Author Topic: Uneven Revving B40 - Any ideas ?  (Read 5728 times)

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Crazy Mike

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Re: Uneven Revving B40 - Any ideas ?
« Reply #15 on: 15 November, 2019, 08:46:15 »
Picture is a zoomed in shot from the original B40 Sales brochure showing the choke lever position.
It looks like the lever is in the Right (clockwise) position, which would now seem to indicate this is the normal running position.

Crazy Mike

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Re: Uneven Revving B40 - Any ideas ?
« Reply #16 on: 15 November, 2019, 08:49:02 »
..zoomed in

JulianS

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Re: Uneven Revving B40 - Any ideas ?
« Reply #17 on: 15 November, 2019, 09:29:51 »
When cable is slackened the slide will go down and put choke on. When cable is tightened the slide will be pulled up and choke will be off.

Crazy Mike

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Re: Uneven Revving B40 - Any ideas ?
« Reply #18 on: 18 November, 2019, 13:56:37 »
Update .. As I now understand the correct position the choke lever should be under normal running circumstances, I took the B40 out for a test ride. To be perfectly honest, I didn't notice much difference in performance. The revs were returning to a normal level for the first mile or so but then … revs stuck high at traffic lights. It seems to happen when the engine is warmed up. Another issue is the engine was cutting out intermittently (felt like electrical but can't be certain) but righted itself once I eased off the throttle. After stopping the revs wouldn't hold at tick-over at all and a twiddle with the throttle screw was needed. So I decided to take the throttle slide assembly out again to check.

Now for those following this, I have previously mentioned my thoughts about the throttle return spring feeling underpowered and after removing the spring and checking against Amal's website, it appears the spring fitted to my (newish) 376 Monoblock is the 'lightweight' spring and not the 'normal' spring. On speaking to Amal they confirm the lightweight spring is only for use where twin carbs are used. For a single carb, the normal spring should be fitted. So I can only guess that the correct carb was supplied with the wrong spring for a single carb use from new. I have ordered a 'normal' spring. Amal indicate this should solve my 'sticking' problem. I shall see ….

Crazy Mike

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Re: Uneven Revving B40 - Any ideas ?
« Reply #19 on: 20 November, 2019, 20:03:17 »
New spring received but is the same as original spring ! Amal say the only difference between the 'normal' and 'lightweight' spring is purely the diameter of the wire 0.040  0.048. The number of coils is now the same - not as their website shows ! So I've now entwined both springs together and fitted them into the carb. This certainly makes the throttle response very quick ! However, after a test ride, no change in the rev problem. I've checked the spark with a Colourtune and it's a even blue colour. Adjusting the throttle stop screw to a normal tickover and then after a few revs the tickover sticks at a higher level. Switching off engine and twisting the throttle, the slide goes down with a positive click. No sticking.
So the next step is again to strip carb, thoroughly clean and blow through, replace all gaskets and hope for the best !
Any additional advice would be gratefully received.
Thanks

KEV441

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Re: Uneven Revving B40 - Any ideas ?
« Reply #20 on: 20 November, 2019, 20:33:15 »
Is the advance/retard bob weights returning properly?, assuming your using points ignition

  Kev

Crazy Mike

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Re: Uneven Revving B40 - Any ideas ?
« Reply #21 on: 23 November, 2019, 17:47:02 »
Thanks for your response here. The distributor was new following a recent full re-build. I've had a look inside and it all looks clean and intact, weights and springs all in place, points gap correct. The bike's been running ok (apart from the high revs).

The Monobloc carb has been removed, dismantled completely (again), thoroughly cleaned with carb cleaner and compressed air, not forgetting the very small holes that are just about visible to me. I have now added a smaller 'inner' spring along with the original throttle slide return spring which gives a satisfactory throttle return feel. The throttle slide does show some signs of marking only very slight in my opinion but the slide returns very positively now.
Started up and set the mixture and throttle stop settings and all seemed well until it warms up and the old problem is still there. The revs do not always return to normal straight away after a twist of the throttle. It takes 2-3 seconds from throttling back to settle back down but as the engine gets warmer this time delay gets worse and in some instances would not settle back at all. The odd thing is that when it's revving high, the throttle stop screw does reduce the revs ok that is, until the next time it's started when it wont tick over !

There are no air leaks at the manifold
All carb components are very clean
The float level has been checked and spot on to the dot on the float chamber cover level.
There is good spring return on the throttle slide.
There is no air filter
The manifold nuts are not overtight.
Using a colourtune, the spark is a decent blue colour at tick over. Not when revving though (goes yellow)
The slide does have some minor marks but moves freely in the body.
With battery cover off and engine revving high, the throttle slide can be seen minutely 'vibrating' within the chamber. Is this normal ? It just takes a touch with a plastic tool to stop the vibrating and then the engine immediately dies.
The engine runs ok when on test rides - no misfires etc
The carb was new in 2016 - only done 300 miles since.

I'm now thinking that maybe there is a problem with the carb body which may be causing this issue. Does anyone have any other thoughts or suggestions ?


DerekAnderson8

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Re: Uneven Revving B40 - Any ideas ?
« Reply #22 on: 23 November, 2019, 19:58:25 »
if carb slide is loose in the body it will be drawing too much air, what colour is the spark plug?

Is it harder to start when engine is hot?

worn carb can cause these problems.

 8)


Crazy Mike

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Re: Uneven Revving B40 - Any ideas ?
« Reply #23 on: 23 November, 2019, 20:39:25 »
The spark is Blue'ish when ticking over but turns yellow when revving up. Hot starting can be temperamental but usually starts within 3-4 kicks. Sometimes it just doesn't want to fire and only by opening the throttle fully and kicking over makes it start.
Is it possible that the slide could be faulty or the Jet Block or both ?

chaz

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Re: Uneven Revving B40 - Any ideas ?
« Reply #24 on: 23 November, 2019, 22:19:37 »
have you done a compression test?
have the valve clearances been checked since its been running?

when you get it running spray some easy start or carb cleaner around carb flange to see if any leaks, revs will rise.
are the cables in screw or ferrules into the cap? may be drawing air in.

Crazy Mike

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Re: Uneven Revving B40 - Any ideas ?
« Reply #25 on: 25 November, 2019, 16:14:57 »
Compression test done (albeit with a very old pop-up type gauge)
Cold = 65 psi
Hot = 95 psi
Hot with a squirt of engine oil in the cylinder = 105 / 110 psi
Valve clearances  Inlet = .20 thou -  now a tight .10 thou   Exhaust = .20 thou -  now .10 thou
No change in revs when carb cleaner / wd40 sprayed around inlet manifold.
Cables are all new. Ferrules into cap with some slack. Cables positioned as vertically as possible into carb.
Original slide return spring with a smaller lighter weight spring fitted inside the original to give a better return feel to the throttle control.

The bike starts a lot better after the valve clearances have been adjusted but still the fault persists. This beings the revs sometimes stick high and won't return to a normal tick over level.
I've noticed when using the colourtune it's impossible to achieve a pure blue spark, best is a blue'ish spark but still with yellow if that makes sense !
When looking into the air intake of carb when ticking over (fast), the slide is vibrating and the lightest of touch of the slide with a plastic tool kills the engine immediately.
What I don't understand is that I can set the tick over quite normally and precisely with the throttle adjust screw, give it a rev or two, the tickover revs then stay high but I can then reduce the revs down with the throttle adjust screw. I don't understand that !

If anyone has any further thoughts then they would be gratefully received.
Thanks for all the comments so far.


royblackburn1@btinternet.

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Re: Uneven Revving B40 - Any ideas ?
« Reply #26 on: 25 November, 2019, 18:18:30 »
Hi Mike I didn't  want to believe it at the time but I must have over tightened a carb on my twin and got same symptoms as you I changed the carb and its back to what it should be,you say your carb is new  if you still got the old one could you try that (if you have I havn't picked up on it) just a thought Roy

DerekAnderson8

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Re: Uneven Revving B40 - Any ideas ?
« Reply #27 on: 25 November, 2019, 18:47:11 »
when the bike is ticking over only the pilot circuit is drawing fuel, the slide is not involved.

When you rev the slide rises and allows more air in and fuel via needle jet.

I still think the carb is excessively worn hence the slide is allowing too much air/fuel to pass and when throttle is shut off there is still mixture getting past.

have you got another carb to try?

 :)

Crazy Mike

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Re: Uneven Revving B40 - Any ideas ?
« Reply #28 on: 25 November, 2019, 21:35:12 »
Thanks for your input.

I've only had the bike for a few weeks now. The previous owner completed a full rebuild over several years. The carb was new in 2016 (I have the receipt) with just over 300 miles on the clock since then. It's unlikely, but possible, that the previous owner may have overtightened the nuts on the carb thus causing some distortion. When I initially removed the carb, the nuts didn't seem excessively tight to me.
It does seem to point to some form of internal wear but after just 300 miles ??
Unfortunately, I don't have access to another carb to try.

Amal say they would be willing to have a look at the carb if I were to take it to them (150 mile round trip for me).
I just want to make sure that I've not overlooked anything else which could possibly cause this problem.

I appreciate your help.

Phil C

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Re: Uneven Revving B40 - Any ideas ?
« Reply #29 on: 25 November, 2019, 21:41:10 »
Mike, When you eventually find out the cause, please be sure to tell us.  Phil.