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Discussion Starter #1
I recently installed an k&n air filter on my prelude B20A4 (european) engine with dual carbs.

something like this: http://www.preludepower.com/forums/showthread.php?t=315285

I notice a big difference in the sound and power in high RPM, but when i go in high rev's, i have sort of ''backfire'' in the carb's.
Some research learned me that this can be a problem if the mixture is to lean.

Is this correct, and is it possible to fix it, without a lot of experience?
Because i know you can't mess around with the PGM-carbs....


Greetz Stijn
 

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...Some research learned me that this can be a problem if the mixture is to lean.
Hi Stijn, a more free flowing air filter can definitely lean out the mixture.
...is it possible to fix it, without a lot of experience?
Before making any adjustments to the carburettors, make a note of the original positions of anything you move. That way, if it all gets into a jumble, you can go back and start again.

The quickest / easiest thing to try is raising the "Jet Needle".
The "Jet Needle" controls fuelling at engine speeds above idle to around three quaters throttle, raising the needle makes the mixture richer (more fuel), lowering the needle makes the mixture leaner (less fuel).
Have a look at the exploded view of the carburettors on page 6-48.

http://media.honda.co.uk/car/owner/media/manuals/PreludeManual/62sf100/6-47.pdf

You need to remove the "Vacuum Chamber" and "Vacuum Piston Spring" to get at the "Vacuum Piston" and "Jet Needle". I have not messed with these particular carb's before, but I would expect the Vacuum piston to simply lift straight out, bringing the Jet Needle with it. Make a note of which way around the Vacuum piston fits into the carburettor body, there is a right and wrong way around, but they usually have a 'lug' somewhere that only allows them to fit the correct way.

Have a look at the "Needle Holder" and see if it has different positions for the "Jet Needle", or the "Jet Needle" itself may have several (usually 5) different 'slots' cut in at the top to allow raising and lowering. If it has different options, raise the "Jet Needle" by one notch, reassemble the carb's and go for a drive, see how it feels. If you still need more fuel, raise by another notch and so on. Any adjustments you make to one carburettor, must also be made to the other.

If the needle is raised too high, it will cause over-fuelling at lower engine speeds; the engine may start to splutter around one quarter throttle opening and not pull as cleanly through its rev range. If you get to this stage, put the needle back down a notch, until the engine pulls cleanly again.

If the problem cannot be solved by raising the needle, or the "Needle Holder" has only one position, or raising the Jet Needle makes the engine run badly, it may be possible to fit a larger main jet. The "Main Jet" controls fuelling in the last quarter of throttle opening, so increasing the size of the "Main Jet" will obviously make the mixture richer at the very upper end of the rev range.


EDIT:

I've just re-read your post, is the "backfire" in the carb's when the engine is under load and accelerating, or when you close the throttle and the engine is decelerating?
 

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Discussion Starter #3
Hey, thanks for the answer! When my friend (mecanic) and i have time we will try to adjust the jets.

I know you can test if the engine is running rich or lean on the color of the spark plugs, should i use this method to adjust the jets?

You asked when the 'backfire' happens. This happens when i am still accelerating and reach 5-6000 rpm.

So we were thinking that it's maybe a problem with the timing of the ignition... but it's strange that this problem occured when i installed the filter.
 

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Hey, thanks for the answer! When my friend (mecanic) and i have time we will try to adjust the jets.
If your mechanic has an air/fuel ratio meter, that will help to determine if the car is going lean in the upper RPM's. From the symtoms you describe, a larger main jet in each carburettor could well cure the problem. I'm not sure how the jets are fitted into your carb's, just looking around the internet, it would seem that some Keihin carb's have press fit jets and others have screw in jets. The links below are for motorcycle carb's, but they are just meant as examples to show what I mean.

