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Eviljohn

Hey polo people. Hope you're all well. To elaborate on the title, i recently swapped from stock carb to weber on a 1983, 1043 breadvan. Has a flatspot which i'm pretty sure is just a case of changing idle jet. Apart from that, she's pretty smooth. Now we have some sun, i've been driving with the window open and noticed a little pop-pop/burble from the exhaust, as i slow down. From what i have read, this could well be down to slightly rich fuel air mix. My question is, can anyone recommend a way for me to accurately check and potentially adjust this myself? I'm familiar with the carb i'm using, i can adjust the idle by ear and i've done the same with the fuel air mix screw. But only ever by ear. I've heard a colourtune mentioned and i have access to one, but are they just for motorcycles? 

Thanks for stopping by and any guidance you can impart.

Take it easy.

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MonsterL35YFP

You could always take it to a tuning company or install a fuel/air fixture gauge but they are pricey and wouldn't really need it after it's set up. 

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dvderlm

Pops can also be lean.

 

Colourtune with 14mm thread will work fine on Polo engine to set stoichometric mixture (bunsen blue flame) with a carb at idle, and spot some low load issues.

 

Or a vacuum gauge to see manifold vacuum - set it for maximum and then reduce a little and revs back to under 1000rpm.. It's useful for driving too as best vacuum is most efficient.

You'll see the vacuum goes very high on overrun when you slow down. Ideally more than 21inchesof Mercury. If throttle flap is too wide at idle (idling on progression hole) 

excess fuel will be pulled and might pass from inlet to exhaust via cam overlap period. The vac gauge will show less than 20 inHg in that condition.

 

(assuming no leaks at carb,  good condition plenum air filter housing).

 

 

 

 

Edited by dvderlm

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Eviljohn

I have no rev counter so that may be an issue. I have a custom carb mount so fairly certain i don't have any leaks there. 

I do have a fuel/air gauge and boss kit that i got for fitting bike carbs but never used. Seems a bit much for what i'm looking to do. 

I'll try and find a carb wizard locally. They seem to be a bit rare though. 

Thanks dudes. 

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dvderlm

Fit the AfR gauge. Honestly, it's the best feedback on fueling and will make later swap to bike carbs easy.

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dvderlm

When does the flat spot occur?

Which Weber? Single choke like TLM or ICH?

Or twin choke progressive like DMTR?

 

 

 

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Eviljohn

Flatspot is at low revs/throttle. When i put my foot down. I've adjusted the throttle pump as much as it will go to allow a little more fuel in at that point, and that really made a massive  difference, but it is still there. 

Single choke, weber 32 ibf. Was still sealed in box when i got it last year, fitted in January of this year.

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dvderlm

24mm venturi. That should suit it well.

40 idle or larger?

Not familiar with that model.

 

Lean bog as air moves faster than fuel. Can you fit a smaller pump drain valve for greater volume, or smaller pump jet for longer squirt of same volume?

 

Or even a closed pump drain spill valve?

https://classiccarbs.co.uk/product/weber-dcoe-dcosp-idf-ida-dcom-ict-carbs-pump-exhaustspill-valve

 

option no hole

Edited by dvderlm

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Eviljohn

Venturi seems to work real well. Apart from that real tiny flatspot, she runs real smooth at the moment. Only considering the swap from 145 to a 150 idle jet because the throttle pump adjuster, (think that's what it is, screw and spring jobby attached to the diaphragm,) is at the end of it's adjustment so i wanna be able to adjust it should i need to. 

I had the same model on a previously owned polo. Again, worked well, real simple to work with. Was prone to icing on the motorway in winter though. 

I haven't looked at any other parts yet. Was hoping to keep it simple as possible. So tune and maybe idle jet. Only noticed the pops when i was driving with the window open. Despite havin a beefy exhaust, i think the mid section may be stainless or just larger bore, not sure but it sounds throaty, i can only hear them with window open. 

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caretakerplus

I'm not familiar with that carburettor model, but if it has a progression jet or compensating jet, it may well be that which could do with being a tad larger.

If it ides OK, and the idle adjusting screw is somewhere in its mid travel position, the idle jet itself is probably about right.

