Xs 750 won’t start after exhaust and intake modifications

[750triples]

Hi all, back again after putting on new pods and a much less restrictive exhaust. I upped the jet sizes by 2 first, which resulted in a little bit of sputtering when I cranked it. I figured it still wasn’t getting enough gas, so I upped the main by 4 and pilots by 3. Now it doesn’t even sputter. I also noticed that the intake slide on the middle carb won’t slide all the way up, it stops about halfway. Any help would be great.

Thanks!

 

[Jan_P]

Not things I know about but I have read here that some pods dont work well with all carburetors
Disturbing the air flow

 

[750triples]

I’m using uni pods, I’ve seen others using them too. They don’t block any of the passageways. I even tried taking off the filters and still nothing.

Not things I know about but I have read here that some pods dont work well with all carburetors
Disturbing the air flow

 

[jpdevol]

If it won't start at all, a few jet sizes probably isn't the issue. If the carbs are BS34 Mikuni's, the faulty slide could be a perforated diaphragm or a choke plunger not sealing. You can test by manually raising the slide, place your thumb over the oval port at the intake bell to seal it closed: it should take ~14sec for the slide to fully drop. If it drops much faster, it indicates a problem that will also keep it from fully raising in operation.

Do you have good spark?

 

[750triples]

Thanks, yes spark is good.

Put my thumb over the ovals on the left and right carbs, both dropped down immediately. On the middle carb, it only goes up about halfway and I don’t want to force it any further. From halfway up with the oval covered, the middle also drops down almost immediately.

What I don’t understand is that it ran fine just a couple weeks ago, so why is this a problem? I left the carbs on my bench during that time (waiting for bigger jets), could that have caused it?

If it won't start at all, a few jet sizes probably isn't the issue. If the carbs are BS34 Mikuni's, the faulty slide could be a perforated diaphragm or a choke plunger not sealing. You can test by manually raising the slide, place your thumb over the oval port at the intake bell to seal it closed: it should take ~14sec for the slide to fully drop. If it drops much faster, it indicates a problem that will also keep it from fully raising in operation.

Do you have good spark?

 

[jpdevol]

If the carbs are Mikuni BS34 that's a problem. You should remove the middle slide and investigate its problem and look for pin holes in the diaphragm on the others. You can also test the diaphragm on CV carbs by blowing air from a blow nozzle or canned air across that oval port. The slide should rise when enough air is present.

Doubtful that this problem arose just sitting a short time.

 

[750triples]

Yes the carbs are BS34s.

I noticed the slides do lift slightly when I’m cranking it, but I’ll try blowing air too.

I’ll crack open the carbs tomorrow.

Thanks for the help.

If the carbs are Mikuni BS34 that's a problem. You should remove the middle slide and investigate its problem and look for pin holes in the diaphragm on the others. You can also test the diaphragm on CV carbs by blowing air from a blow nozzle or canned air across that oval port. The slide should rise when enough air is present.

Doubtful that this problem arose just sitting a short time.

 

[jpdevol]

 

[Jan_P]

Why does it not run on any cylinder all of a sudden
perhaps the triples do that but a twin dont
It will run on one if there is a dirt problem in one carburetor
I would consider stock jetting see if something wakes up + 4 main and + 3 pilot is the largest increase I have heard of

 

[750triples]

Why does it not run on any cylinder all of a sudden
perhaps the triples do that but a twin dont
It will run on one if there is a dirt problem in one carburetor
I would consider stock jetting see if something wakes up + 4 main and + 3 pilot is the largest increase I have heard of

Whoever opened up this carb last really effed up the original pilots… couldn’t back them out with a screwdriver, so I had to knock a small bit into the centre of each of them to back them out. Needless to say those jets won’t be going back in.

The old mains are fine, so I’ll try them, and then I’ll go one step up on the pilots because that’s all I have.

Will update when I’ve made the changes.

