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Discussion Starter #1
New rebuild on 72 tiger. New 930 amal carb with 106 needle and 230 main, 3.5 slide, choke fitted, clip on middle setting. Im at 2500 feet elevation. Carb mounted on fat O ring. I originally set the air screw to 1.5 turns out and ran the bike and got it sorted, timed well etc. So here is the question:

It really likes the choke on for a looooong time. Like 2 minutes of warming up and 5 miles or light riding. If you try to pull the choke off too soon it would backfire lightly on decel and not idle. I finally got the thing to run smooth and idle fine at temp once I put the air screw so its only about 3/4 to half turn out. Does that sound right or should I adjust somewhere else? Thanks
 

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Adjust the mixture screw to where the engine runs best.If it slows down you've turned the screw the wrong way,or too far.
They usually run best with the mixture screw about 1-1/2 turns from bottomed.If it's a lot less,suspect an air leak or blockage in the pilot circuit (either the jet or a discharge hole in the carb body).
If it idles well at 1-1/2 turns but dies when you open the throttle,still check the discharge holes near the front of the slide.If you squirt WD-40 into the idle air bleed at the carb entry (the hole to the side),it should come through both discharge holes.

If the pilot circuit checks out OK,it could be running lean on the slide cutaway or needle-jet.It shouldn't be,if the slide and jet measure up OK.
 

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Carb mounted on fat O ring.
Also check if air is leaking at O ring:
What size is "fat O ring"? Is the carb tightened up properly against the manifold (not too tight to bend the carb but tight enough for no air leaks). I use Viton O ring so it doesn't swell and cause air leaks.
 

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Discussion Starter #4
yes fat O ring. Looks like a good seal. Its cranked down to about half the thickness of the oring with small equal gaps.

Seems to start and run and idle just fine no matter if I turn the screw from .5 out to 1.5 out it just slightly back fires and pops on decel and downshifting. Whilst declerating I can reach up and slightly close the choke and it calms down so thats why I started cranking the air screw in. If its no worry that my air screw is only .5 to .75 turn out I will run it that way a while and see how it goes. I assume this means its tendancy is lean so was considering raising the needle. If I do I will re check the passages for blockages and put in a new O ring.
 

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You say the carb is new, so the fuel level ought to be correct.
However, I think the throttle-valve (slider) should be a #3.
This will richen the mixture, allowing the 1 1.5 turns out of the mixture screw to be achieved.
This is the most efficient setting.
 

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Even though you have a new Amal, the float height does need to be checked while you have it off. It should be about 80 thou, or 2mm, below the top of the bowl when the float tangs are bottomed on the float needle, use an awl or small screw driver to push down on the float tangs. Hopefully, your pilot circuit is clear, unless your gas tank is rusty inside, you could put a fuel filter on the gas feed hoses to catch any impurities. Most engines need several minutes to completely warm up, that is not unusual. Don't over tighten the carb flange to the intake manifold, which it sounds like you have figured out. You are very close to being right, just needs a bit more fiddling. You haven't mentioned what air filter setup you are using, as that would make a difference too, just for reference.
It won't hurt anything to have the air screw at .5 out, but the correct way to tune it is warm it well, at idle, turn the air screw out until the engine runs slower and begins to falter. Then turn the screw in to make engine run regularly and faster. Now lower the throttle adjusting screw until engine idle is almost at a falter. Readjust the air screw to get best slow running, by procedure just mentioned.
You indicated a backfire, is it in the muffler or carb? Banging in the exhaust may be caused by too weak a pilot mixture when the throttle is closed or nearly closed. also it may be caused by too rich a pilot mixture and an air leak in the exhaust system: The reason in either case is that the mixture has not fired in the cylinder and has fired in the hot muffler. If the banging happens when the throttle is fairly wide open the trouble will be ignition -- not carburation. Therefore, double check your timing.
Amals are set up from factory for altitudes to 3000 feet. You can lower your needle if you are too rich in your 1/8 to 3/4 throttle openings and see if that cures it. To lower needle, put the clip in the top, #1 groove.
I hope this helps, keep us posted.

roadduster2:)
 

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New rebuild on 72 tiger. New 930 amal carb with 106 needle and 230 main, 3.5 slide, choke fitted, clip on middle setting. Im at 2500 feet elevation. Carb mounted on fat O ring. I originally set the air screw to 1.5 turns out and ran the bike and got it sorted, timed well etc. So here is the question:

