Sugestions for Jetting carbs for XS750 Carbs for 850 engine?

[skeezo333]

Hi All!
New to the board - and with the wrong model bike no less!

Here's what's been killing me for the last few weeks. I transplanted an 850 engine into a XS750 (the triple) and am having a hell of a time getting the thing jetted - not even close. I've been determining the new main jet size by using the "throttle chop method"
In the main jets I've gone up 5 sizes (from stock 130 to 145!) and it still runs lean! I also went up one size in the pilot jets. I know there is an extra 100cc to deal with (plus a 3 into 1 pipe that the bike was probably never re- jetted for), but this seems excessive.

Just for fun (if you can call taking the carbs on and off a dozen times) I recently pulled the needle up one notch (out of frustration and hoping for dumb luck) and I finally I'm running rich (in the middle) - but still way lean on top.

So - I guess I'm gonna drop the needles back down tomorrow and trade in for the next size bigger jets. Still - it seems excessive the huge jump in main jet size!

While I try to beat my last time taking the carbs off and on again - any thoughts or suggestions?

please, put me out of my misery!
Thanks - Jeff

[ratbyk]

Re-jetting

Suggest that you put the needles back to original height. Leave the big mains in but go down one step on the pilots. Then set your float level 1/8th higher.
The new engine is only 13% bigger than the original. At any given RPM the air flow through the carbs should balance out OK except at WOT at higher RPM. then the higher float level should richen it up over the entire RPM range
Ken/Sooke
78E Ratbyk

[Ken Talbot]

Time for some reading. Try this page for some very detailed info on tuning CV carbs.

[Bill]

Re: Sugestions for Jetting carbs for XS750 Carbs for 850 engine?

Skeezo333 wrote:

New to the board - and with the wrong model bike no less! ...I transplanted an 850 engine into a XS750 (the triple) and am having a hell of a time getting the thing jetted - not even close.

Welcome anyway! There's lot's of good info here and many things are common between the various XS models.

I always thought that jetting was a black art with no concrete guidance availble. Everything I read was focused on how to determine if the jets were right, but not how to get there. The Factory Pro page that Ken referenced is excellent. Also, Denny Zander's Jetting Recommendations provides some rules for estimating what size jet you need. Between the two articles I've just succeeded in jetting the carbs on my '87 GSX-R1100 after avoiding it for 2 years!

You baseline should be the stock configuration. Besides the larger engine and aftermarket exhaust, what kind of filter(s) are you running. You should test with the same setup you plan to run with.

For example, given that you have an aftermarket 3:1 exhaust you would go up 2 sizes from stock. 3 more if you're running individual filters.

All in all, it sounds like you're headed in the right direction. There is also a Yamaha Triples list where you might find specific information about your situation. Some guys use the 750 carbs on 850s because the 850 Hitachi carbs aren't supported.

Good luck and keep us posted on your progress!

XSively,

[skeezo333]

Latest results...

Thanks for all the info guys. I've been researching this stuff on the net for a few weeks - but the Factory page was new to me!

The xs750/850 site is unfortunately a lot less active that this site.

This morning I put the needles back down to stock and raised the fuel level 1.5 mm. The bike seems better (although #1 is still rich #2 less rich and #3 perfect) The bike still will not idle. The engine dies after sounding really un-balanced for a few turns of the crank. It acts a little like an intake leak too- but I know what to look for and there is no leak. (intake boots are in great shape)

That being said - I consider myself pretty skilled with this sorta stuff, and I made damn sure that carbs are set up identical!

I have a feeling I'm dealing with something else here!

I Just noticed:
A very large spread on the timing mark for the 1st cylinder ( the only one I you can time-as viewed with a timing gun on the governor)
Possible causes:
1. bad coils
To eliminate them I just had them put on an actual coil testing machine - they spec fine
2. bad spark plugs - wires
Already added new plugs and wires (I replace spark plug wires every 24 years- weather the need it or not;-))
3.something in the charging system
I have a voltmeter on the bike and the bike is charging fine.
4.bad governor and or timing plate
The governor is "new" (from a salvage yard) and seems to be working fine (i.e. all the parts look clean and move freely - the old one was binding)
the timing plate looks new too. I've removed and re-installed it several times - just for fun.


My gut says this timing thing is maybe electrical (as in that little - probably expensive- black box!)
I guess my main question is: Can the idle mixture be so Off that it would cause the engine to act un-balanced at low speeds and have the timing be so jerky?
More succinctly: Should I chase the timing issue or the pilot jet issue?

You guys have been great. And I appreciate it.

[Bill]

Re: Latest results...

Skeezo333 wrote:

• The bike still will not idle. The engine dies after sounding really un-balanced for a few turns of the crank.
• My gut says this timing thing is maybe electrical (as in that little - probably expensive- black box!)
• Can the idle mixture be so Off that it would cause the engine to act un-balanced at low speeds and have the timing be so jerky?
• More succinctly: Should I chase the timing issue or the pilot jet issue?

Things to consider...

• Do you know if this engine ran right before you got it?
• Are the carbs synchronized? If not, you can pre-synchronize them following this article: Pre-synchronizing Carburetors. You only have three carbs, but the technique should work just the same.
• Have you removed the pilot (idle mixture) screws and checked the metering holes? If you can't see light, some ham-fisted "mechanic" has busted off the tip of the pilot screw. Check the Tech Tip, Idle Mixture Screw - Broken Tip Removal
• If that's OK, are all three set the same? On the XS11 1 1/4 to 1 1/2 turns out from lightly seated is a good start.
• I haven't seen an idle timing problem on an XS11 yet. IMO, if you have spark on all three cylinders it's probably close enough.

More succinctly , I'd make sure that the carb settings and pilot screws are good before I'd chase the timing issue.

Good luck, and keep us posted on your progress!

XSively,

[skeezo333]

Hi. Thanks for your input. All very good suggestions.

In fact carb #3 did have the tip of the mixture screw broken off! It's like you're physic!
Unfortunately - I found that out and fixed the problem several weeks ago. (feeling VERY confident that this was the problem). But still - great guess.

The engine seems fine (low miles 30K), but indeed, I do not know for sure it was a runner.

The carbs are synched as well as I can with the stumble in the idle (mercury carb sticks)

I have spark - and it runs fine at any speeds above 2500.
I guess I'll experiment with a more leaned out idle mixture...
(The XS750 manual says to use 2 1/4 turns out a base setting - so I've been going lean or rich from there... never as lean as 1.5 though)

I'm also going to check the charging system - I've had a Honda that had a bad idle problem that was attributed to a weak output of the stator. The stator was good enough to charge the battery and run the bike at higher RPMs - but didn't put out enough to idle smoothly.

I'll do that testing this weekend- and publish my findings, of course;-)

You guys rock - thanks for all the suggestions.

- Jeff "I want to idle" King