Sq 4 Mk II Solex carb quandary

[pauldridge]

My "new" '53 MK II is fighting me all the way

I did some fuel tank refurbishment earlier this week, and while the tank was off, I took the opportunity to replace the head nuts and spacers with new stainless I got from Draganfly. To do the job completely required removing the Solex carb and intake manifold, which, of course, compelled me to polish it back to its original beauty.

With the intake off, I did notice that a good chunk of the intake-to-head gasket was gone, and I do believe it was that way all along. For some reason the gasket kit I bought did not have that gasket, but it was easy enough to fabricate one using the manifold itself as a guide.

Today I got everything back together, mounted the tank, put some petrol in, and proceeded to crank her up.

First issue I noted was petrol dripping at a good rate from the bottom of the carburetor.. I tried tapping on the fuel bowl, but only if I held the bike upright would it quit leaking. I decided to go ahead and start it... with the choke out, soon as she cranked up, she revved up to an estimated 2,500-3,000 rpms. I shut it down, double checked that the accelerator arm on the Solex was fully seated on its stop, and there was some slack in the throttle cable.

Started again, same thing.. but just as soon as I would try to close the choke, even a little bit, the engine would immediately die, and no amount of throttle opening would save it.

Most certainly I've interjected some gremlin during the process of R&R that carb.

Before I yank it off again and scratch my head, I just thought I'd inquire of you experts to see if someone has a clue as to where to start looking to resolve my problem?

[greg.geonetta]

First off, when you say " choke out " you mean the choke is applied?

I'm wondering if it might have had an intake air leak all along. It must have if there was a good chunk of gasket missing.
Maybe before with the air leak and choke applied, the air/fuel ratio balanced out. Now that you've sealed up the gap, you might not need the choke.
Probably running very rich anyway with the fuel leak. You have to fix that first.

[pauldridge]

Thanks for that comment, and I have thought of that.. and yes, it will only start and run (at high rpm) with the choke fully applied.. before, when I started the engine, I could disengage the choke almost completely within 30-45 seconds, but now, if I push the choke knob in (off) the slightest amount, the engine just quickly dies, almost as though it is getting NO fuel through the primary carburetor, only through the choke.

I'll get the manual out and ponder over the Solex operation... this is a different bird for me. I am used to Amal, Bing, SU and Keihin carbs on bikes, not Solex!

Tomorrow I'll pull the carby off and see if I can see anything awry

[Brian.Fosh]

Hi Paul,

I ran my Mk1 for the first time in my lifetime on its Solex last weekend after rebuilding the Solex at Christmas.
Between then and last weekend, the fuel banjo on the side of the float bowl had become slack and leaked.
It turned out to be corroded & stripped threads in the top of the bowl.
Supporting this in situ, I drilled in situ and tapped in situe for a helicoil, It's M12 x 1.25, shortening the helicoil a few turns first. I ground the taper off the tap once it bottomed to be sure i got the thread to the very bottom of the blind hole. I vacuumed and blew out the swarf as I went and then removed it to rinse it out in petrol.

I still had a drip from the base onto the dynamo. That was because the the threaded plug screw that sits at a steep angle at the bottom of the carb was bottoming out "dead tight" before it fully sealed on the fibre washer. It looked and felt in order normal, so it took me a while to figure this one out..but two washers solved that one.

Your leak sounds like a gush, and the valve above the float could be responsible. My valve is working well and I was impressed when I checked it that it seemingly works so very much better than an Amals reverse-tapered valve through the middle of there floats. I checked it by holding the carb and blowing through the fuel inlet and then turning it upside down to confirm the weight of the float closed the valve. I also sucked on the fuel inlet while upside down and then turned the carb back up he right way. This way I could thoroughly feel for even the slightest air leak, much like sucking the air out the top of your soda-pop bottle and sticking it on your tounge. (Well, come on.. we are all just boys with toys eh?)

I came to learn/read that the bi-starter choke block on the top of the carb is actually a secondary carburettor in its own right, feeding both fuel and air. So if this is not fully closed with its lever full over towards the clutch side of the bike (perhaps its lever has slipped up the choke button piano-wire rod), then the engine could well rev as you describe, perhaps more so if there are still any air leaks.

I hope this helps. Good luck.

Brian

[Brian.Fosh]

PS.. Avoid drinking petrol but most importantly.. Please take care when standing on your head

[paul.jameson]

The symptoms you describe suggest to me that you should look first at the throttle cable. If you have disturbed it or altered its characteristics by, say, including another spacer between carb and engine, the cable stop may need adjustment. These are simply a length of steel strip and you may need to bend it to get the throttle to close fully. Alternatively, it may have been bent incorrectly before, thus holding the throttle open with the cable, rather than the screw stop on the carb.

Get some slack in the cable and work from there. Did you clean out the carb? If not, some debris may have moved into the jet. These carbs are pretty simple and if you can get the jets clean, all should be well. Just don't interchange the jets as this causes all sorts of difficult to find problems.

And finally, you do have plenty of fuel in the tank, don't you?

[Brian.Fosh]

Another thought.

Might the float have a leak and fuel inside?
shake to feel any fuel and if so, Submerge in warm water to find any air leaks.
If its leaky, successively orientate it in warm water to expand the air and push the fuel out, then orientate to let only air in as it cools.
Solder up any leaks.

Regards, Brian.

[pauldridge]

nope.. no leaks, but good thought.. I let it sit 2 days weighted down under water.. dry as a bone afterward.

[paul.jameson]

Have you been able to clean the main jet properly by removing it from its housing? Take the main jet out of the carb, and sit it on a suitable sized piece of steel strip, held in the vice. The steel strip goes in the screwdriver slot for the main jet housing. Having got that sitting on top of the vice jaws with the steel strip in the slot, you should now be able to undo the jet itself and clean it out.

[pauldridge]

Uh.. gulp.. this is a bit embarrassing, and thanks to all of you for your expert opinion. Today, I removed my carb and disassembled, cleaning everything carefully and checking my float for leaks, etc.

Before reinstalling, since I had the tank off, I thought perhaps I should double check the gasket between intake manifold and head. As soon as I looked down there, I could see something awry.

As it turned out, I had foolishly set the rocker box cover gasket in place BEFORE placing the intake manifold gasket. Somehow, the manifold gasket got knocked askew, and wound up overlapping the rocker box gasket. The end result was a gaping hole on one side of the manifold junction.

All gaskets now correctly in place and systems back to normal.

Thanks again for all the helpful comments.

Another lesson learned the hard way!