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PR: Proprietary Items TT carb

vibrac

Well Known and Active Website User
VOC Member
Following on the discussion on TT carbs on a 'welcome too' post I see that there is a rare bird flown in on ebay

I have seen his stuff before and it all looks good he was the source for the BTH (original not electronic) mag on my grey flash
I only have a flange fitting carb on my GF replica because you dont find many clip fitting ones
 

litnman

Well Known and Active Website User
VOC Member
Actually, the slides in both machines are #6 (.375") Can't say that I have seen the TT slides that were measured in 32nd's of an inch. The type 14 float bowls that came with
several Lightnings and they have the bell shaped float needle (14/024) that is pulled up into the seat and it has only one groove. Quite different than the type 302 float bowls.
Both machines have rebuilt KVFTT mags.
 

Cyborg

Well Known and Active Website User
VOC Member
Anyone here experience occasional flooding due to worn needle valve or seat? If so,
how did you deal with it, other than finding new parts. I'm referring to the 14/ series.
Maybe some fine (yellow) Time Saver lapping compound if the needle and seat aren’t too bad.
950A0AB5-B36C-4F40-BE28-30D1873F850E.jpeg
 

greg brillus

Well Known and Active Website User
VOC Member
Interesting about the slide cut away...... I wasn't sure about the number verses the actual height figure........anyway tuning carb's is all about trial and error.......plus knowing which part to adjust to get the correct result.........The absolute first thing you should always do is check the float level with a home made sight/level tube with modified banjo nut........For some reason I have found setting the levels on the larger 302 type bowls is a challenge, either on petrol or methanol fuels.......hard to say why, not sure if the SG of modern fuels is different from those back in the day. There are several mods that need to be done if the level is out.......it is usually too high, thus the engine runs too rich.......For racing it is very important (and no different for a road engine really) that the engine responds cleanly to opening the throttle out of corners, the larger the engine the more important as the power on/off will upset the bike quite badly.......The correct float levels and the slide cut away affect this area directly.
 

Magnetoman

Well Known and Active Website User
VOC Member
not sure if the SG of modern fuels is different from those back in the day.
Well, since you asked...

Making a long story short, almost 20 years ago I did a series of experiments to help a friend get his jetting close if he raced at the bottom of Death Valley one weekend in summer, and at the top of Pikes Peak another weekend in winter. This would let him quickly zero in on the right jetting and spend more time getting to know the course.

Anyway, Amal's jet testing specifications call for fuel with density 0.710 g/cm3 at 15 ℃ (59 ℉). I measured the following densities for four brands of gasoline that I purchased from busy stations (i.e. with a high turnover of fuel) in the summer.

Chevron __ 0.710 ___ 90 ℉
Union 76 _ 0.740 ___ 87 ℉
Texaco ___ 0.749 ___ 88 ℉
Exxon ____ 0.758 ___ 88 ℉

Without going into the (interesting) details or consequences, note that all these measurements were made at temperatures ~30 ℉ higher than Amal specified, so all would have higher densities if measured at 59 ℉, but even at elevated temperatures they have greater densities than the fuel Amal specified back in the day.

The flow rate through a jet depends on the density/viscosity (plus other factors) and, like the density, the viscosity also increases as the temperature is lowered, although with a different slope. I also measured the density and viscosity as a function of temperature of the brand of race gas he used at that time (VP C12), since using it eliminated the variability of "ordinary" gasoline. I programmed all of this into a calculator so if my friend had found his jetting to be perfect at, say, a race in Death Valley when the temperature was 120 ℉, plugging that jet size into the calculator along with the measured temperature and RAD at a different track would give him the new jet size to use, which would be very close to perfect. At least, down to 25 ℉. If he had been racing snowmobiles, I would have extended my measurements to lower temperatures...

Coming back to what Greg wrote, what the above means is if you built a jet-testing rig like Amal specified and used, say, Exxon fuel in it at 59 ℉, the flow you would measure in cc/minute of a, say, 200 Amal jet would not be 200. My guess is Mikuni used the wrong fuel when they set up their calibration rig decades ago, which is why their jets flow 16% different than Amal jets.
 

Magnetoman

Well Known and Active Website User
VOC Member
Plus maybe a slight difference in the way the (entrance to the) jet was bored? Assuming there was any difference.
Amal developed their testing rig as a result of WWI, which predates the memories of all but the oldest (and grumpiest...) VOC members reading this. I looked into it in quite a bit of detail at the time but could not find a mention of viscosity in Amal's specifications, only the density and temperature of the fuel. Another possibility is Mikuni blended a fuel having the necessary density, but the resulting viscosity was different than that of the fuel Amal had used.
 

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