XS750 Carb woes

Aetsh

Coast to Coast
Got a 79 XS750, ran fine when I went over to check it out in January, Been working on a few things, and finally around to firing it up last week..

I had my dad go through the carbs (not rebuild) just clean everything, and make sure none of the jets were clogged, as it was running when I bought it.

Everything back together, it fires up second kick, but it was SCREAMING at almost 8k RPMS, so I cut the choke off, and it goes down to 6k, but still SCREAMIN!

I adjusted the big screw behind the carbs, to adjust the tickover speed (rpms) but nothing happened, in either direction I turn it..


So I've narrowed it down to 2 things..1 being the little brass adjuster on the front top of all the carbs (Seen here) Do adjusting these effect idle speed? I thought I read they were just for the fuel/air mixture. (Keep in mind, my dad didn't touch these, they are still where they were when the bike was running)

Secondly, The linkage that attaches the the carbs to move all three butterflys at once, I tightened them fairly tight..Is this an adjustment for idle speed?



I'm lost..

Thanks guys!
 

hillsy

Over 1,000 Posts
Aetsh said:
Secondly, The linkage that attaches the the carbs to move all three butterflys at once, I tightened them fairly tight..Is this an adjustment for idle speed?


Those linkages are for the balance between the carbs. If you have adjusted them to make them "tight" you have put your carbs WAY out of synch and that's more than likely going to be the problem. Look at the butterflies through the front of the carbs - they should be at the same position and open at the same time. You can adjust these with the carbs off by putting a piece of wire underneath each butterfly and adjusting each one until the wire just falls out (start with the middle carb, adjust it with the idle screw at the rear of the carbs, then adjust the outer carb's butterflies with the linkage screws).

And another thing - you shouldn't be hitting your carbs like that with a rubber mallet :mad: ;D
 

Aetsh

Coast to Coast
You rule!!!! I had no idea those would unsync the carbs and make the idle speed through the roof!

I'll be trying this out tomo!! Thanks again!

And for the record, I just grabbed the pics from the web and drew the little arrows :)
 

Swagger

Putting boot to ass since 1967!
FYI: those old CV carbs blow, never were particularly good in the first place but once they get to be a little venerable (ie; old) they are at best a complete pain as you're finding.

Were it mine I'd go look for a rack of take off carbs from an 94 or later Triumph triple. They have the same cylinder spacing and are a drop in, bolt on easy as pie conversion. They are still CV but less odious than those old ones. I heard about the swap a few years back and sure enough the once I took them off my SuperIII (I put on 39FSRs) they will slot right into one of my XS triples.

The OE Triumph carbs, while still CV are simpler and easier to deal with than the old Keilins (however the hell you spell it) and make the mikuni CVs look like antiquated steam valves. For around $50 you can really help out that old engine.
As well, the header from the same range of Hinkley Triples can be fitted to your XS with a $30 welding job, swapping over to Yamaha appropriate flanges.
 

crazypj

Split personality, I fake being smart
Swagger is 100% correct.
You have Hitachi carbs on XS750, they were almost impossible to get parts for when bike was new. (K&L supplylists main and pilot jets for Hitachi, but you may have to buy packs of 5)
Unless your doing a 10 point restoration, the best bet is swap them for something more modern.

PJ
 

Shawkpilot

New Member
Another thing to look at is your throttle cable. make sure that it is not catching on something. It may be just as simple as that. I am having XS750 carb issues also, but mostly due to switching to pods instead of the stock airbox, and trying to dial in the jets/needle setting. I also took an air compressor and blew through ALL of the hole on the carbs. Some of those jets have tiny holes, and it doesn't take much to clog them.
 

Tripler

Been Around the Block
You have Hitachi carbs

Sorry PJ, these are Mikuni II's. Hitachi came on 80/81 850's. Been running them for 10 years now and trouble free, mind you Seafoam does help. ;) Doug
 

Tripler

Been Around the Block
instead of the stock airbox

This will be a problem, go to the triples site and get the jetting for an xs1100, may prove very fruitful. :eek:
 

crazypj

Split personality, I fake being smart
Should have looked closer, you can see MIK on side (doh)
All the XS750's I worked on had Hitachi carbs.
Maybe it was different in Europe?

PJ
 

Womblezombie

New Member
Well thems my carbs in the pictures. While I realise you should never raise a mallet in anger at a set of carbs it was our last hope before buying replacements. We bought an xs750 that was taken off the road with carb problems and should have realised that while the guy had bought all the carb refurb kits he had not fitted them, thats because the combination of rust and previous slipped screwdriving attempts had left the screw heads trashed. The mallet worked and more amazingly the carbs still worked afterwards, really nicely as well.

