Comerford Cub

[ranspeed]

Hi Fellas

I am new to the forum and am asking for help in setting up my cub.

It is all original comerford cub, one of the first 20 shipped to Comerfords.

It is fitted with an amal 376/314 carb, I have renewed all jets but can,t get it to run without popping. I have checked all joints and they are airtight.

On tick over it surges and pops and all adjustments that i have done have not managed to eradicate these problems. Any ideas, I want to keep the bike as original as possible

[totalshell]

''surges on tickover'' do you mean revs increase by themselves at tickover? if so do they keep rising until its screaming or do the revs rise and fall? ''popping'' do you mean that if you rev the bike and then kill the throttle the exhaust bangs as the revs slow?

[ranspeed]

The bike starts after a bit of persuasion, it hasn't been run since 1982.

When it has warmed up, i set it on a fast tickover and the revs rise slightly and then come back to normal every now and then there is a bang from the exhaust or a blow back through the carb. it will rev out and does pop through the exhaust as the revs settle but i supposed that this would be normal.

I have fitted an electronic ignition on the standard setting.

I recovered the bike as a complete but tatty project and have restored it to a reasonable standard but being 6 ft 5 it is a little small for me to ride far.

Most of the cubs on the forum seem to use amal concentric 6/8 00s but i have checked and this is the carb that it was supplied with.

It has many unusual parts, the headstock strengthening tube is an oil reservoir which drips down onto the chain via a small bore pipe, (an early scotoiler ?)

it is nearing completion and I would like it to run as near as possible to standard

Many thanks

Edited March 12, 2010 by Ranspeed

[totalshell]

the popping is unburnt fuel exploding in the exhaust as the throttle is closed. lean the mixture slightly with the mixture/ air screw

[alan]

the popping is unburnt fuel exploding in the exhaust as the throttle is closed. lean the mixture slightly with the mixture/ air screw

Sorry Mark.... but thats completley the wrong advice..

back fire thro Carb..

Excessively lean carburetor settings can contribute to backfiring. If the mixture is too lean, it may burn very slowly and unevenly. This condition, in turn, may result in burning mixture remaining in the cylinder until the beginning of the next intake stroke when it can ignite the incomming air/fuel mixture.

Backfire in exhaust.....

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.

2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.

3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.

4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.

5) The exhaust must be both free-flowing and have an open exit for the popping to occur.

Other possible causes:

Air Leaks:

Any source of fresh air into the exhaust system can create or worsen the conditions that bring about exhaust backfiring. The most common entry point is the junction of the header pipes and silencer. Even a small air leak can dramatically increase the intensity or likelihood of exhaust system backfiring.

Lean Carburetion:

While exhaust system popping may be considered normal, it is certainly made worse by an overly lean idle circuit.

Be sure that your carburetor's pilot jet is the correct size and that the idle air mixture screw is correctly adjusted before looking for other causes of popping.

Ignition:

If exhaust system popping is very loud, irregular and accompanied by loss of power, then you should suspect that the ignition system is not performing as it should. If, for some reason, the ignition sometimes fires at the wrong time, then exhaust popping can become very energetic (loud). Look for failing high tension leads (plug wires), failing ignition coil and especially switches or connectors as possible causes.

Ranspeed...

what ignition system are you using ?

Edited March 12, 2010 by alan

[totalshell]

my apologies, i'll up my pills . i meant what you said but wrote the opposite.. NURSE!!

[ranspeed]

Thanks for your help guys

I popped up to the offroad bike show at Malvern yesterday and bearing your advice in mind I searched out the Amal carb man, he told me that there is a specific main jet for trials bike applications that doesn,t have the small cross drilling.

This jet is slightly richer and gives a smoother throttle response, more suitable for trials. He also said that the cam in my bike was particularly sporty and that an even tickover, like a road bike would be practically impossible to get, especially with the open sports exhaust.

I pulled the carb as soon as I got home and changed the jet, and also the rubber seal between the carb and the adapter between the head and the carb. I also had a look at the gasket between the head and the adapter, which looked a bit ropey, so i made a new one and changed that as well.

The carb man also said I should ride the bike and see what its like. At 6 foot 5 not an easy feat

I reset the needle to middle groove and the air screw to two turns out. It ticked over a little better but still a bit lumpy.

On riding it up my drive I found that the bike gave plenty of power and went through the gears easily so I may have found a cure / maybe!

Alan, the ignition I am using is the new SPL system I bought at the Stafford show in October, it is trials cub specific and the rotor is replaced by the new system and the coil also replaced, doing away completely with the points, but totally reversable if needed. Getting a great spark and easy starting now the carb is adjusted.

My next job is to change the gearshift splined shaft. The gear lever is double ended, I suppose to help with trials riding, but the spline is damaged and the lever has been bodged on with a screw. It doesn,t look too difficult and can be done with the engine in the frame.

Again, many thanks for your help.....

Edited March 14, 2010 by Ranspeed

[alan]

no problem Ranspeed,

if you are going to change the gear selector shaft there are a coupe of things to watch out for...

As you remove the inner gearbox cover, the cam can fall out, if this happens the cam followers wil drop and the push rods will dis engage from the rockers.... its a cylinder head off job to get everything back as it should be....

I have found it easier to lay the bike over on its side, with the engine on TDC on the combustion stroke, that way there is no load on the cam and gravity is working with you. (you dont even have to drain the gearbox oil) , As you pull the inner cover off keep and eye on the cam and push it back if it starts to come off with the cover.

good luck..

[esteve]

Does anybody know who is making the type of Exhaust/silencer (ha, ha) as fitted to Comerfords Cubs ? Picture below to show the type:

[sparks2]

Does anybody know who is making the type of Exhaust/silencer (ha, ha) as fitted to Comerfords Cubs ? Picture below to show the type:

Hi

I know of no-one today producing that type of silencer.

The type was fitted to the late works and the Comerford cubs.

I believe the type originated with the goodies made for the cub in the mid sixties

and marketed by Comerfords, Sandifords and others. I think Alan Jefferies supplied this

or something very similar up to the mid to late 1980's.

Would be interested if anyone could come up with a supplier or further info.

Regards

Sparks2