Carb or Ignition issue?

[sidecar]

If you are using the car on the track I guess you want to get the best out of the engine, personally I'd ditch the foam filter and fit a 3x14 K+N filter. You might have to fit a 'dropbase' in order to get the bonnet shut, try to keep the top of the filter as far away from the carb as possible. From memory my filter lid is 75mm from the carb.

[Prophead]

What colour is the exhaust smoke when you blip the throttle?

Maybe someone has fitted the weakest piston springs which is holding the carb in its weak mode, i.e. the rods are not lifting at all. take a look at the springs under the pistons that lift the rods, the standard ones are orange which will probably be OK-ish. The orange paint falls off the springs so you may need to look very hard to spot any traces of paint! The springs have not been cut short have they?

Missed these bits earlier.

Exhaust is not smoking so no real colour to report.

I did have the springs out but didnt notice a colour on them. I will pop them out again later when I get a chance and do a close inspection. This had crossed my mind but I am getting decent power at part and full throttle, enough to out drag MK3 Escort rally car down the main straight at Knockhill at least

[Prophead]

If you are using the car on the track I guess you want to get the best out of the engine, personally I'd ditch the foam filter and fit a 3x14 K+N filter. You might have to fit a 'dropbase' in order to get the bonnet shut, try to keep the top of the filter as far away from the carb as possible. From memory my filter lid is 75mm from the carb.

Unfortunately I just dont have the space for that filter, as it is this one sits right inside the hood scoop and takes the full width available. In the future I may get another bonnet and completely redo it.

The step-up springs are orange (or at least one of them is, the other has no paint left on at all).

Think I will order a Thermal Shielding gasket, think I can just squeeze it in. If nothing else should help with the hot starting issue. And may help with this hesitation.

Unfortunately I cant get the car out on the road to test it. Firstly MOT ran out last week, secondly been waiting 2 weeks for engineering firm to finish my custom breather cap (clearance issues again). My origianl setup was too restrictive.

[Prophead]

Just another thought on this, prompted by reading some other posts. What fuel should I be using? Since I've had the car I have been running it on standard 95 unleaded (usually from Esso or BP), should I be using premium 97+ octane unleaded?

[Prophead]

Happy bunny today, the car went straight through the MOT no bother. Not bad for a 35yr old with an engine swap.

Emissions test was the biggest surprise:-
C0 = 0.19 (max pass 4.5), last year 1.774
HC = 106 ppm (max pass 1200) , last year 475ppm

Cleaning the carb out has definitely improved things, hot and cold starts both are now much easier and the engine pulls better through the lower rpms than it did before. The hesitation is much reduced but still there a little, I'm starting to think what I have now is some slack in the throttle cable / linkage. However, since the carb clean something new is happening, when I lift off I am getting some popping and crackling, it also did this on fast idle when I was warming the engine up.

Am I now getting too much fuel through the carb?

[ChrisJC]

Popping and cracking is because SU's run lean on overrun. Some have a small spring loaded valve in the butterfly to solve this issue.

Chris.

[Prophead]

Popping and cracking is because SU's run lean on overrun. Some have a small spring loaded valve in the butterfly to solve this issue.

Chris.

Thanks Chris. It's actually running an Edelbrock 4 barrel, but could be a similar situation. Is this something I need to do something about or consider it as 'normal' ?

[sidecar]

Popping on overrun is basically down to fresh air getting into the exhaust on overrun, it 'seeks out' unburnt fuel and ignites it. It happens more on leaner mixtures because even a lean mixture (richer than 14.7:1) will have unburnt fuel floating about in the exhaust but it allows enough fresh air to be in the exhaust to cause the pop! A rich mixture is so rich that there is no fresh air in the exhaust at all hence no pop! Exhaust leaks such as holes and bad joints also allow enough air in on the overrun to get a pop.

My Cobra has sidepipes, on over run I get BIG pops along with foot long flames shooting out of the ends of each pipe!

[Prophead]

My Cobra has sidepipes, on over run I get BIG pops along with foot long flames shooting out of the ends of each pipe!

very cool, me likes.

Think I'm good for now then. Taking it to a 2hr track session Monday evening for another shakedown. Will report back then. Might also try and get it on a dyno as well next week, see if we can find out some truth about this engine. Will report back!

