Tuning-Timing Questions.

[gareth]

I learnt a lot about my bike on our high-mileage weekend just past and am now in a full-on tinkering jag.

If necessary, I was hoping to add some measure of vacuum advance to the timing on my special which is running ZRX keihin carbs. Last night I went for a fast 5mile burn on the bike to check that the fuel starvation problem that I was having was fixed and it looks good now. No bogging down like before but it still runs hot.
When I pulled over and shut the bike down the engine just felt hot, the metal creaking etc and the exhaust system was very hot all the way along its length.

I'm a bit of a newbie at this tuning stuff so bear with me.

Could the stock special exhaust be just too restrictive for a carb-set up that includes high-flowing pods and more modern carbs? Can this result in a hotter running engine? I blew out a EGT port bolt and an exhaust repair patch this weekend after a hard run.

With the vacuum advance connected at idle on the stock bike , the backplate is set so that the pointer lines up with the F, or 5 degrees of advance. I couldn't find a vacuum line restrictor small enough to make this happen on my carbs. They just pulled too much vacuum at idle and just above. Also, with the vacuum disconnected, the backplate had to be rotated almost all the way clockwise to get the pointer on the T. Is this right?

I have very little leeway if all I want to do is pull that pointer with vacuum just the 5 degrees that the manual suggests. The minimum that I could manage using the tiniest restrictors that I had was 15 degrees of advance at idle. Could I just tap a line at the rear of the carbs in one of the pod boots to pull vacuum from someplace with a lower static pressure?
The reason I am asking is still to do with the hot running engine. Can the extra few degrees of vacuum advance at WOT make a difference in how hot the engine runs?

Before someone asks, I have not done a throttle chop yet at WOT. Its hard to find a place to open the bike up in NYC and still feel safe at the side of the road pulling plugs. The plugs looked good when I got back home, but they might have just colored up a bit from the low speed driving home. I will take a kit on the road this weekend and find someplace to do a series of runs. I was hoping that with unrestricted fuel to the carbs that running lean could be taken out of the picture. But if someone could comment on the questions above it would help me fill out the picture somewhat further. Other riders using these carbs have run the 140 mains that I am using with good results. If anyone on the list is running these carbs I would welcome their comments. I might just fit a set of mains that are a couple of sizes larger this weekend and see if the plugs darken and the bike cools.

Any idea if the symptoms could be relieved by re-oiling the pods? They are Mikes XS k&n clones, and were new prior to this weekend. Just wondering..do these pods actually come pre-oiled from MikesXS?

Lots of questions. The slow economy is leaving me with too much bike tinkering time....

I feel if I can get the bike to cool down a bit then everything will be fine. It runs great, Chugs along all day at 50mph, just hot at WOT.

Regards,

Gareth.

[CatatonicBug]

Maybe this is just my lack of experience, but isn't it NORMAL for the engine and all parts thereof to be HOT after running? I never manage WOT at any point, as I don't have any place that I could go that fast in. I've never gone over 6k RPMs. Every time I park the bike, especially after a longer run, the engine creaks and pops, with the metal contracting, and I would burn my hand if I touched the fins of the engine. The exhaust is normally hot enough to cause blisters If I touched it too. I figure that's how it works on an air-cooled engine, isn't it?

[gareth]

You have a point.. I need to post actual temps.
Warmed up, Running well, Getting Up There, Creaking Hot, Sueing MacDonalds Coffee Hot...

I have a cheap laser thermometer somewhere. I'll pull it out and do some tests. Otherwise the posting reads like Goldilocks with her three bowls of porridge and a few grumpy bears...

G.

[CatatonicBug]

the posting reads like Goldilocks with her three bowls of porridge and a few grumpy bears...

Love it! ROTFLOL!!

[randy]

Gareth,

I think your problem lies in the way the Krikey carbs deliver vacuum. On the stock Mikuni carbs, the vacuum nipple on #2 supplies PORTED vacuum. At idle, maximum vacuum is available on that nipple. (always a good thing, but I digress) When the throttle is cracked open the vacuum signal drops causing the timing to RETARD. Is your timing plate operating that way? Easy to check, just watch it as you crack the throttle. If it's not, you may be running full advance all the time, which would tend to heat things up. If the timing IS retarding under load, it's possible that it's not retarding enough, also would heat things up.

For the same reasons above, you don't want to use the manifold boot nipple for a vacuum signal, it's not ported.

BTW, the timing should never be retarded enough to coincide with the "T" mark, only the "F" mark. The "T" is for Cyl 1 T.D.C. and static cam timing, the "F" for spark firing.

[gareth]

Dear Randy,

I has seen the term "ported vacuum" mentioned a few times. Your post got me to do some searching and I found this link;

http://www.lbfun.com/warehouse/tech_..._explained.pdf

It looks like I need to do a bit of homework if I am going to get this right. And even then there is no guarantee that it will make any difference with the Keihin-deass carbs. I might just pop the old carbs back in for a bit to plot the vacuum pull at different RPMs. I think this was suggested by TC when I first started messing with alternative carburettors.

I'm going to set the idle pointer to the F and just mess with the jetting for a bit.

Small steps..

Thanks for the input. I actually thought that "ported vacuum" was the vacuum at the narrowest part of the carb. Got that wrong..

G.

[randy]

I think 'ported vacuum' is the feeling you get when you drink too much port

[CatatonicBug]

I think 'ported vacuum' is the feeling you get when you drink too much port

I thought it was a Dustbuster powered via USB!

[cywelchjr]

Gareth,

I think your problem lies in the way the Krikey carbs deliver vacuum. On the stock Mikuni carbs, the vacuum nipple on #2 supplies PORTED vacuum. At idle, maximum vacuum is available on that nipple. (always a good thing, but I digress) When the throttle is cracked open the vacuum signal drops causing the timing to RETARD. Is your timing plate operating that way? Easy to check, just watch it as you crack the throttle. If it's not, you may be running full advance all the time, which would tend to heat things up. If the timing IS retarding under load, it's possible that it's not retarding enough, also would heat things up.

For the same reasons above, you don't want to use the manifold boot nipple for a vacuum signal, it's not ported.

BTW, the timing should never be retarded enough to coincide with the "T" mark, only the "F" mark. The "T" is for Cyl 1 T.D.C. and static cam timing, the "F" for spark firing.

I was under the impression that the vacuum port on carbs provided no vacuum at idle (at least on all my cars it was like this) and then once off the idle circuit provided vacuum based on manifold vacuum. This would mean no vacuum advance at idle but varying advance above idle depending on manifold vacuum level.

[randy]

Maybe this will help explain it better:

[gareth]

That's a great .jpg, Randy. It also explains why my over-advanced ignition behaved really well at cruising speeds last weekend and why pulling that same 50plus degrees of advance at WOT wasn't such a good idea.
So, it seems that all I need to do is switch off the vacuum at idle and make sure that it doesn't pull more than 16 degrees extra or so at maximum vacuum. The "off" switch at idle is essential then to get the "ported vacuum". The stock xs switches it with the butterfly throttle across a tiny hole in the carb body. I can probably do it with a small toggle vacuum switch actuated by some part of the throttle mechanisms on the outside of the carbs..
It was making me crazy trying to get the pointer on the F, while pulling too much unported vacuum advance at idle.

I now see the error of my ways.

Good info.

Good stuff.

G.

[randy]

Umm.. one more thing. When setting timing to the 'F' mark, vacuum should be disconnected and plugged for the proper setting.

[randy]

That's a great article, published in Cycle '78. I'll give you a copy sometime. Good reading and lots of tech jargon. Here's my favorite part: