Uneven Revving B40 - Any ideas ?

[Bees]

Hi there.

I had a same problem on my Starfire. It turned out to be the pilot screw and throttle stop adjustments were wrong.

I found an article on here I think that explained it best. 1st of all get the engine properly warmed up. It seems that yours may have been set when engine not at full temperature and therefore when it is at full temperature it's all out.

start with pilot jet. Fiddle around with it to get engine running as fast as possible. Then throttle stop to slow it back down. Then repeat 3 or 4 times.

Try it. It's simple, doesn't involve dismantling or cost anything. Takes about 10 mins.

It worked for me.

[Crazy Mike]

Thanks for all the contributions so far.

After a visit to Hitchcocks, I have now purchased a Anodised Throttle slide for the Monobloc carb which moves much better and more positively than the original slide. They did use a tool to hand 'ream' the inner carb body to ensure it was true.

Before fitting I have, again, removed and disassembled the carb and thoroughly cleaned it out. I'm getting a 'dab-hand' at this now !
The carb flange was slightly distorted (about 15 thou ) and I have rectified this by rubbing down on fine wet n dry on a sheet of glass so it's perfectly flat now.  New slide fitted with new flange gasket & spacer, reverted back to the single original throttle slide spring, checked numbers on jets etc (Main is 190, Needle is 106, Pilot is 20, Slide 30, needle is 'C' type and held in mid position 3). Mixture screw 1.5 turns out.
Once refitted, the bike started first time. Throttle screw adjusted, so time for a test ride to get it up to temperature.

Unfortunately, the same symptoms persist. The revs hold high at tick over when at traffic lights etc and occasionally they do drop to normal but it's not instant as I would expect. The one thing I have noticed is when the revs are high when stationary, just letting the clutch out in 1st, just so there is just enough engagement to slowly move the bike a few inches, then pulling the clutch again, the revs then drop to normal and stay there.

Is this indicative of another area of the engine which could be causing this problem ? I would be really interested in any thoughts !

NB: I did check the plug colour after the ride and it was spot on.

Thanks

[JulianS]

Have you checked the action of the advance/retard in the distributor as per Kevs earlier post. Make sure the action is smooth and not sticking. Sticking advance/retard could cause revs not to drop back.

[Crazy Mike]

Ok. Thanks for the advice.
Will I need to dismantle the unit ? I can see the spring (s) beneath the points but there's not much room to investigate. I have no experience of this type of distributor so any pointers would be really good !
I can't seem to find much on-line about these units and how they function and what the spring / weights actually do and I'm keen to understand this a bit better before I inadvertently do any damage !! (Been there before ...a few times !)
Thanks again.

[Mark]

This may help.

https://www.howacarworks.com/basics/how-engine-timing-works

Regards Mark

[berniej]

Hi Mike,

If you haven't already got a copy, I suggest adding the Rupert Ratio book of BSA singles (engine) to your Christmas list!

This covers just about every process and detail that you will need. Removing the base plate isn't a difficult job and will allow you to check the condition of the springs and for any signs of sticking as Julian mentioned, or wear and tear of the bob-weight mechanism. If there is any slack or stickiness it can result in the bob-weights not being fully pulled in when idling and this will cause the engine to run slightly advanced - and hence at higher revs. Probably worth replacing the springs anyway just to be sure.

While it's out, have a look on the underside of the base plate and see if it has a number stamped - this indicates the amount of advance in degrees. There were a number of variations used across the C15/B40 range of engines and the timing setup varies depending which you have.

Cheers,
Bernie

[Mike Farmer]

 

Go for dead simple. Take the petrol cap off. It may be airlocking. Or take the taps out and make sure they are free flowing. If you have already done that then cant think of anything not already posted.

Mike

[Crazy Mike]

Mark, a very good article. Now a lot clearer ! Thanks
Berniej. Sent my 'Santa' list off 2 weeks ago. Yes, RR's books were on the list ! Good advice from you - Cheers !
Mike. Petrol cap - Now I wouldn't have thought of that one ! See what you mean though. I'll give it a try.

I'll let you know how I get on ....

Thanks all.

[chaz]

Ive just recieved my new tap for the chop's new tank, the old one was badly corroded on the brass pipe nearly eaten through. Ive re made the tube and fitted, not before drilling out the old broken bit of tube, only to find the two holes that go into the tap area were blocked solid with grey sediment, built up, presumably from the ethanol and corrosion of the brass tube and alloy body.
may be worth checking the complete lines.

[beezabob]

I seem to remember weak mixture as a reason for slow return to idle. Maybe a blocked pilot jet or needle is too low? the other thing is the idle screws must be set when the bike is hot. if you set cold it will idle very fast when hot.

[Crazy Mike]

Thanks for the responses. I have checked the fuel cap and feed to the carb and all is well. Fuel flows freely so it's back to the Ignition.

Just a recap. The problem is the revs sometimes do not return to a normal tick speed. After much work on the carb I am now looking at the Ignition and in particular, the advance / retard mechanism. The distributor was completely overhauled by the Distributor Doc during the bike's renovation, this included new springs & weights and on first inspection it all looks ok (to me anyway). I decided to have close look and started to remove the points to gain access to the bob weights below and noticed what I think may be an excessive amount of grease (moly ?) which makes me think this may contribute to my problem.

What I don't know is, is this a 'normal' level of greasing. Perhaps the more experienced may care to comment on this and offer any other comments.
I hope to attach 3 pictures (if it doesn't work, I'll post them separately)
Dist 1 shows the unit with lower point removed
Dist 2 shows central cam removed showing weights and grease
Dist 3 shows weights removed

All comments gratefully received !

Thanks

[Crazy Mike]

Dist 1

[Crazy Mike]

Dist 2

[johnan]

Martin Jay, who is the Distributor Doctor, has been helpful to me via emails quite recently, perhaps drop him a line explaining your situation.

His email address should be on the Distributor Doctor website, I don't have it to hand at the moment.

[chaz]

I dont see the reason for greasing the points assembly at all, in the old days we only ever used to put a drop of 3in1 on the felt pad once a year if we remembered. as I see it, the 'compartment' is oil free but liable to collect dust so excessive grease will attract dust and grit adding to wear and ignition problems