Britbike forum Forums British Motorcycle specific Brands Triumph bulletin board 69 T120 R High RPM miss

I'm almost complete with a restoration of a 1969 Bonneville and have two issues I'm struggling with. The first issue is the bike miss fires at around 4,000 to 4,200 RPM. I have installed new .20 over pistons and rings, valves are seated and set correctly, the carbs are whistle clean, the floats are holding, new Champion spark plugs, new points set at .015 and clean, new condensers, checked plug wires for continuity (good), original coils, white wire on # 1 coil terminal, points on # 15 terminal. Check timing with a strobe, idle is 11 degrees, 38 (I believe it was 38 degrees, but I set where the shop manual stated) degrees at full advance @ 2,000 rpm, clean fresh gas, no air cleaners (at this time), choke off. Attempted to set low speed air screw, but it's idling a little rough at 1,000 rpm. Seems to run OK from 1,000 to 4,0000, then seems to load up and or start missing not wanting to rev any further. Previous Owner let primary case run out of oil, I had to replace the chain and sprockets and the clutch bearings, but the stater looked a little cooked but OK. The amp gauge in the head light shows discharging and charging. The coils seem to get very hot when the ignition is on. A volt meter connected between the positive post of the battery and the ground shows a 3/4 of volt passing when connected.
I planned on chasing down the voltage loss from the battery later but mention it now as it might be related to the miss firing. Any help would be greatly appreciated as it's getting warm outside...
Thanks
Mark

Make sure all your connections are secure. I once had similar symptoms caused by nothing more than loose spade connectors on the coils. Tightened all the connections with pliers and it worked fine.

There shouldn't be any voltage or resistance between battery positive and ground. Maybe an idea to clean up the terminals and check the rest of the red wire system.

I’ll pull a gas tank and check them although I’m pretty certain I wrote very tight
I’ll look forward any other suggestions as well as I’m not very optimistic that that’s the issue
Thanks Mark

I’ll give that a try as well thanks for the thought

Mark

I’ll go through all the wiring connections again
The wife thinks I should be mowing your grass instead of sitting on my stool staring at the Triupmh wondering why doesn’t work right
I’m certain success is just right around the corner I just not want know which corner it is
Thanks Mark

Star the bike at night in the dark and make sure that your coils aren't jumping spark to the frame...seen it before.

This morning I pulled the wire off the center of the rectifier to run just off the battery, no change, still misses at 4,100 RPM
Then I put a donor coil on the LH cylinder and it still misses at 4,100 RPM
I will switch it to the RH cylinder the evening and see if that makes a difference.

I did notice the gas line from the reserve petcock on tank to the LH carb is bent somewhat sharply, and I'm not putting a low of gas in the tank as I'm removing it frequently to check the electrics. I'm going to put a longer gas line on (without the tight bend) and see if the bike has been starving for gas. If that's the case, this thing sure is thirsty....

Mark

My 650 runs better with both taps on. Not thirsty but maybe the gauzes in the taps are a bit blocked.

Definitely best to check electrics first.

Once you have been through the electrics check the tappets and then the carb. Does it miss at 4000 revs whatever the throttle opening? If so it is the electrics.

Dave

I came home from work today replaced the fuel line was a kink in it and move the donor coil from the left-hand side the right-hand side after road testing it with a new fuel line and road testing it was a new donor coil
You’ll rub up to the high fours and start missing and then after that it just starts missing at 4100 RPM and won’t clear out
I’ll take your suggestion and try it with different bottle opening shift at RPM see if it keeps missing
I’m starting to wonder if the PO had messed with the cam timing
When I first got the bike I cleaned up the gas tank with lacquer thinner God it was so clean clean the carbs low speed high-speed main pilot jet everything seems to be clean
I have noticed that there’s a brown or oily substance coming out of breather tube when I fired up in the garage and take it through its paces before I take it for a road test
I don’t think it has too much oil in it oil tanks about 2/3 full
May be an exorcism
Hey more help or suggestions will be appreciated thanks Mark

Ignition key contacts and key ring flapping around in the breeze?

