backfire through SU and rough running

[Rod Shaw]

Hi

Our engine suddenly started to run roughly and backfire through the SU carb part way through a trial this weekend.  No 4 plug was very wet and the others dry so I suspected a problem on 4. 

The engine turns over easily by hand and no apparent loss of compression on any cylinder on testing cold with the starter. However, the tappet on No 8 valve was loose (disconcerting) so I have reset this, checked the others and expected the fault to be cured - no such luck, no change to rough running or backfire. 

I have now taken out and inspected all the valves. They are all straight, no damage I can see and they all slide smoothly in the guides. 

I am not very familiar with the bottom end of 7 engines but could there be a timing gear problem? 

At a recent rolling road exercise we fitted a short trumpet air filter instead of the previous pan one and got the distributor timing correctly set and the car has run very well since, no hint of a problem. I can't see a carb or fuel supply problem.

I have yet to test with another set of plugs. but have checked for a spark on each.

Any other ideas for me to try/check?

Cheers, Rod

[Ian McGowan]

Rod, assuming a standard distributor, I would check points gap and ignition timing. Your problem suggests to me possible movement of distributor etc. leading to over advanced timing??

[Austin in the Shed]

You don't say the spec of your engine,so guesswork.ie
Late Ruby heads tend to oil plugs.
Bosch "type" distributors can fail inside,condensors fail.Arcing though rotor arm (general)
Is it a good spark you have ? What fuel supply,pump ?If so is it pumping correctly.Blocked carburettor jet ?
If you are trialling seriously you need to keep most of these bits as spares.
Unlikely the valve timing has moved.More likely fuel supply or weak spark I think.

[Rod Shaw]

Thanks to Ian and Austin

It is a new Bosch 009 dizzy. I have tried a new capacitor. I will check all the electrical side again and the dizzy timing and report back. Glad its' less likely to be at the bottom end!

Cheers, Rod

[Malcolm Parker]

I had a new Bosch 009 distributor on the Ulster.  It needed a new rotor arm and points before it would run at all.   After a few miles it lost power and started coughing, the whole base plate was loose and revolving in the body.   Consigned to the bin.   I would use a proper Bosch distributor (mine is from an old Skoda) or put a good old Lucas in.
The saloon prefers a Lucas DK4 to any of the various Bosch types I have tried.

[AustinWood]

I had a Bosch 009. It was dreadful. The car ran as you describe. Went back to Lucas DK4A.

[Steve Jones]

Just seen this, Rod. I looked for you at the finish. This explains why you weren't there.

Like others, this sounds like the ignition timing's slipped. That' s the first place to look. In addition, you can easily check the operation of your distributor by fitting mine. You know the engine ran fine on that so it would be a good test. Yes, of course there are some replica 009 units that are of poor quality but yours came from a good source and the same source that all the ones I have came from. Famous last words but mine have never dropped to bits!

Steve

[Ian Williams]

Have you looked at the obvious and replaced the wet plug?

[Rod Shaw]

Thanks to all for the further comments.

The experience that some have had with some Bosch distributors is interesting and I will have a look inside mine and also check against one I have tried previously, as SJ mentions, and report back. IW's point on wet plug also good and I think I have covered this by trying one of the dry ones.

Thanks again - I will keep working on it!

[AustinWood]

My Bosch 'look-alike' had excessive play in the spindle causing wild variations in points gap.