Air jets

[1980cm400t]

I'm currently doing a total rebuild on a 1980 xs1100 sg. I just got to the carbs. I plan on doing pod filters and 4 separate straight pipes. I will need re-jet the carbs i have done this before on a different bike. I am familiar with adjusting main jets, needle height, slow jets, and the pilot jets but this bike appears to have an "air pilot jet" as it is identified in the Clymer manual (part #35 in the diagram if anyone cares to look). It is a horizontal jet that is on the air intake side of the carb (all other jets are vertical). Does this jet need to be swapped out for a larger jet like the others will? I couldn't find any mention of this jet anywhere on the forum.

[DAVINCI]

Short answer is, no. BUT be wary of the internal lip on the pod's velocity stack. The lip on some pods has been known to obstruct the air flow to this jet and it can wreak havoc with tuning. IIRC, the remedy is to trim the lip so it doesn't interfere with the jet

[1980cm400t]

What is the long answer? I'm looking to get peak performance out of this thing

[DAVINCI]

AFAIK, I don't recall anyone here changing that jet, but don't take my word for it. That jet controls the amount of air available for the pilot circuit. So, changing either that jet or the (fuel) pilot jet should have the same net effect, although opposite of each other. I've only read here about main and (fuel) pilot jet changes. There is a guide here about intake / exhaust changes and suggested jet changes to get you in the ballpark performance-wise. Do a search.

Just don't expect rip-your-arms-off performance changes mucking around with jets and pods. The gains will be small. This bike can already perform well enough to throw you off.

[1980cm400t]

Thanks, I haven't actually ridden it yet I got it for free as a fixer upper see attached:
http://forums.sohc4.net/index.php?to...0921#msg850921

[motoman]

FWIW..........performed the best overall in the stock form.......but you do whatever, it's your scoot......

[natemoen]

As people on the forum that you linked to said, your exhaust idea won't really work. There does need to be a decent.amount of back pressure for these engines to work right. As to the flames from the exhaust, not gonna happen naturally. I ran mine without pipes at all for a bit and the most I ever got was a little 2-3 inch flame out the head.

[cywelchjr]

Another point, I'm not aware of the crankshafts being in any way different between the 78 and 79 engines and 80 and 81 engines although the pistons are, so the pistons being a different weight would have not bearing as long as they are all the same, it's not about the pistons matching some magic counter weight in the crankshaft, it's about them (and the connecting rods and other parts of the rotating assembly) all weighing the same so it's rotationally balanced.

[1980cm400t]

That is true it may not necessarily be the crank that is different but somewhere in the engine there is a change in mass to counterbalance the change in the weight of the pistons. On the subject of straight pipes being impossible to tune, I have read the multitude of threads indicating that it is impossible and did some research to figure out why. The physics suggest that the best performance can be obtained by having the least restriction on both the intake and exhaust side of the engine. Experience here suggests that it is impossible to tune the carbs with straight pipes. I read an article that suggested that the cause of this tuning difficulty was due to a poorly designed common intake, i.e. if you have a stock airbox straight pipes won't work. However, if you scrapped the stock airbox in favor of individual intakes (velocity stacks or pod filters) straight pipes would be tunable. Unfortunately, I don't have a copy of the article or I would attach it. Does anyone have an opinion on the whether this is likely to be true for xs1100's (I did this for my 1980cm400t and it runs well)

[1980cm400t]

I do plan on doing straight pipes but I'm going to have them go the full length of the bike. The original headers are to rusty to use so I will have to start fresh.

[DGXSER]

There are some folks who claim they have tuned it quite well with straight pipes. As to the article I do not belive it applies to the XS11 intake as much as you are asserting it would. The XS11 airbox is much less of a common plenum due to the individual velocity stacks to each carb. So I do not think there is a great difference in the POD filters from that stand point. The POD filters do offer much less restriction to airflow than the airbox does. Thats where the gains come from. And even then, POD filters will give soem top end improvement, but they rob some low end performance to get it.

As to the exhaust, I have never tried staight pipes, but I know my bike ran like crap when one of my mufflers cracked at its inlet. I also have rad on here more than a few cases of olks trying and trying to get straight pipes to run right, and eventually putting a muffler on it.

The reality of it is, that almost all of the folks who have tried both will say that these bikes give the best overall performance with the stock airbox and stock exhaust. The engine design (displacement, cam duration and lift, valve sizes and orientation,and the carb selection was made with that setup in mind. Its just impossible at best to find a good stock exhaust that does not cost more than the bike itself.

[natemoen]

If you are going to try it, look on the inside of the head pipes and you will see that the ID of the pipes and you will see they are much smaller than the OD of the pipes, they are double walled and I would suggest going with pipes the size of the ID rather than the OD.