Adjusting Ignition Advance

[D0wn5h1ft]

Problem: Running into stumbling at 2500 rpms

Setup: I have a 79'F with 1980 carbs that have been freshly rebuilt (new gaskets, butterfly seals, the works), stock jetting besides 117.5 on the mains and they work great. I have pod filters (without the problematic lip) and 4-2 straight pipes. Yes, they have been vacuum synced and idle mixture screws are set properly.

- - - - - - - - -

The problem is with my ignition timing not carb related (100% sure of that). Due to the pod filters, I'm not getting enough advance in the mid (2500-3000) rpm range because they don't provide enough restriction for the "factory" advance curve. I've checked my timing with a timing light and I originally set it according to the manual ("F" at 1k RPMs and 36 degrees at 5k RPMs) so my vacuum advance unit is functioning properly and is not leaking.

As a band-aid fix I have advanced my timing plate by 5 degrees. This provides a noticeable improvement in the RPM range I'm having trouble with. Issue is, in order to make it go completely away I'd have to advance the WHOLE timing by more than I'm comfortable with doing. (Too much advance and you get pinging which blows up the motor).

What I really need is a way to adjust the aggressiveness of the vacuum advance. I know other bikes/cars have an adjustable vacuum advance which allows you to "tune" the advance curve to where you need it. In short, I have to reach max advance faster and I don't know how to do that.

Possible Ideas:
I have read about swapping out the mechanical advance springs to achieve a greater advance rate. The problem is, I wouldn't even know where to begin as far as finding replacement mechanical advance springs. I know I could measure the spring rates of the stock ones and find some lighter ones but I'm not sure how feasible that is.

http://www.setyourtiming.com/Timing_Settings.html

Another thing I could try is drilling out the vacuum advance hole on the carb body to make it less restrictive. I do have an extra #2 carb body in case I mess it up but I'd really not like to drill out my freshly rebuilt carbs. I supposed if I overdid it I could put in a restrictor which would allow me to "tune" it by swapping new ones out.

What I'm going to try after work is putting hose line clamps on the vacuum advance port and the carb body port to make sure I'm getting a good seal. I'll also replace the vacuum line just in case it has any leaks I can't see.

Any ideas?

[IanDMacDonald]

WHATEVER MODS YOU'RE THINKING OF, STOP!! This is one area of the bike that requires a little more knowledge. Scott (PO) shortened the slots on my advance to attain a more aggressive curve. The weights on the advance were also lightened.

My advice to you would be to buy a '78 advance unit, and timing plate. The units among the years differed by springs slot length, etc . Steve had a great write-up on it. With the aggressive timing curve was a slightly bigger pilot jet, with a stock 137.5 main. I currebtly have my pod locked at full advance.

[D0wn5h1ft]

WHATEVER MODS YOU'RE THINKING OF, STOP!! This is one area of the bike that requires a little more knowledge. Scott (PO) shortened the slots on my advance to attain a more aggressive curve. The weights on the advance were also lightened.

My advice to you would be to buy a '78 advance unit, and timing plate. The units among the years differed by springs slot length, etc . Steve had a great write-up on it. With the aggressive timing curve was a slightly bigger pilot jet, with a stock 137.5 main. I currebtly have my pod locked at full advance.

How do you not have pinging at low RPMs at full advance? I assume you're saying that you have a 36-ish degree advance at all RPMs????

[D0wn5h1ft]

Also, to clarify, when I said 5 degrees at 1k RPMs and 36 degrees at 5k RPMs that was with the vacuum line disconnected. I have to recheck what the max advance is at 5k RPMs with the vacuum advance connected.|

Found the thread, too:

http://www.xs11.com/forum/showthread.php?t=37189

[motoman]

Another thing I could try is drilling out the vacuum advance hole on the carb body to make it less restrictive. I do have an extra #2 carb body in case I mess it up but I'd really not like to drill out my freshly rebuilt carbs. I supposed if I overdid it I could put in a restrictor which would allow me to "tune" it by swapping new ones out.

Vacuum tube port on carb already has a built-in restrictor

[D0wn5h1ft]

Another thing I could try is drilling out the vacuum advance hole on the carb body to make it less restrictive. I do have an extra #2 carb body in case I mess it up but I'd really not like to drill out my freshly rebuilt carbs. I supposed if I overdid it I could put in a restrictor which would allow me to "tune" it by swapping new ones out.

Vacuum tube port on carb already has a built-in restrictor

Is there a way to remove it? I thought it was just the nipple then a hole that was drilled into the carb body.

[tcoop]

IIRC it's behind the butterfly so that's the restrictor. Butterfly closed Restricted flow. Too big of a tube (i.e. drilling it out) vacuum would not affect the advance as much. (I could be wrong on that)

[IanDMacDonald]

How do you not have pinging at low RPMs at full advance? I assume you're saying that you have a 36-ish degree advance at all RPMs????

There are a couple of threads pertaining to my setup actually posted by the PO. I have more than 36° advance, and less at idle. It's beyond my knowledge, all I know is that it runs much quicker than with stock centrifugal advance. The lag from the advance opening was too boring for me.

[motoman]

IIRC it's behind the butterfly so that's the restrictor. Butterfly closed Restricted flow. Too big of a tube (i.e. drilling it out) vacuum would not affect the advance as much. (I could be wrong on that)

Actually, restricter is part of brass tube. And yes, with open non-restricted vacuum, that vacuum advance will dance to the cyl. pulses big time!

[crazy steve]

First thing, your vacuum advance does nothing at WOT; it's only there to improve fuel economy at partial throttle openings, help with part-throttle response, and improve emissions. It can be adjusted (not easily), but it's rare that that's needed; if you're experiencing pinging in part-throttle-roll-on and your mechanical/static timing is good, that would be about the only time you would want to mess with this.

DON'T drill the restriction in the vacuum line; this will cause the vacuum advance to 'pulse' and create more problems. That restrictor isn't there to limit vacuum (and doesn't) but to 'smooth' the signal so the timing doesn't 'hunt'.

I'll guarantee that the stumble you're getting is due to the straight pipes and you'll find that it's impossible to 'tune away' with timing or carb changes. This is a reversion problem, and some backpressure is the fix... Look here:

http://www.nightrider.com/biketech/e...m#dyno_results

You'll note the big dip at about 3500 rpm. Yours is at a different rpm because of cylinder count, the individual runner intakes, and a 2-1 type exhaust. This is why straight pipes are a bad idea....

[D0wn5h1ft]

Did some reading and mind=blown.

I think Steve was dead on when he mentioned reversion. To share with others noticing this problem I think the following links sum it up pretty well:

http://www.circletrack.com/enginetec...ake_reversion/

http://sense.net//~blaine/pipes.html

TL;DR --> Reversion is when the pipes hit resonance with the intake cycle and make the carbs "fart" or blow spent combustion residue out of the carb (where it mixes with gas) and then gets sucked back into it (getting mixed with gas again) known as double carburetion. The result is an RPM range of usual richness.

I have a theory as to why I was noticing slight improvements to performance by increasing the advance. Most likely, the rich condition created by the reversion was getting more time to burn due to the advance. However, while the additional advance did help, it will never be enough to overcome the overly rich mixture and not adversely affect the rest of the RPM range.

I would like to say, though, that it really isn't an issue of back-pressure according to my readings. It's really an issue of two things: creating high enough exhaust velocity to overcome the reversion pulse and breaking up the reversion pulse as it comes back through the pipe. Both of these things are subsequently what baffles are designed to do. Who Knew!? Simply put, easy the exhaust pulses into the atmosphere to minimize reflection waves.

To quote the second link:
"Repeat after me, folks: BACKPRESSURE HAS NOTHING TO DO WITH ANYTHING. We are dealing with an acoustic phenomenon here, as well as pure wave mechanics. The purpose of the baffles, besides keeping the bike quiet, is to break up the reversion pulse so that it doesn't screw up your carburetion. Period."

I guess this has been more of a "oh, that's what they were talking about" experience than anything else. Straight pipes=reversion and pod filters don't help because they don't resist double carburetion as much as a restrictive airbox would. I see now why Yamaha did what they did. Baffles to beat reversion and noise. Crossover pipe to help with exhaust flow and higher RPMs. Streamlined airbox with velocity stacks to resist any carb back flow.

Weeeee! You learn all sorts of new stuff everyday. Drag Pipes=More HP but Drag Pipes=Big Hole in Your RPM Range. Thanks Steve! I'll most likely be cramming these into the exhaust soon enough:

http://www.ebay.com/itm/2-1-4-V-Slot...2cb909&vxp=mtr

[tcoop]

This is why I like this forum... I learn new stuff all the time.

[crazy steve]

Yes, technically backpressure isn't the 'fix'; but introducing whatever to break up those waves generally introduces backpressure...

Before you spend more money, you might want to try the 'washer fix' as outlined in the link I gave you to see roughly how much of a 'baffle' you need. This why not all mufflers are created equal; lack of noise doesn't always mean lost power any more than more noise always means more power. You may find that a simple baffle such as you linked to won't do the job. That dyno sheet I linked to show a hole in the power curve, but those bikes still ran decent with no stumble, just a very noticeable flat spot. If you have an actual stumble, more drastic measures may be needed... like a muffler of some sort.

[xs650man]

How do you not have pinging at low RPMs at full advance? I assume you're saying that you have a 36-ish degree advance at all RPMs????

Correct me if im wrong but retarding your ignition causes pinging ( detonation ) not advancing it

[Eveready1100]

Just another question to help complete the picture of your issues. What grade of fuel are you running in the bike??