R75/5 Misfire.

talaris · Dec 31, 2017

A bit of help please.

1970 swb R75/5. It misfires ans surges at anything tick over. Not a lot but enough.

I have changed everything. Points condenser coils leads choke bodies plugs and cleaned the carbs and fitted new o rings apart from the throttle as they are very fiddly. Timing is fine and fuel is new and clean.

Apart from the aforementioned o rings what am I missing.

There is a picture of it show us your garage not that it helps but you can see it’s not a minger

nick.ct · Dec 31, 2017

What carbs are you using?

Just seen the pics in show us yours.

Suggest you check the diaphragms in the carbs

Bike looks lovely, I have a 60/5 SWB

talaris · Dec 31, 2017

Thanks.

32 Bings. Did the diaphragms as well. Forgot to mention that.

Driving me nuts it is and costing a fortune!

60/5 and the 75 are getting rare especially in good nick.

nick.ct · Dec 31, 2017

Can you tell if it's missing on just one cylinder or both?

One would indicate a fuelling problem, both an electrical problem

talaris · Dec 31, 2017

I would say one.

If on tickover you take one plug cap off it cuts out. Take the other and it just runs like it’s on one cylinder like it should. A plug did break down though which led me to change the plugs and leads.

I think it is fuel but short of re jetting there isn’t much left to do. All are clear including the little ones in the float bowl. Maybe it’s the throttle o rings.

nick.ct · Dec 31, 2017

I assume that if you swap the plug leads over there is no change?

solidstate100 · Dec 31, 2017

Have you checked the valve clearances?

Edward · Dec 31, 2017

Have you checked to see if the throttle cables are both starting to move the spindles at exactly the same time?

hops · Jan 1, 2018

Compression test?

talaris · Jan 1, 2018

Clearances ok, compression fine on both, throttles open together...

It’s a compre mystery.

Rob Farmer · Jan 1, 2018

Sounds like it's ignition. My 100/7 is poorly at the minute with a similar issue. The nose of my cam that the points run on is so badly worn the points don't open properly. It ticks over beautifully but misses and farts when I Rev it. Not saying this is your issue but it sounds similar.

talaris · Jan 1, 2018

Thanks Rob. Do you mean the cam on the governor?

Kenny · Jan 1, 2018

Ensure OE points are fitted.

solidstate100 · Jan 1, 2018

Also I believe the nose of the cam is very easy to knock out of centre so it can put the points gap/dwell/timing all out of whack.

UturnTony · Jan 1, 2018

How many miles has the engine got on it? I'm thinking inlet valve guides worn allowing air in; weakening the mixture :nenau

Other than that a condenser can cause the same thing but you have replaced that.
What about taking it along to an MOT testing station and see if they will stick their emmisions testing gear on it.
Could give you a read out of what the air/fuel ratio is doing :nenau

Rob Farmer · Jan 1, 2018

talaris said:
Thanks Rob. Do you mean the cam on the governor?

Badly worded on my part. Yes I did. Its also worn where it locates on the nose of the cam. With over 220k on it I'm not too surprised though. Ive also had a lot of issues with the condensors being supplied causing rapid wearing of the points.

talaris · Jan 1, 2018

Really appreciate all the help.

Governor looks ok but frankly I probably wouldn’t recognise if it didn’t. :comfort

Ran it tonight. Idles fine but garage smells of gases quite heavily so it’s obviosly rich.

It’s only on one side as it runs fine on one side if the plug cap is off and won’t run on the other if the cap is off.

Interestingly my temp sensor shows it much hotter on the “good” side than the other.

talaris · Jan 1, 2018

UturnTony said:
How many miles has the engine got on it? I'm thinking inlet valve guides worn allowing air in; weakening the mixture
Other than that a condenser can cause the same thing but you have replaced that.
What about taking it along to an MOT testing station and see if they will stick their emmisions testing gear on it.
Could give you a read out of what the air/fuel ratio is doing

About 85k miles. It’s gassing out really heavily in the garage when run for a few minutes.

talaris · Jan 1, 2018

Kenny said:
Ensure OE points are fitted.

Interesting!

Usually use OEM as it’s the same price as motobins/works/Sherlock. These were from Sherlock’s iirc.

talaris · Jan 1, 2018

hops said:
Compression test?

Yep done that. All good.

R75/5 Misfire.

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