Need help with Solex 3800

BrianSolex, are you out there?

I hope someone can help. I'm having problems getting the motor to run well with the choke off. I've been through all the usual remedies (I think). Replace crankcase, cylinder and head gaskets. Replace fuel pump membrane & diaphram. Checked/cleaned air & fuel jets. Checked for leaks in air intake manifold.

I don't know what else to do?

It runs great full throttle with the choke on, then it will idle with the choke off, but backfires (flames out of the carb) full throttle/choke off. Then fuel will suddenly start pouring out of the carb fuel filter hole and of course will then die. Fuel flows freely from the return pipe. I am at a loss.

Please help!!

Re: Need help with Solex 3800

When you can only run your Solex with the choke on, you either have air leaking into the fuel intake or combustion chamber, or the fuel mixture has too much air. If you have not installed a _new_ muffler, do that first before anything else. Then, make sure the combustion chamber, piston top, exhaust manifold pipe and cylinder exhaust port are totally free of carbon deposits. see attachment. Then, if you still have a problem, while the motor is running, observe the fuel flow going from the carb to the tank. Hopefully you have a clear hose here or else you'll need to get one. The flow should be smooth. If you see any bubbles in the flow, you have an air leak in the fuel intake circuit, like at one of the pipe unions, at the fuel pump cover, or at a hairline crack in a fuel pipe. If the flow is smooth, check for air leaks around the manifold pipe at the carb base, or one of the cylinder gaskets. Spray some non-flammable carb cleaner around suspected areas while the motor is running and see if there is an increase in rpms. If this all checks out satisfactorily, try a new fuel jet. It is sometimes impossible to perfectly clean it. I have had this problem several times. The fuel jet could also be too small or mismarked. There are different sizes from 26cc to 32cc and also 21 and 22cc jets for the very old 45cc motors. It is also possible that the air jet in the top of the carb is missing or too big. I have made different size air jets in the past by using brass 6mm screws and cutting and drilling them to the correct size. It is possible to fabricate a variable fuel jet also, and I have seen them for sale from time to time, but not lately.

Install a new spark plug also. All of that backfiring and fuel has messed it up and you will not be able to clean it perfectly.

Solexly, Brian


Re: Need help with Solex 3800

When re-assembling the cylinder/crankcase/crankcase cover, it is important to tighten the two cylinder nuts on the two cranckcase studs first to set the level of the surfaces, then draw the cover over, then tighten the two nuts on the cover studs, then finally tighten the crankcase cover bolts. If you don't do this in the correct order, the surfaces of the cylinder base, crankcase and cover will not be lined up and there will be a leak. Sometimes the crankcase cover gasket is too short and there is a leak there. Generally when installing a new cover gasket, it should stick out of the top where the cylinder bolts down, and you should have to trim it with a razor blade to the perfect height. Don't forget the copper washers on all the crankcase cover bolts and on the bolt just to the left of the fuel pump in the front of the crankcase.


Re: Need help with Solex 3800

Thanks for the tips BrianSolex. I did notice the trick to putting the crancase gasket back together, however, I didn't think of trying the non-flammable carb cleaner. I will do so this week, and also check out the other items you've mentioned.

Just curious...

Is the carb for a 4600 the same as for the 3800?

I will post my progress when I make some.

Thanks again.


John S.

Re: Need help with Solex 3800

Hi John,

The carbs themselves are the same. The difference is in how they are setup. On the usual Solex, like the 3800, the 5000 and the 4600 V1, the normal position of the throttle barrel is totally open. The carburetor spring is attached to the bottom of the carb throttle lever and holds the throttle barrel full open. The throttle cable approaches the carb pulley from the top. On the 4600 V2 and V3, the spring is on the top of the lever and holds the throttle barrel in a half open position. The throttle cable approaches the pulley from the bottom. There is a bolt in the lever of the 4600 V2 and V3 which limits the travel of the lever and the lever has a hole drilled in it for attaching the spring. see attachment

Solexly, BrianSoleX


Re: Need help with Solex 3800

Hi Brian,

Here's what's happenning with the fuel...

I put a clear hose on the overflow, and the fuel is flowing nicely without any bubbles, then all of the sudden the overflow pipe will actually backup (only a 1/4 full tank) and change flow towards the carb.

I do know that I probably need a new exhaust, however, I tried it with it removed and it made no difference.

