2.0 16v with Twin 40's - On going flat spot

I'm not sure if this is a carb issue or ignition/engine timing related so i've stuck it in here for the biggest audience.

I've recently installed a 2.0 16v 9A with KR cams into my 1.3 mk2 golf. I had issue with the ignition loom, but it's now in and running.

My issue is that it doesn't really accelerate smoothly, it idles quite well, it's not too bad from 2-4k rpm, but when it hits roughly 4.5k it's loses a lot of power and doesn't really accelerate with any haste. The exhaust makes a spluttery spitting sound as the revs go about 4.5k. If you rev the car up with out of gear it hesitates at the same revs, but does go up, but also spitting out of the carbs. I've had this set of carbs on a previous engine and it ran smoothly. I'm currently using:

34mm chokes
140 mains
55 idles
.5 emulsion tubes
175 airs

For the vacuum signal i'm running it ported off cyl 1&2 just behind the carb butterflies then T'd into a single line and into the ignition controller. Perhaps not as good as off all 4 cylinders and then into a plenum, but this should at least let it rev (shouldn't it?) since when WOT it shouldn't really have a vacuum anyway. (?)

My belt timing seems to be perfect, but i havn't yet checked the intercam timing - the engine was built by someone else with more experience of big blocks and 16v's than me, so i didn't feel the need.

Does it sound like a carb, timing or ignition problem to you guys?

apologies for the long post,
Any ideas?

 

I would leave the vacuum advance disconnected and plug the holes in the carbs. The way you have it now will be pulsing the ecu's map signal, and may be the cause of your problem. Vacuum advance is only for high speed low throttle position. ie motorway cruising at 70.

If that makes no difference then the emulsion tubes may not be suited and it is running weak at this point.

 

cheers, i'll give that a go. Does the dizzy have any bob weights? If not, how will it advance at all with the vacuum disconnected


I havn't forgotten about your PM's by the way, i'll get back to you in a bit.

 

The distributor is fixed. The advance curve is controlled by the ecu through the rpm signal.
Vacuum is only present when the throttle plates are nearly closed. If you open the throttle fully the vacuum reduces.

 

That's what i'm getting at, thanks.

I'll disconnect it then, does that mean i will still have a curve, as if the dizzy had bob weights? The vacuum is only there then, for another curve?

 

That is correct. I ran my 16v on carbs with this setup for a while.
IIRC the range of advance is something like 25deg.
You want about 33deg full advance which is more important than having it ticking over perfectly. To check this just rev the car up to about 4000rpm and check the full advance at that point. The ecu will be putting full advance on by then. This means at tickover it will be around 8 or so.

 

righto, i'll give that a go in the morning.

Would the spitting back at rev's also be explained?

 

sounds like a carb issue to me...

Would no doubt benefit from a trip to see mr hillclimber as he has had great results from his carb'd mk1 and knows a thing or two about setting them up....

I personally think the chokes will be strangling it and capping the power output on top of any other issues...

Maybe pm Jason for his thoughts?

 

May have something to do with it.

 

The chokes are 34mm so there is very little restriction there. If anything the lack of venturi effect maybe the issue here as a 40mm carb generally runs a 32mm choke maximum as the auxillary venturi becomes the restriction if you go larger.

I rev mine 7500 occasionally on 32mm chokes so the chokes restricting the engine is not the problem here IMO.

 

Me neither as I said....just think they might also hold the power back from what the motor is capable of.

Will certainly need a good sort through on a rr by someone with decent carb knowledge...

 

They'd almost certainly be capping the power if i moved up to some other camshafts... 160bhp is available on 34mm's though and they're the limit as far as when the aux venturi starts being the cap (it's why i havn't gone for 35/36mm), so i should be ok with them. The choke size certainly won't be the spitting back problem.

I did think the carbs could be at fault though, so i'll also give them a good check tomorrow.

I thought i'd post up incase someone on here had had similar non-revving problems with a KR ignitioned engine.

 

Well today i disconnected the vacuum tube which made little difference at all to the running so i changed the ignition amp, ignition controller and a coolant sensor from the head that snapped while i tightened it. the problem has gone to an extent.

Since my ignition amp was fairly new and worked fine on my old engine i'm ruling that out, the ignition controller was from a corrado (i assume this makes no difference?) and i was assured it worked, i'm ruling that out. Leaving the sensor, which may have been broken despite not looking like it until it failed while tightening.

Anyway, the car is pretty rapid now on WOT. But still has issues at lower throttle positions, it's not very smooth or particually torquey low down. This could well be the carbs, or do you think not having the vac advance hooked up is the issue?

Can anyone recommend some jetting to try, though i don't think i got it too far off with what i had spare(in first post)

 

I found today that my float height was way out on one carb, so that's now set and has given a small improvement.

I also took a vacuum from all 4 ports behind the butterfly, T'd them all together and then put one line into an empty fuel filter - the flow out of it felt pretty constant and not pulsey so i put it on the ignition controller. I havn't noticed any difference to anything, surely i should, since at high vacuum's it should give it some big amount of advance to account for the weaker mixture?

