Got a bit of a detonation problem on the new engine and I think it might be down to the ignition advance. Basically what I think is happening is the new cam runs more advance on idle than the old one did. This means the total advance when the revs are high is too much and that's causing the detonation. On the run up to the nats I backed it off by about 5 degrees and it's much better, not perfect but now it suffers a bit off idle, stumbles a bit which I don't like. I tried a tank of super unleaded but it didn't make any difference so it can't be the fuel and the plugs look fine so I don't think it's running lean at the top end. So I'm going to be altering the mechanical advance on my distributor. What I think I need to do is shave a bit of meat off these weights so they don't fly out as far: Anyone done this before? I have a pair of spare weights... Or those two bits that the springs hook into, I think I remember they were slotted. Is it a matter of shortening the slot with a thin weld and a bit of filing so it doesn't open as far?
First, get a number! Put the thing back together, clamp shaft, rotate rotor in normal rotation direction and back to stp points. Mark off a sector of degrees an a piece of cardboard with a protractor and cut a circle out of center so it can set onto the place where cap mounts; measure arc that rotor can move on advance, 2X that is your amount of centrifugal advance within the limits of slop and measuring small arcs... Find out stock total advance (static setting plus centrifugal) for both original and donor engine... Old Ford chuggers have conservative curves due to bad gas and lazy drivers overusing high gear, so I doubt you have too much total. Possibly curve is coming in too fast, at too low an rpm... Moroso doesn't seem to stock a curve kit for Ford pops, so next step might be gathering a selection of small advance springs from available hotrod kits (1960's Ford and Mopar kits here offer springs right size for Model B) and more from every scrap distributor you can find...then experiment, starting with stiffest.
I'm guessing you already checked this, but it burned me recently, so I'll bring it up. I had a bit of off-idle stumble on our 371 that everyone swore was timing, despite the fact that the timing light said everything was kosher. It turned out that I had used an NOS carb rebuild kit and the accelerator pump had disintegrated, leaving me with a lean spot right off idle. A new kit from Oreilly's and 10 minutes of fishing acclerator pump pieces out of the well and I was in business. Judging from the fact that it occured for you with the new cam, this prolly aint it, but I thought I'd bring it up. Let us know how it works, I'd like to fiddle with my advance a little bit and had thought about adding weight with solder. Just gotta check em on a digi scale to make sure they match...
Redneck I'm still using the stock carb and it doesn't have an accelerator pump. Basically with the ignition advanced between about 5-7 degrees further forward than it is now it idles faster and there's no stumble - but it detonates something chronic when the revs are high. I just backed it off another 2 or so degrees and I've got a good hill to go up on the way home. I was wondering if it was a throttle pump thing, ie with the added cam bump and or advance it needs a bit of richness straight off idle. Bruce your wealth of hints and tips is amazing. I'll measure the advance just like that. Anyone know the total advance of a B motor? My guess is mine has got to be similar?
B curve is tested out on Vince's banger site...but still a puzzle as stock springs are too stiff: http://idisk.mac.com/forever4/Public/pages/bdistributoradvance.htm Note that you can steal some of his techniques... Don't assume same huge advance number (well into the thirties) used by early bangers...I believe they have huge combustion chambers and inadequate use of quench squeeze of piston/head to generate turbulence! Note that flathead V8 tightened the chambers, used quench VERY effectively, and needs very short curve... More tools: Make good drop-on degree scale for distrib measure pulley, compute circumference and size of a degree, mark off maybe thirty degrees in chunks of 5 on pulley rim for testing advance speed and total with a timing light. You might want to steal some Model A tech for testing purposes... kill your centrifugal advance by just wrapping some stout tape around outside. Add a carburetor linkage ball to your timing sector thing on distrib, rig up hand advance...do road testing to find engine happy points for different speeds and conditions...coast to a stop on each setting you want to record, measure advance at that point. Then go study what the centrifugal is actually doing with a timing light. Note that you don't have a vac advance, and engines want more advance at light throttle openings...centrifugal is to get full throttle curve, which is then bumped up by vac can on later cars...so work on the full throttle, as that's what you are equipped for.
I agree with Bruce that you REALLY need to know where you're starting from, especially how much mechanical advance is in the distributor right now. Then you need braze up the slot that limits the total advance. Put it together and check the total advance, adjust the length of the slot with a round file. I usually use a chain saw file. Repeat until you're satisfied. I've reworked a lot of GM point style distributors this way. With this and messing with the springs you can get everything dialed in the way you need it to be. Larry T
Stuff: stiffer springs/lighter weights (drill holes...) gives slower advance. Lighter springs/heavier weights faster advance...drill holes and fill with lead, or if there's pace drill holes and screw on supplements. Total advance is determined by whatever stops weights from moving any more--most likely pins protruding through holes in weights. Discover a nice way to sleeve pins, shorten or file out weights for messing around purposes. Perhaps you can come up with a way to do this with something readily removeable or alterable. Proper advance never gets discussed in the magazines but gives more bang for free than any of the aluminum bits they want to sell you...
AND...springs do not need to match once you get to fine tuning... Springs can be short so they apply pressure from the beginning...or have long end loops so weights go flying out at the beginning and slow down as springs come into play. Many engines like quick advance right off idle, then slower rate with all in at maybe 2,000. This might involve a weak spring on one side, stiff spring with long starting loop on other...
I'm almost unable to rattle the hardware right now due to endless medical probs with wife, but I'm able to do a bit of messing and exploring with small bits I can work on in odd moments...so I've started exploring the B distributor. What I did essentially starts with what Vince shows on his site...springs on a good original B are pretty damn stiff. SO...I can't get to the junk yard, and everything there is computerized ignition anyhow...I just fired up Jeg's and Summit online and ordered all the cheap advance spring sets I found. GM and MSD ones seemed too big and stiff--used on big diameter distribs with heavy weights. The Ford and Mopar ones seemed to meet the general needs...so those plus some messing with end loops will provide the fodder for messing with getting the B to advance faster. On next order, will obtain several sets so I can destry stuff safely...now just need some TIME to myself so I can fire a B and measure. Wish they'd let me bring an engine to work...
Lol, thanks Bruce, it's all intruiging reading this and of course I'm not feeling completely out of my depth, oh no no no... However, I might have cured it. I backed it off a bit more before I left work and the detonation is gone and the stumble is no worse. What I'm going to do is put on the twin carbs hopefully at the weekend and see how it does with the more complex fuelling. I'll also switch back to regular gas because I'm pretty sure my motor shouldn't need premium. You're right, no vac advance and it does stumble on that light throttle opening. I have two spare springs in the toolbox and I noticed they are both different to the ones currently in the dizzy. I might swap out the smaller of the two for the smaller of the spare ones for a bit more advance at low speed and see if it helps. As a side note they let me bring my engines into work, they just have to be in my car
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