I finally took the carb off to install a smaller power (4.5) valve since I am running 9 inch vacuum at idle with mid tier cam and current power valve in the car is 6.5. While doing is I looked at the transition slot and picture one is when choke is fully open and picture number two is when choke is fully closed. I am currently running 11 degrees base timing at 650rpm and when in gear, rpm drops to 500. Is the transition slot opened too much? Fast idle cam runs between 1500 at start and climbs to 2000 in like a minute. Any thoughts?

 

I'm a vacuum secondary guy, so there may be some vagaries having to do with double pumpers that I am not aware of, so take this for what it's worth, but I'm going to go out on a limb here and say that you are showing way to much transfer slot there. Typically, the amount of transfer slot that should be showing is as long as it is wide, so just barely peeking out from under the throttle blade. Then, you adjust the idle speed with the secondaries. There is a little screw that is accessible from underneath on the passenger side of the carb that will open or close the secondaries to let in more or less air, and you use this to set your idle speed, while leaving the transfer slots properly set on the primaries.

Now, like I said, I don't know diddly about double pumpers, so the procedure may be completely different where they are concerned. If so, I'm sure someone who does know will come along and tell me what a jerk I am. You've been a member for a while now, I'm sure you know that we have some very good carb people on the forum. If you're lucky, Lars will come along, and he has probably forgotten more than I will ever know about Holley carbs. Good luck!

Scotty

 

First thought is, you only have 9 inches vacuum at idle, so you have a fair bit of cam. I'm thinking with that much cam your base timing should be more like 18 - 20 degrees. Which would allow you to turn that idle screw down a fair bit!
"Most carb troubles are ignition troubles and the other way around"!
Sounds like you need to recurve your distributor.

 

mine is not a double pumper. It’s 670 street avenger vacuum secondary. I will try to use secondary to adjust idle as you suggested before drilling 3/32 holes in all 4. Do you set the transfer slot gap with idle screw with choke fully opened or closed, I figure opened since idle is at normal operating temp. With the cam I m running, even with the transfer slot this much exposed, I am idling 650 at park and 500 in gear.

 

Yes, with the cam your running, you are not running enough initial timing. Should be up around 18 degrees. Then your idle speed will jump right up and you can close those throttle blades!

 

is my cam considered mild or aggressive?

int/exh - .485/.485
ad duration - 280
.050 - 232
lobe - 108

i currently have msd street fire that came with medium spring with 22 degrees max advance at 4500 rpm. If I ran 18, I ll be at 40 total. Should I set it to 13 to 14 base? Maybe try lighter spring that comes all in at 3000 but have 24 degrees max advance?

 

9 inches of vacuum? Fairly aggressive. However if you add some timing in it it might go up.What is your Compression ratio with that cam? If low compression this is also an issue.
Have you heard of re-curving a distributor? you need to limit that total amount of mechanical advance so you can run 18 initial, 36 all up and all in by 3,000. Then around 12 - 15 degrees of vacuum advance that is all in 2 inches before your idle vacuum number.
Lots of information on here about setting ignition timing.
If you had a stock cam you would have around 18 inches of vacuum. I have a bit of cam in my car, perhaps a bit much I sometimes think. 14.5 inches of vacuum at 800 RPM. I run 18 degrees initial, 36 all in at 2,900. and a vacuum can that tips around 7 inches and is all in by 12 inches. about 15 crank degrees on the vacuum can connected to manifold vacuum.
My throttle blades are nearly closed at idle. I ran a 670 Holley street advenger carb before switching to EFI.
I seriously don't think it's your carb if you are running idle vacuum that low you have a lot of cam for your engine and its needs more base timing.

 

Way too much transition slot. Stock to mild cam specs. You will actually gain vacuum if you close that slot down to a .040 hole and tune the speed and vacuum with the idle set screws. Dont use the throttle set screw. IF you cant get the idle high enough which should be 750 rpm off choke, then bump up the timing. I usually had good luck at 14* to 16* base timing that got me to all in at 3000 rpm. IF any one reading and learning this doesnt know those terms find Lars papers on timing here on the forum or contact him directly for the papers. More timing will liven up your performance dramatically the low single number stock tiiming numbers are for factory emissions and arent what you want.

That cam should be giving you 13 to 18 inches of vacuum in my experience

 

Lars says 38* @3000 rpm should be your goal. I dont think 40 is bad if your car likes it and you arent pinging. IF you are getting your max at 4500 thats way too late thou
I would set up your timing, Then readjust the carb

 

The transfer slot exposure is set without any choke engaged because you want a good idle setting when at operating temperature. To get the engine started and operating when cold, the choke settings "over-ride" the hot idle settings until no longer needed.