Press fit jet
http://4into1.com/keyster-keihin-press-fit-primary-main-jets/

Screw fit jet
http://www.power-barn.com/keihin-99101-116-xxx-main-jet-for-keihin-carburetors/


I know you can test if the engine is running rich or lean on the color of the spark plugs, should i use this method to adjust the jets?
I've only used spark plug colour as a long term indication of how an engine is fuelling. However, I did find this information on tuning Keihin carburettors; the article is aimed at motorcycle tuning, but there is plenty of good information on how the carburettor works and what each jet does, and the author mentions doing a plug check to give a much more immediate sense of how an engine is behaving. He states:

"Plug colour; Ideal plug colour is chocolate brown. A rich plug will be a darkish brown or black, may be oily and sooty. A lean plug will be a light brown or grey (some severe cases may be white). Plug colour must be checked after a plug check has been run. To perform a plug check, run the engine at specific RPM that reading is desired for at least 5 seconds, then turn off engine without letting the RPM's fall. This is a very difficult test to perform at any RPM other than wide open throttle.

It can take years to learn how to accurately and precisely read spark plugs. For amateurs, plug colour should not be the only piece of eveidence used to adjust jetting. Plug reading should be evaluated with other jetting evidence to achieve a proper conclusion on tuning."

Tuning Keihin Carburettors
http://www.duncanracing.com/TechCenter/KeihinCarbJetting.pdf

Animation of how a CV carburettor works.
https://www.youtube.com/watch?v=wyspAHrMbb8

Explanation about tuning a CV carburettor if the air filter and/or exhaust is changed; it shows motorcycles, but cars are the same principle. This guy also talks about using spark plug colour as an immediate indication of fuelling.
https://www.youtube.com/watch?v=y836ZPjvoMI


You asked when the 'backfire' happens. This happens when i am still accelerating and reach 5-6000 rpm.
The reason I asked is, if the backfire occurs when decelerating, it can often be solved by simply opening up the "mixture adjusting screw" a little (half a turn at a time until the problem goes away). However, as your problem occurs during acceleration and high into the rev range, I doubt this is the cause. To check the mixture adjusting screw is set correctly, you could use a Gunson Colour Tune Kit; quite cheap to buy, very easy to use and you can get the mixture adjustment pretty damn close using it. Getting the mixture adjusting screw set correctly will also help to pass emission testing.

https://www.youtube.com/watch?v=xEZ0-uN0NA4


Also, knowing where in the rev range the problem happens, will tell you where adjustments need to be made on the carburettor. Your problem is in the upper RPM's, fuelling in this region is controlled by the main jet. Hence my idea that a larger Main Jet in each carburettor could solve the problem. If you want to modify your car, it may be a good idea to fit a wide band air/fuel ratio meter; this will allow you to see if any modifications you make have produced a rich or lean state, at what throttle opening the engine is going rich or lean, and hence what you need to change in the carburettors to compensate and get the most from your modifications. The link below is again for tuning motorcycles (Honda ST1100), but it is still CV carburettors and a little way down on the left is a small diagramme that shows which part of the carburettor is responsible for fuelling at what throttle opening.

http://www.steinborn.org/st1100.html


In the days of cars with manual chokes, lean fuelling was much easier to diagnose; get the car warm, drive around, when you get to the throttle opening where the problem occurs, put the choke on; if the problem reduces or goes away then you need more fuel. Automatic chokes have made this simple test much more difficult. But, if your mechanic can figure out a way to keep the choke circuit open when the engine is warm, a quick drive will probably tell you if adding more fuel helps the situation.

Choke linkage
http://media.honda.co.uk/car/owner/media/manuals/PreludeManual/62sf100/6-39.pdf

http://media.honda.co.uk/car/owner/media/manuals/PreludeManual/62sf100/6-40.pdf

So we were thinking that it's maybe a problem with the timing of the ignition...
Yes, a poor ignition system can cause difficulty starting and poor running, particularly at high rpm. Check the spark plugs are clean and gapped correctly; check the spark plug leads have a good connection to the spark plugs and the distributor; check the king lead has a good connection at the distributor and the coil; check insulation on the HT leads is not breaking down; check the low tension connections at the coil are sound and not corroded. A simple check is to go somewhere very dark, lift the bonnet and start the engine; if the HT lead insulation is breaking down, it can look like a firework display on top of the engine.

How to check 3G Prelude ignition timing.
http://media.honda.co.uk/car/owner/media/manuals/PreludeManual/62sf100/5-17.pdf

...but it's strange that this problem occured when i installed the filter.
Remove the K&N filter, return to the stock air filter set up and if the problem goes away, voilà le problème.


