380 with Amal carbs

[tooslow]

OK Hi All,
Just finishing off my 380 build, wiring etc to do yet, bought half completed, so it is a GT380 72 reg plate which acording to the previous owner, and the receipt from the engine builder it is "race Tuned" what spec I have no idea.
it is running Amal 2600 MK2 concentric in two stroke spec cleaned by previous owner
anyway it is time to start the engine
Fuel in, pulled the choke out no start,
check its sparking OK on that side,
try again forgot to pull the choke fired right up and ran ok and rev'd cleanly.
pulled the choke just died.
Mixture too rich? adjusted the air bleed screw no effect.

removed carbs to look at the pilot jet sizes no markings but a 1 mm drill fits down snugly
sound too large for me but I dont know.

So my question is has anyone any experience with Amal carbs and jetting, they go by flow rate not mm size
approx what size would be a good start point? or do I just plump for a size and work backward or forwards from there

Martin

20200809_112342 by mbradl12,
20200809_111816 by mbradl12,

[GTandcbr]

Hi and welcome to the forum. Firstly i am no expert on Amal carbs. Two stroke engines need a heavy dose of enrichment to get started but once running the choke should be switched off as soon as possible. I'm not sure why you pulled the choke whilst the engine was running? Thats a sure way to flood it.
It is very warm at the moment it is possible the engine would start without the choke.
Nice looking bike you have there by the way

[Craig380]

Hi and welcome, nice looking bike. Especially like the way the tank's sides have been scalloped.

Can't help with the carbs but the Piper 3-1 exhaust will stifle the bike's top end, they stop the engine revving past 7,000 like you hit a rev limiter. Sound excellent though

[jabcb]

Good looking bike!

The side cover mods are an improvement & get rid of interference with pod filters.
Tank scallops look really symmetrical. Am curious how that was done.

Have no insight into use of those Amal carbs. But I would be worried about the possibility of holed pistons.

[GTandcbr]

Does this help you?
https://www.google.com/url?sa=t&source= ... JC4FLEQMx2" onclick="window.open(this.href);return false;

[tooslow]

Yes I have that info,
Some updates. Swapped the pilot jet for size 30s starts on two outer cyl’s 1st kick will now start on choke as well also revs really crisp and quick,
however middle cyl doesn’t run,
Swapped coils/ plugs and leads etc with the left hand cyl still no fire on middle Cylinder, but everything works on the left hand cylinder.
Compression is ok new plugs point etc timing spot on

Next to restrip, clean and maybe swap the carbs around

Onward and upward

Martin

[Alan H]

First of all check the plug is sparking. If you swap (say) left and centre, at least you'll prove that a known good plug fires or not. The centre one may just be clagged up. Then check that the centre points are working ok.
Where are you based in the UK in case someone is near? I'm near Barnsley in South Yorkshire.
No experience with those carbs, so if they're close, it's worthwhile persevering with them, otherwise try a set of standard carbs te see if they would be easier to set.

[tooslow]

I have swapped plugs, leads and coils, this was the first thing I did fault stays with the center cyl.

Tonight I am going to try and spray fuel in the centre carb to see if it will at least fire on that cyl. if it does then its the carb

I'm in Bedford. UK

I had thought of getting standard carbs as a last resort, but these should run well it does on the other two cylinders anyway

[old racer]

Have you tried swaping the centre carb for one of the others.

[tz375]

It has been decades since I messed with AMAL carbs. Last Amal main jets I ran on a methanol T120 were 400's IIRC. Back in the day I have used GP, TT, concentrics (Mk1) and monoblocs, but that was all a very long time ago in a galaxy far far away, so take all this with a large grain of salt.

According the web, stock 2 stroke set up on those carbs is:

Pilot jet: 25
Needle Jet: 106
Main Jet: 160

Stock GT380 with Mikuni carbs has #80 main jets which flow almost the same as a #80 Amal according to the charts and should be approximately 0.773mm diameter.

The pilot jet should be smaller than 1mm and closer to 0.4mm or close to that if it's equivalent to a #25. #30 sound a lot closer to what we would expect it to need but may still be slightly rich. Tweak the air/mixture screws to fine tune it.

Your bike has no air filters and an aftermarket exhaust so let's start at say Mikuni #90 jet so, we would expect it to use somewhere around a #90 AMAL if they are flow rated or 0.8mm or at least in that ball park.

What mains are in your carbs?