Rf900 Tuning Issues, 0 - 1/8 throttle hesitation
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  1. #1
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    Rf900 Tuning Issues, 0 - 1/8 throttle hesitation

    Hi all,

    I have a 1994 RF900 with about 46,000 miles on it. I change oil, oil filter, and plugs every 4,000 miles and use Mobile 1 Racing 4T 10W-40 & K&N filters. There are no vacuum leaks, fuel filter is clean, I only run 93 unleaded, and I don't redline it, it has a Vance & Hines muffler with the bake removed (previous owner's doing). I put about 20,000 on it with no issue until around 6 months ago.

    After having some carb issues, I cleaned and rebuilt the carbs, cleared all the passageways, installed new teflon spacers, orings, and factory replacement jets (stock sizes, anyway) from factory pro including the aftermarket emulsion tube and needle which were both severely worn (it was explained to me that they're factory spec but more durable and the needle has adjustment slots for tuning).

    So, after putting it all back together, it runs. But that's about it, and it's finicky.

    Inconsistent idle and hesitation from 0-1/8 throttle and severe lack of power from stop are my current problems. It pops on decel and occasionally at idle, but I know that's a product of the exhaust mod.

    I followed factory pro's instructions, verified the main jet size is good by trying the three available sizes and found the best pull at full throttle is the stock size. I don't really know if the float height is set correctly (.27” from the pilot jet to the highest part of the float as per the manual), I've tried .31" and .25" as well with no noticable change in performance. I set the needles at the 3rd position, then 2nd and 4th with no noticable change a so it's currently in 3rd. I've been trying to set the idle mixture screws, but I can't for the life of me figure it out.

    Cylinders 1 and 4 always respond predictably to a mixture change, but 3 and 4... Not so much. It's also damn near impossible to balance the carbs, cylinder 4 always pulls more vacuum than the other cylinders, about 5" or so on the gauges.

    So I'm lost, I love this bike but I can't justify bringing it to a shop to charge me what the damn thing's with just to tell me what's wrong with it.

    I would love to hear any input, thank you.

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  3. #2
    M-J Lifetime Achievement Award
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    Before you do a carb sync you need to check the valve clearances.

    Closed to 1/8 throttle is typically the idle circuits and the tiny jets just after the venturi are prone to clogging....there are often three. If it doesnt idle smoothly it will be difficult to sync the carbs too. A good quality carb cleaner left over night may help.....even if you think you cleaned the idle circuits they may still be clogged.

    One other thing check the top of the carbs each should have a small 'o' ring which is a seal for the idle circuits...many CV carbs have these. Check each diaphragm for any pin holes etc.

    Float height is very important as it affects the amount of fuel drawn up into the air stream....too high and the fuel ratio will be too rich too low and it will be too lean. Many floats have a spring loaded tip which must not be deperessed when taking a measurement.

    Depending on the float action the measurment may be taken from the bottom of the float to the joint of the carb top with the whole thing upside down....others may need the float to swing free. It is vital to know which way to measure this. You may also have a sticking or leaking float needle. The latter will show as a light ring around the needle tip.

    Hope some of this helps
    Last edited by Aussie Steve; 04-22-2019 at 03:01 AM.
    Smoke me a kipper I'll be home in time for breakfast

  4. #3
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    Valve clearances are all well within spec, also compression is excellent since I didn't mention it earlier. I'll double check the venturi passageways. The float valves are all new and springy, the orings have been replaced and the seal for each has been tested, they all pass. The diaphragms were all leak tested, no leaks. As for the float measurement, the owners manulal has a diagram and procedure specific for these carbs, it's a pain, but relatively straight forward.

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  6. #4
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    I strongly suspect that you now have the carbs back near to OEM spec.......but the intake and exhaust are NOT.

    The performance issues you are having are typical for many bikes that are "messed with" by owners who really don't know what they are going.
    Decreased restriction in the intake and exhaust results in a lean mixture, which is especially noticeable at low speeds.

    Putting stock air filters back on it should result in slightly better running.
    Putting mufflers on it might actually make the engine run good.

    Sometimes a quick test is possible.
    If you can get the choke to stay partly ON after it warms up and that makes it run much better.......that pretty much proves the point.
    Don't believe everything that you think.

