Tuning Cold start

[Phild76]

Hi All,

I've done the 70mm cylinder conversion and just rebuilt the carb and the engine turns over but won't rev when cold, if it's left to idle for a few minutes it eventually revs.. any ideas what this is? Could it just be the new cylinder and pistons wearing in. When it's running it revs well, and the upgrade as increased the overall performance.. just a bit confused on the cold start issue

 

[fiat500]

Hi All,

I've done the 70mm cylinder conversion and just rebuilt the carb and the engine turns over but won't rev when cold, if it's left to idle for a few minutes it eventually revs.. any ideas what this is? Could it just be the new cylinder and pistons wearing in. When it's running it revs well, and the upgrade as increased the overall performance.. just a bit confused on the cold start issue

If you haven't changed anything else, and as long as you've assembled everything correctly, it could be as you suggest.
In that case I would try to get the car moving as soon as possible after starting rather than letting it idle with no load.

 

[the hobbler]

If you haven't changed anything else, and as long as you've assembled everything correctly, it could be as you suggest.
In that case I would try to get the car moving as soon as possible after starting rather than letting it idle with no load.

If you have increased the engine capacity, it MAY be that you need to change the idle jet. Amazingly, you might have to DOWN a size of idle-jet.This is because the bigger engine capacity will pull more air through the carb, which will in itself pull more fuel through the carb. On the 126 engines, both of which use a IMB 28carb, the main-jet on the 652cc engine IS one size smaller than in the carb used in the 594cc engine. How long do you keep the choke on when cold starting? It might be worthwhile trying different usage of the choketo see if this makes any difference. The jets for the LMB carbs are not expensive and can be bought from a number of companies. Maybe another Forum member who has also done this conversion can be of help with his experience of the 'up-grade'

 

[Phild76]

If you have increased the engine capacity, it MAY be that you need to change the idle jet. Amazingly, you might have to DOWN a size of idle-jet.This is because the bigger engine capacity will pull more air through the carb, which will in itself pull more fuel through the carb. On the 126 engines, both of which use a IMB 28carb, the main-jet on the 652cc engine IS one size smaller than in the carb used in the 594cc engine. How long do you keep the choke on when cold starting? It might be worthwhile trying different usage of the choketo see if this makes any difference. The jets for the LMB carbs are not expensive and can be bought from a number of companies. Maybe another Forum member who has also done this conversion can be of help with his experience of the 'up-grade'

The Carb is the IMB28 and I've just added the next size up jet.. but have ran it with original size jet too and it had no power and still wouldn't rev until warm. I've put a bigger jet in now, but it still won't rev until warm but has a lot more power and gets up to speed far better on the move. In terms of the choke it runs awful if I try and put the lever fully down.. it was like this before I converted it and always seems to like the choke fully open to around half way when warm. If I close off completely it just dies straight way.

 

[the hobbler]

The Carb is the IMB28 and I've just added the next size up jet.. but have ran it with original size jet too and it had no power and still wouldn't rev until warm. I've put a bigger jet in now, but it still won't rev until warm but has a lot more power and gets up to speed far better on the move. In terms of the choke it runs awful if I try and put the lever fully down.. it was like this before I converted it and always seems to like the choke fully open to around half way when warm. If I close off completely it just dies straight way.

Something sounds wrong with your carb/carb jetting.The "choke" (which to be technically correct ISN'T a choke, but a 'fuel-enrichment' device) should NOT have to be on to make the engine run properly. With it being 'partially on' all the time, that would indicate that something is making the carb running VERY lean as it would seem that you are needing the extra fuel from the 'choke' to give the engine the correct fuel/air mixture to enable it to run correctly. What has the float-level been set too? It should be 7mm between the float and the top-of-carb gasket, with the top of the carb held vertical and the float swinging down. It is easier to get that measurement by using the shank of a 7mm drill (or screwdriver, or piece of rod, in fact anything that has a diameter of 7mm) to put between the float and the gasket. Check that the base of the carb is absolutely flat (and the spacer) to ensure that no air is being pulled in. Something ain't right!

 

[Phild76]

Something sounds wrong with your carb/carb jetting.The "choke" (which to be technically correct ISN'T a choke, but a 'fuel-enrichment' device) should NOT have to be on to make the engine run properly. With it being 'partially on' all the time, that would indicate that something is making the carb running VERY lean as it would seem that you are needing the extra fuel from the 'choke' to give the engine the correct fuel/air mixture to enable it to run correctly. What has the float-level been set too? It should be 7mm between the float and the top-of-carb gasket, with the top of the carb held vertical and the float swinging down. It is easier to get that measurement by using the shank of a 7mm drill (or screwdriver, or piece of rod, in fact anything that has a diameter of 7mm) to put between the float and the gasket. Check that the base of the carb is absolutely flat (and the spacer) to ensure that no air is being pulled in. Something ain't right!

Thanks, the float is 7mm checked that when reassembling it. I've got a feeling it's the base. It was flat on my table and had bit movement in it so this could be the cause. Can these be skimmed to make the the base flat?

 

[the hobbler]

Thanks, the float is 7mm checked that when reassembling it. I've got a feeling it's the base. It was flat on my table and had bit movement in it so this could be the cause. Can these be skimmed to make the the base flat?

The easiest way to flatten the carb's base is to tape a sheet of fine "wet 'n dry" onto something very flat (a thick sheet of glass for example), and keeping it 'wet (WD40 is fine), take the high spots off that way. I would suggest a circular motion so as not to induce a slope onto the base. It can be tedious, but you will then know for sure that no air is being sukked in past the base of the carb. Yo might be surprised as to how much 'bow' there is.