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Hi
I need some help in sorting some timing issues on my 1975 350CSB carb with MSD and HEI distributor.
Previous owner claims there is a more aggressive cam and high comp pistons mounted.
Now to the issue.
The engine seems to run best with at least 19-20 degrees idle timing (no vacuum). Going lower the engine starts yo spike and going down to below 10 degrees i horrible.
But looking at the total timing it passes way over 40 almost upp towards 50 degrees on higher rpms.
I then thought someone fiddled with the distributor and found the mounted one grey spring and one black spring. I do not have the original springs anywhere and therefore i ordered a mr. Gasket 929G advance kit and will probably mount the silver springs to get a mor narrower span.
But i guess i should stay with the stock 139 weights?
Correct me if I'm wrong here but a milder cam would need moore loose/lighter springs and lighter weights but my more aggresive cam would need more heavier/stiffer springs and heavier weights?
Hopefully the heavy springs in the mr.gasket 929g will get me on track. I guessing i need 20 degree idle and then added 15 degrees mech up to 35 degree total.
To set the correct curve you also need the bushings. Set the timing to ~36 degrees then use the bushing to set the range and the springs to control the rate of the timing coming in. Typically go for timing being all in about 2500rpm.
If you are getting 30 degrees of mechanical advance from the dist, that is not the fault of the springs. Are you sure you aren't seeing vacuum advance, too? You want your mech advance all-in at 3000 rpm, 20-22 for a total, as you say, of 35-36, adjusting springs accordingly to set the all-in RPM. This is without vacuum advance.
I'd suggest you email (not PM) @lars for his timing papers and go from there. He explains it much better than I ever could.
Ok, but i guess i can use the bushings in the 929G advance curve kit i ordered? I thought the springs also limited the range but good to know that the bushings is the important part here.
Because as you say, about 34-36 degree of total is the aim regardless of cam and in my case i needto get the range more narrowed in mech addon.
If you are getting 30 degrees of mechanical advance from the dist, that is not the fault of the springs. Are you sure you aren't seeing vacuum advance, too? You want your mech advance all-in at 3000 rpm, 20-22 for a total, as you say, of 35-36, adjusting springs accordingly to set the all-in RPM. This is without vacuum advance.
I'd suggest you email (not PM) @lars for his timing papers and go from there. He explains it much better than I ever could.
The vacuum hose is unplugged from the distributor and plugged on the engine side. Input on dizzy jusy left open...so no possibility for any vacuum advance there.
But you're right that 30 degrees mech seems high... 😯
19-20° initial timing is too high IMHO. Either your balancer has slipped and is showing the wrong initial ignition timing or timing chain is stretched or the camshaft has been incorrectly degreed. Best thing to do here is to find TDC on Cyl. #1 and verify the timing pointer is at 0 ° on the balancer. If timing is correct at the balancer , then your cam is not positioned properly with respect to the crankshaft and requires lots of initial timing to compensate for a "lazy" cam. Also, looking at your photo, your vac. advance has very little advance, but this can be addressed after you sort this out. Another possibility is: Dist. is getting Mech. advance at idle (springs are too light)
19-20° initial timing is too high IMHO. Either your balancer has slipped and is showing the wrong initial ignition timing or timing chain is stretched or the camshaft has been incorrectly degreed. Best thing to do here is to find TDC on Cyl. #1 and verify the timing pointer is at 0 ° on the balancer. If timing is correct at the balancer , then your cam is not positioned properly with respect to the crankshaft and requires lots of initial timing to compensate for a "lazy" cam. Also, looking at your photo, your vac. advance has very little advance, but this can be addressed after you sort this out.
TDC is verified at 0 degree marker.
I agree that 19-20 degree initial would be to high for a standard cam but i thought the more aggressive cams (as mounted) required higher initial timing to run smoothly and that this was common?
So your proposal would be to reposition the mounted cam differently then and stay with the bushings and springs as is mounted in the distributor? So it's not a springthing...
TDC is verified at 0 degree marker.
I agree that 19-20 degree initial would be to high for a standard cam but i thought the more aggressive cams (as mounted) required higher initial timing to run smoothly and that this was common?
So your proposal would be to reposition the mounted cam differently then and stay with the bushings and springs as is mounted in the distributor? So it's not a springthing...
Yes but not 19-20° more like 10-12° initial timing. Well now that we know the balancer hasn't slipped, next we need to know that the dist. rotor is pointed at Cyl.#1. to do this take the dist cap off and look at the rotor tip. there should be a notch in the dist. base that indicates where cyl.#1 is located or you can trace the spark plug lead from spark plug #1 back to the Dist. cap. The rotor should be lined up exactly on this notch or the #1 dist. terminal. This is most likely where your problem lies. Let me know the outcome and if we have to take it further we will.
