PDA

View Full Version : New carb for a Triumph?



grotto
11th October 2009, 15:13
I'm needing a new carb for my 650 Trophy.
Anyone know of a good alternative to the currently fitted amal concentric?
If I'm replacing with a Mikuni or Kehin do I maintain the same size?
cheers

Motu
11th October 2009, 17:08
You could go up a couple of mm without any problem.If using a diaphram carb you'll need to go much bigger...like 38mm or so.

Paul in NZ
11th October 2009, 17:13
Three options really... (well OK, 4)

New AMAL...

Mikuni kit (ring british spares) but you need about 2mm bigger as they don't flow as well as an amal (but are better built)

Fit something 'else' - you are on your own here but the skies the limit.

Repair the AMAL? Whats wrong with it?

grotto
12th October 2009, 13:20
The Amal needs re-sleeving as the bore is worn.
I had this done to one on my 500 which was fine, but one of the jets (idle?) was a little blocked and they couldn't clean it out properly, so it still coughs every now and then.
Maybe it would be best to re-sleeve the amal?

Paul in NZ
12th October 2009, 18:22
Yeah - its a fixed jet on some of them. I think the new ones have remoable ones.

I wouldn't worry because it could be tuned out - I have a shit load of info on them if you want it??

Tbird650
30th October 2009, 21:04
I'd sleeve the carb if it were mine. It will do wonders for the running & will mean you can tune it without chasing your tail. Graeme Cole of Cycletorque, Hamilton has done lots. The idle circuit can be difficult to clear but I've often wondered about soaking in some sort of the solvent. Something aggressive, but not too aggressive if you know what I mean. Pay to check the needle valve, should be viton tip. The needle jet can also get worn over time, yours should be #.106.

Kickaha
30th October 2009, 21:29
New AMAL...



I was just reading about them in classic bike
http://www.amalcarb.co.uk/

Paul in NZ
31st October 2009, 07:58
I've been through this 'coughing' problem on the C and in the end it was little tweaks on several fronts that solved the problem.

Its best to go over everything first and base line the tuning spec's (valves, timing, mech condition etc) even for seemingly unrelated things.

Really, I rode around the issue for a bit but as the miles clocked up, my problems got worse. (remember I have personally ridden this thing 20years and 100,000 miles) Sometimes the thing gets worse slowly and you get used to it.

I eventually realised it was this combination of newer things causing the problem and attacking just one of them was never going to solve the issue. Initially I thought it was my 'upgrade' to a boyer but - probably not.

Problem #1. The 'new' carb. The brand spanker 930 concentric has a fixed idle jet just like the majority of them made (ealiest ones had a removable one). There seems to be a manufacturing flaw with a considerable number of them causing them to run either lean or rich and no amount of fiddling with other settings will help. One solution is to redrill the poxy thing and fit a removable (and thus adjustable) jet. Eventually I just stuck the original slogged out body back on and noticed and immediate improvement.

Problem #2. Careful adjustment of the idle air screw (mixture) is vital and very very easy to get wrong and a lot more snsitive on modern fuels than it was back in the day. I basically lean it off as much as I dare and ride it.
If it pops on the over run when hot, it needs to got a tad richer.

Problem #3. Unleaded fuel. I've ridden the bike through from the leaded era to the early unleaded and into the hi octane unleaded era. The problem is worse on both my Moto Guzzi and the Trophy when using 98 Octane unleaded. 96 or 97 Octane unleaded makes it run much better down low but it's not as happy under unreasonable load (ie hauling us both up hills at full throttle and top gear). Careful fuel selection and using another brand helped a bit. (see below) ie BP is the worst for low speed fouling.

Problem #4 Spark Plugs. I have changed to extended reach plugs one grade hotter than factory for 1970. Triumph went 2 grade hotter and extended reach on the later Rita equipped T140's so I'm pretty safe I think. The theory is that the new fuels burn at a lower temperature and the plugs don't get hot enough to self clean any more.

Not had a problem so far and the bikes going well with several (well 3) 1,000 km plus trips under its wheels summer and winter. It even idles with the flogged out carb (most of the time)

Tbird650
1st November 2009, 01:33
Paul in NZ is right when he says get the tuning specs right first. That was always the first rule... get the plugs & points correct first. Don't forget the air filter and the exhaust shouldn't be leaking either. The carb is always last thing.

A point to consider is the insulator block between manifold and carb. This serves to keep heat from soaking into the carb body when the engine is shut down. Bikes with this original insulator will re-start and idle better than those without. I think what happens is the heat boils the petrol in the bowl which causes rough running till the carb temp normalizes.

The float bowls are notorious from bowing from overtightening. I carefully tap the 2 lugs flat before sanding flat with wet&dry paper on glass. Yes I said carefully tap the lugs flat....using a small dinky hammer.
How many know that float level is adjustable? The needle valve seat will move in the bowl if carefully tapped. The sad part is there's no exact spec from amal as to the proper level. If I recall, we used to set the float "flat" in the uppermost position.

Worn carb slide & body is another amal annoyance. The designers never gave any thought to the wear created by running 2 metals the same together. An engineering disaster! As the slide wears, air leaks past and plays havoc with mixture. When they get bad, there could a be a dead spot right off idle or the engine speed simply won't slow down to idle at all. Sleeving works quite well and I hear there are hard chrome plated replacement slides available now.
More later.

Paul in NZ
1st November 2009, 07:05
Not quite fair. AMAL gave plenty of thought to wear and the original design addressed that but the factories would only pay a set price for a carb so the spec was reduced and we got shitty carbs.

I do have to re adjust the float level on mine. Its a smidge on the low side now and the blasted tickled only just contacts the float. (level is within spec but trying to lean it out)

jellywrestler
1st November 2009, 07:10
The idle circuit can be difficult to clear but I've often wondered about soaking in some sort of the solvent. 106.

find someone with an ultrasonic cleaner these bring them back like new and aren't aggressive etc