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Engine holding back?
Capri18 - 10/6/10 at 11:22 AM

Hi all, I've just got my capri with a 2.0l pinto engine running on honda fireblade carbs and megajolt. The big carbs have had the main jets drilled to 1.7mm, the needle sliders have been raised slightly and the air correctors have been blocked. The problem is when further up the rev range 4000rpm+ the car feels likes it holding back at full throttle, if you back off the throttle slightly it seems to have more power. Can anyone shed some light on where I should begin to tweak?

Thanks

deezee - 10/6/10 at 12:00 PM

Has this been on a rolling road? Or have you just tried to tune the car in your garage? I think its going to have to go onto a rolling road if you want it sorted out.

David Jenkins - 10/6/10 at 12:09 PM

I had similar symptoms with my bike carbs - it turned out that my jets were too big, and it was going seriously rich at full throttle.

Other people have had similar results - you really need a RR session or access to a wideband lambda sensor to check.

franky - 10/6/10 at 12:13 PM

sounds like its over fueling.

norfolkluego - 10/6/10 at 12:20 PM

quote:
Originally posted by David Jenkins
I had similar symptoms with my bike carbs - it turned out that my jets were too big, and it was going seriously rich at full throttle.

Other people have had similar results - you really need a RR session or access to a wideband lambda sensor to check.


Agree with every word of that, swopping to 150 mains from the drilled out jets cured our problem (holding back around 4000)

Capri18 - 10/6/10 at 12:57 PM

Thanks folks, that was some seriously quick replies this site is brilliant! Yeah i agree it will need to get a proper rolling road set-up but i enjoy tinkering away until i get it as close as possible myself....or completely naff it up ha.

I'l try some smaller main jets and see how things go, does anybody know where i can get some jets from?

thanks again

tony-devon - 10/6/10 at 02:04 PM

not going to cost anything and from my experience with bikes it might give a result, I would drop them needles back down.

any bike shop should have the main jets, probably about 2 quid each, I amassed heaps of them over the years.

several good online suppliers, will sort out some links later if you need them

UncleFista - 10/6/10 at 02:25 PM

The CBR600 carbs on our Zetec were drilled to 1.65mm, I left the needles (after experimenting) and I can't remember blocking the air correctors (doesn't mean I didn't) and it runs perfectly.
No stumbling or flat spots.

Do you have the Honda workshop manual for the carbs ? If not, it's HERE

Also, I'm pretty sure Weber jets fit bike carbs, much cheaper too. Have a search (to search this site properly, instead of inserting a space between the words you're searching for, replace the spaces with percentage signs (%) ).

[Edited on 10/6/10 by UncleFista]

Capri18 - 10/6/10 at 04:20 PM

quote:
Originally posted by UncleFista
The CBR600 carbs on our Zetec were drilled to 1.65mm, I left the needles (after experimenting) and I can't remember blocking the air correctors (doesn't mean I didn't) and it runs perfectly.
No stumbling or flat spots.

Do you have the Honda workshop manual for the carbs ? If not, it's HERE

Also, I'm pretty sure Weber jets fit bike carbs, much cheaper too. Have a search (to search this site properly, instead of inserting a space between the words you're searching for, replace the spaces with percentage signs (%) ).

[Edited on 10/6/10 by UncleFista]


Thanks for the search tip there it really helps broaden the search results, yeah i've got the workshop manual you posted the link in another one of my posts a while back so thanks for that I think I'll get two set of 140 jets and then keep drilling one set out until I find a good setting, then the second set can be used in case I go a step too far

quote:
Originally posted by tony-devon
not going to cost anything and from my experience with bikes it might give a result, I would drop them needles back down.


Thats a good idea, i raised the needles because it was running very lean through the mid range but i blocked the air correctors and raised the needles at the same time, so maybe now with the air correctors blocked i will be able to run the standard needle height without it leaning out.

Thanks for the help everyone, if there's any more ideas I'm all ears

jacko - 10/6/10 at 04:44 PM

Do you mean you have lifted the main needles ? if so try lowering then and try again

[Edited on 10/6/10 by jacko]

Capri18 - 10/6/10 at 04:53 PM

quote:
Originally posted by jacko
Do you mean you have lifted the main needles ? if so try lowering then and try again

[Edited on 10/6/10 by jacko]


Yeah i lifted the main needles a tad with with some home made spacers, i'l put the standard spacers back in later and see how it goes. Annoyingly dads got the key for the lockup in his pocket at work, so i cant go and try out any suggestions untill he gets back

David Jenkins - 10/6/10 at 05:01 PM

My approach (after reading lots of stuff) was as follows:

1. Balanced the carbs.
2. Adjusted the idle mixture screws, using a AFR meter to check.

1 & 2 were repeated until the carbs were balanced and the mixture was right.

3. Mixture was checked when at wide-open throttle under load (i.e. accelerating hard up an unused road while someone read the display). Adjusted the jet size until this was OK.
4. Checked the mixture on over-run - not much you can do about that, but it's good to check. Mine was quite lean, but that's OK.
5. Checked the mixture at mid-range revs at steady (light) load - bumbling along the road at 40-ish. This is where you adjust the needles - which I can't, 'cos they're fixed length - I'm still slightly too rich there.

jacko - 10/6/10 at 06:29 PM

quote:
Originally posted by Capri18
Hi all, I've just got my capri with a 2.0l pinto engine running on honda fireblade carbs and megajolt. The big carbs have had the main jets drilled to 1.7mm, the needle sliders have been raised slightly and the air correctors have been blocked. The problem is when further up the rev range 4000rpm+ the car feels likes it holding back at full throttle, if you back off the throttle slightly it seems to have more power. Can anyone shed some light on where I should begin to tweak?

Thanks

What made you block the air correction holes?
is it what you have read on here ?
the air correction holes are blocked to stop the carbs / fuel going week at high revs so if your engine is holding back like on choke it may be because you have blocked the holes
The best and ONLY safe way of setting the carbs up is on a Rolling Road
Jacko

norfolkluego - 10/6/10 at 07:15 PM

quote:
Originally posted by Capri18
quote:
I think I'll get two set of 140 jets and then keep drilling one set out until I find a good setting, then the second set can be used in case I go a step too far
quote:

We run fireblade carbs on a 1.8 Zetec and 150 main jets are about spot on, I would have thought 140s would be too small on your set up