RS 36 Mikuni Carb fuel consumption

[Sandgroper]

Hello Everyone. I’m new here. I’ve just finished a ground up resto on my 79 special and have no money left. That being said, I’ve got a bit of a fuel consumption issue being that I get about 13 km/litre with my pair of RS36 Mikuni carbs. The bike goes like bat out of hell but at this rate I will have to sell one of my good kidneys to pay for more fuel. I’m running short mufflers too which sound great but don’t ask my neighbours. I’m wondering if any of you clever people would know the best jetting set up for my application. Thanks a lot, and great to be here.

 

[Sandgroper]

Ah forgot to mention it’s now a big bore 750

 

[Tinker Taylor]

It is a pretty bike, with a nice custom taillight.
'TT'

 

[grizld1]

I banked a pair of TM36/68 flat slide pumpers and ran them years ago--almost the same carbs, only originally set up for singles (thus the TM designation). The biggest part of the job was linking the throttle shafts and tapping the AP reservoirs for fuel barbs to run both nozzles from one pump. I don't recall the jet list, but differences in motor mods would make it irrelevant anyway. I averaged around 48 mpg in mixed riding (weird US gallons, not Imperial).

The best advice I can give you is to buy a copy of the Sudco Mikuni Tuning Manual, but I can offer a few pointers to get you started. First the needle jets. They're the components with the greatest impact on cruising range fuel consumption. I found that P-6 worked just fine. Next the mains. Roll off testing will tell you a helluva lot more about what you need than staring at spark plugs. Find some open road where you can operate in 5th with revs in the neighborhood of 4 to 5K. Roll the throttle all the way open, then do a quick roll off to the neighborhood of 7/8 to 3/4. If the motor misfires it's a rich signal. Step down the mains until the motor runs clean on roll off. If the motor surges it's a lean signal. Another thing you might do is to remove the plug from the main air jet orifice in the intake bell. Small MAJs and even plugs are installed to prevent excessive aeration of the fuel mix at 5-figure rpms that our motiors will never see.

I was a bit intimidated by the carbs when I first looked at them. A friend of mine who ran a speed shop told me "Relax. You won't believe how easy these are to tune until you've done it." He was right. Isolation of the effects of component changes was so tight that it was almost a straight line march upstream.

There are a couple of old threads by guys who fitted RS carbies to their XS650/750 twins. A quick search should turn them up, the search function on this site is excellent. I'm currently running a pair of Mikuni 33 mm. flat slide 4-stroke pumpers bought cheap on e-bay back when such things could be bought cheap. Set therm up on my cammed and ported 700 XS650D just for yucks. They perform so nicely that I sold the TM36/68s cheap to a guy from Barraboo, WI because I thought his hot motor needed them, but he just stuck 'em on a shelf to collect dust. No good deed goes unpunished.

 

[gggGary]

Sure you don't want to twist that knife a bit Grizzly BEAR?
hah hah I look at those carbs and feel guilty about not installing them about twice a month as it is................
Even picked them up and moved the throttles last week.
Take a trip up to Wisconsin and we'll install them into madness!

 

[grizld1]

Wish I could travel, Gary, but chemotherapy is suppressing my immune system. It's been successful, but my doc is going to follow through with monoclonal antibody treatment just to make sure.

But you don't need anybody's help hanging those carbies, and--hold on, a motopsychic message is coming through. Holy crap, it's Madness. She wants you to know that she wants those carburetors, and she sounds pretty mad--as in pissed off mad. What's that, Madness? You want me to tell him to give you those pumpers or he's gonna see what happens? Ouch! that's enough, I'm saying bye-bye, you calm down and stop the threats!

 

[grizld1]

Sorry, Sandgroper, I got off on a tangent there. I looked for the RS posts and couldn't find them. I'll try to find my TM36/68 jet list, but it's only what worked for one guy on one motor; it might serve as a baseline but not as a recipe. And remember Rule One of carb tuning: Everything else first.

And forgive my bad manners: Welcome to the asylum, it's always good to meet a new inmate!

 

[gggGary]

Hey @Sandgroper No specific info but maybe some ideas?
Yeah I riff on endlessly about madness but she's been my test sled for a long time, anyways.
breathed on 750, Was running good with VM34's but getting atrocious fuel mileage 35MPG ish,
There were changes to exhaust also, my limited understanding is that a sonic back wave can help reduce over scavaging, fuel burning in the exhaust does you little good eh? But in one configuration at least even though performance was great found I was way over jetted, mains ended up going down a lot, it still ran great, but fuel consumption improved a bunch, now can get 50MPG if I avoid excessive wrist twist action. I've also reduced rear sprocket gear count, to get engine RPM down around 4K at cruising MPH
So maybe start reducing jetting a size at a time and see if you can keep performance, avoid detonation, but improve MPG?