  7. #5
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    With exception to the pilot screw (which the setting in the manual lists as "present from factory") everything is set to stock, and of course the exhaust. The exhaust is aftermarket and the baffle was removed by the previous owner (claimed it gave it more horsepower), but it ran like a ray of sunshine until the needle jets wore the hell out of the emulsion tubes, that's where this whole thing started.

    The air box is stock, and I tried the OEM paper filter, made no noticeable difference against the K&N with the problem I'm having now. When it's warmed up and I apply the choke at cruising speed it just inxreases idle and restricts cruising rpm's to about 3k and make it extremely sluggish, which indicates rich, if it were lean then the choke would increase performance to an extent.

    I pulled that carbs off today, and for %%%%'s and giggles I turned in the mixture screws to lightly seated (no two of the four screws we're within half a turn of each other, by the way) I noticed the cone tip on each protruded into the venturi at different amounts. Maybe there's some gunk at the screw seat, I'll investigate further this weekend.

  8. #6
    M-J Lifetime Achievement Award
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    I reread you first post and note you rode it for some time before having carb issues...resulting in a carb rebuild. What was the issue and who did the carb rebuild. Since the problem seems to stem from the rebuild I would suspect something is not correct there....even just one carb can cause the entire engine to perform dismally.

    Also you mention No 4 pulling much more vacuum than the rest and two responding to mixture changes predictably whilst the other two do not. This seems to point to the unreliable carbs being at fault. Unfortunatly I would be rechecking the rebuild....something is not in the correct position or is missing IMO
    Last edited by Aussie Steve; 04-26-2019 at 04:23 AM.
    Smoke me a kipper I'll be home in time for breakfast

  9. #7
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    Quote Originally Posted by OgeyTheTerrible View Post
    When it's warmed up and I apply the choke at cruising speed it just inxreases idle and restricts cruising rpm's to about 3k and make it extremely sluggish, which indicates rich, if it were lean then the choke would increase performance to an extent.
    Not sure we are on the same wavelength here.
    I meant to close the choke slightly at the point and under the conditions where you notice the problem.
    That likely is not "cruising speed".

    Also a vacuum leak can drive you nuts trying to compensate for it.
    That sometimes even occurs in the diaphragm of the fuel petcock.
    Are you sure that you got the vacuum hoses back in the right place on the carbs ??

    Good luck.
    Don't believe everything that you think.

  10. #8
    M-J Lifetime Achievement Award
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    Another thing is if the hoses are old and may begin to split when fitting and removing from the carb nipples.
    Smoke me a kipper I'll be home in time for breakfast

  11. #9
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    Quote Originally Posted by Aussie Steve View Post
    Another thing is if the hoses are old and may begin to split when fitting and removing from the carb nipples.
    And I forgot:
    Since you mentioned trouble at 1/8 throttle, it is possible that your idle speed is just a bit too low.

    This is NOT a Harley and should NOT shake at idle !!!
    Don't believe everything that you think.

  12. #10
    M-J Lifetime Achievement Award
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    I would suggest there are two problems at least. The mixture screw only adjusts the fuel ratio at idle and the transition to 1/8 throttle....exactly the problem you are having. The fact that they were all out at different settings points to someone else fiddling about with them. The setting will depend on what air filters you are running and also the tiny idle circuits being clean and free from blockages....even one of the tiny jets in just one carb being blocked will affect the idle and transition so attention is needed there perhaps.

    The other problem I think is the sluggish performance at higher rpms. This can be affected by the the following. Low float height, Blocked/reduced fuel filter/fuel flow, Needle size and position, Main jet size. The emulsion tubes also need to be clear and functioning.

    I know you said you fitted new bits and jets etc but some crap may have got sucked through into these new parts...esp if the carbs were off for a period of time allowing any gunk inside to dry and flake off. Same applies to the fuel tank and any filters there.

    If this bike has a fuel pump one of the most common faults resulting in poor acceleration is the internal filter being clogged...an extrernal one may also be clogged reducing fuel flow just when it is needed. Revers flushing with raw fuel can help...a good dose of carb cleaner in the tank may also help.

    Hope this helps.....I would be tempted to run some good quality carb cleaner through to see if it makes any difference.