Get Lars' Holley adjustment paper so you can set the secondaries too. You are looking to have the same light gap around the plates as you see in the properly set primaries. (You can't adjust the secondaries via the transfer slot exposure.) These primary/secondary settings should get the carb ready for the engine and start-up.

Keep in mind that timing affects the carb settings. Set the carb adjustments as outlined in the replies you've gotten then start the engine. If needed, adjust the idle speed screw as a temporary adjustment in order to set the base timing and keep the engine running. As you advance the timing, the idle rpm (and vacuum) rises, so you can return the idle screw to the same place it was. Once you have your timing where you want it, and the idle screw is returned to where it should be, adjust the mixture screws. Then only very minor adjustment of the idle screw – as in 1/4 turn – may be necessary to get the correct rpm. It's all a back and forth process.

There are those who swear you shouldn't touch the idle screw after you set it according to the transfer slot but each engine and carb pairing is different so tweaks here and there are expected and necessary. What you don't want to do is attempt to force the carb to get an improperly timed engine running.

 

Your transition slots are open WAY TOO MUCH. The hole should appear as a square rather than a long rectangle.

If you cannot get it to idle with a "square" slot, Holley says to drill the throttle plates for the additional air.

 

Get a good vacuum gauge, like a Bosch, not some harbor freight junk. Hook it up to manifold vacuum port and set timing to produce maximum vacuum and lock it down. Now you can adjust your carb the same way. I turned the primary and secondary screws out 1-1/2 turns, to start, and then adjusted the secondary screws to smooth the idle out. I ended up with 20 in of vacuum, and running a Comp Cam CS 268 with a new Holley double pump and no choke. Idle is 650 in neutral, and 550 in drive.

 

this msd street fire came with medium silver advance springs. On the instruction, it gives out 22 degrees of advance at 4000. I got spring and bushing kit coming where it will be all in at 3000. Once lighter spring is installed I will set base timing around 14 degrees btdc and go from there to reach 38 degrees Lars suggested. Thanks again.

 

since I don’t have stock or mild cam, I may have to do little modification on the current 670. I heard these carbs were built for most of NA engines. It came with 6.5 power valves and 65 main jets. I am running pretty aggressive cam and engine is far from stock 350. I have drilled 1/16 holes in all 4 plates and will try to get it tuned this weekend.

 

i don't understand that cam grind.108 is a very tight LSA. you don't need that tight a sep angle unless you have a lot more duration than that cam has. causes unnecessary overlap. kind of like comp thumpr series.

 

That cam should be about 13-14 inches of vacuum at idle. You need to find out why it isn't.....
This why I do not do HEI distributors for hot camshafts.....they pull 21-22 degrees which is great for stock mild build but big cams want more initial.....my own 406 with 244 degrees @ .050 is at 20 degrees base timing......for 11 inches of vacuum......9 is not enough.

Do this...set the base to 18 degrees.....then set idle/air screws and idle stop......how is it now? Did the vacuum go way up? If it did.....then you need to curve the HEI which is a giant PITA or get a Davis DUI pre curved or MSD ready to run.......

Jebby

 

I suspect previous owner did not know how to set timing correctly. I asked him why is vacuum advanced was not connected and he said distributor was locked out. When I open the cap, it was not locked out. I checked the timing last week and it is set at 10.5 base and only showed 24 degrees at 3K. Checked the advance spring and they were silver medium spring which gives 22 degrees advance at 4K. Right now only way to get 36 all in 3K is ported vacuum advance or rev the engine to 4K and it shows about 33. I have ordered MSD 8428 kit which has lighter spring, bushings and weights that brings 24 degrees at 3K. After spring comes in, I will try adding more on the base timing to around 14 and see how much more vacuum i can pull after adjusting idle and mixture screw. I may invest a little and get the Davis DUI if all else fails. Thank you.

 

Add more initial to get 14*? As Jebby advised, add to get 18* initial and adjust mech'l to get total of 36* (iron heads) at 3000 rpm. Then, you can tweak from there. Your cam is very similar to the Comp Cams Magnum 280 which I had. I was able to get about 14" of vacuum at 900 rpm idle and 18* initial timing.

 

Bumping to 18 is strictly a test…..he will end up with over 40 total….but the idea here is to see how it performs at idle….even low speed running….then figure out what to do to limit the total….

Somebody needs to make an HEI kit with different weights….that pull different amounts….they would sell out!

Jebby

 

I just bought a programmable MSD box and locked out my original HEI which has about 100K miles on it. Man, those GM HEI's are very well built!