From: At my Bar drinking and wrenching in Lafayette Colorado
An aggressive cam will take initial timing in the 18-20 degree range - that's perfectly normal, and it should be set that way. Adding vacuum advance (limited to 12 degrees) will put timing at idle of about 30 degrees. However, to do that, the centrifugal advance curve must be shortened in order to limit the total to 36. On an HEI, this typically involves welding up the limit slot and grinding it to length to achieve the desired total timing - the bushing in the 928G kit is for non-HEI distributors.
An aggressive cam will take initial timing in the 18-20 degree range - that's perfectly normal, and it should be set that way. Adding vacuum advance (limited to 12 degrees) will put timing at idle of about 30 degrees. However, to do that, the centrifugal advance curve must be shortened in order to limit the total to 36. On an HEI, this typically involves welding up the limit slot and grinding it to length to achieve the desired total timing - the bushing in the 928G kit is for non-HEI distributors.
Lars
Hi Lars
Thanks for your response.
Good news that 18-20 initial is ok for aggressive cams, then it was as I expected.
So it seems i need more work than only bushings and springs. It was not the 928G kit i ordered but the 929G for HEI. https://www.holley.com/products/igni...nts/parts/929G
My local store had the 929G or the moroso 72300 in stock i could choose from.
But it sounds like MSD with Accel HEI which i haveis not the best suited setup for my Cam then?
Welding and grinding seems difficult to get it right.
Bigger cams move the torque curve and efficiency up to a higher RPM range. The resulting lower efficiency at below 3000 RPM can be helped with more initial ignition advance in that range.
The springs look a little stiff from here. Need to soften the springs to get full centrifugal advance in by around 3000 RPM.
You have an adjustable vacuum advance limiter plate installed so that might come in handy but wait to play with it until after the centrifugal curve is set up right first.
Bigger cams move the torque curve and efficiency up to a higher RPM range. The resulting lower efficiency at below 3000 RPM can be helped with more initial ignition advance in that range.
The springs look a little stiff from here. Need to soften the springs to get full centrifugal advance in by around 3000 RPM.
You have an adjustable vacuum advance limiter plate installed so that might come in handy but wait to play with it until after the centrifugal curve is set up right first.
Thanks for your response. I'm a bit confused here but your suggestion is to go for the mosten soften springs I can get? I thought it was the opposite to actually limit the mechanical advance as much as possible and therefore have the most stiff / heavy springs?
The thing is I have too much advance and i dont want it nor before or after 3000rpm. 😁
The springs only control the rate of advance. They have nothing to do with the total amount of advance built into the centrifugal advance mechanism.
Generally you want the springs softer so the centrifugal advance starts to begin moving somewhere around 1000 RPM. If the springs are too soft, the centrifugal advance could begin advancing below idle RPM and that would make the idle unsteady. The springs still need to be soft enough to allow the centrifugal advance to be all the way advanced at or before 3000 RPM. You can mix and match different springs to get it where you want it. The curve should start at around 1000-1200 RPM and be all done by 2600-3000 RPM.
Keep this in mind - the goal is to keep the sum of the initial timing setting plus all the centrifugal advance available in the distributor mechanism equal to 36 degrees on a stock head SBC. If you want more initial, you need to reduce the available centrifugal advance in the mechanism. Must maintain the total of 36 degrees here.
This will make wide open throttle and idle work well.
Once the above has been done, the vacuum advance is connected to a manifold vacuum source.
The vacuum can needs to be fully pulled in (advanced) at idle vacuum minus about 2" so it will keep the idle steady. Example: If you have 13" of vacuum at idle, you want a can that will be fully advanced by about 11" of vacuum. Limit the vacuum advance travel to about 12-16 degrees max.
Total of INITIAL plus ALL AVAILABLE CENTRIFUGAL plus ALL VACUUM ADVANCE not to exceed 52 degrees.
Thanks for your response. I'm a bit confused here but your suggestion is to go for the mosten soften springs I can get? I thought it was the opposite to actually limit the mechanical advance as much as possible and therefore have the most stiff / heavy springs?
The thing is I have too much advance and i dont want it nor before or after 3000rpm. 😁
@stingr69 covered it, and @lars has already replied (seriously, email him for his timing papers, PM me if you can't find his email on the Forum, they cover all of this).
The springs basically set the RPM at which maximum mechanical advance occurs. You want this to occur at or just before 3000 rpm.
