1979 XS1100 runs rich through rev range

[LaurenceL]

Hello all,

I am new to the website. I purchased a well-used 1979 XS1100 last fall as a project/second bike to go along with my 1991 Honda CB750. The bike ran extremely rich when I purchased it. Since it had an after market exhaust and I live at about 5,500 ft, I was hoping it might just need to be re-jetted.

These were the symptoms before:

-Bike ran extremely rich. No choke was ever needed. Smoked at idle, low, and mid rpm ranges. Rough idle, slow to rev.
-Was hard to start sometimes when cold. If I couldn't get it started, I could smell gas and the plugs were wet with gas.

I went through the carbs and among other things discovered:

-3 carbs had 137 main jets, one had a 125. I took it out and installed another 137.
-I think the float bowl gaskets were installed backwards, I switched these around.
-2 of the main nozzles had the same amount of holes. 2 had a different amount.
-The bike doesn't have the "octopus" vacuum diaphragm. Left petcock fuel and vent hoses go directly to 1 and 2 carbs. right one goes to 3 and 4. The vacuum advance hose is connected from carb 2 to the left engine case.

This motorcycle has a K&N filter and after market 4 into 1 exhaust. I put everything back together this afternoon and it is still having the same problems. It smokes quite a bit, doesn't idle well, and backfires. Does anyone have any suggestions as to what else might be causing the poor running? I really love the lines of this bike and the way it sounds.

Thanks for any help in advance!

Cheers,

Laurence
1991 Honda CB750
1979 Yamaha XS110

[randy]

Did you set the float hieght? 25mm from the gasket surface is a good starting point. I hope you have a manual.

[TopCatGr58]

Re: 1979 XS1100 runs rich through rev range

Hey there Laurence,

See below quoted bolded text!! I am ASSUMING that you have a Standard since you said it doesn't have an Octopus, but you could have a Special with the Octy removed?? Please check your VIN # against the listing in the MISC Forum tech tips to truly determine your model year and style, it's important for proper suggestions!

HOWEVER, I think you have the lines connected wrong! If the fuel flows this way, then you've probably got a special because it uses gravity petcocks, the Standard uses VACUUM controlled and wouldn't flow IF the vacuum lines were not connected to the Synch ports of the intake boots!

Only 1 fuel line from each of the petcocks goes to each pair of carbs, the other petcock fitting can either be capped off since it's the PRIME port vs. the normal RUN Port!

Next, the LOWER carb inlets are the fuel inlets. The UPPER inlets are VENTS, and need to be routed to a "Y" fitting and then to a simple FUEL or AIR filter to keep bugs out, but needs to VENT to AIR! This is probably why it's running so rich! The PO didn't realize this when he switched from the AIR BOX that has the vent fittings, to the POD filters!!!

Let us know how it behaves once you get this sorted out!
T.C.

Originally posted by LaurenceL
Hello all,

I am new to the website. I purchased a well-used 1979 XS1100

The bike doesn't have the "octopus" vacuum diaphragm. Left petcock fuel and vent hoses go directly to 1 and 2 carbs. right one goes to 3 and 4. The vacuum advance hose is connected from carb 2 to the left engine case.

Thanks for any help in advance!

Cheers,

Laurence
1991 Honda CB750
1979 Yamaha XS110

[TADracer]

If everything is hooked up correctly then I would go after your float heights first like Randy says. But check to see if the jets have 137 or 137.5 stanped on them. The factory Mikuni jets should be 137.5. If your jets have 137 on them then the PO might have used K&N carb kits. If that was what happened and the PO used the pilot jets that came with the kits then I can tell you that you most likely have the wrong pilot jets. While you have the bowls off remove the pilot jets (carefully). The correct factory jets will have 42 stamped on them and SIX HOLES. The K&N pilots may have 42 on them but if they have 8 holes they are the wrong jets and flow way too much. I chased this condition around for a week until I put the stock jets back in and Voila!

BTW, your main nozzles (or emulsion tubes) should be different between the two inner and outter carbs with the idea that the mixture should be different since the 2 and 3 cylinders do not get as much cooling as the 1 and 4. Also, after you get your rich condition sorted out you may need to jet up one on the main for the K&N filter and the 4-1 pipes.

