Britbike forum

Classic British SparesKlempf British PartsBaxter CycleBritBike Sponsor SteadfastCyclesThe Bonneville ShopLowbrow CustomsSRM EngineeringGirling Classic MotorcycleLucas Classic MotorcycleHepolite PistonsIndustrial tec supply

Upgrade your membership: Premium Membership Gold Membership

New Sponsor post
Cyber Monday Sale - 10% off at The Bonneville Shop
by The Bonneville Shop - 11/29/21 5:34 pm
New FAQ post
How to find my own threads?
by reverb - 11/20/21 3:55 pm
Manuals on DVD - Buy 4 for 3
All 4 DVD Manual
Member Spotlight
Maryland, USA
Posts: 7,888
Joined: November 2005
Comment Guidelines: Do post respectful and insightful comments. Don't flame, hate, spam.
Top Posters(30 Days)
NickL 79
Rohan 73
Top Likes Received (30 Days)
Newest Members
old ion limey, muzz750, Motorcycles and Art, SteveDee, Gus_Carrero
11,924 Registered Users
Who's Online Now
9 members (ducati2242, Stuart Kirk, koncretekid, Robert Dentico, Gary Caines, L.A.B., gavin eisler, wilksville, John Harvey), 30 guests, and 15 robots.
Key: Admin, Global Mod, Mod
Thread Like Summary
Allan G, Hillbilly bike, splash
Total Likes: 8
Original Post (Thread Starter)
#842760 03/13/2021 11:02 PM
by splash
So, right spark plug is running rich af. Flat black carbon build up on right spark plug. Left plug is color of toast with very light signs of white ash on arm. After cool primed start up it will idle for a moment without any throttle. It won’t sputter at all but will suddenly stop as if the flooded prime in throat of carb ran out. Kicked over again no prime with some throttle it cranks back up. After warm up it has trouble idling when coming to a stop. It is rather nerve racking having to turn up throttle while squeezing front brake as I come to a rolling stop. This (warm) IS when it sputters to a kill unlike the cool start.

I have:
1. Cleaned and Rotated plugs still same results.
2. Cleaned carb bowl by removing bottom plug and sprayed with carb cleaner.
3. Moved air mixture screw out for leaner mixture (this is when the light ash now on left side while right is still heavy carbon).
4. Sprayed carb cleaner around all carb gaskets looking for leaks via engine kill. No leak.
5. New air filter.

Should I unscrew jet from bottom bowl plug and spray cleaner up? Remove air adjust screw and blow out orifice with cleaner? I’m thinking the prime tickler could not be sealing all the way up after pushed being on that right side. Does the primer flood from center of throat or from somewhere on the right side near button?

Don’t know how important or if there is a connection but before the rebuild the readings where just the opposite with lean on the right and rich on left. Then the right wrist pin cracked piston skirt and came out. This was the reason for nearly complete rebuild.
Liked Replies
by koan58
Should be at 38 deg before TDC on the compression stroke, with the breaker cam locked fully clockwise.

The breaker cam moves at half the rate of the crankshaft, so when 360 deg crank is mentioned, that means the other set of breakers at 180 deg camshaft are concerned.

If you're just guessing your timing you are taking enormous risks, and no diagnosis is possible.
2 members like this
by Allan G
Allan G
Hi splash, set the 0.015” gap when the points are closed, if you try and set the gap when the cam is on the ramp you will never open the points.

You’ll need to strobe time each cylinder after setting the timing, there’s little chance that you’ll get it spot on by eye alone. That said a well setup set of points run sweet as a nut and are still my preferred use... the only reason I don’t use points any more is I haven’t got the time to keep on top of keeping them in time. The points on my little Honda 4 hardly ever need touching and they operate off the crank so work twice as much for the same revs as they do on my beeza, yet still the beeza needed much more attention to keep them in tune.
1 member likes this
by TR7RVMan
Hi splash, It is very common to have back plate at or near limit of travel. It doesn't matter. Getting the timing correct is what matters. You can/will compensate with the sub plates.

Trust me a worn cam will not cause this.

You need a cam advance holding tool. Take a piece of bailing wire. Bend into washer shape that will fit over end of points cam, yet clear center shaft.
Remove AAU bolt. Place your bailing wire over end of AAU. Place larger flat washer over wire. {ut bolt back in finger tight. Hold cam clockwise to stop. Snug bolt & cam will stay at advance.

Don't use thin paper or cellophane. Get an ohm meter. You can use test light if you don't have ohm meter.

Remember you want points opening. Opening is what triggers spark.

Rotate motor until 38b mark lines up on rotor. Note, there are 2 marks on rotor. Only one is for you motor. If you have TDC tool use it to find 38b. Look in spark plug hole. If tool goes in & piston is at top that's TDC back up motor a little. In any case the TDC tool will go into slot at one of the marks. Put X through unused mark on rotor so you know that's wrong mark. The TDC holes in crank are actually slots. You can see them with flash light & mirror & feel them with screw driver. Just so you'll know.

I always start with front points (L cyl). Doesn't matter, but I like to. Turn motor until it looks like front points are ready to open. Hook up ohm meter with alligator clip to the curved points spring. Other lead to head fin. Now, this is important. Read meter with points closed. Record reading. Now turn motor until you see points open. Record reading. Notice you'll see about 3ohm & 70 ohms give or take a lot... the point is you want to see the change when points open. So memorize what meter does when points open. That's what you want to recognize, the moment points open.

