Topic: CB750 Spark Advancers = what are the differences

[stockscreamer]

It does feel a little flat until around 5k according to my ears (tach jumps around a bit) getting the fuel screws to their sweet spot helped from idle to 2-3k but could be better im sure. ill give it a go when my new points, condensors and exhaust gaskets arrive. how well does this modification play out with your ignition system? Im hoping my funds will allow picking one up from you by christmas time.

Thanks a bunch,
Josh

[HondaMan]

It does feel a little flat until around 5k according to my ears (tach jumps around a bit) getting the fuel screws to their sweet spot helped from idle to 2-3k but could be better im sure. ill give it a go when my new points, condensors and exhaust gaskets arrive. how well does this modification play out with your ignition system? Im hoping my funds will allow picking one up from you by christmas time.

Thanks a bunch,
Josh

The Ignition boosts the spark in the low and high range, and removes the wear factor from the points. It makes the bikes start better, too.

[stockscreamer]

Right on she always starts on the first kick, but the other 2 pros will be nice. I cant wait dang-nabbit! haha

[KRONUS0100]

i hate to drag this old post back to the surface, but i need a correct 410 part number for the 78 750F3 spark advancer.  i have just discovered that the hole in the end of the crankshaft for the loccating pin on backside of advancer is larger than the pin on the 300 advancer i have.  advancer walks back and forth.  so i am in ned of the correct 410 advacner unit for the 750f3

[HondaMan]

i hate to drag this old post back to the surface, but i need a correct 410 part number for the 78 750F3 spark advancer.  i have just discovered that the hole in the end of the crankshaft for the loccating pin on backside of advancer is larger than the pin on the 300 advancer i have.  advancer walks back and forth.  so i am in ned of the correct 410 advacner unit for the 750f3

Is that crankshaft hole oval a little bit? They get that way from pulling and pushing the big nut on the end of the crank back & forth with the sparkplugs still in the engine...if it's just a bigger (round) hole, you could shim around the dowel with some thin brass?

[KRONUS0100]

its just a bigger hole.  so i take that means that all the locate pins are same diamter on all advancers then?  shim stock it is then

[edwardmorris]

Interesting read, subscribed.

[flybox1]

Matt, I just measured the the pin on the back of the advancer (AD125-01) from my F3, and an advancer from a K1 (300) and they are both 4mm

[scottly]

I measured the pin of a K7 750 (300) at 4mm, and a 650 (426) at 3mm. I seem to recall reading that some 550's had the 3mm pin, with a 4mm hole in the end of the crank?
Rather than shim stock, see if you can find some thin-wall brass tubing with a 3mm ID and a 4mm OD at a hobby store.

[Nic]

So if the hole in the crank is oval shape like mine, can you just drill it out and fit a brass bush? What I'm wondering is, is the crank soft enough to drill, I suppose it must be if it's gone oval?

[KRONUS0100]

thanks flybox.  i ve got 3 "300" advancers so i had nothing to compare against.   hadnt thought about the brass tube for shim stock.

[HondaMan]

So if the hole in the crank is oval shape like mine, can you just drill it out and fit a brass bush? What I'm wondering is, is the crank soft enough to drill, I suppose it must be if it's gone oval?

Yep, the metals outside of the journal pins are all dead soft. The crank & rod pins are hard.

[Black 750K8]

The K0-K5 versions are covered in the Appendix G of the book, but the post-1975 stuff was removed when the book was shrunk to 268 pages (for cost reasons) and the later bikes left out (removed almost 60 pages). The details in terms of how the bikes are built are minor, but in tuning, there are some differences.

The F0/F1/K7/K8 spark advancers have 10 degrees static advance on their "F" marks, and 23.5 degrees advance angle (published), for a total of 33.5 degrees. The F2/3 bikes have the same static, but 26.5 degrees advance (published) angle, total of 36.5 degrees.

I say 'published', as I haven't measured them myself, like I did the earlier bikes. I fully suspect these figures vary by as much as 2 degrees from bike-to-bike, which was typical of the production tolerances of the day. 

The post-1975 bikes had to cope with new EPA restrictions at 2500 RPM, so their spark advancers have slower advance curves (bigger springs and/or lighter weights) than the earlier ones, which gives them a little bit of a flat feeling in the sub-4000 RPM range, compared to the earlier bikes (along with the later cam opening).

In comparison, the K0 started out with 41 degrees total advance angle, all-in by 2100 RPM when new! The K1 at least held off on the RPM, to about 2500 or so, making the launch a little less sudden. After the heat-annealing of the springs, the K0/K1 could reach full advance by 1500 RPM, causing plug fouling issues from the spitback into the carb throats. This is one reason why I [still] recommend cutting 1/2 turn off those springs, today, to slow it down a little bit. 

I wish you would make a little loose leaf folder with all you have on the 77-78 K&F I'm sure they would sell

[Nic]

So if the hole in the crank is oval shape like mine, can you just drill it out and fit a brass bush? What I'm wondering is, is the crank soft enough to drill, I suppose it must be if it's gone oval?

