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CR throttle body balancing


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Peters posts are very useful and most useful. An education in all things air flow and fuelling!

 

is it worth going back to basics?

 

Andrew,

Nice of you to say that. I got into the basics for supercharging and dual fuelling and came across bald statements in the specialised books that I did not follow, could not understand, and found that infuriating. So I bought Heywood's textbook: "Internal combustion engine fundamentals". It is an education in whats going on in the engine. The thermodynamic stuff is horrendous but can be completely ignored. There are many many useful graphs, and it covers both carbs and injection, and is gratifyingly devoid of digital control emphasis. In short, its hard work but after a month or so dipping in to it you get good feel for the combustion process.

Peter

Edited by Peter Cobbold
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Quentin,

 

From what I can remember, vacuum is at its highest @ idle (makes sense..butterflys closed). Vacuum (inlet manifold depression) decreases as you open the throttle.

 

Therefore if you have a vacuum leak the MU thinks you are actually further up the fuelling track (given a smaller vacuum reading)...therefore a richer mixture.

If you look at this website..there are a few good easy to read graphs which support this.

http://t2000.kvaleberg.org/t_pi.html

 

and from this graph (taken from http://www.lucasinjection.com/Lucas%20Mk2%20manual%20page%2024.htm)

 

Lucas%20page%2024%20fig%2024.jpg

 

Unfortunately this seems to contradict what Peter is saying :(

 

Perhaps worth mentioning as well that if you have the standard dual cable CP/CR choke cable, pulling this out will also increase the revs...perhaps disconnect the cable that goes to the throttle linkage cam to ensure it really is mixture (richening) that's solving the issue.

 

For my money..Id see who has a Gunson CO analyser locally/in your local TR group or go have a quick chat with your friendly local garage (10 quid on a Sat morning perhaps) who can put your car on a CO analyser and find out what co your running at idle. Then give Neil a call.

 

Cheers

Edited by AndrewP
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Tried the vacuum advance off the front TB set. Made no difference to the hesitation. I am confident I don't have a vacuum leak as I have sprayed easy start everywhere and can stall the car by blocking the bleed valve. Also if, after all, my cam is a CP cam, then my 8"HG is spot on no? However, my MU is set up for saloon therefore I would expect I am already rich. I did disconnect the fast idle cam when trying Peter's suggestion. However, as mentioned, for it to reduce the hesitation the choke needed to be out so much the engine was very unhappy. I am therefore not really sure that the lean spike is really the cause but can cleary see how it might be. Andrew, good idea about the CO meter. I'll try a friendly local garage once the new TBs are on. If I still have hesitation well, it's check the cam and ask Neil to reset the MU and/or rolling road.

Thank you once again to everyone for their interest and assistance :)

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Tried the vacuum advance off the front TB set. Made no difference to the hesitation. I am confident I don't have a vacuum leak as I have sprayed easy start everywhere and can stall the car by blocking the bleed valve. Also if, after all, my cam is a CP cam, then my 8"HG is spot on no? However, my MU is set up for saloon therefore I would expect I am already rich. I did disconnect the fast idle cam when trying Peter's suggestion. However, as mentioned, for it to reduce the hesitation the choke needed to be out so much the engine was very unhappy. I am therefore not really sure that the lean spike is really the cause but can cleary see how it might be. Andrew, good idea about the CO meter. I'll try a friendly local garage once the new TBs are on. If I still have hesitation well, it's check the cam and ask Neil to reset the MU and/or rolling road.

Thank you once again to everyone for their interest and assistance :)

 

Quentin

That front take-off is not a throttle-edge take off and wont do the job of retarding transiently.

Have you tried pulling the 'choke' just before ( eg half second) opening the throttle? We need to know it is a lean spike.

Peter

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Unfortunately this seems to contradict what Peter is saying :(

 

 

 

 

Andrew, I dont think theres a contradiction. Your data relate to steady state fuel flows. But I think the hesitation is a transient event. The Lucas PI has no facility for enriching the mixture transiently upon opening the throttle, so cannot do what the throttle pump on Webers does, or the piston damper on SU.

