HS6 carburetors backfire/spitting

[Tr4aJim]

Folks, over the winter I had new throttle shafts fitted to my HS6 carbs. So I disassembled the carbs and just sent them the bodies/float chambers. When I got them back I reassembled them fitting both carbs with new needles/seats and adjusting the float gap to 3/16 inch. I also set the jets 12 flats down below the bridges. I checked the damper oil was correct, and both pistons dropped freely. I also checked the points gap (0.015 inch).

When I started the car, it fired right up. However every 10 seconds or so I would hear a backfire or “spitting” sound from the carbs. When I shut it down,  both carb throats were wet with fuel, not terrible, but visibly wet. I dried them with a towel, and while the rear carb stayed dry, on the front carb the gap between the recessed jet and the bridge refilled with fuel, until level with the bridge. I dried it with a towel and it promptly filled again (not sure why this happens, as both carbs have the same floats/needles/seats).

So, does the carb throats being wet with fuel mean the floats still need a larger gap?

Does the fact that the front carb jet/bridge gap filling with fuel mean it’s float needs an even larger gap than the rear carb? 
 

What say you?

Jim

[Lebro]

Well, either the float setting is too high, or the float valve is not shutting off.

Bob

[Tr4aJim]

Thanks Bob. I’m currently at the maximum recommended float gap (3/16 inch) specified in the Speed Sport “Tuning SU Carburetters” book. However tomorrow I will try increasing it a bit and see what happens. Also both needles/seats are brand new (viton tips) so they should be sealing well. Tomorrow I will also try swapping the needles, seats, and floats between carbs and see if the issues move/change. Fingers crossed.

Jim

[peter clarke]

Try 7/16 as stated in owners manual.

[Lebro]

On HS6's the front float chamber is higher with respect to the bridge of it's carb than the the rear one due to the engine being slanted up at the front.

if your floats are both on the high side then it will be the front one which will flood first. Normally float height is not that critical, but you could (as I have) deliberately set the front one a bit lower than the back one.

Bob

[boxofbits]

1 hour ago, Lebro said:

On HS6's the front float chamber is higher with respect to the bridge of it's carb than the the rear one due to the engine being slanted up at the front.

if your floats are both on the high side then it will be the front one which will flood first. Normally float height is not that critical, but you could (as I have) deliberately set the front one a bit lower than the back one.

Bob

I’ve thought about your theory Bob, is that right? What happens when you drive downhill? So if you had a long line of say 20 carburettors, and the line slanted consistently upwards, you would keep lowering the float level setting in each to effectively create a relatively level fuel line. I would’ve thought that by continuing that compensation, you would lead eventually to no fuel in the float chamber on the forward-most float chamber?
 

Surely the volume of fuel, at a tilted angle or not, is pre-determined by the float chamber setting as a whole assembly (though would need to be set primarily on level ground).  Therefore your theory would suggest a leaner mixture on the front carburettor on your car.
 

Kevin

Edited May 21, 2022 by boxofbits

[Lebro]

It's only an issue with twin HS carbs, because on the front carb the float chamber is above the carb, but on the rear one it is below. H type carbs have both float chambers ahead of the carb, so the same setting will apply to both.

Having said that the actual level is not critical within reason, The 7/16" gap setting should be fine for both.

Bob

[Tr4aJim]

Hmmm, I guess I’m a bit confused on the float setting based on the type of HS carb. My understanding was that the HS carbs that use a “dome” shaped bowl cover, and have curved float fork levers, use the 7/16 spacing.
HS carbs with flat bowl covers,(like my HS6), have flat float levers that are integral with the float, and use 1/8 to 3/16 spacing. Is this not correct?

.

BTW - I checked my carbs and both of the float bowls look at the same height in relation to the carbs.

 

Anyway, I fiddled with both carbs putting the front carb at 3/16 and the rear carb a little less, and now both carbs show the same fuel level. Unfortunately when I started the car, the “spitting” was still there. So I’m now thinking a worse case scenario. Maybe a valve/seat is no longer sealing properly and I’m getting combustion pressure back through the carbs. I will do a compression check today and see.

Jim

 

Edited May 21, 2022 by Tr4aJim

[stuart]

Have you checked your timing?

Stuart.

[Tr4aJim]

Ok folks, the votes are in, and I’m an idiot! While putting things back together after the compression test, I noticed that I had plug wires 2 & 3 reversed on the distributor cap.

I fixed that and the car fired up with no spitting. Who’d of thought? 

Sorry for the wild goose chase. 

On the plus side, the car has had probably the most thorough tuneup of its life! 

But……. there’s still one remaining issue, which I saw at the start of this adventure. With no choke engaged, the carbs disconnected from each other, and the idle screws backed completely off the cams, the car is now idling at 2500 rpm!! Smooth as silk, but flat out. 
Any ideas on this one? 

BTW - Stuart I did verify timing:

- found #1 TDC with piston stop

- using a test light, rotated dizzy until points just opened and test light went on. Locked down dizzy.

- set vacuum vernier to middle position, the rotated a half turn toward “A”, which I believe is 4 degrees before TDC


Jim

[Tr4aJim]

IT’S ALIVE!!! 

I pulled the carbs as a unit and checked the throttle plates. Sure enough the plate on the forward carb had a slight gap when I shined a light through the carb throat. When I looked closely I saw the “lost motion lever” wasn’t exactly square against the throttle lever and was rubbing slightly. I fixed that and the plate closed tight! Reassembled everything (again) and she now idles like a dream! Best idle is at about 1000 rpm, but I’m not entirely convinced my tach is 100% accurate. 

Anyway, some mixture/idle tuning on the carbs tomorrow and she should be back on the road. 

Thank you everyone for coming along on this misadventure! 

Jim