High compression reading 78 xs1100

[markb80]

I have 2 xs11's, an 80 special and a 78 standard. The 78 has 80 carbs, rejetted to just right. The 78 was a barn bike (nice though), and was went through last winter, now a reliable rider. I noticed it seems to lope more at idle, not as smooth as the 80. So I checked compression (hadn't done that since I picked it up last year). The compression read 165 on number 3 and 170 on number 4. I thought my gauge normally read a little low. Holy crap. This was a touring bike, stock exhaust was present when bought. No signs of any customization. So I don't think it has a Wiseco kit in it. But maybe it does. I think I was getting readings of about 150 on the 80 model. I didn't get to check number 1 and 2 yet, been busy working on the house. But that just seemed a bit high. I will get the bore scope out and look at the tops of the pistons to see if they are original I guess. It pulls harder than the 80 special, and the 80 special runs perfect. Any ideas??

[TopCatGr58]

The 78 has a hotter cam than the 80 stock.

How many miles on the 78? You say it's a tourer....what kind of rpm range do you run it in? My thoughts are carbon buildup on the valves/pistons reducing the compression chamber volume.

Stock values should be 142 +/- 14 psi at sea level for new according to the manuals! A search for removal of carbon buildup should yield several good threads that I've seen posted on the subject!

T.C.

[markb80]

I just saw comments in past threads to that effect. Yes it could be carbon. I have some Yamaha combustion chamber cleaner. Darn! Just changed the oil. You have to change oil after using that stuff. It has about 21 k miles on it. It was a touring bike for 10 years, a parade bike used by the mayor of a small town for about 10 years and then a barn bike for 10 years. Someone started to fix it up, but stopped. Rides super nice, shocks and forks are tip top. Engine runs very nice also. 2 sets of 78 carbs had a dead spot at WOT during acceleration at about 5000 RPM. So I put the 80 special carbs from other bike in it, ran perfect. So I rebuilt the spare set for the 80 special and put them in the 78. Much better. I have done carbs for a guy with 2 78's and they ran great. Couldn't get my own 78 carbs to work right though. So, went to 80/81 type. Problem solved.

[SFerinTEXAS]

The 78 has 80 carbs, rejetted to just right.

What would that be?

[markb80]

I think I am at 45 on the pilots and 7.5-10 over on the mains. Would have to pull them to know for sure. If I remember right the 78/79 had bigger jets, but it was mainly due to the carburetor type not the engine. For the higher compression of the 78, you want a little bigger main jet, but not 20 jet units (whatever a jet unit is). So may not need to go as high as the 78,79 stock main jets. I am either running 125's or 130s. I have Jardine cut-old-mufflers-off-and-slip-on mufflers

[natemoen]

78 had a 137.5 for the main

[markb80]

The 80,81 had smaller main jets, but this was due to it being a different type of carburetor (I think). The nozzle design has an effect on main jet size along with other aspects. So a 120 be plenty for an 80 carb in a 78 bike. I think I am at 125. Higher compression generally means you should up the main jet also. But I don't think it's the main reason for the main jet difference.

[DiverRay]

The '78/'79 carbs used the main jet to feed the pilot jet. The '80 up carbs used the tube the pilot jet is located in to feed them. THAT is the reason for the difference in jet sizes. All said, the '78 should run fine with 117.5 mains, unless you have pod filters, or a drilled airbox with a K&N filter.
To get rid of the carbon, you can ride the bike so it's hot, pull the caps off the intakes and run four lines from the intake to a can of Seafoam. Suck up about 1/2 to 3/4 of the can, shut the bike off and let it sit for 20 minutes. While it's sitting, put the caps back on the intakes so you can fire it up and ride it hard in second gear for two minutes. That should take care of any carbon, and you DO NOT need to change oil when you are done!

[markb80]

I think it is more the nozzle design than the pilot jet effect. I say that because some 80/81 carbs had plugs on the pilot jet towers and an opening for fuel to come across after the main jet. Some 80/81 carbs did not have the passage between main jet and pilot jet tower, and so they don't get the plug. I have accidentally plugged 2 sets of 80/81 pilot towers that were the type with no passage. Yet the jetting of 80/81s is all about the same. I believe it may be the 81 carbs that get no plug on the pilot jet tower, and it may have only been the 81 specials, but not sure, as these were replacement/ebay carbs. I do know that this was the difference between the type II BS34 and the type III BS34 carburetors, which were the carbs used in 80/81. The 78/79 had different nozzles (bleed vs emulsion type) and different slides, float valves, and probably other differences too in air jets etc. I like the external thumbscrew for adjusting mixture, but overall I've found the 78/79's to be finicky on my 78. But maybe that was due to my excessively high compression and carbon buildup... who knows.

I think the pilot circuit basically doesn't flow much at wide open throttle, so the mixture setting is almost entirely defined by the needle, main jet, nozzle, and flow resistance for air.

[markb80]

I finally got time to use some Yamaha combustion chamber cleaner on the 78. Compression now reads 155-160 across all cylinders. The bike idles smoother but may need to re-synch carbs again. The 80 special reads 145 on all cylinders but the battery is not great and it doesn't spin the engine as fast as the standard model while doing test. Thanks.