Another carb question = flat spot

[ghost123uk]

This is not a new issue on my DG. It is not even a big problem. In fact I have lived with it for years because I use LPG 95% of the time. However, noticing it yesterday, whilst having to run on petrol, it got me thinking summat. I wondered if it was not a jet blockage, but perhaps a secondary choke opening issue, and if it is, then it might be affecting my performance and MPG on LPG. If I fix it, I might all of a sudden get better running on LPG, though to be honest it does feel OK on LPG now, but it will have been setup with the "fault" present, if you get my drift.

Symptoms (on petrol) =
From ~1/2 to ~3/4 throttle = a noticeable "flat spot". Note:- not the hesitation you get from a faulty accelerator pump, more a simple reduction in power whilst at that throttle opening. If you then go beyond the 3/4 position you feel a fairly sudden jump in power. Otherwise starts, runs, ticks over just fine.

I am wondering if it may be summat to do with the way the secondary butterfly is operating ?
Never (yet !) having had to play with this carb much, I am not very conversant with this aspect of it's operation.



While waiting for any replies, I will get my Haynes out (school boy error = go on the forum before RTFM )

[toomanytoys]

if all vac units and pipe work are ok.. I'd say its dirt in a jet..

[ghost123uk]

Ta for the input Si,

if all vac units and pipe work are ok..

Ah but there is the rub. It's all in the "if". I don't really know how to test that the vac unit which opens the second choke butterfly is working correctly. I know it does open the butterfly of course, but was wondering if it is opening it at the correct time / rate. i.e. it might be a bit sticky or summat.

I did strip and clean the carb out a few years ago and it made no difference, but I may have missed the blockage.

I had a very persistent blockage once on our Scirocco carb (almost the same unit) and that turned out to be a tiny spec of summat that had got into an internal drilling but was loose and moved about. Eventually I had to drill out a tiny core plug type thing, clean the muck out, then used a cut down bolt and some loctite to seal the drilling. Took many carb off operations over weeks before I nailed that one !

[toomanytoys]

well.. if you suck on the pipe and the vac unit doesn't leak and opens the choke when the primary is opened, then its working.. I don't think it matter which port its connected to either as they go to the same spot..
Let me check.. got to rebuild a carb for one of Aidans customers..
I may have another known working carb you could try..

[kevtherev]

well.. if you suck on the pipe and the vac unit doesn't leak and opens the choke when the primary is opened, then its working..

This is the key to it, if this is working ok then the flat spot lies elsewhere.
From experience
Too rich or too weak a mixture will cause a flat spot. A flat spot is just a bit where the torque is affected.
crap in the air jets, fuel jets will contribute to this.
I remember a car I had with a flat spot around 2.5 to 3000 rpm, when it got there I pulled the choke and the car took off, clearly a weak mixture then Unfortunately we have no manual choke, but you can drive it from cold and see if it's still there

[300CE]

I had the same type of issue on mine last week John so will be keeping an eye on this thread - the van has been running beautifully, then the same symptoms. Flat spot at half way, but ticks over & idles fine and picks up when the accelerator is over 3/4's. It was a freezing cold day so wondered if that had something to do with it? Also, the fuel gauge was on the red so I'm wondering if it could've picked some cr4p up as I don't normally let it get that low.

Going to take it out today so will see how she goes.

[ghost123uk]

well.. if you suck on the pipe and the vac unit doesn't leak and opens the choke when the primary is opened, then its working..

Aye, another look see when it's stopped raining perhaps.

I don't think it matter which port its connected to either as they go to the same spot..
Let me check.. got to rebuild a carb for one of Aidans customers..

I think you are right, but it's a while now (2+ years) since I had mine off.

I may have another known working carb you could try..

Well big thanks indeed, but I don't think it's a big enough issue to warrant all the P&P etc

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Aside =

While waiting for any replies, I will get my Haynes out (school boy error = go on the forum before RTFM )

Haynes, on the secondary choke vac unit says "It is not possible to check the secondary throttle vacuum unit without special test equipment" end of

Aside 2, I think it's daft that "we" call the choke, err, a choke, and we call the two mixing chambers in a carb chokes too. Can make it hard to word posts that don't cause confusion.

