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Post by tristram on Mar 3, 2024 5:04:37 GMT -5
Just joined the forum to lean on some more experience than i have!
My SR500 starts and runs evenly for 30-60 seconds but then runs very unevenly and smoke comes from the exhaust.
This all started after I had dismantled and cleaned the carb for the first time in several years to try and stop the engine from running rich (soot on the spark plug). The carb is now very clean (brake cleaner and air hose on all parts and tunnels) and the bottom of floats are set at 23.5mm from the side of the float bowl and I put in a new air filter but the engine is running much worse than before.
The only things that I found unexpected in this process were that the air adjust screw was fairly fully in (ie not 1 3/4 turns out) and the spring inside the float needle seemed very soft (possibly causing the float bowl to over fill?). I have run out of ideas to sort out the problem.
Can you suggest a procedure I could follow to identify and fix what is wrong?
Thanks in advance
Tristram
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Post by stevep on Mar 3, 2024 9:58:38 GMT -5
This is the carb with the bell crank and accelerator pump. What folks have found is that the bell crank linkage gets worn and creates issues. From experience of members on the old forum (now sadly defunct) there are those devoted to this carb and hopefully one of them will be along soon to guide you. What often ended up being the advice was to switch to another simpler 34mm Mikuni, without the added complexity of the accelerator pump. There are flat slide and round slide Mikuni carbs which offer a simpler life! You may find the boot to the airbox needs customising or replacing, or else switch to a cartridge air filter.
If you are in England, Allen's Performance in Nottingham should be able to provide a new carb. Avoid the incredibly cheap copies offered on eBay as they are NOT real Mikuni carbs.
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Post by tristram on Mar 3, 2024 14:23:17 GMT -5
many thanks setevep for the plan b. as it happens the crank at the top of the carb seems tight (and diaphrams intact) so i dont think thats the answer in my case. ideally i would get better understanding of the problem and fix if poss before expence of buying a new carb also i hope to learn a bit of diagnostic procedure from this forum. thanks
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Post by stevep on Mar 3, 2024 15:56:27 GMT -5
I admit I do not have this carb, but hoping this article may yield something useful, maybe something previously overlooked in your carb refresh, that gets you revving again! www.siue.edu/~rsutton/SR/mikuni.htmLooking at what you've changed, namely the air filter, is your new replacement the exact same type? Had the carb been adjusted to make up for the deficiencies of the old filter in some way (ie the old filter was dirty and now with the new filter it breathes easier?) Can you be certain all the parts were reassembled exactly as it was before? (Thinking here of the clip position and the thick/thin spacers on the needle assembly.) Did you add any new parts when you cleaned the carb? When you inspected the jet needle and the needle jet, was there scuffing or a step indicating wear on the needle? Have you compared the jet sizes against the stock parts list? Just trying to get a sense of how 'refreshed' the carb is now and whether it had departed from stock sizes etc. |
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Post by tristram on Mar 4, 2024 3:47:21 GMT -5
thanks stevep for your reply. great to have an objective assessment. thanks also for link to ron sutton's description of my carb.
i reassembled the carb with all the same settings as before and, apart from weak spring in float needle, did not notice any wear. The only change was to replace the old air filter for new (thinking that, as the old air filter was v dirty, it might have caused the original problem of engine running rich). Although I reassembled the 'air adjust screw'* (ron sutton's name for it) in the closed position as before, I have since tried it up to a few turns open with no difference on how the engine runs. It still runs very noisey and uneven with light grey smoke from exhaust.
I will try your suggestion and lower the needle position by raising the clip to see whether that lets less petrol in now that the new air filter is presumably letting more air in.
*would a better description of this be 'idle adjustment screw' as it wd seem to regulate the amount of fuel that gets into the carb when the neddle is low in the main jet?
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Post by andy on Mar 4, 2024 4:56:58 GMT -5
tristram,similar to you, I spent many hours tinkereing with the stock carb many many years back,I got to the point where the inconsistent running/starting was really beyond my logical reasoning  . By all means persevere and maybe it will all come good [be 100% the idle circuit is clear ] but I do not regret for one second replacing mine with a vm34 aftermarket carb and if you go this route there is plenty of help with jetting available on the forum.What I will also add is the stock carb did require a regular tweak/tune and the rubber diaphragms do deterioate over time especially with modern fuel [ethanol kills rubber], on the other hand the aftermarket carb is virtually maintenance free once you've got it set up right. Finally to throw this in :- to make jetting adjustments to the modern mikuni it is so simple compared to the stock item.Food for thought  . Andy |
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Post by tristram on Mar 4, 2024 11:26:34 GMT -5
Many thanks for your message Andy. Changing the needle hight made no difference. Both you and Stevep are pointing me towards a new VM34 carb (mikuni at 247 from allens with new cable and chinese copy at 47 from ebay).
What gets me is that I still dont know whats wrong and would hate to buy a new carb when the carb was not the problem. It also grates that it was running better before i cleaned the carb and replaced the air filter! May have to swallow my pride soon!
