Hesitation, stuttering, misfire - help please (2020)

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I’ve checked using the search facility but I’m not getting answers to my problem

1974 mk2a 10,000 genuine miles only, last ran in anger 15 years ago.

I've been recommissioning this machine over the last few months of lockdown and it‘s now up and running. Nearly. The original annular discharge silencers had already been replaced with peashooters when I got it. It has a balanced exhaust pipe set. Mk3 style plastic air filter unit.
As a consequence of all the years left under a sheet a lot of the perishable items perished and so I’ve had a whale of a time fitting lots of new parts. With relevance to my query I have replaced the following items

Spark plugs BP7ES
HT leads
Resistor plug caps
932 Amal carbs, new purchased recently, these are fitted with no3 slide, 106 needle jet, needle on the middle clip, 260 main jets, viton tipped float needles. ( the ones on it were jetted to suit the annular silencers and would not run very well at all)
air filter element, both.
fuel taps, both sides were leaking
fuel hoses, the originals were very hard and opaque.

that’s about it, I think. The valve clearances have been checked, as has the original points.

It starts easily now, fuel on, tickle and then second kick. First kick when hot. I still need to fine tune the idle I know.

When out on its test rides it pulls nicely, changes gear well, and is strong and responsive. All the way up to 4000+ rpm in 4th gear. Then it stutters, hesitates, misfires etc. I can’t ride through it, it’s like it’s hitting a primitive limiter. Open the throttle in 2nd and it romps up to near the redline, change into 3rd and it’s the same, change into 4th and it’s now gone over the 4000+ rpm and it keeps pulling strongly.
Ease back on the throttle to below 4K and open the throttle and it’s back on the “limiter”

hm, perhaps I’ve got a dodgy coil? I’ve swopped out the 46 year old originals for a pair of new Wassell Lucas units, in addition I also put in a 2nd new pair of plugs.

Now, there is less of the problem, but it is still there. On the occasions where I’ve managed to stall it, always at the traffic lights it restarts immediately first kick.
Your suggestions are welcomed.
 
I'm guessing somebody cleverer than me (my dog?) might jump in with a: 'throttle position?' request/suggestion. Sounds like not enough fuel at part throttle, perhaps? (fido agrees)

(Mind you, seeing the thread heading I thought Nicholas Parsons had risen from... )
 
Last edited:
Check the fuel level in the float bowel. 260 mains, 106 jet is the same as my peashooter MK2a, not sure of the needle position but I raised mine one notch from the MK2a spec to cure a flat spot in mid range and opening throttle.

Resistor plug caps or HT leads are not needed on points, or analogue EI, only digital EI ie the expensive ones. Solid copper HT and non resistor caps and plugs are best for non digital EI bikes.

air filter element, both.

Not sure what you mean on this, there is only one filter element, a foam covered steel mesh on a MK2a with plastic air box.
 
Landspeedracer said:
is strong and responsive. All the way up to 4000+ rpm in 4th gear. Then it stutters, hesitates, misfires etc. I can’t ride through it, it’s like it’s hitting a primitive limiter.
air filter element, both.
Click to expand...

Your description sounds like it might be fuel starvation..did you dismantle your new carbs and check air passages and jet sizing the seller claimed?

Had a similar issue and changed the needle position as KOMMANDO described. It worked.
 
Thanks.
a. The air filter has a main filter and a secondary foam bit covered by the front plate, I just wanted to show I’d fitted everything

b. Yes, I dismantled the carbs and checked the jets and pilot passages just in case.

c. I’ll swop over the plug caps they were suggested to me to fit, I’ve got normal ones and some more new HT leads coming soon.

d. I’ll lift the needle to the upper position and see what it does.

e. I’d better double check the fuel cap as I fitted a new rubber seal to it as well.
 
Wouldn't hurt to check electrical grounds and the condition of the ignition switch. and kill switch. Things may be good at lower speeds and conditions, but go bad with changes in vibration, etc.
90% of carburation problems are electrical.
90% of electrical problems are fuel related.

George
 
I had a similar problem, after I mounted the secondary air filter with my MK3 (260 mains and peashooter and balanced pipes).
("secondary foam bit covered by the front plate")
Before it was running fine with 260 mains, but afterwards it always hesitated during
acceleration about 3000 rpm. After I went to 240 or 250 mains it was fine.
I thought first it was the ignition. But it was really the Carb-setup, also the Sparkplugs look alright now.



Regards Marcel
 
I had a similar problem and cured it by reducing spark plug gap. Mine would begin to miss fire at around 5000 RPM. I have no idea why this worked.
 
Oh, I forgot. I have added several additional Earth straps, from the coil mounts to head steady, cylinder head to head steady. All of the terminals have been brushed shiny.

kill switch has been checked on the multi-meter. I guess I could disconnect it temporarily to rule it out. Ignition switch does seem sensible, but it is tight bearing in mind this thing has had minimal use of the years.

I still wonder why it is that if I go to 5000/6000rpm in 2nd and 3rd very easily and then select 4th it will continue to run hard without an issue.
 
The higher the gear the larger the throttle opening for the same revs as the load is higher. By marking your throttle you can see what part of the carb is in play.
 
Landspeedracer said:
I’ve checked using the search facility but I’m not getting answers to my problem

1974 mk2a 10,000 genuine miles only, last ran in anger 15 years ago.

I've been recommissioning this machine over the last few months of lockdown and it‘s now up and running. Nearly. The original annular discharge silencers had already been replaced with peashooters when I got it. It has a balanced exhaust pipe set. Mk3 style plastic air filter unit.
As a consequence of all the years left under a sheet a lot of the perishable items perished and so I’ve had a whale of a time fitting lots of new parts. With relevance to my query I have replaced the following items

Spark plugs BP7ES
HT leads
Resistor plug caps
932 Amal carbs, new purchased recently, these are fitted with no3 slide, 106 needle jet, needle on the middle clip, 260 main jets, viton tipped float needles. ( the ones on it were jetted to suit the annular silencers and would not run very well at all)
air filter element, both.
fuel taps, both sides were leaking
fuel hoses, the originals were very hard and opaque.

that’s about it, I think. The valve clearances have been checked, as has the original points.

It starts easily now, fuel on, tickle and then second kick. First kick when hot. I still need to fine tune the idle I know.

When out on its test rides it pulls nicely, changes gear well, and is strong and responsive. All the way up to 4000+ rpm in 4th gear. Then it stutters, hesitates, misfires etc. I can’t ride through it, it’s like it’s hitting a primitive limiter. Open the throttle in 2nd and it romps up to near the redline, change into 3rd and it’s the same, change into 4th and it’s now gone over the 4000+ rpm and it keeps pulling strongly.
Ease back on the throttle to below 4K and open the throttle and it’s back on the “limiter”

hm, perhaps I’ve got a dodgy coil? I’ve swopped out the 46 year old originals for a pair of new Wassell Lucas units, in addition I also put in a 2nd new pair of plugs.

Now, there is less of the problem, but it is still there. On the occasions where I’ve managed to stall it, always at the traffic lights it restarts immediately first kick.
Your suggestions are welcomed.
Click to expand...
I just fought and won exactly the same symptom battle.

I wrote in a different thread about my coils causing a similar problem at 2900 PRM. New coils fixed that problem.

When I went for a test ride it was perfect until I tried to go higher than 4000 RPM. When I twisted to what should have been about 6000 RPM, it would sputter, backfire, speed up and slow down. I was sure it was related to the Tri-Spark ignition so I put in a replacement - same problem. Then, I disconnected the PODtronics and the problem disappeared. I have the Tri-Spark noise suppressor installed and have used the Tri-Spark/PODtronics combination on a bunch of other bikes without problem - In fact, this bike had no problem earlier in the year. I replaced the PODtronics with the Tri-Spark MOSFET regulator and it's fine.
 
1. It's been awhile since I did points, but each set of points are timed separately to each cylinder.
Verify all this is correct, ditch the resistor caps, then move to carbs.
2. New Amals? What was the source?
850's have 3 1/2 slides, 750 has 3. Probably not the issue, but leads to other questions.
What spray jet? Notched spray jet is stock for 850, straight spray jet for 750.
The needle needs to be matched to the spray jet. Top of needle has rings, notched spray jet works with 4 rings, straight spray jet works with 2.
These rings are above the 3 'Grooves' for the clip. Needle needs to be matched to the spray tube.
Mismatched needle / spray jet will cause issues.
3. I suspect the 260 is too fat for the Black Box, maybe not. Try removing and running open. See what happens.
 
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I also had a needle circlip getting hung up on the spring inside the slide, only it was doing so intermittently. So, performance would be satisfactory or conk out at a higher RPM but return to normal when I would back off on the throttle.
Figuring that one out took a while.
 
My 850 mkII was giving serious misfire and back fires at around 4k under hill climb loads. Turned out to be one main jet had started to unwind in left side body. A good snug up and issue cured.

Are you seeing and blue smoke out the back when throttle blipping from idle and higher up? Plugs fouling? My next misfire issue came along some months after the jet issue. Seems to have been a plugged oil drain hole in inlet rocker cavity such that excess oil was getting pulled into left cylinder, giving misfires. This was more or less across rpm range so not as you describe.
 
Personally I would not bother raising the needles at this point in the analysis as it is a pain in the ass to do and really
only enriches the flow in a tuning like altitude sense whereas your problem sounds like a total shut off of fuel or ignition
at the reported 4000rpm level. If you are basically sea level then leave the needles and other carb basic setting at stock for now
until you can verify the rest of the checklist first.
Does the motor take over and run smoother when it fails at 4000 and then you back if off? And that really is the only issue?
What is often first viewed as carburation and fuel problem is often diagnosed as electrical!
 
This could easily be a weak spark. Besides possibly bad coils, it could simply be electrical resistance introduced by aging electrics. Easy to find this. Suggestion—take your multimeter and measure your voltage at the battery and at the coils. It should only drop a few tenths of a volt. I had a very elusive similar problem on a Commando and it turned out to be internal corrosion in the ignition Switch introducing a drop of maybe 1 -1/2 volts. Also as these bikes age the connections in the wiring loom deteriorate and introduce resistance. If you don’t have a multimeter make a jumper wire to run from the battery to the coils and see if that helps.
 
Lineslinger said:
Your description sounds like it might be fuel starvation..did you dismantle your new carbs and check air passages and jet sizing the seller claimed?

Had a similar issue and changed the needle position as KOMMANDO described. It worked.
Click to expand...

Cut a round piece out of a sheet of rubber gasket material, same od as clip, punch holes for needle and throttle cable and sandwich between clip and bottom of spring. Will eliminate the problem
 
Sorted I think.
basically it was due to a loose nut holding the spanners.
I removed the fuel tank so I could get at the carbs, and to tidy up a little bit of wiring. I took out the slide assembly on both sides and discovered the needles were 2 ring ones, rather than the 4 ring ones it should be. There is a significant difference in length between the two types. In addition I have the square spray tubes and not the sculpted ones.

so, I quickly reassembled it all using 2 off 4 ring needles and went out for a brief ride as it was rather thundery and looked like rain. It runs fine, not hesitating or stuttering or misfiring as before.

How relevant is the difference between the 2 types of spray tubes as I wasn’t able to notice anything?
 
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