The V8 Owners Forum -

Hi. I have just installed a turbo on my 3.5L RV8 engine which is currently fitted to my rx7.

I have a few questions that I was hoping that maybe somebody could help me with.

basically I have a single turbo running through a decent intercooler and then blowing through the 2 standard SU carbs with a few little mods so that they can cope. I also have a rising rate fpr installed. As the engine comes on boost it is starting to lean out quite badly as I have not yet adjusted the fuelling.

My question is this- I dont know too much about how to adjust the carbs so is it just a case of running the car on a RR with the exhaust gas analyser and filing down the main needles until i get the fuelling to a better ratio under boost? I have access to the college RR for free so in theory I can spend all day on it.

I have posted this on other bits of the forum in the past but the basic peramiters on the SU are
1/ spring in the dash pot
2/ Main jet (very rarely changed)
3/Needle profile
4/ Needle height
5/If you are well read up you can also change thr redius into the Venturi, particulally the front of the piston BUT you need to know what you are doing it has a big effect on full throttle mixture especially when the piston is near the top of it's travel and through to max revs.
6/ Viscosity of oil in the dashpot.

Sorry got to go to a meeting I'll finish this off later.
Mike

Are you using a turbo SU plenum ? which has a built in restrictor.

If not, TBH, you are wasting your time, or else you will have to replicate that restrictor.

otherwise, it will be very very difficult to adjust the carb to get enough fuel into the engine.


Ask about tuning here.

http://www.turbominis.co.uk/forums/index.php

But the restrictor is vital. I tuned my Mini years ago, and made my own plenum, without realising how important it was. I had to make some serious modifications to the carb, to fuel my Mini engine. I may have had to do some of them anyway, as I was pushing things quite hard. But it would have been soooo much easier with the restrictor in place.

Also, there are 3 different types of spring, colour coded. Cant rem what they are though.

std n/a is one, mettty turbo another, and montego turbo the strongest. Stronger spring is required on the turbo carb, to stop the dashpot lifting quite as much, which also assists with richening things up.

Thanks for the fast responses.
So far all I have done is plumbed the turbo into the carbs, which are the standard rover v8 hif style SU carbs, plumbed boost into the floats and fitted a rising rate fuel pressure regulator. I am not planning on running very high boost levels as the engine is still standard internally and the turbo I am using is fairly small.
I take it the restrictor that you mentioned is to create a larger pressure difference before and after the carb throat to help with the carb seeing the difference in pressure whilst under boost?

RX7-V8T wrote:Thanks for the fast responses.
So far all I have done is plumbed the turbo into the carbs, which are the standard rover v8 hif style SU carbs, plumbed boost into the floats and fitted a rising rate fuel pressure regulator. I am not planning on running very high boost levels as the engine is still standard internally and the turbo I am using is fairly small.
I take it the restrictor that you mentioned is to create a larger pressure difference before and after the carb throat to help with the carb seeing the difference in pressure whilst under boost?

Not so much to help. But the pressure differential it creates over the jet, will cause it to lift more fuel.

FWIW, I ended up with a huge modified needle valve from a Weber, and a 0.125" jet and filed 0.125" needle in mine to get enough fuel in !!!! ( 0.100 is std in a 1.75" SU, the 0.125 comes from the 2" carbs. )
Add to that an extended float bowl and pickup pipe, to hold more fuel.

it was probably rich as anything, but no such things as widebands back in those days.

Ok the car is now running fairly well. I have however just failed my MOT for emissions due to having a CO value that was off the scale, it maxed out at 9.99%

Can anyone please tell me what causes this and what is the best way to go about fixing it?

too rich obviously.

Incorrect needle, or more likely, the fuel jet height needs adjusting.

There is a recessed screw to adjust it, on the left hand side of the carb, looking at the throttle butterfly.

From memory...screw in to richen, out to weaken

Cheers for the rapid response there. The guy at the garage thought it might be something worn out like jets etc.

Is it possible that because I am pressurising the float chambers with boost pressure that maybe this pressure is slightly too high? Or is it more likely to be the mixture settings. I dont know how the test is carried out but if it was too high a pressure wouldnt it be fine at idle when the float chamber does not see boost?

There is one jet, in each SU carb. It could be worn, but chances are slim. Even if it is warm, adjusting the mixture screw will usually be enough to bring things to a sensible level.

If your carb is seeing boost at idle, then yes, you do have problems. But fuel mixtures are certainly the least of them !!!

OK I have access to the rolling road this thursday so with aid of the college exhaust gas analyser I should be able to adjust the carbs to get the right air/fuel ratio.
I take it if I can get close to the ideal ratio then the CO levels will also drop to acceptable levels?[/list]

If you want to get CO levels to the correct levels, then AFR's while linked....are irrelevant.

Use their exhaust gas analyser and adjust the mixture screws, until the CO is where you want it.

Do ensure both carbs are adjusted the same though...... so remove dashpot and piston, and adjust the jet til its flush with its holder, then start from there. At least you know both carbs have the same starting point.