Backfire and idle jumps with engine heat????

Status
Not open for further replies.
Captain B

Captain B

Country flag
Here's the situation for reference: http://www.accessnorton.com/norton_commando3668.html


So, I checked the timing and it's OK, but as I did so, I noticed that as the engine warmed the idle advanced up to 3500 (after 5 minutes) and the backfires increased. Any Ideas??? :?

I'm currently running without an airfilter (stock sponge element) because the old one is falling apart and the new one isn't here yet.
 
If you still have the old points auto advance ignition, I suspect the auto advance is stick on. But the revving and backfiring may also be something like loose gaskets on the carby joints, sucking air. Gets lean, hot and angry.

Mick
 
Auto advance seems OK. I looked at it and everything seemed clean and moved easily. I was thinking maybe some gunk got in the carbs or something, but I've never had them out and don't really know what to do first.

Does (can?) a Norton running lean backfire (flame out the muffler) if the timing is OK?
 
backfire and high idle with warm engine

Sounds like a classic worn out Amal carb slides. Do you have Amal concentrics with high mileage?

David
 
With the backfiring and no air filter you run a huge risk of catching on fire. Don't run it like that.

Check that all your tiny idle passages are perfectly clean. Below is the best article on Earth ever written for any Amal owner:

http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

Someone may have cranked in the idle speed (slide lifting) screws to compensate for a clogged idle circuit. The clogged idle circuit on mine made it backfire like crazy so it is the first thing I think of whenever someone says backfire...

Russ
 
My thoughts are along the same lines as batrider -- clogged idle circuit is the likely culprit. You can use fine copper wire from a CAT 5 cable to probe the pilot jets clean.
 
L.A.B said:
If you remove the pilot adjuster screw from each carb ...
Click to expand...

Before you remove the screw, use a Sharpie pen to mark the slot position on the carb body. Then, screw the pilot screw very gently until it just bottoms out with very light finger pressure. Count the turns to do this.

When you put the screw back in, first screw it gently until it bottoms with light finger pressure. Then, counting the turns, back it out the same amount you previously screwed it in (i.e. before you removed it) and line your slot up with your Sharpie mark.

I find that this limits the amount of adjustment needed, i.e. you don't have to start from scratch with your idle adjsutment. With a bit of luck you wont have to adjust it at all.
 
Corona850 said:
L.A.B said:
If you remove the pilot adjuster screw from each carb ...
Click to expand...

Before you remove the screw, use a Sharpie pen to mark the slot position on the carb body. Then, screw the pilot screw very gently until it just bottoms out with very light finger pressure. Count the turns to do this.

When you put the screw back in, first screw it gently until it bottoms with light finger pressure. Then, counting the turns, back it out the same amount you previously screwed it in (i.e. before you removed it) and line your slot up with your Sharpie mark.

I find that this limits the amount of adjustment needed, i.e. you don't have to start from scratch with your idle adjsutment. With a bit of luck you wont have to adjust it at all.
Click to expand...

Good point -

When I got my carbs adjusted just right, I took a fine brush and some black touch-up paint, and painted a mark on the air screw and idle adjustment screw on each carb. I found that the air screws tended to back out a little at certain engine vibration points, and a little teflon tape tightened the screws up enough to keep their position.

with the screw positions marked, I can easily rule out misadjustment if there's a starting or running issue.
 
If the idle circuit is clogged then the current position of the idle mixture screws will probably be irrelevant and you will likely have to start afresh once everything is cleaned out. Who knows how much adjustment has previously be made to these screws in an attempt to get the bike running properly. I have found that somewhere between 3/4 and 1 & 1/2 turns out is a good starting point. I have learned with old machinery never to assume something is correct simply because this is the way you found it.
 
BillT,
It's time to replace the mixture screw "O" ring. Teflon tape may find it's way into the idle circuit.

Cash
 
I'm takin' notes. Thanks guys! I'll be tackeling this soon I hope. I'm just putting together a shopping list for Old Britts. (add O rings :wink: )
 
cash said:
BillT,
It's time to replace the mixture screw "O" ring. Teflon tape may find it's way into the idle circuit.

Cash
Click to expand...

Teflon tape was used AFTER replacing the O-rings, and I still had an issue with settings staying put. I understand that a strand of tape could find its way
into the idle circuit, and was very careful to apply a bit at the back couple of threads to minimize the possibility.
I figured it was better than trying lock-tite, and worked like a champ for me.
 
This is exactly the problem I had yesterday while out for a spin. My bike has a new amal mk 2 single carb conversion, new Pazon and filter and the bike started to pop and bag on the over run after about 30-40 minutes running. i filled up with fuel as I wondered if i was getting low and the the over-run issue stopped but the engine was running a higher rpm when idling (1500). The bike would have cooled a bit while waiting at the petrol garage?
The set up is has less than 400 miles on it, and I thought the engine was hot when I stopped for the obligatory cream cakesi had to collect as it was my birthday . Later in the day, when the bike had cooled down it idled fine.
Should i get the carb off and check the manifold bolts are tight as these were a wotsit to get on in the first place (had to use a flexi screw driver with an allen key head as the bolts are allen heads and it was the only way to reach the heads through the manifold)?
Thinking it through, it could be air getting in as the engine gets hot and the metal expands?

Any ideas?
 
Update:

Carbs cleaned up and reassembled + new K&N filter + 230 main jets = runs 100%better :D

Still seems lean at WOT so, I'll go with 260 mains.

Now... setting the idle. Is it desirable to get it to idle correctly at the lowest throttle adjust screw setting possible? What's the secret? I've read the book 20 times :? the first two sentences are the ones I don't understand.

The books says to: "throttle up to 1/8 open." (OK)
Set it to run at a fast idle speed" (what is considered fast?) "with the throttle shut by using the throttle adjust screw." (I thought they just said 1/8 open?!?!)

So, is that just a long way of saying to make it idle hight by cranking in the throttle adjust screws a bit?

To what RPM?

Thanks for putting up with my newbish questions :wink:
 
Captain B said:
Update:

Carbs cleaned up and reassembled + new K&N filter + 230 main jets = runs 100%better :D

Still seems lean at WOT so, I'll go with 260 mains.

Now... setting the idle. Is it desirable to get it to idle correctly at the lowest throttle adjust screw setting possible? What's the secret? I've read the book 20 times :? the first two sentences are the ones I don't understand.

The books says to: "throttle up to 1/8 open." (OK)
Set it to run at a fast idle speed" (what is considered fast?) "with the throttle shut by using the throttle adjust screw." (I thought they just said 1/8 open?!?!)

So, is that just a long way of saying to make it idle hight by cranking in the throttle adjust screws a bit?

To what RPM?

Thanks for putting up with my newbish questions :wink:
Click to expand...

Throttle up to 1/8 open means the idle circuit is the dominant fuel delivery.

Normal idle should be around 1000 RPM. Fast idle maybe 1500. The idea is to have the engine run fast enough to keep from stalling while you set the air screws, then drop the idle down to normal. You want the engine to turn at the lowest RPM at idle where it is running smooth with no hunting or surging. There's no set number, as each engine may have its own idle 'sweet spot' dependent on timing, cam, wear, fuel, etc.

Try this:
http://www.amalcarb.co.uk/documents/Click here to download Mk1 600 900 Hints and Tips.pdf

There's a pretty good set of illustrations on page 7 showing the sequence of fuel control and a step-by-step process for tuning your Amals
 
Status
Not open for further replies.
Back
Top