jetting

DELLORTO JETTING

I am having problems with throttle response on a 907 engine fitted with Cosworth pistons.

At engine speeds of less than 2000RPM the vehicle will act as though the brakes have been applied when opening the throttle for sharp acceleration. Above 2000RPM the vehicle feels as though it would accelerate faster than the Schumacher brothers and never misses a beat.

The problem has been somewhat improved by a carburettor rebuild including all parts being cleaned in an ultrasonic bath. Further improvement has been made by retarding the ignition timing but this is unsatisfactory as it robs power.

Current carb jetting is as follows:

Main: 110 - 7772.5 - 142

A quirk of the symptoms is in that if the throttle is pressed more than once in quick succession, the hessitation is reduced. The acceleration cicuit appears sound and meets the volume delivery requirement to number of pumps.

All this said, it would appear that the idle circuits are lean. Can anyone help with jet selection for todays low octane fuels.

Federal settings are as follows:

Idle: 7850.6 - 56 (Less air more fuel)

Main: 110 - 7772.5 - 130 (Less fuel)

Can anyone help before I spend vast amounts of time road testing.

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I've had a similar problem on my Excel which improved greatly (but was not cured completely!) by rejetting.

I knew very little about dellortos but ended up finding an excellent website by a guy called Dave Andrews. http://hometown.aol.com/dvandrews/index.htm

Have a browse round, there is some pretty good info on jetting dellortos.

After talking to Dave Andrews and the guys at Contact Developments (UK dellorto importers) I ended up changing idle jets from 58 (standard on '83 Excel) to 62 and idle holders from .9 to .4 (richer, despite what the numbers imply!). First I only changed the holders but found I changing the jets also helped noticable.

From what I have learn't the main jets will not make much difference below 2000rpm, at these revs the progression is controlled by the idle jets/holders.

Unfortunately, the only way of really telling is by experimenting and it can get pretty expensive if you are not careful!

One thing I forgot to mention....

One of the reasons I started looking at re-jetting was that I'd always had a very lumpy idle and could never really adjust the carb balance and mixture to my liking. I finally borrowed a friends colour-tune (thing that lets you look at the spark as it runs) and found that the only way of getting a clean, regular blue flame at idle was wind the idle mixture screws out as far as they would go. This implied the jets were very weak for some reason. I rebuild the carbs which made no difference so then I started playing with jets/holders.

After rejetting I now find the mixture screws are in a much better position (about 3 turns from closed) so this re-inforces the diagnosis.

If you haven't already tried it might be worth trying a colour-tune. I found I got a much better idea of what was going on when I used it.

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Thanks Toby, that sounds promising and the Web Site was better than the book I bought. I had previously only found Dellorto tuning for Ducati and Harley Dave bikes.

There is one thing that still intrigues me and that is what the air bleed and locking nut assembly is for situated between the cold start drilling and choke.

Also, how much did you pay for Jets ? (approx).

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I can't remember exactly but I went to Contact Developments (near Reading) and negotiated (ie. Pleaded poverty!) for a set of jets plus holders.

I changed the jets in their yard, went for a drive, went back and then changed the holders. (they even lent me a screwdriver to do the job!) A lot easier than mail order!

I think I ended up paying around £15 to £20 for 4 jets and 4 holders although I could be wrong.

Contact developments are very helpful over the phone, their details are at http://www.racecar.co.uk/dellorto/

not sure what the other assy you are refering to is. I've got a dellorto manual at home so I will try and dig it out and find out what all the bits do again.

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I had the pilot jets re-jetted to one size larger than spec. to overcome flat spots and hesitation. This worked well, but has increased fuel consumption to 22mpg from 28 when it was very lean.

Paul Mattty's did miine for £20 during a normal service and were very helpful.

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I have just found my dellorto manual. Not suprisingly I can't work out what you mean by "the air bleed and locking nut assembly is for situated between the cold start drilling and choke."

Working back from the manifold end on top of the carb you have the mixture screws, a blanking plug, a blanking screw, the accelerator pump jet and then the choke assy. Probably doesn't help much anyway, if you do want more info let me know :-)

fwiw, the Contact developments price list shows idle jets at £2.75 each and the holders at £5 each.

Simon is right, re-jetting will not do you mpg much good. It might also cause problems at MOT time but this can be solved by careful adjustment of the mixture screws.

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Have you tried any of the following?

Check float chamber level heights these are essential for good idle mixture control.

Balanced carbs against each other with vac gauges?

Made sure that there are no air leaks? (run lean as anything when there is high manifold depression, i.e. tickover and low revs then picks up OK on wider throttle openings)

Made sure that the carbs are not torqued up too tight on the manifold so as they can't move anymore...this is bad they should be able to wobble up an down a little (stops the fuel in the float chambers from foaming....giving lean running)

Are the float jet sticking?

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I note the issue of flat spot etc and have found that this really in all instances is down to carb balancing venturi to venturi not just the main coupling between them.What you require is 4 mercury manometers which actually fit via factory adapators in each barrel.This actually measures the vacuum between the carb butterfly and inlet values. The actual air box etc stays in place.When the engine is started and idling 4 columns of mercury will individually raise to a height in i,e mm of mercury.What I have found is that all the loti i have owned are out of balance.What you have to achieve is 4 columns at the same height and to achieve this read the service notes.Please note before you touch any screws spray wd40 on all the o rings to check for air leaks etc, if the vacuum increases you have an air leak on that barrel(o rings),do not forget the servo take off also.

When you have achieved balance check exhaust co from 1 pipe with the other blanked off because each pipe will give a different reading.As per the service notes screw each idle screw in completly and each screw each 3 complete revs and adjust the co in to lean off or out to enrich.If you change 1 screw 4 revs all the screws should be 4 revs.

Following this ensure the igtn timing is correct for your engine build

please not i have tried all types of balancing aids, but not 1 is as sensitive as the mercury manometer and is actually used at the factory.it has been assumed that your engine is in a good state of health i,e compression test before balancing.

Rather a lot to state here, please give me a call on 01773 521388 to clarify.

P.s the reason cooling fans seize is that water runs down the bonnet and spills directly onto the fan assemblies.Each fan does not have any external sealing and therefore water enters and seizes the bearing at the fan end.Remember the fan comes from early escort/fiesta and the bonnet completly covers the fans in this instance and seals are not required in this application.

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Very useful info, thanks!

I have been on the lookout for a secondhand manometer, perhaps I need to stump up for a new one.

I have balanced my carbs using a gunson carb-balancer. You are right, they don't work too well as they restrict the airflow a lot and change the mixture completely.

One word of warning to others. If the mixture is too lean when you balance the carbs you will get very strange readings as it won't necessarily fire on every spark and this gives wildly varying vacuum! I found this out the hard way and it was only when I gave up balancing and used a colour-tune device to adjust the mixture that I realised what was happening (don't have a CO meter either!).

It's worth pointing out that my experiences might not be typical as my engine is far from perfect and I am probably compensating for more fundimental problems (lack of compression, coked up from burning too much oil etc etc)