Backfiring through the carb

[Carpet]

Hi all. Looking for a bit of help with this one. Just fitted a HS4 carb and LCB exhaust. The car is now backfiring quite viciously through the carb. I've checked the compression and it's all good. Just ordered some new spark plugs as the old ones look dirty so might be that. I've tried swinging the dizzy and adjusting the air/fuel mixture. I know that people say that a lean mix can cause this but I'm well beyond the 2 turns back to start the mixture adjust. Anything people can think that I'm doing stupid? Anything I can try if the plugs don't work? TIA

[svenedin]

In order for the engine to backfire through the carburettor the engine has to fire when an intake valve is open. So either a valve is sticking open or the ignition timing is wrong.

Was the car running well before you changed the carburettor? Have you changed anything else?

First check (and set) the ignition timing.

1) Take the spark plug out of cylinder number 1 nearest to the front of the engine.
2) Using the starting handle, turn the engine over whilst feeling for compression with your finger over cylinder number 1. Compression will try to push your finger off.
3) Go under the car and line up the notch in the crank pulley with 3 degrees before top dead centre (BTDC). The long pointer is Top Dead Centre, the next shorter pointer is 5 degrees BTDC.
4) Take the cap off the distributor and check that the rotor is pointing to cylinder number 1 HT lead. If not, loosen the distributor clamp bracket nut and rotate the distributor as needed, 7/16" AF.
5) Check that the HT leads are arranged on the distributor cap 1,3,4,2 ANTICLOCKWISE.
6) Start the car. It will need tweaks of the timing but this is close enough.

If this does not solve the problem then you will need to check the valve clearances next.

For setting the fuel mixture my personal preference is a Gunson Colortune. You really can't go far wrong except with a very worn engine that burns oil.

Stephen

[Carpet]

The engine was running OK prior to me putting on the new carb but I had noticed coolant in the oil. As I was taking the carb off I thought it was a good time to replace the head gasket. I'll have a look and see if any valve is stuck open. I doubt that it's the timing as that was fine but if the new sparks don't work and I cant see a stuck valve I'll give that a try. Thanks.

[svenedin]

Could it be as simple as the HT leads are on the plugs in the wrong order? 1,3,4,2. Or where you started replacing the leads was not actually the correct lead for cylinder 1?

Stephen

[Carpet]

Just double checked the order. Is correct. I'll have a look at the valves tomorrow. Would be really annoying if I have to take the head off again

[svenedin]

Ok do also check that lead 1 is lead 1 or it will all be wrong.

I sympathise if you have to take the head off again. I had to do it twice for a leaking bypass hose stub pipe. Very tedious. Let's hope this is not necessary. I would certainly be checking all of the valve clearances before considering taking the head off. You can also check that all of valves are opening and closing properly by turning the engine over on the starting handle and watching the rockers, push rods etc. You did say that the compressions were all OK so this is against the idea of a valve stuck open because that would surely affect the compression.

Stephen

[oliver90owner]

Questions:

Is the engine running, or back-firing and won’t start?

You have the correct direction for the dizzy leads around the cap?

[svenedin]

I assumed this car was not starting as the OP said backfiring viciously through the carb. My assumption may be incorrect!

[Carpet]

So I've taken the rocker cover off and hand cranked the car. The pushrod in the centre of the photo doesn't go up or down. I took the rocker assembly off and was able to remove the rod. It's not bent and moves freely so I'm assuming its whatever is on the other side. This explains why I had good compression as the rod goes to the exhaust valve which is stuck shut. The question is now where do I go from here? Any help much appreciated.

Edit: Not sure how to post an image. Here's a link https://i.postimg.cc/c4yxkZSS/IMG-20250 ... 59-HDR.jpg

[svenedin]

So without the rocker assembly installed, the pushrod moves up and down as you turn over the engine on the starting handle? The pushrod is operated by the camshaft via a cam follower. Sometimes these come out stuck to the pushrod or are displaced.

I'd be inclined to give the valve stem, where the rocker operates on the valve, a firm tap with a hammer (ideally a copper or nylon faced one) and see if it frees off. If it doesn't you are going to have to take the head off and investigate why the valve is stuck. Is it stuck in its guide for instance? Then you are going to have to ascertain why the valve got stuck. Was there oil getting to the rocker assembly?

Stephen

[Carpet]

So the valve itself moves. It's the rod that doesn't. I'm assuming it has come off of the follower.

[svenedin]

What capacity engine is this? If it is 949 or 1098 you can remove the pushrod cover on the side of the engine and investigate. 1275 does not have pushrod covers. I can't see that there's room for the cam follower to be lost behind the cover but who knows? Any chance it came out with the pushrod and you dropped it?

https://www.morrisminorspares.com/engin ... ly-p830252

Stephen

[geoberni]

Any help much appreciated.

Edit: Not sure how to post an image. Here's a link https://i.postimg.cc/c4yxkZSS/IMG-20250 ... 59-HDR.jpg

Not difficult, see this, and once you've done that and it's attached, click [Place inline] because that stops it having a white square around it as an attachment.

Image add.JPG (144.58 KiB) Viewed 1322 times

[Carpet]

Thanks everyone for all your help, especially Stephen. I removed the tappet cover and found that one of the cam guides was completely free of its hole. Popped it back in and reassembled and now ip and running again with no back fire. Now I just need to tune the carb and bleed the brakes and she should be back on the road.

[svenedin]

Really good to hear. Well done

Stephen