bm82
New Member
Posts: 20 
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Post by bm82 on Nov 23, 2007 1:54:25 GMT -5
Hi all,
I was just contemplating on buying a new carb for my 1978 sr500, which was experiencing unreliability before i started rebuilding my bike a few months ago. It currently has the stock carb which i believe to be the VM34SS. Can anyone suggest an alternative thats relatively hassel free and reliable, and where the best place to obtain one would be. I am pretty hopeless with carbs and am looking for something that takes minimal setup and tuning. Im running a supertrapp muffler and aftermarket pod style air filter.
thanks,
ben
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Post by andy on Nov 23, 2007 13:24:17 GMT -5
hi ben,i've been down that route myself.my advice is replace it it with a mkuni vm34 or vm36.i bought mine from allensperformance in the u.k [where are you?] and they were very good to deal with.make sure you also buy the rubber manifold with the vacuum pipe stub.these carbs are simple to play with and they dont have the accelorator pump,they are an ideal replacement.my only regret is i should have gone upto a 36 because like you i have a bigbore silencer[straight thro] and a large k&n filter.hope this helps.nearly forgot,they are a single cable carb which comes in at the top so you will need a new throttle cable which allens also supplied.it is a universal cable that you can cut to suit,once you have done this you will need to solder the supplied nipple on the end,its not to tricky.
regards.
andy
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bm82
New Member
Posts: 20
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Post by bm82 on Nov 23, 2007 18:47:30 GMT -5
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Post by mattb on Nov 23, 2007 22:14:16 GMT -5
Hi Ben. I'm in the same position as yourself. '78 SR500's been in the garage for almost two months (being off it in this weather is driving me mad!), just had the cam shaft, rocker arms and everything up top replaced, to deal with a loud tapping up there (everything was worn), and now the bike won't start, or dies very easily, and the mechanic is almost certain it's the carb. Anyway, I did some research this morning, just google and you'll find plenty of places (eg Mikunioz) who'll do a MIkuni VM34 for around $200. At that price, I'm thinking that for a 1978 motorcycle it makes sense to kill the gremlins once for all by just putting a new carb on! Let us know how you go...
Matt
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Post by andy on Nov 24, 2007 14:12:45 GMT -5
hi ben,o.k i've been looking into jetting.the vm i've got has a 35 pilot and a 260 main,needle set on mid position.the vm36 is jetted the same but for a bigger mainjet 320.these sizes are a good reliable start point.extra jets for experimentation are quiite cheap.remember,the spark plug colour reveals a lot that is happening inside!let me know how how it goes.
andy
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Post by troutthunter on Nov 25, 2007 11:35:31 GMT -5
FYI here's a link to a german site about carby alternatives. I used this one to get the jetting on my Dellorto PHM 38. The jets they had listed were all right on or within 1 jet size. I suggest using the babel fish translator linked from: www.altavista.com and copy and paste this link to the url bar in the translator. www.motorang.com/bucheli-projekt/vergaseralternativen.htmgood luck on the carby! |
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bm82
New Member
Posts: 20
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Post by bm82 on Nov 25, 2007 22:21:49 GMT -5
Thanks guys for all the helpful suggestions. I have purchased the VM36 roundslide and now await its arrival. I just couldnt bare to keep stuffing around with the original carb. I know the bikes previous owner cleaned it up and rebuilt it, and it was fine initiallly, but it got progessively more unreliable. So ill have a go with the new carb and share the result. Ill just need to work out the other things i need to install the new carb. I will order a new throttle cable to suit, i already have a new air filter. Im not sure what else i need?
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Post by caferacercarl on Dec 3, 2007 2:12:09 GMT -5
There is a guy on this site, curlyone? great bloke, he has just had all this done and would gladly share the jetting data with you, he lives near Sydney airport, or phone me 02 63377599, but I'm in love with the dellorto's myself,or FCR"S, or the stock pumper rebuilt. regards Carl
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bm82
New Member
Posts: 20
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Post by bm82 on Feb 10, 2008 5:45:50 GMT -5
Hi all,
Just an update on the carb change. I have installed the vm36 carb, and got the bike running quite easily now, starts on first or second kick, which I am very happy with. I have had a few brief test rides, and I still havent been able to tune it the carb successfully. Now, I know little about carbs, so Ive made a few attempts at trying to simply adjust the idle and airscrews.
The bike initially produces black smoke, when revved, and with adjustment of the air screw, im able to reduce this. So I took the bike for a first test ride, and encountered some issues.
