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By black fingernail
#84115
Since I got the bike, a 1998 500, it was nearly impossible to 'rev up' without horrible spitting back through the carb, The bike had been fitted with a new carb, a 'Mikcarb', when I got it. I stripped it and blew it through to make sure it was clean, no difference.
I set the ignition timing spot on, and a bit advanced, and a bit retarded, no difference, so I re-set it back to 'spot on', I also checked the advance/retard mechanism, all good.
I squirted carb cleaner around the carb rubber mounting and inlet manifold while it was ticking over, no difference, if there was an air leak, the engine would have revved up. I checked the tappets, all ok.
So, back to the carb, as a shorter less restricted exhaust, and a cone type air filter is fitted, I have up jetted as per our hosts jetting guide, all to no avail.
I stripped it down again, this time, I got the magnifying glass out and examined the casting for defects, all appeared good until I looked at the drilling that the pilot jet feeds into, it was not round, it had a minute 'flat spot', this hole must have been cast. I got a ' micro' drill, I can't remember the size, but it just slid into the lower part of the hole, I twiddled it around with my fingers until it exited through nicely, removing the 'flat spot'.
I re-fit the carb..... drum roll..... CURED, it revs up lovely now, I was getting a bit worried, I must admit.
User avatar
By Adrian
#84116
What with Wheaters' problem with fuel starvation there seems to be an issue with more recent Mikarbs in that they're not always being properly checked/finished . Well spotted.

A.
User avatar
By Wheaters
#84117
Glad it's not just me! But also glad you found the problem; certainly spitting back is a sure sign of weak mixture.

I can only wonder if the reason my 350 had previously seen off four owners in only 3,000 miles was the ongoing carburettor problem causing the bike to cut out - mine had a fuel flow restriction problem at the top end; a similar problem to this one, but with the metal casting/drilling at the fuel inlet at the top of the float chamber.

I'm much happier with the concentric type carb (albeit in my case a Wassell 26mm) I've now fitted. It's made to a higher quality compared to the Mikcarb. In particular it has a much better fuel inlet arrangement, from the bottom of the float chamber rather than the top and also has a "tickler" button. It starts first kick now and has more top end.
User avatar
By Presto
#84158
Considering that the Mikcarb is based on the (very satisfactory) Mikuni VM series I’d not be too quick to dismiss it on account if its design and top feed arrangement. (Just a passing thought.)
User avatar
By Adrian
#84165
Well the Mikuni VM design is good, but the manufacturing quality of the Mikarb copy, maybe less so?

A.
User avatar
By Wheaters
#84168
I agree that the design is possibly adequate, but quality control, in the case of the 24 mm one on my bike, not good.

But too quick to dismiss it? The fuel starvation problem on my bike is documented on this forum. It goes back almost two years and has cost me many hours of work, two new fuel taps, a service kit and a second fuel line filter change (none of which made any difference because I eventually discovered the problem was a manufacturing/machining fault in the casting itself).

Once I cured the initial severe valve seat recession problem I had on this bike and got it running once it kept compression for more than about fifty miles at a time I soon discovered it also had this annoying and potentially dangerous fuel flow problem. I think the four previous owners gave up with the bike because all of them sold it on after a few hundred miles each; it had done a total of just 3,000 miles in twelve years before my ownership. I've since put 6,000 miles on the bike in three years but until recently I was unable to find the cause of the same fuel problem. I'd almost learned to live with it but it annoyed me every time I went out on the bike and opened the throttle to keep up with main road traffic.

The 26mm concentric carb I've now fitted is much better made, works better all round and in my opinion well worth £75!
The Mikcarb is now in the spares cupboard.
By Clement
#84175
This all sounds similar to the crusade which I undertook: clean carb; replace points, sparkplug, condenser, re-set timing several times etc all to make starting easier, prevent kick back and farting through the carb which is a (new) VM28.
Black Fingernail, can you give me some additional info: so remove pilot jet and then (by hand?) poke around with drill bit? Which size?
Many thanks. Clement
User avatar
By black fingernail
#84191
Hi, Clement,
I just removed the pilot jet,( the slide was still hanging on the cable ), I held it up to the light and looked through a magnifying glass through the hole.
I do not know what size the micro drill was, I just selected one that slid into the first part of the hole, then I just gently 'twiddled' it between my fingers until it came out into the venturi.

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