Now back up motor & come forward rotation. Turning rear wheel on center stand is easy way. Spark plugs out. You always want to measure with motor rotating forwards, never backwards. If you pass the opening too fast. Back up motor a lot, then come at it again. This compensates for backlash in timing gears.

So AAU locked in advance. Ohm meter hooked up. Rotate motor forwards until meter shows points just opened. Go easy it happens quickly when you're at the sweet spot.

Look at line on rotor. Remember your sub plates & slot are centered. Your gap .015". Now move back plate until you are on time or close as you can get.

It can take several trial & error adjustments at first. If you run out of slot, then move sub plates. However double check point gap first as it can change moving primary plate. It just does...

Now go back & turn motor until points open again per meter. Move sub plate to trim the timing until you see meter read points open at same time line is on pointer.

Now move to rear points. Again verify gap. Now, trim timing on rear points my moving sub plate.

Expect to take a few hours until you get the hang of it. Probably all day Saturday actually. Takes time & practice. I can do all this in about 10-15 min. Like riding a bike or surfing. Looks easy & it is once you get the hang of it.

After all this cut some strips of copy paper. Dip in gas or carb spray. Place between point faces & clean off any oil or grease residue. Dry with dry strips of paper. Put a dab of point cam lube on the rubbing block. Lubricam SL-2. Ebay if local parts store doesn't sell it. This is very important. Or rubbing block will get hot, melt/wear quickly & gap will change. Properly serviced the points will last many thousands of miles & should cover 3k miles between servicing without problems. I do it all the time. Have for years.
1 member likes this
by triton thrasher
triton thrasher
Originally Posted by splash
So I finally figured out the points with timing light. Fires right up on first kick. Still have low idle issue when from high RPMs then to pulling clutch in to coast or put in neutral to coast to a stop. Usually by bumping the throttle it will settle at idle OR if I leave it in gear to slowly whin down the RPMs. Is this common?

If we can assume that the points gaps and timing are now correct and equal on both cylinders (I’m dubious of that, after reading your previous posts) and the engine has warmed up, then an engine that slows down too much when you shut the throttle, then picks up slightly, can be a sign of a rich pilot mixture.
1 member likes this
by TR7RVMan
Hi Splash, A ‘70 Tiger would have come with Amal MKI. A round float bowl, with pressed in .017 pilot (idle) jet, behind mixture screw.

If you remove mixture screw, look in hole with flashlight you’ll see tiny brass bushing with .017 hole in it. Strip back some 18g stranded wire a few inches. Stick 1 strand into hole. You have to be steady as hole is tiny. This will push dirt into passageway. Shoot carb cleaner into hole & it usually will blow dirt back into float bowl. Remove drain from bowl first. Open fuel tap & let some fuel flush bowl.

A newer premier version concentric will have a cross head brass plug on opposite side of mixture screw. This brass is head of removable pilot jet. The very first version had removable pilot jet inside above float bowl. Highly unlikely you have earliest carb.

There is also new non premier concentric with silver cross head screw. It has pressed in jet. The silver cross head can be removed to make jet cleaning easier.

Lots of changes & carbs shaped the same.

MKII concentric has square bowl & looks Japanese like Honda carb. No tickler.

To check for worn out carb/slide bore, heat soak motor. Pull over & let motor idle 5-10 seconds. Open throttle slowly as possible. No matter how slow you open throttle motor should not falter or die. Some wear will falter, bad wear will die. This is tiny opening only .005” or so. You can pass the bad spot by opening more quickly, then motor will rev as expected. Don’t jump to conclusions. Do this test several times over a few rides. Cheating mixture about 1/4 lean (out with screw) will make idle feel worse, but improve dying somewhat. New carb is best cure. New slide may help temporarily.

Make sure idle is not too low in every case.

Street bikes tend to not like straight pipes. Not to mention peace of the island. Emgo repro original mufflers work well & not too costly. The motor is sensitive to air filter changes also. Paper elements can make make mixture do odd things.

Factory settings only work for factory stock bike & ‘70s fuel.

You have much going on at the same time. Will take some sorting to work this out.

Remember the basics start with good reliable power to both coils, good points & point wires to coils. Good condensers, good plug wires & good coils. Good advance unit & timing.

On paper it’s easy. Real life is lots of hard work, often frustrating if you don’t have experience.
1 member likes this
by koan58
Plugs now look the same after running for a few days after setting timing. Electrode is burnt toast color but arm and circumference where threads begin is black carbon.

That sounds good enough Splash, no need to mess with it further. Ride it and enjoy!
1 member likes this
by triton thrasher
triton thrasher
I can tell you for sure that mixture bias happens on single carb Triumph twins too.

I don’t know if slanted spacers fix it, but I know that twin carburettors can fix it.
1 member likes this
Job CycleBritish Cycle SupplyMorries PlaceKlempf British PartsPodtronicVintage MagazineBSA Unit SinglesBritBike SponsorBritish Tools & FastenersBritBike SponsorBritBike Sponsor

© 1996-2021
Powered by UBB.threads™ PHP Forum Software 7.7.5