The problem is now lack of power/flat spots like lack of fuel from about 15%-50% throttle, and very juddery due to the big flat spots, more noticable in higher gears. It clears if you floor it and above 4k it seems better. I've had a look at the progression holes, they seem clear, but i'd guess this is roughly where the problem is so its odd. I thought it's the idle circuit, but since it runs nicely cruisingon the flat (so no need for more than a smidge throttle) with only very light throttle.

Anymore ideas? I know this is more a carb issue now probs, but since i have the thread i may aswell keep this one going.

 

As said before I would completely disconnect the vacuum line and blank it off.
By fitting twin sidedraught carbs the "cylinder filling" is now different from what the ecu was ever programmed to work with anyway.
As soon as the throttle is over 1/ 3 open there will be no vacuum advance anyway. The ecu puts on the advance curve. The vacuum is purely for high speed low throttle opening and is absolutely not the problem here.
Set the ignition to around 33 deg full advance as described previously.
The carbs just need balanced and correctly setup too run on this engine. This may require a number of changes including idle, main, air and pump jets and possibly an emulsion tube change if an adequate fueling situation cannot be found with jet adjustments on the current tubes.

I will post my cat down if you are still having problems!

 

Lol cheers, i had taken your posts in, advice has not been disregarded. The vac advance was a seperate note, i wasn't expecting it to sort the progression issue.

The car will be used for motorway driving, so i hooked it up for that, any advance extra over what the ECU will do at low throttle positions must be worth having?

I have the jetting at a position where i wouldn't expect massive flat spots and struggling, it's what i took at a stab at and had spare which turns out to be pretty much bang on what dellorto have recommended as a good starting point.. I've adjusted the idle holders to no gain and i will try the emulsions tomorrow with some other spares. I'm pretty sure the carbs will have to be coming off again for a good checkup, but i can't see what can have gone wrong in the week they were off a perfectly running engine, apart from the float height adjustment which i did this morning.

I'll keep this updated incase anyone in the future gets similar issues.

 

Ok, timing is set to 8degrees BTDC.

My issue is still here, not really noticable in 1st, 2nd and not much in 3rd, but i took it for its first dual carridgeway and it's noticable in 4th/5th.. so much that you can't cruise at 70, you have to accelerate from 63 upto 75 and then use light throttle to cruise as well as you can, which brings you back down to 63 before the problem goes away again. etc.

I've tried changing my emulsions from .5's to .6's (so from the leanest to richest of the most commonly used emulsions which should cover most eventualities) hasn't made much if any difference.

I've also tried leaning up the idle mixture to a point where it is reluctant to idle, so in theory leaning the idle jet allowance in the progression phase. This made it worse really, but a bit more spritely when you put your foot down. Doing the same but in the rich direction made it more sluggish, maybe slightly better (masking the problem??), but it's already on 55 idle jets about the limit for 500cc/cyl... So that leads me to believe i was probably right with how it was, and the problem is elsewhere... unless i'm overlooking something?


Any more ideas? fwding to MrHillclimber..

 

Ok, i think i've solved it. After being convinced my jetting wasn't so far out either way i'd have issues i looked at other things...

I looked through 26 pages of the carb section for anything that might help, one thread (on the 25th out of 26 pages!!!) mentioned that without filters the air rushing at the carbs since they are on the front caused them problems. I hadn't read anything else about problems apart from when people are talking 100+ MPH.. anyway, i tried fixing an old number plate to the back of the grille and the problem is cured, or pretty much gone. good stuff. I can only assume it was leaning the mixture up far too much/disturbing the flow terribly at those revs and then cleaning up when the engine was taking more air at higher revs than the speed was throwing at them...?

I'll make some kind of air block for the grille all the way along to the rad. Maybe i'm having this issue more since i'm running a single headlight grille? I've got an airbox but i need to make a backing plate for it, doesn't help that i'm doing a 300 mile journey tomorrow.

I might now revert back to my .5 tubes and try 50 idles in stead of 55's to see if i can get something like economy... What should i expect on a run with it?

 

Hi Rich

Sorry I've missed all this, been flat out getting a customers hillclimb car ready (2nd in class on a hill he's never seen before..!).

Sounds like your getting there Rich. I was going say... fuel pressure.. checked it?

Failing that if I can have it while you go off on yr travels this week we could get it on the rollers.. though if I were you I'd wait till the new inlet manifold is on.... I'd best get that finished as well had'nt I!

 

That would have been awesome Jason, and i'm happy for it, but you're right it should probably wait until it's the final setup with the new manifold, saves doing it twice! Something to look forward to anyway.

FP is set at 3psi with a filter king regulator, is the stalling with direct airflow into the carbs something you've seen before? It is fine WOT after 4k, but i've noticed it does put some atomised fuel/air out the front of them aswell.. could be the manifold design, or maybe intercam timing?