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I am having some trouble with advancing my ignition timing. I replaced my old Q-jet with a Holley Street Demon 625 #1901 carb.
My Q-jet was hacked up by the previous owner and was causing me a lot of problems (flooding, fuel percolation, vacuum leaks, warped airhorn)
My new carb is a major improvement and has ported and full manifold connections for my vacuum canister (my Q-jets were missing so I could not use them).
I read an article that I should use manifold vacuum for a cooler engine and improved fuel economy, since I drive 100% on the street.
Before hooking my vacuum lines I wanted to measure my inital and total timing. So I bought a dial back timing gun to do some testing.
Measured with vacuum lines disconnected and plugged
Idle vacuum 12.5"
Initial timing 10° BTDC
Total timing 26° BTDC
Total timing of 26° seems a bit low. I read several threads that total timing should be around 34-36°. So I adjusted it to 34° total timing (double checked at 3500 RPM), 18° initial timing with vacuum disconnected and plugged.
No hard starting problems, idles more smoothly and idle vacuum is 14".
Took it out for a drive and experienced a lot of pinging at part throttle and WOT (usally when the secondaries open). I am running RON95 fuel (equavalent to 91 in US).
I retarted the timing to 30° and still pinging, then to 28° and still pinging, then 26° and no more pinging.
I hooked up the vacuum canister to full manifold vacuum and measured again.
The car drives much better with the vacuum canister hooked up to full manifold vacuum. The idle is very smooth, but I am bit worried that initial is quite high. Also there is no pinging when driving part throttle.
I don't understand that I get pinging with more than 26° of total timing and no pinging with 48° of vacuum advanced total timing.
Am I running lean on the secondaries? Should I use fuel with a higher octane rating?
Could it be that my cam is too small in combination with the 10.25:1 compression?
Should I buy an adjustable vacuum canister?
BTW I did not verifiy if #1 piston TDC aligns with the 0 mark on my timing tab.
I am also planning on buying a wideband sensor and rejetting the carb if necessary.
Specifications:
Engine is Oldsmobile 425 10.25:1 compression
Small crane camshaft (specs unknown)
Stock intake
Stock heads
Accel coil and I believe HEI distributor
Hooker headers 1 3/4" + 3" exhaust with flowmaster delta 40 mufflers
Knowing what distributor you have is very important to setting it up. Pictures work well for us. And as Joe stated, making sure your 0 timing mark lines up at TDC is also very important. You also need to run the euro98.
You are not going to be able to run a 22* vacuum advance canister, it will need to be limited with a stop. I would disconnect it for now and plug the line until you get the mechanical and initial advance sorted out. It is probably a major contributor to you pinging.
This is my GP Olds street timing spec. It should get you in the ballpark:
Check for vacuum leaks good, fuel system pressure and overall condition of the ignition system before diving into this.
12-14* Base timing at curb idle, no vac advance can, and its supply plugged off.
Adjust A/F mix to obtain highest vacuum signal possible. Take this reading from a manifold vac port or on the carb base plate below the throttle plates, at curb idle.
Hook up the advance canister. It should add in another 8-10-12* at idle, hooked up to an unported full vac signal source. If it's putting in to much or not enough timing get an adjustable travel(not rate) canister and limit it to 10* to start out. Make sure the timing plate which the canister is connected to in the distributor moves freely on both types of distributors.
Lastly is the mechanical advance...The centrifugal advance should be "all-in"(total timing) at ~ 2700- 3000 RPMs adding another 10-12-14* ish. For a total of 34-38* at 2700-3000RPMs. If it's coming in too quick the springs need to be heavier. You can mix and match the springs (heavy medium light) to achieve the desired results. make sure the cent adv is clean and working freely and the stops are functioning.
These are hypothetical numbers. Each engine wants something slightly different depending on many factors. Cam, Compression, ambient temps, fuel octane, feet above sea level, driving habits, cylinder conditions et..al... The goal is to give it as much timing as it will tolerate prior to preignition.
Read the plugs to look for lean conditions.
Only use 90+ ethanol free gas. Ethanol and anything below 90 octane aren't good for these old carbureted cars.
So the 625 is close to the edge on CFMs but according to the math, it should work. A lean AF ratio may not be the problem. Lets get the ignition timing events and vacuum signal dialed in first before analyzing which side of Stoic your engine is running.
Another consideration is heat. If the carb is absorbing too much heat in conjunction with high under hood and ambient temps; this can aid in pre-ignition. This can be alleviated with a carb to intake insulator, blocking off the intake heat riser crossover passages either in the heads or the intake and eliminating the thermostatic stove flap in the exhaust manifold.
But Im piling too much on. One step at a time starting with the timing and AF ratio.
Last edited by droldsmorland; July 3rd, 2018 at 12:35 PM.
GM HEI. Are you running a full "keyed" 12 volt source to this distributor??? You should not be using the resistive wire originally hooked to the points distributor coil.
Also verify the grounds. The neg wire from the battery should have a good clean tight connection to the engine block(drivers side head). There are also ground straps that attach between the head(s) and firewall. These need to be clean and tight.
