View Full Version : Twin 40 Tuning
I used to run a Mechanical Fuel pump and a 265deg Kent NVO camshaft. The carbs ran, and were setup perfectly.
I've now installed a AST2 290deg Camshaft and an Electronic Fuel pump and Regulator. So i went for an RR setup. Without the carbs being setup, the car was **** under 3k, but once it got onto the powerband, it pulled very well from 3-5k, and i expect beyond that. However, i didn't load it past 5 untill the setup was done properly.
After the first visit, the Air Correction Jets and the Mains jets were all swapped. The car was now responsive and great under 3k, but after the initial 3/4k "pull" it flattened off, and didn't really pull in an exponential way that you'd expect.
I returned, and said i wasn't happy and they said they'l happily look at it to try and improve this.
Now, there is a noticeable gain in power on the top end, and it carries on pulling like it should. However, it's gone alot worse under 3k (maybe to be expected) BUT i've now got a missfire/splutter on light acceleration around the 3.2k mark (i.e when you sit at 74 on the motorway, which is 3200rpm, and lightly accelerate there is about a 0.5-1 second splutter/misffire.
This is obviously not 100% right. However, i'm being told that i can't expect a set of "old carbs" to beable to be setup perfectly. I.e to have a smooth pulling setup, i need to buy Brand new carbs !
Can anyone suggest a place in the north west that could sort this, Or suggest the problem... ?
Thanks Nick
might have a bent spindle...
but even old carbs are fine.... wont drag the usual suspects out but you know who they are.
Maybe take it to someone who knows howto set carbs up lol
greg1.4
14-08-07, 06:29 PM
you should expect it to be lumpy/hard to drive under 3k with the 290 cam in it.. but when it hits 3k or more it should just take off and pull like you would expect..
as for the flat spot i had a similar problem and it was the accelerator jet pumps that were damaged. i replaced them and goes well now. still has the tiniest flat spot when you put the foot down but thats expected with carbs that are not brand new or 100% set up correctly..
It wont be at all lumpy/hard to drive.
My carbs were setup be feel/listening, with a ast2 fitted.
And it pulled from 1000rpm in 5th gear.
Maybe take it to someone who knows howto set carbs up lol
I think that's probably the answer. But where in the northwest ?
As far as i know, only the jets were checked. What else should be on the "checklist" ?
might have a bent spindle...
There were fine with his old cam fitted tho, its deffo down to jet setup.
you should expect it to be lumpy/hard to drive under 3k with the 290 cam in it.. but when it hits 3k or more it should just take off and pull like you would expect..
as for the flat spot i had a similar problem and it was the accelerator jet pumps that were damaged. i replaced them and goes well now. still has the tiniest flat spot when you put the foot down but thats expected with carbs that are not brand new or 100% set up correctly..
I love the voice of inexperience, or one that has been led down the garden path. I presume you are talking from personal experience and not what you have read/gone by what your mates have thought etc.
I dont know muich about carbs or tuning in general, but i do know from running twin 40's for the last 7 1/2 years and having previous experience of them that tuning them correctly is the key issue (and when i say tuning them i mean down to the simple things like having the throttle cable tensioned correctly which has been seen as a lost art amongst many r/r's imo)
The cam in question 'should' pull as mentioned above by LLTN. obviously alot depends on whether the car is tuned for balls out power or like mine which has a good spread of power everywhere which imo is far more beneficial and as such had mine turned down abit to equal out the torque/bhp split, and that wasnt down to the carb tune.
The small cough when sat at constant speed and then demanding a surge isnt uncommon, occasionally mine even doesnt like being sat static and that has happened since my carbs were brand new.
Tbh the two variants you have described dont seem 'that' far wrong tbh, if you have a vernier has it been dicked around with? (i dont even bother running one as no need) aswell as messing with the carbs that were probably close enough the first time the dizzy can have a large effect on how it drives as they tweek it. I did have 2 marks on mine, one for abit more top end and one for how i prefer it, needless to say it never moved and the marks are long gone lol.
