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Pod filter tuning question

  • Thread starter Thread starter J Paster
  • Start date Start date
J

J Paster

Guest
Hi everyone. This is a photo of the project I just finished. It is a 1978 GS550 with an 82 750 motor installed. I had a no title T model, and used the motor and engine cradle to achieve this. I am very happy with the result, and am now sorting out the little details. When I initially planned this modification, it was with the expectation that I would use these Mikuni TM33 carbs that I traded for on Craigslist. They are about all that will fit in there due to space concerns. I have them mostly sorted out except for a bit of lean stumble at very low RPMs, just above idle till about 2500 RPM. From the research I've done, these are pretty rare carbs that only came OEM on a couple of later models. I've inspected the needle jets and jet needles, and nothing appears worn out. I am wondering, since these came on an OEM bike with an airbox, and I am using pods (I have to!), and I know they ran like this on PO's bike, should I change the air jet on the back? If so, does someone know if I should go with a bigger or smaller one? Any help would be greatly appreciated.
 
Greetings and Salutations!!

Greetings and Salutations!!

Hi Mr. J Paster,

You might try going up one notch on the jet needles to richen up that transition between idle and the low mid-range.

Anyway, let me dump a TON of information on you and share some GS lovin'. :D

I just stopped by to welcome you to the forum in my own, special way.
big_hi.gif


If there's anything you'd like to know about the Suzuki GS model bikes, and most others actually, you've come to the right place. There's a lot of knowledge and experience here in the community. Come on in and let me say "HOoooowwwDY!"....
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Here is your very own magical, mystical, mythical, mind-expanding "mega-welcome". Please take notice of the "Top 10 Common Issues", "Top 15 Tips For GS Happiness", the Carb Cleanup Series, and the Stator Papers. All of these tasks must be addressed in order to have a safe, reliable machine. This is what NOT to do: Top 10 Newbie Mistakes. Now let me roll out the welcome mat for you...

carpet.jpg


Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike! :D

Thanks for joining us. Keep us informed. I will put you on my prayer list.

Thank you for your indulgence,

BassCliff
 
Thank you for the welcome, BassCliff!
BTW, I failed to mention in my initial post that these are flat slide carbs, the precursor to the modern Mikuni HSR carbs. I have them tuned to run quite well, actually. Wide open throttle response is great, idle is good and I have spent alot of time swapping main jets, pilot jets and raising / lowering the jet needles to get to the point where it is. Any bigger pilot jet and I get low speed / stall at idle. Any bigger main jet and it stumbles momemtarily upon sudden hard acceleration. These have an accelerator pump circuit, so it took awhile to find the sweet spot... My issue does act alot like the needles are too low, but if I raise them, the problem persists. I've proceeded carefully through raising needles, then enrichening the pilot circuit if no change is detected, until I ended up grossly over - enrichening the pilot circuit. I am considering replacing all the needle jets and jet needles, but close inspection doesn't show me any radical pitting, oval shaping or excess wear. I am also wondering, since I don't have any idea what these carbs were originally installed on, if the shape of the jet needles just don't jibe with the GSX 750 motor. I don't have any pod filter experience, that's why I was wondering if this low speed lean condition could have anything to do with the air jets. But I'm definitely wondering about the shape of the jet needles as well. I guess I'm a bit over my head here, but hey, I got the thing this far! Surely there's a fix. Thank you again for the welcome! I have been a frequent reader of the GS Resources for a long time and rely on you to help keep my GS's on the road!
 
It may be the fuel level in the bowls. I have a set of RS34s on my 700 and they are pretty sensitive to fuel level. I had a rich condition that I finally corrected with less fuel in the carbs. You may need a tiny bit more. With the flatslides I can't just whack the throttle open without killing intake velocity. With the pumps you want to roll the throttle anyway so you get that squirt slowly instead of flooding the intakes with a bunch of extra fuel. Once you get the revs up you can just hammer the throttle through the gears.
 
Hey, thanks! I hadn't considered that. I have an 82 750 that I was having problems getting set up with a V&H pipe and a K&N filter, would die after a few blocks and take a few minutes to start back up - it turned out that I had to raise that float level 2mm to alleviate the starvation. BTW, these carbs are pretty sweet - bike is a fresh rebuild with 816cc Wiseco kit and some custom valve seat work done by Pro 1 cylinder heads out here. No porting but the seat work really seems like it brings out the torque in these motors. If I yank on it, this thing pulls the front end like no 550 or 750 I've ever had! Not having any tech info on these carbs, I just checked their intial setting and went with that. I'll give that a try.
 