Initially, I fouled the plug on the first run. The bike moved away fine, but when coming to a stop, it would cut out and die. This kept happening until finally as I came to a stop, the bike stopped again and could not be started. Took the plug out and discovered that it was a sooty black. I replaced the plug, and attempted to re-adjust the air screw and idle together. Bike started again easily. Second test ride, and the same issues. Through the gears, the bike is fine, although power seems to dwindle (seemed to splutter) a bit at the top of the rev range in first and second gears, third gear seems to pull quite well. However, as i come to a stop, at an intersection, ect, the bike tends to cut out and die. This seems to happen every time, however i am able to keep it alive by bliping the throttle as i come to a full stop.
I know my adjustment of the carb is incorrect, but im not sure how to rectify it. I have made no changes to the original jets, ect, that came with the carb standard, as i didnt want to tinker with the carb itself, I have limited experience with these things.
These are the standard specs of the carb, as purchased and installed:
36mm bore, 43mm spigot mount, 310 main jet, 35 pilot jet, Q-5 needle jet, 2.5 throttle valve, 6FJ6 jet needle 2.0 air jet.
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Post by andy on Feb 10, 2008 12:40:13 GMT -5
Hi bm82,this is interesting and very similar to my problems with my vm34,although perhaps not as bad as yours.occasionally when i roll off the throttle coming to a stop i get cut out problems which is a pain at junctions.also in the low rev range it is a bit fluffy and not crisp,i'm talking 'just off throttle',once underway its spot on and pulls and sounds just right.i'm gonna put a post up re-pilot jet problems as my thoughts are this could be the problem.no doubt a short session on a dyno would reveal all and if i can't crack it myself thats the route i may take.keep this thread going i'm interested to see what remedies you come up with.
Regards,Andy
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Post by pablosrfivehundred on Feb 15, 2008 20:53:34 GMT -5
i'm having the same problems.
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Post by andy on Feb 16, 2008 4:53:42 GMT -5
Pablo,try this;-warm the bike up before you get on board.choose a route that takes in loads of slow riding,try not to open the throttle anymore than approx 1/8th [this is the range the pilot jet operates in i.e 0 to 1/8th]after 3-4 miles take the plug out and inspect,it should be between a tan to grey colour ideally.adjust your airscrew and tickover if needed and repeat the process.I noticed on mine that it ran just right from 1/8th throttle and above but the plug was sooty black below this.mine is now better but i'm still considering trying a couple of smaller pilots as my airscrew is almost out of adjustment,i reckon that if all is well it should be 1and 1/2 turns out.REMEMBER plug colour reveals all!
Regards,Andy
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Post by pablosrfivehundred on Feb 16, 2008 13:35:49 GMT -5
yes black and sooty.
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Post by StewRoss on Feb 17, 2008 2:17:35 GMT -5
I had problems with one of my 34mm standard carbs on my road SR the other day that had me knackered...at first...I rode the bike and it was running as rich as anything at low to midrange...black plug, black exhaust, running roughly and generally a real pain in the ***.  This problem had come at the same time as a possible electrical problem as well, and all in all I wasn't very happy with it. I changed the carby over with one that I had spare, which had a seized mixture screw...and it wasn't much better. So after riding it a bit this morning I decided that I'd had enough . I came home and removed the second carby and put them both on the bench. I then played around with the second carby and looked at the operation of the accel pump...move the quadrant, the pump lever goes down and stays down...keep moving the quadrant the lever stops and the rod travels through it. Looks about right... I looked at the carby that I'd first had problems with; ...' move the quadrant'...the lever didn't move  I tried it again and same result...I had another look at the parts and found that the split pin that went through the rod was gone and so was the bottom washer...  The rod was still situated through the lever though...just...hmmm, wonder what that does to the carby?  I put another washer and pin on it, a stainless type this time, and put it on the bike. Took it for a ride of about 120ks...it ran perfectly. Great found the problem but why? If the accelerator pump is disconnected surely the pump just won't operate...unless it somehow allows fuel to run through the carby all the time with the lever disconnected from the rod (suction?)...which would explain the really rich running problems I was experiencing...now that it's running again I will keep it on...for now! Must read up on the operation of the carby some more... Now...back to the Mikuni VM carbies...(...got a bit sidetracked...) the mixture screw controls an air bleed...if it runs at the highest revs adjusted nearly all the way in...PJ is too small...if it runs at the highest revs a long way out...PJ is too big...as you say it should run at the highest revs about 1.5 turns out...in an ideal World...only change them in 'next size up/down' increments... Plug chops are great for determining the correct MJ size... I'm going to have to brush up on all of this when I change the standard SR carby for a VM38...which may be sooner rather than later if this carby doesn't behave. I also have a Mikuni accessory pump kit that I can fit to a standard carby as well...might be fun to play...heh, heh... ;D SR  |
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Post by marlon on Feb 17, 2008 5:43:30 GMT -5
Completely off topic, but bring on EFI!
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I tried it again and same result...I had another look at the parts and found that the split pin that went through the rod was gone and so was the bottom washer... 