    Good luck.

    Ps the higher rpm problems are not electrical are they?
    Last edited by Aussie Steve; 04-26-2019 at 11:11 PM.
    Smoke me a kipper I'll be home in time for breakfast

  13. #11
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    The original issue was sudden, literally. It ran fine on the way to work (about 40 miles with plenty of action through all throttle positions) and on the way home it abruptly wanted to stay in stop-and-go traffic. Out sounded like a cylinder was down and I noticed a steady stream of fuel leaking under the bike, turned out to be carb 4. In order to get going rpm's had to be near 5k to get it going and it wouldn't go faster than 30. I traced it to a bad float valve and replaced them all. While u was in there I noticed heavy resin buildup all over the interior of the carbs, checked a few o-rings and found them to be lightly cracked, so I ordered a rebuild kit with all new seals and such, installed everything after a good clean an put them back on the bike. It ran fine for about 2000 miles and then it started stumbling at 1/4 throttle. I pulled everything again and found the emulsion tubes to be very worn and that's when u replaced the emulsion tubes, jet needle and the other jets in the kit from factory pro. It got rid of the 1/4 throttle problem but I haven't been able to tune it for idle AND acceleration.

    Easy Rider:
    Adding any choke at any throttle position makes it stumble and reduces power, also, there are no vacuum leaks nor any vacuum hoses to leak from.

    Aussie Steve:
    See above, I re-re-rebuilt the whole thing, there is absolutely nothing in there that shouldn't be. I might not have been clear before, when warmed up it accelerates poorly up to 1/8 throttle, after that it pulls like a rocket. High rpm performance is as good as it's going to get in my opinion. There are no electrical issues that I've found.

    I just finished the third rebuild and put it on the bike, I still don't know what the mixture screws settings are supposed to be, so I'll start at 1-1/8 turns and go from there, similar carbs from different models (the rf900 was manufactured with about 30 different country codes, each one has different mixture settings from 1-1/8 to 1-3/.

  14. #12
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    Quote Originally Posted by OgeyTheTerrible View Post
    also, there are no vacuum leaks nor any vacuum hoses to leak from.

    See above, I re-re-rebuilt the whole thing, there is absolutely nothing in there that shouldn't be.
    How exactly do you know that there are no vacuum leaks ? Does it not have a vacuum operated petcock ??

    Then....unless you know exactly how all of the hidden internals of the carb jets are made and you run fine wires through all of them, you can not know beyond a shadow of a doubt that there is "nothing in there that shouldn't be".

    I am not saying these things because I necessarily think that either really has a bearing on your problem......but to try and emphasize that assumptions can take you down a rocky path sometimes.

    As was suggested earlier, I suggest a healthy dose of Berrymans B12 Chemtool in the gas and running it a bit to clean out the hidden passages that make up the low speed circuit.

    Beyond that......I wish you good luck.

    An exhaust gas analyzer might be useful in a case like this but they are hard to find these days.
    Don't believe everything that you think.

  15. #13
    M-J Lifetime Achievement Award
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    'accelerates poorly up to 1/8 throttle,' This can only be the idle circuits and or pilot jet settings nothing else is effected until post 1/8 throttle opening....other than the aformentioned and air intake, vacuume leaks, fuel supply and fuel level. Also a base setting for mixture screws is 1 1/2 turns out or used to be....
    Last edited by Aussie Steve; 04-28-2019 at 03:17 AM.
    Smoke me a kipper I'll be home in time for breakfast

  16. #14
    Ditch Magnet
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    i've got a 02 suzuki 650 and the mixture screw are set about 2.5 to 3.0 when i got it i set them at 1.5 after going thru the carbs and it would only start on one cylinder.

  17. #15
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    Alright, I took apart the carbs, cleaned them, ensured all the passageways were clear and all the gaskets/orings aren't broken/rotted, reassembled them, bench synced them, installed them along with new coil packs and wires, and then live synced them within 1/2 of Mercury apart. Set the pilot screw out to 1-1/4 turns (per similar model settings).

    Same.

    Damn.

    Problem.

    I'm thinking of just buying a set of used (untouched) carbs and see if that fixes it ..


 
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