[ghost123uk]

well.. if you suck on the pipe and the vac unit doesn't leak and opens the choke when the primary is opened, then its working..

This is the key to it, if this is working ok then the flat spot lies elsewhere.

Ah, good, that kinda confirms what I thought originally, up until my musings on it yesterday.

Now, have I got any carb top to base gaskets left (the book over the carb on booting it didn't clear anything last time I tried that trick. btw Kev, I always shy'd away from using my hand to do that...)

[kevtherev]

There could, in fact, be many reasons, so here they are... carbon has developed on the back of the valves. Water in the fuel, dirty jets, fuel octane rating incorrect, vacuum leaks about the manifold area, the timing being out, advance mechanism fault, slightly burned valves, the throttle butterfly worn carb linkage wear, water/ corrosion on the connector from the spark control etc.

I would be swapping it out for a known good one

[ghost123uk]

I had the same type of issue on mine last week John... Flat spot at half way, but ticks over & idles fine and picks up when the accelerator is over 3/4's.

Sounds the same symptoms as mine then.

It was a freezing cold day so wondered if that had something to do with it?

Personally I doubt it. On very cold days some vehicles get issues with icing up, but our vans don't seem to suffer from that. Likely because then engine is at the back etc so in warmer environment than on conventional cars.

Also, the fuel gauge was on the red so I'm wondering if it could've picked some cr4p up as I don't normally let it get that low

I am always at odds on this one, often with folk with equal or more experience than I. My take is that any carp will always fall to the lowest point in the tank and the fuel feed pipe is for obvious reasons also very close indeed to the lowest part of the tank. So it would suck up carp whether the tank was half full or running dry. But that is a debate for another thread

[ghost123uk]

There could, in fact, be many reasons, so here they are... carbon has developed on the back of the valves. Water in the fuel, dirty jets, fuel octane rating incorrect, vacuum leaks about the manifold area, the timing being out, advance mechanism fault, slightly burned valves, the throttle butterfly worn carb linkage wear, water/ corrosion on the connector from the spark control etc.

I would be swapping it out for a known good one

I take your point, at least it would eliminate the carb from the issue. Mind you, it does run fine on LPG, so that takes a few things out of the equation, plus it has been like this and has got no worse for many years (note = it is only a slight flat spot, not very troublesome). Also it has had several plug/lead/cap/rotor changes, and in fact even a whole different engine !! (thanks Billy739) though I used my original carb. Had a different ign amp too.

Next bright dry day I will take the carb off and look harder for a blocked jet, I bet you both will be proved correct (and it does run just fine on LPG which points to a blocked jet too). Now, what to talk about next

[itchyfeet]

Yep had the same too this week, not used petrol for a while except to start, after a bit of a run it was better, I have had similar before after not using petrol for a while which just came good on its own, so I'm going to add some petrol and run it tomorrow again for allonger run. Lpg is fine.

I wondered about moisture in fuel or even old fuel

Surely second choke vac unit must work if it runs on lpg as this is not fuel dependant.

[ghost123uk]

Agreed ^^^ but mine has been like this for years, mostly on fresh fuel.

Surely second choke vac unit must work if it runs on lpg as this is not fuel dependant.

Aye, but when my LPG has been set up, at various times, (remember the open loop/closed loop/BLOS//me doing it. local garage helping/campershack checking it posts), the settings on the vap unit might have been set up to give the nearest correct readings with the secondary "choke" fault (if it exists).

[300CE]

Went out in mine today John & the problem has gotten worse, really kangarooing now and needing full throttle to pull away & smells rich. Put some fresh fuel in & got a mate coming round tomorrow to help us have a look at the carb (wondering if the choke is sticking) & fuel pump. I know your symptoms aren't as bad as this but will keep you updated if i can resolve it.

[CovKid]

I had annoying probs with flat spots mid-range. Turned out to be the pump. If you've not used petrol in a long while John, the pump diaphram may be cracked.