Cracking and banging of of exhaust

[advinnie]

Hi all I’m having a bad time with my 1600 draw through turbo bug. She starts fine idle fine and if you accelerate normally all is good but when cruising at any rpm in any gear and you floor it there is lots of cranking/popping noises coming out of the exhaust and the engine is also not responding great either when this is happing it’s as if it’s lost all its power. This is happening on boost and off boost. Like I say if you was to slowly/ normally accelerate she is as smooth as silk of boost then the boost kicks in and she is off it just goes wrong if you floor it. Any help would be great cause I don’t what to blow the engine for the second time

[buguy]

What does your AFR do when that happens?

[advinnie]

It stops reading mate. It drops to 8.2:1 then shoots up to 19.8:1. It’s as if it’s got no idea if is honing lean or rich.

[madmike]

Try pumping the throttle while it's doing the popping, if it improves you know it's lean , if it don't it's 'FAT'
one of the DOCE guys showed me his mod of adding another 'Progressive hole' (search)

[advinnie]

Just tried pumping the the throttle when it’s playing up and it just stalls the engine. I’m wondering if it has something to do with the pump jet may be bing to large ( I’m running two .70 jets) or the pump exhaust/spill valve I have fitted witch is a non return one. The reason I’m thinking this is because if you accelerate nice and easy she is good but the accelerator pump is not really kicking in but if you floor it the accelerator pump kicks in and may be over fuelling the engine ?
What do you think?

[66brm]

Sounds like a good diagnosis

[advinnie]

Well he’s some news I sept down from 70 pumps to 60 and it made no difference so step down to 50 and it went very wrong. It would nose drive and jump around so went back to the 70’s and things went back to normal. Now what I did find it that looking down the carb at idle there was no signs of fuel leaking but as soon as the butterflies where open there is a very high flow of fuel coming ing out of the auxiliary Venturi if this is normal or not I’m not sure but the craking and popping from the exhaust and the hesitation from the engine starts as soon as this extra fuel start pouring out of the auxiliary Venturi.
Also if I’m at around 2800 rpm or higher this fuel pouring out the auxiliary vent does not affect the engine but when at idle or below 2800 rpm and I open the throttle it’s as if the engine is not turning fast enough for all this extra fuel. So are the mains coming in to early? Are the progression ports being uncovered to early ?

[Dan Dryden]

I think you’re running lean on your accelerator circuit.
The progressive acceleration will mainly be using your idle and main jets, but flooring it will need your accelerator circuit to come into play.
Try upping the size of your accelerator pump jets.

[advinnie]

Ok, should I go from the two 70’s to 75 or take a big jump up to 85? I only ask cause I can only find 75’s and 85’s but nothing in between

[Dan Dryden]

Could you buy both?
If 75’s are too small and 85’s too big you could ream the 75’s out.

[buguy]

Could you do one of each to split the difference?

[advinnie]

Right I’ve just wasted £20 on pump jets
Going smaller than 70’s on the pump jets causes some really bad hesitation and popping so went and got a pair of 75’ & 80’s. I really didn’t notice any difference between the 70 & 75 jets, I still have hesitation and popping from the exhaust when I rapidly go from half throttle to WOT and when I installed the 80’s I get lots of black smoke coming out of the exhaust and some amazing flames and exhaust backfiring. So in a nut shell the 70 pump jets gave me the best performance although it’s still not great. I really do have a lot of fuel pouring out the auxiliary vents as soon as I open the throttle could it be the main kicking in to fast/early? If so I’m sure I’ve read somewhere the chancing the float height will chance when the mains come in?

[buguy]

It's not a waste. And it probably won't be the last of it. You need to have a variety to tune a carb. I don't know how many are here but they are about $10 a pair for a Holley carb. Gotta have em.

[advinnie]

Wow that is a good size collection of jets

[advinnie]

Now this is just an idea and I know I’m most likely going to be wrong but can the main vents be to small causing to much of a vacuum and sucking to much fuel out once the throttle is opened?