Mechanical Advance problem
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Mechanical Advance problem
Hello, I've just built a 431 stroker engine with a Comp 268h cam, S code intake, Edelbrock 1405 (which is a 600cfm, I wonder if this is also too small) stock rebuilt Duraspark distributor. I got it running and broke in and took it for a ride and it started to pop and sputter at higher rpms, usually at cruising speed. I checked the weights and they were 16l. So I took some out of another Duraspark I had, which had 13l in it. I checked the mechanical advance using a timing light, vacuum plugged. I have my initial set at 12 and it doesn't go any higher than that when I rev the motor up. I have the weights inside set at 13l which gives me 26 mechanical plus the 12 = 38. Any ideas on what's going on here? Are my springs too strong? I am able to advance the shaft with my fingers but it doesn't seem like it's throwing the weights out. Any help would be appreciated.
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Yes as far as I know the timing light is good, I timed the 351 with it and a Volkswagen 1.8 and they never had timing issues afterwards. I was already thinking the carb might be too small but when I watched for the advance through the rpm range there was none. It stayed at 12.
Post Fiend
Joined: Apr 2008
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From: Big Sky Country
I know the stock Duraspark springs are pretty stiff but you should see some advance.
If you can rotate the mechanism by hand it should also rotate under centrifugal force.
I would try the electric drill idea just so I could rule out the dizzy and focus elsewhere if need be.
If you can rotate the mechanism by hand it should also rotate under centrifugal force.
I would try the electric drill idea just so I could rule out the dizzy and focus elsewhere if need be.
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Alright, I will take another look at it tomorrow. I have a few other Durasparks laying around I can rob for parts if I would need to. If I remember correctly before I put this dizzy in I could hear the weights opening by spinning the gear driven shaft with my fingers. But those were the ones that came with it. It still did the same popping sputtering thing with those. But I'll look at what you suggested tomorrow.
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I was almost convinced it was all carb related but I wanted to check everything else before I pulled the trigger on a larger one. And then this mechanical advance issue came about. I figured I was just running out of fuel at higher rpm. I'll still end up switching out carbs.
Not to switch gears, but would a 650cfm be sufficient for my application or would I need a 750? I don't know much about cfm, I used the calculator summit has on their site, but I know I don't want to get something too big either.
Not to switch gears, but would a 650cfm be sufficient for my application or would I need a 750? I don't know much about cfm, I used the calculator summit has on their site, but I know I don't want to get something too big either.
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Laughing Gas
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Deffinatly check the advance. You could hook up your timing light to another engine to make sure it is working correctly, and if it is showing advance on your other engine, then you know its dist. related.
I'm running a 650 mighty demon (or trying to but thats another story) on the 390 i built for a sled puller. But its not stroked, and only running a 230 degree cam with a real word max up the track RPM being no more than 5k RPM. I was affraid of the same thing, going too large and having it oversized and not want to pull up the R's when its coming off the line.
Just when you use the calculators be realistic on what RPM you will be in and where your going to peak. And i would use a few different calculators and average what they say to pick your carb.
I have run a 1506 i beleive is the part # edle (600cfm) on this same engine, it worked fine but when i put the demon on even with stock exhaust manifolds i could feel and hear a difference of how much better it ran.
Edit: i would try to find a different intake if you are going for a higher RPM. If i remeber correctly the s code falls off pretty early in stock form.
I'm running a 650 mighty demon (or trying to but thats another story) on the 390 i built for a sled puller. But its not stroked, and only running a 230 degree cam with a real word max up the track RPM being no more than 5k RPM. I was affraid of the same thing, going too large and having it oversized and not want to pull up the R's when its coming off the line.
Just when you use the calculators be realistic on what RPM you will be in and where your going to peak. And i would use a few different calculators and average what they say to pick your carb.
I have run a 1506 i beleive is the part # edle (600cfm) on this same engine, it worked fine but when i put the demon on even with stock exhaust manifolds i could feel and hear a difference of how much better it ran.
Edit: i would try to find a different intake if you are going for a higher RPM. If i remeber correctly the s code falls off pretty early in stock form.
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Post Fiend
Joined: Apr 2008
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From: Big Sky Country
I'd take some red nail polish and make a "witness mark" on the 13L reluctor arm and distributor body, you should be able to see about a 1/2" of movement if it advances.
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Well, I found some smaller springs and put them in last night and checking it with a timing light, the advance seems to start to kick in around 2000 rpms and only jumps up to 20 degrees advanced, I took it up to 3000 rpms and it made it to 30, but isn't it supposed to reach total at 2500? Should I get a smaller spring set yet?
Post Fiend
Joined: Apr 2008
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From: Big Sky Country
In a big heavy truck i'd recommend delaying total advance to 3,000 rpm, i have a similar setup to you... F250 4x4 FE (mine's a 390 though) S code intake, 600 Eddy carb, comp cams 268H, long tube headers, duraspark dizzy, 9.5 CR, etc. etc.
I experimented with the Mr. Gasket dizzy re-curve kit, part #925D, but i found the springs to be way too light, i was "all in" around 2,500 rpms, so i went back to my stock springs, i'm currently using the 10L slot, running 18 degrees initial, 20 degrees mechanical, for 38 total, all in at 3,000 rpms.
My curve looks like so....
800 rpm 18* (idle)
1,000 rpm 18*
1,500 rpm 26*
2,000 rpm 30*
2,500 rpm 34*
3,000 rpm 38* (all in)
Hopefully you can get yours squared away soon.
I experimented with the Mr. Gasket dizzy re-curve kit, part #925D, but i found the springs to be way too light, i was "all in" around 2,500 rpms, so i went back to my stock springs, i'm currently using the 10L slot, running 18 degrees initial, 20 degrees mechanical, for 38 total, all in at 3,000 rpms.
My curve looks like so....
800 rpm 18* (idle)
1,000 rpm 18*
1,500 rpm 26*
2,000 rpm 30*
2,500 rpm 34*
3,000 rpm 38* (all in)
Hopefully you can get yours squared away soon.
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That's darn near exact what I'm running. I've got it stroked to more cubes but that's about it. I'll mess around with recurving it the way you mentioned and see what it does.
I do, however, think I found my backfiring problem, the old horse shoe clip was loose on my new coil and I believe it was losing spark when I got up to cruising speed.
I do, however, think I found my backfiring problem, the old horse shoe clip was loose on my new coil and I believe it was losing spark when I got up to cruising speed.
Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
That's why I figured i'd mention it, our truck/engine combos are very similar...I was using the 13L slot for the longest time, 12 initial, 26 mechanical, 38 total.....
But i'm always tinkering and just recently re-curved using the 10L slot, 18 initial, 20 mechanical, 38 total...the truck has much better off-idle response with this curve, so far i'm pretty pleased with it.
What's your gearing/tire size? i'm running 4.10's and 35's
But i'm always tinkering and just recently re-curved using the 10L slot, 18 initial, 20 mechanical, 38 total...the truck has much better off-idle response with this curve, so far i'm pretty pleased with it.
What's your gearing/tire size? i'm running 4.10's and 35's