Misc: Carburettors Amal TT Carburettors

[Cyborg]

Been doing a bit of reading on Amal TT carbs with regards to the air valve. I found some info written by Amal that states the air valve can be used (amongst other things like tuning) to richen the mixture for starting. My question is... does it really do much of anything at small throttle openings and at kickstarting speeds. Several folks have told me it's pretty much pointless for cold starts and doesn't really do much unless there is a larger throttle opening and the engine is turning slightly faster than kickstarting speed.

 

[davidd]

I was like most folks and blocked it off. Maurice Candy told me I should not do that as it was handy, so I installed everything. It certainly does work! If I forget about it as I am idling in the pits the bike will bog down as I enter the track until I open it. So, it seems effective as a choke. As for starting, I can't say because I am using rollers and the bike starts easily, on or off. But I do think it could be handy for kick starting based on what I am seeing.

David

 

[greg brillus]

It is basically an air bleed (bypass) around the slide, some one told me that using it was like changing up or down a full jet size, but what does that mean...... It would have been most handy for instances where there is a change in altitude, where climbing higher would cause the mixture to go rich, so using the slide would lean off this affect. In practice I found I would start the engine with the slide fully down (to enrichen the mixture) then bring the handlebar levers back about half way once the engine started to warm up, and left it there. I never had the chance to alter it during a race, as there is too much to concentrate on. This was with a set of genuine Lightning Amal TT 's 32 mm with the large 302 float bowls mounted on a 1000cc Twin running on Methanol.

 

[Cyborg]

I read some interesting stuff about using it to help determine jetting etc and that part of it makes sense. In tuning for speed it says fully open to fully shut is equal to 3 or 4 jet sizes and you can use it to determine correct jet size as well as adjust the mixture for altitude. I can see it helping with cold starts using a roller starter and slightly wider throttle openings, but have difficulty seeing how it would work very well with kicking it over with the throttle barely open, as it appears to affect the main jet. Looking forward to finding out for myself, although at the rate I'm progressing, I'll need a roller starter anyway.

 

[greg brillus]

It does affect the slow running as it is a slide bypass, remember these carbs don't have an idle stop screw, so generally when the throttle is closed the airflow is basically near "Cut off" hence the engine will generally die once the throttle is closed because there is insufficient air entering the engine to keep it running. In racing this is actually what you want so the engine quits if you part company with the bike. I don't know what engine you are running, but the jet sizes for a given engine size and rev range are quite well known, and the need to run excessively rich is not necessary, so aim for several sizes larger than what is recommended and come back from there. It is good to find people who have done what you are trying before and ask them. It saves a lot of wasted time, and unnecessary guess work, and it might just save your engine ............And your wallet............

 

[vibrac]

I think you are all on the right path the choke slide is not much help in starting but was a great help in suck it and see situations at different tracks with one practice and a jet guess before. When it comes to road use filing a hole about 1/4 inch square in back of slide will give you a tickover you will find this elsewhere in forum but Ron Kemp set up his racing comet like that so finely that at MCC races his kick starts would beat bumps one kick with foot returning to hit first and away so fast he had to demo it to officials when they said he jumped the start.

 

[John French]

I read or heard somewhere that the air slide was developed for use on the IOM and allowed the jetting to be adjusted between the highest and lowest altitudes of the course, I don't remember the source but it seems reasonable. The GP carb on my Velo has that feature blocked off and it works fine for street useage.

 

[greg brillus]

It has always been a tuning aid, and some have thought it was a type of choke. I have seen carb's with the whole piece cut off, such a shame, perhaps lack of room caused it. There is generally always a way around that kind of treatment, though shortage of time would definitely be a big contributor here.

 

[davidd]

It has always been a tuning aid, and some have thought it was a type of choke. I have seen carb's with the whole piece cut off, such a shame, perhaps lack of room caused it. There is generally always a way around that kind of treatment, though shortage of time would definitely be a big contributor here.

Amal itself felt it was a starting enrichment device and said so, at least in their History of Amal section found on Burlen's site.

I also noted in some GP literature that it was referred to as an "air choke."

I ran across these items about a decade ago when I was trying to figure out how the TT carb worked. I don't know if either is correct, but I realized that I was not going to figure out what it was all about unless I tried it. For me, it seemed to work better as a starting aid than a tuning device because the Vincent is not very sensitive to jet changes. However, because the enrichener works throughout the range of the carb, it will enrich the mixture at start also assuming the throttle is not held closed. (The fact that it works throughout the range can help you tune the needle and throttle valve.)

I thought it would be more useful for tuning with the large altitude changes I was riding in, but it was not, at least for main jets. At Bonneville I asked Marty Dickerson what jets he used at Bonneville and at Daytona as he was running an 1-3/8" GP (which was not legal in AHRMA). He said he never changed the main jet anywhere he raced.

The procedure for tuning with the air slide is to hit red line on the straight and note the rpms. On the next pass richen up the mixture (lower the air slide) and see if the rpms increase. This was not very useful because I could always pull 7000 rpm or higher with the jet I was running. As mentioned, the bike was an easy starter, so I felt the air slide was not useful to me, but it did enrich the starting mixture, which might be useful to others.

David

 

[Magnetoman]

My BB Gold Star came to me with a well sorted Concentric but, while on one hand I should leave well enough alone, on the other hand it would look better with the TT it had come with from the factory. Although I have two TTs of the correct size, and have collected information about these carburetors from every source I could locate, for now 'well-enough' has the upper hand so they sit on the shelf. However, my recent acquisition of a 1928 Ariel for next year's Cannonball cross country rally has me thinking of the TT again.

In their contemporary TT literature Amal pointed out that greatest power at full throttle wasn't the only concern in racing because "there are 'Governor's Bridges', and also that you have to 'get up' to full bore." The TT's needle has seven grooves and slides were originally supplied with six cutaways, allowing fine steps in tuning the carburetor. Presumably, these fine steps mean it would be possible to get the jetting closer to "perfect" over the entire range than can be achieved with a Type 6, Monobloc or Concentric. "Perfect" jetting over the full range would result in the highest possible fuel economy which is important to me on a bike whose tank only holds 2.4 U.S. gallons of fuel and where ~250 miles/day will be covered.

Note that a Mikuni on the Ariel isn't an option because of regulations, and a Mikuni on a Gold Star isn't because of aesthetics.

So, those of you who have lots of practical experience with TTs, can they be tuned to work better over their entire range of throttle positions than any of the "conventional" Amals? Even if this is the case, are they more sensitive to atmospheric conditions than conventional Amals? That is, if the jetting is perfect at 72 oF at sea level, will a TT be further away from perfection when it is at, say, 85oF and 2500 ft. than a conventional Amal would be?