Does this weird fuel flow give you any ideas? Maybe the pump?

Re: Need help with Solex 3800

Hi John,

The first thing that pops into my head is the fuel tank cap. There is usually a small breather pin hole in the top of the cap. I have seen some without holes and some on which the hole is clogged. If you don't see a hole on the cap, perhaps you can loosen the cap a little so that it is not sealed tight and try it like that.

Yes, I suppose the pump could be causing some problems also. The plastic seat that goes between the crankcase and the fuel pump diaphragm must be installed with the concave side facing forwards (towards the pump), and it must be pressed/tapped onto the nub on the crankcase so that it is secure on the nub. If it is not secure, the concavity will flatten out and the pump will not function properly.

While you have the pump off the motor, cover the hole in the back of the pump and blow through the bottom port of the pump. Air should come out of the top port. Now, with the rear hole still covered, blow through the top port. No air should come out of the bottom port.

Let me know what you find with these three things.

Solexly, Brian

Re: Need help with Solex 3800

Thanks for more tips, Brian. I ought to be an expert with all your help. Your extensive knowledge & willingness to share it is an inspiration! I will be sure to keep the same helpful attitude that you've shown to many here on the Moped Army board.

I will let you know what happens. Thanks again.

Re: Need help with Solex 3800


Thank you for the kind, encouraging words. It is truly my pleasure to try to help.

Please keep me updated.

Solexly, Brian


Re: Need help with Solex 3800

Bruce Lindsay /

I have exactly the same problem with my 3800. It starts fine with the choke but won't run with the air open. I suspect that the pump isn't delivering enough fuel, but it is brand new. How can I check that all is well, and what actuates it?

Thanks, Bruce

Re: Need help with Solex 3800

does it run good with any amount of choke, your trying to get to 17mph.

Re: Need help with Solex 3800

Bruce Lindsay /

No it doesn't. It starts but lacks power and won't rev out when pulled off the front wheel. I have a clear return pipe to tank and there's hardly any flow. As soon as I move the choke lever it splutters and dies.

Re: Need help with Solex 3800

rebuild the pump, brian has a utube video, n u can get the parts off him or treats. whatchout for those invisible check balls.

Re: Need help with Solex 3800

Bruce Lindsay /

The pump is new. There was a blockage of the pick up pipe from the tank but that has been cleared. However, I will have a look at the pump to make sure there hasn't been some transfer of debris. Bloody frustrating!

Re: Need help with Solex 3800

The pump being new means nothing when its most likely 40 years old or even older.

Re: Need help with Solex 3800

The pumps are new,... not forty years old,... made in Hungary using the original Velosolex machinery if you got it from a knowledgeable parts supplier (like me) China otherwise (garbage).

Perhaps the fuel jet is dirty...common problem on a Solex because the passageway through the jet lengthwise is's only a 28 cc jet!

Also, think of the Start lever as a choke when first starting the engine, but as a mixture control once the engine is started.

When the engine has warmed up, move the lever left and right to find the best running position.

As long as the lever is somewhere between center and full right, you are fine, no problem.

But, if you have to keep the lever to the left of center for good running, then you have some problem...either too much air in the mixture at the carb (air being sucked into the fuel circuit), or not enough fuel in the mixture at the carb (fuel restriction like a dirty fuel jet or filter).

It is the membrane that does the fuel pump's work. The fuel pump merely contains two check valves.

The crankcase compression and vacuum makes the membrane fluctuate, and suck and push fuel through the pump.

The plastic fuel pump seating piece needs to have a concave side and a flat side. The flat side must face the crankcase. If you cannot easily tell which side is flat and which side is concave, you need to replace the seat, and it must be secured to the crankcase.

See video:

Never raise the running engine from the tire.

The clutch will lock up and you will not be able to control the engine speed.

You will not get a true idea of how the engine is running.

And, if you then lower it down onto the tire, you will damage the tire, drive-roller and drive-roller bushings.

DO NOT use any fuel / gas that contains ethanol.

It is damaging to 2-stroke engines in general, and is especially damaging to the plastic and rubber parts of the Solex fuel system.

DO NOT use full synthetic 2-stroke oil.It will leave a film on the sloppy, low-rev Solex engine that will cause over-heating.

Use only semi-synthetic or synthetic blend.

If you e-mail me, I can send you some helpful literature, manuals, guides, etc.

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