Last edited by droldsmorland; July 3rd, 2018 at 12:39 PM.
Pull #1 spark plug. Stick a finger in the #1 hole and turn over the engine with a remote or a helper until it pops your finger out. Short bursts on the starter. Now see where the timing mark is. If it's off a little bar the engine one way or the other to line up the marks to verify TDC. (remove the distributor cap so to disabled the ignition so it won't run).
With the distributor cap removed the rotor should be pointing in the general direction of #1 cylinder.
Last edited by droldsmorland; July 3rd, 2018 at 01:35 PM.
I've seen a few different HEI conversions over the years to a remote coil. The large cap HEI distributors are usually coil in cap. How old is that distributor and the parts for the conversion? If the parts are worn and corroded, this may be contributing to some of your issues.
A 650 is too small for a big block, who cares what the math says. If a 455 didn’t need more than that you can be quite assured GM would not have bothered casting the Q-Jet with 750 cfm airflow. The Q-Jets on the later 307 and 260 engines were still based on the same castings, the secondary air door was mechanicals limited to restrict airflow. Get a 750 minimum carb on there, make sure TDC is accurate on the balancer, and see what happens
A 650 is too small for a big block, who cares what the math says. If a 455 didn’t need more than that you can be quite assured GM would not have bothered casting the Q-Jet with 750 cfm airflow. The Q-Jets on the later 307 and 260 engines were still based on the same castings, the secondary air door was mechanicals limited to restrict airflow. Get a 750 minimum carb on there, make sure TDC is accurate on the balancer, and see what happens
Those cfm calculators are totally bogus, the Street Demon 625 is too small but may only be slightly lean out of box. The 750 Street Demon is set up out of box for a larger cubic inch engine, like your 425. The Olds V8 usually have the vacuum advance in the opposite position but he obviously has the wires in the right position. Check the balancer position, weird that more timing cures pinging.
If I want to know if my carb is too small, can I check it with a vacuum gauge hooked up to manifold vacuum and run at WOT to see if the vacuum level is 0" or near 0"?
I am having some trouble with advancing my ignition timing. I replaced my old Q-jet with a Holley Street Demon 625 #1901 carb.
My Q-jet was hacked up by the previous owner and was causing me a lot of problems (flooding, fuel percolation, vacuum leaks, warped airhorn)
My new carb is a major improvement and has ported and full manifold connections for my vacuum canister (my Q-jets were missing so I could not use them).
I read an article that I should use manifold vacuum for a cooler engine and improved fuel economy, since I drive 100% on the street.
Before hooking my vacuum lines I wanted to measure my inital and total timing. So I bought a dial back timing gun to do some testing.
Measured with vacuum lines disconnected and plugged
Idle vacuum 12.5"
Initial timing 10° BTDC
Total timing 26° BTDC
Total timing of 26° seems a bit low. I read several threads that total timing should be around 34-36°. So I adjusted it to 34° total timing (double checked at 3500 RPM), 18° initial timing with vacuum disconnected and plugged.
No hard starting problems, idles more smoothly and idle vacuum is 14".
Took it out for a drive and experienced a lot of pinging at part throttle and WOT (usally when the secondaries open). I am running RON95 fuel (equavalent to 91 in US).
I retarted the timing to 30° and still pinging, then to 28° and still pinging, then 26° and no more pinging.
I hooked up the vacuum canister to full manifold vacuum and measured again.
The car drives much better with the vacuum canister hooked up to full manifold vacuum. The idle is very smooth, but I am bit worried that initial is quite high. Also there is no pinging when driving part throttle.
I don't understand that I get pinging with more than 26° of total timing and no pinging with 48° of vacuum advanced total timing.
Am I running lean on the secondaries? Should I use fuel with a higher octane rating?
Could it be that my cam is too small in combination with the 10.25:1 compression?
Should I buy an adjustable vacuum canister?
BTW I did not verifiy if #1 piston TDC aligns with the 0 mark on my timing tab.
I am also planning on buying a wideband sensor and rejetting the carb if necessary.
Specifications:
Engine is Oldsmobile 425 10.25:1 compression
Small crane camshaft (specs unknown)
Stock intake
Stock heads
Accel coil and I believe HEI distributor
Hooker headers 1 3/4" + 3" exhaust with flowmaster delta 40 mufflers
I haven't even read all the replies but you hit the nail on the head with verifying true tdc and installing a wide band gauge. Verify you heat range on your plugs as well
If I want to know if my carb is too small, can I check it with a vacuum gauge hooked up to manifold vacuum and run at WOT to see if the vacuum level is 0" or near 0"?
