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Jetting Database?

3K views 20 replies 5 participants last post by  javadog 
#1 ·
I've spent a lot of time looking around for information on tuning flatslides. I was hoping there might be some kind of jetting database on this site, but have been unable to find one in a sticky or searching otherwise.

I've seen jetting and suspension databases on other sites. Do we have any?
 
#2 ·
I haven't seen any. the closest thing would be a member here having a similar setup as you and they give their jetting specs, but even then you can't expect to have it spot on just from that since every bike will be different.

lots of trial and error, plug readings, dyno runs with AFR readings, and just sound / smell / feel
 
#6 ·
Thanks for the reply, guys.

Here's what I have:
1987 750; 1mm over; 1990 (L model) intake cam; Yosh Duplex; Mikuni RS 36 flatslides

Carb specs-
125 mains
P-4 needle jets
Jet Needles - whatever was stock from Mikuni
15 pilots
Pilot Screws - 1/8, 1/4, 1/2, 1 turn out all present issues below; 1/4 & 1/8 turns work better than 1/2 or 1

The tip of my Duplex is sooty and up to 1/4 throttle it sounds fat, blubbery, stumbles.

However, it takes a while to get it warmed up to where it won't die coming off of idle. Once it does come off of idle, it won't hang nor dip below idle when I blip the throttle.
But it does sound fat at low throttle setting as one might use cruising through neighborhoods keeping a low profile.

Also, I pulled a plug to see what's going on plugwise. If I hadn't ridden the bike and someone showed me the plug, I'd tell them it was spot on. No soot, nor any white deposits or other lean indicators. actually very clean. This was after riding for about 5 miles at low throttle settings through town.

Above 1/4 throttle it is as clean as it can be. Pulls smooth to redline - no hesitation, no stumbles.

These have always been problematic at low end since I bought them 21 years ago. I fiddled with them, had very accomplished tuners fiddle with them, and back when they were on my LTD I paid a guy to spend a day on the dyno with it.

It has always been a choice between crappy bottom end running or crappy everywhere else regardless of who tunes it.

I can't imagine I have a reversion problem due to the larger intake cam; it's not THAT radical. I even used these carbs on a 90 motor and they gave me the same fits, so I don't suspect it's a head/cam issue/low port speed/reversion, etc.

I've looked over the Mikuni manual - it looks a lot like the one that came with the carbs except the old had a picture of Schwantz ripping a wheelie on the cover.

I did look at the thread you linked, Slabby1100, and all I can say is that the dudes who have seamless carburetion on their RSs should buy lottery tickets...or mine have been hosed from the get-go.
Also, my pilot screws are fine, everything is perfectly clean, etc. These have been a problem since the day I bought em.
 
#15 ·
Thanks for the reply, guys.



I did look at the thread you linked, Slabby1100, and all I can say is that the dudes who have seamless carburetion on their RSs should buy lottery tickets...or mine have been hosed from the get-go.
Also, my pilot screws are fine, everything is perfectly clean, etc. These have been a problem since the day I bought em.
Mine on the 1100 run just like you describe. Crappy down low, but pull clean from 4000 rpm. I agree with your statement above:wink2:
 
#7 ·
I was hoping perhaps I could look at similar set-ups and see if I'm way off anywhere. What I gather is that many who are pleased with their RSs, are 1100 riders, and those engines should show a much stronger signal to the carbs. Despite what I said above, I'm sure there is a port speed issue at low throttle, but I'm curious to know if this is a typical 750 problem, or I am some sort of outlier.
 
#10 ·
Good question. I'll take a look once I open up the tops of the carbs to do a synch...not sure when I'll get to it.

I had these completely apart 7 years ago and replaced every rubber bit I could order for them from Sudco. Put 'em all back together and synched them. It was right after I did that I thought I'd do a compression and leakdown check which prompted what ended up being a full overhaul from the babbits up...all for a leaking o-ring on an oil return pipe.

To my knowledge, I set everything back up the way they were...which produced good running. Of course, with larger slugs things will change, but they are acceptable except for the off idle to 1/4 throttle running. This is a lot like my LTD ran after the dyno tuning.

Just wanted to explain why I haven't fully investigated the needles.

FWIW, I opened up the float bowls, before posting, to see what I had in there and everything was still sparkly - I was sure to drain everything before I put it in the shed 7 years ago.

I'll get the needle info out within the week. Thanks
 
#11 ·
7 years is a long time. If I were having trouble with these, I'd pull them apart again and clean them. It's one thing for the float bowls to be clean, it's another for all of the very small passages to be clean.

I'd also replace all of the rubber parts, including all of the o-rings. These things don't like to be used, then stored, then put back into service. Float bowl gaskets that old will be flat and hard-pressed to seal again.

Your pilot jet is already one size smaller than stock, so I'm surprised you feel like you have a too rich mixture down low. One thing I'll caution you on is reading the mixture by looking at the tail pipe...

Have you synched these things?

JR
 
#12 ·
I appreciate your admonishment regarding the age of the rubber. I did clean out all the passages after noticing this issue then opening up the float bowls to verify my jetting.

Everything checked fine.

I, in fact, wanted to run them a bit in hopes of possibly letting the O-rings get wet before opening up the dry carbs. I guess we'll see about new leaks when I put them back on.

I mention the tail pipe because it appears to diametrically oppose the plug reading. And of course, crappy California pump gas won't be the best indicator anyway.

I synched them 7 years ago and have yet to synch them since I put it back together. I wanted to get a little of time of the engine before it spent time idling and heating up with fresh bores, et al.

Granted, I may be premature in asking about the jetting without a recent synch, but these characteristics are nothing new with or without a synch; however, I do expect a synch will clean things up a bit.

I guess the crux of my original post was a hope of finding others with flatslides on a 750 and comparing what they have and where they live with what I have and where I live.

I'm at least as surprised as you are by the stumbles with the small pilots. I'm happy for any advice, though.

Like I said, I'll let you know what my needle is at and report any improvement with a synch.
Thanks again, I appreciate your interest!
 
#13 ·
My observations from life:

I've never been happy with a set of carbs that sat for that long between runs.
I've never had a problem with a set of carbs that was freshly rebuilt.
Reading anything from plugs or any other visual cue is a waste of time.
Synching is the first thing you should do.
Tuning these on a dyno and measuring the CO is the way to tune them.
No idea where you live but temperature might be an issue. Where I live, what works in the middle of summer won't run worth a crap today. It's cold outside...

JR
 
#21 ·
No problem, just trying to help. Mikuni suggested the middle needle clip position for both the 750 and 1100 and I don't recall ever needing to raise the needle on any relatively stock-ish bike I've played with. The reason I mention the throttle position is that the needle will become a factor if you open the throttle too far, regardless of rpm. I can't see what you're doing from this far away, so I mention it for what its worth. When tuning a flat-slide carb, the only thing I consider is throttle position.

I mention the fuel level, since a lot of guys get that wrong. Since I don't know you, I have no idea of your experience level. No insults are intended. If you can't fix the problem any other way, making small changes to the fuel level will have an effect in the response.

Maybe it's time to find a dyno that you can use. It's so much easier tuning a bike on a dyno.

Have fun,
JR
 
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