stubborn flat spot under acceleration

[brit01]

Been getting a stubborn flat spot under hard acceleration.

I have 53 idles
142 mains.

I had 33 pump jets.
I changed them to 40 pump jets last night to experiment - got a nosedive and then good acceleration but obviously it was washing my cylinders as I lost some oil pressure.

With the smaller 33's I didn't get the nosedive, just a slight hesitation.

Should I increase the idle jets to 55 and keep with the smaller 33 pump jets?

OR keep the 53 idles and try 35 pump jets maybe?

Above 3000 rpm it responds just great.

[Alfasixnut]

Hi Brit
The idle jets shouldn't have a significant effect when accelerating - or even when running normally. The Pump jets are to briefly act under acceleration, as the air is pushed into the engine quicker than the denser fuel, so the mixture would go lean unless the pump jets act when the throttle/butterflies are opened quickly. I doubt that bore washing would affect the oil pressure so quickly - if there was that much petrol in the bores then the rings would be absolutely worn out very quickly.

[brit01]

The idle jets shouldn't have a significant effect when accelerating - or even when running normally. The Pump jets are to briefly act under acceleration, as the air is pushed into the engine quicker than the denser fuel, so the mixture would go lean unless the pump jets act when the throttle/butterflies are opened quickly.

Yes I'm aware of the function of the pump jets to compensate for the influx of air coming in.
If the mains are set ok then where can the transition be made smoother under hard acceleration?
I would imagine it's the idle jets as they work up until 2500-3000 rpm.

[woody14]

Does it pop and rasp on over-run?

[brit01]

Does it pop and rasp on over-run?

yep, always done this. thought this was a characteristic of the 8v boxer
too lean on the idles?

[woody14]

Yes! try going up by 5!! Also what ignition advance are you running? i run mine at 12deg BTDC no pinking issues im sure you would get away with 10 even with the heat you have over there!!

[brit01]

Yes! try going up by 5!! Also what ignition advance are you running? i run mine at 12deg BTDC no pinking issues im sure you would get away with 10 even with the heat you have over there!!

cheers mate - I'll pop in those 55's then and drop back to the normal pump jets.

timing is 8 degrees at 850 rpm now (right on the mark). I know if I put it more 1/2mm or more then it's ok with just me in the car but with extra bodies it starts pinging.

I know I really need a timing gun with the degrees clock. I would just tend to turn it, test it under load until it pings then drop it back a touch.

But don't you have a 1.5 sud? I would imagine there would be slight differences.

Mate we had 40 degrees in the shade this summer.
Maybe ok in the cold winters but not in the summer. (you couldn't even touch the paintwork if it was out in the sun)

[woody14]

Sud, but same engine as yours at the moment! On with building a tuned 1600!!

[brit01]

idles changed from 53 to 55. pump jets back to 33 ish (made locally!).
Started her up and she sounded smoooth. (all in under 30 mins)
Tomorrow if I get the chance I'll take her for a spin. May have to adjust the idle mixtures a little.

Also going to throw in some Motul 300v 20w/60 oil this week hopefully. (as opposed to 10w/60 as this will need to be changed more frequently and the car will never be started up in cold conditions so 10w is not really needed)
As my crank was on the limits of it's factory tolerance I want to bump up the pressure a little with the 60 viscosity oil.

[brit01]

i run mine at 12deg BTDC no pinking issues

woody: As I only have a standard timing gun, how would you estimate each degree increase on the flywheel as the light flashes? I mean in mm from the BTDC mark.

Mine is about 6mm advanced from the 8 degree mark. This would be about 9 degrees or more?


Also trying to find an old post about increasing the main jet size. Beats me where it is.
1 size up from 142 would be 145?

[woody14]

Yeh difficult that way, i suppose half the distance again from the 8 degree mark would be about 12!!!

[brit01]

I noticed while removing the jets the other weekend that one emulsion tube is slightly loose where it's attached to the air corrector jet, so it hangs I guess a few mm lower than the others.

Can this screw the progression circuit up and possibly be causing the unstable acceleration under WOT?

Are these reliant on all being the same height? Fixed tightly into the air correct jets.

[brit01]

If the emulsion tube that is connected to the top of the air corrector jet, is slightly lower than the others can this effect this cylinder in question?

Are the holes in the e-tube dependent on the height/level of the fuel level in the float chamber?

If so then yes I guess if one e-tube is slightly loose and lower than the others this may be causing a slight stumble as one piston lags behind the others.

[brit01]

chaps? anyone?

carb pros? I'll try and tighten up the loose e-tube before the weekend and see if it makes a difference.

[brit01]

Does anyone remember if their jets (mains and idles) sit flush with the emulsion tubes and main bodies like in this picture? Or is there a small gap where it is squeezed together?

Cheers

[brit01]

I feel lonely on this thread

I think I probably solved my own issue.
I will remove the jets and push in the jets to their bodies until they are flush/seated correctly. Seen many pics and all show the jets seated firmly/flush with the main bodies.

This may have been the simple solution all along.

[brit01]

Checked all jets this weekend.
All bottomed out and flush as they should be. I was mistaken in the previous post.

All ok.

Didn't have much time but I'm suspecting it's running rich on the 55's and not lean. Can be a bit misleading sometimes.

If I had the original air intake attached (63mm pipe) to the airbox, it would hesitate under acceleration from light to 3/4 throttle. Only under full throttle it would kick in.

Then I opened the airbox to allow no restriction and the power kicked in much earlier leading me to believe it was starved of air and not lean at all. But under wide throttle it would hesitate a little(going lean).

I will install the smaller idle jets with the 63mm original air intake and see how it behaves.
Maybe going back to factory specs with the original airbox intake is what it needed as it was designed for. If I had open filter boxes on the carbs I'm sure I would need the 55's and larger mains but these suck in hot air from the engine bay.

(another issue occurred coming back during the last 2/3 kms) - a little knocking from the left front wheel hub area when turning the steering wheel either way. Have to remove the wheel and inspect all bolts. Hope it's not the outer drive shaft.

Had a bad migraine most of the weekend so productive work on the car was pretty much impossible.

regards

Chris