Quirks of Mikuni/Solex's and Webers

[jeffro ra28]

Rodger, im interested to see why you dont use soft mounts on your engines?

[GeeB]

Thanks Rodger, i definately followed the correct tightening sequence last time but maybe took it a bit easy on the actual tightening, didn't want to over tighten if thats possible. Was used in a pretty rough khanacross so a bit more vibration than regular road

[GeeB]

BTW how do you know what size jets to use

[Rodger]

Jeffro,

I use the OEM spacer blocks. In the limited dyno runs I have done I have not observed any drop off in performance at the top end that can be contributed to fuel frothing and associated A/F changes. I notice though I see a lean off at high speed, full throttle (in competition) more to do with lack of fuel flow. A combination of the OEM tank/electric pump and lines I used.

There is a definate need to use some sort of spacer, OEM or softmount, as this reduces heat transfer into the carbies from the manifold. I ran my show Celica for a while without any spacers and I could feel how hot the carbies got.

GeeB

Most of my jetting choice is based on Mikuni's original recomendations, especially if the engine has been modified. Then changes are done using road driving and Idle adjustment. If I work on a set of Carbies I'll make a judgment call based on how it drives, get it as drivable as I can and then advise the owner to have the car dyno'ed to better set the balance, idle mixture and speed and to show what the main jetting is like and ask the dyno guys what they think the jetting needs to be.

There are definate improvments for all 18R-G runners with the OEM Type T Solexes. Most are well under jetted (Air and Main) simply because Toyota had to meet the anti-polution levels at the time and being used in road cars they rarely were run hard and fast so no need for top end power. Running unleaded, removing the anti-polution gear and re- jetting can actually make the motors run cleaner.

Regards

Rodger

[sillycar chick]

Something has me stumped. 51LII has developed a little issue when cruising along at light throttle. It seems there is not quite enough fuel when cruising at lower revs, but when you give it a bootful she's more than happy to get up and boogie. It runs fine, it just has this slight miss (for want of a better word, it doesnt feel like a miss it feels like a fuel issue) Ive checked the venturies and they dont seem to be loose. This minor problem has only started to arise since I had the carbies balanced and re-tuned. My boyfriend's dad tuned them, and by god he did a better job without a dyno than the carby mob did with a dyno. The thing now idles so smooth its not funny, and I have noticed a big difference in response when you put your foot down. The only thing is this slight issue that is really difficult to explain.

I know rodger has suggested my jetting sizes seem lean compared to what the manuals recommend, but the exhaust pipe on my car and how it is running is indicating it is actually slightly rich. Plugs looked good last time I checked (though they are well overdue for a change - I have been too busy ) Maybe the plugs could be the issue, im not sure And there are no problems with it dieing out up hills, 51LII just wants to power up them.

Any suggestions as to what is causing this?

[jeffro ra28]

Speaking from experience, It is extremely hard to judge an engines tune by looking at exhaust ports or exhust pipe. It also is very hard to determine the correct tune by looking at a plug when running on regular unleaded. This is because of all the impurities in it.

I am only saying this because i dyno performance/race engines atleast once a week. And from taking readings individualy from EGT's and lambda, the plugs in an engine running unleaded fuel is a fairly irellavant tool to tune from. Premium unleaded is a different matter. Also, the only real way to get a proper reading is with a new plug and the engine must be shutdown instantly after a full throttle hold.

[Rodger]

Dana,

If the top end power is good with the main jetting you have I would look at going up one size in Pilot jet from #57.5 (OEM) to #60. It is running just a tad lean at cruise. Mine did it. It is the slightest miss when out on the highway.

When the "dad in law" tuned them I'd bet he leaned off the idle mixture a bit which has an effect on the overal pilot mixture.

The next size up on the Main Jet will just about do the same thing. At the normal 100-110km cruising the carbies are barely on the main system. This is why the car picks up again when you open the throttle some more to accelerate.

Regards

Rodger

[sillycar chick]

Jeffro: Car runs on 98 octane, nothing less

Rodger: will try drilling the pilot jets to 60 see if that makes a difference. I have a set of number drills so should be easy.

[sillycar chick]

Was doing some research on the DCOE webers, and came across a very informative site. Its dcoe.net. If you follow the links to "tuning/guides" and then at the top of the contents "Selection and tuning of Weber DCOE carburettors" you will find a very informative article about selecting sizes of venturis, jets and emulsion tubes etc to suit the size engine you have. Well worth the read.

[sillycar chick]

Bought a new silly with 45mm webers on it, and had a few hiccups tonight. Hoping someone can shed some light. This morning I had some issues with it cutting out and running rough on throttle. Played around with it this arvo and decided to replace the fuel filter, as I happened to have a new one here. Started it up and it seemed ok, so went down the road for a test drive. As I get 50ish metres down the street the throttles get stuck on! So I turn it off, pop the bonnet and check but the linkages are moving freely and dont seem to be sticking. So I start it up again and it screams again at about 5000rpm. Turn it off and check again. Nothing is sticking! Keep checking the linkages for a few more cycles, start it up and she runs at normal revs. Drove it down the road and the cutting out problem didnt show itself, so figured the fuel filter was the trick. Took it out again later to get some dinner, and its still running fine.

Is there something I should be checking or lubricating? i will give the linkages a quick lube before i go to work tomorrow, should i be doing something about lubing butterflys or something? im just concerned about the sticking coming back, could be very unsafe in traffic

[Starfire]

If the throttles are opening past 100% open, the flow of air can hold them open. Make sure that the stops are correctly set.

[Rodger]

A couple of things to check Dana that can stick Webers open. The accel pump mechanism is unlike the Solex but a pump plunger type arrangement. There is a return spring sitting inbetween the barrels. A pump plunger can get stuck and not be released by the spring. This spring also is the throttle shaft return spring so if the plunger is stuck so is the throttle.
Webers often in competition use one or two more external springs added to the linkages to ensure they close.

The Throttle shafts run on bearings which could be binding either from wear on the shafts or the bearings themselves.

Check the linkages again, they could be loose on the shafts and so cock sideways a bit and catch on the body or are sloppy on the shafts so one carb is open just a touch when the other is closed.

All these linkage problems I also see on the Solex and the Dellortos.

BTW Dana I am posting the filters tomorrow.

Regards

Rodger

[Billzilla]

A couple of things to check Dana that can stick Webers open. The accel pump mechanism is unlike the Solex but a pump plunger type arrangement. There is a return spring sitting inbetween the barrels. A pump plunger can get stuck and not be released by the spring. This spring also is the throttle shaft return spring so if the plunger is stuck so is the throttle.
Webers often in competition use one or two more external springs added to the linkages to ensure they close.

More accurately, the first thing you remove in a Weber is the return spring as they break too often and jam.
Then add two external return springs.

[Rodger]

Damn fickle Webers hey Billzilla

Sometimes makes us wonder why we just don't use those complicated programable thingies that everyone else has problems with

Regards

Rodger

[airfireman1]

Hi,Can someone please give me a starting point for jet and specs for a set of 40dcoe webers to go onto a 18rgeu. I have no idea what these have but were supposed to be from a 2tg. At the moment it is idling extremely rich and adjusting the idle mixture screws right in makes very little difference. No rush as I have had to pull the head due to a BHG but some reference points would be a great start.
Cheers