Carb question

[radioactiveman]

Hello!

For a while I've been bothered about having an 'undulating' idle, comparing myself (or my car, rather) to the smooth sewing machine-like rhythm of some of the cars I see on YouTube. It's generally satisfactory, but the ignition light does tend to pulse a little, and it could be improved.

Anyway, recently after I started the car and when I went to rev it it 'faltered' (it was a little like I'd lifted the lift pin all the way up). It occurred to me that maybe the jet/needle wasn't as free as it should/could have been (my theory being revving was introducing lots of air but not enough extra fuel). Is it possible for things inside the carb to get gummed up? It seemed to sort itself out relatively quickly and get back to normal.

[svenedin]

Yes the carburettor can get gummed up. Less likely on a car that is used regularly; more likely on a car that sits unused for periods of time.

The needle and jet can (and do) wear, the spring that pushes the piston down can get corroded and weak and the jet might never have been properly centred.

Have you done the piston lift and let go? It should come back down with a metallic thunk. Also have you checked the dashpot oil?

Having the engine run like a Singer sewing machine is usually down to having the rocker clearances correct, the fuel mixture correct and the ignition timing spot on.

Stephen

[JOWETTJAVELIN]

Check for air leaks, all carb manifold flange gaskets in order, insulating piece is not cracked, and finally check tightness of manifold bolts and the two carb bolts with spring washers underneath.

Don’t rule out an ignition fault such as extreme wear in the distributor, or even wrong distributor fitted.

[oliver90owner]

Have you compared the actual idle speed of your engine with these others? Think, here, that if you continue to slow the idle, eventually one cylinder will be ‘helping’ the others to allow the engine to continue running at all.

If one starts the engine by means of the starting handle, even the compression on one cylinder may make it the optimum time, to crank more energetically, in order to get the engine running. From that, I might suspect the real compression pressure at very slow idle may be an important factor. Even air flow within poorly balanced manifold ducting can make a difference - have the ports been matched, for instance?

[les]

I think the suggestion of an air leak is well worth considering, I suppose, time permitting cleaning the carb internally would also be a plan. Sometimes you just have to eliminate things one at a time. Hope you get it running well.

[myoldjalopy]

To add to all the above, sometimes symptoms of that sort can merely indicate a thorough service is due! Plugs and points in need of attention can cause difficult starting, poor idle etc.

[radioactiveman]

Thanks all for your thoughts. I appreciate that no one on this forum has any obligation to help a stranger on the Internet yet you always continue to do so!

I imagine part of my problem is lack of use, but servicing the points/dizzy and valve clearances might be a good next step. I've been a bit reluctant so far for fear of messing up the timing (with regard to the points) or doing it wrong but so far I've managed not to do too much damage to the old boy

Interestingly, the spacer bit between my carb and manifold has some sort of residue at the join, and I've always assumed that that is just some sort of jointing gunk, but perhaps it's the sign of a leak. I will investigate!

[myoldjalopy]

The other easy thing to check is whether there is enough oil in the carb dashpot - too little can cause hesitation/stalling on acceleration.

[ManyMinors]

Another point - since you suggest that the car is seldom used. How old is the fuel? Modern petrol goes past its best quite quickly and that will also cause a car to run poorly.

[radioactiveman]

The fuel is a little old, but I've not noticed much of a difference whilst it's been in. I've also topped up the dashpot oil, although maybe it's worth trying a different viscosity.

I suspect it's a case of just making sure everything is good/clean/tight one at a time as there's not a specific fault.

Talking of servicing, am I correct in my understanding that if one wants to service a distributor (new points/gap) it's easier/best to do it out of the car and that won't impact timing if the bolt on the top of the distributor isn't loosened? Are there any precautions to take here?

Ta

[svenedin]

Yes you leave the pinch bolt alone and take out the distributor by removing the 2x bolts that hold the clamp bracket. The timing is not affected. It is much easier to change points with the distributor off the car and you’re less likely to lose screws! You can give things a clean, inspect properly and lubricate.

When re-installing the distributor ensure it goes fully home. If you have not changed points before check the order of installation as if you put the insulator in the wrong place on the post your car will not start. Remember to clean the contacts on even brand new points.

Stephen

[myoldjalopy]

If you do remove the dizzy to service it (most people don't), in addition to leaving the clamp/pinch bolt alone, make sure you don't turn the engine over at all before re-fitting the dizzy or you will lose the timing!

[svenedin]

If you do remove the dizzy to service it (most people don't), in addition to leaving the clamp/pinch bolt alone, make sure you don't turn the engine over at all before re-fitting the dizzy or you will lose the timing!

I can’t see why you would want to turn the engine over but it will not affect the timing. It just means you’ll have to rotate the distributor spindle so that the drive dog engages properly. Providing the bracket is not rotated on the distributor body it will be fine.

Stephen

[oliver90owner]

new points/gap) it's easier/best to do it out of the car

Nope. Perhaps easier to actually reach and change parts but, if you think that altering the points gap is not going to affect the timing, you are mistaken. The points gap will definitely change the timing, particularly if you may be changing the make of the points. Small differences can create timing changes.

I have never removed a distributor for changing the points - even where the distributor is horizontal (contact breaker plate vertical), but I have removed umpteen magnetos for service. I always check, and adjust, the timing as necessary.

[myoldjalopy]

Let's say you fit a new set of points properly and check the timing, the ignition should be set up correctly. As the points wear the gap will alter, but if you have not messed with the timing in the meantime, when you re-set the gap/fit new points gapped correctly, then the timing will be back to where it was originally.

[radioactiveman]

Thanks, interesting stuff!

I’ve always been reluctant (so far) to get involved with the timing as it seems a complete mystery. Particularly when a ‘strobe’ is involved!

[radioactiveman]

Good news! I was able to change and gap the points. I actually changed all the replacey bits as I had a set that had everything connected together (in fact I have two…). I kept the dizzy on the car as I wasn’t feeling very brave… it was a little tricky but it seems to have worked.

I did, of course, forget to replace the rotor arm… which did initially cause some consternation.

I’m not sure it’s made much difference, but it did seem much more eager to start, which is something!

I also popped the idle speed up a smidge as the slightly nicer sound makes me feel better

Thanks for the help everyone.