No expert on these bikes, as you can tell from the malleting, so this could be total nonsense but we replaced the vacuum hose to the fuel taps and had a similar problem, engine would rev madly because the carbs were being flooded with fuel, put back the original tube (looks like it was stolen from an aquarium) and it is fine again.
 

Moto Noob

I have no clue what I am doing...
Swagger said:
FYI: those old CV carbs blow, never were particularly good in the first place but once they get to be a little venerable (ie; old) they are at best a complete pain as you're finding.

Were it mine I'd go look for a rack of take off carbs from an 94 or later Triumph triple. They have the same cylinder spacing and are a drop in, bolt on easy as pie conversion. They are still CV but less odious than those old ones. I heard about the swap a few years back and sure enough the once I took them off my SuperIII (I put on 39FSRs) they will slot right into one of my XS triples.

The OE Triumph carbs, while still CV are simpler and easier to deal with than the old Keilins (however the hell you spell it) and make the mikuni CVs look like antiquated steam valves. For around $50 you can really help out that old engine.
As well, the header from the same range of Hinkley Triples can be fitted to your XS with a $30 welding job, swapping over to Yamaha appropriate flanges.

stumbled upon this on google... is this for real? I hate my old xs750 carbs. You are saying I can buy new triple carbs for a triumph and they will bolt strait onto my yamaha with no problems or adjustments and work better?
 

Swagger

Putting boot to ass since 1967!
Not 100% tweak free....
Mechanically they are a drop in deal. There may be some gymnastics required to accommodate the throttle cables but if that's a problem you may want to look into professional assistance. Barring that, a little jetting will be required. Not too much though, depending on your engine/Setup. They are FAR more flexible for use with pods......though still cv+pod=Bitchy.

In short, very doable and pretty worth it.
 

Moto Noob

I have no clue what I am doing...
Swagger said:
Not 100% tweak free....
Mechanically they are a drop in deal. There may be some gymnastics required to accommodate the throttle cables but if that's a problem you may want to look into professional assistance. Barring that, a little jetting will be required. Not too much though, depending on your engine/Setup. They are FAR more flexible for use with pods......though still cv+pod=Bitchy.

In short, very doable and pretty worth it.

that sounds awesome, my carbs are in decent shape but no matter what i do the bike just doesnt idle right. You are not the first person I have heard say that the stock xs750 carbs are not great... but I had no clue I could branch off to the triumph triples so easy.
 

Basic Amateur

New Member
I don't know if anyone still follows this thread - but if anyone sees this and has some knowledge, I'm open to learning.
I purchased a 79 XS750s a couple years ago. It's fun, but it never idled right and would occasionally die. I took it to a couple professionals, who installed a new Rectifier Regulator and diagnosed bad coils, but they never fixed the problems. I read this thread and finally tracked down some Mikuni carbs off a 95 Triumph Triple. They mounted right up, but wouldn't start. I tried some starter fluid and they started up and idled perfectly. Take off from a stop with smooth direct torque, but when I give it gas it sounds like an air compressor with lots of noise but no power at all. It didn't backfire so I assumed it was running lean rather than rich. The jets that came on the carbs were 110 - no idea what they were running on.

I bought some main jets and jumped on a Triumph Triple thread. People were saying they were running 137 main jets with after market hi-flow exaust and modified airboxes; two users said they were planning to go to 140 the next time they had the carbs off.
1) I have stock exhaust, but I'm running foam UNI pods so I figured I would start even higher -145. Wouldn't start and when I took them back off the intakes were full of fuel plugs were soaked.
2) I decided maybe I was wrong and it was originally running rich at 110 due to the stock exhaust, so I tried 100 main jets had to use starting fluid, but the performance was the same. Ran it about 3 miles and the plugs are as pictured.

So; am I doing this completely wrong? How do I figure out what jets to try?
 

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Basic Amateur

New Member
Incase anyone still wants to ride a 42 year old three cylinder the secret is Triumph carbs with 140 mains and stock pilot & needle jets and stock exhaust. This bike is a hoot now!
Yamaha.jpg
 

der_nanno

Faster!
I am one of those guys from back in the day and I found 137.5 with pods to be even better. My high-flow exhaust was a very rotten stock item that may or may not have the lost the odd baffle due to rust eating it away.

Also be aware, there's Mikuni and Keihin carbs on those T300-Triumphs depending on model year. I am only referring to the Mikunis.
 

Basic Amateur

New Member
Thanks for the info. I didn't know there were Triumph running Kiens; mine are Mikunis. On the otherhand, I didn't know anything about any of this until I jumped in the middle. lol!

140 actually run pretty well, but I feel like I should be able to get a bit more at full-throttle and it smooths out when I throttle back - so I'm thinking too much air & not enough fuel at the top end. I could be wrong.

I'll just ride it this way and monitor my plugs for a while.
 

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