1978 XS650 SE will not rev

livetoride21 · Mar 25, 2016

Hi everyone, I'm currently working on a 1978 XS650 as some of you may know.
I'm having an issue with the bike, as it will idle fine, but it will not rev unless on partial choke.
I'm sensing I have a fuelling issue.
Here's what has been done / tested so far;
-Exhaust taken off - no real difference except easier start.
-Float levels checked with Clear tube, both .5mm below bowl gasket.
-Carbs fully rebuilt with new floats (checked for floatation), new float needle/seats, new pilot and main jets. new mixture screw and choke plunger.
-Diaphragms appear to be working correctly as they rise when the bike is revved with choke half on.

I'm pretty certain that the engine is starving for gas when I go to rev, as with the choke half on it will rev. I can also deduce I don't have any electrical gremlins if it will rev with choke on.
Any Ideas where could still be clogged?
I wouldn't think it would be the pilot circuit, as it idles fine, but if not please help!

TwoManyXS1Bs · Mar 25, 2016

If it idles fine, run the idle screw up a bit, and pull a plug wire. Running on one cylinder, do your choke on/off tricks to get it to rev up. Now reverse, replace the plug wire, pull the other plug wire, repeat test. Is it the same both sides?

5twins · Mar 25, 2016

Don't rule out ignition related, or in this case, ignition advance related problems. If the ignition isn't advancing as it should, the motor will be reluctant to rev up. Take a look at the advance unit and make sure the weights pivot out and return freely. Also make sure the advance rod rotates as it should and freely. Have you ever serviced those parts?

livetoride21 · Mar 26, 2016

TwoManyXS1Bs said:
If it idles fine, run the idle screw up a bit, and pull a plug wire. Running on one cylinder, do your choke on/off tricks to get it to rev up. Now reverse, replace the plug wire, pull the other plug wire, repeat test. Is it the same both sides?

It idles fine, but as soon as I run the idle screw up a bit (the same as pulling on the throttle) it starts to spit and pop and want to die. The one thing I forgot to say I had done, was I backed off the mixture screw until it was around 3.5 turns out on both sides and managed to get it to rev up a bit. (which once again leads me to believe I'm starving for fuel somehow. Any thoughts?

5twins said:
Don't rule out ignition related, or in this case, ignition advance related problems. If the ignition isn't advancing as it should, the motor will be reluctant to rev up. Take a look at the advance unit and make sure the weights pivot out and return freely. Also make sure the advance rod rotates as it should and freely. Have you ever serviced those parts?

I don't want to rule this out yet, but my thoughts are if it was ignition related, it would not rev just about perfect with the choke half on. correct me if I'm wrong ?

TwoManyXS1Bs · Mar 26, 2016

Ok, then let's try this. While it's idling, and without using the choke, gently cup a hand over a carb intake bell, just enough to add a little restriction to the incoming air. Feather/jiggle the throttle, while adjusting the cupped hand, to try to get it to rev up. Note the response. Then do the same for the other side. Note any differences between the two.

Trying to ascertain if the problem is universal/common to both carbs, or isolated to just one carb...

retiredgentleman · Mar 26, 2016

Go up one size on the pilot jets.You're not getting enough fuel/air mixture through the pilot circuit.

weekendrider · Mar 26, 2016

My money is on the two un-metered slow jet holes being clogged.
OP did you actually see fluid flowing from the pj holes in the throttle bodies?

livetoride21 · Mar 26, 2016

weekendrider said:
My money is on the two un-metered slow jet holes being clogged.
OP did you actually see fluid flowing from the pj holes in the throttle bodies?

If you're talking about these two holes

-photocredit bmwbobz

they appear to be clear. I ran a fine wire through them to make sure, as well as blowing air through them.
if not, which spot are you talking about?

also I have verified over and over that the choke circuit is clear.
any ideas?

weekendrider · Mar 26, 2016

Open the throttle plate and look at the top side.
Those holes are where the fuel enters. One will be metered with the screw. The others open. I'll get you a pic later.

5twins · Mar 26, 2016

In my pic above, note the pilot inlet labeled in yellow. That is where the fuel/air mix from the pilot jet enters the main carb body. It comes out these 4 tiny holes in the main bore. They are right around where the butterfly plate closes so you will have to flip it open to see them .....

The 3 clustered on top flow directly from the pilot jet. The one to the side comes from the pilot jet but through the mix screw. You can adjust it's flow with the screw. Remove the mix screw and it's associated parts. On your '78, that would just be a little tension spring. Dribble some carb cleaner or solvent of some sort in that inlet hole on the bottom of the carb and blow it through with compressed air. Block the mix screw hole while doing so and the cleaner should come out all 4 of the little holes in the main bore. Block the mix screw hole and it's outlet into the bore, and the cleaner should come out just the 3 holes. Block them and the mix screw hole, and the cleaner should come out only the mix screw inlet. Squirt some cleaner in the mix screw hole, block all the main bore holes, and you can back flush out through the inlet hole on the carb bottom. Switch all around like this and you can flush from several different directions.

livetoride21 · Mar 26, 2016

5twins said:
In my pic above, note the pilot inlet labeled in yellow. That is where the fuel/air mix from the pilot jet enters the main carb body. It comes out these 4 tiny holes in the main bore. They are right around where the butterfly plate closes so you will have to flip it open to see them .....

