Backfiring though the carburettor

what does it mean?

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Rob D
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Backfiring though the carburettor

#1

Post by Rob D »

We're having great trouble starting our 1942 UL after leaving it unused for some weeks. It starts using a squirt of "easy start" sprayed through the carburettor, and runs for a couple of seconds, but the engine doesn't catch, there's a worrying tendency when using "easy start" for the engine to "backfire" with a small bang through the carburettor causing a puff of smoke (this is when the air intake is removed).
Is it normal to see this, and what does it signify if it's not normal?
Before the bike was set aside, it was running very well.
Thanks
Rob
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Re: Backfiring though the carburettor

#2

Post by WLA_Flattie »

Drain the sump and measure how much is in there. If you're wet sumping it would be hard to start. I wouldn't worry about the backfire with starting spray unless it also starts doing it on fuel too.
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Re: Backfiring though the carburettor

#3

Post by hd74cid »

Just my take on this...

I'm thinking what you're using as a starting fluid is probably ether. Highly combustible which makes it work so well. Problem is use too much and too much isn't much. It has a tendency to wash the cylinders and pistons dry causing premature wear. Here in the states we have WD-40 lubricant in an aerosol. Most any spray lubricant that warns about being highly flammable is better going in your cylinders than something that will wash them clean and dry.
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Re: Backfiring though the carburettor

#4

Post by animal12 »

The Premium Quick start has som lube in it for the top end of the engine , but it's still should be used sparingly . In the good ole days we used to use WD40 instead of quick start . & Never use quick start on a diesel engine .
animal
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Re: Backfiring though the carburettor

#5

Post by panheadrider1961 »

hard starting issue first to check as always low battery voltage ? if it was running fine before
Rob D
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Re: Backfiring though the carburettor

#6

Post by Rob D »

Thanks for all the info above!
As I have learned, the bike only needs three things to run: compression, spark, and gas. We've confirmed [with "easy start"] we've got the first two, so I'm suspicious the fuel is not getting through.
Now I need to hear from those with experience:
The carburettor has been refurbished including a new bowl and float, but I've kept the original float valve needle [27382-33] and float lever (27381-33 - the repro float lever was a casting with a rough finish on all 3 curved fingers, looked wrong to me), but I've used the repro float valve seat. So the float valve has a mix of original and repro parts which worries me.
Also worrying me: the new float is very light, so I wonder if it has enough weight to drop down against any resistance of the moving parts and lift the float valve needle. The original float was hollow brass, in good condition.
So I'm tempted to use the original float and original float valve needle and seat .
A combination of repro and original parts can surely cause trouble.
Does anyone have experience of repro carburettor parts?
Are original hollow brass floats OK?
Thanks in anticipation.
Rob
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Re: Backfiring though the carburettor

#7

Post by panhead_kicker »

I stay away from brass floats; they are much heavier than stock, and can develop pinhole leaks and log. I use a Durable float on my 1964's M74B. It's given many years of trouble free service, and is installed and adjusted per OEM maintenance procedures. As to your current float, it could be hanging up slightly on the bowl stem. Some sloppy stems may require the float to be indexed to prevent hanging. Depending on its manufacture, it could be less than fuel resistant and has swollen, sticking on the stem.
Does your bowl have a drain screw you can open to see if fuel is there?
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Re: Backfiring though the carburettor

#8

Post by panheadrider1961 »

drop fuel line away from filter turn on fuel valve good flow ? then pull up to reserve any change?
crappie float lever is not good polish, and deburr edges, set needle nub to arch at 3 thousandths gap full arch travel up and down
, also light polish on nub to stop hanging in arch ,swing must be good before reassembly
set float to specs, and offset is critical ,so float does not grab side of bowl and hang up
use new nitrophyl black float made from today,s material for crap gas
Rob D
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Re: Backfiring though the carburettor

#9

Post by Rob D »

Panheadrider, thanks - can I check a few details-
- do you mean there should be only a very small clearance (you mention 3 thou gap) between the top of the float needle and the middle finger of the float lever? How does a close tolerance help prevent the needle sticking? Can you explain how I should measure it (a piece of printer paper is about 2 - 3 thou).
- when I polish the sliding surfaces: is a dremel with felt disc OK? Using polishing compound like jeweller's rouge, not valve grinding paste.
- How about bedding the needle to the seat - any tips?
thanks again
Rob
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Re: Backfiring though the carburettor

#10

Post by panheadrider1961 »

YES TO QUESTION 1
polishing with some scotchbrite if it is brass and should be thats all that is needed (red preferred) also light polish on needle nub to help it slide
3 thousandths feeler gauge , or machine gauge shim stock is very bendable to set the arch
if needle is solid on point should match seat, if you have coated needle point get a solid material needle and seat it is better in my H.O.
it is a take your time and it can be done, check the swing before assembling and follow float setting offset from bowl as outlined in book
QUESTION 2
you want needle to follow arch line of the 3 fingers without catching on shoulder of the nub area of the needle ,so you may have to work all 3 fingers to get it to slide smooth ,this is what you are trying to accomplish
Carry on
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Re: Backfiring though the carburettor

#11

Post by panheadrider1961 »

hd74cid
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Re: Backfiring though the carburettor

#12

Post by hd74cid »

Rob D wrote: Mon Jan 20, 2025 12:13 pm - How about bedding the needle to the seat - any tips?
thanks again
Rob
I polished the needle and seat in a motel parking lot once. I used white toothpaste. It has a light abrasive unlike a clear jell which has none. Rinsed with water and dried. Still works today. The modern riders were loosing their minds watching me and I had gathered a sizeable crowd by the time I finished.
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Re: Backfiring though the carburettor

#13

Post by RooDog »

hd74cid wrote: Thu Jan 23, 2025 6:37 pm
Rob D wrote: Mon Jan 20, 2025 12:13 pm - How about bedding the needle to the seat - any tips?
thanks again
Rob
I polished the needle and seat in a motel parking lot once. I used white toothpaste. It has a light abrasive unlike a clear jell which has none. Rinsed with water and dried. Still works today. The modern riders were loosing their minds watching me and I had gathered a sizeable crowd by the time I finished.
Here in Tennessee just having toothpaste can draw a crowd...
hd74cid
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Re: Backfiring though the carburettor

#14

Post by hd74cid »

RooDog wrote: Thu Jan 23, 2025 6:46 pm Here in Tennessee just having toothpaste can draw a crowd...
For the absolute win... :D
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Re: Backfiring though the carburettor

#15

Post by Eb74UL »

You may not like my answer here but are you sure the issue is your carburator?

I run a Mikuni VM38 on my UL and it starts one kick...
When cold, I turn on the "enrichner", kick slowly the engine until it takes a gulp, then turn ignition on and its a one kick...

You could try running a VM38 to see how your bike starts and runs with it?
You can likely purchase a new Mikuni on Ebay for about $150 (real Mikuni) and then you need an adapter and cable... not too expensive for a test...

Then if it runs fine, you can take the time to set up your Linkert properly.

Do you time your ignition static? I set mine at about 5 deg advance static, with the auto advance at rest (not advanced). not enough advance will make the bike hard to start.

If you have an intake leak it will be hard to start... backfiring through the carb is great to cause an intake leak...

Eb
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