I do have a little tab on the keys it does flapper around but engine breaks down the first year or second year or third year so the wind speed variesWhat engine always breaks down around 4100 RPM

Vibration certainly changes at different rpms and your 4100 could be affecting something. The ignition key contacts as suggested by previous poster are indeed a prime candidate. I dismantled mine and cleaned them.

Dave

Hi Dirtbikerider, Sounds like you've covered your bases on tune up & electrical for the moment.

At this point I might suggest mark your grip so you know exactly the throttle opening when bike falters. Don't look at RPM for this test. Just throttle opening.

Print this tuning guide by John Healy & put in your pocket. Go for some road tests & study guide. This will help eliminate carb problems.

https://www.princeton.edu/ssp/65-cub-data/library/amalbritbike.pdf

Maybe carbs are good. I've found the carbs can mimic electrical problems & vise versa.

The key is marking the grip so you accurately know the throttle opening. I find it impossible to guess opening as you ride. I must mark it.
Don

I'm trying to check the static timing (11 degrees BTDC) and the full advance (38 Degrees BTDC). I created a degree whee for the stater, and lined it up with TDC mark and the pointer on the primary case. I put the TDC tool in the crankcase and confirms TDH alines with the pointer and the mark on the stater rotor. Then I plotted the 11 degree and 38 degree points on the rotor and put small center punch marks on the rotor for each. I pulled the plugs, rotated the engine to 11 degrees on compression and then put an ohm meter on the points to check when they began to open. When doing so, I got 0 ohms when the points were closed and some degree of ohms when they were open. This indicated to me the current can pass even when the points are open. is this normal?

Thanks
Mark

You might expect to go through a couple of sets of plugs while breaking in. There's a fair bit of oil getting into the combustion chamber. My personal experience with this leads me to suspect Champion plugs that seem to oil/gas foul and can't be brought back by cleaning. So, I like NGK plugs initially as these seem to hold up, but then a switch to Champions because I'm more familiar with them and how they react to different carb settings.

Sounds odd, I know, but I never took it further than that. Perhaps just one heat range hotter Champions for break-in would have been the answer.

I just had this problem on my old T-bird. I rode around here on Sunday to keep everything working right. I went to pass a slow truck and the engine just quit at a wide throttle opening. Ran fine under that. I got home slower, but OK and cleaned the plugs. Problem solved. However, I have "unknowns" in my bike which are Bosch Iridium plugs since my local NAPA doesn't have the Champions I normally use. I'll be looking for a set of Champions now...

My problem was throttle and load related, not RPM. But, the solution to either is usually in the ignition.

Cheers,
Bill

i havent used champions in anything in almost 40 years. found a fouled set of N3G plugs in my tankbag a while ago, like a bad dream from the past. champions would always foul in my old T120, and would never come back.

NGK B8ES plugs always worked fine in the same machine. but i'm curious, so maybe i'll try the champions again. could be that whatever fouled them so long ago is fixed.

When the set of points that the meter leads are across are closed, there is the direct, virtually zero resistance route through the points.

When the points open, there is still a route to ground, which is back to the open points' coil, through its primary windings, then through the common supply to the other coil, through its primary windings to the other set of points which are closed, so to ground.

The resistance of that route will be twice the coil primary resistance plus any wiring losses. At a rough guess, I'd expect to measure something like 10 ohms.

Take care using the resistance function for this task. If you accidentally turn the ignition on, it may damage the meter. So it could be a good idea to take the fuse out or disconnect a battery terminal.

A safer way is to use the volts function with ignition on, in which case you will see 0V points closed, and ~12V points open.

Also, this should be done with the AAU locked at full advance.