[LaurenceL]

Thank you all for the responses. There are some very knowledgeable people on this board.

To clarify:

- Float heights are set to 25mm. These carbs are well-worn. A couple of the floats are dented and one of the posts that holds the float pin on one carb is broken off. The soldering on them looks to be intact and I don't see any holes in them.

- I have been using a Clymer's manual which may have caused the confusion about the "octopus". The pictures in the manual show this "octopus" and don't distinguish between a standard and special. My apologies for the confusion. I checked my VIN and it is indeed a 1979 XS1100F, not a special.

- regarding the fuel/vent lines, the bike has a K&N filter in the original airbox, not individual pod filters. I've got a vent line going from the left petcock to the #2 engine intake boot, a fuel line going from the left petcock to the lower nipple between 1 and 2, and an air line going from the air box to the upper nipple between 1 and 2. The right side is identical. Also, there is a vacuum line from the body of #2 to the motor.

- the main jets were definitely 137 and not a half number. I have not checked the number of holes in the pilots but they are size 45. For full disclosure, the air pilot jets are size 210 and the pilot screws are set at one turn out. The jet needle is set at the second notch from the top of the needle (one up from the middle notch.)

- While checking the VIN last night, I also noticed the #1 header was pretty loose, at the motor and at the collector. Also, there is a leak at the collector, between the headers and muffer.

Any ideas of what else to check if none of the above are obvious culprits?

Thank you again for the continued suggestions.

Laurence
1991 Honda CB750
1979 XS1100

[DiverRay]

Laurence,
I would suspect the jets!!! IF you can get them easily, try and get the MIKUNI JETS for the carbs. Four of the pilot jets, and four 137.5 mains should do the trick. A lot of people have put in the K&N carb kits, and replaced the jets when they did. The pilots run to about 3K RPMs, with the mains after that. The K&N jets are NOT sized the same as the stock jets, even though they have the same number stamped on them. Almost eveyone on this board, including me, run ONLY Mikuni jets, and have little problems with that.
My "Daily Ride" is a '79 F with almost 100K, K&N in the airbox with 8 1/2" holes drilled in the bottom middle, and a Kerker header on it. IIRC I am running 137.5 mains, stock pilots, middle position on the needle, and two turns out on the A/F screw. This is at sea level, so you may only be out 1 at elevation.

[randy]

What DiverRay said is on the money. Go with the Mikuni jets. I run a MAC 4-into-2 exhaust with 140 main jets, standard pilot jets, the floats set slightly rich at 24mm, Pilot screws 2 turns out and a UNI filter in the stock airbox (similar to K&N). Mine bike is very happy with this combination.

[TopCatGr58]

Hey Again Laurence,

Okay, now that you know your model/year, it will be helpful to put it in your signature or profile info(Edit profile) so that you won't have to state it in future requests/posts, and folks can easily see what you have!

Well, after seeing that you DO indeed have the fuel/vacuum hoses routed properly, Ray is most likely onto the other main possibility for your overly rich running.

However, you'll want to fix your float post, you can use JB WELD, or some of that 2 part stuff that comes in a roll that you cut and kneed together to activate...magnasteel or such, and you can mold/make a replacement post to hold the other end of the pin securely. You don't want the floats to be titled.

Secondly, you will want to take the floats out, and place in almost boiling water, and submerge them to check for pinholes/bubbles, and you can hold it there with a strainer/slotted spoon, etc., and the heat may also help to expand the dents!!

However, if they are leaking, then you can try to repair with solder, or you can get replacements from MikesXS, the plastic ones, the metal ones will NOT fit! You'll need to get a complete set of 4 to be able to set the float heights evenly IF you have to get the plastic ones!

And you'll definitely need to fix your exhaust before trying to tune. Good luck and keep at it, and keep us posted!
T.C.

[planedick]

And finally

After you get those carbs sorted out the secret to a great running machine is the synchronizing of those carbs.

[skids]

If you have a loose head pipe, it will make it run screwey. Check for leaks around the head manifold boots.