PDA

View Full Version : Very General Advice on Carbs



fouraceswes
01-10-2010, 11:14 AM
Seems like this come up a lot lately, so here goes.

Couple things

Mystery Carbs are the gateway to a lot of tuning problems on Triumphs and other motorcycles. To me, and maybe to others as well, Mikunis all look the same. I have seen British stuff at the shop with Mikunis designed for two stroke applications and the customer has fiddled with it ad infinitum already. They are not gonna work. Also sometimes the carb is way too big for the displacement of the bike. "The guy told me it came off of a Triumph" is not a good way to buy a carb.

Number two, old, worn out, beat up carburettors of any manufacture are gonna give you problems. Amals and Mikunis both are suseptible to bad gas sitting in them for years and clogging up jets, I find Amals easier to clean in this condition. I have way more experience with Amals than Mikunis and so I can tweak an Amal when it is worn out much better than a Mikuni, but they both still have to be massaged and dealt with when they are old and beat up. Slides get worn out, bores get dented, springs get weak, jet passages get clogged.

Assess your carbs, try to figure out some of the issues with them and when they get old, save your dough and get new carbs, whether you decide to get Amals or Mikunis for your Brit Bike, get them somewhere like MAP cycle, they are probably not gonna be pre-jetted EXACTLY to match your elevation, etc, but they will be in the ball park an you can call and ask tech questions. Baseline jetting on Amals is in the part book for your year bike. You should have one.

Don't buy mystery carbs and don't replace worn out old carbs with another type of old worn out carb.

PS I see a chromed float bowl and I unsnap the strap on my revolver.
__________________

TRUMPNUT
01-10-2010, 12:22 PM
I don't know chromed out mono blocks are the shit ! LOL

Jethro
01-10-2010, 2:33 PM
If anyone has to start from a new Mikuni carb, I can supply ALL my settings on a pair of VM-32's running on a 71 T120 stock 650 with drag pipes. Bike runs GREAT, 50 miles to the gallon on the highway, and starts one or two kicks. I also highly recommend the tuning book on mikuni's from Sudco. You can often fin\d it on Ebay fairly cheap. I agree that Mikunis are often more difficult to dial in, but I honestly believe it is because of the MULTIPLE variables you can tune, which can be a real advantage. Plus the carbs are built like brick shit houses. My opinion only, and opinions are like assholes.. everyone has one.

http://i11.photobucket.com/albums/a172/jettoes1/Chopper%20Sep09/IMG_2995.jpg

fouraceswes
01-10-2010, 2:50 PM
Opinions like Jethro's come from experience, those are the opinions you want to listen to. When a guy's opinion starts out, "I heard. . . " that is when you want to listen politely, then expect problems.

Jashole
01-10-2010, 2:58 PM
If anyone has to start from a new Mikuni carb, I can supply ALL my settings on a pair of VM-32's running on a 71 T120 stock 650 with drag pipes. Bike runs GREAT, 50 miles to the gallon on the highway, and starts one or two kicks. I also highly recommend the tuning book on mikuni's from Sudco. You can often fin\d it on Ebay fairly cheap. I agree that Mikunis are often more difficult to dial in, but I honestly believe it is because of the MULTIPLE variables you can tune, which can be a real advantage. Plus the carbs are built like brick shit houses. My opinion only, and opinions are like assholes.. everyone has one.

http://i11.photobucket.com/albums/a172/jettoes1/Chopper%20Sep09/IMG_2995.jpg

is that a sticker on the gas tank? hahahah its fucking sick.

Jethro
01-10-2010, 8:06 PM
Something I forgot to mention. If you run drag pipes like mine... here is a shot of them...

http://i11.photobucket.com/albums/a172/jettoes1/Chopper%20Sep09/IMG_2998.jpg

I have found that a little bit of back pressure makes a huge difference in how the bike picks up from a stop-torque--and that it wont hesitate so much off the throttle (remember we have to rely on venturi with these older carbs). I mention this because some people blame the carb for these two issues. What I did, is to drill a hole about 1 and 1/2 inches up from the back of the exhaust, and put in a one inch stainless steel bolt with the bolt head on the outside and the threads inside the exhaust pipe. Then, I threaded a wingnut on the threads with a locking nut. This way you can adjust backpressure by how much you turn the wingnut perpendicular to the exhaust flow. I would strongly recommend you do this with open pipes before getting into the "tuning" loop with carbs.

framus
01-11-2010, 4:22 PM
Good advice all around here. Every carb really has its place and one has to evaluate what they want said carb to do. With a lot of old carbs, there is a point of deminishing returns...especially with Amals. We all know the drawbacks of each carb so I'm not going to beat that dead horse. Wes is spot on about the jetting. I really have no idea what some of the VM-32 and VM-34's were meant to be on but the jetting is so far off for what we've got it isn't funny. I think that is where Mikunis get a bad rap...a lot of people just think they can slap one on there out of the factory box and be alright. This is the furthest from the truth in all aspects.

One of these days, I'm going to have to go through all of the needles I save from each Mikuni carb I re-jet for our kits. I'll bet I've done well over 250 of 'em, not counting the triple kits which use the stock needles.

Jethro
01-11-2010, 6:43 PM
These are the setting I'm using now...

Main Jets 220
Jet needle 6DH2 (the clip is in the middle)
Slide 3.0
Needle Jet P4
Pilot Jet 40

framus
01-12-2010, 8:24 AM
It depends on the combo, cams, altitude, and whatnot but that doesn't seem like a bad combo. We run a slightly different jet needle and needle jet depending on the carb/quantity of carbs.

gravelveins
01-12-2010, 10:16 PM
this is great info all around.
i for one, have nothing to add to this conversation
other than saying thanks for sharing ya'lls experience and knowledge.