Won't run on rear carb

[Ian]

I can't get my L26 to run on the rear carbs, well not me personally but the mechanic the car is with.

New float assemblies have been installed to stop the overflow problem I was having, the carbs have been cleaned out, its getting fuel and spark. The front carb works fine but not the rear.


I'm think he mentioned something about if he lifts the damper it runs ok, but this doesn't make sense to me. Give me fuel injection anyday.


Anybody got an ideas?

 

[MikeB]

Ian

I've replied on your build thread, with a phone nr if you want to talk

 

[MikeB]

Ian

If you need a guy who knows SUs, try Alan Roddy of Roddy's Garage Ballymoney

From what you've said the jets should be set, as the car ran perfectly before, hence it's most likely to be the carb balance that needs sorted

Here are some tips on setting up the linkage. If you don't have a carb balancer, use a piece of tube held to the ear with the other end at the car mouth and you can hear the difference in the air speed in each carb, you'll be surprised how accurate this can be

OK, here is a pic of my linkage (won't be as clean as yours I'm sure !!). Anyway the important bit in this pic, is the screw at the centre top of the pic, which sets the idle speed, screw this in until it touches the stop on the linkage and then give it another turn or maybe two, until you get the carbs balanced and then you can turn it back or forwards to achieve the preferred idle speed

The next shot shows the Idle screw on the carb itself these are actually redundant as the previous screw does their job, so screw them back until you get the carbs balanced, you can then turn them back down to provide an additional throttle stop later on. They are on the linkage side of each carb

OK on to the Balance bar, Screw A pushes on the front carb, and this has a double prong that fits snuggly over the throttle lever of that carb, on my carbs the screw is turned to be finger tight on the lever, just about clamping it, not screwed really tight.

Screw B pushes the rear carb throttle and this screw has only a prong over the top of the lever and this is the screw you use to balance the carbs, by screwing it up to reduce the rear carb opening or down to open it up

So if the rear carb is not sucking as fast as the front carb, Screw B needs to be screwed down until it brings the air speed up to match the front carb.


Good luck

 

[johnymd]

If it was running fine before the float assembly was changed then I would think that's the first place to look. Hopefully the guy doing the work has has not fiddled with various settings to try and get it to work.

I seam to remember that the front and rear car floats were different, or am I going mad?

 

[Mr.F]

front and rear carb floats were different

The height of the inlet valve assembly is different between the two carbs to compensate for the slight tilt on the engine. Floats and other parts remain the same. Float bowl lids may be different on position of pivot rod.

 

[Rob Gaskin]

As I said in your 'Project' Thread, the piston should rise even if the engine is running only on the front carb. (even if the carbs are out of balance and there is no fuel in the rear carb.) The pure act of the engine sucking air through the carb should make the piston rise on it's own. Something basic is wrong IMO.

This might be useful:

http://en.wikipedia.org/wiki/SU_Carburettor

 

[Ian]

Thanks for the guide Mike, unfortunately I can't get to my car until Monday, if on Monday he's had no luck with it I'll try and get it home and try myself, if I can't get it going I'll give Alan a call, funnily enough I was actually at his garage the other day, looking for Steven as he will be mapping the new engine in my civic. Neither Alan or Steven were in when I called though. The Datsun actually needs to go there anyway to get the caster, camber and toe setup correctly as they are the only place close by I've found that can set all three.


My linkage is the same as yours however I think my carb balancing setup is slightly different. I'll find out for sure on Monday.

If it was running fine before the float assembly was changed then I would think that's the first place to look. Hopefully the guy doing the work has has not fiddled with various settings to try and get it to work.

Floats should be ok, they were almost the same as the ones I replaced, apart from they now use metal to attach the float, this makes them easier to adjust. The front float works fine and its the same as the rear, I could always swap them over though, that way I can rule that out as a fault.



Rob - So, if its not an air leak then its likely something is causing the piston to stick?




Not having much luck with cars at the moment, my other cars just failed MOT today on a track rod end, at least I can fix that easily. I should have noticed it anyway but I'd been more concerned with getting my new engine in the bay and running, need to get MOT asap as I have another 300 miles to do before an oil and filter change and reset the tappets, then another 600 miles to do to finish running in the engine before I can get it mapped.

 

[Rob Gaskin]

....Rob - So, if its not an air leak then its likely something is causing the piston to stick?....

Any joy? Are there holes in the bottom of the piston to link the venturi with the chamber above the piston? Are they blocked?

 

[MaximG]

Ian

If you need a guy who knows SUs, try Alan Roddy of Roddy's Garage Ballymoney

From what you've said the jets should be set, as the car ran perfectly before, hence it's most likely to be the carb balance that needs sorted

Here are some tips on setting up the linkage. If you don't have a carb balancer, use a piece of tube held to the ear with the other end at the car mouth and you can hear the difference in the air speed in each carb, you'll be surprised how accurate this can be

OK, here is a pic of my linkage (won't be as clean as yours I'm sure !!). Anyway the important bit in this pic, is the screw at the centre top of the pic, which sets the idle speed, screw this in until it touches the stop on the linkage and then give it another turn or maybe two, until you get the carbs balanced and then you can turn it back or forwards to achieve the preferred idle speed

The next shot shows the Idle screw on the carb itself these are actually redundant as the previous screw does their job, so screw them back until you get the carbs balanced, you can then turn them back down to provide an additional throttle stop later on. They are on the linkage side of each carb

OK on to the Balance bar, Screw A pushes on the front carb, and this has a double prong that fits snuggly over the throttle lever of that carb, on my carbs the screw is turned to be finger tight on the lever, just about clamping it, not screwed really tight.

Screw B pushes the rear carb throttle and this screw has only a prong over the top of the lever and this is the screw you use to balance the carbs, by screwing it up to reduce the rear carb opening or down to open it up

So if the rear carb is not sucking as fast as the front carb, Screw B needs to be screwed down until it brings the air speed up to match the front carb.


Good luck

I don't want to say what your doing is wrong but what you describe contradicts Nissans service manual in how to set up and tune these carbs.

Mike B

 

[Ian]

Yes, she is running now, thankfully. Running smoother than before, but power feels a bit lacking, especially lower down, but I don't know if this is how it was and I'm just used to my Civic which pulls really hard from idle.


I'm sure a carb expert with experience in finely tuning carb setups to gain maximum power could extract more from them but for all the time I'll be using the engine it will do fine.

Only real issue is it doesn't run well on cold startup, choke doesn't seem to do as much as it used to, but if I apply more choke at the engine it doesn't help the running at all. Funs well once it warms up a bit.





I got the tracking and camber set today and it feels much better, toe was way off and front camber was around +1.5, its now set to -0.42, which is as much as I could get it until I turn my camber plates round as they are on backwards and are stopping it from moving in any further.