Starting problems

[mattfrme]

Hello all,

I have a 79 xs100sf.

Problem is that when it is hot it wont start. Actually it barely starts when it is cold. Seems I can get it to run once a day.

I have cleaned the carbs, replaced jets, diaphrams look fine. New fuel hoses, Octopus installed correctly.

Idle adjust screws are almost entirely out.

Obvoiusly it runs hot, with alot of carbon build up, but seems to be the only way to get it to start.

I have had problems with coils, but they have been moved to a better location, although closer to the top of the engine.

some specific questions:
1. Carbs rebuilt with aftermarket kit. noticed that on original 2 of the jets were smaller then on the other carbs. Replacement set all jets were the same.

2. did not replace float jet or change float height (they were all pretty much the same)

3. There was too much oil in the bike by about the filter's capicity. noticed last night, changed oil, not filter, replaced with 20w50. pulled spark plugs, soaked in gasoline, cleaned, then replaced. No obious change in start or run.

4. With the Accel coils, does it matter which plug us attached to which tower? I know 1/4 and 2/3. but what about the towers themselves?

I am rather certain this is a spark problem, but why does it come and go? The primary wires look fine, red stripped to +, grey/orange to - . voltage and impediance check out.

Any ideas?

Thanks

Matt

[buffalo]

Does not matter which tower on Accel coils... Check your ground wire from engine to frame under battery box, don't just look, take the bolts out and make sure there is not corrosion under the wire terminals. I had this same problem a while back, crank but no start when hot. It would usually start cold and run fine. What happens is starter is using all of the available current flow not leaving enough to fire the coils properly.

[ratbyk]

Start Problems

An engine that is running too rich or flooded will be hard to start when warm.
From some of your comments I would guess that the carbs are away to rich. The first clue is after market jets. We have found that the jets provided with these kits, even though they may have the same number as the jets you removed have holes in them far larger than the original Mikuni's. Time and again someone new to the Forum will come on with exactly the same complaint as you.
Pull the carbs back off and compare the physical size of the holes in the mains and pilot jets from your originals to the aftermarket kit jets. It will leave you PO!
After you have put the original jets back in, make sure that the float level is not set to high. Also the idle mixture screws should be initially set at 1 1/2 turns out starting from a gentle seat.
Lots of luck.
Ken/Sooke
78E Ratbyk
82 FT500 "lilRat"

[Jerry]

A poll was taken a while back on starting...turns out a lot of us crank the throttle wide open when starting a warm engine. Never tracked down the reason this helps, but is a very common technique amoung XS/XJ owners.

[mattfrme]

Update

Hello all,

Ok, Rebuilt the carbs with the old pilot jets. Noticed however that the hole at the end of the nipple on carb 2 and 3 is larger then the ones with the rebuild set. Used the larger originals.

Adjusted to 1.5 turns and bread tie test. New plugs.

Still massive carbon buildup after 5 min. also idled rough (much rougher then before. Lots of black smoke too.

Thinking that the smaller jets need to go back in, and the OEM need to come out.

any thoughts?

Matt

[ratbyk]

Hard Staring

Matt, sorry to hear that a jet change did not solve your problem. Your black smoke clue still indicates that the engine is getting away to much fuel. Check out the following chart and compare the jet sizes to what you have in the carbs now. Remember that the numbers on the original Mikuni carb jets are entirely different than the aftermarket jet number/sizes.

Here is the list of carburetors and associated jetting for the various years. NOTE- many bikes have been equipped with high velocity exhausts and accordingly will have larger pilot and/or main jets installed.

Models and Jetting Year

1978 E 137.5 45.0 40 140 180 135 Mikuni BS34-II
1979 F/SF 137.5 42.5 32.5 140 180 135 Mikuni BS34-II
1980 G
1981 H 115(1&4)
120(2&3) 42.5 25 140 185 135 Mikuni BS34-II
1980 SG/LG
1981 SH/LH 110(1&4)
120(2&3) 42.5 25 140 185 135 Mikuni BS34-II
1982XJ 112.5 47.5 25 140 170 ? Hitachi-BS34

If your present jets are comparable to the above I think we are down to four other possibilities.

1. Carbs flooding because the float needle and seats not sealing.
To check turn the carb rack upside down and blow into the fuel line "T" between carbs 1/2 and 3/4. The weight of the floats should seal the needle and seat. If you can blow any air (lung pressure) then these valves are not allowing the fuel to be shut off at proper fuel level causing the rich mixture.

