40 replies to this topic

[Chris1992]

Hi guys.

 

Lately I've been having an absolute mare with my twin 1/4" SU carbs. I've just finished a restoration (complete with metro 1275 transplant) so the car hasn't seen the road yet, but it's already blowing black smoke and I just can't get the engine to run right; lots of coughing and spluttering. The points are good, and the plug gaps are spot on, but every time I check the plugs they're completely black. (plugs and HT leads are brand new by the way).

 

The general consensus between me and my dad is that the car running way too rich, but the after tightening the nuts under the carbs that adjust the mixture (the technical term has escaped me) as far as they go, assuming this will make the mixture leaner, it seems to have made little to no difference... On top of this, if I adjust the timing to it's correct position, the engine nearly dies and seems much happier running with the distributor rotated slightly clockwise... 

 

I'm scratching my head here, could this be a case of needing longer needles for the carbs? Can you even buy longer needles? 

 

Any advice would be much appreciated!

 

Cheers

Chris

[ACDodd]

Have the carbs been properly rebuilt?

Are the jets in the fully up position? Ie the choke mechanism not holding them open.

Ac

[Chris1992]

Have the carbs been properly rebuilt?

Are the jets in the fully up position? Ie the choke mechanism not holding them open.

Ac

 

Not completely rebuilt, but I've done a basic rebuild with a service kit, including new jets which seem to be working correctly. I've checked the choke and it's not interfering.

[dyshipfakta]

What car were the carbs off?

[A-Cell]

..and what needles are in them. (Check both carbs)?

[Alex_B]

if it was me I would be resetting jet heights, and checking the choke isnt holding on, check the float level is correct, ensure the jets are centered correctly and the needles are the right type or at least close to correct for the engine spec, top up dash pot oil and set the timing correctly once the engine is running. 

From there adjust the jets to get the fueling closer to optimum. It sounds like something is a miss, assuming you're not suffering from high fuel pressure then checking all the variables in the carbs would be the best bet. The SU book or website should show you what initial settings and needle spec you need. 

[Chris1992]

Cheers for the help so far chaps. Some good advice there!

 

What car were the carbs off?

 

They were second hand off ebay. Apparently they came off an MG Midget, but I have no idea what size engine that car had.

 

..and what needles are in them. (Check both carbs)?

 

No idea yet, this will be the next thing I check. I'll have a look tomorrow and report back.

 

if it was me I would be resetting jet heights, and checking the choke isnt holding on, check the float level is correct, ensure the jets are centered correctly and the needles are the right type or at least close to correct for the engine spec, top up dash pot oil and set the timing correctly once the engine is running. 

From there adjust the jets to get the fueling closer to optimum. It sounds like something is a miss, assuming you're not suffering from high fuel pressure then checking all the variables in the carbs would be the best bet. The SU book or website should show you what initial settings and needle spec you need. 

 

I've check the choke and it doesn't appear to be interfering, I have also checked the float level, but just in case, what level should the float be at? I would assume pretty much horizontal at the point the valve cuts the flow? If that's the case then mine seems to be fine. How are the needles supposed to fit into the jets? Should they be tight or slightly loose? As mine are still a little loose inside the jet even with the mixture nut tightened up completely. I'm beginning to think that I may have the wrong needles fitted, but how would I know which needles I should have?

Edited by Chris1992, 17 October 2016 - 08:10 PM.

[AlexMozza]

I'd check the needles before anything else.

Could be worth getting it to run as best as you can, then putting it on a rolling road to get it 100%.

[petey81]

As ac said get them rebuilt including the spindles and check what needles you have.

ac did the spindles on mine after i used a rebulid kit and the difference is amazing. No point using worn carbs to be honest.

[Alex_B]

On the carbs I work on regularly (larger h8s and such on old jags) the float level is 7/16th or 11mm, I have a turned down ally bar at that dia for setting the level. But double check this is the case on yours. Also if these aren't mini carbs the float bowl angle is wrong I believe so worth checking that

Edited by Alex_B, 17 October 2016 - 10:17 PM.

[Spider]

I've only just now looked at this thread.

 

On the carbs I work on regularly (larger h8s and such on old jags) the float level is 7/16th or 11mm, I have a turned down ally bar at that dia for setting the level. But double check this is the case on yours. Also if these aren't mini carbs the float bowl angle is wrong I believe so worth checking that

 

Yeap, first thing after checking the mixture nut would be to check the float level. If it's high, it'll do exactly this.

 

Check it in the normal way, but also, remove the dashpot after a run and look at the fuel level in the Jet, it needs to be ~10 - 15 mm down from the bridge. Remember, just sliding a needle in there and doing nothing else will raise that level a fair way, not because of displacement by the needle, but a lower pressure on the fuel.

 

Next thing would be the fuel delivery pressure.

Edited by Moke Spider, 18 October 2016 - 12:06 AM.

[finch661]

do you not have to change the float chamber spacers to adjust the angle. i am sure midget twin carbs sit slightly differently. not sure this is necessarily relevant but thought i would mention

[Spider]

Are the Jets 0.090" types and not 0.100"?

[Dusky]

Jets centered?
Not Running Ultra cold plugs by any Chance?

[Chris1992]

Thanks for the continued help guys, much appreciated! I've just been out to the garage and got the needles out of the carbs, and what I have found is a bit puzzling. The needle from the first carb (one on the right) is stamped 'EB', where as the needle from the second carb (one on the left) is stamped 'AN'... Now I'm assuming that these should be matching? Are either of these the correct needles for my setup or are they both wrong? 

 

As ac said get them rebuilt including the spindles and check what needles you have.

ac did the spindles on mine after i used a rebulid kit and the difference is amazing. No point using worn carbs to be honest.

 

I have considered getting them professionally refurbished, but £280 is a lot of money for me at the moment! 

 

On the carbs I work on regularly (larger h8s and such on old jags) the float level is 7/16th or 11mm, I have a turned down ally bar at that dia for setting the level. But double check this is the case on yours. Also if these aren't mini carbs the float bowl angle is wrong I believe so worth checking that

 

Ok once I get the needles sorted then this will be my next port of call. Regarding the angle of the float bowl, when mounted to the engine, the float bowls seem to be sitting perfectly upright, which is a good thing I guess?

 

Are the Jets 0.090" types and not 0.100"?

 

I have absolutely no clue. How can I tell this? Is there a marking on them anywhere that will tell me? 

 

Jets centered?
Not Running Ultra cold plugs by any Chance?

 

I'm not sure if they're centered, but the needles seem to be a bit loose inside the jets even with the mixture nut done all the way up which I guess isn't a good sign. I can't say I'm familiar with ultra cold plugs, the ones I'm using are just standard plugs as far as I know!