Help with carb needle

[akuchanny]

Hi I have just finished putting together a Frankensteine style engine, prior to sourcing a 1275 later on, and need some help with picking the right needle for the car in it's current state so I get the best of both power delivery and economy. Basically to make thecae as drivable as possible, currently it's a utter dog, after the standard manifold and carb where changed this week.

The spec so far is

A standard 948 block with no modifications
standard 1098 head
Lcb exhaust
Water heated inlet manifold
Hif38 1.5 inch carb
K&n cone filter
K&n stub stack in filter.

It's running exceptionally lean, and needs full choke or it will die or stall, so far I hav had the BDL needle suggested but have no idea if it is really the right choice.

Any help would be grateful received.

Andy.

[bmcecosse]

The 1098 head MUST be skimmed 60 thou before it is suitable for a 948....... Who suggested BDL needle ??? It won't fit that carb..... DO NOT connect the 'water heating' on the manifold, and try an AAA needle.....

[akuchanny]

Why should you not connect the water inlets on the manifold? Not too worried about the head skim, it's a temp head on loan until I decide what direction I'm taking, and the approx 1 horse power drop I can live with. Have played with the idle and it's running much better, but will source a needle and try it out.

[bmcecosse]

I suspect you will lose more than 1 bhp.....(more like 3 or 4) and the economy will suffer. Best head is a 12G 940 - from a 1275 engine. The water heating is not a good idea, certainly in summer - the ingoing charge wants to be as cold as possible.

[akuchanny]

Okay thanks for the info. The engine is going to be replaced by a 1275 at some point. But will take a look at the needle options for its current state of build.

[akuchanny]

Okay I tried a AAR needle and it was better but still awful, have filed it down a bit, which made an improvement, but still not great. The car is more drivable. I'm pretty sure that a AAA needle will be too extreme, but could give it ago when my supplier has one in stock.

Does anyone have any other suggestions? I'm even leaning to towards a rolling road session to sort it out as I do a lot of miles in the car, and it will be awhile before the 1275 swap will be happening. If I do go this route does anyone have anyone they could recommend who is not too expensive.

Thanks again.

Andy.

[bmcecosse]

You say it's a dog after the standard manifold was changed........so.....go back to the standard manifold?? The comp ratio will be costing you power - possibly less than a standard 948 with that head unless you sort out the CR. And it doesn't warrant the larger carb and exhaust etc in that state of 'tune' - and I use that word reluctantly!

[akuchanny]

I would but these are the parts I have, as the old manifold died prematurely. There are moments when it runs wonderfully, but then others when it seems possessed. Im hoping it's just a case of finding the right needle now, or having a tuner work their dark and mystic arts on the current set up, it is showing great promise when it is running well though.

[bmcecosse]

Carb piston sticking? Mech advance sticking? Vacuum advance sometimes working/sometimes not ? If the performance comes and goes -it should be easy enough to track down the reason......

[mike.perry]

If you want to play around with needles go to http://www.mintylamb.co.uk/suneedle and you can compare all the profiles

[bmcecosse]

If the power comes and goes -it's nothing to do with needle profile........

[akuchanny]

I've fitted an aaa needle, and although only roughly tuned, it's running much much better, still need go do some playing wit it, but for no what will have to wait as it's MOT time, and I have a nasty looking sill to deal with. But I will keep you posted on what happens when I get back to the carb, but in comparison to before it flew today.

Thanks for all the help so far

Andy

[chickenjohn]

DOES sound to me like there is some intermittent problem causing the performance problems. Try a new old stock condensor and set the points and timing. Also, do you have a manifold leak somewhere?? perhaps a loose vacuum breather pipe.

personally, I would just go back to the standard 1098 set up.

[PAULJ]

I tried three or four needles in my wifes car with an Ital engine. I settled for a HIF38 with AAA and it runs sweet as a nut. Not fast 80 tops in 5th but acceloration is good and is a rearly nice drive. 3.7 diff

[bmcecosse]

Yes - an AAA or sometimes an AAU is usually pretty close for that sort of engine set up - but on a 948 I have my doubts!!

[akuchanny]

Hi guys, im only using the 1098 head as my 948 head has warped and is no use at the moment. And i has planned a engine swap so dont want to spend money on repairs to the 948 head, The 1098 head is also on loan so I can not skim or make any permeant adjustments to it.

My eventual plan is to replace the 948 block with a 1275, but funds don't allow for all of this at the moment, so I have to make do with the 948 block until a 1275 appears in the right price range.

How would a 12g940 head work with the current set up? Keeping the lcb and the hif38, as these are the only parts to hand at the moment, I am unsure how this head would effect the engine, as it's from a 1275, would compression be an issue? And would valve clearances be a problem, I did have sone roller rockers offered to me but blive these would cause me no end of issues.

I have almost sorted the running of the engine in it's current state of tune, the carb manifold had worked loose and was the source of the intermittent problem, so has been nipped up now it's just fine tuning the carb, may try a AAU if I dint get further, but think that the problem power wise lies with the compression ratio with the head, as stated earlier. Although up to 60 it's very drivable, but then dies out more that it did in standard form.

Thanks again.

[bmcecosse]

A 940 head will be fine - nice big valves. It would be much better if you could find a camshaft from a 1098 engine (AEA 300)... You must have 320 thou clearance from head of valve to surface of the head. The CR will be fine - the 940 has small (21.4cc) combustion chambers. And you are right - DO NOT even consider non-standard rockers.......complete waste of money.

[akuchanny]

Hi again, sorry have not replied in a while, things have been hectic. I have got a 12g940 head, that is ported and polished and an 11 stud layout, and another that I think is standard, tried test fitting the ported head today, and am I right in saying that all the value seats need to be ground down around 40-50 thousand? Also I belive it currently has double springs, will this cause a problem? I''m guessing that the water pump will need to be filed down, or is there a suitable replacement, otherwise in going to have a waterfall down the front of the block.

I also wanted to find out if the head would then be suitable for a 1275 after the modifications, as I'd like to use it on the new block when sourced. I want to avoid any block work on the 948 as I don't want the engine out untill it's replaced.

The 1098 head in the current set up has lost a large amount of mpg (15!) and has never run massively well, also it's on loan and I'm not massively up for paying for it, then paying for more mods for it to work.

Thanks again for all the info so far, it's been really appreciated.

A.

[bmcecosse]

Use the standard 940 head on a 948 - the valves/ports etc are massive. Only sink the valves IF you don't have 340 clearance. Measure first! Save the fancy head for a 1275 engine.

[akuchanny]

Thanks will use the standard head, how do you measure clearance? As all books say I need to cut pockets into the block for this head to work on a 948.