The bushings in the weights set the total mechanical advance. If you are getting 30 degrees of mechanical advance, that's too much. Stock HEI is about 21 degrees. Your weights are opening too much, and need stops to keep them in check. Perhaps 16 degrees of total mechanical advance and 20 degrees of initial is correct for your engine. The bushings should let you set that.
Both the mechanical advance and the vacunm advance movements can be welded and ground back to achieve the amount of movement you need. Minus a welder, I would try JB weld to do the job.
to grind it back use a dremal or small rat tail file.
having a higher duration cam requires some tweaking to the stock timing set up to get it to both idle and run well.
next up Is the carburetor. That may need some tweaking as well.
Both the mechanical advance and the vacunm advance movements can be welded and ground back to achieve the amount of movement you need. Minus a welder, I would try JB weld to do the job.
to grind it back use a dremal or small rat tail file.
having a higher duration cam requires some tweaking to the stock timing set up to get it to both idle and run well.
next up Is the carburetor. That may need some tweaking as well.
Yes, I started to dismantle the dizzy weights/springs and planned to both weld and change bushings on both your and @lars recommendations.
But it seems as previous owner already been there with a weld. If I limit it even more it will be more or less stiff 😏
Added pic below on current setup.
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by Valle
If I limit it even more it will be more or less stiff ��
No, you still have lots of advance there. See my timing paper on how to figure out how much you need to weld it up. Here is the basic extract on the process:
If your advance curve is too long, you will need to shorten the advance stop slot by welding a dab of metal into the end of the slot and grinding it to suit your needs. Here’s how:
Either in your engine or on a distributor machine, determine how long your centrifugal advance curve is: Easiest way to do this is to install a heavy set of springs to assure that the advance is at “0” at your idle speed and then removing the springs to observe the max total advance. Let’s say that initial timing with the springs installed is 10 degrees, and removing the springs puts the timing at 35. Your centrifugal advance curve is 25 degrees long. As an example, let’s say you want to run 20 degrees of initial timing with 35 total: You want your curve to be limited to 15 degrees.
Here’s how to calculate exactly how much to shorten the slot:
Using a set of calipers, measure the travel distance of the advance stop pin/bushing in the slot. This will be the distance from the side of bushing or pin to the contact surface in the slot. Let’s say this distance is 0.20”.
Now we know that 25 degrees = 0.20”. How do we find what is equal to 1 degree? 7th grade math tells us to divide both sides of the equation by 25:
25/25 = 0.20/25
Dividing this out we get that 1 degree = 0.008”
Since we want to shorten our centrifugal advance from 25 to 15 degrees, we want to shorten it by 10 degrees:
10 x 0.008 = 0.08”. We want the stroke to be shorter by .080”.
Measure the total actual length of the slot without the pin in it and subtract 0.08”. This is the new target length of the slot. Weld a dab into the slot’s end, and use a Swiss file or a ball end grinder on a high speed to shape the slot to the new calculated length.
Are you accepting mail-order distributor work again?
Originally Posted by lars
No, you still have lots of advance there. See my timing paper on how to figure out how much you need to weld it up. Here is the basic extract on the process:
If your advance curve is too long, you will need to shorten the advance stop slot by welding a dab of metal into the end of the slot and grinding it to suit your needs. Here’s how:
Either in your engine or on a distributor machine, determine how long your centrifugal advance curve is: Easiest way to do this is to install a heavy set of springs to assure that the advance is at “0” at your idle speed and then removing the springs to observe the max total advance. Let’s say that initial timing with the springs installed is 10 degrees, and removing the springs puts the timing at 35. Your centrifugal advance curve is 25 degrees long. As an example, let’s say you want to run 20 degrees of initial timing with 35 total: You want your curve to be limited to 15 degrees.
Here’s how to calculate exactly how much to shorten the slot:
Using a set of calipers, measure the travel distance of the advance stop pin/bushing in the slot. This will be the distance from the side of bushing or pin to the contact surface in the slot. Let’s say this distance is 0.20”.
Now we know that 25 degrees = 0.20”. How do we find what is equal to 1 degree? 7th grade math tells us to divide both sides of the equation by 25:
25/25 = 0.20/25
Dividing this out we get that 1 degree = 0.008”
Since we want to shorten our centrifugal advance from 25 to 15 degrees, we want to shorten it by 10 degrees:
10 x 0.008 = 0.08”. We want the stroke to be shorter by .080”.
Measure the total actual length of the slot without the pin in it and subtract 0.08”. This is the new target length of the slot. Weld a dab into the slot’s end, and use a Swiss file or a ball end grinder on a high speed to shape the slot to the new calculated length.
Aggressive Cam and ignition timing