 

[Sandgroper]

Sorry, Sandgroper, I got off on a tangent there. I looked for the RS posts and couldn't find them. I'll try to find my TM36/68 jet list, but it's only what worked for one guy on one motor; it might serve as a baseline but not as a recipe. And remember Rule One of carb tuning: Everything else first.

And forgive my bad manners: Welcome to the asylum, it's always good to meet a new inmate!

That’s terrific information! I did the roll off testing between 4 and 5 k in rpm and if anything noticed that it had slight misfire so will begin by getting that Mikuni manual that you were writing about so I can identify what I’m looking at properly. My trade background is fabrication and panel beater so everything I’ve ever done mechanical is from making mistakes and then fixing them. I just don’t want to break anything. It took me a year and a half to restore my bike from the ground up. I’ve been reading everyone’s posts on here for ages and given all ideas a go and I am extremely greatful as it got me this far. Thanks everyone!

 

[Sandgroper]

Hey @Sandgroper No specific info but maybe some ideas?
Yeah I riff on endlessly about madness but she's been my test sled for a long time, anyways.
breathed on 750, Was running good with VM34's but getting atrocious fuel mileage 35MPG ish,
There were changes to exhaust also, my limited understanding is that a sonic back wave can help reduce over scavaging, fuel burning in the exhaust does you little good eh? But in one configuration at least even though performance was great found I was way over jetted, mains ended up going down a lot, it still ran great, but fuel consumption improved a bunch, now can get 50MPG if I avoid excessive wrist twist action. I've also reduced rear sprocket gear count, to get engine RPM down around 4K at cruising MPH
So maybe start reducing jetting a size at a time and see if you can keep performance, avoid detonation, but improve MPG?

Thanks gggGary yes I wil be experimenting with the jetting sizes very soon. I also have a 17 tooth counter shaft sprocket and a 32 tooth rear sprocket. Thanks for your help.

 

[grizld1]

OK, the misfires in roll off are telling you that your WOT mixture is too fat. Now here's some good news. If you remove the float bowl drain plug you can remove the main jet (series N100.604, large round pattern) with a flat blade screwdriver. The main jet threads into the needle jet, and the NJ itself threads into the carb body and can be removed with a 1/4" drive socket, either 8 or 10 mm., can't recall which off hand. Anyway, removal and installation are easy, so drop a plug, extract the bits, and let me know what you find.

Did a bit of digging but couldn't turn up the TM36/68 jet list. But I did turn up a few notes in my old Sudco Mikuni Tuning Manual. I used P-6 NJs instead of the P-8's that Mikuni supplies with the TM36/68, with the 9DZH6-50 needles as supplied by Mikuni, with clip in #3 notch. In the intake bell I ran the main air jet hole open and used a 1.0 air jet in the pilot hole. IIRC I used a #15 PJ and 135 MJ.

I don't know the history of your carbs, but the RS series was offered only in banks of 4 and were typically jetted for cylinders with smaller size and higher compression than our motors. The combination of smaller volume and higher vacuum usually called for leaner jetting than our twins can use. The rich condition makes me suspect that Monkeyboy the Tree Wrench had his hairy mitts on your carbs somewhere back down the line.

Can't really offer any more without seeing a jet list. Get the book, strip the carbies, post the list, and I'll do my best to help. Relax and enjoy, you'll tame those unruly pots!

 

[Sandgroper]

Got the main jets out and they are a 130, have ordered some more jets next size down. Was definitely running rich. Cheers grizld1

 

[grizld1]

The needle jet is the heart of the carburetor. Remove it and show what you have. The misfire on rolloff does indicate a rich condition, but don't make assumptions. If needles and NJ's are right and correctly installed, 130 MJs would not be rich enough with those carbs to cause the kind of godawful fuel consumption you've experienced. When you get the manual, strip those carbs and examine everything, from correct seating of jets to clearing of main and pilot air jet passages. But don't assume that your carbs are to blame! While you're waiting for the book, check ignition timing with a strobe and make sure that the ignition is advancing correctly. Then check power to the ignition: turn on main and kill switches and make sure that there's no more than 1/2V difference between battery voltage and voltage at the + terminal of the ignition coil. The last time I saw extreme rich symptoms with jetting in place that should have been fine, the trouble was caused by a bad key switch. The owner rejected the idea that weak electrics could cause his rich symptoms. I finally got my hands on his bike at a rally, bypassed his key switch with a jumper wire, and sent him for a ride. Rich symptoms were gone. I advised him to baby the bike during the group rides to avoid holing a piston, since he had it jetted leaner than a winter coyote!

