7 replies to this topic

[crf150]

Hi, I bought my mini quite a while ago and was planning to build it back up, it's ground o a halt at the moment because I'm short of time but I managed to buy a waxstat to conventional conversion jet and some gaskets because I have been having problems with it idling/running properly. When it was running and was warm, if I left it to idle the revs would pick up quite high and then drop again, this would continue to happen until it would end up cutting itself out.
I started off by taking the float chamber off to find that there was and orangey coloured fluid in there ? After I drained it it it was gunky in the bottom, I cleaned it all out with some brake cleaner I though the only possible reason could have been rust from the fuel pipes?

Apart from that I changed the waxstat to a conventional, will I need to make any modifications to the mixture or anything? Would be very helpful if I could get some info tonight or early morning as I am going back tomorrow to see if it will run(battery is on charge tonight)

Much appreciated.

[crf150]

Forgot to say that I have got a single su HS4 carb

[dklawson]

The orange goo is very likely to have been old fuel gum/varnish. While brake cleaner may cut it, spray carb cleaner works much better. Any goo you found in the float bowl would also likely to have been in the flexible tube connecting the jet to the bowl. However, since you replaced the jet that will no longer be an issue. One other place to check/clean/adjust is the float valve built into the bowl lid. Make sure you take care of this and use new rubber fuel line when you put the pieces back in the engine bay. Old fuel line can shed rubber crumbs that block the float valve.

You have replaced the jet. Therefore you MUST make some basic tuning adjustments just to get the engine started. Once the engine is up to temperature you will be able to adjust the mixture. Please visit the Burlen web site and check out the links (left bottom part of the opening SU pages) for their technical documents. They will walk you through how to service the carb and how to make the initial adjustments. Make sure the choke is loose before making mixture and idle speed adjustments. The choke and fast idle adjustments are generally made last.

[crf150]

I could only find the tuning for the carb when the carb is running, I could find anything saying tuning after carb service ? Have you got a link ?
Thanks

[crf150]

A

Edited by crf150, 01 December 2012 - 01:14 PM.

[dklawson]

It appears that the online info at the Burlen web site focuses on tuning after the reassembly and basic adjustments are made. I am sure there are other online resources that discuss this but your best course will be to follow the instructions in your service manual or the SU book printed by Haynes.

I will give you some brief statements below but encourage you to get the Haynes book on Weber/SU/Stromberg carbs and perhaps the SU tuning manual also. You can never have too many resources for such things.

Center the jet.
If you have an early HS4 its metering needle will be fixed (rigid).
(Later needles are spring loaded and floppy. The statements below do not apply to spring loaded needles).
Remove the damper from the top of the carb.
The needle clamping screw comes in from the side of the piston. Loosen the screw, move the needle in/out until its shoulder is flush with the bottom side of the piston and then tighten the screw again.
Put the piston/needle back in the carb body along with the spring and fit the suction chamber and install its screws.
On the bottom of the carb loosen the jet bushing nut securing the jet assembly to the bottom.
Lift the piston through the carb throat and allow it to fall sharply. Tighten the bushing nut a tiny bit and repeat the piston lifting/dropping.
Keep lifting/dropping/ tightening until you can fully tighten the nut and the piston assembly continues to smack the bottom of the carb body with a solid "click". If the piston does not fall smoothly and land with a click, the jet is not centered. Keep working at it until it does.

Initial jet setting.
Make SURE the choke linkage is 100% loose, disconnected, and not adjusted. You do not want the choke interfering with the jet adjustments.
AFTER you have the jet centered, use the jet adjusting nuts to bring the jet up flush with the inside of the carb body. You can place a finger against the piston and slowly raise the jet by turning the nut.... stopping when you just feel the piston being contacted. From that point, turn the adjusting nut "back out" (down) full two rotations. That will create the initial mixture setting.


Float valve.
Remove the bowl cover and turn it upside down. Use the shank of a drill bit to measure the gap between the float and the bowl lid. This should be about 1/8". You will either need to gently bend the metal tab the float is on to achieve this or you will need to add/remove fiber washers between the brass float valve body and float bowl lid to achieve this.
Test the valve by blowing in the fuel pipe inlet while the lid and float are upside down. No air should come through until you lift the float up.

Choke setting.
Sorry... you are going to have to read or Google for that one. I cannot remember the basics for that but it must be made LAST.

These steps will all be detailed (better) in the Haynes and SU manuals. They are also covered fairly well in the Haynes manual for the early Mini.

[crf150]

Thanks for the advice, got it running in the end after about 2/3 months of it not being started. It was idling a little high for the first start but after I let it cool down I started it back up again it was idling a little high so only need to do some small adjustments, I need to replace the dashpot oil because it looks a bit old. Does the oil damp the revs going up or down ? I'm unsure, I got told that it damps it revving up ?

Apart from that it all seemed to be running fine now just need to get it all fixed up ready for the respray ! I might be thinking of selling it on here as a project, do many people put projects on here ? And do they sell ?

[dklawson]

The oil slows the lifting of the carb piston on acceleration. You will find lots of preferences for what oil to use in the dashpot. The original SU oil is a straight 20W oil. The thinner the oil the faster the piston will rise. After needle changes some people prefer to run thinner oil. Really... use whatever dashpot oil gives you the driving experience you want.

I seldom venture out of this section of the board so I cannot advise you on selling projects. I can tell you that over here (in the U.S.) a lot of people buy cars and start to restore them. Overwhelmed by the task they put the unfinished project up for sale and expect lots of money because they have bought so many "new parts". They almost never get their asking price. That is not to say that finished projects get their asking price either. A car has to be truly rare for an owner to recover his/her restoration costs. If you rebuild a car it is generally best financially to drive it for a few years to enjoy it and recover some of the cost of restoration.