Yep, the metals outside of the journal pins are all dead soft. The crank & rod pins are hard.

Thanks

[700504]

Decided to revive this thread for it's great information contained within. I'm having trouble with timing, I've tried shimming the plate, and I shimmed the advance pin hole in the crank as it was ovaled as Hondaman has described. I've put a dial gauge on my advance post and its warped, I pulled three more posts from spare engines and they were all warped as well. My question is, what would be the tolerance for the run out on the advancer post? I have a junk crank that I'll try and straighten it in, but no precision tools to do the straightening itself. The bike is a 77 cb750f with a 300 advance.

[HondaMan]

Decided to revive this thread for it's great information contained within. I'm having trouble with timing, I've tried shimming the plate, and I shimmed the advance pin hole in the crank as it was ovaled as Hondaman has described. I've put a dial gauge on my advance post and its warped, I pulled three more posts from spare engines and they were all warped as well. My question is, what would be the tolerance for the run out on the advancer post? I have a junk crank that I'll try and straighten it in, but no precision tools to do the straightening itself. The bike is a 77 cb750f with a 300 advance.

I do the straightening with a dial indicator ($25 at Harbor Freight nowadays) and a magnetic base to hold it to the clutch cover (see page Appendix G-3 in the book), with a nut on the threads to prevent damage. I turn the engine over (plugs out, please...) with the kickstart and watch the dial indicator. Tapping (use a small hammer, like 6 ounce ballpeen) the shaft a little at a time, then re-testing, takes an hour or so. Make it less than 0.004" (0.1mm) for best results: I almost always can get them to 0.001". Above 0.006" (0.15mm) the 2-3 points timing 'jitters' under a timing light, and above 0.008" it becomes almost impossible to set the timing on the 2-3 side.

[700504]

Decided to revive this thread for it's great information contained within. I'm having trouble with timing, I've tried shimming the plate, and I shimmed the advance pin hole in the crank as it was ovaled as Hondaman has described. I've put a dial gauge on my advance post and its warped, I pulled three more posts from spare engines and they were all warped as well. My question is, what would be the tolerance for the run out on the advancer post? I have a junk crank that I'll try and straighten it in, but no precision tools to do the straightening itself. The bike is a 77 cb750f with a 300 advance.

I do the straightening with a dial indicator ($25 at Harbor Freight nowadays) and a magnetic base to hold it to the clutch cover (see page Appendix G-3 in the book), with a nut on the threads to prevent damage. I turn the engine over (plugs out, please...) with the kickstart and watch the dial indicator. Tapping (use a small hammer, like 6 ounce ballpeen) the shaft a little at a time, then re-testing, takes an hour or so. Make it less than 0.004" (0.1mm) for best results: I almost always can get them to 0.001". Above 0.006" (0.15mm) the 2-3 points timing 'jitters' under a timing light, and above 0.008" it becomes almost impossible to set the timing on the 2-3 side.

Thank you HM, I was hoping you'd chime in again. I was able to deduce that all my shafts that I have are bent by using the dial indicator that I have. I knew you have stated the above previously but I couldn't find the thread that I read last night. I'll be using a junk crank and an extra set of cases, so I don't risk damaging any of my running bikes. So tapping seems alright, would you also recommend putting a small length of metal tube over the shaft to gain leverage on it and bend it slowly? Or just precise tapping to straighten?

[PeWe]

I found new shaft at yamiya750.com. The o-ring too

[700504]

I found new shaft at yamiya750.com. The o-ring too

Thanks PeWe, $47.13 shipped to me, I'll try truing one of the many first.

[HondaMan]

Decided to revive this thread for it's great information contained within. I'm having trouble with timing, I've tried shimming the plate, and I shimmed the advance pin hole in the crank as it was ovaled as Hondaman has described. I've put a dial gauge on my advance post and its warped, I pulled three more posts from spare engines and they were all warped as well. My question is, what would be the tolerance for the run out on the advancer post? I have a junk crank that I'll try and straighten it in, but no precision tools to do the straightening itself. The bike is a 77 cb750f with a 300 advance.

I do the straightening with a dial indicator ($25 at Harbor Freight nowadays) and a magnetic base to hold it to the clutch cover (see page Appendix G-3 in the book), with a nut on the threads to prevent damage. I turn the engine over (plugs out, please...) with the kickstart and watch the dial indicator. Tapping (use a small hammer, like 6 ounce ballpeen) the shaft a little at a time, then re-testing, takes an hour or so. Make it less than 0.004" (0.1mm) for best results: I almost always can get them to 0.001". Above 0.006" (0.15mm) the 2-3 points timing 'jitters' under a timing light, and above 0.008" it becomes almost impossible to set the timing on the 2-3 side.

Thank you HM, I was hoping you'd chime in again. I was able to deduce that all my shafts that I have are bent by using the dial indicator that I have. I knew you have stated the above previously but I couldn't find the thread that I read last night. I'll be using a junk crank and an extra set of cases, so I don't risk damaging any of my running bikes. So tapping seems alright, would you also recommend putting a small length of metal tube over the shaft to gain leverage on it and bend it slowly? Or just precise tapping to straighten?

