14 replies to this topic

[VNOMUS]

Model: Mini 1000
Year: 86
Description of problem (please be as in depth as possible):

I got Mini 1000 which runs great but wanted to switch to twin SUs mainly for aesthetics. So recently, I bought a set of used HS2s and manifold, cleaned them up and rebuilt them (new standard jets, blue springs, butterflys and shafts), set the initial mixture at 1.5 turns out. The motor started right up and runs ok except for a bit smoke (rich)and a couple of problems which are probably related...

1. The balance is way off. One carb's flowing a lot, the other hardly at all?
2. The mixture's too rich even after turning the mixture all the way in on both. When I raise the pin the high-flowing carb the idle goes up, on the other there's no change (no matter what the mixutes set at).

Any non-standard parts that might be involved with the problem?

None...

Any ideas on what would cause the balance to be off so much? (the motor was running fine with a single carb so I don't think there's anything wrong with it)....

Edited by VNOMUS, 04 October 2009 - 03:47 PM.

[bmcecosse]

You obviously have to balance them up after installation! Slacken the joining link on the throttle spindles - then listen to each intake through a length of 1/2" tube - held up to your ear - and to the SAME spot on each carb. Adjust the idle screws until both sound exactly the same. Then adjust the mixture on each carb - then re-adjust the idle etc etc. Finally caerfully re-tighten the joining link - without moving either spindle! Are the carbs and manifold definitely for a Mini ? If from a Spridget (or an 1100/1300) they will be wrong angle and will be hitting the bulkhead! Check the float bowls are dead vertical - if not vertical they will be flooding over all the time - hence perpetual rich mix.

Edited by bmcecosse, 04 October 2009 - 03:54 PM.

[dklawson]

Roy has given you good advice. The only thing I would add is that when you are balancing the air flow between two carbs it is necessary to not only loosen the throttle linkage between them, you must also loosen the choke linkage AND back off the choke adjusting screws. All of this is spelled out pretty well on various web sites and in most Haynes manuals.

The listening to the tube method mentioned by Roy is a tried and true method of balancing carbs. There are two others that come to mind. The first alternate method is to use a UniSyn gauge or similar to measure the air flow. Car parts stores used to sell these routinely. Today, you're more likely to find them at a motorcyle shop and of course... on eBay. The second alternate method I only know as the pointing wire method. I'm sure it has a proper name. The dashpot dampers are removed from both carbs and with the engine off, wooden or plastic dowels are pressed lightly into the tops of the carbs. Thin wires are then inserted into these dowels and bent so they are pointing at each other. (One dowel and wire in the front carb, one dowel and wire in the back carb, the two wire tips pointing at each other). Once the engine is running the unbalanced carbs will no longer have the wire tips pointing at each other. The balance is adjusted until the wires are again "tip to tip". It was a very effective way to cheaply balance carbs. (Not as cheap as a tube stuck in your ear though!)

[VNOMUS]

Thank you both for the info & advice! It's my first experience syncing multiple carbs so I guess I was expecting them to be more closely matched right from the start. I've got a Unisyn tool as well so I'll follow the instructions above. Thanks again!....

[bmcecosse]

Not heard of the 'pointing wires' trick! Sounds good. I only ever connect 1 choke up on twins - saves all sorts of hassles - and never failed to start even in coldest weather.

[Pooky]

Gunsons Carbalancer

[valve bounce]

make sure you have the right needles also

[zef]

I thought initial setting for su's was 2.5 turns out and screwing the mixture screw IN makes the mixture richer?

[VNOMUS]

Ok, I did as listed above and although I can match the sound up above idle, they still don't match when I reset the idle. The fuel bowls aren't hitting the bulk head and are both vertical. The mixture screw and push pin seem to have no effect on the "weak" carb. I think because at idle, it's simply not flowing much (I can lift the piston all the way up with no change, when I do the same to the "stronger" one, the motor stalls)...

[lawnmowercrap]

Ok, I did as listed above and although I can match the sound up above idle, they still don't match when I reset the idle. The fuel bowls aren't hitting the bulk head and are both vertical. The mixture screw and push pin seem to have no effect on the "weak" carb. I think because at idle, it's simply not flowing much (I can lift the piston all the way up with no change, when I do the same to the "stronger" one, the motor stalls)...

Are the air filters the same make? A re the needles the same? Have you blanked off the breathers? Do the daspots have oil in them? Are the fuel lines from the t piece to the carbs the same length? What pump are you using?

Edited by lawnmowercrap, 04 October 2009 - 11:06 PM.

[VNOMUS]

I haven't put the air filters on yet. I'll have to check the needles. I bought them from Mini Spares a week ago but I just "assumed" they were identical. Not sure what the "breathers" are so I didn't I blocked them off. The daskpots have oil. The line from from the stock fuel pump tees off to both carbs but they are not the same length...

Edited by VNOMUS, 05 October 2009 - 12:43 AM.

[dklawson]

I thought initial setting for su's was 2.5 turns out and screwing the mixture screw IN makes the mixture richer?

That may be for later HIF type carbs. On the HS2s mentioned above, the default "start" position is two full turns of the nuts (no screws) "down" from flush with the bridge. Turning the nuts "out" lowers the jets relative to the carb needles and opens up the flow area (richens the mixture).

From what you're describing, it sounds like the engine may be running on only one of the carbs at idle. There are two things that come to mind. Please take no offense at any of the suggestions and observations... none is intended.

The nature of some of your questions and response implies that you have not gone through a complete adjustment of the carbs. You pretty much have to start from scratch when you fit them. Assume nothing is set correctly. Complete descriptions of how to set and adjust SUs should be in most Haynes manuals.

Specifically, as I mentioned in my first post, if the choke linkage is not loose and the choke's fast idle screws are not completely backed out you can get into situations where you can not make changes to the mixture at idle. Make sure both choke and throttle linkages are loose as you make your adjustments.

Again returning to items that must be adjusted "from scratch", it is necessary to center the jets on HS2 carbs. If you didn't take this step, it could be that one of the carbs needles is not centered and therefore, it is not falling freely to the bridge and binding.

[Spitz]

Are you sure the fuel valve on the "non-working" carb isn't stuck closed?
There's a chance the needle valve in the float bowl is stuck and no, or little, fuel is getting into the bowl.

[VNOMUS]

Ok finally got back to working on the Mini!....Thank you all for the tips and advise. I pulled both carbs, matched the piston and domes (they were mixed up!), reset the float height on the weak carb(it was a bit low), recentered the jets, and resynched(choke linkage disconnected this time). Results were much better(balance-wise), but still not right. The front carb's still runs rich (black smoke & missfire) even with the adjustment nut all the way in. Also, I still can raise the rear piston all the way up with no affect on rpm, but on the front one it'll increase rpm initially, and stall if I raise it all the way?...

Edited by VNOMUS, 12 October 2009 - 12:42 PM.

[VNOMUS]

Hmm, looks like the floats were the problem. The little tab at the rear of the float was too high which allowed the float to pivot down so far that the needle would jam open. The carbs are still too rich though. I've currently got "M" needles installed and according to another post those should be swapped for "GY's to start off. Is there a listing of particular sets that work well in a stock 998?...

Edited by VNOMUS, 14 October 2009 - 09:13 AM.