2.Is the carb fuel enrichment (choke) actually coming off when you move the lever to the off position?

3.An engine has to breath freely in and out. On the in side, is the air filter plugged? Is there a rats nest built inside your air box? Some sort of blockage on the intake

4. Same story on the out side. Any blockage on the exhaust will back up the flow of fuel/air through the cylinders and cause an overly rich mixture.

Don't know what else to say. Keep us posted. We all would like to know the final problem/solution.
Ken/Sooke
78E Ratbyk
82 FT500 "lilRat"

[ratbyk]

Hard Stating

Matt, sorry forgot to incude the model/jet size/ number sequence.

Models and Jetting
Model Main
Jet Pilot
Jet Start
Jet Main
Air Pilot
Air Throttle

Ken/Sooke
78E Ratbyk
82 FT500 "lilRat"

[mattfrme]

Ok,

Here is an update of what I have done:

Reinstalled aftermarket jets. The ports were smaller for all the holes, I have to assume that the ones that were in there were incorrect.

Pre calibrated carb via bread tie test.

Pilot screws 1.5 turns from gentle seat.

discovered both petcocks were leaking, repaired both.

New spark plugs were immediately covered with heavy carbon build up.

Nothing in airbox, running with cover on and off.

I cant think of any other adjustments that are relevant to the fuel system.

However, I am again comeing back to a spark problem. Wouldn't the same carbonization happen with a weak spark?

when I put new NGK spark plugs in, the bike will start, run for a while, not long enough to shut the choke, and it runs hot.

I have not gotten it started since the most recent adjustments, I need new spark plugs. That happens in the AM. I will let you know what happens.

Matt

[randy]

Going back to your original thread, one of the items that stands out is:

.2. did not replace float jet or change float height (they were all pretty much the same)


At what hieght were the floats set? 25mm or 1" is pretty close to right. Even a couple of mm can make a big difference. If the floats are set too high, she will run rich.

Randy

[mattfrme]

Greetings all,

The floats were all within 1/16 of an inch (inch being the maximum)

What effect would timing have? would that cause the carbon build up and hard starts?

still trying to figure it out...

Matt

[randy]

More likely a rich mixture is your problem.

Randy

[TopCatGr58]

Hey Randy and Matt,

Yeah, I would agree that an overly rich mixture will definitely run rough, but if the timing is way off, like not advancing properly, it will run hot and poorly, poor revs, not able to rev very high or very quickly. When I had a sticking float valve on 1 carb, it was horribly rich in the low rpms, but I could crack the throttle wide open and it would eventually respond and rev up.

It might not hurt to investigate the timing, the vacuum advance unit and such, as well as the exhaust to ensure it isn't clogged up!! I had a little old 4cyl turbo Tbird and one day the power just dropped off like crazy. The catalytic converter innards had gotten broken up and turned and twisted all around inside and down the exhaust pipe severely restricting the outflow and thereby also the intake and breathing of the engine.....remember the potato in the tail pipe gag?? Good luck!
T.C.

[mattfrme]

Finally?

Hello all,

Ok, got it running and rode it. I decided that the idle was too high (when I pushed the choke all the way in, the idle reved to 4K)

when I adjusted it down to 1000 RPMs, it would die at stop signs, and seemed to loose power at the top of 2nd gear. I pulled the carbs, recleaned them, new plugs, readjusted pilots to 1.5 turns, bread tie tip, and it started. Adjusted idle to 1200, runs fine. just came back from a 25 mile ride.

It even starts after being shut off and sitting a while.

However, there is a clicking in cyl. 2. This is a new sound, after replacing the spark plugs this AM.

Question I am using NGK plugs with a suffix of 11. (the ones I used previously were - r) what is the 11 for?

Also, before I start to stress about lifter's and valve clearance, any other ideas what could be causing the clicking? any simple remedies?

Finally, with the Accel coils, do I need to disconnect the ballast?

Thanks,

Matt

[Geezer]

Re: Update

Originally posted by mattfrme
Hello all,

Ok, Rebuilt the carbs with the old pilot jets. Noticed however that the hole at the end of the nipple on carb 2 and 3 is larger then the ones with the rebuild set. Used the larger originals.

Matt

Somewhere along the way Mikuni changed the design of the primary jets. The new jests with the large hole in the end run way to rich for our old bikes.

Geezer