Good to have you here!
You are my kind of GS guy.
750 4-valve into a 550 2-valve frame.
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It looks great!!

Daniel
 
The only info I can find on float height is from some of the thumper forums. 14mm seems to be what everyone quotes. It's also quoted on the Sudco site. There's usually a +/- 1mm that goes with that. This page is for the RS series so don't use that height, it just shows where to measure from..
http://www.sudco.com/vol33/32.pdf
 
well that was weird

well that was weird

OK, tried dropping the float height from 14mm to 12mm tonight. Sounded great when I started it - good idle and seemed to want to rev through the range, so I took a ride around the block - it was definitely loading up when I wanted to accelerate slowly from 2000-2500 rpm but I can yank up the throttle and it revs fast enough to pull the front wheel! Once most of the way around the block (about 3 miles here) it was slow to return to idle. I'll check for a leak but all this stuff is new, so I doubt it. BTW, new Dyna S, coils and wires. Valves are .04mm - .05mm per spec. Valve job, new seats and seals, new pistons, rings, intakes, all except these old carbs... I can get parts for 'em though... starting to wonder if I'll save any over a new set...

Guess I'll go the other way a touch, maybe set them at 16mm and see if I can screw it up the other way and learn something. I'll keep you posted. At least these things are easy to remove and work on. Oh and thanks for the compliment on the bike!
 
Hi,

What carbs are you using? Most CV type carbs are set with 22.4mm float height (+/- 1mm). See the chart HERE.

float_height_bwringer.jpg


float_height_steve1.jpg


float_height_steve2.jpg



Thank you for your indulgence,

BassCliff
 
Last edited:
These are Mikuni TM33 flat slide carbs. Pretty rare. I think the established previous float level of 14mm worked pretty good, but I'm going to try going to 16mm for fun. Originally, I was curious about the air jets, i.e. when you get a dynojet kit for CV style carbs, don't they replace those air jets if you are going to use the stage 3 kit w/pods? And if so, do the air jets get bigger or smaller?
 
Hi,

J Paster said:
These are Mikuni TM33 flat slide carbs. Pretty rare.
Click to expand...

Oops. Sorry, I've never even seen a set of those carbs. I don't think I've seen any documentation on them either.

The DJ3 kit for my bike contains "main air correctors", but I don't know which way they are correcting. Probably a little larger. But since I'm obviously naive in this area, I'm just going to be quiet now. :o

Please forgive me this useless post. :p

But maybe it will bump the thread so somebody with the right knowledge will see it and share their experience.


Thank you for your indulgence,

BassCliff
 
You'll be able to dial the TMs in. You'll never be able to just whack the throttle open from low rpm. The accelerator pump should be set up to start pumping at 1/4 throttle and shut off at 3/4 throttle. You can see the pushrod do its thing and tell when the pumping starts and stops.
 
Billy Ricks, you're right about the accelerator pump. I noticed that when I was rebuilding them. They're kinda like VM carbs - you can't just expect to keep going from idle to full throttle without some hesitation. So when I speak of how hard it will accelerate, I'm referring to only once and probably from about 2500 rpm on. Hey, BassCliff - thanks for the point out about how to measure stepped floats. I always measured from the highest point (carbs inverted)... Gotta re-check that. Also, If the DJ3 kit does indeed change the air jet, that's good info for me if I can't find a happy place for these guys. I thought I remembered that those had to be changed on CV carbs for running pod filters. I have no idea if messing around with different size air jets will help, but that at least confirms the thought I had. I believe on CVs the air jet gets smaller with pods, but am only armed with a weak / fuzzy memory of checking out the kit that I changed to a stage 1 on my other GS. Thanks for your input! Gotta leave the bike sit for a week, but I'll report back on this thread as soon as I can make more changes..
 
Update on air jet

Update on air jet

Hi everyone,

I was reading through the carb/fuel threads and noted 5 azzed GS's remarks following a question he posed regarding the DJ3 kit he got for his CV carbs on his GS. It seems I did remember that correctly, that DJ sends you a leaner air jet to richen the mixture, and I recall that these pertain to the DJ3 kit where you're gonna use pods, no the DJ1 kit that retains the airbox. So, with this knowledge, I'll also give that a shot. The air jets are readily available from Bike Bandit, but the smallest they offer is 0.5, which is what is currently in there. I'll try capping them altogether and see if that cleans up my small area of lean running and post back. Of course, I've still got a couple of other things to check too....
 
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