Your Timing Advancer May Be Screwing You

Seriously. The one on my 750 twin has never been quite right. When new, if you set it to the "F" mark at idle, it's a little slow at high RPM. Over time, it got worse. The problem is, the timing advance has to track the design curve pretty close from about 1500 RPM to 4000 RPM to run right. On mine, if I set it right for high RPM, the idle was about 10 degrees too fast and it was hard to start, idled rough, and kicked back a lot.

If I set the idle on, the high RPM timing was about 10 deg slow and the engine was balky and sluggish.

The culprit is the SPRINGS in the advancer. The ones on mine are very stiff, made from .035" diameter wire. To work right, the advancer must have a little "pre load" spring tension to hold the timing fully retarded at idle and up to about 1500 RPM. Above that, centrifugal force starts to pull the weights out against the springs until it is fully advanced at 4k RPM.

The hard springs stretch over time until they are loose (no preload tension) and the weights can move about 1/3 of the way out before reaching spring tension. That means at idle, the weights were already about 10 degrees advanced which eats up 1/3 of the total 30 degrees advance available.

Bottom line, I ended up having to make new springs. The final results are it advances a bit faster than the FSM design specs, but runs WAY better than it did. Much better throttle response between 1K and 4K RPM, feels like it has more power.

Pretty amazing, and it's basically free. You may want to check your spark advance against the FSM specs. If your advancer is a little aged it is probably dogging out a little, there may be some easy to grab power improvement there.

Nice info. How did you make the springs, and how did you calculate the rate, etc? It sure would be nice if the hardware store had them...lol

OKC_Kent wrote:

Nice info. How did you make the springs, and how did you calculate the rate, etc? It sure would be nice if the hardware store had them...lol

Finding the springs is the hard part. I measured the wire diameter, coil diameter and immediately found out I couldn't get those. Mine have .035 wire and the coil diameter is 7/32" (0.22") and it has about ten coils of wire in the spring itself.

I found some springs at Orchard Supply that were made of .035 wire but the coil diameter was 0.25" (a little too wide). But, they fit into the unit so I tried them.

The spings were more than twice as long as needed, which was good because that gives me two tries per spring. I used the pre formed loop end to go on the advancer's slotted post, but had to make the other end to go into the hole in the weight.

That is really the hardest part: straightening out a couple of loops of the wire and then bending it to get the right angle hook to fit in the weight without deforming the spring too much. 035 wire is really hard to bend.

The other thing is, you want just a bit of pre load so you need to get the finished length just right so it has just a bit of stretch installed. I straightened the end out, set it in the advancer, and used a sharpie to make a small mark where I though the bend should start and got it pretty close.

I got them pretty good. The wider coil diameter will mean the spring constant is a bit lower. I checked the timing advance and it was good down low: did not move until about 1500 RPM then started up. I got the full advance by about 3200 RPM, it's supposed to be at 3900 which means the lighter spring constant was letting it unwind faster. But, it runs really well and doesn't ping so I am leaving it alone.

I made another set with stronger spring tension and it ran worse, because it was not letting it advance until about 2000 so it was balky.

In tweaking, you can make the spring stronger (higher spring rate) by reducing the number of coils. You can increase pre load by bending the loop so it stretches the spring a little more when you install it. No preload and your timing will start advancing too soon. Matching the curve means you have to get both just right.

you should be looking at metric spring suppliers i would think. im not sure but maybe mcmaster carr has em

I scoured the net and found a couple of places with a wide selection of springs, but they typically have minimum order requirements and/or only sell to businesses.


www.centuryspring.com/


www.leespring.com/browse_catalog.asp?springType=E



www.mcmaster.com/#extension-springs/=2ovx0t

Bringing up this old topic because I was asked how to make advancer springs for a 750 twin. This thread has all the info I used to make the set that I am running in mine right now and they are working really well. If your timing weights are "loose" and don't have any tension on them so they can wiggle free, your bike's timing is getting screwed up by that.

I had an advancer that was worn ( loose springs )

The loose or worn springs let bike advance fully at a much lower rpm.

Do to this, the bike was unpredictable and impossible to tune. The bike ran hot and would idle poorly.

If you were to look and feel the advancer, it seemed fine? but noticed the weights didn't have a snap-back when I opened them and let go.

I purchased a new one and it was like night and day. The bike idled great, ran much better and I finally was able to find a correct tune.

I chased this problem for a year, via carbs, timing and other things before finding it.

My timing advancer screwed me bigtime !

4TheKZ1000 wrote: I had an advancer that was worn ( loose springs )

The loose or worn springs let bike advance fully at a much lower rpm.

Do to this, the bike was unpredictable and impossible to tune. The bike ran hot and would idle poorly.

If you were to look and feel the advancer, it seemed fine? but noticed the weights didn't have a snap-back when I opened them and let go.

I purchased a new one and it was like night and day. The bike idled great, ran much better and I finally was able to find a correct tune.

I chased this problem for a year, via carbs, timing and other things before finding it.

My timing advancer screwed me bigtime !

EXACTLY what happened to me. hard to start, wouldn't idle, and sometimes random backfired at idle which scared the crap outtta me.

Getting a new advancer is great, I couldn't find one and the only used ones I saw were ridiculously expensive.

Runs great with the new springs. Actually accelerates with less throttle opening than before, feels quicker.

thanks bountyhunter for posting this it was a huge help . I alreay attempted to fix my current springs by tightening them up and that backfired on me . I started it and it backfired like crazy . Will try your new spring method , thanks !

This might be a dumb question but what exactly happens when the advancer opens . What is its purpose and why do some people weld there advancer in a single postion ? alot of questions but this is interesting .

Schmeck wrote: This might be a dumb question but what exactly happens when the advancer opens . What is its purpose and why do some people weld there advancer in a single postion ? alot of questions but this is interesting .

The advancer advances the timing which gives the engine the response it needs to keep up with added fuel and rpm.

If you weld or lockout the timing it all comes in at once. Race cars(some street cars also) and bikes can benefit from having all the timing(initial and centrifugal) come in at once. They just stab the throttle and go all out.