Yes, if your manifold vac is near 0 at WOT you are perfect
A 650 is too small for a big block, who cares what the math says. If a 455 didn’t need more than that you can be quite assured GM would not have bothered casting the Q-Jet with 750 cfm airflow. The Q-Jets on the later 307 and 260 engines were still based on the same castings, the secondary air door was mechanicals limited to restrict airflow. Get a 750 minimum carb on there, make sure TDC is accurate on the balancer, and see what happens
You guys would all be surprised on true CFM of your engine. I have operated a superflow 901 measuring true airflow, BSFC, vac readings, exhaust temps and read pressures in various areas of intake and exhaust for finding your cork in an engine. Your only assuming unless you have had done or done any of the above
i stand corrected. I have owned several Cutlass with the mighty 260, all but one car had that slug pulled out almost as soon as I got it home. The one that remained 260 powered was too nice and ran too well to mess with. In any case, I don’t recall what carb thru had? Maybe the one I thought was a Q-Jet really was just a especially weak 307?
The 625 Street Demon can produce well over 500 horsepower, and not many 455s will ever see close to that much in the real world, though you can dream and dynos can and do lie. Besides this 99% of the time you are driving around on the primaries, which are only about 200 cfms with the quadrajets or Street Demons or thermoquads. This is why they make excellent street carbs because they are not over carbing. With a conventional holley, or other typical carb you don't get those benefits, as the primaries might be much bigger losing air speed, and you start getting bad mileage and its hard to tune for part throttle. They are not meant for part throttle as much, but for wot all out racing.
Now here is one for you and we will see if anyone knows why. the L78 375hp rated (425 actual) BBC got a 780cfm Holley, but the smaller BBC 350hp or 325hp 396s got the lager CFM Quadrajets. Hmmm LOL
To richen up the Street demon just pull the primary rods. if you have a digital caliber and measure they diameter on the middle larger size from the needle tip. You can now emery cloth or sand that section down some a little at a time ( a few thousandths), just make sure you do they are the same size on the other metering rod, and see if its now rich enough on the afr meter and drives good. This is very old school and can be more precise tuning then buing different size rods. The small part at the tip is full throttle and for full throttle can be done the same. The secondary requires jet and flapper tension tweaks. Too soon and you get a bog. Too late and suddenly you feel them kick in and open and a surge of power.
In picture #1 the piston stopped after coming up on the compression stroke.
In picture #2 the piston is still at the same point, if I turn it more it will go down again.
What is my TDC mark? The moment the piston reaches TDC? Or is the delta of coming up and going down? Is my tab off by a few degrees?
TDC is when the piston comes up to the top of the compression stroke, on the #1 cylinder for this conversation. So the piston is at its max height in the #1 cylinder.
Your timing mark is on the timing tab as shown in the pics but is that the compression stroke? Did you put your finger in the spark plug hole and did it spit out your finger while cranking and land where the pics show in your post?
If yes you are on the compression stroke of #1. You can reposition the balancer to get the yellow groove to line up with 0.
Now, where is the rotor in the distributor pointing?
I think zero would be the center groove on the tab? I don't see numbers on that tab(correction see my edit post #28). If that's the compression stroke of #1 you are close enough to verify the distributor's position.
Last edited by droldsmorland; July 5th, 2018 at 02:47 PM.
It's very difficult to sense exactly when the piston is at TDC. The usual way to find TDC is to use a piston stop set at something before TDC. Turn the engine one way until the piston contacts the stop. Mark the balancer. Turn the engine the other way until the piston hits the stop and mark the balancer again. Split the difference between these two marks and that's TDC.
In picture #1 the piston stopped after coming up on the compression stroke.
In picture #2 the piston is still at the same point, if I turn it more it will go down again.
What is my TDC mark? The moment the piston reaches TDC? Or is the delta of coming up and going down? Is my tab off by a few degrees?
When you check for TDC with the heads on using a piston stop, full revolution counter clock wise till stop, mark, then full clockwise until stop. They end up in diff stops due to timing chain slack etc. The mark between the two stop lines is true TDC or as close as you'll get with the heads on and it'll work on either cyl 6 or 1
Yep, 70 agreed, as you and Joe mentioned this method will get him at absolute zero. But where it is now will get him in the ballpark with the simplified version of timing an Olds V8.
The balancer zero mark can be anywhere on that timing tab for us to know if he's on the right track. Again we need to see where the rotor is pointing now.
I do see a distributor installed 180 out. Sure it will still run but I'd prefer to see it installed correctly in order to continue to properly split the dictionary for the OP.
Once we and Robert are happy that he is on or very close to #1 tdc and the rotor is pointing towards #1 we can further assist and ascertain that the engine is assembled correctly, timing chain, distributor is installed correctly etc...
When I was adjusting the timing, I fueled up a day before. I don't know how old the fuel at the gas station is. (RON 95)
OK. I was just making sure it wasn't gas from before you did the motor from maybe years ago.If there was that was really old a gas in there and you filled it us it might be bringing the octane down.
Today is a hot day and I decided to put premium fuel in the car. I advanced the total timing up to from 26 degrees to 32 degrees and did not notice any detonation. I still have a bit regular fuel in the tank, so I am going drive around a bit and see what happens. So far I am happy with the results.
Eventually I want to advance it up to 36 degrees (if that is possible).
You may be fine with regular, once the old fuel is burned out more and is not bringing the average octane down so much. Glad it seems to have gone away.
July 3rd, 2018, 06:20 AM
. Robert let's get a look at the distributor rotor position.