 

[Sandgroper]

Far out you know your stuff mate! Well I downloaded the book however I think it’s missing quite a few pages. I want one out of America but they want more in shipping costs than they do for the book! It’s a bit like buying fuel at the moment I’m getting stung from all directions. I’m too scared to walk outside in case they hit me with another cost.
I had no idea about the voltage being possibly responsible for running rich(surprise) and yes I have been mucking around with wires looking like the chef from the muppets. I assumed that because it starts everything was good…
Well I’ve got lots to do in between trying to make a dollar or two. One for me, two for them.
Thanks!

 

[grizld1]

I'm not claiming that ignition grief due to weak current is definitely your problem, only that it's a likely candidate. Be sure to check valve lash too. If an aftermarket alternator was installed, verify your timing marks; there are good posts on that topic, search and you will find.

The free online stuff from Mikuni literature is not at all the same as the Sudco tuning manual, which was written by the folks at Sudco, not Mikuni. RS designates radial smoothbore 4-stroke flat slide pumpers configured in banks of four. Members of the same design group include 34 and 36 mm. singles bearing the TM designation, and the HS and HSR carbs for Hardly Ablesons. Tuning procedures are more detailed in the HS and HSR sections, presumably because Sudco noticed that Harley owners need more help than RS buyers. (That's a fair judgement. I once had a professional Harley mechanic inform me that my TM36/68s were strictly race kit and would never be sufficiently tractable for the street. When I saw him outside his shop one day I made sure to beep the horn so he'd look up and hear the motor purring smooth as silk at 1500 rpm as I cruised by in 4th gear. I don't make a habit of lugging the motor, but there was a lesson to be taught.) Anyway, there's a lot of valuable information in each section that applies to tuning your carbs, and it pays to read it all.

One thing you don't want or need to do in disassembly is to remove the air intake horn. The gasket is thin and tears easily, and it's hard to find replacements. And --one more time-- there's no need to remove it!

 

[Jim]

And just to echo what Griz said, retarded timing can play havoc on mileage.
An old mentor of mine was fond of saying "never underestimate the ignitions ability to fool you into blaming the carbs."

 

[jay760]

It depends on how hard you are riding it to get that consumption, I only run flat out racing and get about 15 miles per gallon with 36mm dellortos

 

[Sandgroper]

You fellas are champions. Really top site. So glad I joined up.

 

[650Skull]

Far out you know your stuff mate! Well I downloaded the book however I think it’s missing quite a few pages. I want one out of America but they want more in shipping costs than they do for the book! It’s a bit like buying fuel at the moment I’m getting stung from all directions. I’m too scared to walk outside in case they hit me with another cost.
I had no idea about the voltage being possibly responsible for running rich(surprise) and yes I have been mucking around with wires looking like the chef from the muppets. I assumed that because it starts everything was good…
Well I’ve got lots to do in between trying to make a dollar or two. One for me, two for them.
Thanks!

I have a PDF of the Mikuni-Sudco tuning manual. Its 19mb so to big to post up here. I can get it to you with an email. Send me a conversation and we can sort out the E-mails.

 

[grizld1]

That's the item, Skull! Thanks for stepping up. I'm not a lawyer, but the Sudco book is copyrighted material, so I suspect that posting it on the site could be a problem, to put it mildly, so best to use email anyway. You're a good man, I don't care what they say about you.

Read slowly and carefully, SG, and post some pics of your carbs, preferably removed from the bike. A lot of custom work has to be done to fit RS carbs for the XS650, with some opportunities for error. The only lines that should be joined are the fuel lines and the accelerator pump reservoir lines between the float bowls. All other lines are vent lines and there are lots of those. They need to be open to atmosphere.

Note that the 34 mm. TM 4-stroke flat slide pumper doesn't appear in the Sudco manual. That's because the bean counters at Mikuni America don't import it, so Sudco, the larger of their two U.S. distributors, don't get it. It's available in Europe and elsewhere. Have a look here: https://www.mikuni-topham.de

A few questions: who did the carb mods, and what if any motor mods were done in addition to the displacement increase? In particular, were ports modified, was an aftermarket camshaft installed, was deck height altered, was cam timing set correctly, and if an aftermarket cam was installed, where did you set valve lash? Also who did the carb and motor work?

Don't let this stuff stress you. Troubleshooting is a process of elimination. It's usually a mistake to assume that your trouble has only one cause and that you can logic your way to it. For one thing, a problem can be caused by more than one defect. The more you think in terms of checking things off a list one inspection at a time, the farther ahead you'll be. Try to derive a sense of accomplishment from every item you eliminate, and have fun with the project.