I've done that successfully, too! It does take a small ID and thick tube, though.

[700504]

Alrighty, here's the skinny of what I did with good results: I had a spare crank and set of cases with bearings installed. I set it up on the work bench with my dial gauge on the advancer shaft. Initial reading of indicator gave almost 0.015" run out, It was thoroughly bent. After about an hour of slight bending with my modified vice grips (vice grips with flat plate welded on either either side. I was able to get the run out just under 0.001". I had to test and bend starting at the base and moving out, I think it had been bent multiple times in numerous places on the shaft. I put it back on the bike, and both 1.4 and 2.3 F are dead under the timing light, no movement at all, I had never seen this on one of these bikes. Come to find out I tested 5 other shafts that I had laying around and all were out of tolerance. SO in summation: straightened shaft and no more movement of the F mark under timing light. Points plate is advanced as far as it will go but both 1.4 and 2.3 are perfect on the timing mark.

[Brian G]

There seems to be an awful lot of work involved here and if authenticity is the prime consideration, then it makes sense to go that way. I have no end of respect for those who have the skill and knowledge to make these worn, 35-odd year old systems run well. If, however, all you're concerned with is a smooth running motorcycle, why not go with an electronic ignition which does away with the mechanical advance mechanism - for instance

http://www.trispark.com.au/images/FireBox%20Pro%20Flyer.pdf ?

This particular unit is around $335 USD and while not cheap, it incorporates a rev limiter, laptop adjustable settings and is not particularly voltage sensitive. I use one of their ignition systems on my BSA triple and one of the steps of the installation procedure was to remove the mechanical advance mechanism entirely. Other than a quick timing light check last year, I haven't touched the timing since 2007!

Regards,

[HondaMan]

There seems to be an awful lot of work involved here and if authenticity is the prime consideration, then it makes sense to go that way. I have no end of respect for those who have the skill and knowledge to make these worn, 35-odd year old systems run well. If, however, all you're concerned with is a smooth running motorcycle, why not go with an electronic ignition which does away with the mechanical advance mechanism - for instance

http://www.trispark.com.au/images/FireBox%20Pro%20Flyer.pdf ?

This particular unit is around $335 USD and while not cheap, it incorporates a rev limiter, laptop adjustable settings and is not particularly voltage sensitive. I use one of their ignition systems on my BSA triple and one of the steps of the installation procedure was to remove the mechanical advance mechanism entirely. Other than a quick timing light check last year, I haven't touched the timing since 2007!

Regards,

Most electronic units have a very short (some are VERY short) spark duration. Part of the design of these engines is: they are swirl-charge stratified bores, and during the compression stroke the "swirl" (Honda called it a 'whirlwind') reaches nearly 700 MPH. The particular values Honda/TEC worked out for the Fours includes a 1.5+ mS spark discharge duration to ignite the widest-possible flamefront: the wider this initial arc is, the longer the burn pushes down on the piston. Since many of these engines are undersquare bores, this is important to low-mid range torque. Less spark equals less torque in the 3500-6000 RPM range, and this has been both verifiable (by riding) and dyno-shown (from time to time) to be true.

The other feature: my particular design was intended for the long-distance, low-maintenance rider. If the box should ever die for some reason, you merely unplug the 4 points-coils wires form it and plug them back together (2 minutes tops, on the side of the road) and you're back on the pavement instead of looking around for a trailer. Mine also increase cold-spark duration with post-1995 points (which are tungsten-plated instead of pure tungsten) and stops ALL arcing, so the points and advancer simply stop wearing each other out.

So far, no other all-electronic version can boast this particular set of facts. While I meant for it to stop me from bending over every 3 months to adjust my points, it quickly spread because of the performance improvement (initially) over the Dyna S triggers, which lose considerable spark duration, and even more above 5500 RPM. Their "fix" was to use higher-output coils, but those have EVEN LESS duration: with the Dyna S and their 3-ohm coils, it makes just 0.9mS spark (for oscilloscope waveforms of this stuff, search for the long thread where we re-developed this box here, showing all the details as we went). Pamco claims 120 degrees of dwell to reduce this effect: I haven't seen their waveforms though, to know how true it may be. OEM points are near 180 degrees dwell, for reference: this is preserved with my little box.

[itsahonda4]

This post timing is perfect...my 71 CB750 has what I call a hanging idol and wont completely slow down when the throttle is released......
I suspect part of the problem is that I cant get the timing marks perfect as I run out of adjustment so the ignition is retarded....my bike has a 300 advancer, with standard springs but the fact that Im out of adjustment has me wondering why that is the case....
Even if my problem is a carb problem, until I get my timing right its all academic.....
Any suggestions? How do I 'twist' the advancer as described by Hondaman to try to get more advance to get my timing marks to line up?
Al

[kmb69]

.....
Any suggestions? How do I 'twist' the advancer as described by Hondaman to try to get more advance to get my timing marks to line up?
Al

Gapping the points a little wider will advance the timing.