It really likes the choke on for a looooong time. Like 2 minutes of warming up and 5 miles or light riding. If you try to pull the choke off too soon it would backfire lightly on decel and not idle. I finally got the thing to run smooth and idle fine at temp once I put the air screw so its only about 3/4 to half turn out. Does that sound right or should I adjust somewhere else? Thanks
My '72 TR6 has a Mikuni. My first 2 miles are downhill. I leave the richener on for the 1st 5 miles in cool weather or it will spit under hard acceleration.. My '73 TR7 has an Amal with a choke. In cool weather, I use 1/2 choke to get a 1st kick start. I run with the choke 1/4 on for the first 5 miles. In warm weather, over 70 degrees, I have no problem before they warm up. Once warmed up, I never use the choke/richener for the rest of the day unless I stop for over 2 hours. I don't warm them up at the house because of the steep downhill to the road. My air screw is 1 1/4 out. Bob
 

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Discussion Starter #8
THanks all. Its supposed to be dry and over 40 here Saturday so I will clean out idle circuit, check O ring mount, bring it up to temperature and do the slowest best idle adjusting and give it a couple more miles and then check strobe timing. Will report back from there.

Carb, and all connective gear is brand new. Im running the aftermarket tuna can filter right on the back of the carb. I checked float height and jets when I first mounted the carb back in late August. Idle circuit maybe a little gummy.

How much difference will the slide size make as Caulky indicates? Huge deal or very subtle? Thanks again
 

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How much difference will the slide size make as Caulky indicates? Huge deal or very subtle? Thanks again
Seat-of-the-pants impression is that 1/2 a size on the cutaway affects mixture strength about 5-10% at just under 1/4 throttle (less at any other throttle opening).Enough to foul plugs,if you started with the right slide and went to a richer one.

3-1/2 cutaway is correct for a single carb 650/750.If it's lean at 1/4 throttle,I'd try changing the cross-drilled hole on the needle-jet.It's cheaper than a new slide.Solder up one side of the 0.035" cross-drilled hole,and re-drill it to 0.020".You need a number 76 drill or a 0.50mm drill. If you don't like it,just drill it back out to 0.035".
 

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Discussion Starter #10
Well, it snowed so couldnt test ride but....I took the carb off and the fat oring was all cut up and out of position somehow. Must have been leaking some air. I must have got the wrong material.

Went back to the standard small o ring and gasket. warmed it and strobe time it and adjusted the slowest best idle as per the amal directions. Its at about 1.25 turns out now. I will have to wait until spring now to road test.

I did notice something else while strobe timing everything was looking good as the rpms come up but when it get over 3000 or so the strobe starts jumping around. The bike runs fine. Is it the bike or the strobe? What causes that? Vibration on loose connections or does the induction start picking up interference somehow? I dont understand electricity. Its a boyer with tympanium and all new wiring. Till spring....
 

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spark plugs

Also, what plugs should I be running? 72 tiger, boyer, tympanium. I will pick up some new ones before spring. Thanks
I highly recommend the NGK BR8EIX iridium spark plug. These give the best horsepower on a dynamometer. When you rev the bike to 3000 rpm to time the bike on its center stand, it will walk backwards. Set it on a rubber mat or get an assistant to hold it steady. Bob
 

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That O-Ring sounds like something I'd do! Did you hookup your strobe to another battery and not the one in your bike? It helps keep out any interference in the strobe that way. Any 12v works.
Sounds like you've got it fixed, hope so anyways.
Good Riding to You,

roadduster2
 

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Discussion Starter #16
Yes i took it up to 5k
From about 3k to 5k is where the strobe starts firing sporadically
Im using seperate car battery
Up to 3k the strobe is perfect only firing on timing
Rotar is tight and square
Dont know if its bike or strobe interfering
 

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....I took the carb off and the fat oring was all cut up and out of position somehow. Must have been leaking some air. I must have got the wrong material.

Went back to the standard small o ring and gasket.
dl.allen,

Once you go fat, you never go back. PM me with an address and I'll mail you an extra one I have of the O-ring I use. I use -219 viton (extra-extra fat) and I am very pleased with the sealing, gas resistance and heat insulation properties.

P.S.
sounds like your strobe wires may be affected by the higher vibes or the strobe is defective. Double check all your connections. Try to borrow a different strobe if you can. I strobe while sitting on bike on side stand so it doesn't dance. Don't store your bike just yet. Snow may be gone in a day or so. Really warm here for another couple of days.
 
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