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Post by stevep on Mar 4, 2024 11:52:10 GMT -5
It can take two bites of the cherry to get a carb all the way clean. If you know anyone with a sonic bath, that's worth trying. Please avoid the chinese copy carbs, as they won't have the true tolerance and are expensive paperweights at best!! They are forgeries in effect and will not perform the function of a precision instrument.
Andy pointed you at the idle circuit and given the tiny passages in the carb, these are best cleared and cleared again to ensure previous cleaning hasn't just moved, rather than removed any dirt particles. If you have the correct carb cleaning tools use those rather than fuse wire which will scratch the passages and cause a secondary issue!!
I once had the carb off three times in a weekend before I got it perfect, but I got there. Patience and persistence are your friends, as well as us forum members of course!
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Post by tristram on Mar 5, 2024 11:19:12 GMT -5
Another day, another stripdown and carb clean. Brake cleaner squirts well through all tunnels (unfortunately i dont have a friend with sonic bath) and checked settings are stock (ie. needle jet clip on 2nd groove from top, float hight at 23.5mm and air adjust screw 1 3/4 turns out) the 3 diaphrams and needle jet seem ok/ unworn and a new float needle is installed as the old one had a weak spring. I particularly checked the idle circuit (ie pilot jet and air adjust screw). It is now running with slightly less smoke but noisey and rough as anything and wont tickover without a bit of revs.
I cd now remove and dismantle the carb with my eyes closed. (perhaps thats where im going wrong!). I wish i could spot whats going wrong. Will try one more time tomorrow before throwing ££ at the problem.
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Post by andy on Mar 5, 2024 12:00:09 GMT -5
Tristram, I’ve been down the same road! Last roll of the dice are replacement diaphragm’s unless your happy there all good. Just to add, Allen’s sound pricey at £247, have a shop around, I’m sure you can find genuine mikuni’s on flee bay cheaper if you end up down that road.
Andy
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Post by tristram on Mar 5, 2024 15:50:16 GMT -5
Many thanks for your support. £247 includes the new throttle cable i wd need. Here are vids i took of before and after I cleaned the carb. The before is on pavement and the after is on cobbles.
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Post by stevep on Mar 6, 2024 8:36:44 GMT -5
I know videos can sound extra tinny and noisy compared being next to the machine running, but I'm tempted to say there's a lot of mechanical noise likely unrelated to your carb woes. Is the cam chain adjusted correctly? Are the valve clearances in spec? (Given that it can prove hard to get a correct clearance with feelers if the valve tips/adjuster screws are not in good order.)
Worth checking those are in spec first, before going on to look at carb issues.
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Post by tristram on Mar 6, 2024 13:34:05 GMT -5
many thanks for comments and suggestions. more of a 'rattler' than a 'thumper' at the moment. valve clearances are spec but cam chain tensioner is set at maximum so may indicate loose cam chain, ill add checking that to the list. i fitted the non-spec exhaust a few years ago and had more noise and more power since.
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Post by gotsron on Mar 6, 2024 19:09:21 GMT -5
I admit I've only scanned the exchanges here, but sometimes it escapes notice that there is a diaphragm in the fuel shut-off valve on the fuel tank. This setup is responsible for shutting off the fuel flow whenever the engine is not running, If it gets a puncture, it can leak fuel into the venturi on the carb and cause over rich symptoms. You would need to block off this pipe (so it doesn't suck vacuum into the venturi when disconnected) in order to test this theory. (run the tap in the Prime position)
Otherwise you are in good hands, I switched carbs and Andy has saved my sanity on several occasions in the process.
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Post by andy on Mar 7, 2024 13:30:18 GMT -5
tristram,just been checking a few old posts on fuelling woes with the stock set-up,have a look at what I've cut and pasted below ,the petcock can very easily be overlooked: As we spoke about earlier I've took some pics of my mikuni set-up etc.etc.
I'll ping them over shortly.
Andy
Turn petcock to Prime to test the running, that is direct flow not dependent upon the vac function.
Note:
When you pull the vac line off the petcock look for any gas in that line and lower it to see if any runs out. If there is gas there the diaphragm is leaking thus allowing gas to be sucked into the carb and messing with the jetting. Really Hard to tune the carb if that is happening; btdt! Gave me FITS until I found it.
The above is about my last roll of the dice but worth a minor exploration !!
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. By all means persevere and maybe it will all come good [be 100% the idle circuit is clear ] but I do not regret for one second replacing mine with a vm34 aftermarket carb and if you go this route there is plenty of help with jetting available on the forum.What I will also add is the stock carb did require a regular tweak/tune and the rubber diaphragms do deterioate over time especially with modern fuel [ethanol kills rubber], on the other hand the aftermarket carb is virtually maintenance free once you've got it set up right. Finally to throw this in :- to make jetting adjustments to the modern mikuni it is so simple compared to the stock item.Food for thought 
.