The 'lean spike' phenomenon is common to all si engines except for direct injection. The cause of the 'lean spike' is not widely known but happens because of the fuel distribution between surfaces and air flow in the manifold. Thus in normal running only some of the injected fuel is entrained in the air flow, a portion forms a film on the manifold walls where it evaporates, but after a delay compared with the entrained droplets. In steady running this does not matter, the mixture derived from entrained droplets and surface evaporation combine to be correct. But transients present a problem. The liquid fuel thet wets the walls has to build up in the second or so after opening the throttle from tick-over and this depletes the mixture entering the cylinder to cause the lean spike. Webers and SU enrich the mixture briefly to cover those losses to the surfaces, the Lucas PI cannot..... unless the driver pulls the choke out for a second or so while flooring the throttle.

Clearly some PI engines do not suffer the transient and I suspect this is because the MU is set up rich so that transient leaning does not slow combustion so much that the engine stumbles. But that rich setting is not good for economy. Why the deficiency in the PI design? I reckon it's a reflection of the racing heritage of the PI where standing starts happened just once a race.

Transient spark retard could be the cure to the hesitation, and could allow MUs to be tuned leaner.

Peter

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Quentin,

 

From what I can remember, vacuum is at its highest @ idle (makes sense..butterflys closed). Vacuum (inlet manifold depression) decreases as you open the throttle.

 

Therefore if you have a vacuum leak the MU thinks you are actually further up the fuelling track (given a smaller vacuum reading)...therefore a richer mixture.

If you look at this website..there are a few good easy to read graphs which support this.

http://t2000.kvaleberg.org/t_pi.html

 

and from this graph (taken from http://www.lucasinjection.com/Lucas%20Mk2%20manual%20page%2024.htm)

 

Lucas%20page%2024%20fig%2024.jpg

 

Unfortunately this seems to contradict what Peter is saying :(

 

Perhaps worth mentioning as well that if you have the standard dual cable CP/CR choke cable, pulling this out will also increase the revs...perhaps disconnect the cable that goes to the throttle linkage cam to ensure it really is mixture (richening) that's solving the issue.

 

For my money..Id see who has a Gunson CO analyser locally/in your local TR group or go have a quick chat with your friendly local garage (10 quid on a Sat morning perhaps) who can put your car on a CO analyser and find out what co your running at idle. Then give Neil a call.

 

Cheers

 

Interesting Lucas setting-up instructions - wish I'd had access to that 30 years ago...

If I had Quentin's problem I'd try screwing A3 on the MU top richer, to try to get tickover richer without affecting load and wot mixtures. And I'd fit an AFR guage in the exhaust so I had measurements of the mixture.

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Joy!! :) Decided to tweak the mixture. Carefully marked the position of all adjustment screws them richened A1 by 1/4 turn. Neil had advised to richen across the board which I understand a1 does. Hesitation gone! Tried another 1/4 turn which I have left it on at the moment. This has also allowed me to advance the timing from about 8 to about 11 deg. Smoother at lower rpm now. Not much change at wot from before. There is a very faint residual hesitation, but probably only noticeable to someone who has become fixated on it. The new TBs may fix that anyway when they come.

 

Honestly I cant believe this ended up being a non cost 5 min fix after all my tribulations :lol: Peter, You were correct all along. Thanks for your patience and sticking with me. I will now monitor the conditon of the plugs and try and borrow a CO monitor to make sure I have not overdone it, then maybe next year the rolling road.

 

Thanks again to everyone for their help. This has been a huge learning experience for me.

Quentin

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Quentin,

Glad that you've got rid of the hesitation. At last! (Pity really - I was convinced the throttle edge trick would do the job.)

But richening the mixture across the board to cure a lean spike does not fill me with enthusiasm as it will hit fuel consumption. I'd get yourself an AFR gauge and save cash on the RR and set the mixture yourself - that instruction sheet is all you need to decide which spring(s) to adjust, as you already have the vac.gauge.

I bought an AEM-UEGO but other wideband kits will do the job too.

 

Peter

Edited by Peter Cobbold
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Quentin,

Glad that you've got rid of the hesitation. At last! (Pity really - I was convinced the throttle edge trick would do the job.)

But richening the mixture across the board to cure a lean spike does not fill me with enthusiasm as it will hit fuel consumption. I'd get yourself an AFR gauge and save cash on the RR and set the mixture yourself - that instruction sheet is all you need to decide which spring(s) to adjust, as you already have the vac.gauge.

I bought an AEM-UEGO but other wideband kits will do the job too.

 

Peter

 

 

Yup good point, I can see that and I will do that I think. Meantime, I am just chuffed to bits. I even just cleaned the engine bay :P

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