ??? mine starts fine but has 15 inches vacuum timing at 13 degrees to initial. when trying to "drive it", it acts like the back 2 barrels are not even opening ! manually opening the carb under the hood, They are indeed operating, but it's like the engine is not drawing enough air to open the vacuum secondaries ! thanks for any help
You didn't mention what carb and distributor you are running in your 71? Also what brand timing set? Some timing sets are way out to lunch as far as accuracy. The Qjet opening I controlled by the secondary air door setting and the point distributor has a different timing curve than the HEI. You want a lot of part throttle timing with 8 to 1 compression. I would also suggest manifold vacuum for the vacuum advance.
"You didn't mention what carb and distributor you are running in your 71? Also what brand timing set? Some timing sets are way out to lunch as far as accuracy. The Qjet opening I controlled by the secondary air door setting and the point distributor has a different timing curve than the HEI. You want a lot of part throttle timing with 8 to 1 compression. I would also suggest manifold vacuum for the vacuum advance."
Cloyes timing set (3 keyway) Edelbrock carb (acted the same with the Q-jet), new HEI distributor,18 degrees initial, it is using manifold vacuum. everything was fine till the 204/214- .447/.478 cam installation.( I've installed a few cams over the years, all brands into all makes.)
I did not Degree this one. I hope I don't have to open it up again.
Which Cloyes set? I know one member found the now discontinued Street roller set a couple of degrees retarded. My Street roller set degreed fine but the chain fit loose from day one, glad they don't make them anymore. Check for vacuum leaks and make sure that crappy Edelbrock carb is in tune. It is probably pig rich, which will drop your vacuum reading. Too many threads on here with poor running Edelbrock carbs. The regular Performer carb's secondary opening is not easily adjusted. I would bump the timing another two to four degrees. Eliminate all these 3 things, it will be time to check your cam timing. Unfortunately no 8 to 1 350 will be a power house. I also ran that cam in a 76 stock 350, advanced 2 degrees on the Cloyes set but not degreed, 140 to 142 psi cranking compression, ran low 15's with 2004R and a 3.42 gear. Also in a 8.5 to 1 Olds 350 with 307 5A heads in a truck, ran great. I just used a stock Cloyes replacement set dot to dot and had around 150 psi cranking compression. I did not degree it either but would never install another cam without degreeing. My last cam was within a half degree of where it supposed to be.
Last edited by olds 307 and 403; July 17th, 2020 at 06:40 AM.
July 25th, 2018, 11:14 AM