Colour Tune are great BUT, with the spark plugs being well and truly recessed, it can be a bit of a problem to see what is going on.

Why not fit your AFR gauge anyway, it can't do any harm.

You don't really need a tachometer (rev counter if you like) to set the carb by vacuum gauge - just set the carb for maximum vacuum and adjust (if need be) on a road test - making sure that you remember what position that the adjuster screw was in after setting by vac, and turning it no more that 1/4 turn at a time to make any corrections required.

As said, if you can achieve 20''Hg fair enough, but just aim for the highest vacuum - if the engine sounds too fast, slow it down a bit on the throttle screw.

 

A vacuum gauge is a great tool, it is even possible to adjust the ignition timing by it if you don't have a timing light. Retard the ignition slightly and at idle speed, slowly advance it until you JUST reach maximum vacuum (whatever it is) Retard the ignition again and then advance it until it is EXACTLY half inch of Hg below the maximum that you achieved in the first place.

Setting the idle mixture should ALWAYS be done after setting the timing.

 

By the way, it is quite possible that your 'beefy' exhaust system is the real cause of the 'pop pop burble.'

 

Regards

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Eviljohn

I contacted webcon when i had the weber fitted and they sent me quite an in depth fitting and spec guide compared to the hand drawn 1piece of A4 that came in the box. Best i can make out, all jets are one fixed size. 

There is a VERY good chance it's down to the exhaust itself. Depsite having a lot of paperwork and receipts with it when i bought the car, i can't see anything that relates to the exhaust. The back box looks standard as far as i can make out, i think the down pipe looks standard, but the mid section is the only part that doesn't look stock. On my previous breadvan there was a section of flexi pipe to account for movement of the engine. This exhaust doesn't have that. Just a section of very clean pipe. No idea about the bore. 

I have access to a timing gun so can get that done. I do have access to a very basic vacuum gauge too. I'll look into fitting the afr gauge as i do still have all the bike carb stuff, and may look into that in the future. 

The idle is a little odd if i'm honest. I've had issues with previous carb mounts leaking so i'm aware of what "hunting idle" sounds like. There is a very slight variation in the revs as it idles when choke is disengaged, but nowhere near the amount of fluctuation as i've encountered before. 

Thanks for the input so far guys. Very informative. 

Screenshot_20180430-060423.png

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dvderlm

Check fo leaks at all vacuum hoses.

 

I assume you mean 45 to 50 idle not 145 to 150 idle.

50 will be too much - mixture screw needed to be nearly closed on my 1.3  first stage of 24/25 DMTL. Can you get 47?

 

 

Or lower the float (0.5mm) is an option and up idle and main to match.

Edited by dvderlm

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caretakerplus

It might be a good idea to keep an eye on your exhaust system that doesn't have the flexi section.

Engine movement may well be accommodated by the rubber exhaust mounts without too much of a problem. However, these same mounts may have a shortened life as a result.

Certain grades of stainless steel are prone to 'work hardening', so any exhaust system without some 'give' in it is likely to end up cracking that grade of stainless.

 

Regards

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Eviljohn

I'll be sure to do that. Thank you.

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Eviljohn
On 30/04/2018 at 08:41, dvderlm said:

Check fo leaks at all vacuum hoses.

 

I assume you mean 45 to 50 idle not 145 to 150 idle.

50 will be too much - mixture screw needed to be nearly closed on my 1.3  first stage of 24/25 DMTL. Can you get 47?

 

 

Or lower the float (0.5mm) is an option and up idle and main to match.

I have replaced all rubber vacuum hoses with silicone hose.

45-50 not 145-150. My bad. Spent a long time looking into jets for bike carbs so put the 1 at the front out of habit. 

I have looked into getting hold of a 47. I couldn't seem to find one but admitedly, i only looked briefly. I haven't contacted Webcon about it yet. They're normally pretty helpful.

I can adjust the float height and see what that does to the flatspot/idle. 

Idle isn't "hunting" or "lumpy" but it also isn't smooth and regular. Thinking i should also check the seal on the custom mount just to be sure. 

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