I took Tr7RVman suggestion And investigated a carburetor problem in Lou of a ignition problem and he was right. I checked the carburetor main jetand it had 190s and the book said it needed 260s. I called Baxters to order new main jet and they told me the 190s or correct and the book is wrong. They then asked how many miles on the motorcycle I told him 60,000 and they said the carburetors all wore out and I need a new carburetors. After some convincing from them I lay down my $400 and bought two new carburetors which made a huge difference. Now the motorcycle will idle and rev up but doesn’t seem to have the correct amount of power for A freshly rebuilt engine. i’ve been checking the plugs and they seem to be running lean, they have a very very light gray look. i’m wondering if the 190 jet size horse for emissions and engine really want something richer. Does anybody have a device for re-jetting a fresh engine?
Thanks Mark

I meant does anybody have advice for rejecting I eat what jet size should try next
Thanks Mark

I meant does anybody have advice for rejecting I eat what jet size should try next
Thanks Mark

How old are the condensers/capacitors?

He says they are new at the opening post.

Are you still running with no air filters,? if so up the main jet 2 sizes.


Its a fresh rebuild , right?During the first 1,000 miles it will free off a lot, and will continue to do so for a few more.

Typical signs of leanness , if at full throttle ( surely not yet on a fresh motor!) the motor picks up a little as the throttle is rolled off , thats lean.on the main jet. Buy two pairs, 200s and 210. Not too spendy

At half throttle ,if acceleration could be better, no 8 stroking, clean running but slow to pick up maybe spits back as the throttle is opened, thats lean at the needle jet or needle position, try lifting the needle one notch and see , this is free, and what I would try first.

Most important, to seat the rings , a couple of bursts to 5K or more pulling hard on a hill, just there and back, this forces the rings to bed in, do not sustain this "hard work" just a couple of times is enough in the first 50 or so miles, after that run in as per normal.
Leave messing with the main jet until fully run in, it only has real influence beyond 3/4 throttle.

Mark your throttle twistgrip somehow, small triangles of coloured insulting tape is easy and removable when satisfied, , mark at 1/4, 1/2 , 3/4 openings, you will be surprised how much time you spend in the 1/4 throttle range, at this point the main jet has almost zero effect, this is the domain of the pilot jet and the throttle slide cutaway, adjusting the pilot mix to give optimum running will pay dividends here, if your carbs and pipes are stock its unlikely that the throttle cutaway will be far off, if its to the book.

Hi Dirtbikerider, Gavin again suggested marking your grip.

Did you do that before you got new carbs? What opening did you have when bike started misfiring? I don't want to know RPM but how far throttle was turned.

If you didn't mark grip how do you even know if you were on main jet? If you just rode in town, with main jets removed, you may ride for days & not even know it.

What slide is in new carb & old carb?

As was suggested get the next to sizes richer. Go too rich until it 8 strokes, then go down 1 size until it stops.

If you don't know what 8 stroking sounds like the only real way to know is go too rich until you hear it. 60-65 mph in high gear up a little hill will show it. It will show in lower gears, but not as obvious to hear.

I don't mean to sound harsh, but there is no short cuts on carb tuning. Study the tuning guide & take it while you ride. Memorize the feeling of bike.

Regarding your timing, you verified the 38deg. mark is lined up with pointer when flywheel locator tool is notched in. That is good. You're ready for strobe timing with a timing light.

I just did a '69 motor overhaul. The timing cover should have a pointer. Start it up & use a timing light & see what the timing is. Adjust the line to line up with pointer. Rev motor 3-4k rpm. It will probably line up near 2k rpm, but give it some more to be sure. Static timing is only needed to get motor started after a overhaul. While static timing can be very accurate, strobe timing with light is a sure bet. If the line on rotor is lined up with pointer 3-4k rpm that is what it actually is.

Put a rubber mud flap from a truck under center stand to keep bike from walking backwards. Put a large fan in front of motor to keep it cool.

Remember, carb adjustments are last. All the tuning things like valve adjustment & timing must be correct first. Throttle cable sync must have been done first too. At least as best you can. If bike will idle you should get very close on carb sync before getting into jetting.