dgas/jets/rolling road ideas?

[paolow]

can anyone help? ive got a 2 litre pinto in my car in surprisingly good nick - ie, no oil burning and factory spec compression, but im having some teething troubles with my carb. i binned my old weber 32/36 and whacked on a weber 38 dgas which i downjetted to suit but my downjetting was done at 'best guess' rather than with any real evidence so im after some advice!
the car runs smoothly when hot but in a narrow band between, say 2800 and 5000 revs. before then it just bogs down and after that it has a sharp power drop off.
i dont think ive got the full whack of horses because of the fuelling so im after some jetting tips as im laying the blame at that door!
the car runs very well in the narrow power band but im after some tractability and im reluctant to stick 100 quid into a rolling road session if there are easy answers.
the question then is this: does there exist a simple listing of dgas jet settings for my carb and if so, where is it! i know the power is there to be had, i just want to make the most of it! the car is no slouch, but, its got a piper filter and a complete custom exhaust and that, coupled with the carb should make it a sporty beast no? as it is, its not bad, but no flier.
bizarrely, one symptom i have is that after heavy acceleration, when i lift off, the car surges forward for a second. is this indicative of overfuelling? might a RR be easiest? before i do so though, should i bite the bullet and buy a vernier cam cog to make it more productive? im not afraid of spending a bit of cash on it, but, as ever, the less the better!
a final question then, if anyone can help, i live in S/E london / kent. anyone got any personal RR recommendations to help me set my car up if thats what really is needed?
any advice would be very gratefully recieved.

[Pigeon]

Sounds like you're running too weak. Try putting the original jets back in for a start - it may still not be right, but it'll at least give you another data point for making comparisons.

[paolow]

Pigeon said:

Sounds like you're running too weak. Try putting the original jets back in for a start - it may still not be right, but it'll at least give you another data point for making comparisons.

not so easily done - i havent got them anymore. you know how it is, you get something, put it in a safe place, and thats the last you ever see of it . what i will do tonight tho is pop my plugs out and see if they give me any clues. i did also speak to one company who will rolling road it, but they are such a complete shower, id rather they didnt have my money. ill post again if i find anything interesting when i sort my plugs.

[350zwelgje]

It is possible to make a calculation for the proper carburettor setup (forgot it my self as not playing with carbs anymore for a long time), and if it is oversized the symptoms look like you are having. At low revs a carb needs enough 'flow' to be able to supply enough fuel. So if you went a size up, it won't be only the jets or needles. Don't know your carb but if it has two stages check if the first one (for low flow of air) is working/setup correctly.

Rob

[paolow]

350zwelgje said:

It is possible to make a calculation for the proper carburettor setup (forgot it my self as not playing with carbs anymore for a long time), and if it is oversized the symptoms look like you are having. At low revs a carb needs enough 'flow' to be able to supply enough fuel. So if you went a size up, it won't be only the jets or needles. Don't know your carb but if it has two stages check if the first one (for low flow of air) is working/setup correctly. Rob

i getcha, youre saying my carb is simply over choked for my engine. fair point. the two barrels open in a synchronous fashion rather than one then the other so thats probably not helping matters. interestingly i couldnt find my spark plug spanner so i had a crack at adjusting the idle jets by ear which yielded surprisingly good results. its smoother now so thats a start anyway. i will get the plugs out tomorrow for a look. hopefully they should make plain whats going on under there!

[paolow]

hmmm - this is interesting- i pulled the plugs out and, while the circular part of the business end was slightly blackened, the insulator was almost white and the electrode was sparkling clean. that would point to underfuelling right? hmmm - i can see me buying a colortune to sort this out!
is it actually necessary to change the air correctors etc or can i just play with the mains? what do air correctors do anyway?
have i got this right?
pump jets richen the mix temporarily when you accelerate to prevent the engine leaning off too much.
idles regulate the idle mixture, but work all the time so that they are always fuelling.
mains regulate the amount of fuel that can be drawn into the main venturi.
is that right?
anyone point me in the right direction?
thanks in advance

[Pigeon]

Yep, too weak. Your understanding of what the jets do seems to be correct. Googling for weber tuning, or weber tuning dgas, will probably be useful, as you'll come across things like this:

www.redlineweber.com/html/Tech/38_dgas_tunning.htm

www.preloved.co.uk/fuseaction-forums.showdiscussion/thread_id-3229/2ab9add5.html

[paolow]

great links pigeon. what ill do is to follow those instructions to the letter and give it another tune (if only you knew how much my neighbours love me ). interestingly though it would seem that my idles, for a start, are too small as you have to really back em out a long way before itll run properly.
probably my main problem stems from my initital overdoing of the undersizing to downjet it. probably what i should have done is just changed the pump jets and started there rather than thinking i could guess it all!

[steve_D]

The jets fitted to the replacement carb provided the correct ratio of fuel to the volume of air that could pass through that carb.
The only reason for putting a larger carb on an engine is if you have made modifications to the engine so that it now flows better and therefore needs more airflow.
If you have not made these mods then put the original carb back on.
Your engine will love you for it.
Steve