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Long stroke crankshaft for 500/535cc EFI engine including the GT

All English crankshaft for the fuel injected 500/535cc EFi engine including GT

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Mikarb float height measurement

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Mikarb float height measurement
Started at 03:00pm on the 10th October, 2017 by stinkwheel
stinkwheel Subject: Mikarb float height measurement
I'm a tad confused. In our hosts technical notes it states float height should be measured from the body of the carb but then shows a picture of it apparently being measured from the lip round the carb body.

Which is it? Should I measure from the lip (blue line) or the recess the gasket sits in (red line)? There is a roughly 3mm difference, which is a lot.

Posted: 03:00pm 10th October, 2017
Alan R Subject: Mikarb float height measurement
Hi Matey !!--------- I think the clue is in their instructions}--- Quote..."and with the float chamber gasket removed measure from the carb body surface to the top of the float, as shown below. The standard measurement is between 24-26mm"

So remove the gasket then measure from your RED line...at least that's how I set mine..
Posted: 07:30pm 10th October, 2017
stinkwheel Subject: Mikarb float height measurement
Thanks. It was the "As shown below" bit that was confusing me because it shows it being measured from the top of the lip, effectively contradicting the written instructions.

I have it a bit on the low side then because I did it like the picture. Hopefully this will fix the high rev spluttering.

Posted: 11:15pm 10th October, 2017
Leon Novello Subject: Mikarb float height measurement
It should not matter; you have 3mm of leeway, the carburetor is not that sensitive to 2 mm up or down.
Posted: 07:05am 11th October, 2017
Presto Subject: Mikarb float height measurement
Leon is right - the tolerance is sufficient to allow for the discrepancy. But the recommended method is to measure to the lip on which a straight edge would rest if placed on the joint surface. In other words as per the illustration in the notes. But although some care needs to be taken, the measurement is not absolutely critical to the last mm in this application.
Posted: 08:41am 11th October, 2017
Presto Subject: Mikarb float height measurement
PS - the illustration in the technical note (on this site and reproduced above) corresponds with the illustration in the Mikuni tuning guide - i.e. float level measurement is the distance from the bottom of the float assembly to the bottom of the carburetor body (what becomes the top surface of the carburetor body when it is inverted) with no reference being made to the recessed surface on which the gasket is placed.
Posted: 08:51am 11th October, 2017
Alan R Subject: Mikarb float height measurement
Hi Guys------ yes, those instructions can seem to be a bit contradictory at first, can't they.........I've just had a look at my spare carb. to refresh my memory and it seems we're all correct.......ie}-- remove the gasket, place the steel ruler's edge on your RED line ( Which is a good broad surface compared to the thin lip ) but take the reading starting from the BLUE line ( the lip )...As mentioned it's not that critical and bending the Brass tag is a matter of guesswork at best.....I have an old 6" steel ruler that I've pre-marked those two settings on in the form of little nicks on the ruler edge...So it's become a form of "Go"..."No-go" gauge if you like.....Saves straining the old eyes, that's for sure

...Whilst you have the carb. apart why not inspect the float needle tip for ridges ( or fit a viton tipped one ), roll the main needle on a flat surface to check for straightness, the slide for wear or "Rattle" in it's bore and all jets for damage---especially the slow running one..
Posted: 10:02am 11th October, 2017
Presto Subject: Mikarb float height measurement
PPS [!] - to me the tech note seems as clear as daylight. the reason the gasket is removed is that it is possible that it may obtrude above the carb body surface. but simply measure as the tech note explains and all will be well. :-)
Posted: 10:26am 11th October, 2017
Alan R Subject: Mikarb float height measurement
"Many are the roads that lead to Rome"
Posted: 10:33am 11th October, 2017
stinkwheel Subject: Mikarb float height measurement
Aye, already replaced the needle valve because it was leaking. Going to throw a new main jet in there too. Needle itslef looks pretty ship-shape.

I'm actually wondering if my high rev stutter is points chatter, despite it feeling like fuel starvation. I solved the first misfire by replacing the 1950's condenser that had been fitted by the previous owner (probably not happy playing with 12v). When I look, the "spring" on the points is also way longer than on my other bullet suggesting this is also wrong and perhaps can't stay shut properly at higher rev speeds.

This is what was fitted. It's an indian stator plate but I think the points and condenser are off an old brit model. Condenser was definately duff in any case.:

Posted: 10:34am 11th October, 2017
stinkwheel Subject: Mikarb float height measurement
Thought I'd just post an update on this.

I'd pretty much fiddled with everything I could think of and was still getting a high rev stutter then cutting out on the overrun, almost like fuel starvation. I was at the point of giving up and wheeling it into the naughty corner for the rest of the winter when it started making an odd misfire at idle. Lots of clattering and noise through the exhaust, like a tightened exhaust pushrod.

Couldn't see anything, tappets were fine. When I built the engine, I had a choice of heads. One was shiny, polished and obviously freshly rebuilt with new valves, stem oil seals and twin plugged, the other was a bog-standard, well used one. Obviously, I fitted the shiny new-looking one.

I swapped out the heads, if fired up first kick, settled to a steady idle then rode out for 5 miles without a stutter. Might need to tweak the high rev mixture but it's now tuning, not fault finding.

I've not pulled the shiny head apart yet. I don't really need it but might have a go out of curiosity. Looking at it now, the valve guides look shiny and new and the stem oil seal is now floating about on the stem. I now wonder if the person who built this head used our hosts replacement valve guides then forced oil seals on top (which do not actually fit on the replacement guides). If they did that, I also wonder how carefully they fitted the guides. My suspicion is that the exhaust valve has been binding in the guide, sticking open and that it's also been pummelling the ill-fitting oil seal at high revs, possibly preventing the valve from opening fully.

It'll teach me to judge a book by its cover. Every other part of the engine the shiny head came with has been in some way subtly wrong or badly done to the point it makes it not work.

Posted: 10:29am 26th October, 2017
stinkwheel Subject: Mikarb float height measurement
Picture of the "shiney" head.

Posted: 10:30am 26th October, 2017
Alan R Subject: Mikarb float height measurement
Hi matey !!-------- is that twin spark plugs I see before me ??........Did you fit an HT coil capable of handling them ??.......I also had the problem re}--- valve stem seals not fitting onto a new valve guide..In the end I took a new guide along to my local Cyl. head engineering firm and they found a seal to fit properly......I don't fit them onto exhaust guides as they need all the lubrication they can get...
Posted: 02:38pm 26th October, 2017
stinkwheel Subject: Mikarb float height measurement
Aye, it came with our hosts recommended twin coil and two brand new iridium tip plugs. I bunged an old Lucas on I had lying about back on to change back to single HT lead (which was a total pain having re-fitted the toolboxes and tea caddy).

Closest I found for oil seals that would fit my tuned 350 head were off an Austin mini 1100 but they are the all rubber type, no spring/band on them.
Posted: 03:29pm 26th October, 2017

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