EDIT:
Found this wonderful video produced by Shell in 1985, explaining the operation of a carburettor. Simpler times.
https://www.youtube.com/watch?v=vErfnY5bPrY
 

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Thanx for the information, you're helping me a lot! After reading and try to understand it was time to try it out.

This evening I had some time so I tried to raise the needle. When we opened it, I realised that there were was no notch to raise or lower the needle. Only a screw with a spring. We tried to turn the screw 2 turns out and tested it, but without a noticable effect. Then we tried to put a roundel(or how you translate it) around the needle to raise it with 3mm. The idle was a bit higher, around 1000rpm instead of 800. But the ' backfire' is not gone...

Tomorrow we will try to raise it further, but i think you are right, that we need to replace the main jet.

I will keep you updated!
 

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Yesterday i raised the needle a bit furter, i turned the adjustmentscrew of the needle 3turns out. When i went testdriving the 'backfire' suddenly disapeared, but now i have a big lack of power... especially in high rpms i don't get the 'kick' i'm used to. Yet it does not seem that i'm overfuelling now.

this evening i will try to lower it again.
 

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...we tried to put a roundel(or how you translate it)...
Washer


Yesterday i raised the needle a bit furter, i turned the adjustmentscrew of the needle 3turns out. When i went testdriving the 'backfire' suddenly disapeared, but now i have a big lack of power... especially in high rpms i don't get the 'kick' i'm used to. Yet it does not seem that i'm overfuelling now.

this evening i will try to lower it again.
Sounds like it is lack of fuel at the top end, and opening up the other fuel options has compensated enough to stop the top end misfire. Unfortunately, this has made the mixture overly rich through the rest of the rev range and so lost power. Try dropping the needle again (to where you get the best engine response) but leave the mixture screw3 turns out; see how that goes.

Opening the mixture screw will give you more fuel throughout the rev range, but I don't know if it will be enough to stop the top end misfire on its own. Leaving the mixture screw open could also produce a rough idle and high emissions at idle.

Making adjustments to stop the misfire appears to be messing up fuelling generally, which I'm sure makes the car less fun to drive. It could be the best option is to put the needle and mixture screw back to their standard positions and fit larger main jets. Good job on being willing to experiment, I'm sure a lot of people would have looked at those carb's and just closed the bonnet. :emthup:

It could be worth asking on the "2nd Gen" forum, just post them a link to this thread. There are a few guys on there who have experience with carburettors.
 

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Yes indeed, I'm learning a lot!

Well, that's the strange thing, yesterday i putted the needle back in standard position.
And still the backfire is gone, together with the power above 3000rpm...

So now I'm starting to think that main problem is not the tune up, but something with the valves or so?

i never messed with the mixture screw, but with the screw off the neelde, something like this :
https://www.google.be/search?q=scre...screw+spring+jet+needle&imgrc=k2yqCr2pmeIcLM:

Ok I will ask the 2nd genners their opinion.

I'm very gratefull for your help!
 

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Yes indeed, I'm learning a lot!

Well, that's the strange thing, yesterday i putted the needle back in standard position.
And still the backfire is gone, together with the power above 3000rpm...

So now I'm starting to think that main problem is not the tune up,
When you get spurious results you can start questioning anything and everything; which is where this comes in:
"Before making any adjustments to the carburettors, make a note of the original positions of anything you move. That way, if it all gets into a jumble, you can go back and start again."

If you're able to put all your adjustments back to stock and the engine returns to its previous state, you will know it was simply the adjustments that caused the change in performance.

The carburettors on your vehicle may not have been touched for 20 years, grime can buid up. They can also get worn and temperamental. You may find that simply cleaning the carburettors helps the situation.


...but something with the valves or so?
Does your friend the mechanic have a compression tester?

i never messed with the mixture screw, but with the screw off the neelde, something like this :
https://www.google.be/search?q=screw+jet+needle&safe=active&biw=1768&bih=1006&source=lnms&tbm=isch&sa=X&ved=0ahUKEwik9t_PuN7KAhXHCw8KHXYpDtEQ_AUIBigB&dpr=0.95#safe=active&tbm=isch&q=screw+spring+jet+needle&imgrc=k2yqCr2pmeIcLM%3A

Ok I will ask the 2nd genners their opinion.

I'm very gratefull for your help!
Can you tell me what model your carburettors are?
I'll see if I can find some good pictures of them.
 
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