The 3 clustered on top flow directly from the pilot jet. The one to the side comes from the pilot jet but through the mix screw. You can adjust it's flow with the screw. Remove the mix screw and it's associated parts. On your '78, that would just be a little tension spring. Dribble some carb cleaner or solvent of some sort in that inlet hole on the bottom of the carb and blow it through with compressed air. Block the mix screw hole while doing so and the cleaner should come out all 4 of the little holes in the main bore. Block the mix screw hole and it's outlet into the bore, and the cleaner should come out just the 3 holes. Block them and the mix screw hole, and the cleaner should come out only the mix screw inlet. Squirt some cleaner in the mix screw hole, block all the main bore holes, and you can back flush out through the inlet hole on the carb bottom. Switch all around like this and you can flush from several different directions.

Thank-you for the explanation 5twins, I had probed through these holes before to make sure they were not blocked, but I'm going to do so again.
I'm currently soaking the carbs (fully disassembled) in lemon juice/vinegar solution and will boil them later tonight once I'm back from easter dinner.
Hopefully once that's done all the passageways will forsure clear out.
I'll report back once this is all done.

5twins · Mar 26, 2016

You really need compressed air blown through the passageways to ensure that they're clear. Just soaking usually isn't enough.

livetoride21 · Mar 26, 2016

5twins said:
You really need compressed air blown through the passageways to ensure that they're clear. Just soaking usually isn't enough.

Oh yes, I understand that. I've been blowing 120psi air through them quite frequently haha.
but at this point I'm thinking soaking definately will not hurt.
Also, I just thought of something;
If the three clustered holes were blocked, wouldn't it not idle at all when the choke is off?

5twins · Mar 26, 2016

That would depend how high you set the idle speed screw. It opens the butterfly plate more so you could actually get an idle from just that plate opening.

retiredgentleman · Mar 26, 2016

I don't think livetoride21 has told us what exhaust and air filters he is using. We don't even know if he has the stock carbs for a 1978??

livetoride21...........When you replaced the pilot jets, what size were they? Stock size being #27.5.

5twins · Mar 26, 2016

There's also a question on if the proper type (VM22/210) of pilot jet was installed. The other style, the BS30/96, will fit but it probably won't work right.

livetoride21 · Mar 27, 2016

GOT IT!
Thank you for the tips everyone. It turns out my problem was due to one of two things, #1 the pilot jets that I received from Mikes XS were slightly different than my original jets. They had many small holes around, and not one hole straight through, so I cleaned the old jets and put those in.
#2 my carbs are now fully cleaned once again.
One of these two things was causing my no REV issue, unfortunately I changed both things at the same time, but I suspect it was a small passageway somewhere in the carbs causing the issue, as I don't really see the holes in the jets making much difference.

For curiosity sake, I'm going to switch the jets one more time and see if it will rev or not.

retiredgentleman · Mar 28, 2016

livetoride21 said:
GOT IT!
Thank you for the tips everyone. It turns out my problem was due to one of two things, #1 the pilot jets that I received from Mikes XS were slightly different than my original jets. They had many small holes around, and not one hole straight through, so I cleaned the old jets and put those in.
#2 my carbs are now fully cleaned once again.
One of these two things was causing my no REV issue, unfortunately I changed both things at the same time, but I suspect it was a small passageway somewhere in the carbs causing the issue, as I don't really see the holes in the jets making much difference.

For curiosity sake, I'm going to switch the jets one more time and see if it will rev or not.

Its possible Mikesxs sent you the wrong carb rebuild kit. What is the number on the side of the Mikesxs pilot jet....................it should be 27.5.

Yes, the jets that Mikesxs sells have holes around the bottom. They are aftermarket
jets, and the holes are not needed with the stock Mikuni 78/79 carbs. It does no harm, but they aren't required. Some other brand carbs may need the holes to add air from the bottom.

The stock Mikuni jets you have in now, have no holes around the bottom. No holes are needed because the air is added above the jets.

If the Mikesxs jets were #27.5, then they would have worked just as well as the stock Mikuni jets. So, that means you must have cleaned out some debris somewhere in the pilot circuit.

Having said all that, even if the aftermarket jets say 27.5, its still possible they were drilled too small in the factory. I've bought lots of parts from Mikesxs that just don't fit or work properly. It seems the only way to know if you have a high quality jet is if it has the genuine Mikuni trade mark stamp on it (a small square inside a large square).

livetoride21 · Mar 28, 2016

retiredgentleman said:
Its possible Mikesxs sent you the wrong carb rebuild kit. What is the number on the side of the Mikesxs pilot jet....................it should be 27.5.

Yes, the jets that Mikesxs sells have holes around the bottom. They are aftermarket
jets, and the holes are not needed with the stock Mikuni 78/79 carbs. It does no harm, but they aren't required. Some other brand carbs may need the holes to add air from the bottom.

The stock Mikuni jets you have in now, have no holes around the bottom. No holes are needed because the air is added above the jets.

If the Mikesxs jets were #27.5, then they would have worked just as well as the stock Mikuni jets. So, that means you must have cleaned out some debris somewhere in the pilot circuit.

Having said all that, even if the aftermarket jets say 27.5, its still possible they were drilled too small in the factory. I've bought lots of parts from Mikesxs that just don't fit or work properly. It seems the only way to know if you have a high quality jet is if it has the genuine Mikuni trade mark stamp on it (a small square inside a large square).

The jet size from mikesXS is in fact 27.5. However, what I'm yet to verify, is the location of the restriction in it ; (VM22/210) vs other.

I'm going to try it out with the mikes xs jets one more time and see what happens.
then it's on to exploratory surgery to see how the cam guides are, and everything else inside.

5twins · Mar 28, 2016

It's probably the correct type. I don't think Mike's sells anything smaller than a #30 of the BS30/96 type. Take a careful look through those MikesXS pilots to make sure the metering orifice is clear. They are aftermarket and the quality isn't always the greatest on them. Maybe one wasn't drilled through correctly (or at all, lol). As mentioned, genuine Mikuni jets are your best bet.

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