If i have missed the post regarding nicks tune apologies and please dont take offence this is said in the best intentions from a similarly paranoid person lol but are you sure its just the figures you were given didnt tally to expectations, as r/r are full of rubbish anyway, and have seen a few similar spec engines run a massive array of numbers. Changing somthing basic like a cam on its own doesnt make a huge difference in the grand scheme of things, and after 4 or 5 miles any gains that feel great would soon seem to disappear. Stu and others would agree i am terrible with my car, i let it stand for months, jump in a drive it expecting it to feel fast again, but nope its still as crappy and normal as ever (to me), so i go fault finding, yet everyone else cant see the problem as it doesnt exist
Apologies for the essay lol lol lol but i think that covers most aspects of the issue that may be at hand, as said been there done that and worried twice as much.
I don't run a Vernier.
The top end felt flat as anything. the first time.
The bottom end is now terrible. It's tempremental and very unpredictable.
Not right when you're Paying ?100 for 40mins ! I'm not expecting miracles, but i'm exprecting it to at least driver better than before. All i expect of it is to pull smoothly onto the powerband, and then pull steadily up to 7k. Neither attempt got close to this.
My previous setup on the NVO cam felt better than this, and the carbs were perfect then.. Absolutley nothing wrong with them.
tom reid
14-08-07, 10:44 PM
You can't expect good low down driveability with a 290 cam, also the gearing can have a marked effect on the way the car responds to throttle inputs.
No offence mate, but it sounds to me that you have over-cammed the engine, for what you want out of it.
was the top end actually flat, or was it with the extra gained grunt low down when acceleration stayed static it just felt flatter as the rush was over :confused: where as the old setup probably built its power up further up the range, ok not as much power but in the manor it was put out.
Either way if your not happy then it should be sorted, just get it backed off a touch to get a happy medium by the sounds of it
womble sri
14-08-07, 10:49 PM
mine was a bit rough at lower revs an it has a similar cam to yours it wasnt very good at 2k in a high gear. the power band on the cam is quite high and as said maybe the rr operator isnt that great with carbs its best to get a veteran to set then up who as been tuning then since they were the best thing to do to an engine :thumb:
It was dead responsive low down, felt great... But as soon as it came onto the powerband at 3-4k, it then just flattened off and seemed to bog. It even struggled reving around 6.5k in 4th !
I had to do a 60 mile round trip to get it looked at the 2nd time. So i wouldn't have done it unless it was pretty unnacceptable.
I checked the advance after the 1st attempt myself, it was running about 20 at 1.3k rpm and pulled about 32. when the revs fell to about 600ish it was 14.
I'm not sure wether they changed this, but he said he changed the mixture for the 2nd time, and then cleaned some "crap" out of a jet.
You can't expect good low down driveability with a 290 cam, also the gearing can have a marked effect on the way the car responds to throttle inputs.
No offence mate, but it sounds to me that you have over-cammed the engine, for what you want out of it.
Sorry tom, i have as others have, it pulls in fifth nice and strong from as low as 16mph when i am super lazy This is the cam i have driven with for years and on an everday case for the first couple. Drivability wise its spot on whether on track as people have seen down to poodling in traffic jams.
Just saying as i have first hand exp of this cam in a similar engine
No, i have not overcammed the engine. I want 3k+ pull for track use, But it's clearly obvious that a good low end is also achievable with the right setup !
even a piper BP300 isnt over cammed either......ok not got quite the same low down pull as an AST2 but its fricking close on a PROPERLY SETUP engine.
Gotta love old wives tales hey :D
tom reid
15-08-07, 04:28 PM
Ok, I get what your saying, but, this cam is designed to "work" from 3-7k, now you could time it in earlier to get it "on" a little earlier. Assumming that the people that set it up know their stuff, there shouldn't be too much to do to get it working better.
What advance are you seeing at 4000rpm?
What is the fuel pressure set at?
shame that bike hasnt got a reverse gear huh
tom reid
15-08-07, 06:00 PM
This bike don't need a reverse gear to go up your A$$
Fuel pressure is set @ 3Psi, and i've adjusted it to 3.5 with no noticable differences. I was getting 32 max Advance, but this may have been altered on the 2nd attempt.
I think i've found my answer though - Take it to someone else who knows what they're doing.
May have found a Local Pinto King who's very handy with Carbs !
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