[DaveEFI]

Popping on overrun is basically down to fresh air getting into the exhaust on overrun, it 'seeks out' unburnt fuel and ignites it. It happens more on leaner mixtures because even a lean mixture (richer than 14.7:1) will have unburnt fuel floating about in the exhaust but it allows enough fresh air to be in the exhaust to cause the pop! A rich mixture is so rich that there is no fresh air in the exhaust at all hence no pop! Exhaust leaks such as holes and bad joints also allow enough air in on the overrun to get a pop.

My Cobra has sidepipes, on over run I get BIG pops along with foot long flames shooting out of the ends of each pipe!

The CO figure says it's running weak. Carb RV8 are usually set to 2-2.5% at idle.

[sidecar]

Popping on overrun is basically down to fresh air getting into the exhaust on overrun, it 'seeks out' unburnt fuel and ignites it. It happens more on leaner mixtures because even a lean mixture (richer than 14.7:1) will have unburnt fuel floating about in the exhaust but it allows enough fresh air to be in the exhaust to cause the pop! A rich mixture is so rich that there is no fresh air in the exhaust at all hence no pop! Exhaust leaks such as holes and bad joints also allow enough air in on the overrun to get a pop.

My Cobra has sidepipes, on over run I get BIG pops along with foot long flames shooting out of the ends of each pipe!

The CO figure says it's running weak. Carb RV8 are usually set to 2-2.5% at idle.

I don't set the idle mixture on a carb according to 'the book' figures, the engine will tell you what mixture strength it wants, all you have to do is to listen to the engine whilst adjusting the idle mixture screws. Also as soon as the engine becomes 'non-standard' by fitting a different manifold, carb, gas flowed heads, different cam, ect, then the book figures have even less relevance to the engine.

[DaveEFI]

Popping on overrun is basically down to fresh air getting into the exhaust on overrun, it 'seeks out' unburnt fuel and ignites it. It happens more on leaner mixtures because even a lean mixture (richer than 14.7:1) will have unburnt fuel floating about in the exhaust but it allows enough fresh air to be in the exhaust to cause the pop! A rich mixture is so rich that there is no fresh air in the exhaust at all hence no pop! Exhaust leaks such as holes and bad joints also allow enough air in on the overrun to get a pop.

My Cobra has sidepipes, on over run I get BIG pops along with foot long flames shooting out of the ends of each pipe!

The CO figure says it's running weak. Carb RV8 are usually set to 2-2.5% at idle.

I don't set the idle mixture on a carb according to 'the book' figures, the engine will tell you what mixture strength it wants, all you have to do is to listen to the engine whilst adjusting the idle mixture screws. Also as soon as the engine becomes 'non-standard' by fitting a different manifold, carb, gas flowed heads, different cam, ect, then the book figures have even less relevance to the engine.

I only quoted the book figure as reference. But I've yet to come across a modified engine that idles happily on a weaker mixture than a standard one. And even standard ones might idle more happily on a slightly richer mixture. But I also like my idle speed as slow as it will go reliably.

[sidecar]

The CO figure says it's running weak. Carb RV8 are usually set to 2-2.5% at idle.

I don't set the idle mixture on a carb according to 'the book' figures, the engine will tell you what mixture strength it wants, all you have to do is to listen to the engine whilst adjusting the idle mixture screws. Also as soon as the engine becomes 'non-standard' by fitting a different manifold, carb, gas flowed heads, different cam, ect, then the book figures have even less relevance to the engine.

I only quoted the book figure as reference. But I've yet to come across a modified engine that idles happily on a weaker mixture than a standard one. And even standard ones might idle more happily on a slightly richer mixture. But I also like my idle speed as slow as it will go reliably.

Fair enough, I don't know the CO emissions for my engine at idle but I do know that it is not happy if the mixture is any leaner than 12.2:1. I put this down to my 'free flowing' exhausts and longer than standard duration camshaft.

I hope that the OP has at least twiddled the mixture screws on his carb in order to find out which setting his engine idles best on!

[DaveEFI]

There used to the tables online that gave the rough CO versus AFR.

IIRC, 1.5% CO is very roughly 14:1. That's what my standard Vitesse RV8 is happy with.

A tuned engine needing 12.2:1 for best idle is of no surprise.

[sidecar]

There used to the tables online that gave the rough CO versus AFR.

IIRC, 1.5% CO is very roughly 14:1. That's what my standard Vitesse RV8 is happy with.

A tuned engine needing 12.2:1 for best idle is of no surprise.

Yeah, I was looking a table online, I have books with tables in them but I can't remember whether they were tables for Lambda to AFR or AFR to CO!