Sunbeam S7 Running Problems

Thought I would try here as I’m struggling to get my Dads old Sunbeam S7 running. It really was a “running when parked” bike 6 or 7 years ago. Sadly Dad has now gone so I’m on my own. It starts easily and idles nicely. Try and give it some throttle and it coughs and splutters and even spits back through the carb. When I first tried to get the bike started there was no spark so I replaced the condenser, coil and points. I checked the advance and retard mechanism was free. I now have a big fat spark. The original carb was leaking so I fitted a new Amal 624 - a common upgrade for Sunbeams. All the jets were clean and verified. Battery is new as is the fuel. Timing is set at the mid point of adjustment and seems to make no difference when I adjust it. Fuel is fresh and flowing through new petcocks. Thoughts on what to check next?

Does it have an air slide (choke) and do you know how it works?

I took it out of the 624 and the original carb did not have a slide choke. You know, I’ve never stopped to consider how it works. Doesn’t it just restrict the air passage thus richening the mixture?

I asked because a lot of parvenues try to operate the choke backwards.

Maybe it just needs to warm up a bit before it will take some throttle.

Maybe you need a smaller slide cutaway.

Maybe the main jet is blocked with the maker’s notorious debris.

A lot of bikes that are carbureted on the lean side will not take applied throttle until they get a little heat in them. That can be tuned out with adjustment,, likely by a smaller slide like TT mentions. I have honda's that require choke for quite a while before they will run without it. Sounds like you may not be far off.

ilikevelos said:

A lot of bikes that are carbureted on the lean side will not take applied throttle until they get a little heat in them. That can be tuned out with adjustment,, likely by a smaller slide like TT mentions. I have honda's that require choke for quite a while before they will run without it. Sounds like you may not be far off.

Click to expand...

On the flip side
Any bike that you can tickle to start or use a choke etc that starts and runs and idles straight away with the choke off is running way too rich
I see and smell this a lot at vintage bike events

Nice looking bike. Had a similar issue with my old Matchless after it had sat a while. Seemed to be combination of crap in the carb jets and inconsistent fuel level caused by sticking float.

Is the Amal carb a Premier model?

If so I had a very similar problem with transition of idle to low throttle on a Commando. Took some time to solve. I did the following.

Firstly as suggested take out all jets and check they are correct and clean. One of my jets in the matched pair of carbs was incorrect from new.

Next carefully clean all the internal passages in the carb with carb cleaner and air pressure. The two tiny holes at the back side of the side must be clear and flow when you blow carb cleaner through. This is critical and needs cleaning from both directions.

Next check the fuel level in the bowl is correct. One of mine was several mm different from the other. Amal have a technical sheet on how to do this.

Next check the float has no rough finishes and that the metal tag slots are smooth without metal flash that catches on the float needle. There was a bad lot of floats made so check that. I'll have a look for the post that describes that.


Next check the spindle that holds the float is smooth and doesn't jam.

Next check the gasket on the carb bowl doesn't catch the float.

I keep the choke and beleive the bike should be tuned such that it is needed for the first few mins of warm up. As mention make sure it is being operated correctly. Cable tight the choke is off. Cable loose the choke is on.

Follow the detailed Amal instructions for setting the carb.

Check all this about 5 times and in the end you will have it sorted. Be thankful your not using twin carbs.

You can also try increasing idle jet size. Do you have a 17 two ring fitted. You could try a 19 3 ring.

Thank you for taking the time to write all that down. I have a 19 idle jet fitted. The choke slide is eliminated. I read in the Sunbeam Bedside Table Book that the points insulators can crumble so I’ve ordered new ones and will have at it one I return from the UK back to France.

Eliminating the choke is not necessarily a great idea in my opinion. What are your thoughts behind that ?

2 things,.. There's what's happening and what's causing it....

If your normal inspection doesn't turn up a cause, then you are on to do some testing to see if you can figure out what is happening..... and thereby eventually discover the cause...

Since it starts and idles, that is a huge help in narrowing down what is happening. Since it won't accelerate and it coughs like it's overly lean, that might indicate that the secondary fuel circuit might be involved if the cause is fuel related. Here's what you do......

You remove the air filter so you can see the opening of the carb.
You grab the throttle grip with one hand and reach into the carb with a finger of the other hand.
Gently raise the throttle and feel with your finger that the needle is rising and falling with the inner slide body. If it's not, then your needle clip has come undone and your needle isn't being pulled up inside the jet so you aren't getting more fuel when the inner body lifts and gives you more air... Getting a lean kickback would be the expected result.

If your needle is rising with the carb inner body, you can still test for what's happening even if the most obvious cause of lean might be eliminated ( an unfastened needle clip)

My favorite test for fuel starvation uses a squirt bottle with a right angled straw. With the air filter off, I put a little fuel in the bottle and would gently squirt it into the mouth of the carb as I lifted the throttle. If it accelerates with the addition of fuel, then you might have found what's happening "fuel starvation", but you haven't found the cause.

I had a manifold air leak from an overtightened intake manifold that had an invisible crack that allowed air into the manifold. I found it by spraying carb cleaner on the outside of the manifold as the bike ran, which made the bike stumble. I did the test a few times and it stumbled every time. When I took the manifold off, it fell into two pieces...

The point of the squirt botttle is to test for fuel delivery. If the squirt bottle doesn't stop the lean kick back, then I would persue a faulty ignition advance or something ignition related since the fuel bottle test is just to see if you can figure out which way to go in your trouble shooting.

If the squirt bottle helps the problem, the problem is fuel delivery. If the squirt bottle doesn't help, I would start looking at ignition...