Why won’t my XS Start

[Sowiuk1]

So I’ve got a 77 xs. I’ve just had the carbs fully stripped cleaned and rebuilt. Put them back on, kick it over and I get loads of back pressure to the point where the carbs pop off. They’re even clamped on! The valves look clear. They open and close and the exhaust isn’t blocked.

Does anyone have any suggestions or thoughts??

 

[Jan_P]

If it is an ignition pulse that ejects the carburetors
The gnition is off --is to advanced

 

[grizld1]

Carbs typically pop out of boots when the tips of bob weights in the advance governor (AKA automatic timing unit) wear, stretching the interval between full advance and full retard. Usually what I find is that the PO (or in some cases current owner) has set advance timing correctly, leaving retard timing at startup close to TDC, which is far too retarded, not too advanced. Check static ignition timing with a light or ohm meter. Timing is set the same way in all breaker point XS650 models, so if you need instructions, download what you need from the 1978 factory manual, free at https://www.amckayltd.com/carbguide.pdf. There are a number of methods for correcting the advance curve. The one I like best is from Gary Goetz: shim the limiting tabs on the advance rotor.

If all this sounds like a foreign language to you, look at the advance governor under the chrome cover on the right side of the cylinder head and the points cam under the left side chrome cover and observe how they work, then reread. It will all make sense, I promise.

 

[Sowiuk1]

Thankyou guys, one issue I should have mentioned. I have a PMA CDI ignition, so no points. Could it still be advanced timing issue?

 

[Jan_P]

Thankyou guys, one issue I should have mentioned. I have a PMA CDI ignition, so no points. Could it still be advanced timing issue?

Yes ....
Not much experience on PMA but otherwise
If the spark comes before the intake valve has closed fully and smashing the pressure into the intake line.
On the other end the spark comes to late and fires when the exhaust valve has opened and shoots in the exhaust
But again someone with better knowledge on this setup perhaps can come in.

But a check of valves and ignition timing is not wrong . Do you have any filters and normal exhaust on
So the mixture is in the right region.

 

[grizld1]

I assume you have a PMA that incorporates a crank fired ignition. If so, was the motor running well after installation? If so check fasteners, and if nothing has come loose your problem isn't likely to be timing. If at all possible, get the thing started so you can put a strobe to it. If not, locate TDC the right way, by indexing, and do the needful to get your timing straightened out.

 

[5twins]

It appears you have '78-'79 carbs there, not '76-'77 ones. If the jetting was changed to stock '77 specs during the carb rebuild, that might be the issue. It's quite different compared to what the '78-'79 carb set should have .....

 

[grizld1]

Good catch, 5twins, should have seen those big ole vent holes over the main and pilot air jets. Should have asked for some history, Sowiuk. Did you install the PMA and ignition? If so did the bike run afterward? Has it run for you at any point?

 

[Sowiuk1]

Yes ....
Not much experience on PMA but otherwise
If the spark comes before the intake valve has closed fully and smashing the pressure into the intake line.
On the other end the spark comes to late and fires when the exhaust valve has opened and shoots in the exhaust
But again someone with better knowledge on this setup perhaps can come in.

But a check of valves and ignition timing is not wrong . Do you have any filters and normal exhaust on
So the mixture is in the right region.

Currently there are no filters on, and a straight pipe exhaust, (temporary). Primary focus is to get it running then it’s coming out to get the frame painted. It’s jetted for foam filters and a twin side exit exhaust.

 

[Sowiuk1]

It appears you have '78-'79 carbs there, not '76-'77 ones. If the jetting was changed to stock '77 specs during the carb rebuild, that might be the issue. It's quite different compared to what the '78-'79 carb set should have .....

View attachment 213894

My mistake, they are the 78’s and have been jetted to 150 for foam filters and a straight through exhaust.

 

[Sowiuk1]

Good catch, 5twins, should have seen those big ole vent holes over the main and pilot air jets. Should have asked for some history, Sowiuk. Did you install the PMA and ignition? If so did the bike run afterward? Has it run for you at any point?

I’ve never had the bike running.
originally it was a un-registered US-import, it had a half complete points system that I have removed and replaced with a PMA CDI system. This gets a good spark, just a question if whether it sparks at the right time.

 

[Jan_P]

I can say that much that here on the Forum was a Big man i believe + 200 lb with a PMA setup -- i think he was in Philadelphia
I promised we would help him but have not heard of him in a while and I have forgot what name he used here if anyone remember please give the name ill check in

Almost destroying his knee trying to kick start getting severe Kick back a XS 650 with no exhaust and no intake Filter
The mixture and pressure waves in the Exhaust and intake gets ruined so even if the jetting is right
I dont believe you can start it unless the intake air fuel ratio is close .. assuming a correct jetting the no filter means a leaner mixture and you can be
in trouble without a knee .. trying to start it But again others have better knowledge on PMA

 

[grizld1]

Timing is a question that you need to resolve, Sowiuk. For what it's worth I've never seen an XS650 motor spit the carbies out of the boots for any other reason than defective boots from the People's Big Joy Condom Recycling Collective of Tientsin and incorrect ignition timing.

Your first step is to locate TDC accurately. Near TDC the piston "dwells;" that is, it moves very little in relation to crank rotation. Correction for piston dwell is done by indexing. A tentative TDC mark is made, the degree wheel is zeroed on that mark, then degree readings are taken at equal drops before and after TDC. The midpoint of these readings gives you true TDC; for example if the reading BTDC is 4* and the reading ATDC is 8*, the tentative mark needs to be advanced by 2* for the final mark. The drop doesn't have to be large, .040" is plenty. If you use a dial indicator with a long stem directly on the piston it's a simple matter to pull back the plunger while you turn the engine over the top. Once you have the TDC mark scribed, follow directions from the manufacturer of the PMA kit.

Alternatively, reinstall the OE alternator, buy a cam driven ignition, and sell the PMA kit. Good luck to you.

 

[gggGary]

Some of those PMA rotors have no key so any timing marks must be CONFIRMED to match piston TDC. With in spitting range by simple methods should be enough to confirm. Advanced (or other) timing issue spitting the carbs off IS a thing for the XS. ;^) @grizld1

 

[grizld1]

I dunno, Gary; if "spitting range" error puts advanced spark much before 40* BTDC and the rider doesn't know what detonation sounds like or what to do when he hears it, very bad things can happen. What's safe to do for somebody who knows how to tune by ear can produce a disaster for guys who don't.

 

[5twins]

150 mains are quite large, probably even for straight pipes, but shouldn't make the bike not run. I tried them in the past when sorting my carbs so I know the bike will run, just not that well, lol (black plugs, gurgling through the midrange).