 

[jpdevol]

The BS34's have a peculiarity in that the nozzle that holds the pilot jet is capped with a plug. It's a rubber plug on our 650's, on your 750 the parts diagram shows a brass threaded plug (#20). If those are missing it would be a problem. https://www.cmsnl.com/yamaha-xs750-1979-usa-canada_model8715/partslist/C-07.html

The other thing critical is the fuel level in the float bowl - I dunno what the spec is for yours, but, generally, with a sight tube it must be within ~ 1mm of the gasket surface

 

[gggGary]

The choke needs to be fully seated (off) for the slide tests to work.

 

[jetmechmarty]

Rubber plug:
My XS11 runs no rubber plugs. There’s no channel between pilot and main circuit. Also BS34. There has to be a circuit if it needs the plug. Not all BS34s are the same.

 

[jpdevol]

Rubber plug:
My XS11 runs no rubber plugs. There’s no channel between pilot and main circuit. Also BS34. There has to be a circuit if it needs the plug. Not all BS34s are the same.

All I know is that a XS750F (or SF) is supposed to have plugs - rather strange looking ones - on the pilots

I once had an xs1100G; it had rubber plugs. I figured if the PO fluggled the pilots, maybe he's missing the plugs

 

[jetmechmarty]

I once had an xs1100G; it had rubber plugs.

Yes, the XS1100s are a mixed bag. I know almost nothing about an XS750, but I have ridden one. My XS11 carbies don't have a circuit between the pilot and main jets. The XS650 carbies do. It takes a close look to see the difference. I just thought I'd throw it out there. I have a good story about this that I'll tell you in Townsend. Otherwise, I really don't have more to offer on the XS750.

 

[750triples]

Hello all, thanks for the patience, here’s the update:

All I know is that a XS750F (or SF) is supposed to have plugs - rather strange looking ones - on the pilots

View attachment 250576
I once had an xs1100G; it had rubber plugs. I figured if the PO fluggled the pilots, maybe he's missing the plugs

I do have the plugs, and they are installed.

The choke needs to be fully seated (off) for the slide tests to work.

Thanks for letting me know about the choke, after putting the choke in the slides go down very slowly with my thumb over the oval (30+ seconds each). So I don’t think the diaphragms are a problem at all.

I fixed the stuck slide on the middle carb, it seems like the spring was hitching on something, but now the slide moves up and down freely.

I also moved back down to the stock main jet, and one size up on the pilot (the stock pilots are too damaged to go back in).

After all this, the bike still shows no signs of starting.

However, I also noticed that the petcock on the left side of the tank (left if you’re sitting on the bike) seems to be supplying fuel very slowly. Even on prime with empty fuel lines, the fuel inches it’s way back up to the petcock very slowly. The right petcock seems to flow very quickly and freely. Could that be part of the problem?

 

[jpdevol]

 

[Jan_P]

I would start working with the spark and ignition timing
I fail to se how all carburetors should stop working with different jets tested
I feel there should be some signs of life
Battery Voltage ?
Kick starting ?
Connectors ?
Wires ?

I don't like them but sometimes I use Starter fluid. for test ---Be careful though

 

[750triples]

I would start working with the spark and ignition timing
I fail to se how all carburetors should stop working with different jets tested
I feel there should be some signs of life
Battery Voltage ?
Kick starting ?
Connectors ?
Wires ?

I don't like them but sometimes I use Starter fluid. for test ---Be careful though

I was thinking about starter fluid too. That’s a scary thing to use on these old bikes though.

Battery voltage, wiring, lights, all the electrical is 100%. Getting good spark, starter motor is working fine, kick starter working fine.

While waiting for the new jets, I had the tank off the bike and emptied out. When I went to reinstall it, I noticed one of the petcocks was not moving, but I pressed a little harder and it kind of snapped back into motion, like cracking a tight bolt. I don’t know if that’s the same one that’s now flowing really slow, but I feel like even with a slow flowing petcock I should still at least get a sputter out of the engine.

View attachment 250578

I don’t have a nipple like that to put on the bottom, but I’ll try to rig something.

 

[Jan_P]

I dont know how the triple looks but is there separate wiring to colis mixed up -- or they are mixed for the position sensor on the crank wires are flipped

I Have a Boyer Bransden system and once connected the wires froom the pickuip wrong ..fine spark at TDC but at the wrong stroke
Spark at the exhasut stroke TDC ..Or something if that type